Evaluation of the levels and causes of performance degradation in a UK based PV roof integrated array

Understanding the nature of the long term degradation of photovoltaic (PV) systems is essential for calculating their lifetime performance and financial returns. However limited data exists for real world analysis of systems installed under UK irradiance and temperature conditions. The Centre for Alternative Technology (CAT) has had PV systems installed for over 20 years, and as part of this has had one of the first Building Integrated PV systems in the UK, installed in 1996. As part of its ongoing maintenance and repair programme, with the cooperation of Dulas Ltd, this 13.5kWp roof was stripped, modules individually flash tested and refitted. From the 180 modules, performance ranged from 80-96\% of rated power (Pmax), with an average figure of 91.2\%. This shows an annual power degradation of 0.63\%, which is lower than would be expected using standard predictions. The results of each module test have been compared against location and levels of degradation to calculate the conditions that the different modules have experienced in order to attempt to understand the range of degradation.