Ralph Gottschalg

On the importance of considering the incident spectrum when measuring the outdoor performance of amorphous silicon photovoltaic devices

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.

The influence of the measurement environment on the accuracy of the extraction of the physical parameters of solar cells

This article concerns the influence of measurement conditions on the extraction of a solar cells equivalent circuit parameters. Influences previously not investigated are considered, thus helping to define suitable measurement strategies. The influences of measurement environments are investigated, as is the influence of the fitting algorithm chosen. It is shown that the number of measurement points for the current-voltage characteristic can have an important effect on the accuracy of the parameters extracted. The stability of the system is of minor importance, as long as the variations in the measurement conditions are monitored. We also show that the Marquardt-Levenberg algorithm using a least squares error criterion and a hybrid algorithm employing an area criterion outperform other choices of fitting algorithm.

Modelling long-term module performance based on realistic reporting conditions with consideration to spectral effects

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.

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.

The Effects of Solar Cell Capacitance on Calibration Accuracy When using a Flash Simulator

A model for investigating the dynamic behaviour of solar cells, which allows the simulation of the measurements process with flash solar simulators, is presented in this paper. This model is then applied to the simulation of measurement processes currently employed by common solar simulator types. It is shown that the cell capacitance, the series resistance and cell area can all influence the measurements, giving rise to transient errors, especially in short flash solar simulators. It is also shown that point distribution as well as scan time also has significant importance

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.