Optimizing Array Voltage for Battery-Based Systems: Page 2 of 2
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While a module’s voltage drops when the cell temperature rises, the voltage will increase as the temperature falls, so a second calculation is required. NEC Article 690.7 provides correction factors for calculating maximum voltage based on STC open-circuit voltage and lowest expected ambient temperature. The 2008 NEC requires you to use the PV module’s voltage temperature coefficient in this calculation, if it is available. Using the record low temperature for the array’s location is common practice during system design.
Record low temperatures approaching -40°C are not uncommon in some US locations. For example, according to weather.com, the record low temperature is -27°C for Santa Fe, New Mexico, -31°C for Denver, Colorado, and -38°C for Lander, Wyoming, while average summer high temperatures are typically near 30°C.
Modules at -40°C are 65°C colder than their STC specification parameter. Continuing to use the -0.35%/°C temperature coefficient, the cold temperature voltage multiplier is: 1 + (-65°C x -0.35%/°C) = 122.75%.
In order to remain compliant with NEC 690.7 and keep the temperaturecorrected maximum voltage below the controller’s absolute 150 Vdc limit, the array’s Voc at STC is calculated as: 150 Voc (absolute limit) ÷ 122.75% (cold ambient temperature multiplier) = 122.2 Voc.
Vmp is typically about 80% of Voc. Therefore, the array’s Vmp limit for this exercise is: 122.2 V x 80% = 98 V.
Accordingly, the suitable array Vmp at STC range has narrowed considerably to a minimum of 73 V and a maximum of 98 V.
Finding modules with the right STC voltage specifications for the site temperature range in the above example used to be a daunting challenge. Two 72-cell modules in series (about 68 Vmp at STC) results in an array voltage that is below the minimum 73 Vmp at STC value. Three 72-cell modules in series (about 102 Vmp at STC) results in an array voltage that is greater than the maximum 98 Vmp at STC value. One popular solution was to configure five 36-cell modules in series, for an array voltage of approximately 85 Vmp at STC that fit comfortably between the 73 V and 98 V limits. However, 36-cell modules have low power ratings and a high cost per watt compared to 72-cell modules. The combination of higher specific-power and labor costs resulted in a relatively expensive array.
Fortunately, the marketplace has addressed and solved these issues of module specifications and cost. Wiring five 36-cell modules in series creates strings with 180 cells in series operating at about 85 Vmp at STC. Several manufacturers now offer large-format 60-cell modules. Configuring three of these modules in series results in an equivalent 180-cell string. These manufacturers include BP, Canadian Solar, REC, Sharp and SolarWorld. Other solutions are also available. Canadian Solar, Day4Energy, Kyocera and Sharp produce 48-cell modules, which create 192-cell strings when wired four in series. This approach works quite satisfactorily over broad temperature ranges as well.
—Jim Goodnight / Schneider Electric / Vienna, VA / schneider-electric.com