Abstract
Given the large demand nowadays for domestic hot water, and its impact on modern building energy consumption, air source transcritical CO2 heat pumps have been extensively adopted for hot water production. Since their system efficiency is limited by significant irreversibility, a CO2-based mixture could offer a promising drop-in technology to overcome this deficiency without increasing system complexity. Although many CO2 blends have been studied in previously published literature, little has been presented about the CO2/R32 mixture. Therefore, a proposed mixture for use in transcritical CO2 heat pumps was analyzed using energy and exergy analysis. Results showed that the coefficient of performance and exergy efficiency variation displayed an “M” shape trend, and the optimal CO2/R32 mixture concentration was determined as 0.9/0.1 with regard to flammability and efficiency. The irreversibility of the throttling valve was reduced from 0.031 to 0.009 kW⋅kW−1 and the total irreversibility reduction was more notable with ambient temperature variation. A case study was also conducted to examine domestic hot water demand during the year. Pure CO2 and the proposed CO2 blend were compared with regard to annual performance factor and annual exergy efficiency, and the findings could provide guidance for practical applications in the future.
Highlights
Accepted: 21 July 2021Nowadays, with rapid urban construction and development, building energy consumption is increasing rapidly in China [1]
Compared with the pure CO2 cycle, which is considered as the baseline system, the annual performance factor and annual exergy efficiency are increased by 5.39% and 3.11%, respectively, with the CO2 /R32 mixture cycle
A CO2/R32 mixture refrigerant for use in a transcritical CO2 heat pump is investigated, and a case study is presented to show the benefit of the determined optimal CO2/R32 concentration
Summary
With rapid urban construction and development, building energy consumption is increasing rapidly in China [1]. Domestic hot water (DHW) requirements dominate and account for 40–50% of energy consumption [2]. As an environmentally friendly and efficient solution, the air source heat pump (ASHP) is advocated as an alternative method to conventional solutions [3]. Following publication of the Montreal Protocol and the EU F-Gas Regulation, the use of traditional hydrofluorocarbons is gradually being phased out. Given its superior non-flammable and non-toxic properties, the use of natural CO2 as a safe refrigerant has attracted the attention of researchers [4]. Its temperature glide matches well with the water temperature lift. An air source transcritical CO2 heat pump (TCHP) water heater should be the preferred choice for DHW
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