Solar-assisted heat pumps vs. air-source heat pumps

An international group of scientists conducted a one-year comparative analysis of two types of heat pumps for water heating – direct-expansion solar-assisted heat pumps (DX-SAHPWHs) and air-source heat pumps (AHPWHs). Both systems were examined via numerical modeling, assuming they were deployed with identical parameters in Tehran, the capital of Iran.

“To make those water heaters comparable, all design parameters for both heat pumps are assumed to be the same with identical components,” the research group said. “For the solar-assisted heat pump hot water system, the evaporator is the flat-plate solar thermal collector while for the air-source heat pump water heater, the evaporator is a low-temperature liquid-to-air heat exchanger with the same area and configuration as the unglazed solar collector, however, the plate is removed from the top.”

The thermal collector and liquid-to-air heat exchanger were assumed to have a surface area of 4.21 m2. In the case of the DX-SAHPWH, the condenser comprises a 60-meter serpentine copper tube immersed in the domestic hot water tank and acting as a thermosyphon heat exchanger. The chosen working fluid is R-134a.

“In the formulation of the air-source heat pump water heater, the thermodynamic relations for the components as well as the parameters are the same as the solar-assisted heat pump,” the academics added. “Only the equation of the evaporator needs to be modified, assuming the fan speed equals 10 m/s.”

Modeling the two systems, the researchers calculated their monthly coefficient of performance (COP) and electricity consumption over a 12-month period. For every month, they used as input the average day data about the cloudiness factor, horizontal radiation, ambient temperature, and wind speed. Target hot water in all cases was 50 C, 60 C, and 70 C.

“Comparison of the COP between those systems for all three hot water temperatures for all months shows that there is less than 0.1 difference in COP,” the results showed. “In other words, both systems perform almost the same during different seasons and demand water temperatures. For both systems, COP has the lowest value of 2.0 in the coldest month of January, and the highest value of 2.8 in the hottest month of August. The DX-SAHPWH system operates with a very slightly better in terms of COP compared to the AHPWH system in all months.”

As for electricity consumption, the analysis showed that both systems consume almost the same electricity during different seasons and various water temperature demands. “For both systems, energy consumption has the lowest value of 3,850 MJ in the coldest month of January, and the highest value of 4,900 MJ in the hottest month of August. The DX-SAHPWH system consumes very slightly less electricity compared to the AHPWH system in some months while it is opposite in some other months,” they said.

Doing sensitivity analysis, the scientific group found that when the irradiance doubles from 500 W/m2 to 1,000 W/m2, solar heat gain increases by 49 % in the DX-SAHPWH for hot water at a temperature of 50 C. Also, for the same rise in irradiance and the same water temperature, the evaporator temperature increases by 55% from 22.32 C to 34.65 C.

“As the weather condition changes in terms of irradiance and ambient temperature along the year, the DX-SAHPWH performance changes dramatically for most operating parameters,” they added. “For example, the difference in evaporator temperature between January and August is 21.8 C (from 4.9 C to 26.7 C) for hot water temperature 50 C. Similarly, the compressor work varies between 2,850 and 5,868 MJ annually, i.e., 106 % change. However, COP showed less variation among different months as it varies between 2.04 and 2.79 for water tank at 50 C.”

Concluding their research, the academics said for lower temperatures and higher levels of solar radiation, the DX-SAHPWH is recommended. However, they noted that for higher temperatures and lower radiations, AHPWH operates more efficiently.

Their findings were presented in the paper “Annual comparative performance of direct expansion solar-assisted and air-source heat pumps for residential water heating,” published in the International Journal of Thermofluids.  The research was conducted by scientists from Ireland’s University College Dublin, the MaREI Centre for Energy, China’s China University of Petroleum, and the United States’s Rice University.