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Building Efficiency, Load Flexibility Can Reduce Back-up Battery Size: Study

Building efficiency and load flexibility can significantly reduce the size of the battery system required to provide backup service to a house, researchers at Lawrence Berkeley National Laboratory found in a new report.

The premise of the report is that residential behind-the-meter solar-plus-storage systems are growing rapidly, driven in large measure by customer demand for backup power, while the energy consumption patterns for homes are changing as they become more energy efficient and home owners embrace devices that allow for more efficient electricity use, such as switching to heat pumps, the LBNL researchers said.

The report, Solar +Storage for Back-up Power: Implications of building efficiency, load flexibility, and electrification for backup during long-duration power interruptions, is the second in a series of studies developed in collaboration with the National Renewable Energy Laboratory analyzing solar photovoltaic and energy storage systems used for backup power.

While the earlier study characterized solar PV-plus-energy-storage-systems (PVESS) backup power capabilities in existing building stock, the latest study explores how those capabilities may evolve as homes become progressively more efficient, flexible, and electrified, the Berkeley lab researchers said.

Both studies focused on the technical potential rather than the economics of PVESS backup power during long-duration power interruptions, that is, power loss of one day or more. Other and ongoing work at LBNL has explored PVESS backup power for short-duration events.

The most recent study uses NREL’s ResStock building modeling platform to create statistically representative distributions of the existing building stock in 10 locations across the United States. The LBNL researchers then analyzed the data to determine how much battery storage would be required as a series of energy efficiency, load flexibility, and electrification measures are applied across homes in each region.

Among the key findings, the researchers found that efficiency and load flexibility measures can reduce battery sizing, particularly in hot locations and for homes in cold-winter locations with electric heating. The largest effect was observed in the Dallas-Fort Worth area where efficiency and load flexibility measures reduced required median battery sizing by roughly 50 kilowatt hours.

The researchers also found that efficiency, load flexibility, and – in mild winter climates – heat pumps significantly expand the addressable market for PVESS backup power. A residential PVESS system with a battery system at the upper end of the typical 10- to 30-kWh range could provide backup power over a three-day power interruption to some portion of all homes in each location, the researchers said.

The LBNL researchers also noted, however, that heat pump retrofits can either increase or reduce required battery sizing, depending on the climate and on the existing heating and cooling equipment.

In hot climates, efficient heat pumps can significantly reduce storage sizing when replacing inefficient air-conditioning equipment units with the largest effect in Phoenix where heat pump retrofits reduced median battery sizing by roughly 30 kWh, they said, adding that heat pumps also reduce required storage sizing in cold locations when replacing electric resistance-based heating.

However, for fossil-heated homes in cold climates, heat pump retrofits can require significantly larger amounts of battery storage for backup power, the study found.

Other electrification measures have relatively small impacts on backup battery sizing, the study found.

For instance, homes retrofitted with heat-pump water heaters, induction ranges, electric ovens, and heat-pump dryers can have either an increase or decrease in required battery sizing, depending on whether the measures are replacing fossil-based or less efficient electric based appliances.

LBNL plans to host a webinar on the study on December 7.


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