CO2 Heat Pump Water Research Articles

Pinch point characteristics and performance evaluation of CO2 heat pump water heater under variable working conditions

• A faster and effective method for seeking the optimal discharge pressure is proposed. • The pinch point characteristics in the gas cooler are comprehensively revealed. • The relationship between the pinch point and the optimal discharge pressure is clarified. • The thermal matching and COP are no longer related at high inlet water temperatures. • The performance differences between the CO 2 heat pump and R134a are clarified. The pinch point in the gas cooler significantly impacts the optimal discharge pressure and system performance of the CO 2 heat pump water heater, however the pinch point characteristics are still not fully revealed. Given this, a comprehensive thermodynamic investigation is conducted. The thermodynamic model is developed and validated, and a faster and effective method for seeking the optimal discharge pressure is proposed and proven accurate. The pinch point variation pattern with working conditions and its influence on the system performance are investigated. Besides, a comparative study on the CO 2 heat pump water heater with R134a is conducted under variable working conditions. The results show that there are four pinch point variation patterns. The pinch points may appear at the hot end, the interior, and the cold end of the gas cooler. At low inlet water temperature and large temperature lift conditions, two pinch points appear exactly at the optimal discharge pressure and are at the interior and the cold end. The appearance of the two pinch points enables the optimal thermal matching of CO 2 and water, resulting in a maximum coefficient of performance (COP). The two pinch points are still possible at high inlet water temperatures; however, the thermal matching and COP are not closely related anymore. The CO 2 heat pump water heater is more competitive than R134a for low inlet water temperature and large temperature lift conditions, but not vice versa. The results obtained may be helpful for the optimization and efficient control of the CO 2 heat pump.

Comparison of Transcritical CO2 and Conventional Refrigerant Heat Pump Water Heaters for Domestic Applications

Although CO 2 as refrigerant is well known for having the lowest global warming potential (GWP), and commercial domestic heat pump water heater systems exist, its long expected wide spread use has not fully unfolded. Indeed, CO 2 poses some technological difficulties with respect to conventional refrigerants, but currently, these difficulties have been largely overcome. Numerous studies show that CO 2 heat pump water heaters can improve the coefficient of performance (COP) of conventional ones in the given conditions. In this study, the performances of transcritical CO 2 and R410A heat pump water heaters were compared for an integrated nearly zero-energy building (NZEB) application. The thermodynamic cycle of two commercial systems were modelled integrating experimental data, and these models were then used to analyse both heat pumps receiving and producing hot water at equal temperatures, operating at the same ambient temperature. Within the range of operation of the system, it is unclear which would achieve the better COP, as it depends critically on the conditions of operation, which in turn depend on the ambient conditions and especially on the actual use of the water. Technology changes on each side of the line of equal performance conditions of operation (EPOC), a useful design tool developed in the study. The transcritical CO 2 is more sensitive to operating conditions, and thus offers greater flexibility to the designer, as it allows improving performance by optimising the global system design.

CO 2-heat pump water heater: characteristics, system design and experimental results

CO 2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO 2 process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO 2, makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.