The summer time is the best season of the year to maximize solar energy production. However, as irradiance values increase, higher ambient temperatures generally come along with it. These ambient temperatures have a deep impact in the solar panel efficiency and also in the operating temperature of the modules.
Solar panels are tested under controlled conditions to evaluate their performance and compare with other modules under the same environmental parameters. Nominal temperature values for solar panels are rated at 25°C for STC and 45°C for NOCT conditions.
Nevertheless, these are just nominal values under a controlled environment; in real life scenarios, solar panels are capable of reaching temperatures that can exceed 70°C in areas with good solar irradiance. Moreover, desert areas in regions like the Middle East or Africa, like for instance Ethiopia, can reach more than 2000kWh/m2/yr of solar irradiation, causing module temperatures to reach around 100°C (212°F).
Before jumping ahead into details, we must first learn what are the actual nominal values under which solar panels are rated to operate. For that, we must know what are the operating conditions that modules are subjected to when they are tested.
What are STC and NOCT test conditions?
Every new solar panel model that wants to come into the market, needs to perform a set of lab tests better known as Standard Test Conditions (STC) and Nominal Operating Cell Temperature (NOCT). These tests evaluate the temperature performance, power output, efficiencies, current and voltage values of the module under specific environmental conditions.
STC are associated with these lab conditions:
- Irradiance: 1kW/m2
- Temperature: 25 °C
- Air Mass: 1.5 AM
On the other hand, NOCT conditions test the modules under the following parameters:
- Irradiance: 800W/m2
- Wind Speed: 1m/s
- Ambient Temperature: 20°C
- Surface Temperature: 45°C
As can be seen above, there are some important differences between the two test conditions. NOCT conditions are referenced to better represent real-life conditions than what STC conditions do. For instance, irradiance values are considered at 800W/m2 which would be closer to what would be expected in most locations with good but variable solar radiation. Moreover, wind speed is considered to represent cooling effects in the cells while operating temperatures are established around 45°C unlike STC where they are considered at 25°C.
Impact of Temperature on Solar Panel Performance
Now that we know the difference between test conditions, the next important fact to consider is the effect that temperature has on solar panel performance.
Temperature is an environmental parameter that has an important effect in the voltage of the solar panel. Since voltage is hindered by temperature variations, the power output of the solar panel is also affected. Meanwhile, the temperature can also have an effect on electric current values, however, it is so small that it can be considered negligible.
Now, it is important to know that cold temperatures have a positive effect on voltage, increasing its value. On the other side, hot temperatures have a negative effect, reducing voltage. In other words, cold ambient temperatures favor the performance and energy production of the solar PV module, while high temperatures hinder power output and generate what is known as thermal losses.
An example of this impact can be seen in the figure below where the I-V curve of a solar PV system can be detailed. The I-V curve represents variations of voltage and current in the solar panels. The figure below shows the variations in electrical current and voltage for different operating temperatures that oscillate between 0°C and 100°C. If we compare, voltage values for 50°C compared to 100°C, we can see that the knee of the 50°C curve would be close to 140V, while the knee of the 100°C curve would result close to 120V. This 20V difference in this case, can represent some important difference in the power output of the system. On the other hand, as can also be seen in the curve, variations in electrical current regarding temperature variations are negligible.
How do Ambient Temperatures Affect Operating Temperatures of the Solar Panel?
Some important difference to be aware of is that ambient and operating temperatures in a solar system are very different parameters, however, they are correlated.
To generate electricity and electrons movement, some energy loss in the form of heat must be experienced. This is intrinsic to the photoelectric effect, however, there is another component that plays an important role in the operating temperature, this is of course, the ambient temperature. This matter is very intuitive, the hotter the climate, the hotter the solar cells will get. Nevertheless, there is a way to estimate how hot the solar panel will get according to the ambient temperatures by following the expression below.
As can be seen above, having the NOCT temperature value is important, generally should be specified in the datasheet of the module but should be close to 45°C. The Ic represents the estimated irradiance that the module receives according to local solar resource, this can be found in NREL map resources. Cell operating temperature (Tcell) will increase or decrease accordingly to the expression above.
What are temperature coefficients and why should we care about them?
Another important parameter regarding thermal performance in solar panels is the coefficient temperatures of the module. These parameters establish the rate at which the solar panels will change their voltage output according to an increase of 1°C in the operating temperature of the solar panel.
This parameter is the one that allows to establish the equivalent operating voltage as well as maximum and minimum voltage for the solar panels. It can be typically found in the datasheet of the module as well. The expression below shows how to estimate open-circuit voltage of a module based on the temperature coefficient β.
What is more important is to know that the lower voltage temperature coefficient of the module, the lower impact the solar panel will have on performance due to rising temperatures.
How do solar panels perform in hot climates?
Generally, people tend to think that the hotter the climate, the better it is for solar power. However, we have learned that this is not necessarily true as the high ambient temperatures generate higher thermal losses. Solar radiation has two components, light and heat, of which only light can be used to generate electricity with traditional solar PV modules. The heat component is not actually beneficial for solar electricity generation.
Besides, heat is generally linked to high relative humidity values. Humidity is not good for solar PV production either because it can lead to other types of issues such as moisture, leakage currents, delamination and even reduction in electric current values.
Sloped roofs also have a big disadvantage in hot climates when compared to ground mounts. The reason is because air circulation behind the module is hindered when installing solar panels on sloped roofs, this causes the cooling effect of air is minimized and therefore increases thermal losses. Typical installations with sloped roofs in average temperature climates will not be much affected in the thermal aspect as in locations with very hot climates.
Temperature has a deep effect on the efficiency and power output of the solar panels, which is why it is important to consider the thermal losses as part of the solar energy production estimations.
Solar panels under typical weather conditions with good sunlight, can reach to temperatures of 70°C. However, in more extreme climates temperatures can rise up to 100°C. As you can imagine, it is unwise to directly touch solar modules without protection under the sun, this is one of the reasons why maintenance procedures must generally be carried out in the late afternoon or early mornings.
Frequently Asking Questions
Solar panels can get very, very hot. However, it is very unlikely that they get damaged due to high temperatures.
PV modules are also tested for damp heat tests under which the solar panels must withstand up to 1,000 hours of high temperatures at 85°C with up to 85% relative humidity. Moreover, they will also need to pass through a test in which 200 cycles of temperature variations between -40°C and 85°C will be experimented by the module. As you can see, if a module passes these tests, it is rated to endure even under hot climate conditions.
In any case, manufacturers specify the maximum tolerable temperature by the module that you can double check just in case needed, however, to surpass these values, conditions would need to be extreme.
If you want to minimize the effect of high temperature conditions, then you might want to keep in mind these considerations.
First of all, consider purchasing a high quality module with a low temperature coefficient. HIT modules from Panasonic have the lowest temperature coefficient at -0.258 % / °C, that can be a good starting or reference point.
Second, consider installing a ground mount instead of a roof mount to maximize air cooling. Finally, for desert areas, a water cooling system may be a useful addition to the PV system to bring temperatures of the module down and make it operate more efficiently.
Are there any solar panels rated for specialized heat resistance?
Yes, some module manufacturers focus on designing solar panels for very hot and desert areas. For instance, J.V.G Thoma, designs modules to withstand operating temperatures of up to 145°C. Almaden also manufactures high-heat resistance modules with no PID losses as well.
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