DENVER - Residential thermal energy storage (TES) systems have been discussed since the 1970s and 80s. Is the time right for another push in the direction of residential-light commercial cooling systems?

A very large audience turned out to at least learn about them at the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 2005 Annual Meeting in Denver. Doug Hittle, Ph.D., of Colorado State University, spoke on refrigerant-based residential energy storage. The system he described is designed for residential and light commercial applications from 3 to 10 tons.

What It Doesn't Have

Perhaps the most notable features of the new residential TES system are those it doesn't have. For instance, these systems are refrigerant-based, and he explained - there is no brine involved.

Doug Hittle describes energy use of an installed residential TES system in Anaheim, Calif., and compares it to the building’s energy use with a more conventional system.
Condensing units used in the systems are those already being used today. The refrigerant management system is integrated onto the side of the ice storage tank. Ice forms on coils inside the tank.

Both the tank and the condensing unit are installed outside the home or light commercial building. "We tried to make it no more aesthetically obnoxious than any of the other crap we put up on the roof," Hittle said.

Pressure drop through the system is nominal, Hittle explained. The system doesn't use an expansion valve. A small pump meters refrigerant. The system operates in a flooded-evaporator condition, he said. The refrigerant temperature is kept below freezing (25 degrees F) to make ice. There is an "extensive" heat transfer surface.

Furthermore, there is no superheat involved in its cycle, Hittle said. Refrigerant is pulled in a liquid-vapor state.

Energy And Cost

Is there an energy penalty tied to the system's operation? "Comparing a typical system operating during peak hours and a TES operating at night to make ice, there is no penalty," Hittle said. Its condenser does not run during peak hours; only the fan runs.

Unitary TES boasts essentially the same coefficient performance as today's high-efficiency unitary systems, Hittle said. However, this may vary according to climate. "For most climates, you'll save energy," he said. "In some warmer climates, you'll break even. In extremely warm climates, you would use more energy."

The main benefits would come from peak load reductions. In a typical unitary system, he explained, "as the temperature outside goes up, condenser capacity goes down. Designers bump up the size of the condenser to accommodate the highest possible temperature. You wouldn't have to do that with this system, because the condenser runs at the coolest time of day.

"The condenser isn't even running during peak load hours."

The advantage of a reduced peak load would be on the straining, aging electrical grid, Hittle pointed out.

"The best applications are those where everyone wins" - in other words, where utilities encourage peak load reduction through reduced off-peak rates.

"There is a significant market," Hittle said. The residential unitary market accounts for 3.6 million installed units per year.

"This product, I think, opens up the game for thermal energy storage." In conclusion, he asked, "How much is it worth not to brown out?"

Publication date: 08/08/2005