Measurements of the Actual Efficiency of PV Panels Depending on Their Orientation and Environmental Temperature

Polycrystalline photovoltaic (PV) panels of various orientations were installed in Riga, Latvia, for long-term monitoring and determination of the efficiency in real operation conditions depending on the environmental parameters. Results of the first couple of years of monitoring are summarised in this paper. Analysis of the available energy and environmental data allows for the quantitative estimation of actual efficiency depending on spatial orientation and following comparison with manufacturer’s values. The cloudiness that characterizes Riga's climate from October to January significantly reduces energy production. The temperature dependence of the PV module’s electrical efficiency is also clearly determined. The detailed data analysis indicated that the experimentally measured efficiency indicators for panels correlate well with the values that were determined for standard conditions. The energy produced during the winter months is determined to be less than 10% of the annual energy production. At the same time, the efficiency of PV modules (%) during the winter increases due to an increase in the open-circuit voltage in a solar cell. The interesting result is the impact of cloudiness on the solar radiation intensity over different years, resulting in high variations in produced energy for different years.