Optimizing Array Voltage for Battery-Based Systems
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Before the advent of modern maximum power point tracking (MPPT) photovoltaic controllers, configuring a PV array for a given battery bank’s nominal voltage was a fairly simple exercise. Older series-type pulse-width-modulation charge controllers had a simple 1:1 relationship with the battery bank. For example, a 12 Vdc-nominal module operating at approximately 17 Vmp could be used to charge a 12 Vdc-nominal flooded-cell lead acid (FLA) battery, which typically requires about 14.4 V during the absorption cycle and about 15 V for equalization charging. Because of this simple relationship, array voltage received minimal concern during the design stage, other than correctly sizing the PV-to-controller wiring to minimize voltage drop. Similarly, calculating maximum circuit current to specify the PV controller and overcurrent protection devices was fairly straightforward.
With today’s more advanced charge controller technology, MPPT controllers are the industry standard. They offer significant advantages over non-MPPT controllers, including optimized energy harvest and the option to configure the array at higher voltages than the nominal battery voltage. Most MPPT controllers have dc-to-dc voltage stepdown functionality, which allows you to use a high-voltage array for lowvoltage battery charging. The benefits of higher array voltages include lower array currents, reduced power loss in the homerun wiring, reduced conduit size and cost, and the option to locate the array farther from the battery pack to minimize any potential shading issues. Installers always prefer smaller-gauge conductors, because they are easier to work with.
By nature of their operation, MPPT controllers deliver improved system performance and can reduce copper costs when compared to non-MPPT controllers. However, for optimal performance, they also present greater system design complexity. Factors that must be considered during the design phase include battery charging voltage requirements, the use of high-voltage arrays, the evolving requirements of the National Electrical Code, the impact of a site’s annual ambient temperature range on array voltage and the characteristics of the modules specified.
At a battery reference temperature of 25°C (77°F), a 48 Vdc-nominal FLA battery bank typically requires net charging voltages of approximately 59 Vdc for the absorption stage and 62 Vdc for the equalization stage. MPPT controllers typically have temperature-adjusted operational limits between 140 Vdc and 145 Vdc, with absolute voltage limits of 150 V. In addition, most readily available circuit breakers used in dc applications carry a 150 Vdc rating. At first glance, the 91-volt span between 59 Vdc and 150 Vdc may seem fairly large, but operational and NEC considerations shrink that range into a fairly narrow “sweet spot” in short order. Consider, for instance, a relatively extreme example where the array will be operating in an environment that is very hot in the summer and very cold in the winter.
A key operational trait of the dc-todc buck-type converter used in many MPPT controllers is that the output voltage is always lower than the input voltage—a 2 Vdc drop is common. Combine this loss with another 2 Vdc drop in the conductors between the array and the battery, and the array’s operational maximum power point voltage must always be about 4 V higher than a battery bank’s target charging voltages.
In addition, a module’s maximum power voltage (Vmp) at STC is based on an illuminated cell temperature of 25°C in a laboratory environment. Because PV cells and modules frequently operate at approximately 25°C to 35°C above ambient temperature, a PV module really cannot be expected to produce fullrated power unless the ambient temperature is approximately -10°C. A module’s temperature-related power variance is manifested primarily as a change in output voltage. For crystalline modules, a typical open-circuit voltage temperature coefficient is -0.35%/°C. The module’s voltage drops as the cell temperature increases. In many locations, it is not uncommon for cell temperature to reach 65°C or higher at mid-day in the summer (30°C to 35°C ambient plus 30°C to 35°C cell temperature rise above ambient). In this case, the actual cell temperature will be about 40°C above the 25°C STC temperature. With these assumptions, the resulting voltage drop percentage due to elevated cell temperature is: 40°C x -0.35%/°C = -14%.
In this example, the array’s operational voltage is about 86% of the STC specifications (100% – 14%). Put another way, the array’s STC Vmp specification needs to be 116% of the estimated minimum operational voltage required. With this percentage in hand, you can now specify the array’s minimum voltage at STC: (59 V (minimum target battery voltage) + 4 V (loss in CC and wiring)) x 116% = 73 V.
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