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Utility-Interactive Battery Backup System Design

Many photovoltaic system designers and installers have considerable training and experience with utility-interactive, grid-direct (UIGD) systems. Comparatively few have training or experience with utilityinteractive, battery backup (UIBB) systems and their nuances. This technical introduction is intended for the latter audience and highlights the differences between the two system architectures, components, design details and NEC requirements. Incorporating battery backup increases a system’s parts count, complexity and cost per installed watt, but it may prove to be a desirable - if not an essential - feature to some customers.

As with gird-direct systems, some jurisdictions, like California, require minimum performance specifications for UIBB inverters. The list of eligible inverters published by the California Energy Commission includes UIBB inverters manufactured by OutBack Power Systems and Xantrex Technology. Accordingly, the generally similar UIBB system architecture employed by these two companies is the only one addressed in this article. In this architecture, UIBB systems include an array combiner, charge controller and a battery bank, along with other balance-of-system components.

THEORY OF OPERATION

Like grid-direct systems, UIBB systems reduce energy consumption from the utility grid, export excess energy back to the grid and utilize inverters that meet UL 1741 anti-islanding requirements. As with grid-direct systems, UIBB systems harvest energy from the sun during the day, and the grid powers all loads at night. The battery bank is maintained at or near float voltage and is primarily charged by the PV subsystem and, as necessary, from the grid via the inverter’s built-in charger. This architecture results in a fairly efficient system that avoids most of the inefficiencies associated with daily battery cycling (discharging and recharging).

When the grid fails, the UIBB system’s inverter immediately disconnects from the grid, just as with a grid-direct PV system. But instead of the home or office going dark and quiet, the system transfers to battery power select loads previously assigned to a subpanel. This is the UIBB system’s key advantage.

The batteries can power the backup loads for a few hours or days, depending upon several variables: the power demands of the combined loads being backed up, the size of the array, the available insolation and the size and health of the battery bank. In residential applications, backed-up loads often include lighting, refrigeration, home electronics, cell phone chargers, computer equipment and the like. In business applications, lighting, data and computer equipment are often deemed critical loads that require an uninterruptable power source. Once grid power is restored, all loads are transferred back to the grid, and the inverter’s built-in charger and the PV array recharge the battery bank. Once the bank is recharged, the system returns to normal operation.

UIBB SYSTEM COMPONENTS

PV array. In almost all cases, PV arrays in UIGD systems are configured for higher dc output voltage than in UIBB systems. In grid-direct systems, modules are configured in series strings to achieve temperature-corrected open-circuit voltages as high as 600 Vdc. The maximum power voltages of these strings may be as high as 550 Vdc, depending on the operating characteristics of the specified inverter. However, the operational input voltage limit for OutBack and Xantrex charge controllers is in the 140 to 145 Vdc range, with absolute maximum limits of 150 Vdc. The 150 V hard stop limit of these controllers requires that UIBB arrays be configured using shorter, lower voltage strings with several strings wired in parallel. Apollo Solar manufactures controller models with an absolute maximum voltage of 200 Vdc. Note that these controllers cannot be networked on a communications level with OutBack and Xantrex UIBB inverter systems.

To illustrate configuration differences between typical UIGD and UIBB arrays, consider the following examples. In a grid-direct application, a 4 kW array of 20 Evergreen ES-A 200 modules (11.05 Imp, 18.1 Vmp, 12.0 Isc, 22.5 Voc) might consist of a single 4,000 W series string. In some locations, it is possible to connect 20 of these modules in series and still have headroom for the low temperature voltage correction factor covered in NEC 690.7. In a UIBB application, the same 4 kW array might be configured with five 200 W modules per series string with four 1,000 W strings in parallel. Similarly, a 2.76 kW array consisting of 12 REC SCM-230 modules (7.8 Imp, 29.4 Vmp, 8.3 Isc, 37.1 Voc) could be wired with three 230 W modules per series string and four 690 W strings in parallel. Either of these array configurations will satisfy NEC 690.7 and practical voltage calculation requirements for arrays exposed to both cold and hot environments. (For additional details on optimizing array, charge controller and battery matching in UIBB systems, see sidebar.)

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