Abstract
The energy yield delivered by different types of photovoltaic device is a key consideration in the selection of appropriate technologies for cheap photovoltaic electricity. The different technologies currently on the market, each have certain strengths and weaknesses when it comes to operating in different environments. There is a plethora of comparative tests on-going with sometimes contradictory results. This paper investigates device behaviour of contrasting thin film technologies, specifically a-Si and CIGS derivatives, and places this analysis into context with results reported by others. Specific consideration is given to the accuracy of module inter-comparisons, as most outdoor monitoring at this scale is conducted to compare devices against one another. It is shown that there are five main contributors to differences in energy delivery and the magnitude of these depends on the environments in which the devices are operated. The paper shows that two effects, typically not considered in inter-comparisons, dominate the reported energy delivery. Environmental influences such as light intensity, spectrum and operating temperature introduce performance variations typically in the range of 2–7% in the course of a year. However, most comparative tests are carried out only for short periods of time, in the order of months. Here, the power rating is a key factor and adds uncertainty for new technologies such as thin films often in the range of 10–15%. This dominates inter-comparisons looking at as-new, first-year energy yields, yet considering the life-time energy yield it is found that ageing causes up to 25% variation between different devices. The durability of devices and performance-maintenance is thus the most significant factor affecting energy delivery, a major determinant of electricity cost. The discussion is based on long-term measurements carried out in Loughborough, UK by the Centre for Renewable Energy Systems Technology (CREST) at Loughborough University.
Highlights
The most critical factor determining the suitability of deploying photovoltaics is the cost of energy, or service, delivered and not the power rating of the devices
The focus is on thin film technologies, namely different modules produced from amorphous silicon (a-Si) and Copper Indium Gallium Diselenide (CIGS), in particular on the energy yields of these devices which are susceptible to variations in the operating environment, have a wider design window and less availability of field experience data than conventional wafer-based crystalline silicon (c-Si) devices
The uncertainty in the specific energy yield is split into two parts, the actual measurement of the energy yield and the determination of the power rating used for the normalisation
Summary
The most critical factor determining the suitability of deploying photovoltaics is the cost of energy, or service, delivered and not the power rating of the devices. The effects of the modifications in series resistance and temperature coefficients on the specific energy yield depend on the particular location environments and the device responses relative to STC. The sub-optimal design, i.e. the one with two bus bars which causes a higher series resistance and high ohmic losses, typically has a lower power at STC but may have a higher specific energy yield than those with three bus bars due to relatively higher efficiency at lower irradiances It can be seen that in certain environments the ranking in terms of performance ratio or specific energy yield changes for the different series resistances assumed or temperature coefficients. The aim in the following is to derive some general factors, which excludes device specifics and gives an estimated ranking of the importance of the different effects on the performance, identifying the optimisation potential for future devices
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