Dispatchable Pre-Coolers

Dispatchable Pre-Coolers


Cooling and ventilation account for more than 50% of the summer time peak electrical draw by California commercial buildings. Almost 50% of the connected generation in California is reserved to operate for less than 10% of hours on the hottest summer afternoons. Peak demand in California is growing at about 2.4 percent per year, roughly the equivalent of three new 500-megawatt power plants. Consequently, a significant portion of electricity cost results from construction and operation of capital intensive peak power plants.

Goals & Results


With funding from Southern California Edison, WCEC evaluated the transient response from turning on a condenser-air pre-cooler to achieve load reduction in a laboratory and field setting. Response time is a key factor in deciding if pre-coolers are a practical tool for achieving dispatchable demand reduction. Laboratory testing measured the time response impact of a pre-cooler installed on a 4-ton RTU during startup, operation, and shut down of the pre-cooler. Researchers conducted field testing between July-October 2017 on a big box retail store in Corona, CA. The store had six RTUs with existing pre-coolers and an additional five RTUs were retrofitted with condenser-air pre-coolers as part of the project. WCEC simulated 25+ demand events that lasted from 60 – 240 minutes at times when outside air temperature was 95°F or higher.

When the pre-cooler was activated, laboratory testing showed the power (with respect to the steady-state power draw) was reduced by 15% within 16 minutes. When the pre-cooler was turned off, partial cooling benefits remained for up to 20 minutes.


Condenser-air pre-coolers evaporate water into the airstream used for heat rejection from an air conditioning system. The evaporative process reduces the air temperature and allows the air conditioner to operate more efficiently at higher outdoor air temperatures. WCEC’s previous work has illustrated the energy and demand savings potential of condenser-air evaporative pre-coolers in the hot-dry California climate (up to a 30% energy savings from cooling during peak hours). In this project, researchers evaluated the performance of condenser-air pre-coolers being used as a dispatchable asset to reduce electrical demand during peak demand events.

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