Performance maximization by temperature glide matching in energy exchangers of cooling systems operating with natural hydrocarbon/CO2 refrigerants

Abstract

• Temperature glide matching increases COP of hydrocarbon + CO 2 refrigeration systems. • Exergy destruction decreases in energy exchangers and increases in expansion valve. • Dimethyl ether + CO 2 and propylene + CO 2 blends have the highest performance. • Performance maximum is observed at a lower concentration of CO 2 in the blends. • A model is developed for temperature glide matching without experimental input. This research is part of an effort in finding viable alternative refrigerants to mitigate anthropogenic climate change. In this study binary zeotropic mixtures of selected hydrocarbons (HCs) with carbon dioxide (CO 2 ) are evaluated as alternative refrigerants for a vapor-compression refrigeration cycle. A thermodynamic analysis of these mixtures is performed for a cycle with a variable temperature of energy transfer fluid in both evaporator and condenser/gas cooler. A temperature glide matching method with a specified minimum temperature difference between refrigerant and energy transfer fluid in the energy exchangers was developed for performance maximization. This method does not require any experimental input or an educated guess for the temperature difference between refrigerant and energy transfer fluid and can simulate both sub-critical and trans-critical cycles. Performance metrics like the coefficient of performance (COP) and volumetric refrigeration capacity (VRC) are calculated and discussed for various mixture compositions (0 to 100% CO 2 ). Furthermore, non-dimensional exergy destructions in different components in the cycle are determined to estimate the effect of individual components on the total performance. An increase up to 40% in COP for zeotropic mixtures of HCs and CO 2 is observed when compared with pure HCs. The VRC is also shown to increase with increasing CO 2 concentrations in HC + CO 2 mixtures. Exergy analysis shows a decrease in non-dimensional exergy destruction in the energy exchangers due to temperature glide matching.