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A group of researchers from Iran have analyzed the coefficient of performance and energy consumption of a solar-assisted heat pump and an air-thermal heat pump and found that there are three crucial factors in determining their annual performance: changes in irradiance, ambient temperature and wind speed.
An international group of scientists has carried out a year-long comparative analysis of two types of heat pumps for heating water: direct expansion solar-assisted heat pumps (DX-SAHPWH) and aerothermal heat (AHPWH). Both systems were examined using numerical modeling, assuming they were deployed with identical parameters in Tehran, the capital of Iran.
“In order for these water heaters to be comparable, it is assumed that all the design parameters of both heat pumps are the same and with identical components,” explains the research group. “For the solar assisted heat pump hot water system, the evaporator is the flat plate solar thermal collector, while for the air fed heat pump water heater, the evaporator is a liquid heat exchanger. -low temperature air with the same surface and configuration as the non-glazed solar collector, however, the top plate is eliminated.”
It has been assumed that the thermal collector and the liquid-air heat exchanger have a surface area of ??4.21 m2. In the case of the DX-SAHPWH, the condenser is formed by a 60-meter serpentine copper tube submerged in the domestic hot water tank and which acts as a thermosiphon heat exchanger. The working fluid chosen is R-134a.
“In the formulation of the aerothermal heat pump water heater, the thermodynamic relationships of the components, as well as the parameters, are the same as those of the solar-assisted heat pump,” the academics add. “You just have to modify the evaporator equation, assuming that the fan speed is equal to 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 each month, they used as input the average days data on cloud cover factor, horizontal radiation, ambient temperature, and wind speed. The hot water target in all cases was 50 ºC, 60 ºC and 70 ºC.
“Comparison of the COP between these systems for the three hot water temperatures during 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, the COP has the lowest value of 2.0 in the coldest month of January, and the highest value of 2.8 in the warmest month of August. The DX-SAHPWH system performs with a very slightly better COP compared to the AHPWH system in all months.”
Regarding electricity consumption, the analysis showed that both systems consume almost the same electricity during different seasons and different 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 than the AHPWH system in some months, while in others it is the opposite,” they noted.
Through a sensitivity analysis, the scientific group discovered that when the irradiance doubles from 500 W/m2 to 1,000 W/m2, the solar heat gain increases by 49% in the DX-SAHPWH system for hot water at a temperature of 50 ºC. Furthermore, for the same increase in irradiance and the same water temperature, the evaporator temperature increases by 55%, from 22.32 ºC to 34.65 ºC.
“As weather conditions change in terms of irradiance and ambient temperature throughout the year, the performance of the DX-SAHPWH changes dramatically for most operating parameters,” they add. “For example, the evaporator temperature difference between January and August is 21.8ºC (from 4.9ºC to 26.7ºC) for a hot water temperature of 50ºC. Similarly, the compressor work varies between 2,850 and 5,868 MJ per year, that is, a 106% change. However, the COP showed less variation between the different months, since it varies between 2.04 and 2.79 for the water tank at 50 ºC."
As a conclusion to their research, the academics stated that for lower temperatures and higher levels of solar radiation, the DX-SAHPWH is recommended. However, they noted that for higher temperatures and lower radiations, the AHPWH works more efficiently.
Their conclusions are presented in the article “ Annual comparative performance of direct expansion solar-assisted and air-source heat pumps for residential water heating ” residential), published in the International Journal of Thermofluids . The research was carried out by scientists from University College Dublin (Ireland), the MaREI Energy Center, the Chinese University of Petroleum and Rice University in the United States. |
