During the design phase, you will need to estimate the temperature of the PV modules to determine the temperature-adjusted voltages from the array. The exact values to use will be based on the array location, the proximity of the array to a structure (such as a roof), and the designer’s own estimation of ambient temperatures.
For adjusting the array’s Voc value, use the record low temperature as the minimum PV cell temperature. The array will go to full Voc once the sun strikes the array and before the array has begun to warm up. This is a conservative estimate, justified because high DC input voltage can damage an inverter.
There is a little more latitude when designing for the Vmp of the array—components won’t be damaged, but the inverter runs the risk of shutting down. Once the PV cells have been sitting in the sun, they will be hotter than the ambient air temperature. Cell temperature estimation typically relies on using either the highest average temperature or the record-high temperature in your location.
Another method is to use the American Society of Heating, Refrigerating and Air-Conditioning dry-bulb temperature data (Appendix E of Expedited Permit Process for PV Systems—see Access). The appendix shows a “2% design temperature,” where the recorded temperature exceeded the listed value 2% of the time, averaged from June through August. If you use this data, the result is an accurate PV cell temperature estimate for 98% of the summertime conditions. This is acceptable, considering there will be little time when the array operates at a higher temperature, which may result in inverter shutdown, but does not result in inverter damage.
The final method is estimating the cell temperature for high ambient-temperature conditions. This considers the ambient temperature, as well as the PV array mounting method. As with the ambient temperature selection, this is not a set value but ranges from adding 25°C to 35°C to the ambient temperature (see “String Theory” in HP125).
Voltage Correction Example
To help clarify, here are example calculations for adjusting the two module voltages. The example module has the following specifications:
Voc = 33.6 V
Temperature coefficient for Voc = -0.114 V/°C
Vmp = 26.4 V
Temperature coefficient for Vmp = -0.124 V per °C
The site has a record cold temperature of -18°C, so for adjusting Voc, this temperature will equal cell temperature.
To calculate the adjusted voltage for cold temperatures, use the equation:
Voc at STC + ((Tcell - TSTC) x coeff)) = Max Voc
33.6 V + ((-18°C - 25°C) x -0.114) = 38.5 V
The 2% summer temperature is 37°C and the array is mounted parallel to the roof so the module’s high cell temperature is estimated at 37°C + 35°C = 72°C. To calculate for the adjusted Vmp value in the summer:
Vmp at STC + ((Tcell - TSTC) x coeff)) = Min Vmp
26.4 + ((72°C - 25°C) x -0.124) = 20.6 V
These new adjusted voltage values can now be used to calculate the minimum and maximum number of modules required by a particular inverter. For example, if a SMA SB5000US inverter was chosen, the inverter can accept up to 600 VDC and requires at least 250 VDC to operate. Using the example modules:
600 VDC ÷ 38.5 Voc = 15.5 modules, or a maximum string length of 15 modules
250 VDC ÷ 20.6 Vmp = 12.1 modules, or a minimum string length of 13 modules
Example ASHRAE 2% Design Temps
Cell Temperature & Inverter Operating Range