S.R. Williams

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.

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.

Actual PV module performance including spectral losses in the UK

STC efficiencies are not sufficient to compare photovoltaic devices of different semiconductor material or device configurations. The energy yield changes as the variables of STC deviates from their original values when the modules are placed in various climatic conditions. The magnitude of this change for different modules is not always clear and needs to be investigated and modelled. A modeling and analysis method named site specific conditions (SSC) is demonstrated which is a measure-correlate-predict approach. It allows an accurate estimation of the actual energy yield for different sites based on the measurements at one single site. The method takes into account the effect of the physical operating environment and translates this to other meteorological conditions on the basis of physics related formulae. Our results show a large seasonal variation for modules for the different effects. For crystalline modules losses of up to 12% in the summer is due to the temperature effect while the multi-junction thin film losses of more than 30% in the winter is due to spectral changes and incidence angle effect for the UK.

Performance of photovoltaic modules in a temperate maritime climate

The energy production of photovoltaic modules is a topic of growing importance. Under real operating conditions two modules with the same name-plate efficiency may have very different energy productions, despite being installed at the same site. In this paper we investigate the performance of a variety of modules (crystalline, polycrystalline and single and multi junction amorphous silicon and cadmium telluride) installed in Loughborough in the UK. The paper investigates the reasons for the significant difference in performance of modules of the same technology and efficiency. The analysis is carried out on the basis of influence of device temperature, magnitude of irradiance, incident spectrum and age of device.

Effects on photovoltaic solar module performance in the UK climate

SYNOPSIS Presently, PV devices are rated on the basis of a standard test condition (STC) efficiency and not on their energy production, which would be a much better parameter for their cost per generated kwh. There is a lack of knowledge of factors affecting performance of thin film PV devices, especially operating in maritime climates such as the UKs. Modules studied in this investigation include crystalline, polycrystalline and amorphous silicon, cadmium telluride and copper indium diselenide. The different factors affecting the performance of PV modules operated in the UK climate are investigated. The main effects identified are changes in the spectrum, operating temperature and irradiance, each affecting device performance according to the type of cell material. The spectral effect is predictably more pronounced for the wider band gap materials (amorphous silicon and cadmium telluride) and includes two separate identifiable effects, termed primary and secondary. The primary spectral effect depends on the availability of spectrally useful irradiance and is more pronounced for the amorphous silicon (a-Si) single junctions, where it varies by + 5% to—9% with respect to the annual average. There is also evidence of a secondary effect in the a-Si multi-junctions, due to mismatch of current between the series connected sub-cells, the magnitude of which is much less than the primary spectral effect, but is noticeable nevertheless. The materials with narrower band gaps (polycrystalline, crystalline and copper indium diselenide) suffer more significantly from thermal effects and less from spectral effects than do devices with a wider band gap. The effect of radiation intensity is device-specific; some devices (crystalline and polycrystalline) benefit from increased irradiance, although some thin films show a deterioration of efficiency with increasing irradiance. The importance of shunt and series resistance is discussed in this context.

Accuracy of Energy Prediction Methodologies

In the current market, the specific annual energy yield (kWh/kWp) of a PV system is gaining in importance due to its direct link to the financial returns for possible investors who typically demand an accuracy of 5% in this prediction. This paper focuses on the energy prediction of photovoltaic modules themselves, as there have been significant advances achieved with module technologies which affect the device physics in a way that might force the revisiting of device modelling. The paper reports the results of a round robin based evaluation of European modelling methodologies. The results indicate that the error in predicting energy yield for the same module at different locations was within 5% for most of the methodologies. However, this error increased significantly if the nominal nameplate rating is used in the characterization stage. For similar modules at the same location the uncertainties were much larger due to module-module variations

Educating the world: A remote experiment in photovoltaics

The increasing deployment of photovoltaic (PV) systems requires large numbers of skilled engineers with a greater understanding of all aspects of PV technology both theoretical and practical. Developing experimental rigs at universities is expensive and limited to students physically attending the university. One recent approach to increase access to laboratories is the development of remote experiments. Here students can control real experimental equipment using a visual interface via the Internet. In this paper we explore the development of a photovoltaic laboratory to enable users to access and remotely control experimental equipment based at Loughborough University, UK, from anywhere in the world.