Standard Test Conditions of PV Module
All PV module manufacturers test their modules under standard test conditions (STC). The three main elements to the STC are cell temperature, irradiance, and air mass – all of which are variable conditions that the PV modules will be exposed to after they’re installed. Because these conditions affect the modules’ power output, PV manufacturers had to establish a value for each of these elements that everyone could test to and report their results. Those standard values are as follows:
Cell temperature
The STC for cell temperature is 25 degrees Celsius or 77 degrees Fahrenheit (Note that it’s cell temperature, not air temperature.) When a PV module is operating in the sun, it typically gets much hotter than 25 degrees Celsius. Depending on the location and the way the module is mounted, cell temperatures of 75 degrees Celsius aren’t uncommon when the modules are in full sun.
Irradiance
Irradiance, simply stated, is the intensity of the solar radiation striking the earth. The STC value for irradiance is 1,000 watts per square meter (W/m2). Irradiance values vary from 0 W/m2 to 1,250 W/m2. The 1,000 W/m2 value represents the full sun, or peak sun, which is common to many terrestrial locations.
Air mass
Air mass is a representation of how much atmosphere sunlight must pass through to strike the earth. The STC value for air mass is 1.5 (AM 1.5). Actual air mass values vary widely depending on one’s location on the globe, the time of year, and the time of day.
Of these three elements, you should concern yourself with compensating for differences in cell temperature and irradiance values because these two variables directly and measurably affect a PV module’s voltage and current in the following ways:
- Voltage: The higher the cell temperature, the lower the module voltage – and vice versa.
- Current: The higher the irradiance value, the more current is pushed through the module.
You maximize a PV module’s electrical output by keeping the module as cool as possible and pointing it directly at the sun as much as possible.
Environmental effects on standard test conditions
When you’re in the field and need to verify a PV module’s power output, you must be able to apply voltage temperature coefficients and calculate the module’s current output based on the irradiance received at that time; the following sections introduce the info you need to know.
The temperature of PV module
You need to be able to calculate the change in voltage due to cell temperature because every electrical component connected to a PV array has input voltage requirements that must be met. Regardless of the type of PV system used, if you apply a voltage in excess of what the equipment is rated for, you may damage it beyond repair; if you don’t send enough voltage, the electronics will shut down. The excessive voltage is considered a safety concern, whereas the lack of voltage is considered a performance concern.
There are two distinct voltage coefficient values, one for Voc and one for Vmp. The coefficient value for Voc is always less than the coefficient for Vmp. After all, there’s a greater change in the voltage when the PV module is operating (Vmp) than when it isn’t doing any useful work (Voc). When working with a crystalline PV module, check the module’s spec sheet for the Voc and Vmp values. If they’re not there, the manufacturer should be able to supply them because they’re known quantities; you just have to ask either the person you’re buying the modules from or the manufacturer for this information. If the information isn’t readily available, you can use the following values and be within reason:
- Temperature coefficient for Voc = –0.3%/°C
- Temperature coefficient for Vmp = –0.5%/°C
Notice the negative number in front of the coefficient; it represents the inverse relationship. So as the temperature increases (a positive change in temperature), the voltage decreases. As the temperature goes down (a negative change in temperature), the voltage increases. Your reference temperature is always going to be the STC temperature of 25 degrees Celsius. So when you calculate the change in voltage, you have to compare the cell temperature in that scenario to 25 degrees Celsius.
For example, if you want to find out what percentage change the module’s Voc will have when the cell temperature is 58 degrees Celsius, you need to subtract the STC temperature from the cell temperature: Cell temperature – STC temperature = Difference in temperature 58°C – 25°C = 33°C You now know that the cell temperature is 33 degrees Celsius more than the STC temperature, which means you can use this information in conjunction with the given temperature coefficient to determine the percentage change.
The difference in temperature × Voltage temperature coefficient = Percentage change in voltage 33°C × –0.35%/°C = -13.2% change in rated Voc.
Irradiance on PV module
The change in irradiance directly affects a module’s current output. You may need to verify the performance of a single module or an entire array. By measuring the irradiance value and the current output of the PV module (or array), you can compare the two to see whether the modules are operating as expected. Without knowing the exact irradiance measurement, you can’t accurately determine whether the current values are reasonable.
To calculate this change (which is an important task when commissioning, or turning on an array), you first need to know what the irradiance value is. You can figure this out by using an irradiance sensor that points in the same direction as the PV module (or array). If you’re using a handheld sensor, place the bottom of the sensor on the frame of the module and read the digital display to see the irradiance value at that moment. Because the module’s output is directly proportional to the irradiance and the STC value for irradiance is 1,000 W/m2, you can estimate the percentage change of current by measuring the irradiance and dividing that by 1,000 W/m2.
For example, if the measured irradiance is 650 W/m2 and you want to know what the effect is on the current value, then you’d perform the following calculation:
650 W/m2 ÷ 1,000 W/m2 = 0.65
The current is therefore 65 percent of the module’s STC value. The irradiance sensor reports the values in terms of W/m2, and because the STC is 1,000 W/ m2, this makes the conversion easy on you. When you’re in the field, you can expect to see irradiance values at or below 200 W/m2 and up to 1,250 W/m2. Regardless of the value you see, you can simply multiply the current value at STC by the percentage you calculate to determine the estimated current value under those irradiance conditions.
