Your Solar Panels May Be Even More Cost-Effective Than You Thought

Because solar radiation has increased, PV systems in Germany are yielding 5% more power than predicted. ©Fraunhofer ISE

When it comes to solar, the U.S. is learning a lot from Germany these days. A group of U.S. utility executives is visiting Germany to learn from its experience with a rapid shift to renewables. In the midst of this, another interesting piece of solar news has come in from Germany.

PV systems in Germany, it turns out, are regularly generating 5% higher yields than expected. Naturally, that has implications for other countries too.

Why is this happening? An investigation by Fraunhofer ISE links these higher yields to an upward trend in solar radiation. The current radiation level in Germany is considerably higher than the averages in the last 30 years. These average values have generally been used in yield predictions up to now. This means that actual PV system yields are exceeding their forecasts by approximately 5%, making them even more cost-effective than anticipated.

Based on these findings, Freiburg researchers have adapted their quality assurance services for PV systems and are now basing their yield predictions on solar radiation data from the last 10 years.

Until now, experts have based their solar resource assessments on the assumption that possible radiation levels in the future will not significantly differ from the average values of the previous few decades. However, solar radiation is undergoing long-term trends that cause average radiation values to change over time. These changes are caused by factors like increases and decreases in air pollution and the concentration of aerosols in the atmosphere.

In collaboration with ETH Zurich and Germany’s National Meteorological Service (DWD), researchers at Fraunhofer ISE have conducted research on the extent to which past average values deviate from today’s radiation levels and how these deviations will affect yield predictions for solar installations. The team analyzed DWD’s radiation measurement data from 1951 to 2010 from selected locations in Germany. The results show that solar radiation has been increasing since around the mid-1980s, with current radiation levels at the locations under observation measured at around 5% higher than the average value between 1951 and 2010.

These findings prompted researchers at Fraunhofer ISE to analyze yield predictions for PV power plants being monitored by the institute, which uncovered similar deviations between past yield assessments and actual radiation and yield measurements. “Relying on average radiation values from the past 30 years causes a systematical underestimation of actual PV system yields in Germany by around five percent,” said Björn Müller, project leader at Fraunhofer ISE. “We expect that other regions experiencing the brightening effect are seeing similar underestimations.”

The investigation results are now being included in Fraunhofer ISE’s yield predictions. In order to more realistically assess investments in future PV system projects, the quality assurance specialists have decided to use a modified data base for their yield predictions. They now use satellite-derived solar radiation time series from the last ten full years, which depict the increase in solar radiation, thus reducing underestimations.

“Our yield forecasts are based on satellite data from the past decade, which enables us to determine the profitability of PV systems more effectively than before,” said Klaus Kiefer, head of the department Quality Assurance PV Modules and Systems at Fraunhofer ISE. “They provide an improved decision support, for example when buying or reevaluating a PV power plant. Conducting further analyses when reevaluating PV systems can also help to significantly reduce investment risks.”

Fraunhofer ISE will present more research results on this topic at the European Photovoltaic Solar Energy Conference and Exhibition 2014 in Amsterdam.