Potentials for COP increase in vapor compression systems.

摘要

This work presents an experimental and numerical study of three different opportunities for COP increase in vapor compression systems: work recovery ejectors, condenser subcooling and increase of relative size of heat exchangers (HXs).;Regarding work recovery ejectors, experimental results showed that the COP of the ejector system increased between 8.2% and 14.8% when compared to a conventional expansion valve system operating with R410A. The two major mechanisms of improvement of the ejector system were quantified separately: COP gains between 1.9% to 8.4% were solely due to the work recovery, while liquid-feeding the evaporator alone was responsible for 4.9% to 9.0% of COP gain. Overall ejector efficiencies from 12.2% to 19.2% were achieved.;A major portion of this dissertation explores the effects of condenser subcooling on the COP of vapor compressions systems. It is shown that, as condenser subcooling increases, the COP reaches a maximum as a result of a trade-off between increasing refrigerating effect and specific compression work. A thermodynamic analysis pointed out that refrigerants with large ratio of liquid specific heat to latent heat of vaporization tend to benefit more from condenser subcooling. Numerical results suggested that R1234yf systems would benefit the most from condenser subcooling in comparison to R410A, R134a, and R717. Experimental results obtained with a vehicular air conditioning system revealed COP gains between 6% and 44% for R1234yf and 2% to 21% for R134a due to condenser subcooling. Results also indicated that the larger the air-refrigerant temperature difference in the condenser, the higher the COP maximizing subcooling and the COP gains from condenser subcooling. Additional experiments showed that the presence of an internal heat exchanger reduces the benefits of the condenser subcooling. With a unique set of microchannel condensers, an experimental comparison between subcooling generated in non-designated area (NDA) and designated area (DA) of the condenser showed that both configurations yielded similar values of maximum COP improvement within the operating conditions considered. It was also demonstrated that condensers with a higher air-refrigerant temperature difference would require a larger COP maximizing area ratio allocated for subcooling. Nevertheless, a fixed designated area yielded near maximum COPs within a reasonable range of operating conditions. The effect of refrigerant mal-distribution on the performance of a two-pass parallel-flow microchannel condenser as well as on the overall system COP is numerically investigated. The results showed a COP deterioration of not more than 3% for the worst case of mal-distribution considered.;The effect of the size of the heat exchangers (HXs) relative to the cooling capacity on the performance of actual residential air conditioning systems has been experimentally and numerically investigated for R410A and transcritical R744. Experiments were carried out in a limited range of operating conditions but an experimentally validated model was used to extrapolate trends. As the relative size of the HXs was increased, a maximum COP of 9.5 (without fans power) was experimentally measured with R410A, a value much higher than off-the-shelf units but that is only 28% of the ideal Evans-Perkins COP. Lower compressor isentropic efficiency, condenser subcooling and evaporator superheat were pointed out as constraints for further COP improvement by oversizing the HXs. These constraints were removed in the numerical model and results showed that further oversizing HXs can improve COP but sensitivity is reduced significantly as HXs become larger relative to the cooling capacity. Experimental and numerical results indicated that the COP of an air conditioning system operating with transcritical R744 is less sensitive to the relative size of the HXs than that of the same system operating with R410A. Transcritical R744 becomes more competitive in terms of COP as the size of HXs is reduced while R410A by far outperforms transcritical R744 in a larger system. In addition, it is suggested that the COP ratio to the theoretical limit of Evan-Perkins cycle could be used as a measure of the heat exchangers relative size.