How pre-cooling homes could dramatically reduce energy usage
It’s a typical, sweltering August day in Los Angeles, with temperatures pushing 95° downtown. When people get home from work in the late afternoon, the first thing they do is crank up the AC to cool their overheated homes.
Unfortunately, this surge in demand taxes the already stretched power grid, with the energy supply having great difficulty meeting the growing demand. At 5 p.m., the California Independent System Operator, or CAISO, which runs the bulk of the state’s wholesale energy market, issues a Flex Alert, asking consumers to voluntarily cut their electricity usage. Because of the tepid response, rolling blackouts follow, cutting off power in rotating one-hour intervals to millions of Angelenos. During the outages, people can’t cook, wash their clothes, watch TV or use their air conditioning.
Researchers at the USC Viterbi School of Engineering are experimenting with a new approach to keeping the AC running, even on the hottest days—one that saves consumers money, reduces CO2 emissions from the generation of electricity, and lowers the demand for power during peak hours, thereby decreasing the likelihood of Flex Alerts and rolling blackouts.
“We want to use houses as batteries by using their thermal mass to store cool air,” said Kelly Sanders, the Dr. Teh Fu Yen Early Career Chair and associate professor of civil and environmental engineering. “And we do that through a concept called precooling.”
In the most comprehensive study of its kind, Sanders and Stepp Mayes, a USC Viterbi Ph.D. student in civil and environmental engineering, have found that if people sufficiently cool their homes in the afternoon when solar energy is plentiful, they will use less electricity between 4 p.m. and 9 p.m., when the power grid is most at risk of rolling blackouts. That five-hour window is the time when energy is most expensive, and, with solar vanishing from the grid as the sun goes down, electricity is often generated by gas powerplants that belch climate-warming CO2.
On sweltering summer days, for instance, the USC researchers said residents could lower their thermostats to 72° beginning at 2 p.m., when clean energy is abundant. When the peak demand period begins two hours later and the electricity is dirtier, they might reset their thermostats to 78°. And wait. After the peak demand period passes and the grid becomes less stressed, people could reset their thermostat to a comfortable 75°.
“We’d like to see people shift some of their electricity consumption, to really aggressively use air conditioning at one time of day in order to use less later by relying on a building’s ability to store energy,” Mayes said.