How Does Heat Affect Solar Panel Efficiencies?
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 solar panel might be -.258% per 1 degree Celsius. So, for every degree above 25°C, the maximum power of the 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)
Look at how heat debilitates the 10 panels that are mounted above asphalt shingles by ~ 10% beginning in mid-May thru August (temp about 49C 120F---space clearance for cooling is important! We know there is much more sun in July than the highest output time in the merry, cool month of May. What a waste.
"So, for every degree above 25°C, the maximum power of the Sharp solar panel falls by .485%, for every degree above, it increases by .485%."
This should read "for every degree below, it increases by .485%."
Great article though! Thanks..
Great article. Acknowledges what we have known, and worked around for years here at Falmouth Solar, LLC on Cape Cod, MA. What really would be helpful would be some hard numbers, tested in my area. What output loss is experianced at what height / and roof temps for example. WE don't like the trend of the local installers here to put the array within a couple of inches of the roof, then put a sheet metal cover along the bottom row. How can heat convect? It's a nice look from the ground, but stupid from an engineers view. We don't install railess racking because we generally will even provide 1.5" between rows to further let the excess heat evaluate. This, as far as I know is an original thought, and I am unaware of any other installers doing it. Testing is needed to validate the effects of side skirts, row spacing, and height above roof for validation of our efforts, but a potential loss of 10-25% is HUGE in this industry, and generally ignored (except by the wise few reading this article!)
I'm not sure I entirely understand your question. Are you circulating water from tanks to help cool your modules? If yes, perhaps it would be best to increase the volume of water your circulating or the frequency in which the water is cooled.