Air-source heat pumps with thermal storage cut upfront costs by 29.7%

A research team from China’s Hebei University of Technology has proposed combining an air-source heat pump (ASHP) with thermal energy storage for buildings to cut upfront and operating costs while improving energy efficiency and reducing carbon emissions.

The proposed system stores thermal energy during off-peak electricity tariff periods and uses it when demand and prices are higher, shifting peak loads and lowering building operating costs. The researchers said the integrated configuration also reduces the required heat pump capacity, lowering initial investment while maintaining high heating and cooling performance.

“This study proposes an innovative air-source heat pump coupled energy storage (ASHPCES) system based on reversible air-to-air heat pumps that can switch between heating and cooling modes to provide indoor thermal regulation for buildings,” the research team said. “Equipped with a stratified thermal storage tank that can operate in both heat and cold storage modes, the system stores energy during low-price nighttime periods and supplies heating or cooling to buildings during high-price working hours.”

The system comprises reversible air-to-water heat pump units, a stratified water storage tank, circulating pumps, control valves, and fan-coil terminals. The researchers sized the configuration according to the maximum cumulative daily heating or cooling demand that could be met during an eight-hour off-peak tariff period.

They designed the system for a six-story, 21.6 m office building in Tianjin with a conditioned floor area of 10,030 m².

The ASHPCES system uses six customized ASHP units, each with a rated heating capacity of 146 kW and a cooling capacity of 130 kW. Each unit requires 37 kW of electrical input in heating mode and 41 kW in cooling mode. The heat pumps use R410A refrigerant and operate at ambient temperatures from -25 C to 45 C, supplying water at 45 C for heating and 5 C for cooling.

The heat pumps are coupled to a 1,010 m³ stratified water storage tank, modeled as four vertical thermal layers and insulated with a 100 mm foam layer. The researchers simulated the system for office buildings in northern China’s cold-climate region and compared its performance with that of a conventional ASHP system without thermal storage.

The reference system consisted of heat pump units, circulating pumps, and fan-coil terminals operating continuously between 08:00 and 18:00 to meet real-time heating and cooling demand. Unlike the proposed configuration, it was sized for the building’s maximum instantaneous load, requiring 11 heat pump units.

The simulations showed that the proposed system operated its heat pumps at full capacity between 23:00 and 07:00 during low-tariff periods, storing hot water in winter and chilled water in summer based on the following day’s predicted demand. During occupied hours, from 08:00 to 18:00, the building was supplied primarily by the storage tank, while the heat pumps only operated during intermediate tariff periods if additional energy was needed. The system was designed to meet peak hourly loads of 831.44 kW for heating and 1,307.46 kW for cooling.

“Compared with the conventional ASHP system, the ASHPCES system combined with peak-valley pricing can significantly reduce annual operating energy costs and improve economic performance,” the researchers said. “The annual operating cost is reduced by 51.4%, the dynamic payback period is approximately 3.57 years, and the life-cycle cost is reduced by 38.37%.”

Compared with the conventional system, the storage-assisted configuration reduced annual electricity consumption by 21.9 MWh and cut carbon emissions by 8.8%, equivalent to about 17.6 metric tons of CO₂ annually. Annual electricity consumption fell to 248.1 MWh from about 270 MWh for the reference system.

The thermal storage approach also reduced the number of heat pump units from 11 to six, cutting upfront investment by 29.7%, from CNY 3.43 million ($510,208) to CNY 2.41 million. It also reduced annual heat pump operating time by 43.6%.

“This design not only makes full use of time-of-use tariffs to slash operating costs, but also downsizes heat pump capacity by avoiding peak-load-based sizing, thus reducing the upfront investment,” the research team concluded.

The research is presented in “Techno-economic performance of air source heat pump and thermal energy storage considering peak-valley time-of-use electricity prices,” published in Cleaner Energy Systems.