Solar Photovoltaics and Energy Yield

Article written by Arne Jacobson and Stephen Kullmann

pv testing array

Energy Yield Test Array at Humboldt State University. The energy yield study involves detailed measurements of the performance of amorphous silicon and crystalline silicon PV modules. (Photo Credit Arne Jacobson)

Thin film solar photovoltaic (PV) modules are emerging as a lower cost alternative to the more conventional crystalline silicon (c-Si) PV modules. Amorphous silicon (a-Si) PV is the most mature of the thin film technologies and worldwide, a-Si modules make up approximately 15% of total solar PV sales. In some developing country markets, a-Si PV has become the dominant technology.

The growing use of amorphous silicon PV technology has led to a controversy about solar PV module performance ratings. The debate is related to the relative performance of c-Si and a-Si PV technologies. Manufacturers of a-Si modules claim that their products produce 10-15% more electrical energy per rated Watt of power output than c-Si technology. The reason for the variation, the theory goes, is related to the differential influence that real world weather conditions such as temperature have on the performance of the respective module types. With this in mind, a-Si PV manufacturers say that they should be allowed to adjust their power ratings to account for this extra energy production. Manufacturers of c-Si modules dispute this claim.

Over the past two years, we at SERC have been researching PV performance with an aim of resolving the controversy. Our work has involved detailed ‘energy yield’ measurements of the performance of 12 solar PV modules from Kenya, which is one of the countries where a-Si modules are widely used. Six of the modules in the sample were amorphous silicon PV modules and six were crystalline silicon PV, and all had power ratings ranging from 14 to 25 Watts.

We measured the average performance of the a-Si and c-Si PV modules over a 12 month period. Our results do not support the claims of the a-Si manufacturers, as they indicate that the per Watt energy output of the a-Si modules was only 3% higher than the output of the c-Si modules over the course of one year. However, these results are specific to the prevailing weather regime here on the North Coast of California. The difference in performance between the a-Si and c-Si modules is likely to be somewhat larger in a warmer location. We are currently working to develop a collaborative project with partners elsewhere in California that will allow us to expand the study to include additional sites.