PVESS can backstop critical loads during long grid outages, study says

As the 2022 Atlantic hurricane season churns away and disrupts electric power systems, attention focuses on how effective solar-plus-energy-storage systems can be in reducing the impacts of extended outages.

A new report from the Energy Department’s Lawrence Berkeley National Laboratory and National Renewable Energy Laboratory looks at the backup power capabilities of behind-the-meter solar-plus-energy storage systems (or PVESS). The analysis finds that backup performance depends, “first and foremost,” on PVESS sizing and the set of critical loads selected for backup. 

The study simulates PVESS performance in providing critical-load or whole-building backup power across a wide range of geographies, building types, and power interruption conditions. It also considers 10 historical long-duration (greater than 24-hour) power outages, and evaluates how PVESS could have performed in providing backup power during those events.

Anytime, anywhere… almost

It said that if heating and cooling loads are excluded from backup, then a PVESS with as little as 10 kWh of storage (the lower end of sizes currently observed in the market) can fully meet basic backup power needs over a three-day outage in virtually all U.S. counties and in any month of the year.

But, if critical loads include heating and cooling, then a PVESS of that size would meet 86% of critical load. A larger PVESS with 30 kWh of storage (the upper end of sizes currently observed in the market) would meet 96% of critical load. 

Backup coverage of heating and cooling loads varies across regions, depending on climate and building stock characteristics. The study said performance tends to be lowest in regions where electric heating is common (the southeast and northwest), and also in regions with large cooling loads (the southwest and parts of the southeast). 

Backup performance can also vary within regions, based on differences in the building stock. The report offers performance differences based on heating technology (electric resistance vs. heat pumps vs. fossil heating), building infiltration rates (the leakiness of the building), air-conditioner efficiency, and temperature set-points.

Properly sized solar-plus-storage systems can back up critical loads during extended outages.

Backup performance for homes with electric heat or high cooling loads is also sensitive to weather variability. The report said that among counties with high penetration of electric heat, between 53% and 96% of critical load is served during winter months, depending on which specific day the outage begins in each month.

A similar but “less dramatic trend” was observed for homes with high cooling loads. Even greater variability would occur under more extreme weather conditions than explored in the analysis, the report said.

Backup performance was fairly insensitive to outage durations beyond one day. In general, backup performance declines as outage duration increases, though the effect is relatively modest given the ability of PV to recharge the batteries each day, the report said.

For a PVESS with 30 kWh of storage and critical loads that include heating and cooling, backup performance drops from a population-weighted average of 100% of critical load served for a one-day outage to 92% for a 10-day outage.

The study said that in 7 of the 10 historical outage events that it analyzed, the majority of homes would have been able to maintain critical loads with heating and cooling using a PVESS with 30 kWh of storage. 

The lowest performing event was during Hurricane Florence, during which almost no PV generation occurred over the first three days of the roughly eight-day outage due to cloud cover. 

For the two winter storms analyzed, all critical load was served in the median case, but a sizeable fraction of customers—those with electric heating—saw much lower performance.

The study was supported by the Energy Department’s Solar Energy Technologies Office.

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Author: Renewable Energy World

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