Exergy-based analysis of heat pump using surplus heat from data centre for medium sized district heating and CO2/propane-based mixtures

Conventional exergy-based analysis is a power tool for revealing the location, the magnitude and the sources of thermodynamic inefficiencies within an energy conversion system. However, the estimation of the avoidable exergy destruction is needed to provide engineers with the most useful information for improving the overall thermodynamic efficiency of the investigated system. In this study the thermodynamic performance of a heat pump aiming at recovering the waste heat from a data centre in favour of medium sized district heating was assessed by computing avoidable irreversibilities. Supply and return water temperatures of 75/30 °C were assumed for district heating. CO2/propane (R744/R290)based mixtures were considered as eco-friendly (negligible global warming potential and no ozone depletion potential) working fluids. Such blends also offer lower flammability risks than pure R290 and lower operating pressures than pure R744, resulting in better performance. For the investigated scenarios mass fractions in R744/R290-based mixtures were equal to 0.70/0.30, 0.80/0.20, 0.85/0.15 and 0.95/0.05. Pure R744 was also included for comparative analysis. For the selected operating conditions, the values of heat rejection pressures providing the lowest total exergy destruction rates and the highest coefficient of performance (COP) values coincided. For cases with the optimal discharge pressures a good matching of temperature glides within the gas cooler was observed. The results obtained showed that the mass fraction of R744/R290-based mixtures significantly affects the system total exergy destruction rates and the removable ones. The scenario involving the mass fraction of R744/R290-based mixture of 0.85/0.15 presented the lowest value of the total exergy destruction rate (i.e. 43.2 kW) and the highest COP (i.e. 4.02). The system with pure R744 provided 45.8 kW of the total exergy destruction rate and COP equal to 3.86. For the blend R744/R290 with 0.85/0.15 mass fraction the temperature profiles of the working fluids in evaporator also had good matching. The conventional exergy analysis indicated that both the compressor and the expansion valve had the biggest values of exergy destruction rate, i.e. the highest priority for improvement. As for the outcomes in terms of removable irreversibilities, it was found that only the compressor is the component with the highest priority for thermodynamic improvement providing decrease of exergy destruction rate between 12.08 kW and 14.68 kW and increase for values of COP between 4.33 and 4.84. The results obtained from the analysis of removable irreversibilities showed that the mutual interactions between the compressor, gas cooler and evaporator were weak. The thermodynamic efficiency of the expansion valve depended mostly on the irreversibilities within the gas cooler. Taking into account the flammability safety and potential of the system thermodynamic improvement the mixture involving the 0.85/0.15 R744/R290 mass fraction was the most preferable with achievable COP equal to 4.84, which was 3.6 % higher compared to the heat pump with pure R744 after thermodynamic improvement.