Power-Conditioning Equipment for Residential Battery-Backup PV Systems

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Storms, wildfires, and overloaded or faulty electrical grid transmission and distribution networks result in power outages that impact tens of millions of US residences each year. Hurricane Sandy, the nation’s most recent large-scale storm, resulted in utility outages that affected 8.5 million customers across 21 states at its peak, according to the US Department of Energy.

Large-scale grid failures inevitably lead to surges in demand for PV systems with battery-backup capabilities. However, many solar design and installation firms have determined that battery-based systems are too expensive to sell effectively or too complex to design, install and service. As a result, many firms have little, if any, experience with these systems. Offering battery-backup options to residential customers is profitable for integrators who invest the time, resources and training to come up to speed with, and stay current on, battery-based system equipment selection, design and installation. The falling costs of PV modules, and ongoing advancements in battery-based inverter and BOS technologies, can make PV with battery backup an attractive offering for integrators who serve the residential market.

The owner of a grid-tied PV system with battery backup gets the best of both worlds: a reduced electric bill and the ability to live comfortably and safely without the grid when necessary. Installers of backup systems can access a market with fewer competitors, better margins and higher equipment and installation values than installers who offer grid-direct systems only. In this article, we introduce electricians and integrators who are new to battery-based grid-tied PV installations to basic system topologies and the power-conditioning equipment used to build these systems. While the learning curve may seem steep, experienced installers will need very few additional skills or tools to design and install battery-backup PV systems, particularly if they employ a preconfigured and prewired powerconditioning system.

Primary System Configurations

Integrators can configure a battery-backup PV system as either dc coupled or ac coupled. Each of these primary system configurations has benefits and drawbacks to consider, and each influences equipment selection as well.

DC coupling. Traditionally, the majority of battery-backup PV systems have been dc coupled. In dc-coupled systems, PV array source circuits are typically configured at relatively low dc voltages of less than 150 Vdc maximum. Individual source circuits are routed to a combiner box that provides overcurrent protection for each string and combines the individual dc inputs, allowing a single pair of larger transmission conductors to be run from the combiner box to the system’s dc charge controller. Because the charge controller is usually installed in close proximity to the system’s inverter/charger and battery bank, the wire run distance between the array combiner and the system’s charge controller can be significant, especially if the array is ground mounted rather than on the roof. With a groundmount system, the conductor cost may be significant due to the transmission distance and the dc-coupled array’s relatively low-voltage and high-current characteristics.

In dc-coupled systems, the output of the charge controller is connected to the system’s main low-voltage (typically 48 Vdc nominal) dc bus via overcurrent protection and disconnect equipment. When the utility grid is functional, the system’s utility-interactive battery-based inverter converts the dc energy generated by the PV array to alternating current, synchronizes the ac waveform with the utility grid and exports any surplus energy to the utility. During power outages, the battery-based inverter disconnects from the utility grid and energizes select household electrical circuits using energy stored in the batteries or provided by the PV array. To prevent battery overcharging, the charge controller regulates the dc current that the PV array generates during daylight hours.

AC coupling. In ac-coupled systems, a grid-direct string inverter effectively replaces the PV charge controller that is used in dc-coupled systems, and typically the source-circuit combiner box as well. In these systems, the PV array is configured at relatively high voltages of up to 600 Vdc, and the string inverter converts the array output to 240 Vac. In accoupled systems, the string inverter’s ac output is connected to the battery-based system’s ac bus. When the utility grid is functional, ac electricity provided by the string inverter supplies energy to household loads, and excess energy is exported to the utility grid. In the event of a grid failure, the battery-based inverter disconnects from the utility grid and energizes select household electrical circuits using energy stored in the batteries or provided by the PV array via the string inverter.

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