Sizing MPPT Solar Charge Controllers and the Effects of Temperature
When attempting to determine how many solar panels one can put on a string for a given MPPT charge controller, it is important to account for the temperature coefficient of the solar panel. The what, you might ask? The temperature coefficient is the amount the voltage will change depending on the temperature a given solar panel is exposed to. In colder weather, the voltage of a panel will go up, and in warmer weather the voltage goes down. By taking into account the temperate coefficient, one can design a system so that the voltage of the strings of the panels does not increase in cold weather to the point where they can cause damage to the charge controller.
So how does one measure the maximum possible voltage for a given panel in a given area?
1. Find the open circuit voltage (Voc) of the solar panel in question. That is the highest voltage it can operate at under standard test conditions. The Voc is usually located on a sticker on the back of a solar panel.
2. Find the temperature at standard test conditions. This would be found in the spec sheet of a given solar panel. Usually it is 25°C or about 77°F.
3. Find the temperature coefficient for the panel in question. Also found in the spec sheet of the panel. Usually it’s expressed in the form of a percent change per degree of temperature, like -0.34%/C, for example.
4. Determine the coldest it is ever likely to get at the installation site. This is not the average cold temp, it’s the record cold temp, because it can only take once for the strings of panels to operate at a voltage above the capacity of the controller and ruin it.
5. Make your calculation using this formula:
Voltage open circuit (Voc) at ambient temp of installation at ambient temp of interest =
Voc (at STC) + (Module Temp Coefficient x Ambient Temp of Interest – 25°C)
Example of Sizing an MPPT Charge Controller Knowing Lowest Temperature
Let’s say I live in Warsaw, Missouri and I want to install 16 SolarWorld 240 watt solar panels on my house. What is the highest voltage each panel will ever see?
The coldest temp recorded in that area is -40°C.
The open circuit voltage of the SolarWorld 240W panel is 37.3 and the temperature coefficient is -0.37%/K. Kelvin? That is what’s on the spec sheet, and fortunately the conversion from Kelvin to Celsius is easy – it’s a one to one ratio. 1 degree Kelvin = 1 degree Celsius.
Now we need to find the difference in temperature between the standard test condition and the record low temp at the installation site. STC is 25°C. Subtract -40 from that and we get -65°C.
Next, multiply that by the temperature coefficient. -65°C X -0.37% = 24.05%.
Finally, calculate the max increase in voltage due to the drop in temp. The Voc is 37.3 volts, but the voltage could increase by 24.05% if the record low was ever reached. So 37.3 x 24.05% = 8.97 V. Add 8.97 to the 37.3 Voc, and we get 46.27V.
That is the voltage for each panel that one would use when determining the voltage of a string of solar panels. In this case, I hope to have strings of 4 panels, so 4 x 46.27 = 185.08 Volts. So if I want a charge controller that can handle strings of 4 solar panels, then it will have to have a voltage capacity of at least 185.08 volts.
In conclusion, including a voltage coefficient calculation is a necessary part of the process of sizing solar panels to MPPT charge controllers and even a PWM controller. The change in voltage is greater with colder weather, but whatever the climate, it’s better to make the calculation in the design stage of a project than to experience a surprise once you’ve got the system up and running.