Excessive heat can significantly reduce the output of a PV system. This article gives some pointers on avoiding unexpected energy loss in an array.
It may seem counter-intuitive, but solar panel efficiency is affected negatively by temperature increases. Photovoltaic modules are tested at a temperature of 25 degrees C (STC) – about 77 degrees F., and depending on their installed location, heat can reduce output efficiency by 10-25%. As the temperature of the solar panel increases, its output current increases exponentially, while the voltage output is reduced linearly. In fact, the voltage reduction is so predictable, that it can be used to accurately measure temperature.
As a result, heat can severely reduce the solar panel’s production of power. In the built environment, there are a number of ways to deal with this phenomenon.
Different module designs and different semiconductor compounds all react to temperature – here’s a brief intro into what to expect.
Determining Your Efficiency
The best way to determine your panel’s tolerance to heat is by looking at the manufacturer’s data sheet. There, you’ll see a term called the “temperature coefficient (Pmax.)” This is the maximum power temperature coefficient. It tells you how much power the panel will lose when the temperature rises by 1°C above 25°C. @ STC (STC is the Standard Test Condition temperature where the module’s nameplate power is determined).
For example, the temperature coefficient of a Panasonic HIT 330W N-Type 96 solar panel is -.258% per 1 degree Celsius. So, for every degree above 25°C, the maximum power of the Panasonic solar panel falls by .258%, for every degree below, it increases by .258%. What this means no matter where you are, your panel may be affected by seasonal variations. However, the temperature coefficient also tells you that efficiency increases in temperatures lower than 25°C. So, in most climates, the efficiency will balance out over the long run.
For a geographic region where temperatures higher than 25 degrees C. are the norm, one can consider alternatives to Mono or Polycrystalline modules, which have the highest efficiency (At 1:1 concentration), but also the highest temperature coefficient at PMAX. Project designers may want to consider a thin film or CdTe module – or in the case of a very large project, High Concentration PV, which is designed for hot climates, but not applicable for small projects.
How to Reduce the Effects of Heat
After the module technology is selected for installation, there are several ways to minimize the negative effects of high temperatures:
• Install panels a few inches above the roof so convective air flow can cool the panels.
• Ensure that panels are constructed with light-colored materials, to reduce heat absorption.
• Move components like inverters and combiners into the shaded area behind the array.
Further information about temperature coefficients can be found in this research paper, produced by Sandia National Laboratories: "Temperature Coefficients for PV Modules and Arrays" David L. King, Jay A. Kratochvil, and William E. Boyson" (PDF)