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• Published: 22 September 2020

Impacts of solar intermittency on future photovoltaic reliability

• Jun Yin   ORCID: orcid.org/0000-0003-2706-0620 1 ,
• Annalisa Molini   ORCID: orcid.org/0000-0003-3815-3929 2 , 3 &
• Amilcare Porporato   ORCID: orcid.org/0000-0001-9378-207X 4 , 5  

Nature Communications volume  11 , Article number:  4781 ( 2020 ) Cite this article

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• Energy modelling

As photovoltaic power is expanding rapidly worldwide, it is imperative to assess its promise under future climate scenarios. While a great deal of research has been devoted to trends in mean solar radiation, less attention has been paid to its intermittent character, a key challenge when compounded with uncertainties related to climate variability. Using both satellite data and climate model outputs, we characterize solar radiation intermittency to assess future photovoltaic reliability. We find that the relation between the future power supply and long-term mean solar radiation trends is spatially heterogeneous, showing power reliability is more sensitive to the fluctuations of mean solar radiation in hot arid regions. Our results highlight how reliability analysis must account simultaneously for the mean and intermittency of solar inputs when assessing the impacts of climate change on photovoltaics.

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Introduction

Increasing the use of solar energy is widely regarded as one of the most effective approaches to reduce CO 2 emissions, yet the short-term intermittent nature imposes definite limitations to its reliability. While this problem may be partially solved by power storage, geographic dispersion, load control, and radiation forecasting 1 , 2 , 3 , it still has significant impacts on the grid integration of solar energy. For instance, photovoltaic power plants in Northwestern China (capacity of 43.87 GW in 2019, 1/3 of China’s total) were punished for providing intermittent energy to the Northwest Grid with fines of $28 million US dollars in 2017, $42 million in 2018, and $28 million for the first half of the year 2019, whereas coal-fired and hydropower plants were rewarded for their constant and even dispatchable sources of electricity 4 , 5 , 6 . Similarly, the example of Kauai island, Hawaii, a world pioneer in using renewable energy 7 , currently relies on diesel generators on overcast days 8 , 9 . While the solar radiation varies across a range of timescales, here we focus on the daily level, which accounts for a significant portion of the penalty in the case of the Northwestern China 4 , 5 and is closely related to the power reliability in Kauai, Hawaii 9 .

The daily radiation is expected to change in future climates due to altered cloud and aerosol patterns 10 , 11 , 12 , 13 , 14 , presenting additional challenges for the long-term planning and management of solar energy. Previous studies have focused mostly on the relative change of long-term mean radiation input 15 , 16 , 17 , 18 , 19 . While mean metrics are essential, the portion of time with energy supply lower than the demand, termed loss-of-load probability (LOLP) 20 , which is related to the reliability and the market values of power output, cannot be captured by mean values alone. Power reliability is vital for grid planning and management. For example, the solar plant from Tesla is expected to provide 52 MWh of electricity every evening to the power grid in Kauai, Hawaii 7 . Tesla’s design of 13 MW solar array and 52 MWh effective battery storage result in an LOLP of 0.12, possibly maximizing the net profit while still satisfying the reliability requirement 9 . In a grid-connected system, LOLP is directly associated with the operating cost of the peaking plants (e.g., diesel generators in Kauai, Hawaii 8 , 9 , hydropower stations in Northwest of China 21 , gas turbines in the Great Plains, United States 22 ) and thus linked to the market values of the solar energy.

To investigate the impacts of future climates on LOLP, we combine here satellite-derived data and climate model outputs. In particular, we focus on the impact of incident solar irradiance, one of the dominant factors controlling solar power generation 15 , 17 , 18 . We show the nonlinear behaviors of LOLP in response to climate change, pointing towards a tradeoff between the potential power outputs and the power reliability.

Characterizing solar energy intermittency

We begin our investigation with an analysis of the clearness index, K , defined as the ratio between the near-surface global horizontal irradiance (GHI, including direct and diffuse irradiance) and the corresponding extraterrestrial horizontal irradiance (see “Methods” section). This index accounts for the scattering, absorption, and reflection of solar radiation from all optically active constituents in the atmosphere, such as clouds and aerosols, and is often used in solar energy industry 23 , 24 , 25 , 26 . For example, we consider Southeastern Romania’s case, where climate change has shown strong regional impacts 27 and the case of Dubai, UAE, which is pursuing an ambitious plan to foster solar energy development in the region 28 . Romania and UAE, located in the continental and desert climatic zones, also have two contrasting cloud seasonality (see Supplementary Fig.  1 ) and drastically different solar energy production potentials. We use satellite data from Clouds and the Earth’s Radiant Energy System (CERES), which are based on column-model estimates and have been already used for solar power assessment 29 , 30 . Such multi-decadal records allow us to characterize the empirical distributions of daily K . As can be seen in Fig.  1 , the K distributions for larger mean values (denoted as μ and also referred to as the mean clearness index) tend to have longer left tails, which are associated with the weaker solar radiation and lower power generation.

The distribution of clearness index ( K ) derived from the CERES data in ( a , c ) January and ( b , d ) July during 2001–2009 (blue lines) and during 2010–2018 (red lines) in ( a , b ) Southern Romania and ( c, d ) Dubai. The hatched areas indicate the probability when power generation does not meet the demand, the loss-of-load probability (LOLP). The averages of clearness index are marked by the vertical dash lines and the values are reported in Supplementary Table  1 . Source data are provided as a Source Data file.

From the K distribution, the LOLP of a solar power plant operating at daily basis (e.g., the Tesla’s power plant at Kauai, Hawaii) can be estimated as the fraction of days with solar radiation lower than the demand value,

where f ( K ) is the probability density function (pdf) of K , and K D is the value of K that is just sufficient to meet the energy demand (see “Methods” section). LOLP is, therefore, the cumulative density function (CDF) of K at K D . This metric has long been used for designing a stand-alone (off-grid) photovoltaic power system 31 , 32 , 33 and is also a critical reference for evaluating a grid-connected system 20 . The constant demand K D in (1) is similar in spirit to the regulation from Northwest Grid of China, which was originally issued for coal plants considering their relatively constant power output but was recently extended to solar and wind power plants. A thorough characterization of the global solar power intermittency and its response to climate change using the LOLP is a fundamental starting point to assess the future reliability of photovoltaic.

Climate-change impacts on power reliability can be assessed by considering the change of LOLP during the lifespan of typical photovoltaic modules. Going back to the case of the Southern Romania, a solar plant designed under historical climate records of 2001–2009 is assumed to have a design LOLP, LOLP D , of 0.3. Over the following nine years (2010–2018), the mean of K increases in both January (Δ μ = 0.015) and July (Δ μ = 0.03), which may be associated with the change of climate seasonality 34 . The corresponding values of LOLP drop from the design value of 0.3 to 0.27 in winter (ΔLOLP = −0.03) and to 0.21 in summer (ΔLOLP = −0.09), respectively (see the hatched and shaded areas in Fig.  1a, b ). For the case in Dubai, aerosol optical depth trends 35 may account for the increase of μ in winter, leading to a decrease of LOLP (Fig.  1c ), while the monthly mean clearness index remains relatively constant in summer (Fig.  1d ). The comparisons between these two periods (2001–2009 and 2010–2018) objectively quantify not only the increase in mean surface solar radiation, but also the increase in its reliability.

With this methodology, we now move to the future climate scenarios and use climate model outputs (see “Methods” section) to calculate the changes of μ and LOLP between 2006–2015 and 2041–2050, consistently with the typical lifespan of photovoltaic modules. As shown in Fig.  2a, b and in agreement with previous studies 15 , the change of solar radiation is evident in some regions and show marked seasonal variations. The solar radiation in Europe is projected to decrease in January and increase in July, which may be associated with the projected changes in rainfall seasonality and the corresponding cloud variations 34 . The decrease in solar radiation in the Middle East may be associated with large-scale circulation 36 , cloudiness trends 37 , or the positive trends of aerosol optical depth as documented over large parts of the Middle East for the period 2001–2012 35 .

The color at each grid point represents the ensemble means of ( a , b ) the relative change of mean clearness index (Δ μ / μ ) and ( c , d ) the change of loss-of-load probability (ΔLOLP) between 2006–2015 and 2041–2050 in the month of ( a , c ) January and ( b , d ) July from 11 climate model outputs. The LOLP during 2006–2015 (i.e., design LOLP) is set as 0.3; maps with other design LOLP show similar patterns (see Supplementary Figs.  2 and 3 ). The dots show the ensemble mean of the corresponding variables are statistically different than zero, suggesting consistent variations of solar radiation or reliability from most climate models ( t -test, 5% significance level; statistics of the sign of the changes are given in Supplementary Figs.  4 – 6 ). Source data are provided as a Source Data file.

This redistribution of the Earth’s energy and shifts in climate seasonality 38 have direct impacts on solar power reliability as quantified by the corresponding variations of LOLP (see Fig.  2c, d ). Although it is apparent that increasing solar radiation (Δ μ  > 0) often leads to more reliable power output (ΔLOLP < 0), this relationship is clearly nonlinear. For example, the slight decrease of solar radiation in the Middle East and Northern Africa results in a significant increase of LOLP; an increase of solar radiation in the west of Amazon rainfall forest in July leads to a sharp decrease of LOLP; strong variations in both radiation and power reliability are shown in the Northern United States in January. In what follows, we will investigate this nonlinear relationship to quantitatively link the previous reports on mean solar radiation to one of our major concerns on power reliability.

Theoretical framework for power reliability

The case studies in Fig.  1 and geographical patterns in Fig.  2 suggest that LOLP may be linked to the distribution of K , which in the solar industry is often associated with the mean clearness index, μ 39 , 40 . To systematically and theoretically assess this linkage, we consider in detail satellite data as well as climate model outputs under the historical climate conditions. We obtained the statistics of K from all regions over the world with μ ranging from 0.3 to 0.7 with a binning interval of 0.05 (see dark color curves in Fig.  3a and Supplementary Fig.  7 ). As can be seen, f ( K ) tends to be positively skewed in regions with smaller μ and negatively skewed in regions with larger μ (see Fig.  3a ). Since the diffuse radiation has the largest variations for moderate K 39 , which includes direct and diffuse radiation, it is logical to expect σ first increases and then decreases with rising μ as presented in Fig.  3b . Overall, such empirical distributions even under changing climate conditions turn out to be well described by beta distributions (see “Methods” section).

a Probability density functions (pdf) of daily clearness index ( K ) in different regions over the world (binning width of 0.05) from the satellite data in January during 2001–2009 (dark color) and during 2010–2018 (light color). b Relationship between mean ( μ ) and standard deviation ( σ ) of daily K . The black/blue/red dots correspond to the lines in the a ; the grey dots are from 11 climate model outputs during 2006–2015; the dash green curve shows the best quadratic fit. c dσ / dμ calculated as the derivative of the corresponding σ ~ μ relationship in ( b ). Source data are provided as a Source Data file.

One may wonder whether these characteristics can vary in response to changing climates. To address this point, we checked the statistics of K at different periods (see the light-color curves in Fig.  3a , Supplementary Figs.  7 and 8 ). The results show that the distributions of K appear identical and the μ ~ σ relationships almost remain unchanged. These behaviors essentially describe how the intermittency of solar radiation (i.e., σ ) will adjust after the change of mean solar radiation (i.e., μ ), providing valuable information for solar power planning and management.

The invariant characteristics of K allow us to link Δ μ / μ to ΔLOLP between different periods and thus, in turn, to obtain power-reliability information from previous reports on long-term mean solar radiation. Operationally, this can be accomplished by Taylor expanding Eq. ( 1 ) to first order as

where L s is the sensitivity of LOLP to μ and can be derived analytically for the beta distribution of K (see “Methods” section), and the change of μ in percentage format is usually consistent with other reports. In Eq. ( 2 ), the first term evaluates the climate impacts in terms of LOLP, whereas the term in the bracket assesses the future solar radiation in the conventional apporach 15 , 16 , 17 , 18 , 19 . The relation between the two, ΔLOLP and Δ μ/μ , is clearly associated with the sensitivity parameter L s , a nonlinear function of μ and K D (or design LOLP, see Eq. ( 11 ) in “Methods” section). Particularly interesting is the fact that the absolute values of L s are larger in sunny regions/seasons with larger μ (see Fig.  4a ). This may be accounted for by the fact that the small perturbation of μ in sunny regions tends to have larger change in the variability of solar radiation (i.e., large absolute values of dσ / dμ , see right side of Fig.  3c ), which is obviously associated with the intermittency of solar energy. Since these are also the regions of the world where the largest solar plants are expected to be deployed in the future, this fact should be considered with great attention in reliability analysis.

Contour plots of L s is calculated ( a ) analytically from Eq. ( 2 ) and ( b ) numerically from climate model outputs. The red and blue dots in ( b ) are corresponding to the examples in ( d ), which compares the change of loss-of-load probability (LOLP) and the change of mean clearness index ( μ ) from 2006–2015 to 2041–2050 in January with design LOLP of 0.3 in regions where 0.3 <  μ  < 0.35 (red dots) and 0.65 <  μ  < 0.7 (blue dots) as projected by climate models. The red and blue lines are the corresponding best fit lines and their slopes (i.e., ΔLOLP / (Δ μ / μ )) numerically represent L s . ( c ) As in ( d ) but only for Bulgaria, Cyprus, Greece, Hungary, and Romania (i.e., region 7 defined in ref. 18 ) in January (red dots) and July (blue dots). The red and blue circles correspond to the example of Southern Romania in Fig.  1 . Source data are provided as a Source Data file.

Climate model outputs corroborate the previous analytical results. Figure  4d shows Δ μ / μ and ΔLOLP between 2006–2015 and 2041–2050 for given values of μ and design LOLP. The slopes of these two quantities are reported in Fig.  4b , showing similar patterns as their analytical counterparts (Fig.  4a ).

With the obtained nonlinear function of L s , one can readily infer the power reliability. To facilitate this, we mapped the analytical solution of L s in Fig.  4a to each location over the world with monthly mean clearness index from CERES data (see Fig.  5 and Supplementary Figs.  9 and 10 ). These maps could serve as lookup tables to assess power reliability in future climates. For example, Fig.  5 shows that L s is approximately −0.8 in January and −1.6 in July in Southern Romania for a design LOLP of 0.3. The mean solar radiation in this region is projected to vary around −15~0% in winter and around −5~5% in summer toward the end of the century 18 . Multiplying these variations by L s , one can find the impacts of these variations on LOLP (i.e., 0~12% in winter and −8~8% in summer). While the winter season has larger variations in solar radiation, it also has a small absolute value of L s so that the impacts on future power reliability in winter are reduced. This analysis is corroborated by the results from climate-model outputs as shown in Fig.  4c , which suggests larger spread of ΔLOLP but slightly smaller change of Δ μ / μ in summer in the surrounding of Romania.

This sensitivity in ( a ) January and ( b ) July is obtained from analytical solutions with design LOLP of 0.3 and solar radiation climatology from CERES. Source data are provided as a Source Data file.

The heterogeneous distribution of LOLP sensitivity in Fig.  5 essentially stems from the nonlinear relationship between μ and σ , which remains relatively constant under changing climates. Lower absolute values of L s with smaller clearness index suggest that the solar power in humid subtropics may have lower potential for large variability in future climates. This is consistent with the observed σ ~ μ relationship in Fig.  3b , where these slopes are flatter for smaller μ . Meanwhile, the humid subtropics are predicted to have relatively more solar radiation in the future climate scenarios 15 , 17 . The multiplication of small negative L s with positive Δ μ yields small but negative ΔLOLP, suggesting slightly higher power reliability. On the other hand, the arid hot regions are predicted to have less solar radiation but could yield much lower power reliability due to the strong LOLP sensitivity.

Our proposed framework may be further extended to diagnose the impacts of power storage, which is regarded as one of the most important solutions to the intermittency problems. Power storage smooths the power output to provide reliable energy. In our analysis, this effect may be considered by reducing the daily variability of future solar radiation and evaluating its impacts on LOLP (see “Methods” section). As expected, solar radiation with reduced variability has smaller LOLP, showing that increased storage can be used to mitigate the intermittency’s impacts in most parts of the world. However, this may not be sufficient in a few regions such as the Middle East (see Supplementary Fig.  11 ). When mean solar radiation is significantly reduced as predicted by climate models, it may require increasing both the power storage capacity and solar module size.

To investigate more detailed grid operation and conduct cost-benefit analysis of various mitigation strategies, the proposed framework may be extended by statistically downscaling the daily solar radiation to the hourly timescale 39 , 41 and involving multiple power sectors for power generation, storage, transmission, distribution, marketing, and technology development 1 , 2 . Our framework could also be used to analyze the temperature impacts on power reliability (see “Methods” section), although it is argued that the temperature impacts on photovoltaic power generation appear much weaker than the solar radiation impacts over the lifespan of photovoltaic modules 36 .

In summary, our results have shown how the impacts of this radiation change on power reliability could be significant due to the large absolute values of LOLP sensitivity, which had not emphasized previously. The sensitivity analysis points towards a tradeoff between the mean solar radiation that quantifies the total potential solar power and the power reliability, which being related to intermittency remains a major concern in the absence of large power storage options. This contrasting behavior between solar power availability and reliability requires special attention in assessments of future solar energy scenarios.

Clearness index (K)

The daily clearness index, K , is defined as

where T is the length of 1 day, GHI is the near-surface global horizontal irradiance, which is the sum of the direct and diffuse irradiance, and EHI the extraterrestrial horizontal irradiance. Daily GHI are obtained from CERES SYN1deg during 2001–2018 and from 11 climate model outputs (ACCESS1.3, BCC-CSM1.1 m, CanESM2, CCSM4, CMCC-CMS, CSIRO-Mk3.6.0, EC-EARTH, GFDL-CM3, INM-CM4, IPSL-CM5A, and MPI-ESM) in “rcp45” experiment during 2006–2015 and 2041–2050. All these data have been used to obtain the empirical distributions of K for calculating the loss-of-load probability as explained next.

Loss-of-load probability (LOLP)

The photovoltaic power output is related to the incident solar radiation and other factors controlling the solar cell efficiency 15 . Each month, the Sun’s declination angle has small variations; the daily incident solar radiation on a fixed or tracking array can be approximated as a monotonic function of daily clearness index 42 . Factors such as soiling and tree shading on solar modules could have notable impacts on power generation but can be controlled by regular maintanence. The solar cell efficiency factors such as air temperature and wind speed usually have only secondary impacts and are discussed in the following “Methods” section. Regarding climate change impacts, the incident solar radiation has been identified as the dominant factor for photovoltaic power generation. For this reason, we model the power output as a monotonic function of the clearness index, say p  =  g ( K ). This function can be used to estimate the LOLP. Similarly to the off-grid version of a photovoltaic software 43 , 44 , LOLP can be defined as the fraction of days when daily energy supply ( p ) is lower than the daily demand ( p D ). We obtain LOLP as the derived distribution of K ,

where K D is the specific value of K that is just enough to generate the demanding energy p D , f (·) and F (·) are the probability and cumulative density function of K . These functions are estimated from multi-year historical climate records, and thus the corresponding LOLP already captures the interannual variability of daily power generation. Such estimates are referred to as design LOLP, LOLP D . For the lifespan of typical photovoltaic modules (20–30 years), one can then quantify the climate impacts on power reliability as the change of LOLP from its design value.

LOLP sensitivity ( L s )

The distributions of K enters the LOLP expression in Eq. ( 4 ). As presented in Fig.  3a , the distribution of K tends to be positively skewed for smaller mean value of K (denoted as μ ) and negatively skewed for larger μ . These behaviors may be described as beta distributions naturally bounded between 0 and 1. This is confirmed by the results of the Kolmogorov-Smirnov goodness-of-fit tests over most regions in the world in different climate zones (see Supplementary Fig.  12 and Supplementary Table  2 )

where β 1 and β 2 are the shape parameters. Note that this beta distribution is a parsimonious choice which we prefer to other unbounded distributions (e.g., Weibull and extreme value distributions) used in the literature 45 , 46 . We stress however that our framework is not limited to the use of beta distributions but can easily adopt other distributions if they appear more suitable in some regions (e.g., Australia and Western Sahara). These shape parameters can be expressed by the mean ( μ ) and standard deviation ( σ ) of the distribution 47 ,

As described in Fig.  3b , the standard deviation may be modeled as a function of mean (e.g., \(\sigma = - 0.83\mu ^2 + 0.65\mu + 0.03\) , the best quadratic fit) so that the distribution of K can be written as

Substituting ( 8 ) into ( 4 ) and performing a Taylor expansion to first order yields

where \(F_b( \cdot )\) is the cumulative beta distribution and K D is equivalent to design LOLP,

The corresponding analytical solutions of L s (Fig.  4a ) are very similar to its counterpart calculated numerically as \({\Delta}{\mathrm{LOLP}}/({\Delta}\mu /\mu )\) (Fig.  4b ). The approximation of Taylor expansion to the first order is justified by the fact that L s is relatively constant for a small perturbation of μ (see Fig.  4a ). Clearly, one can insert other distributions suitable in some specific regions into Eqs. ( 4 ) and ( 9 ) to obtain the corresponding analytical expression for the sensitivity of power reliability.

Impacts of temperature change on power reliability

Temperature influences the energy conversion efficiency and can have significant impacts on power generation in hot climates 48 . It is estimated that photovoltaic power output reduces by 0.45% for each degree increase in temperature 49 , 50 . Therefore, we may treat the temperature rising as equivalent to the increase of power requirement in our original framework and redefine the parameter K D as

where the temperature factor, γ T , is about 0.0045/K, T r is the reference temperature, and K D , r is the specific value of K that is just enough to generate the demanding energy at the reference temperature. With this change, the corresponding LOLP becomes,

The change of LOLP from current to future climate conditions can be expressed as

This expression suggests that the change of LOLP has two parts. This first part is in Eq. ( 9 ) and the second part can be obtained analytically by substituting Eq. ( 5 ) into Eq. ( 15 ). The sensitivity for temperature, L T , is always positive (see Eq. ( 15 )), meaning that rising temperature increases the LOLP.

Impact of power storage on power reliability

Power storage at multiday timescale, if feasible, would obviously help improve power reliability. To explore this issue within the scope of the present analysis, as a proof of concept, we simply smoothed the daily clearness index to roughly estimate the impacts of power storage on power reliability

where the clearness index K is smoothed into K b . The corresponding standard deviation becomes

where the coefficient b controls the reduction of the variability. This coefficient b is set as 0.75 and 0.5 for two future scenarios corresponding to the 25 and 50% variability mitigation.

We applied Eq. ( 17 ) to recalculate the clearness index from 11 climate model outputs during 2041–2050, which were then used to numerically calculate the LOLP. We showed the change of LOLP with no variability mitigation, 25% mitigation, and 50% mitigation in Supplementary Fig.  11 . Reducing the variability leads to a decrease of LOLP and thus more reliable power output as expected. This is generally sufficient for addressing some of the challenges of intermittent solar power and the uncertainties related to climate change. In some regions, however, climate models also predict decreasing trends of mean solar radiation, which may not be compensated only by the power storage. This is the case of the Middle East, where solar power is projected to be significantly reduced, so that LOLP increases even with variability mitigation measures (see Supplementary Fig.  11 ).

Data accuracy

To provide information regarding the data accuracy, we compared these satellite data and climate model outputs with the data from National Solar Radiation Database (NSRDB). The latter are produced by ground observations, satellite data, and meteorological models and are arguably one of the most reliable datasets for assessing the long-term spatial and temporal variability of the solar resource 51 . It should be noted that validating the global solar irradiance and surface energy balance is one of the biggest challenges in the climate science community 52 , 53 .

Two typical outputs with different assimilation models, METSTAT and SUNY, are achieved in NSRDB [ https://rredc.nrel.gov/solar/old_data/nsrdb/ ] and both are recommended by NREL. We compared SUNY and METSTAT during 2001–2010 when both products are available (see Supplementary Fig.  13a, b ). Of 1415 sites over the United States (sites with missing data are excluded), the root mean square errors (RMSE) between these two outputs are around 0.05, which may be considered as the systematic biases from NSRDB. When further compared these measurements with satellite data (CERES SYN) in the same locations during the same period (see Supplementary Fig.  13c–f ), one finds similar ranges of RMSE, suggesting that the satellite products are as accurate as these reliable data.

We then compared the long-term clearness index from the satellite data and the climate model outputs during 2006–2015 averaged at 280 km equal-area grids over the world (see Supplementary Figs.  14 , 15 , and Supplementary Table  3 ). The RMSE for some climate models (e.g., CCSM, GFDL) are similar to these SUNY-METSTAT differences from NSRDB, while for others the RMSE is at least of the same order of magnitude.

Besides these data comparison, it is also important to note that aerosol is a key climate component and future aerosol emissions are usually described as different scenarios such as Representative Concentration Pathways (RCPs) 54 . Our results are from RCP45, which includes the projected decline in aerosols during the 21th century because of the emission controls 55 . While the future aerosol emissions are prescribed, not all models include their indirect effects related to the aerosol-cloud interaction (see Supplementary Table  4 ), which could have an impact on cloud formation and the prediction of solar radiation 56 . However, these indirect effects do not seem to have strong impacts on the relationship between the mean and standard deviation of the radiation (see Supplementary Fig.  8 ), a key feature in our analysis of power reliability.

Data availability

The climate model data were downloaded from the fifth phase of the Coupled Model Intercomparison Project website [ http://cmip-pcmdi.llnl.gov ]. The satellite data from CERES were obtained from website [ https://ceres.larc.nasa.gov/order_data.php ]. Source data are provided with this paper.

Code availability

Matlab code for calculating the analytical solutions of power reliability sensitivity is available at [ https://github.com/jy8/solar ]; other codes are available upon request.

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Acknowledgements

We would like to thank Professors Robert Socolow and Tiejian Li for their constructive comments on this work. J.Y. acknowledges support from the National Natural Science Foundation of China (41877158, 51739009), NUIST startup funding (1441052001003), Jiangsu distinguished faculty program, and NUIST’s supercomputing center. A.P. acknowledges support from the USDA Agricultural Research Service cooperative agreement 58-6408-3-027; and National Science Foundation (NSF) grants EAR-1331846, EAR-1316258, FESD EAR-1338694, and the Carbon Mitigation Initiative at Princeton University. A.M. acknowledges support from the Khalifa University Competitive Internal Research Award, CIRA-2018-102.

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Yin, J., Molini, A. & Porporato, A. Impacts of solar intermittency on future photovoltaic reliability. Nat Commun 11 , 4781 (2020). https://doi.org/10.1038/s41467-020-18602-6

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Recent Developments and Challenges in Solar Harvesting of Photovoltaic System: A Review

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Solar energy is a green and renewable energy source which is commonly used in photovoltaic and thermal cells. Solar power systems are among the fastest developing alternatives to fossil fuels, extending to commercial and industrial applications. As the position of the sun and other significant aspects fluctuate constantly, only a percentage of the sun’s energy potential is a harness. Solar trackers are ideal for enhancing solar panels’ conversion effectiveness by following the sun’s position all day long. This paper is an overview to take full advantage of the PV system by enhancing the solar panel’s conversion efficiency and choosing effective solar tracking. This review describes the basics of PV material and their achieved efficiency for laboratory, commercial, and industrial applications. The key objective of this paper is to create a roadmap of sun-tracking methods, their pros and cons to build an effective, low-cost, and reliable PV system for maximum solar energy harvesting. The study revealed that the active dual-axes closed-loop control based on non-conventional control algorithms could be the best tracking method to maximize the PV system’s output.

• Photovoltaic system
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Alok Sanyal, MD Faiyaz Ahmed & J. C. Mohanta

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Sanyal, A., Ahmed, M.F., Mohanta, J.C. (2023). Recent Developments and Challenges in Solar Harvesting of Photovoltaic System: A Review. In: Li, X., Rashidi, M.M., Lather, R.S., Raman, R. (eds) Emerging Trends in Mechanical and Industrial Engineering. Lecture Notes in Mechanical Engineering. Springer, Singapore. https://doi.org/10.1007/978-981-19-6945-4_18

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Photovoltaic Cell Generations and Current Research Directions for Their Development

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The purpose of this paper is to discuss the different generations of photovoltaic cells and current research directions focusing on their development and manufacturing technologies. The introduction describes the importance of photovoltaics in the context of environmental protection, as well as the elimination of fossil sources. It then focuses on presenting the known generations of photovoltaic cells to date, mainly in terms of the achievable solar-to-electric conversion efficiencies, as well as the technology for their manufacture. In particular, the third generation of photovoltaic cells and recent trends in its field, including multi-junction cells and cells with intermediate energy levels in the forbidden band of silicon, are discussed. We also present the latest developments in photovoltaic cell manufacturing technology, using the fourth-generation graphene-based photovoltaic cells as an example. An extensive review of the world literature led us to the conclusion that, despite the appearance of newer types of photovoltaic cells, silicon cells still have the largest market share, and research into ways to improve their efficiency is still relevant.

1. Introduction

Concerns about climate change and the increase in demand for electricity due to, among other things, an ever-growing population, necessitate efforts to move away from conventional methods of energy production. Rising carbon dioxide levels in the atmosphere caused by the use of fossil fuels is one of the factors causing ongoing climate change. Switching to renewable energy will produce energy with a smaller environmental footprint compared to fossil fuel sources. We are able to harness the full potential of sunlight energy to develop the best possible energy harvesting technologies capable of converting solar energy into electricity [ 1 ].

The currently used solar energy is very marginal—0.015% is used for electricity production, 0.3% for heating, and 11% is used in the natural photosynthesis of biomass. In contrast, about 80–85% of global energy needs are met by fossil fuels. The difficulty with fossil fuels is that their resources are limited and hostile to the environment due to their CO 2 emissions. For instance, for every ton of coal burned, one ton of carbon dioxide is released into the atmosphere. This emitted carbon dioxide is toxic to the environment and is a primary cause of global warming, the greenhouse effect, climate change, and ozone depletion [ 2 ].

The necessity of finding new renewable energy forms is extremely relevant and urgent today. That is why mankind must find alternative sources of energy to provide a clean and sustainable future. Within this context, solar energy is the best option among all alternative renewable energy sources due to its widespread accessibility, universality, and eco-friendly nature [ 3 ].

The most common metric used to evaluate the performance of photovoltaic technologies is conversion efficiency, which expresses the ratio of solar energy input to electrical energy output. The efficiency combines multiple component characteristics of the system, such as short-circuit current, open-circuit voltage, and fill factor, which in turn are dependent upon basic material features and manufacturing defects [ 4 ].

The cost-effectiveness of making a photovoltaic cell and its efficiency depend on the material from which it is made. Much research in this field has been carried out to find the material that is the most efficient and cost-effective for building photovoltaic cells. The specifications for an ideal material for PV solar cells include the following [ 5 ]:

• The cells are expected to have a band gap between 1.1 and 1.7 eV;
• Should have a direct band structure;
• Need to be easily accessible and non-toxic; and
• Should have high photovoltaic conversion efficiency [ 5 ].

A key problem in the area of photovoltaic cell development is the development of methods to achieve the highest possible efficiency at the lowest possible production cost. Improving the efficiency of solar cells is possible by using effective ways to reduce the internal losses of the cell. There are three basic types of losses: optical, quantum, and electrical, which have different sources of origin. Reducing losses of any kind requires different, often advanced, methods of cell manufacturing and photovoltaic module production. An upper efficiency limit for commercially accessible technologies is determined by the well-known Shockley–Queisser (SQ) limit, taking into account the balance between photogeneration and radiative recombination [ 6 ].

However, the greatest potential lies in the ability to reduce quantum losses, as they are intimately connected with the material properties and internal structure of the cell. Relevant here is the concept of band gap, which defines the minimum required energy of a photon incident onto the cell surface for it to take part in the photovoltaic conversion process. There is a relationship between the efficiency of the cell and the value of the band gap, which in turn is highly dependent on the material from which the photovoltaic cell is made. The basic, commonly used material for solar cells is silicon, which has a band gap value of about 1.12 eV, but by introducing modifications in its crystal structure, the physical properties of the material, especially the band gap width, can be affected [ 7 ].

The dominant loss mechanisms in conventional photovoltaic cells are the inability to absorb photons below the band gap and the thermalization of solar photons with energies above the band gap energy. Third-generation solar cell concepts have been proposed to address these two loss mechanisms in an attempt to improve solar cell performance. These solutions aim to exploit the entire spectrum by incorporating novel mechanisms to create new electron–hole pairs [ 8 ].

Major development potential among these concepts for improving the power generation efficiency of solar cells made of silicon is shown by the idea of cells whose basic feature is an additional intermediate band in the band gap model of silicon. It is located between the conduction band and the valence band, and its function is to allow the absorption of photons with energies below the width of the energy gap, resulting in higher quantum efficiency (a higher number of excited electrons in relation to the number of photons incident onto the surface of the cell) [ 9 ]. Currently, many directions of research development on the introduction of intermediate bands in semiconductors can be identified. One of them is the use of ion implantation, where two methods can be distinguished: introduction of dopants with extremely high concentrations to the substrate of the semiconductor, and implantation of the layer of silicon with high-dose metal ions [ 10 ].

The improvement of solar cell efficiency involves reducing various types of losses affecting the resultant cell efficiency. The National Renewable Energy Laboratory (NREL) runs a compilation of the highest verified research cell conversion efficiencies for different photovoltaic technologies, compiled from 1976 to the present ( Figure 1 ). Cell efficiency results are given for each semiconductor family: multi-junction cells; gallium arsenide single-junction cells; crystalline silicon cells; thin film technologies; emerging photovoltaic technologies. The latest world record for an individual technology is indicated by a flag across the right edge containing the efficiency and technology symbol [ 11 ].

NREL Best Research-Cell Efficiencies chart [ 11 ].

Photovoltaic cells can be categorized by four main generations: first, second, third, and fourth generation. The details of each are discussed in the next section.

2. Photovoltaic Cell Generations

In the past decade, photovoltaics have become a major contributor to the ongoing energy transition. Advances relating to materials and manufacturing methods have had a significant role behind that development. However, there are still numerous challenges before photovoltaics can provide cleaner and low-cost energy. Research in this direction is focused on efficient photovoltaic devices such as multi-junction cells, graphene or intermediate band gap cells, and printable solar cell materials such as quantum dots [ 12 ].

The primary role of a photovoltaic cell is to receive solar radiation as pure light and transform it into electrical energy in a conversion process called the photovoltaic effect. There are several technologies involved with the manufacturing process of photovoltaic cells, using material modification with different photoelectric conversion efficiencies in the cell components. Due to the emergence of many non-conventional manufacturing methods for fabricating functioning solar cells, photovoltaic technologies can be divided into four major generations, which is shown in Figure 2 [ 13 ].

Various solar cell types and current developments within this field [ 14 ].

The generations of various photovoltaic cells essentially tell the story of the stages of their past evolution. There are four main categories that are described as the generations of photovoltaic technology for the last few decades, since the invention of solar cells [ 15 ]:

• First Generation: This category includes photovoltaic cell technologies based on monocrystalline and polycrystalline silicon and gallium arsenide (GaAs).
• Second Generation: This generation includes the development of first-generation photovoltaic cell technology, as well as the development of thin film photovoltaic cell technology from “microcrystalline silicon (µc-Si) and amorphous silicon (a-Si), copper indium gallium selenide (CIGS) and cadmium telluride/cadmium sulfide (CdTe/CdS) photovoltaic cells”.
• Third Generation: This generation counts photovoltaic technologies that are based on more recent chemical compounds. In addition, technologies using nanocrystalline “films,” quantum dots, dye-sensitized solar cells, solar cells based on organic polymers, etc., also belong to this generation.
• Fourth Generation: This generation includes the low flexibility or low cost of thin film polymers along with the durability of “innovative inorganic nanostructures such as metal oxides and metal nanoparticles or organic-based nanomaterials such as graphene, carbon nanotubes and graphene derivatives” [ 15 ].

Examples of solar cell types for each generation along with average efficiencies are shown in Figure 3 .

Examples of photovoltaic cell efficiencies [ 16 ].

2.1. First Generation of Photovoltaic Cells

Silicon-based PV cells were the first sector of photovoltaics to enter the market, using processing information and raw materials supplied by the industry of microelectronics. Solar cells based on silicon now comprise more than 80% of the world’s installed capacity and have a 90% market share. Due to their relatively high efficiency, they are the most commonly used cells. The first generation of photovoltaic cells includes materials based on thick crystalline layers composed of Si silicon. This generation is based on mono-, poly-, and multicrystalline silicon, as well as single III-V junctions (GaAs) [ 17 , 18 ].

Comparison of first-generation photovoltaic cells [ 18 ]:

• Solar cells based on monocrystalline silicon (m-si)

Efficiency : 15 ÷ 24%; Band gap : ~1.1 eV; Life span : 25 years; Advantages : Stability, high performance, long service life; Restrictions : High manufacturing cost, more temperature sensitivity, absorption problem, material loss.

• Solar cells based on polycrystalline silicon (p-si)

Efficiency : 10 ÷ 18%; Band gap : ~1.7 eV; Life span : 14 years; Advantages : Manufacturing procedure is simple, profitable, decreases the waste of silicon, higher absorption compared to m-si; Restrictions : Lower efficiency, higher temperature sensitivity.

• Solar cells based on GaAs

Efficiency : 28 ÷ 30%; Band gap : ~1.43 eV; Life span : 18 years; Advantages : High stability, lower temperature sensitivity, better absorption than m-si, high efficiency; Restrictions : Extremely expensive [ 18 ].

The first generation concerns p-n junction-based photovoltaic cells, which are mainly represented by mono- or polycrystalline wafer-based silicon photovoltaic cells. Monocrystalline silicon solar cells involve growing Si blocks from small monocrystalline silicon seeds and then cutting them to form monocrystalline silicon wafers, which are fabricated using the Czochralski process ( Figure 4 a). Monocrystalline material is widely used due to its high efficiency compared to multicrystalline material. Key technological challenges associated with monocrystalline silicon include stringent requirements for material purity, high material consumption during cell production, cell manufacturing processes, and limited module sizes composed of these cells [ 19 ].

A picture showing ( a ) the Czochralski process for monocrystalline blocks and ( b ) the process of directional solidification for multicrystalline blocks [ 21 ].

Multicrystalline silicon blocks are produced through melting high-purity silicon and crystallizing it in a big crucible by directional solidification process ( Figure 4 b). There is no reference crystal orientation in this process, as in the Czochralski process, and therefore, silicon material with different orientations is produced. The most commonly used base material for solar cells are p-type Si substrates doped with boron. The n-type silicon substrates are also used for the fabrication of high-efficiency solar cells, but they present additional technical challenges, such as achieving uniform doping along the silicon block in comparison to p-type substrates [ 20 ].

In the production of crystalline solar cells, six or more steps need to be carried out sequentially. These typically include surface texturing, doping, diffusion, oxide removal, anti-reflective coating, metallization, and firing. At the end of the process, the cell efficiency and other parameters are measured (under standard test conditions). The efficiency of photovoltaic cells is determined by the material quality that is used in their manufacture [ 21 ].

The theoretical efficiency threshold for first-generation PV cells appears to have been estimated at 29.4%, and a sufficiently close value was reached as early as two decades ago. At the laboratory scale, reaching 25% efficiency was recorded as early as 1999, and since then, very minimal improvements in efficiency values have been achieved. Since the appearance of crystalline silicon photovoltaic cells, their efficiency has increased by 20.1%, from 6% when they were first discovered to the current record of 26.1% efficiency. There are factors that limit cell efficiency, such as volume defects. Breakthroughs in the production of these cells include the introduction of an aluminum back surface field (Al-BSF) to reduce the recombination rate on the back surface, or the development of Passivated Emitter and Rear Cell (PERC) technology to further reduce the recombination rate on the back surface [ 22 ].

2.1.1. Al-BSF Photovoltaic Cells

Silicon solar cells with distributed p-n junctions were invented as early as the 1950s, soon after the first semiconductor diodes. Originally, boron diffusion in arsenic-doped wafers was used to form p-n junctions, but now, the industry standard is phosphor diffusion in boron-doped wafers. After the transition in the 1960s from n-type wafers to p-type wafers, the implementation of an aluminum back-surface field (Al-BSF) by fusing the back contact to the substrate made it possible to reduce recombination on the back side ( Figure 5 ). This fairly simple contact screen printing design held a dominant position, with 70–90% of the market share for the past several decades [ 23 ].

Silicon solar cell structure: Al-BSF [ 1 ].

Standard aluminum back surface field (Al-BSF) technology is one of the most widely used solar cell technologies due to its relatively simple manufacturing process. It is based on depositing Al entirely on the full rear-side (RS) in a screen-printing process and forming a p+ BSF, which helps repel electrons from the rear-side of the p-type substrate and improves the cell performance. The process flow of Al-BSF solar cell fabrication is shown in Figure 6 . Standard commercial solar cell design consists of a front side with a grid and a rear-side with full area contacts [ 24 ].

Al-BSF solar cell manufacturing process [ 21 ].

2.1.2. PERC Photovoltaic Cells

The efficiency of the industrial Al-BSF cell, however, reached about 20% around 2013. It has therefore become attractive to replace the fully contacted Al-BSF cell with a PERC (Passivated Emitter and Rear Cell) structure with local back contacts to achieve enhanced electrical and optical properties ( Figure 7 ). The passivated emitter and rear contact (PERC) solar cell improves the Al-BSF architecture by the addition of a passivation layer on the rear side to improve passivation and internal reflection. Aluminum oxide has been found to be a suitable material for rear side passivation [ 25 ].

Silicon solar cell structure: PERC [ 1 ].

The capability of this cell structure was demonstrated as early as the 1980s, although it was limited to laboratory processing because of its high cost relative to the yield gain. Moving the PERC technology into mass industrial production in theory involved a comparatively small industry threshold, as only two steps needed to be added to the Al-BSF process, i.e., passivation of the back surface and precise calibration of local back contacts. Nevertheless, decades passed before a profitable PERC process could be developed. A number of reasons led to the implementation of PERC in low-cost, high-volume production, and the increase in productivity to levels ranging from 22% to 23.4% [ 26 ]:

• Introduction of aluminum oxide back surface passivation by plasma-enhanced chemical vapor deposition (PECVD) and formation of local back surface field (BSF) by laser ablation of back passivation layer and Al alloy;
• Introduction of a selective emitter process in low-cost manufacturing, a “back-etching” process, or through a laser doping process;
• Reducing the width of front metallization fingers from about 100 μm to less than 30 μm in high-volume production while reducing contact resistance for lightly phosphorus-doped silicon;
• Adding a low-cost hydrogenation step at the end of the cell formation process to passivate volume defects and inactivate boron–oxygen complexes responsible for light-induced degradation (LID); and
• Reappearance of monocrystalline silicon wafers as a result of cost reduction in silicon ingot production by the Czochralski method and the introduction of diamond wire cutting [ 27 ].

2.1.3. SHJ-Type Photovoltaic Cells

In parallel with PERC cells, other high-performance cell designs such as interdigitated back contact (IBC) solar cells and heterojunction solar cells (SHJ) have been introduced to mass production. Silicon heterojunction solar cells (SHJ), otherwise referred to as HIT cells, use passivating contacts based on a stack of layers of intrinsic and doped amorphous silicon ( Figure 8 ). Among the major technological challenges associated with this promising cell structure is that once the amorphous silicon layer is deposited, processes above 200 °C cannot be used. This rules out the well-known burned-in screen-printed metal contacts, and thus demands alternative methods using low-temperature pastes or galvanic contacts [ 28 ].

Silicon solar cell structures: heterojunction (SHJ) in rear junction configuration [ 1 ].

There are currently intensive efforts to develop high-capacity production lines that could be competitive with present production standard lines. For SHJ technology to become widespread, there will be a need to overcome the challenges of increased cost of cell manufacturing tools, reducing the use of silver or replacing it with copper by developing Cu electroplating technology, as well as reducing the use of indium in the transparent conductive oxide (TCO) layer [ 29 ].

Moreover, as shown in Figure 9 , the HIT solar cell has a symmetric structure, which has two advantages. One is that the cell can be used in what is known as a bifacial module, which can generate more electricity than a regular module, and the other is that the structure is less stressed, which is important when processing thinner wafers [ 30 ].

Structure of an HIT solar cell [ 30 ].

2.1.4. Photovoltaic Cells Based on Single III-V Junctions

GaAs-based single III-V junctions are reviewed at the end of this section. The III-V materials give the greatest photovoltaic conversion efficiency, achieving 29.1% with a GaAs single junction under single sunlight and 47.1% for a six-junction device under concentrated sunlight. These devices are also thinner (absorption layers typically being 2 to 5 µm thick) and thus could be fabricated as lightweight, flexible devices capable of being placed on curved surfaces. The III-V devices have high stability and have a history of high performance for challenging applications such as space [ 31 ].

The dominant III-V layer deposition process, metal–organic vapor phase epitaxy (MOVPE), holds the responsibility behind practically every performance record for III-V devices. Yet, historically, this process has been considered as a costly growth technique because of the high cost of precursors, the comparatively low usage of these precursors, and batch growth cycles that require many hours to be completed. Latest studies have significantly improved the growth rate and demonstrated much greater use of precursor chemicals using both MOVPE and hydrogen vapor phase epitaxy (HVPE) techniques, with HVPE also solving the precursor cost problem. Finishing currently includes a great number of labor-intensive, high-priced, and comparatively inefficient process steps, involving photolithography, manual application of spin coating, contact alignment, and metal evaporation and lifting [ 32 ].

2.2. Second Generation of Photovoltaic Cells

The thin film photovoltaic cells based on CdTe, gallium selenide, and copper (CIGS) or amorphous silicon have been designed to be a lower-cost replacement for crystalline silicon cells. They offer improved mechanical properties that are ideal for flexible applications, but this comes with the risk of reduced efficiency. Whereas the first generation of solar cells was an example of microelectronics, the evolution of thin films required new methods of growing and opened the sector up to other areas, including electrochemistry [ 33 ].

The second-generation photovoltaic cell comparison [ 18 ]:

• Solar cells based on amorphous silicon (a-si)

Efficiency : 5 ÷ 12%; Band gap : ~1.7 eV; Life span : 15 years; Advantages : Less expensive, available in large quantities, non-toxic, high absorption coefficient; Restrictions : Lower efficiency, difficulty in selecting dopant materials, poor minority carrier lifetime.

• Solar cells based on cadium telluride/cadium sulfide (CdTe/CdS)

Efficiency : 15 ÷ 16%; Band gap : ~1.45 eV; Life span : 20 years; Advantages : High absorption rate, less material required for production; Restrictions : Lower efficiency, Cd being extremely toxic, Te being limited, more temperature-sensitive.

• Solar cells based on copper indium gallium selenide (CIGS)

Efficiency : 20%; Band gap : ~1.7 eV; Life span : 12 years; Advantages : Less material required for production; Restrictions : Very high-priced, not stable, more temperature-sensitive, highly unreliable [ 18 ].

2.2.1. CIGS Photovoltaic Cells

A key aspect that needed improvement was reducing the high dependence on semiconductor materials. This was the driving force that led to the emergence of the second generation of thin film photovoltaic cells, which include CIGS. In terms of efficiency, the record value for CIGS is 23.4%, which is comparable to the best silicon cell efficiencies. It should be noted, however, that the efficiency of the research cells does not directly translate to industrially achievable efficiency due to the nature of large-scale processing. Nevertheless, module efficiencies above 20% are already a reality. There has been a significant increase in the efficiency of CIGS cells in recent years and further increases are expected, for example, as a result of further research into alkaline treatment after deposition [ 34 ].

Group I-III-VI semiconducting chalcopyrite alloys (Ag,Cu)(In,Ga)(S,Se) 2 , commonly known as CIGS, are particularly favorable absorber materials for solar cells. They have direct band gaps ranging from ~1 to 2.6 eV, high absorption coefficients, and favorable internal defect parameters that allow high minority carrier lifetimes, and solar cells made from them are inherently stable in operation. The first recorded yield was 12% in a monocrystalline device in the mid-1970s. Subsequently, CIGS thin film absorbers, processing, and contacts were greatly improved, resulting in thin film cells with a small area and an efficiency of 23.4%. Current record module efficiencies are 17.6% on glass and 18.6% on flexible steel [ 35 ].

CIGS solar cells have been developed in a standard substrate configuration; however, deposition of CIGS at comparatively low temperatures on metal or polymer substrates to form flexible solar products is also possible. CIGS thin films are mainly being deposited by co-evaporation/devaporation or sputtering, and to a minor extent by electrochemical deposition as well as ion beam-assisted deposition. Since these are quaternary compounds, it is critical to control the stoichiometry of the thin film during fabrication. Work is also underway to produce fully or partially solution-deposited CIGS solar cells, and some predict that they could be the ultimate path to ultra-thin, coiled, and flexible PV modules [ 36 ].

The steps to improve the efficiency of CIGS cells may be described in the following way: (1) evaporation of CIS compound; (2) reactive elemental bilayer deposition; (3) selenization of sputtered metal precursors; (4) chemical bath deposition of CdS with ZnO:Al as emitter; (5) gallium alloying; (6) sodium alkali incorporation; (7) three-step co-deposition; (8) post-deposition treatment involving heavy alkali ion exchange; and (9) sulfurization after selenization (SAS). Progress is far from linear, with the complete potential for the optimization of the complex interactions between those techniques, along with others under development (e.g., silver alloys), yet to be achieved. A large number of scientists who specialize in CIGS think that efficiencies of 25% can be reached [ 37 ].

CIGS is a versatile material that can be produced by many processes and used in a variety of forms. There are currently four main categories of depositing methods used to fabricate CIGS films: (1) metal precursor deposition followed by sulfo-selenization; (2) reactive co-deposition; (3) electrodeposition; and (4) solution processing. All recent world records and the greatest commercial successes have been achieved by two-step sulfo-selenization of metal precursors or reactive co-deposition. CIGS can be deposited on a variety of substrates, including glass, metal films, and polymers. Glass is suitable for making rigid modules, while metal and polymer films allow applications that require lighter or flexible modules. With the evolution of global energy markets toward an appreciation of greenhouse gas reduction and circular economy aspects, the comparatively benign environmental impact of CIGS (especially without CdS) in comparison to different photovoltaic technologies is becoming the next competitive advantage [ 38 ].

Photovoltaic cells based on CIGS technology are composed of a pile of thin films deposited on a glass substrate by magnetron sputtering: a bottom molybdenum (Mo) electrode, a CIGS absorbing layer, a CdS buffer layer, and a zinc-doped oxide (ZnO:Al) top electrode. The co-evaporation and CdS buffer layer deposit the CIGS active layer by means of a chemical bath in a regular procedure ( Figure 10 ) [ 38 ].

Demonstration of the CIGS-based standard solar cell stack [ 38 ].

2.2.2. CdTe Photovoltaic Cells

Second-generation photovoltaic cells also include CdTe-based solar cells. An interesting property of CdTe is the reduction in cell size—due to its high spectral efficiency, the absorber thickness can be reduced to about 1 μm without much loss in efficiency, although further work is needed ( Figure 11 ). Super-thin cells are particularly attractive for flexible applications, particularly in building-integrated photovoltaics (BIPV) due to their lighter weight, and transparent photovoltaic panels with CdTe can be developed due to the choice of transparent coating. Their transparency varies from about 10% to 50%, with the disadvantage that an increase in transparency necessarily decreases efficiency. Still, the transparent panels could replace window panels in buildings, not only generating electricity that could be used to power itself, but also contributing to noise reduction and thermal insulation, since most panels are encased in double glass [ 39 ].

Schematic of a CdTe solar cell [ 1 ].

The technology of CdTe solar cells has developed considerably with the passage of time. In the 1980s, the efficiency of certified cells reached 10%, and in the 1990s, the efficiency was above 15% with the use of a glass/SnO 2 /CdS/CdTe layer structure and annealing in a CdCl 2 environment, and subsequent Cu diffusion. By the 2000s, efficiency of the cells hit 16.7% using sputtered Cd 2 SnO 4 and Zn 2 SnO 4 as transparent conductive oxide (TCO) layers. Over the past decade, new cell efficiency records have reached 22.1%. CdTe technology is increasingly used in rooftop systems and building-integrated photovoltaics [ 40 ].

In 2001, NREL produced a cell with an efficiency of 16.5%, which remained the benchmark for about 10 years. The record efficiency has been improved several times in the past 2 years by First Solar and GE Global Research. Currently, CdTe thin films account for less than 10% of the global PV market, with capacity expected to increase. Most of the commercial CdTe cells are manufactured by First Solar, which has achieved record cell efficiencies of 22.1% and average commercial module efficiencies of 17.5–18% [ 41 ].

The history of research and development and production of CdTe-based PV cells begins several decades beyond the first studies conducted by Bell Labs (Murray Hill, NJ, USA) in the 1950s on Si crystalline cells. The leading companies have been working on the commercialization of the underlying technology: Matsushita (Kadoma, Osaka, Japan), BP Solar (Madrid, Spain), Solar Cells Inc.—predecessor to First Solar (Tempe, AZ, USA), Abound Solar (Loveland, CO, USA) and GE PrimeStar (Denver, CO, USA). The top manufacturer of thin film CdTe PV is currently First Solar Solar (Tempe, AZ, USA), having fabricated 25 GW of PV modules since 2002 [ 42 ].

A range of comparatively easy and inexpensive approaches have been used to produce solar cells with 10–16% efficiency. Examples of several promising cheap deposition techniques include (1) close-space sublimation, (2) spray deposition, (3) electrodeposition, (4) screen printing, and (5) sputtering [ 43 ].

Recently, a record efficiency of 16% was reported in a CdS (0.4 μm)/CdTe (3.5 μm) thin film solar cell in which CdS and CdTe layers are deposited using metal–organic CVD (MOCVD) and CSS deposition techniques, respectively. Most of the high-performance solar cells use a device configuration of the superstrate type, where CdTe is deposited on a window layer of CdS. Typically, the structure of the device is composed of glass/CdS/CdTe/Cu-C/Ag. Most of the time, post-deposition heat treatment of the CdTe layer in the presence of CdCl 2 is necessary to optimize device performance [ 44 ].

The recent increase in efficiency is due partly to almost maximum photocurrent by optimizing the optical properties of the cell, deleting parasitically absorbing CdS and introducing CdSe x Te 1−x with a lower band gap. CdSe x Te 1-x extends the bandwidth of the absorber from ~1.4 to 1.5 eV and increases the carrier lifetime, thus improving photocurrent collection with no proportional loss of photocurrent. The use of ZnTe in the rear contact also improves the contact ohmicity significantly, and thus the efficiency [ 45 ].

2.2.3. Kesterite Photovoltaic Cells

In recent years, kesterite thin film materials have attracted more interest than CdTe and CIGS chalcogenide materials. Cu 2 ZnSnS x Se 4−x (CZTSSe) thin film photovoltaic material is attracting worldwide attention for its exceptional efficiency and composition derived from the Earth. A lot of research is being conducted on material engineering or designing new architecture to achieve high-performance CZTSSe thin film solar cells. Until recently, the most advanced thin film CZTSSe solar cells have been limited to 11.1% power conversion efficiency (PCE), with these efficiency levels reached using the hydrazine suspension method. Further vacuum and non-vacuum deposition techniques also proved effective in producing CZTSSe solar cells that had a PCE above 8%. Yet still, even record equipment with a PCE of 11% is significantly below the physical limit, generally referred to as the Shockley–Queisser (SQ) limit, which is around 31% efficiency under the Earth’s conditions [ 46 ].

A hydrazine-based pure solution method is used to prepare CZTSSe layers, and a Cu-poor and Zn-rich stoichiometry is adopted in the starting solution (Cu/(Zn + Sn) = 0.8 and Zn/Sn = 1.1). Multiple layers of components are spin-coated onto Mo-coated soda-lime glass and annealed at temperatures above 500 °C. Regarding the fabrication of devices, CZTSSe layers are deposited on Mo-coated glass substrates, then 25 nm CdS is deposited in a standard chemical bath and sputtered with 10 nm ZnO/50 nm ITO. A 2 μm thick Ni/Al top metal contact and 110 nm MgF 2 should be deposited on top of the devices by electron beam evaporation. The area of the device should be determined by mechanical scribing [ 47 ].

2.2.4. Photovoltaic Cells Based on Amorphous Silicon

The last type of cells classified as second-generation are devices that use amorphous silicon. Amorphous silicon (a-Si) solar cells are by far the most common thin film technology, whose efficiency is between 5% and 7%, rising to 8–10% for double and triple junction structures. Some varieties of amorphous silicon (a-Si) are amorphous silicon carbide (a-SiC), amorphous germanium silicon (a-SiGe), microcrystalline silicon (μ-Si), and amorphous silicon nitride (a-SiN). Hydrogen is required to dope the material, leading to hydrogenated amorphous silicon (a-Si:H). The gas phase deposition technique is typically used to form a-Si photovoltaic cells with metal or gas as the substrate material [ 48 ].

A typical manufacturing process for a-Si:H cells is the roll-to-roll process. First, a cylindrical sheet, usually stainless steel, is rolled out to be used as a deposition surface. The sheet is washed, cut to the desired size, and coated with an insulating layer. Next, a-Si:H is applied to the reflector, after which a transparent conductive oxide (TCO) is deposited on the silicon layer. Finally, laser cuts are made to join the different layers and the module is closed [ 49 ].

Amorphous silicon is usually deposited by plasma-enhanced vapor phase deposition (PECVD) at comparatively low substrate temperatures of 150–300 °C. A 300 nm thick a-Si:H layer is capable of absorbing about 90% of photons above the passband in a single pass, allowing the fabrication of lighter and more flexible solar cells [ 2 ].

Figure 12 shows the step-by-step fabrication process of an a-Si-based photovoltaic cell. Photovoltaic cells based on thin films are cheaper, thinner, and more flexible compared to first generation photovoltaic cells. The thickness of the light absorbing layer, which was 200–300 µm in first-generation photovoltaic cells, is 10 µm in second-generation cells. Semiconductor materials ranging from “micromorphic and amorphous silicon” to quaternary or binary semiconductors such as “cadmium telluride (CdTe) and copper indium gallium selenide (CIGS)” are used in thin films of photovoltaic cells [ 50 ].

Manufacturing process of a-Si-based solar PV cell [ 2 ].

2.3. Third Generation of Photovoltaic Cells

The third generation of solar cells (including tandem, perovskite, dye-sensitized, organic, and emerging concepts) represent a wide range of approaches, from inexpensive low-efficiency systems (dye-sensitized, organic solar cells) to expensive high-efficiency systems (III-V multi-junction cells) for applications that range from building integration to space applications. Third-generation photovoltaic cells are sometimes referred to as “emerging concepts” because of their poor market penetration, even though some of these have been studied for more than 25 years [ 51 ].

The latest trends in silicon photovoltaic cell development are methods involving the generation of additional levels of energy in the semiconductor’s band structure. The most advanced studies of manufacturing technology and efficiency improvements are now concentrated on third-generation solar cells.

One of the current methods to increase the efficiency of PV cells is the introduction of additional energy levels in the semiconductor’s band gap (IBSC and IPV cells) and the increasing use of ion implantation in the manufacturing process. Other innovative third-generation cells that are lesser-known commercial “emerging” technologies include [ 52 ]:

• Organic materials (OSC) photovoltaic cells;
• Perovskites (PSC) photovoltaic cells;
• Dye-sensitized (DSSC) photovoltaic cells;
• Quantum dots (QD) photovoltaic cells; and
• Multi-junction photovoltaic cells [ 52 ].

Third-generation photovoltaic cell comparison [ 18 ]:

• Solar cells based on dye-sensitized photovoltaic cells

Efficiency : 5 ÷ 20%; Advantages : Lower cost, low light and wider angle operation, lower internal temperature operation, robustness, and extended lifetime; Restrictions : Problems with temperature stability, poisonous and volatile substances.

• Solar cells based on quantum dots

Efficiency : 11 ÷ 17%; Advantages : Low production cost, low energy consumption; Restrictions : High toxicity in nature, degradation.

• Solar cells based on organic and polymeric photovoltaic cells

Efficiency : 9 ÷ 11%; Advantages : Low processing cost, lighter weight, flexibility, thermal stability; Restrictions : Low efficiency.

• Solar cells based on perovskite

Efficiency : 21%; Advantages : Low-cost and simplified structure, light weight, flexibility, high efficiency, low manufacturing cost; Restrictions : Unstable.

• Multi-junction solar cells

Efficiency : 36% and higher; Advantages : High performance; Restrictions : Complex, expensive [ 18 ].

2.3.1. Organic and Polymeric Materials Photovoltaic Cells (OSC)

Organic solar cells (OSCs) are beneficial in applications related to solar energy since they have the potential to be used in a variety of prospects on the basis of the unique benefits of organic semiconductors, including their ability to be processed in solution, light weight, low cost, flexibility, semi-transparency, and applicability to large-scale roll-to-roll processing. Solution-processed organic solar cells (OSCs) that absorb near-infrared (NIR) radiation have been studied worldwide for their potential to be donor:acceptor bulk heterojunction (BHJ) compounds. In addition, NIR-absorbing OSCs have attracted attention as high-end equipment in next-generation optoelectronic devices, such as translucent solar cells and NIR photodetectors, because of their potential for industrial applications. With the introduction of non-fullerene acceptors (NFAs) that absorb light in the NIR range, the value of OSC is increasing, while organic donor materials capable of absorbing light in the NIR range have not yet been actively studied compared to acceptor materials that absorb light in the NIR range [ 53 ].

The most advanced BHJ structure by combining organic donor and acceptor materials showed tremendous hope for low-cost and lightweight organic solar cells. Over the past decade, enormous progress was made, with power conversion efficiencies reaching more than 14% for a single-junction device and more than 17% for a tandem device through the design of new NIR photoactive materials with low bandwidth. Compared to wide-band organic photovoltaic materials, low-band donor and non-fullerene acceptor materials with wide-range solar coverage extended to the NIR region typically exhibit more tightly superimposed electronic orbitals, easier delocalization of π electrons, higher dielectric constant, stronger dipole moment, and lower exciton binding energy. These properties make low-bandwidth photovoltaic materials play an important role in high-performance organic solar cells, including single-junction and tandem devices [ 54 ].

A clever strategy in active layer design could be summed up as optimizing the weight ratio of donor to acceptor materials, using ultra-low band gap materials as a third component to improve NIR light utilization efficiency, and adjusting the thickness of the active layer to achieve a compromise between photon collection and charge accumulation. Much effort has gone into optimizing the translucent top electrode: well-balanced conductivity and transmittance in the visible light range, increased reflectance in the NIR or ultraviolet (UV) light range, and better compatibility with active layers. In terms of device engineering, photon crystal, anti-reflection coating, optical microcavity, and dielectric/metal/dielectric (DMD) structures have been placed to realize selective transmission and reflection for simultaneous improvement of power conversion efficiency and average transmission of translucent OSC visible light [ 55 ].

2.3.2. Dye-Sensitized Photovoltaic Cells (DSSC)

Conjugated polymers and organic semiconductors have been successful in flat panel displays and LEDs, so they are considered advanced materials in the current generation of photovoltaic cells. A schematic representation of dye-sensitized organic photovoltaic cells (DSSCs) is shown in Figure 13 . Polymer/organic photovoltaic cells can also be divided into dye-sensitized organic photovoltaic cells (DSSCs), photoelectrochemical photovoltaic cells, and plastic (polymer) and organic photovoltaic devices (OPVDs), differing in mechanism of operation [ 56 ].

Schematic representation of a DSSCs [ 2 ].

Dye-sensitized solar cells (DSSCs) represent one of the best nanotechnology materials for energy harvesting in photovoltaic technologies. It is a hybrid organic–inorganic structure where a highly porous, nanocrystalline layer of titanium dioxide (TiO 2 ) is used as a conductor of electrons in contact with an electrolyte solution also containing organic dyes that absorb light near the interfaces. A charge transfer occurs at the interface, resulting in the transport of holes in the electrolyte. The power conversion efficiency has been shown to be about 11%, and commercialization of dye-sensitized photovoltaic modules is underway. A novel feature in DSSC solar cells is the photosensitization of nanosized TiO 2 coatings in combination with optically active dyes, which increases their efficiency to more than 10% [ 57 ].

DSSCs hold promise as photovoltaic devices because of their simple fabrication, low material costs, and their benefits in transparence, color capability, and mechanical flexibility. The main challenges in commercializing DSSCs are poor photoelectric conversion efficiency and cell stability. The highest attainable theoretical energy conversion efficiency was estimated at 32% for DSSCs; however, the highest efficiency reported to date is only 13%. Intensive work is underway to understand the parameters governing the DSSC to improve its efficiency. Numerous attempts have been made to optimize the redox pair and absorbance of the dye, modify a wide band gap semiconductor as a working electrode, and develop a counter electrode (CE). In addition to increasing the efficiency of DSSC, the cost of materials is another major issue that needs to be solved in future work [ 58 ].

2.3.3. Perovskite Photovoltaic Cells

Perovskite solar cells (PSCs) are a revolutionary new photovoltaic cell concept that relies on metal halide perovskites (MHPs), e.g., methylammonium iodide as well as formamidine lead iodide (MAPbI 3 or FAPbI 3 , respectively). MHPs integrate a number of features favored in photovoltaic absorbers, including a direct band gap with a high absorption coefficient, long carrier lifetime and diffusion length, low defect density, and ease of tuning the composition and band gap. In the year 2009, MHP was first described as a sensitizer in a dye cell based on liquid electrolyte conducting holes. In 2012, MHP demonstrating ~10% efficiency of PSCs based on a solid-state hole conductor sparked an explosion of PSC studies. In about a decade of research, the efficiency of a single PSC junction increased to a certified level of 25.2% [ 59 ].

The development of PSCs has been heavily influenced by the improvement of material quality through a broad range of synthetic methods designed under the guidance of a fundamental understanding of MHP growth mechanisms. Comprehension of the complex and correlated processes of perovskite growth (e.g., nucleation, grain growth, as well as microstructure evolution) has aided in the development of a broad range of high-efficiency growth modes (for example, single-step growth, sequential growth, dissolution process, vapor process, post-deposition processing, non-stoichiometric growth, additive-assisted growth, and fine-tuning of structure dimensions). The latest efforts were concentrated on interface engineering, focusing on reducing open-circuit voltage losses and improving stability, particularly by introducing a two-dimensional perovskite surface layer. With progress in synthetic control, the perovskite composition is becoming simpler, mainly toward FAPbI 3 . This will undoubtedly contribute to the simplification of scale deposition methods and a basic understanding of the properties of these cells [ 60 ].

2.3.4. Quantum Dots Photovoltaic Cells

Solar cells made from these materials are called quantum dots (QDs) and are also known as nanocrystalline solar cells. They are fabricated by epitaxial growth on a substrate crystal. Quantum dots are surrounded by high potential barriers in a three-dimensional shape, and the electrons and electron holes in a quantum dot become discrete energy because they are confined in a small space ( Figure 14 ). Consequently, the ground state energy of electrons and electron holes in a quantum dot depends on the size of the quantum dot [ 61 ].

( a ) A scheme of a solar cell based on quantum dots, ( b ) solar cell band diagram [ 64 ].

Nanocrystalline cells have relatively high absorption coefficients. Four consecutive processes occur in a solar cell: (1) light absorption and exciton formation, (2) exciton diffusion, (3) charge separation, and (4) charge transport. Due to the poor mobility and short lifetime of excitons in conducting polymers, organic compounds are characterized by small exciton diffusion lengths (10–20 nm). In other words, excitons that form far from the electrode or carrier transport layer recombine and the conversion efficiency drops [ 62 ].

The development of thin film solar cells with metal halide perovskites has led to intensive attention to the corresponding nanocrystals (NCs) or quantum dots (QDs). Today, the record efficiency of QD solar cells was improved to 16.6% using mixed colloidal QDs with perovskites. The universality of these new nanomaterials regarding ease of fabrication and the ability to tune the band gap and control the surface chemistry allows a variety of possibilities for photovoltaics, such as single-junction, elastic, translucent, controlled cells with heterostructures and multi-junction tandem solar cells which would push the field even further. However, a narrower size distribution has the potential to enhance the performance of QD solar cells through more ways than one. Firstly, electron transport might be better in smaller QDs, as larger QDs function as a band tail or shallow trap that makes transport more difficult. Secondly, the open-circuit voltage (V OC ) of QD solar cells could be limited by the smallest band gap (largest size) QD near the contacts. Enhancing the homogeneity and uniformity of QD size would also improve PV performance by the minimization of such losses. Although controlled experiments such as these have not yet been reported, it is possible that more controlled synthesis might provide benefits to QD cells [ 63 ].

2.3.5. Multi-Junction Photovoltaic Cells

Multi-junction (MJ) solar cells consist of plural p-n junctions fabricated from various semiconductor materials, with each junction producing an electric current in response to light of a different wavelength, thereby improving the conversion of incident sunlight into electricity and the efficiency of the device. The concept to use various materials with different band gaps has been suggested to utilize the maximum possible number of photons and is known as a tandem solar cell. An entire cell could be fabricated from the same or different materials, giving a broad spectrum of possible designs [ 65 ].

Usually, the cells are integrated monolithically and connected in series through a tunnel junction, and current matching between cells is obtained through adjusting each cell’s band gap and thickness. The theoretical feasibility of using multiple band gaps was examined and was found to be 44% for two band gaps, 49% for three band gaps, 54% for four band gaps, and 66% for an infinite number of gaps. Figure 15 illustrates a scheme of an InGaP/(In)GaAs/Ge triple solar cell and presents crucial technologies to enhance efficiency of conversion [ 66 ].

Schematic illustration of a triple-junction cell and approaches for improving efficiency of the cell [ 65 ].

Grid-matched InGaP/(In)GaAs/Ge triple solar cells have been widely used in space photovoltaics and have achieved the highest true efficiency of over 36%. Heavy radiation bombardment of various energetic particles in the space environment inevitably damages solar cells and causes the formation of additional non-radiative recombination centers, which reduces the diffusion length of minority carriers and leads to a reduction in solar cell efficiency. The sub-cells in multi-junction solar cells are connected in series; the sub-cell with the greatest radiation degradation degrades the efficiency of the multi-junction solar cell. To improve the radiation resistance of (In)GaAs sub-cells, measures such as reducing the dopant concentration, decreasing the thickness of the base region, etc., can be used [ 66 ].

2.3.6. Photovoltaic Cells with Additional Intermediate Band

The National Renewable Energy Laboratory (NREL) estimates that multi-junction and IBSC photovoltaic cells have the highest efficiency under experimental conditions (47.1%). The main feature of these cells is precisely the additional intermediate band in the band gap of silicon. Currently, two types of these cells are specified in the world literature: IBSC (Intermediate Band Solar Cells) and IPV (Impurity Photovoltaic Effect) [ 67 ].

Impurity Photovoltaic Effect (IPV) is one of the solutions used to increase the infrared response of PV cells and thus increase the solar-to-electric energy conversion efficiency. The idea of the IPV effect is based on the introduction of deep radiation defects in the structure of the semiconductor crystal structure. These defects ensure a multi-step absorption mechanism for photons with energies below the band gap width. The addition of IPV dopants into silicon solar cell structure, under certain conditions, increases the spectral response, short circuit current density, and conversion efficiency [ 68 ].

A major direction of study with great potential for development is Intermediate Band Solar Cells (IBSCs). They represent a third-generation solar cell concept and involve not only silicon, but also other materials. The idea behind the intermediate band gap solar cell (IBSC) concept is to absorb photons with an energy corresponding to the sub-band width in the cell structure. These photons are absorbed by a semiconductor-like material that, in addition to the conduction and valence bands, has an intermediate band (IB) in the conventional semiconductor’s band gap ( Figure 16 ). In IBSCs, the silicon layers are implanted with very high doses of metal ions to create an additional energy level [ 69 ].

Energy band diagram of an intermediate band solar cell (IBSC) [ 69 ].

Based on the research conducted on the effect of defects introduced into the silicon structure, a model was developed according to which introducing selected deep defects into the charge carrier capture region results in improved PV cell efficiency. Of particular interest are defects that facilitate the transport of majority carriers and defects that counteract the accumulation of minority carriers. This contributes significantly to reducing the recombination process at the charge carrier capture site. Finally, by introducing defects into the structure of the silicon underlying the solar cell, we combine effective surface passivation with simultaneous reduction in optical losses [ 70 ].

The introduction of intermediate bands in semiconductors, using ion implantation, can be executed using two methods: by introducing dopants of very high concentration into the semiconductor substrate, or by implanting the silicon layer with high-dose metal ions. The increasing use of ion implantation in the photovoltaic cell manufacturing process has the potential to reduce the cost of deployment and increase the cost-effectiveness of silicon cells by increasing their efficiency. The use of ion implantation technology provides increased precision of silicon layer doping and generation of additional levels of energy in the band gap, as well as shortening the individual stages of cell fabrication, which ultimately translates into improved quality and lower production costs [ 71 ].

Lately, the technique of ion implantation is gaining popularity in the solar industry, gradually displacing the diffusion technique that has been used for many years. As can be seen in Figure 17 , cell performance is expected to continue to improve as the technology evolves toward higher efficiencies. In addition to local and reference doping, the major benefits of this technology involve high precision control of the amount and distribution of dopant doses, which results in high uniformity, repeatability, and increased efficiency (above 19%), with a significantly narrower distribution of cell performance [ 72 ].

Stabilized cell efficiency trend curves [ 72 ].

In the method of ion implantation, chosen ions with the required impurity are inserted into the semiconductor by accelerating the impurity ions to a high energy level and implanting the ions into the semiconductor. The energy given to the impurity ions defines the depth of ion implantation. Contrary to the diffusion technology (where the impurity ion dose is introduced only at the surface), in the ion implantation technique, a controllable dose of impurity ions can be placed deeply into the semiconductor [ 73 ].

2.4. Fourth Generation of Photovoltaic Cells

Fourth-generation photovoltaic cells are also known as hybrid inorganic cells because they combine the low cost and flexibility of polymer thin films, with the stability of organic nanostructures such as metal nanoparticles and metal oxides, carbon nanotubes, graphene, and their derivatives. These devices, often referred to as “nanophotovoltaics”, could become the promising future of photovoltaics [ 74 ].

Graphene-Based Photovoltaic Cells

By using thin polymer layers and metal nanoparticles, as well as various metal oxides, carbon nanotubes, graphene, and their derivatives, the fourth generation provides excellent affordability and flexibility. Particular emphasis was placed on graphene because it is considered a nanomaterial of the future. Due to their unique properties, such as high carrier mobility, low resistivity and transmittance, and 2D lattice packing, graphene-based materials are being considered for use in PV devices instead of existing conventional materials. However, to achieve adequate device performance, the key to its practical applications is the synthesis of graphene materials with appropriate structure and properties [ 75 ].

Since the properties of graphene are fundamentally related to its fabrication process, a judicious choice of methods is essential for targeted applications. In particular, highly conductive graphene is suitable for use in flexible photovoltaic devices, and its high compatibility with metal oxides, metallic compounds, and conductive polymers makes it suitable for use as a selective charge-taking element and electrode interlayer material [ 76 ].

In the past two decades, graphene has been combined with the concept of photovoltaic material and is showing a significant role as a transparent electrode, hole/electron transport material, and interfacial buffer layer in solar cell devices. We can distinguish several types of graphene-based solar cells, including organic bulk heterojunction (BHJ) cells, dye-sensitized cells, and perovskite cells. The energy conversion efficiency exceeded 20.3% for graphene-based perovskite solar cells and reached 10% for BHJ organic solar cells. In addition to its function of extracting and transporting charge to the electrodes, graphene plays another unique role—it protects the device from environmental degradation through its packed 2D lattice structure and ensures the long-term environmental stability of photovoltaic devices [ 77 ].

Semi-metallic graphene having a zero band gap creates Schottky junction solar cells with silicon semiconductors. Even though graphene was discovered for the first time in 2004, the first graphene–silicon solar cell was not characterized as an n-silicon cell until 2010. Figure 18 schematically shows a graphene–silicon solar cell with a Schottky junction. Graphene sheets (GS), cultured by chemical vapor deposition (CVD) on nickel films, were wet deposited on pre-patterned Si/SiO 2 substrates with an effective area of 0.1–0.5 cm 2 . The graphene sheet forms a coating on the exposed n-Si substrate, creating a Schottky junction. The graphene sheet was contacted using Au electrodes [ 78 ].

Graphene–silicon Schottky junction solar cell. ( a ) Cross-sectional view, ( b ) schematic illustration of the device configuration [ 75 ].

Graphene synthesis uses mainly two methodologies, which are the bottom-up and top-down methods. In the top-down approach, graphite is the starting material, and the goal is to intercalate and exfoliate it into graphene sheets by solid, liquid, or electrochemical exfoliation. Another approach under this categorization is the exfoliation of graphite oxide into graphene oxide (GO), after which chemical or thermal reduction takes place. A bottom-up approach is to produce graphene from molecular precursors by chemical vapor deposition (CVD) or epitaxial growth. The structure, morphology, and attributes of the resulting graphene, including the layer numbers, level of defects, electrical and thermal conductivity, solubility, and hydrophilicity or hydrophobicity, are dependent on the manufacturing process [ 78 , 79 ].

Graphene can absorb 2.3% of incident white light even though it is only one atom thick. Incorporating graphene into a silicon solar cell is a promising platform since graphene has a strong interaction with light, fulfilling both the optical (high transmittance) and electrical (low layer resistance) requirements of a typical transparent conductive electrode. It is important to note that both the layer resistance and the transmittance of graphene change with the number of layers. As the layer resistance decreases as the number of graphene layers increases, the optical transparency decreases as well [ 80 ].

For PV technology, graphene offers a lot more because of its flexibility, environmental stability, low electrical resistivity, and photocatalytic features, while having to be carefully and deliberately designed for the targeted applications and specific requirements [ 78 , 80 ].

One problem for graphene application is the absence of a simpler, more reliable way to deposit a well-ordered monolayer with low-cost flakes on target substrates having various surface properties. The other problem is the adhesion of the deposited graphene thin film, a subject that has not yet been studied properly. Large-area continuous graphene layers with high optical transparency and electrical conductivity may be fabricated by CVD. As an anode in organic photovoltaic devices, graphene holds great promise as a replacement for indium tin oxide (ITO) because of its inherently low-cost manufacturing process and excellent conductivity and transparency properties [ 81 ].

Graphene’s major disadvantage is its poor hydrophilicity, which negatively affects the design of devices processed in solution, but that fact may be overcome through modifying the surface by non-covalent chemical functionalization. Given graphene’s mechanical strength and flexibility, as well as its excellent conductivity properties, it can be anticipated that new applications in plastic electronics and optoelectronics will soon emerge involving this new class of CVD graphene materials. The discovery paves the way for low-cost graphene layers to replace ITO in photovoltaic and electroluminescent devices [ 82 ].

3. Prospects and Research Directions

Since the beginning of photovoltaic cells, crystalline silicon-based photovoltaic technology has played a dominant role in the market, with crystalline PV modules accounting for about 90% of the market share in 2020. In recent years, there has been a rapid development of thin film solar cells (such as cadmium telluride (CdTe) and indium–gallium selenium compounds (CIGS) cells) and new solar cells (such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), quantum dot solar cells (QDSCs), etc.) [ 83 ].

The growing interest in BIPV systems has contributed to the overall development of photovoltaic technology, which has led to lower costs, increasing the feasibility of investment. Most of the standard second-generation technologies show efficiencies of 20–25%, and while they are expensive, the cost of silicon cells has come down and it is the improvement of silicon technologies that is now one of the key research directions [ 84 ].

Graphene and its derivatives are a promising area of research as they are in the early stages of research and development. The goal of using carbon nanostructures is to produce energy-efficient products that combine transport, active, and electrode layers. Many researchers in contemporary graphene research are now focusing on new graphene derivatives and their novel applications in manufacturing devices [ 85 ].

Nevertheless, the technologies used for third- and fourth-generation cells are still in the prototyping stage. Production-scale prototypes have also been built and have been successful (10–17% efficiency). In contrast, third-generation multi-junction cells are already commercially available and have achieved exceptional conversion factors (from 40% to over 50%) that place this alternative as the best [ 85 ]. Considering the market trends of increasing use of intermediate energy levels in PV cell production, it makes perfect sense to conduct research in this direction, which is exactly what our research team is doing.

The practical realization of the idea of energy-efficient IBSC-type silicon solar cells with intermediate energy levels in the band gap of the semiconductor, produced by ion implantation, needs more studies directed at the search for the optimal implantation parameters, which is the energy, type, and dose of ions, adjusted to the substrate material properties, particularly the level and type of dopant [ 86 ].

It appears that implantation can also lead to a reduction in the optical losses present in the cell. Impurities and defects introduced into the silicon crystal lattice under the right conditions can create additional intermediate band gaps, which realistically contributes to the reduction in the energy gap width. As a result, some photons with energies lower than the band gap value cause the formation of additional electron–hole pairs. The existence of this additional energy band contributes to the increase in the value of the photoelectric current, which results from the absorption of photons not previously involved in the photovoltaic conversion process. The range of absorbed light radiation increases toward the infrared, and after absorbing a photon from this range, the electron goes first to the intermediate band and then to the conduction band [ 87 ].

Our long-standing studies on changing the electrical parameters of silicon through the use of neon ion implantation have resulted in the development of the authorial methodology for the generation and identification of additional levels of energy in the silicon band structure, improving the efficiency of photovoltaic cells made based on it [ 88 ].

The research has been directed at determining the effect of the degree and type of silicon defect in terms of the possibility of producing intermediate energy levels in the semiconductor’s band gap, thereby increasing the efficiency of solar cells by enabling a multi-step transition of electrons from the valence band to the intermediate band and then to the conduction band.

The object of our research is a method of producing intermediate energy levels in the band gap of n- and p-type silicon, with a specific resistivity ρ ranging from 0.25 Ω·cm to 10 Ω·cm, by generating deep radiation defects in the crystal structure of the semiconductor by implantation of Ne + neon ions. The research material is doped with elements such as boron, phosphorus, and antimony.

Neon ions were chosen because the ions primarily produce point defects, the deliberate introduction of which into the crystalline lattice of silicon in the process of implantation makes it possible to alter its fundamental electrical parameters, including energy gap width and resistivity. The parameters significantly affect internal losses in photovoltaic cells [ 89 ]. Experimental studies were conducted to provide details for determination of the optimal dose of implanted neon ions because of their ability to generate intermediate energy levels in the semiconductor band gap.

The Results of the Author’s Research

The silicon samples were implanted with neon ions of energy E = 100 keV and different doses D using a UNIMAS 79 ion implanter and then isochronically annealed at 598 K for 15 min in a resistance furnace. The electrical parameters of the silicon samples were tested using a Discovery DY600C climate chamber using the proprietary PV Cells Meter computer program and the Winkratos software. A GW Instek LCR-8110G Series LCR meter was used to measure capacitance and conductance values, while sample temperature values were measured using Fluke 289 and Lutron TM-917 multimeters ( Figure 19 ).

Silicon samples laboratory stand. ( a ) Schematic diagram of the laboratory stand: 1—solar cell, 2—supporting construction, 3—temperature sensor, 4—pyranometer, 5—light source, V1—Fluke 289, V2—The LCR-8110G Series LCR meter, RC—shunt resistor, RL—adjustable load. ( b ) Special measuring holders inside the climate chamber to hold silicon samples. ( c ) Discovery DY600C climate chamber [ 90 ].

The resulting capacitance and conductance measurements allowed us to determine the position values of the additional energy levels in the band gap. Two methods were used for this purpose. The first is the Thermal Admittance Spectroscopy (TAS) method, by which it was possible to determine the e t ( T p ) rate that determines the thermal emission, followed by the Arrhenius curves. By using the Arrhenius equation, it was possible to determine the activation energies of the deep energy levels by approximating the experimental data with a linear function [ 86 ]. An example of the results obtained by the TAS method is shown in Figure 20 a.

The Arrhenius law approximation ranges for silicon implanted with neon Ne + ions of energy E = 100 keV ( a ) P-type silicon doped with boron, ρ = 0.4 Ω·cm, D = 2.2 × 10 14 cm −2 , Δ E = 0.46 eV. ( b ) N-type silicon doped with phosphorus, ρ = 10 Ω·cm, D = 4.0 × 10 14 cm −2 , Δ E = 0.23 eV [ 86 , 87 ].

Another method of determining the activation energy is the approximation of selected parts of the course C p = f(1000/ T p ) with the function of the equation ln(y) = Ax + B, where C p is the unit capacitance of the tested sample, and T p is the temperature of the sample during the measurements performed at the frequency of the measuring signal f = 100 kHz. This in turn allowed the calculation of the conduction activation energy Δ E , which determines the depth of the additional intermediate energy level [ 87 ]. An example of the results obtained by the Arrhenius curve approximation method is shown in Figure 20 b.

On the basis of the conducted research, it was possible to identify radiation defects that create additional energy levels in the silicon band gap, with corresponding activation energies, where the results are shown in Table 1 . Our research proved that the implantation of Ne+ ions results in generating radiation defects in the crystal lattice of silicon as a photovoltaic cell base material and enables the generation of intermediate levels of energy in the band gap, improving the efficiency of photovoltaic cells made on its basis.

Determination of intermediate energy levels for boron and phosphorus doped silicon samples implanted with Ne + ions and energy E = 100 keV, isochronically annealed at 598 K [ 86 , 87 ].

4. Conclusions

Solar energy is one of the most demanding renewable sources of electricity. Electricity production using photovoltaic technology not only helps meet the growing demand for energy, but also contributes to mitigating global climate change by reducing dependence on fossil fuels. The level of competitiveness of innovative next-generation solar cells is increasing due to the efforts of researchers and scientists related to the development of new materials, particularly nanomaterials and nanotechnology.

It is noted that the solar cell market is dominated by monocrystalline silicon cells due to their high efficiency. About two decades ago, the efficiency of crystalline silicon photovoltaic cells reached the 25% threshold at the laboratory scale. Despite technological advances since then, peak efficiency has now increased very slightly to 26.6%. As the efficiency of crystalline silicon technology approaches the saturation curve, researchers around the world are exploring alternative materials and manufacturing processes to further increase this efficiency. Polycrystalline and amorphous thin film silicon cells are seen as a serious competitor to monocrystalline silicon cells. However, their disadvantage is their disordered nature which results in low efficiency.

In this paper is a comprehensive overview of various PV technologies that are currently available or will be available in the near future on a commercial scale. A comparative analysis in terms of efficiency and the technological processes used is presented. Over the past few decades, many new materials have emerged that provide an efficient source of power generation to meet future demands while being cost-effective. This paper is a comprehensive study covering the generations of photovoltaic cells and the properties that characterize these cells. Photovoltaic cell materials of different generations have been compared based on their fabrication methods, properties, and photoelectric conversion efficiency.

First-generation solar cells are conventional and based on silicon wafers. The second generation of solar cells involves thin film technologies. The third generation of solar cells includes new technologies, including solar cells made of organic materials, cells made of perovskites, dye-sensitized cells, quantum dot cells, or multi-junction cells. With advances in technology, the drawbacks of previous generations have been eliminated in fourth-generation graphene-based solar cells. The popularity of photovoltaics depends on three aspects—cost, raw material availability, and efficiency. Third-generation solar cells are the latest and most promising technology in photovoltaics. Research on these is still in progress. This review pays special attention to the new generation of solar cells: multi-junction cells and photovoltaic cells with an additional intermediate band.

Recent advances in multi-junction solar cells based on n-type silicon and functional nanomaterials such as graphene offer a promising alternative to low-cost, high-efficiency cells. Currently, multi-junction cells, which benefit from advances enabled by nanotechnology, are breaking efficiency records. They are still quite expensive and represent a complex system, but there are simpler alternatives that may eventually provide a path to the competitiveness of the highest efficiency devices. Another significant advance is being made in the generation of additional energy levels in the band structure of silicon. In both cases, more research evidence, policies, and technology are needed to make them accessible. Therefore, it remains crucial to develop silicon-based technologies. The use of these new solar cell architectures would provide a new direction toward achieving commercial goals. Multi-junction based solar cells and new photovoltaic cells with an additional intermediate energy level are expected to provide extremely high efficiency. The research in this case focuses on a low-cost manufacturing process. Therefore, commercialization of these cells requires further work and exploration.

Nanotechnology and newly developed multifunctional nanomaterials can help overcome current performance barriers and significantly improve solar energy generation and conversion through photovoltaic techniques. Many physical phenomena have been identified at the nanoscale that can improve solar energy generation and conversion. However, the challenges associated with these technologies continue to be an issue when they are incorporated into PV manufacturing. Thanks to initial successes in recent years, nanomaterials are one of the most promising energy technologies of the future and are expected to significantly reform the future energy market. Carbon nanoparticles and their allotropic forms, such as graphene, are expected to offer high efficiency compared to conventional silicon cells in the near future and thus contribute to new prospects for the solar energy market.

Funding Statement

This research was funded by the Lublin University of Technology, grant number FD-20/EE-2/708.

Author Contributions

P.W. proposed a study on photovoltaic cell generations and current research directions for their development and guided the work. J.P. conducted a literature review and wrote the paper. J.P. and P.W. described further prospects and research directions and outlined conclusions based on the collected literature. P.W. reviewed and edited the work. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Informed consent statement, data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

100 Best Solar Energy Case Studies of 2019

The adoption of solar energy in the world is growing at a rapid pace in the world.

More and more consumers, businesses and governmental organizations are considering solar energy.

But it can be sometimes difficult to convince your family, friends, boss or colleagues to adopt solar energy?

To make it easier to convince people to adopt solar power we selected the best and most complete 100 solar energy case studies.

The case studies included in this list contain key information about the return on investment and annual savings of solar energy systems built all over the world and different sizes.

The list is divided in three categories:

Residential Solar Energy

Commercial solar energy, public sector solar energy, 1. home lavallee family.

Country: Cumberland, Rhode Island, United States Installer: Renewable Energy Service of New England Inc. Solar PV: Suniva Inverter: Enphase Size: 9.5 kW Return on Investment: 34.9% Annual Savings: $3845

RES installed 33 solar modules for the Lavallee Family. The projected return of investment is 6 years.

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2. Home Middle Franconia

Country: Bavaria, Germany Inverters: SMA Size: 5 kWp Cost reduction: €875 per year

One family of five installed a solar energy system with batteries. The whole system included a SMA pv inverter, a SMA battery inverter and a SMA sunny home manager for system monitoring and energy management.

3. Home Götz Family

Country: Wetzlar-Hermannstein, Germany Installer: Gecko Logic Solar PV: Yingli Inverters: SMA Size: 8.5 kWp Cost Reduction: €3936 per year

A colleague convinced the family to invest in solar energy. The solar modules exceed the predicted energy yield. This system was installed by Gecko Logic.

4. Home Tan Family

Country: Jalan Kelawar, Tanglin, Singapore Installer: ReZeca Renewables Solar PV: Yingli Solar Size: 18.6 kWp Estimated Annual Savings: SGD$6000

The Tan Family wanted to reduce their footprint and their energy bills. In total 62 solar panels were installed.

5. Home Pappalardo Family

Country: Viagrande, Italy Installer: Etnergia Solar PV: Yingli Inverters: SMA Size: 8.58 kW Cost Reduction: €5533 per year

After seeing solar pv installation in other countries the family decided to switch to solar energy. The company Etnergia installed 39 solar panels on roof with south-east orientation. The system is performing better than expected.

6. Absolute Coatings

Country: New Rochelle, New York, United States Installer: Sunrise Solar Solutions Inverter: Enphase Size: 82 kW Savings over system life: $442 866

Sunrise Solar Solutions designed and installed 313 solar modules for Absolute Coatings on a new roof. The mounting system is ballast only. This project is part of 200 kW solar energy system that will completed in a next phase.

7. Rehme Steel

Country: Spicewood, United States Installer: Freedom Solar Power Solar PV: Sunpower Size: 81.6 kW Estimated savings over 25 years: $338 883

Rehme Steel wanted to reduce their operating cost and their carbon emissions.

8. Birkhof Horse Stables and Riding School

Country: Waldsoms, Germany Installer: Gecko Logic Solar PV: Yingli Inverters: SMA Size: 34.68 kWp Cost Reduction: €12 954

Birkhof choose for solar energy, because of environmental and cost reduction reasons. Gecko Logic installed the system in 2008.

9. Ryan and Ryan Insurance

Country: Kingston, New York, United States Installer: Sunrise Solar Solutions Solar PV: Conergy Inverter: Enphase Size: 16.3 kW Savings over lifetime system: $69 654 Years to breakeven: 5.9

The roof of Ryan and Ryan Insurance was big enough to place enough solar panels to cover their whole energy consumption. The solar panels are mounted with a fully ballasted racking system.

10. Powerplant Poggiorsini

Country: Poggiorsini (Bari), Italy Installer: SAEM Company Solar PV: Yingli Inverters: Siel Size: 3 MWp Return: €1 412 000 per year

The solar power plant was built by SAEM Company and is made up of 13 500 units. The plant is oriented to the south. The plant produces enough energy to power the homes of 1500 families.

11. Huerto Solar Villar de Cañas II

Country: Villar de Cañas, Spain Installer: CYMI Solar PV: Yingli Inverters: Siemens Size: 9.8 MWp Return: €6 336 000 per year

Prosolcam bought a 22 hectare site to invest in solar energy. The company CYMI designed and installed the system that consist of 56 180 pv modules. The plant has an south facing orientation.

12. Amcorp Gemas Solar Plant

Country: Gemas, Negeri Sembilan, Malaysia Installer: Amcorp Power Sdn. Bhd. Solar PV: Yingli Size: 10 269 MWp Return: MYR 11.88 million (about $2.6 million)

Amcorp Power is a solar farm developer in Malaysia. The solar plant has a power purchase agree with Tenaga Nasional Berhad for 21 years. The plant that consists of 41 076 pv modules, produces enough energy for 3315 residential homes.

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13. Jackson Enterprise LLC

Country: California, United States Installer: CM Solar Electric Solar PV: Sunpower, LG Inverter: SMA Size: 26kW Average Annual Savings: $11 556 Return on Investment: 23.6%

The solar energy system provides at least 100% or more of the energy consumption of the building. And the total net investment of the system was $49 000.

14. Diab Engineering

Country: Geraldton, Australia Installer: Infinite Energy Solar PV: Conergy Inverter: SMA Size: 100 kW Year 1 return on investment: 34% 10 Year Net Present Value: $139 000 Annual Savings: $41 000

Diab Engineering choose Infinite Energy to install a solar energy system on there roof of their workshop. Diab Engineering used government funded solar programmes to finance their system.

15. GAL Manufacturing

Country: New York,United States Installer: Solar City Size: 237 kW Annual Savings: $50 000

GAL Manufacturing is a family owned company that builds elevator parts. The system will generate almost half of the buildings energy consumption. The project is partly funded by government funds.

16. Hewlett Packard

Country: Palo Alto, California, United States Size: 1 MW Estimated Lifetime Savings: $1 million

HP installed 1 MW of solar modules on its roof. The system will provide 20% of the buildings usage. HP doesn’t own the system, but will purchase the energy produced from Solar City.

17. Velmade Prestige Sheet Metal

Country: Osborne Park, Australia Installer: Infinite Energy Solar PV: REC Solar Inverter: SMA Size: 31 kW Year 1 return on investment: 18% 10 year Net Present Value: $7 400 Annual Savings: $8 500

In 2014 Velmade installed 120 solar modules on its roof. As a small-to-medium business it wanted to reduce its operating costs. The project is expected to payback in 5.3 years. Velmade used outside funding for its solar system.

18. Bella Ridge Winery

Country: Herne Hill, Australia Installer: Infinite Energy Solar PV: REC Solar Inverter: SMA Size: 40 kW Year 1 return on investment: 21% Annual Savings: $18 300

Bella Ridge Winery is a energy intensive company and was suffering of rising electricity prices in Australia. Infinite Energy installed 156 REC Solar modules on a ground mounted rack. The projected payback period is 4.4 years.

19. Cheeky Brothers

Country: Osborne Park, Australia Installer: Infinite Energy Solar PV: REC Solar Inverter: Fronius Size: 40 kW Year 1 return on investment: 28% Annual Savings: $13 500

Cheeky Brothers is a Food company that installed 152 REC Solar panels on its roof. The system produces 28% of electricity consumption.

20. Seven Acres Business Park

Country: Suffolk, United Kingdom Installer: Enviko Solar PV: CSUN Inverter: SMA Size: 40 kW Yearly Income and Savings: £8 079

This business park decided to install 120 solar panels on its roof just in time before feed in tariffs were reduced in 2012. The project was completed just in time by Enviko.

21. Broad Oak Cider Farm

Country: Clutton Hill Industrial Park, Bristol, United Kingdom Installer: Enviko Solar PV: Conergy Inverter: Solaredge Size: 100 kW Yearly Income and Savings: £15 894

Enviko helped Broad Oak Cider Farm install 400 solar panels that covered the whole roof of the building. Power optimizers were used to reduce the effects of shading on the panels.

22. Glebar Inc.

Country: Franklin Lakes, New Jersey, United States Installer: Solar Energy World Solar PV: Schuco Size: 55.5 kW Yearly Savings: $8000

Glebar Inc was looking for a way to reduce its energy bills and reduce its carbon footprint. Solar Energy World helped achieving their goals. The system is partly funded with a tax break and Solar Renewable Energy Credits.

23. Metuchen Sportscomplex

Country: Metuchen, New Jersey, United States Installer: Solar Energy World Solar PV: LG Size: 312 kW Yearly Savings: $33 397

The developer Recycland LLC decid to add Solar Energy to its building to reduce energy costs and to reduce its carbon footprint.

24. Alfandre Architecture

Country: New Paltz, New York, United States Installer: Sunrise Solar Solutions Solar PV: Conergy and Hyunday Inverter: Enphase Size: 33.4 kW Savings over lifetime system: $190 000

Alfandre Architecture is applying for the LEED GOLD Certification. Adding solar energy to the project is a logical step. Sunrise Solar Solutions did the installation of the new building.

Country: San Jose, California, United States Installer: Solar City Size: 650 kW Annual Cost Savings: $100 000

Ebay wanted to make its campus in San Jose more sustainable. Solar City designed and installed the 3248 solar panel system on five different buildings located on the campus.

26. Heritage Paper

Country: Livermore, California, United States Installer: Solar City Size: 528 kW Annual Cost Savings: $26,950

Heritage Paper is the packaging supplier of big companies like Nordstrom and Cliff Bar. Their huge facility uses huge amounts of energy and installing solar panels was a no-brainer.

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27. Batth Farms

Country: San Joaquin Valley, California, United States Installer: Solar City Size: 1.5 MW Estimated lifetime savings: $9 000 000

The Batth farm uses a lot of energy for the irrigation of the land and running waterpumps. To reduce their operating costs Solar City installed a solar energy system on their farmland.

28. Advance Auto Parts

Country: Enfield, Connecticut, United States Installer: Solar City Size: 1.17 MW Annual Cost Savings: $100 000

Advance Auto Parts is a distribution company of after-sales auto parts. Solar City installed the solar system with little to no disruption to daily operations.

29. Roofmart

Country: Kewdale, Australia Installer: Infinite Energy Solar PV: REC Solar Inverter: SMA Size: 100 kW Year 1 Return on Investment: 25% 10 year Net Present Value: $103 200 Annual Savings: $37 600

Roofmart design, manufactures and distributes steel constructions that are used for garages, patios and sheds. The system was installed in december 2015 and the cost will be returned in under 4 years.

Country: Osborne, Australia Installer: Infinite Energy Solar PV: Winaico Inverter: SMA Size: 100 kW Year 1 Return on Investment: 32% 10 Net Present Value: $240 300 Annual Savings: $45 300

Imdex is listed on the ASX and produces and manufactures fluids and instruments for the mining, oil and gas industries. The projected payback period the solar energy system will be 3.1 years.

31. Audi Seattle

Country: Seattle, United States Installer: A&R Solar Solar PV: Sunpower Size: 235 kW Estimated 25 year savings: $2 million

Audi Seattle is a dealer of high performance electric vehicles. The company wanted to power their vehicles with a sustainable energy source, solar energy.

32. Boulder Nissan

Country: Boulder, United States Installer: Independent Power Systems Solar PV: Sunpower Size: 50.25 kW Estimated 25 year savings: $384 000

Boulder Nissan is a high volume seller of the electric Nissan Leaf in the Boulder area. The adoption of solar energy is a logical step.

34. Microsoft

Country: Mountain View, United States Solar PV: Sunpower Size: 551861 kW Estimated annual savings: $120 000

Microsoft is one of the biggest software companies in the world with a commitment to the environment.

35. Rivermaid Trading co.

Country: California, United States Installer: Sunworks Solar PV: Sunpower Size: 1.7 mW Estimated annual savings: $300 000

Rivermaid Trading is a grower, processor and distributer of fruit. The company has facitlities that are huge and with solar energy they wanted to reduce their energy bills.

36. Lake County Sanitation District

Country: Lakepoint, United States Solar PV: Sunpower Size: 2.17 mW Estimated savings over 20 years: $5 million

The Lake County Sanitation District wanted to reduce their environmental impact.

37. Dobinsons Spring & Suspension

Country: Rockhampton, Australia Solar PV: Hanwha Q Cells Size: 510 kWp Estimated annual savings: AUD$160 000

In the past decade Dobinsons saw their energy costs grow with 100%. With an solar energy system Dobinsons is now protected from increasing energy prices.

38. Austchilli

Country: Bundaberg, Australia Solar PV: Phono Solar Size: 300 kWp Estimated payback period of 4-5 years

Rising energy costs made the business model of Austchilli less feasible and that is why they choose solar energy.

39. Enmach Industries

Country: Bundaberg, Australia Solar PV: Q-Cell Size: 100 kWp Estimated annual savings: AUD$40 000 Estimated payback period of 3.5 years

Like a lot of Australian manufacturing companies, the energy bill of Enmach Industries was rising. Solar energy was the only logical solution.

40. Advantage Welding

Country: Rockhampton, Australia Solar PV: Phono Solar Size: 33 kWp Estimated payback period of 4.2 years

To reduce their electricity bill Advantage Welding worked together with Gem Energy to install solar energy panels on their roof.

41. Bridge Toyota

Country: Darwin, Australia Solar PV: Q Cells Size: 100 kWp Estimated annual savings AUD$35 000 Estimated payback period of 3.5 years

Bridge Toyota has a huge energy consumption for its showroom, office, workshop and warehouse. To prevent huge energy bills cutting in their operating margins they switched to a solar energy system on the roof of their facility.

42. Great Western Hotel

Country: Rockhampton, Australia Solar PV: Q Cells Size: 57 kWp Estimated payback period of 3.2 years

The Great Western Hotel used a renovation to make their operation more green with a solar energy system that is connected to the grid.

43. Luther Auto Group

Country: Midwest, United States Solar PV: Sunpower Size: 454 kWp Estimated saving over 25 years: $2.1 million

The Luther Auto Group used their large flat roofs of their dealerships to generate cheap solar energy.

44. Turtle Bay Resort

Country: Kahuku, United States Solar PV: REC Solar Size: 702 kWp Estimated saving over 20 years: $2.5 million

The Turtle Bay Resort won the Leader in Sustainability Award in Hawaii. The Turtle Bay Resort worked together with REC Solar to install a roof mounted system and a ground mounted system.

45. Zurn Industries

Country: Paso Robles, United States Solar PV: REC Solar Size: 552.7 kWp Estimated annual savings: $110 000

Zurn Industries is a manufacturer of irrigation equipment and want to reduce their operating expenses with the installation of a roof mounted solar energy system.

46. San Antonio Winery

Country: Paso Robles, United States Solar PV: REC Solar Size: 517 kW

Estimated saving over 30 years: $4 million The San Antonio WInery will produce 80% of the power they need for their wine production facility and their hospitality center.

47. Ballester Hermanos

Country: San Juan, United States Solar PV: REC Solar Size: 874 kW Estimated annual savings: $100 000 Ballester Hermanos is located on Puerto Rico that has high energy prices. Solar energy through a power purchase agreement made a lot of economic sense. 

48. Sonoma Mountain Village

Country: Rohnert Park, United States Solar PV: REC Solar Size: 1.16 mW Estimated annual savings: $680 000 Sonoma Mountain Village improved their Leed Premium status by expanding their solar energy capacity.

49. Haas Automation Inc.

Country: Oxnard, United States Solar PV: REC Solar Size: 1.74 mW Estimated annual savings: $500 000

Haas automation wanted to reduce their carbon footprint and reduce their energy costs and opted for two solar roos systems in partnership with Renusol.

50. Niner Wine Estates

Country: Paso Robles, United States Solar PV: REC Solar Size: 388.47 kW Estimated payback period of 5 years

Niner Wine Estates is a Sustainability in Practice Certified winery and has an LEED status. Through their solar energy system they generate 100% of their energy needs.

51. Valley Fine Foods

Country: Benecia and Yuba City, United States Solar PV: REC Solar Size: 1.14 mW Estimated annual savings: $250 000

Valley Fine Foods used a roof mounted and ground mounted solar system to reduce their energy cost.

52. Tony Automotive Group

Country: Waipahu, United States Solar PV: REC Solar Size: 298 kW Estimated savings over 25 years: $5.3 million

Tony Automotive groups has Honda, Nissan and Hyundai dealerships in Hawaii. The need for solar energy was great, because Hawaii has the highest energy costs in the nation.

53. Windset Farms

Country: Santa Maria, United States Solar PV: REC Solar Size: 1.05 mW Estimated annual savings: $245 000

The Windset Farms installed more than 4000 solar energy panels on their roof to curb their rising energy bill.

54. Vintage Wine Estates

Country: Santa Rosa & Hopland, United States Solar PV: REC Solar Size: 945 kW Estimated savings over 30 year period: $10 million

Vintage Wine Estates used a combination of roof mounted and ground mounted solar panels to reduce their utility costs.

Country: Bibra Lake, United States Solar PV: Conenergy Size: 350 kW Estimated annual savings: AUD$169 000

AWTA is the largest wool testing organization in the world. The installed 1085 solar panels on their roof and produce 32% of their energy consumption.

56. Transmin

Country: Malaga, Australia Solar PV: Suntech Size: 40 kW Estimated annual savings: AUD$15 200

With the help of the AusIndustry Clean Technology Investment Program, Transmin made their operations more sustainable with 174 Suntech panels and 2 SMA solar inverters.

57. Mining & Hydraulic Supplies Pty Ltd

Country: Malaga, Australia Solar PV: Solarpower Size: 7 kW Estimated annual savings: AUD$1900

Mining & Hydraulic Supplies has reduced their electricity bill significantly and generate 80% of their energy with solar panels.

58. T&G Corporation

Country: Perth, Australia Solar PV: Suntech Size: 33 kW Estimated annual savings: AUD$9800

In the preceding years T&G Corporation saw their utility bills rise 28%. With solar energy the made their future energy bills predictable again.

59. Firesafe United Group

Country: Bibra Lake, Australia Solar PV: Hanwha Size: 80 kW Estimated annual savings: AUD$23 500

Firesafe United Group installed 3 solar energy systems on their roof to optimize their energy costs.

60. Pacific Nylon Plastics Australia

Country: O’Connor, Australia Solar PV: Canadian Solar Size: 20 kW Estimated annual savings: AUD$10 700

Pacific Nylon Plastics Australia used the redevelopment of their buildings to make their operations greener with the installation of 80 solar pv panels

61. Sheridan’s

Country: West Perth, Australia Solar PV: Daqo Size: 15 kW Estimated annual savings: AUD$6 100

Sheridan’s installed with their installation partner Infinity Energy 60 solar panels on their roof and one fronius solar inverter.

62. Signs & Lines

Country: Midvale, Australia Solar PV: Q Cells Size: 40 kW Estimated annual savings: AUD$13 500

Cost control was a major reason for Sign & Lines to choose for a roof mounted solar energy system.

63. Slumbercorp

Country: Welshpool, Australia Solar PV: REC Solar Size: 40 kW Estimated annual savings: AUD$16 100

64. WA Glasskote

Country: Landsdale, Australia Solar PV: REC Solar Size: 40 kW Estimated annual savings: AUD$10 200

WA Glasskote generates 12% of its energy consumption with their solar energy system.

Country: Malaga, Australia Solar PV: REC Solar Size: 200 kW Estimated annual savings: AUD$82 854

Dobbie wanted to reduce their impact on the environment and their energy costs.

Country: Belmont, Australia Solar PV: REC Solar Size: 30 kW Estimated annual savings: AUD$15 100

Pindan, a construction company, generates 7% of their energy usage with solar panels.

67. Wallis Drilling

Country: Midvale, Australia Solar PV: REC Solar Size: 67 kW Estimated annual savings: AUD$28 900

Wallis Drilling wanted to reduce their costs and make their operations more sustainable. They choose for a roof mounted solar energy system with four Fronius solar inverters. Their solar energy electricity consumption represents 47% of their total energy consumption.

68. Geostats

Country: O’Connor, Australia Solar PV: REC Solar Size: 20 kW Estimated annual savings: AUD$6 600

Geostats wanted to make their operations more environmentally friendly and optimize their energy costs.

69. Eilbeck Cranes

Country: Bassendean, Australia Solar PV: Canadian Solar Size: 40 kW Estimated annual savings: AUD$15 800

Eilbeck Cranes installed 156 Canadian Solar on their roof connected to two Fronius inverters monitored with Fronius Remote Monitoring Solution.

70. Arbortech

Country: Malaga, Australia Solar PV: Poly Solar Panels Size: 40 kW Estimated annual savings: AUD$13 000

Arbortech wanted to reduce its dependency on the utility prices by switching to rooftop solar.

71. Australian Safety Engineers

Country: Canning Vale, Australia Solar PV: REC Solar Size: 40 kW Estimated annual savings: AUD$22 100

Australian Safety Engineers wanted to decrease their utility bill. They opted for a rooftop solar energy system.

72. Stylewoods

Country: Kewdale, Australia Solar PV: Winaico Solar Panels Size: 40 kW Estimated annual savings: AUD$31 500

Stylewoods wanted to reduce their energy bill to free up more working capital for their operations.

73. Plas-Pak

Country: Malaga, Australia Solar PV: Winaico Solar Panels Size: 100 kW Estimated annual savings: AUD$31 500

Plas-Pak wanted to maintain competitive prices for their clients and to make their company more environmentally friendly.

74. John Papas Trailers

Country: Welshpool, Australia Solar PV: REC Solar Size: 40 kW Estimated annual savings: AUD$13 300

John Papas Trailers reduced their dependence on grid electricity through the decision for a solar energy system.

75. Quality Blast and Paint

Country: Welshpool, Australia Solar PV: Sunpower Size: 40 kW Estimated annual savings: AUD$12 550

Quality Blast and Paint wanted to become more competitive through the adoption of solar energy.

76. Pelagic Marine Services

Country: Freemantle, Australia Solar PV: Sunpower Size: 40 kW Estimated annual savings: AUD$15 530

Pelagic Marine Services wanted to make their business more sustainable and more cost efficient and choose for a solar energy system installed by Infinity Energy.

77. Twenty Two Services

Country: Neerabup, Australia Solar PV: Sunpower Size: 13 kW Estimated annual savings: AUD$4 200

Twenty Two Services wanted to reduce their yearly CO2 emissions and their utility bills. Infinity Energy helped them install solar energy system containing 38 solar panels and one Fronius inverter.

78. Yolo County

Country: California, United States Solar PV: Sunpower Size: 6.8 mW Estimated savings over 30 years: $60 million

Yolo county wanted to reduce their energy bill and supply their residents with green energy.

79. AC Transit District

Country: California, United States Installer: Sunpower Size: 177 kW Estimated savings over 25 years: $5 million

ACT Transit District is is Sunpower helped AC Transit District with the installation of two solar energy projects.

80. US Airforce Academy

Country: Colorado Springs, United States Solar PV: Sunpower Size: 6 mW Estimated savings annual savings: $500 000

81. Department of Mines and Petroleum

Country: Carlisle, Australia Installer: Infinite Energy Solar PV: Winaico Inverter: SMA Size: 40 kW Year 1 Return on Investment: 31% 10 Year Net Present Value: $69 200 Annual Savings: $15 400

Infinite Energy installed 153 solar panels on the roof of the Department of Mines and Petroleum. The projected return is 2.8 years.

82. Sacred Hearts Academy

Country: Hawaii, United States Installer: Hawaiian Energy Systems Solar PV: Centrosolar America Solar Inverter: Enphase Size: 243 kW Cost Reduction: 33% annually

Sacred Hearts Academy is a private school in Honolulu, Hawaii. Hawaiian Energy Systems inc. and Centrosolar America installed 1023 panels on three different sun orientations and was completed in 2013.

83. Ina Levine Jewish Community Center

Country: Arizona, United States Installer: Green Choice Solar Solar PV: Centrosolar America Size: 1.3 MW Cost reduction: $6.8 million lifetime system

The Ina Levine Jewish Community Center delivers services to the Scottsdale community. Green Choice Solar installed 5685 solar panels on two locations. One part of the panels was installed on the roof and the majority was installed on 400 carports.

84. Fire station Gifhorn

Country: Germany Installer: Elektro Ohlhoff Solar PV: Yingli Solar Inverter: Kaco Powador Size: 60.86 kWp Cost Reduction: €25900

The roofs of the fire station in Gifhorn presented a perfect solar energy investment opportunity. It was an easy decision for the local government of Gifhorn.

85. University of Colorado

Country: Boulder, Colorado, United States Installer: Eco Depot USA / Solarado Energy Inverter: SatCon Technology Corporation Size: 100 kW Average Annual Savings: $21 750 Return on investment: 7.9%

In septembre 2009 the University of Colorado installed solar panels on a solar carport. This project was part of a LEED Platinum certificate process for which the University applied. The LEED platinum status is the highest green building status that can be achieved in the LEED program.

86. Rotary Residential College

Country: Kensington, Australia Installer: Infinite Energy Solar PV: REC Solar Inverter: SMA Size: 40 kW Year 1 return on investment: 33% 10 year Net Present Value: $69 000 Annual Savings: $20 400

Rotary Residential College is a high-school with a lodging service to their students. Infinite Energy helped the Rotary Residential College with the installation of 153 REC solar panels on their roof.

87. Solar Carport Santa Cruz

Country: Santa Cruz, California, United States Installer: Swenson Solar Size: 386 kW Annual Savings: $73 000

The city of Santa Cruz choose Swenson Solar to build two solar carports with 834 and 936 solar panels installed on them.

88. Hurstpierpoint College

Country: Hurstpierpoint, United Kingdom Installer: Enviko Solar PV: Conergy Inverter: SMA Size: 53.75 kW Yearly Income and Savings: £10 151

Hurstpierpoint is a college home to more than 1000 students. The college wanted to reduce their energy bill and demonstrate their green credentials. The solar panels are installed on three different roofs. Because of the feed-in-tariff the cost of the installation will be recovered in 6 years.

89. San Ramon Valley Unified School District

Country: Danville, United States Solar PV: Sunpower Size: 3.3 mW Estimated savings over 25 years: $24.4 million

The San Ramon Valley Unified School District was confronted with the reduction of their budgets and growing energy bills. Getting solar energy was their solution.

90. University of California Merced

Country: Merced, United States Solar PV: Sunpower Size: 1.1 mW Estimated savings over 20 years: $5 million

The university wanted to reach their sustainable goals and with no upfront cost the adopted solar energy through a power purchase agreement.

91. Stonehill College

Country: Easton, United States Solar PV: Sunpower Size: 2.8 mW Estimated savings over 20 years: $1.8 million

The Stonehill College started the Stonehill Goes Green campaign to reduce their gas emmission with 20% by 2020. That is why they switched to solar energy paid through a power purchase agreement.

92. Inland Empire Utilities Agency

Country: San Bernardino County, United States Solar PV: Sunpower Size: 3.5 mW Estimated savings over 20 years: $3 million

The Inland Empire Utilities Agency has the objective to be 100% powered by renewable energy by 2020.

93. Phelan Piñon Hills Community Services District

Country: San Bernardino County, United States Solar PV: Sunpower Size: 1.5 mW Estimated savings over 30 years: $13 million

The Phelan Piñon Hills Community Services District was confrented with fast growing electricity prices and lowered their cost with solar energy.

94. Bundaberg Christian College

Country: Bundaberg, Australia Solar PV: Hanwha Q Cells Size: 193.98 kWp Estimated annual savings: AUD$100 000

The Bundaberg Christian College has opted for a solar energy system with battery backup, the largest system of its kind at an Australian school.

95. Cathedral College

Country: Rockhampton, Australia Solar PV: Q Cells Size: 85 kWp Payback period of six years

Because of it strong commitment to sustainability, Cathedral College opted for solar energy.

96. Emerald Marist College

Country: Central Highlands, Australia Solar PV: Q Cells Size: 100 kWp Estimated annual savings: AUD$40 000

Due to high air conditioning usage and electricity bills during the summer months, Emerald Marist College, choose to install a solar energy system on its roof.

97. Pleasanton Unified School District

Country: Paso Robles, United States Solar PV: REC Solar Size: 1 mW Estimated saving over 25 years: $2.2 million

The Pleasanton Unified School District made the switch to solar energy through a power purchase agreement. The solar panels were placed on solar carports.

98. Roseville Joint Union High School District

Country: Paso Robles, United States Solar PV: REC Solar Size: 1.02 mW Estimated saving over 25 years: $8 million

The Roseville Joint Union High School District installed solar panels over their parking structures.

99. St Catherine’s College

Country: Crawley, Australia Solar PV: Sunpower Size: 200 kW Estimated annual savings: AUD$84 000

100. City of Perth – Depot

Country: Perth, Australia Solar PV: Sunpower Size: 39 kW Estimated annual savings: AUD$16 100

The city of Perth wanted to make their depot more sustainable and more cost efficient. 

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