By: dr. Asterios Bouzoukas – O&M manager
Thursday 25/07 was announced from the Dutch meteorological service, KNMI that a new temperature record of 40.2oC was reached. This should have indicated for the solar industry in the Netherlands a record in power production from PVs from the moment it was a day with clear sky and high irradiation. Unfortunately this is not the case.
Photovoltaic solar panels convert sunlight into electricity, so the more sunlight, the better. That’s not always true, because sunlight consists not only of the light that you see, but also of invisible infrared radiation, which carries heat. So solar panels will perform great if get a lot of light, but as it gets hotter, its performance degrades.
So How Does Heat Affect PV modules?
Solar panel manufacturers test their products at standard conditions of 25 degrees Celsius with an insolation of 1,000 watts per square meter. Insolation is a measure of how much solar power is hitting each square meter perpendicular to the direction of the sunlight. The insolation can be higher than 1,000 watts per square meter around noon on very clear days, and that will make your solar panel generate more current, which means more power. Unfortunately, it’s a different story with temperature affecting this thought.
The current generated by a solar cell is a function of the amount of sunlight that hits it. The more sunlight that hits it, the more current it will generate. But electrical power is the product of the current times the voltage.
As the temperatures of the solar cells rise above 25 degrees Celsius, the current rises very slightly, but the voltage decreases more rapidly. The net effect is a decrease in output power with increasing temperature. Typical silicon solar panels have a temperature coefficient of about -0.4 to -0.5 percent. This means that for every degree Celsius above 25, the power output from the array would drop by that percentage. At 50 degrees Celsius, a 250-watt solar panel with a temperature coefficient of -0.4 would produce less than 225 watts.
So it is very important when we chose a solar module for our plant always to look the temperature characteristics of the module. Lower temperature coefficients means lower losses cause of increased temperatures.
The effect of temperature on the voltage could be seen in the following picture. In this curve you can see the voltage of a module (pink line) during one day. You can see as well how the temperature (black line) on that day influenced the voltage: the warmer the modules, the lower the voltage. At this graph we see that the voltage started at around 600V in the morning and dropped down up to 525V, a 12% decrease in voltage. If assume that the current was the same in these two periods of the day (same amount of irradiation) then a power loss of 12% would be caused from the increased temperature.
Unfortunately the increase of ambient temperature can cause a similar effect on the inverters.
So How Does Heat Affect Inverters?
What is not as well understood is that heat also affects solar inverters. The reasons are not the same – although the solar inverter has semiconductor parts in it which lose efficiency as they heat up.
As the inverter works to convert DC power to AC power, it generates heat. This heat is added to the ambient temperature of the inverter enclosure, and the inverter dissipates the heat through fans and / or heat sinks. The heat needs to stay below a certain level at which the materials in the inverter will start to degrade.
In order to keep the heat low, the inverter will stop generating power or reduce the amount of power it generates by “derating”. Derating is the controlled reduction of the inverter power to prevent the sensitive semiconductors in the inverter from overheating. Figure below shows how an inverter handles temperature derating. At about 45-50 degrees C. it starts to ramp down power.
This ramp-down of power can be prevented with six key system design considerations:
- Install inverters in cool locations (shaded wall rather than the roof).
- Choose locations with sufficient air exchange. Ensure additional ventilation when necessary.
- Do not expose inverters to direct sunlight. For outdoor installations, use existing shadows or covers for inverters.
- Maintain the minimum clearance to neighbouring inverters or other objects.
- Increase the clearance when it is foreseeable that higher temperatures could occur at the installation location.
- Arrange multiple inverters so that they do not draw in the warm air of other inverters.