Thermal-fluid performance modelling of a transcritical carbon dioxide heat pump cycle

Abstract

The continuous augmentation of environmental laws for refrigerants has forced the HVACR sector to re-consider natural refrigerants, with Carbon Dioxide (R744) being a promising candidate due to its competitive heat transfer properties and negligible environmental impact. When operated in a transcritical heat pump cycle, R744 can heat water to above-conventional temperatures such as 90°C without a severe downgrade in cycle efficiency. Recent literature revealed that 18.8% of the total industrial energy demand in South Africa is integrative with this system when operating at such high water temperatures. Although the system’s ability and potential market have been pointed out, actual cycle performance data is limited to low ambient temperatures only, which is unusual for South Africa. Thus, the need exists to predict the cycle performance at the mentioned water outlet temperatures over a range of high ambient temperatures, typically beyond 25°C. In this study, a thorough literature study was carried out to evaluate the component types, methods and correlations typically used to numerically model the cycle in question. The relevant theory that governs the thermo-physical behaviour of each component was compiled and integrated into a simulation model. The verification revealed that the component models were able to predict their reference data within an absolute maximum deviation of 3.17%. The individual component model validations (relative to experimental data) and the full cycle simulations (relative to literature data) proved the model to deliver satisfactory accurate and tangible results for the purpose of this study. After sizing the system to criteria that provide a sufficient capacity to satisfy all intended operating conditions, various simulations followed. At a water outlet temperature of 90°C, subject to an ambient temperature rise from 25°C to 40°C, the best-recorded COP value enhanced from 3.02 to 3.37 (+11.6%), whilst the heating capacity rose from 32.7 kW to 41.1 kW (+25.7%). This was accompanied by a moderate rise in optimal discharge pressure. Other parameters such as the heat transfer rates, mass flow rates, and power consumption were also studied. Normalised to the most appropriate literature, the noted efficiencies were found lower than Yamaguchi et al. (2011), yet, higher than Wang et al. (2013a). It was concluded that the high ambient temperatures did not result in significantly higher efficiencies than those of literature, which is linked to lower ambient temperatures, yet, at higher humidity. Hence, the important influence of ambient humidity was also considered. At an ambient temperature of 40°C, a rise in relative humidity from 30% to 60% led to a COPH improvement from 3.37 to 3.77 (+11.9%). At such high ambient conditions, it was also revealed that the evaporation temperature needs to be maintained at a low enough value (in this case 16.4°C) for the R744 discharge temperatures to still be satisfactory for the application of such high water temperatures. Supplementary, at the conditions of interest, it was found that an increase in water inlet temperature heavily degrades the cycle efficiency.