Sensitivity Analysis Of Vapour-Compression Refrigeration System

Sensitivity analysis of exergy efficiencies of vapour compression Refrigeration System in heating and cooling application

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Abstract: The objective of this paper is to investigate how changes in temperature levels affect the efficiency of thermal exergy transfer occurring at near-environmental temperatures. In this regard, this paper focuses on building ventilation as an application example involving heat transfer relatively close to ambient conditions. The analysis uses exergy efficiency and heat transfer effectiveness, combined with exergy consumption and warm/cool exergy, to gain insight into the effect of varying temperatures in heat exchange at near-environmental temperature. A dimensionless

There are mainly two types of exergy efficiency definitions: the universal ones in which gross exergy inputs and outputs are considered, and the functional ones in which net exergy flows are considered respectively. This work focuses on exergy analysis of space cooling by application of a vapour-compression heat pump using air cooling and heating. This system is modelled as simple as possible. In this case there are four independent parameters which determine the behaviour of the system: the temperatures of the air streams entering the heat exchangers, the environmental temperature and the second law efficiency of the heat pump. The results show that the functional exergy efficiency is more sensitive to parameter changes than the universal exergy efficiency. These efficiencies show not much difference when the exergy value of the hot air stream can be used in a meaningful way in the building. There are four contributions to the overall exergy losses in the case of space cooling. Two determined by heat exchange in both heat exchangers, the amount of exergy of the hot air stream, and the irreversibility’s within the heat pump itself, expressed in its second law

They analyse variations of exergy transfer effectiveness with number of transfer units (NTU), with the ratio of the heat capacity of cold fluid to that of hot fluid (Cc/Ch) and with finite pressure drops. They note that there is not an optimal combination of NTU and Cc/Ch for maximising exergy transfer effectiveness. They donot elaborate on the effects of temperature variations.Johannessen et al. (2002) examine temperature profiles and local entropy productionprofiles in heat exchangers. They show that the standard counter-current heat exchanger is the best first approximation to optimal heat exchange conditions in practice, as it has, mqualitatively the same properties as the optimal solutions presented in their study; when the temperature difference Th – Tc between the hot and the cold fluids is approximately constant. In the present study the authors assume Th – Tc to be constant throughout the heat exchanger, for the sake of simplicity.

Hesselgreaves (2000) studies entropy generation in heat exchangers, and develops a new relationship for optimising balanced counter-flow heat exchangers for a ‘long’ duty, i.e. with significant temperature changes of the working fluids.

Yilmaz et al. (2001) present