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A research group has designed a solar-assisted CO2 heat pump system that appears to be able to achieve a lower levelized cost of energy than gas boilers in UK residential buildings. The system has the potential to reach a coefficient of 5.1.
Researchers at Brunel University London have developed a new system design for residential heat pumps that uses transcritical carbon dioxide (CO2) as a refrigerant.
CO2 has a very low boiling point and is considered an ideal cooling solution for heat pumps. Additionally, it is non-toxic, non-flammable and has a low global warming potential (GWP). “CO2 has a much lower GWP and ozone depletion impact than commonly used synthetic refrigerants and, used in its transcritical cycle, can maintain heat pump performance at lower source temperatures and higher supply temperatures than most synthetic refrigerants,” the scientists stated.
In the study “ Application of transcritical CO2 heat pumps to boiler replacement in low impact refurbishment projects ,” published in Heliyon , the group research explained that CO2 heat pumps have been on the market since the beginning of this century and noted that a well-known product currently on the market is the Eco Cute model for domestic water heating (DWH), introduced by the Japanese manufacturer Sanyo in 2001. These devices typically achieve coefficients of performance (COP) of up to 4 and deliver hot water at temperatures up to 60ºC.
The scientists propose a transcritical CO2 heat pump design that includes system modifications external to the heat pump cycle and aimed at recovering low-temperature heat. Their goal was to create a system with a COP of 3 with the same costs as a gas boiler.
Using a series of simulations, the group designed a 1kW heat pump system for a 91m2 semi-detached house in the UK with hydronic heat distribution and 50mm of external wall insulation. It consists of a compressor, a gas refrigerator, an internal heat exchanger, an evaporator, an expansion valve and a receiver.
The heat pump also integrates a subsystem that includes a freeze-thaw air source heat exchanger and phase change materials (PCM) heat storage tanks. PCMs can absorb, store and release large amounts of latent heat in defined temperature ranges. They have often been used at a research level for cooling photovoltaic modules and heat storage.
“The heat source side uses a thermal liquid,” the academics explain, noting that the liquid creates an extra heat exchange stage in the heat pump, but allows multiple heat sources to be used, while reducing the pressure and plumbing costs. “The heating part models the transfer of heat from humid air through a layer of frost, if any, to the heat exchanger. The frost models build and melt the frost layer according to the mathematical models of da Silva et al. “They incorporate the impact of the frost layer on heat transfer and air flow.”
Simulations showed that the heat pump system can reach a COP greater than 3 during a full year of operation, with maximum values ??ranging between 3.5 in winter and 3.7 in summer. “The results show that the total annual demand for heating and DHW is satisfied with COP between 3.14 and 3.27,” the researchers stressed. “The defrost in winter was in operation up to a third of the time. The heat pump was never used more than 25% of the time. "Recycling was only significant in summer, reaching 16% of the heat pumps operating time."
Their analysis also showed that, if using photovoltaic panels or thermal collectors, the COP of the system can potentially be raised to 5.1. “With solar collectors, and current UK Government subsidies and energy prices, the CO2 heat pump system can achieve a levelised cost of energy (LCOE) of £0.20 ($0.22)/ kWh, 23% less than the 26 pounds/kWh of a replacement condensing gas boiler,” they further explained. |
