Assessment of Residential Air-to-Water Heat Pumps Coupled with Thermal Energy Storage

Optimizing HVAC demands for residential cooling systems is becoming increasingly important in California as the electric grid becomes increasingly powered by renewable energy generation. Hydronic systems in general are particularly amenable to load shifting because they can be coupled to thermal energy storage (TES) tanks. TES systems can satisfy on-peak cooling load using off-peak compressor operation to chill the tanks. Indoor air-precooling methods using conventional air-to-air systems are prone to comfort issues due to the resulting large indoor temperature swings during the day. Storing thermal energy in water in insulated tanks allows for maintaining comfort through the peak period.

Tests were conducted in the summer of 2018 at a California laboratory test house to determine how effectively on-peak (5PM to 9PM¹) cooling loads could be shifted to off-peak periods by TES storage coupled to hydronic low-mass radiant ceiling panels. These tests compared the energy and comfort impacts of two load shifting methods: precooling using an air conditioner vs. incorporating chilled water storage with the air-to-water heat pump (AWHP)/Radiant system. Water storage was chilled during the night, to take advantage of cooler condensing temperatures.

Key Findings

1. Load shifting energy use: Reference AC precooling energy use (6.4 kWh) was double that of the radiant system (3.2 kWh), and nearly double the peak period energy use for the reference AC on a similar day without precooling (3.4 kWh). The peak period energy use of the radiant system without precooling was 2.9 kWh.
2. Load shifting effectiveness: The reference AC used 2.2 kWh to shift 1 kWh of electrical load off peak, while the radiant system used 1.2 kWh to shift 1 kWh of electrical load off peak. Only the radiant system was able shift the entire peak period load.
3. Comfort: Air Conditioning Contractors of America (ACCA) Manual RS comfort standards were used to quantify and compare comfort across system types. The Radiant system provided much better comfort than the Reference AC during both precooling and load shifting hours. The radiant system was able to maintain the indoor temperature within 3 °F of setpoint on load shifting days while the reference AC could not.

[1] Consistent with Pacific Gas and Electric time of use peak period definition at the time of the project.

This initial test focused on the successful demonstration of the load shifting potential of an AWHP coupled with TES and using radiant ceiling panels for delivery. With electrification efforts proceeding in California, it is important to expand the research towards more mainstream fan coil delivery options, as well as evaluate the performance and cost effectiveness of AWHP systems which can also provide domestic hot water. Additional research should also be conducted to evaluate different tank sizes, types, and installation methods, including industry outreach, to reduce market barriers for residential thermal energy storage systems. Research could also explore optimizing the compressor runtime during hours of peak PV input to the grid.