Thomas R. Betts

Optical filtering of solar radiation to increase performance of concentrator systems

Solar cell overheating due to high irradiation levels is a significant problem facing concentrator systems. Some form of cooling is needed to maintain the highest possible performance of such systems. Liquid filters may be used to inhibit unwanted solar radiation from reaching the cell and thus limit cell-operating temperatures. The performance of the cooling will depend on the optical properties of the liquid filter applied, as illustrated in this paper, on the basis of different filters. An ideal filter is identified and its effects on the systems are described. It is shown, on the basis of system modeling calculations, that cell performance could be increased by up to 25% using an ideal filter. Such a system can reach an efficiency of 22% in a realistic working environment compared to a STC value of 16%. The absorbed part of the incident radiation can be used as a heat source, so adding to the potential value of the system.

Impact of spectral effects on the electrical parameters of multijunction amorphous silicon cells

The influence of spectral variation on the efficiency of single-, double- and triple-junction amorphous silicon cells has been investigated. The average photon energy (APE) proves to be a useful device-independent environmental parameter for quantifying the average hue of incident spectra. Single-junction devices increase in efficiency as light becomes blue shifted, because more of the incident spectrum lies within the absorption window and less in the red/infra-red tail; this is denoted the primary spectral effect. Double- and triple-junction devices also exhibit a secondary spectral effect due to mismatch between the device structure and the incident spectrum. These both reach a maximum efficiency, which drops off as light is red or blue shifted. The effect is more pronounced for triple-junction than double-junction devices, as mismatch between junctions is statistically more likely.

Performance of amorphous silicon double junction photovoltaic systems in different climatic zones

To date the majority of investigations into the performance of amorphous silicon photovoltaic systems have been limited to single sites, and therefore the conclusions from such studies are unlikely to be as generic as they might at first appear. This paper compares data collected from different systems across the world in Brazil, Hong Kong, Spain, Switzerland, and the United Kingdom. All systems have been operating for a number of years, and are employing double junction amorphous silicon devices of a similar age manufactured by RWE Solar. The data are analysed for performance variations reflecting the different climatic zones, and the variations are explained on the basis of operating temperature, incident irradiation and seasonal spectral shift.

Seasonal performance of a-Si single- and multijunction modules in two locations

Module performance data collected at two sites are analyzed in order to identify the respective magnitudes of seasonal annealing and degradation in comparison to spectral effects. It is demonstrated in this paper that at one site (Loughborough, UK) the spectrum dominates and very little seasonal annealing is observed. In contrast, at the other site (Golden, US), half of the seasonal variation can be attributed to spectral changes while the other half must be attributed to thermal annealing of defects. Differences between multi-junction categories are investigated and it is shown that single-junction devices exhibit a greater seasonal annealing than multi-junctions, while the latter tend to be more influenced by spectral effects.

Experimental investigation of spectral effects on amorphous silicon solar cells in outdoor operation

The effect of varying spectrum on PV output is often underestimated in the case of amorphous silicon photovoltaic devices. This paper gives an indication of the order of magnitude of the seasonal variation of the useful irradiance in a maritime climate and also shows that this will involve a direct change in efficiency. This can be expected to be in the range of 15 percent around the annual average, thus explaining the seasonal performance. The spectral effects are investigated by distinguishing between a primary and a secondary effect. The primary effect is dependent on the availability of useful spectral irradiance while the secondary effect depends also on the spectral composition of the light in the useful range. It is shown that the secondary effect is especially significant for double junction devices.

Investigating the seasonal performance of amorphous silicon single- and multi-junction modules

The seasonal performance fluctuations observed in amorphous silicon solar cells are investigated. The dominant forces driving the increased efficiency in summer are identified, from long-term measurements, to be thermal annealing and spectral variations. A method for correcting for changes in the incident spectrum is applied in order to correct for the seasonal changes. In a second step, the fill factor is investigated in order to establish the magnitude of thermal annealing seen by these devices. The magnitude of each effect is investigated.

ADVANTAGES IN USING LEDS AS THE MAIN LIGHT SOURCE IN SOLAR SIMULATORS FOR MEASURING PV DEVICE CHARACTERISTICS

Advances in photovoltaic technology resulted in increased complexity of device calibration, largely being affected by deviations of test spectrum from natural spectra. While the output spectrum of some solar simulators is adjustable, generally only light intensity and module temperature can be varied. This is due to the light sources used in current simulators. LEDs offer an additional degree of freedom, when using an appropriate combination of wavelengths. This paper presents the advantages of this lighting technology for solar simulation and backs these up through results of the prototype unit developed at the Centre for Renewable Energy Systems Technology. The ability to keep LEDs stable for a long time and dim them with minimal changes in the spectrum allows generation of a spectrum closely matched to AM1.5G standard test spectrum or indeed even realistic variations of the outdoor spectrum. LEDs can be controlled very fast within microseconds or operated continuously, combining a steady state and a flash solar simulator with additional functions such as variable flash frequencies and flash shape. Combined with the life expectancy exceeding 50.000h, LEDs are a strong candidate for solar simulator light sources introducing a significant improvement in calibration lifetime as well as significantly reduced running cost. The usage of LEDs can enhance todays characteristic measurement functions and even opens possibilities to fully characterise solar cells indoors within a much shorter time than is possible today, over a range of conditions previously only available through outdoor characterisation.

ASPIRE - a tool to investigate spectral effects on PV device performance

A new model for calculating spectral irradiance from standard meteorological inputs is presented, named ASPIRE (All-sky SPectral IRradiancE). The Bouger law-based transmittance approach for modelling clear-sky spectra has been augmented with an extended empirical process to account for variation with cloud cover. Unlike previous clear-sky models, this new tool separates the effects of air mass from cloud cover within the clearness index by comparing measured solar spectra with the results of clear-sky modelling. The intended use of the ASPIRE model is to allow investigation of spectral effects on PV device output under realistic operating conditions, for which measured spectral irradiance data is rarely available. These investigations are important to refine the energy yield calculations for PV systems operating under seasonally varying spectra, especially wide band gap materials.

Effect of shading caused by dust on Cadmium Telluride photovoltaic modules

The effect of dust on Cadmium-Telluride photovoltaic (PV) thin film modules is investigated by the application of a spatial 3 dimensional model developed with the circuit analysis software PSPICE. The effect of dust concentration and tilt angle variation on the PV modules performance was investigated. The probability of hotspots in different installation positions is investigated. The simulation results showed a reduction in the samples performance with increased dust concentration and reduced tilt angle. The variation between cell positions showed that a horizontal orientation of the cells has an increased risk of hotspots with cells with localized lower parallel resistances than cells identified with uniform high parallel resistance.

Outlier identification in outdoor measurement data - effects of different strategies on the performance descriptors of photovoltaic modules

Outdoor measurement campaigns of PV module performance are normally affected by a relatively large number of outliers. The aim of this paper is to develop a statistically sound approach of obtaining a dataset that allows one to analyse continuously monitored devices for use of high volume data measurements. This paper uses ISC as a self-reference parameter to measure the incident irradiance on the module, which largely eliminates the error due to differences in spectral and angular response between test and reference detector. The outlier identification procedure is based on statistical distribution analysis of different performance descriptors and it assures 0.99 confidence level and the same skewness for the remaining data. This approach can be applied to whole datasets as well as for data in specific irradiance-temperature bins. The developed methodology will be used to analyze outdoor data from different devices at different locations with reduced uncertainty. It is shown that this approach is particularly useful for obtaining lower uncertainties in low irradiance measurements.