Cooling our Homes with the Sun: Exploring Precooling Strategies to Reduce Greenhouse Gas Emissions

Abstract:

The CO2 emissions associated with consuming electricity (measured in terms of an emissions factor, kg CO2 per MWh) varies across time as a function of the fleet of power plants generating electricity at the time of electricity usage. During summer in California, the emissions intensity of the grid during midday hours can be less than half of the intensity during evening hours when high fractions of solar generation are replaced with natural gas generation. Demand for electricity in California typically peaks in the evening hours, creating a daily electricity consumption pattern that is more carbon intensive than it would be if consumption occurred in hours when more renewables are available.

Air conditioning (AC) is a major driver of residential electricity consumption that can constitute as much as 70% of a building’s total energy consumption while operating, and it is often used in early evening hours when people return home from work. However, altering the timing of AC usage to better align with less emissions intensive hours might be an effective mechanism to reduce the amount of greenhouse gases and pollution associated with cooling a home. This concept of shifting AC usage from evening hours to midday hours, known as “precooling”, has been studied largely in the context of demand response, but its potential for reducing emissions has not been explored with rigor.

This study focuses on quantifying the total emissions associated with electricity generation for various precooling/AC-usage patterns. Hourly electricity consumption data were generated by simulating a residential building prototype from NREL, representing an average American home, in the DOE’s EnergyPlus program with hundreds of distinct precooling schedules for a typical meteorological year within California Climate Zone 9. After simulation, results were filtered by thermal comfort conditions, and hourly electricity consumption data for AC operation during summer months were multiplied with hourly grid-averaged emissions factors for the California Independent System Operator to calculate the emissions associated with each schedule. The results of this analysis provide insight into the feasibility of precooling as an emissions reduction strategy in the residential sector and the characteristics defining optimal precooling strategies.