Abstract:— The electrical performance of photovoltaic (PV)cells, modules, and systems are rated in terms of their maximum electrical power with respect to a total irradiance, temperature, and spectral irradiance. The impact of the reference conditions, measurement procedures, and equipment on the performance rating is discussed.
Abstract:Estimation of the electrical yield of a PV module is expected to be a more useful predictor of performance for installers than Wp alone. A method for the energy rating of PV modules based on performance surfaces under development at the ESTI laboratory uses the module temperature and incident irradiance as independent variables and has been successful in prediction of real energy production for crystalline Si modules. However, it was found to be more difficult to accurately predict the performance of thin film modules and it was therefore necessary to explore the reasons. One potentially significant parameter not included in the standard performance surface is the effect of spectral variations, and this has been studied during indoor and outdoor testing on CIS and a-Si modules. The outdoor measurements were performed on a tracker so as to preclude angle of incidence effects. Module I–V curves and the solar spectrum were measured at frequent intervals over a range of air mass values during the course of a number of days. A crystalline Si reference device and a pyranometer were used as irradiance sensors in order to explore the effect of the choice of reference device used. The spectral mismatch factor is calculated from measurements of the solar spectrum and device spectral responses and is applied to correct the individual module measurement points.
The dependence on air mass, i.e., the details of the solar spectrum of these devices, has also been shown, so employing only total irradiance and device temperature may not be sufficient when an energy rating is being made. This effect is most pronounced for the a-Si module tested, for which a significant part of this dependence was corrected by the application of the relevant mismatch factors.
Abstract:Photovoltaic (PV) cells and modules are often rated in terms of a set of standard reporting conditions defined by a temperature, spectral irradiance and total irradiance. Because PV devices operate over a wide range of temperatures and irradiances, the temperature and irradiance-related behavior must be known. This paper surveys the temperature dependence of crystalline and thin-film, state-of-the-art, research-size cells, modules and systems measured by a variety of methods. The various error sources and measurement methods that contribute to cause differences in the temperature coefficient for a given cell or module measured with various methods are discussed
Theoretical analysis of the optimum energy band gap of semiconductors for fabrication of solar cells for applications in higher latitudes locations.[edit | edit source]
Abstract:In this work some results of theoretical analysis on the selection of optimum band gap semiconductor absorbers for application in either single or multijunction (up to five junctions) solar cells are presented. For calculations days have been taken characterized by various insolation and ambient temperature conditions defined in the draft of the IEC 61836 standard (Performance testing and energy rating of terrestrial photovoltaic modules) as a proposal of representative set of typical outdoor conditions that may influence performance of photovoltaic devices. Besides various irradiance and ambient temperature ranges, these days additionally differ significantly regarding spectral distribution of solar radiation incident onto horizontal surface. Taking these spectra into account optimum energy band gaps and maximum achievable efficiencies of single and multijunction solar cells made have been estimated. More detailed results of analysis performed for double junction cell are presented to show the effect of deviations in band gap values on the cell efficiency.
- Study discusses how to achieve maximum conversion efficiency for a specified solar spectrum absorber material with optimum band gap for solar cell fabrication. - Proper matching of the solar cell absorber band gap to light spectrum as being fundamental for efficient energy conversion. - Article also discusses the application of graded gap semiconductors for band gap engineering. - 3rd generation solar cells aiming to obtain structures with higher than one QE due to the introduction of the band gap multiple intermediate bands (IB). - It is expected that in such structures photons may generate more than one electron–hole pair. - Conclusions. - The IEC 60904-3 standard with recommended AM1.5 solar spectrum distribution seems to be nonsuitable for performance prediction of multijunction solar cells for terrestrial applications. - It is as well nonsuitable in the case of single junction devices with energy band gap significantly different from the optimum 1.39 eV value, e.g.,a-Si cells.. - Presented results merely serve only as a useful guide to the range of band gap values of interest since calculations are based on simplified cell model.
A practical method for the energy rating of c-Si photovoltaic modules based on standard tests.[edit | edit source]
Abstract:The performance of a photovoltaic module at Standard Test Conditions (STC) is valuable for comparing the peak performance of different module types. It does not, however, give enough information to accurately predict how much energy a module will deliver when subjected to real operating conditions. There are several proposals for an energy rating for PV modules which attempt to account for the varying operating conditions that one encounters in the field. In this paper, we present an approach with the emphasis on simplicity and practicality that incorporates existing standard measurements to determine the energy output as a function of global in-plane irradiance and ambient temperature. The method is applied to crystalline Si modules and tested with outdoor measurements, and a good accuracy of prediction of energy production is observed. Finally, a proposal is made for a simple Energy Rating labeling of PV modules
Modelling long-term module performance based on realistic reporting conditions with consideration to spectral effects.[edit | edit source]
Abstract:A model for the annual performance of different module technologies is presented that includes spectral effects. The model is based on the realistic reporting conditions but also allows for secondary spectral effects, as experienced by multi-junction devices. The model is validated against measurements taken at CREST and shows a good agreement for all devices. Combining this relatively simple model with ASPIRE, a spectral irradiance model based on standard meteorological measurements, allows the translation to other locations. The method is applied to measurements of different devices deployed in Loughborough University and the significance of certain effects is discussed.
Experimental solar spectral irradiance until 2500 nm: results and influence on the PV conversion of different materials.[edit | edit source]
Abstract:In this work, results are presented concerning solar spectral irradiance measurements performed in Madrid in the wavelength range 250–2500 nm, that is, extending the spectral range far away from the wavelengths where PV semiconductors are active. These data were obtained considering a horizontal receiver surface during selected clear days covering the four seasons of the year. PV materials having different spectral responses (m-Si, a-Si, CIGS, CdTe) have been considered to calculate spectral factors (SF) taking as reference the standard solar spectrum AM1.5 defined in standard IEC 60904-3. From these SFs, the influence of natural solar spectral variations in PV conversion has been established. It is shown, for example, that PV technologies based on a-Si are highly favored, from the spectral point of view, in spring–summer compared to other technologies having broader spectral responses, which are more favored in autumn–winter. From the experimental measured solar spectra, we have calculated Weighed Solar Spectra (WSS) corresponding to the four seasons of the year and also to the whole year. The WSS represents, for a certain period of time, the solar spectrum weighed over the irradiance level. SFs have been calculated for different WSSs showing spectral gains for the four PV materials during almost the full year. Otherwise, it is also shown in this work how the near-IR part of the solar spectrum affects the evaluation of the solar resource as a whole when reference solar cells made of different PV materials are used. For typical m-Si, a-Si, CIGS, and CdTe solar cells, the ratio of Isc over global irradiance is not constant along a given day showing variations that depend on the season and on the PV material considered.
- Study presents the spectral responses of the different PV materials; m-Si, a-Si, CIGS and CdTe using experimental solar spectra from 250 - 2500 nm - most studies limit themselves to the 300 - 1100 nm spectral range. - Data is for 1 year period covering the four seasons in Madrid Spain. - The study compares the short circuit current, Isc data from literature with corresponding integrated experimental solar irradiance (250 - 2500 nm) data for the four commercial PV cells . - Solar spectral radiance measurements were performed using a spectroradiometer MONOLIGHTTM model with 1 nm resolution and 100 - 300 s scan time. - Two detectors used, the Si (250 - 1095 nm wavelength range) and InGaAs (for 1095 - 2500 nm range) thermoelectrically cooled photodiodes. - All measurements were rooftop, 620 m above sea level, on clear days. - Study concludes that PV conversion efficiency of semiconductors is seasonal dependent due to natural solar spectrum variations: - a-Si and CdTe (narrow spectral response) - suitable for summer time , and m-Si and CIGS (wide spectral response) - winter time. - Natural solar spectral variations cause a non-linearity of Iscvs integrated global irradiance that can lead to some errors if spectral corrections are not made.
On the importance of considering the incident spectrum when measuring the outdoor performance of amorphous silicon photovoltaic devices.[edit | edit source]
Abstract:Conventional measurement practice for the outdoor performance evaluation of solar cells does not make use of the complete spectrum, relying instead on the total irradiance as measured, say, with a pyranometer. In this paper it is shown that this can result in significant errors for solar cells having wide band gaps, in particular, for amorphous silicon solar cells. Two effects are investigated. The first relates to quantifying the typical errors associated with instantaneous measurements; what one might term the calibration of devices. The second relates to quantifying the impact of neglecting variations in the spectrum on the estimation of the annual energy production. It is observed that the fraction of the spectrum falling in the spectrally useful range for amorphous silicon can vary by as much as +10% to −15% with respect to standard test conditions at the test site used in this study, which translates directly into performance variations of similar magnitude. The relationship between changes due to spectral variations as opposed to variations in device temperature is also investigated. The results show that there is a strong case for investigating spectral effects more thoroughly, and explicitly including the measurement of the spectral distribution in all outdoor performance testing.
- Article investigates the error associated with neglecting spectral variations in the outdoor calibration of PV devices (i.e. the instantaneous response), and the errors associated with the estimation of annual energy yield. - Data collected over 5 yrs from 300 - 1700 nm in 10 nm steps at Loughborough, UK. - Srong correlation with AM and the degree of cloud cover demonstrated, and can affect instantaneous performance calibration by up to 20% depending on the time of day and the time of year and climatic conditions. - There is a seasonal variation in the incident energy distribution that is strong enough to explain the seasonal performance of a-Si devices in a maritime, high latitude climate. - Spectral effects sometimes confused with other factors such as changes in device temperature.' - Study concludes that a more thorough understanding of the performance of a-Si devices is only possible if spectral data are available. - article less relevant to our research focus since it does not look at bandgap realted issues.
Abstract:We present a method for calculating the spectral albedo of snow which can be used at any wavelength in the solar spectrum and which accounts for diffusely or directly incident radiation at any zenith angle. For deep snow, the model contains only one adjustable parameter, an effective grain size, which is close to observed grain sizes. A second parameter, the liquid-equivalent depth, is required only for relatively thin snow.
In order for the model to make realistic predictions, it must account for the extreme anisotropy of scattering by snow particles. This is done by using the "delta-Eddington" approximation for multiple scattering, together with Mie theory for single scattering.
The spectral albedo from 0.3 to 5 μm wavelength is examined as a function of the effective grain size, the solar zenith angle, the snowpack thickness, and the ratio of diffuse to direct solar incidence. The decrease in albedo due to snow aging can be mimicked by reasonable increases in grain size (50–100 μm for new snow, growing to 1 mm for melting old snow).
The model agrees well with observations for wavelengths above 0.8 μm. In the visible and near-UV, on the other hand, the model may predict albedos up to 15% higher than those which are actually observed. Increased grain size alone cannot lower the model albedo sufficiently to match these observations. It is also argued that the two major effects which are neglected in the model, namely nonsphericity of snow grains and near-field scattering, cannot be responsible for the discrepancy. Insufficient snow depth and error in measured absorption coefficient are also ruled out as the explanation. The remaining hypothesis is that visible snow albedo is reduced by trace amounts of absorptive impurities (Warren and Wiscombe, 1980, Part II).
Visible and near-ultraviolet absorption spectrum of ice from transmission of solar radiation into snow[edit | edit source]
Method to Determine Snow Albedo Values in the Ultraviolet for Radiative Transfer Modeling[edit | edit source]
Abstract:For many cases modeled and measured UV global irradiances agree to within ∓5% for cloudless conditions, provided that all relevant parameters for describing the atmosphere and the surface are well known. However, for conditions with snow-covered surfaces this agreement is usually not achievable, because on the one hand the regional albedo, which has to be used in a model, is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method is given to determine the regional albedo values for conditions with snow cover by use of a parameterization on the basis of snow depth and snow age, routinely monitored by the weather services. An algorithm is evolved by multiple linear regression between the snow data and snow-albedo values in the UV, which are determined from a best fit of modeled and measured UV irradiances for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18–0.5 range. Since the constants of the regression depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with different snow conditions improves the accuracy of modeled UV irradiances considerably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average deviations between measured and modeled UV global irradiances.
Progress and challenges for next-generation high-efficiency multijunction solar cells[edit | edit source]
Abstract:Multijunction solar cells are the most efficient solar cells ever developed with demonstrated efficiencies above 40%, far in excess of the performance of any conventional single-junction cell. This paper describes paths toward next-generation multijunction cells with even higher performance. Starting from fundamental multijunction concepts, the paper describes the desired characteristics of semiconductor materials for multijunction cells; the corresponding challenges in obtaining these characteristics in actual materials; and materials and device architectures to overcome these challenges
Four-junction spectral beam-splitting photovoltaic receiver with high optical efficiency[edit | edit source]
Abstract:A spectral beam-splitting architecture is shown to provide an excellent basis for a four junction photovoltaic receiver with a virtually ideal band gap combination. Spectrally selective beam-splitters are used to create a very efficient light trap in form of a 458 parallelepiped. The light trap distributes incident radiation onto the different solar cells with an optical efficiency of more then 90%. Highly efficient solar cells including III–V semiconductors and silicon were fabricated and mounted into the light trapping assembly. An integrated characterization of such a receiver including the measurement of quantum efficiency as well as indoor and outdoor I–V measurements is shown. Moreover, the optical loss mechanisms and the optical efficiency of the spectral beam-splitting approach are discussed. The first experimental setup of the receiver demonstrated an outdoor efficiency of more than 34% under unconcentrated sunlight.
Abstract:Planar converters containing quantum dots as wavelength-shifting moieties on top of a multi-crystalline silicon and an amorphous silicon solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.
Effects of Gallium-Phosphide and Indium-Gallium-Antimonide semiconductor materials on photon absorption of multijunction solar cells[edit | edit source]
Abstract:The main challenge in the photovoltaic industry is making the solar cells more cost effective. Single junction solar cells can only absorb a certain wavelength of the solar spectrum, hence produce less efficiency. In contrary multijunction solar cells direct sunlight towards matched spectral sensitivity by splitting the spectrum into smaller slices. The high efficiency multijunction photovoltaics made up of III-V semiconductor material alloys with high optical sensitivity and ideal combination of band-gaps increase absorption of photons, creates more electron-hole pairs, and hence increase the efficiency of the solar cell. National Renewable Energy Laboratory (NREL), US Department of Energy (DOE) and many leading research organizations all over the world are investing money in the design of III-IV multijunction solar cell projects. In this paper, we introduce a novel multijunction photovoltaic cell based on GaP/InGaAs/InGaSb, and compare it with existing single-junction and multijunction cells. We observe that the inclusion of GaP and InGaSb layers in our design has made a significant improvement in absorption of solar energy in the entire spectral range, thus resulting in higher efficiency.
Band gap-voltage offset and energy production in next-generation multijunction solar cells[edit | edit source]
Abstract:The potential for new 4-, 5-, and 6-junction solar cell architectures to reach 50% efficiency is highly leveraging for the economics of concentrator photovoltaic (CPV) systems.The theoretical performance of such next-generation cells, and experimental results for 3- and 4-junction CPV cells, are examined here to evaluate their impact for real-world solar electricity generation. Semiconductor device physics equations are formulated in terms of the band gap-voltage offset Woc [TRIPLE BOND] (Eg/q) − Voc, to give a clearer physical understanding and more general analysis of the multiple subcell band gaps in multijunction cells. Band gap-voltage offset is shown experimentally to be largely independent of band gap Eg for a wide range of metamorphic and lattice-matched semiconductors from 0.67 to 2.1 eV. Its theoretical Eg dependence is calculated from that of the radiative recombination coefficient, and at a more fundamental level using the Shockley-Queisser detailed balance model, bearing out experimental observations. Energy production of 4-, 5-, and 6-junction CPV cells, calculated for changing air mass and spectrum over the course of the day, is found to be significantly greater than for conventional 3-junction cells. The spectral sensitivity of these next-generation cell designs is fairly low, and is outweighed by their higher efficiency. Lattice-matched GaInP/GaInAs/Ge cells have reached an independently confirmed efficiency of 41.6%, the highest efficiency yet demonstrated for any type of solar cell. Light I-V measurements of this record 41.6% cell, of next-generation upright metamorphic 3-junction cells with 40% target production efficiency, and of experimental 4-junction CPV cells are presented.
Abstract:Both for photovoltaic and photovoltaic/thermal applications insight is required in the mechanisms that determine the effective absorption factor Aeff. Aeff is the part of the incident irradiation that is converted into heat, taking into account that part of the energy is withdrawn as electricity. Aeff was studied for five different solar cell technologies using an optical simulation model and ranges from 74% for single junction amorphous silicon solar cells to 82% for CIGS solar cells. The simulations also show that the longer wavelength part of the spectrum is hardly absorbed by the active semiconductors, but mostly by free carrier absorption in the transparent conductive oxide film present in these devices.
References[edit | edit source]
- K. Emery, "The rating of photovoltaic performance," IEEE Transactions on Electron Devices, vol. 46, no. 10, pp. 1928 –1931, Oct. 1999.
- R. P. Kenny, A. Ioannides, H. Müllejans, W. Zaaiman, and E. D. Dunlop, "Performance of thin film PV modules," Thin Solid Films, vol. 511–512, no. 0, pp. 663–672, Jul. 2006.
- K. Emery, J. Burdick, Y. Caiyem, D. Dunlavy, H. Field, B. Kroposki, T. Moriarty, L. Ottoson, S. Rummel, T. Strand, and M. W. Wanlass, "Temperature dependence of photovoltaic cells, modules and systems," in , Conference Record of the Twenty Fifth IEEE Photovoltaic Specialists Conference, 1996, 1996, pp. 1275 –1278.
- T. Zdanowicz, T. Rodziewicz, and M. Zabkowska-Waclawek, "Theoretical analysis of the optimum energy band gap of semiconductors for fabrication of solar cells for applications in higher latitudes locations," Solar Energy Materials and Solar Cells, vol. 87, no. 1–4, pp. 757–769, May 2005.
- R. P. Kenny, E. D. Dunlop, H. A. Ossenbrink, and H. Müllejans, "A practical method for the energy rating of c-Si photovoltaic modules based on standard tests," Progress in Photovoltaics: Research and Applications, vol. 14, no. 2, pp. 155–166, 2006.
- S. R. Williams, T. R. Betts, T. Helf, R. Gottschalg, H. G. Beyer, and D. G. Infield, "Modelling long-term module performance based on realistic reporting conditions with consideration to spectral effects," in Proceedings of 3rd World Conference on Photovoltaic Energy Conversion, 2003, 2003, vol. 2, pp. 1908 –1911 Vol.2.
- J. J. Pérez-López, F. Fabero, and F. Chenlo, "Experimental solar spectral irradiance until 2500 nm: results and influence on the PV conversion of different materials," Progress in Photovoltaics: Research and Applications, vol. 15, no. 4, pp. 303–315, 2007.
- R. Gottschalg, T. R. Betts, D. G. Infield, and M. J. Kearney, "On the importance of considering the incident spectrum when measuring the outdoor performance of amorphous silicon photovoltaic devices," Measurement Science and Technology, vol. 15, no. 2, pp. 460–466, Feb. 2004.
- Wiscombe, Warren J., and Stephen G. Warren. "A Model for the Spectral Albedo of Snow. I: Pure Snow." Journal of the Atmospheric Sciences 37, no. 12 (December 1980): 2712–2733. doi:10.1175/1520-0469(1980)037<2712:AMFTSA>2.0.CO;2.
- Wiscombe, Warren J., and Stephen G. Warren. "A Model for the Spectral Albedo of Snow II: Snow Containing Atmospheric Aerosols." Journal of the Atmospheric Sciences 37, no. 12 (December 1980): 2712–2733. doi:10.1175/1520-0469(1980)037<2712:AMFTSA>2.0.CO;2.
- Warren, Stephen G., Richard E. Brandt, and Thomas C. Grenfell. "Visible and Near-ultraviolet Absorption Spectrum of Ice from Transmission of Solar Radiation into Snow." Applied Optics 45, no. 21 (2006): 5320. doi:10.1364/AO.45.005320.
- Schwander, Harry, Bernhard Mayer, Ansgar Ruggaber, Astrid Albold, Gunther Seckmeyer, and Peter Koepke. "Method to Determine Snow Albedo Values in the Ultraviolet for Radiative Transfer Modeling." Applied Optics 38, no. 18 (June 20, 1999): 3869–3875. doi:10.1364/AO.38.003869.
-  D. J. Friedman, "Progress and challenges for next-generation high-efficiency multijunction solar cells," Current Opinion in Solid State and Materials Science, vol. 14, no. 6, pp. 131–138, Dec. 2010.
- B. Mitchell, G. Peharz, G. Siefer, M. Peters, T. Gandy, J. C. Goldschmidt, J. Benick, S. W. Glunz, A. W. Bett, and F. Dimroth, "Four-junction spectral beam-splitting photovoltaic receiver with high optical efficiency," Progress in Photovoltaics: Research and Applications, vol. 19, no. 1, pp. 61–72, 2011.
- W. G. J. H. M. van Sark, A. Meijerink, R. E. I. Schropp, J. A. M. van Roosmalen, and E. H. Lysen, "Enhancing solar cell efficiency by using spectral converters," Solar Energy Materials and Solar Cells, vol. 87, no. 1–4, pp. 395–409, May 2005. .
- I. Bhattacharya and S. Y. Foo, "Effects of Gallium-Phosphide and Indium-Gallium-Antimonide semiconductor materials on photon absorption of multijunction solar cells," in IEEE SoutheastCon 2010 (SoutheastCon), Proceedings of the, 2010, pp. 316 –319.
- R. R. King, D. Bhusari, A. Boca, D. Larrabee, X.-Q. Liu, W. Hong, C. M. Fetzer, D. C. Law, and N. H. Karam, "Band gap-voltage offset and energy production in next-generation multijunction solar cells," Progress in Photovoltaics: Research and Applications, vol. 19, no. 7, pp. 797–812, 2011.
- ] R. Santbergen, J. M. Goud, M. Zeman, J. A. M. van Roosmalen, and R. J. C. van Zolingen, "The AM1.5 absorption factor of thin-film solar cells," Solar Energy Materials and Solar Cells, vol. 94, no. 5, pp. 715–723, May 2010.