Unfortunately almost all residential and light commercial solar installations need utility power to operate. Therefore they may not be able to provide power during a grid-outage when needed most. Traditional backup power options, such as using a generator or using AC coupling or DC coupling, have many drawbacks such as maintenance, safety and inefficiency issues. But a direct DC transfer solution can provide a better way for grid-tied solar systems to provide battery backup power. This is because the DC transfer solution keeps the grid-tied system intact like an AC coupled solution when the grid is working, and transfers over to direct DC coupled backup power during a grid failure. Thus a DC transfer solution combines the best of both AC coupling and DC coupling.
Adding a small battery bank to a 600-V controller with an installed DC transfer switch, such as that from Morningstar, offers a back-up solution that can be installed outside, in a garage or in a basement. This can be done without any roof work or modifying the PV array.
When the grid is working, the DC transfer switch is kept in the grid-on controller-off position. DC power from the solar array moves to power the string inverter. Power flows to the main service panel and excess power is credited to a utility bill. Then the main service panel supplies power to an installed critical load subpanel to power loads during an outage. Power also goes to a backup inverter/charger which converts AC power to DC power to charge a backup battery bank so the batteries will be charged and ready in the case of a grid outage. During a grid failure, the DC transfer switch can be turned to the battery-backup, controller-on position. In this position, the DC power from the array flows through the controller and charges the battery, which sends power to the inverter/charger. The charger converts the DC to AC power and directs power to the critical load subpanel so that critical loads can be run, such as refrigerators and lights. When the grid power is restored, the transfer switch is turned back to the grid-on, controller-off position.
Three main design considerations for a DC transfer approach are sizing the array, sizing the battery, as well as choosing the inverter/charger.
Typically, using one or two strings of a 600-V PV array will work well. If the maximum input voltage is 600 V, the Voc operating window would be between 100 to 525 V. Use low operating input voltage for arrays that are partially shaded or covered in snow. Keep array Voc less than 570 V for production during cold temperatures. With a 600-V transfer switch rated to a 30-A max current, the max. Isc (STC) should be less than or equal to 24 A.
Sub-array configurations can be managed with a fused transition box or dual-output combiner box with larger inverters, so part of the array gets wired through the controller and part is wired directly to the inverter. The inverter/charger keeps the batteries in float during grid-tied operation. During backup, the grid-tied string inverter remains off and disconnected from array and backup AC circuit. Therefore, inverter/charger configuration and sizing is independent from the grid-tied inverter. Single-phase, split-phase or three-phase inverter-charger configurations can be used. Backup inverter/charger sizing is determined by calculating the maximum continuous and startup surge power requirements for the backup loads.
The battery bank is also scalable regardless of the size of the array. Battery sizing calculations can be based on the kWh/day that is required for critical load needs and number of days of backup power that is required. Minimum battery bank size is based on a maximum charging current which is 60 A for each controller. Batteries commonly have a maximum charge rate of 15 to 30% of the C/20 Ah rating. The controller can be custom programming for a reduced maximum charge current for a smaller battery bank. The inverter/charger’s max charging current can also be reduced.
On sunny days the controller uses a limited amount of the array power to fully charge the batteries and “ignores” excess power which might damage a smaller battery bank. On cloudy days, full array power is accepted when it is needed to provide the same amount of backup power as a larger AC-coupled system.
Critical load backup requirements are typically significantly lower than daily grid-tied usage. The DC transfer solution can provide scaled down backup options, independent from the size of the grid-tied PV system.
Retrofit installation overview:
- Install battery bank, inverter/charger, BOS and backup sub-panel according to manufacturer requirements
- Disconnect PV Array from grid-tied string inverter
- Connect PV Array to 600-V controller and grid output to PV string inverter
- Make controller to battery bank connection to main battery terminals, or directly to the battery bank
Multiple MPPT charge controllers can be installed with a larger PV system. Integrating a DC transfer solution with an AC coupled system is also very cost effective and can provide better performance with the direct DC charging.
This tip was contributed by Mark McHenry, marketing manager for Morningstar Corp.