DBPapers
DOI: 10.5593/SGEM2015/B41/S17.016

COMPARATIVE EXERGY ANALYSIS OF A SINGLE STAGE HEAT PUMP AND A HEAT PUMP CASCADE OPERATING AT DIFFERENT TEMPERATURE LIFTS

I. Dosa, N. Diaconu
Friday 25 September 2015 by Libadmin2015

References: 15th International Multidisciplinary Scientific GeoConference SGEM 2015, www.sgem.org, SGEM2015 Conference Proceedings, ISBN 978-619-7105-38-4 / ISSN 1314-2704, June 18-24, 2015, Book4, 123-130 pp

ABSTRACT
For increased temperature lift, heat pumps operating with a single-stage cycle become progressively more inefficient. These disadvantages may be overcome by operating a two or multi stage heat pump. A great variety of two stage heat pumps with enhanced efficiency were designed. The addition of extra components such as flash tank, expansion valves causes additional sources of irreversibility; the entire irreversible loss of the whole cycle is still lower than that of the single-stage cycle. A single stage cycle is simpler, with fewer components, therefore lower purchase price and maintenance costs. An alternative for two stage heat pumps can be the heat pump cascade. This paper presents a comparative exergy analysis of a single stage heat pump and a heat pump cascade operating at different temperature lifts: 60 °C, 55 °C and 50 °C, using ammonia refrigerant. Results reveal that for high temperature lifts (60 °C) COP of cascade heat pump can be 3,74% higher, while for 50 °C is 0,28% smaller, resulting that for smaller temperature lifts using cascade heat pump is no longer an option. Variance of exergy efficiency is not significant. Losses due to due to irreversibility of compression and expansion are lower since the temperature lift is divided between two heat pumps, resulting lower compression ratios, therefore lower pressure and temperature, and even different refrigerant flow rates. Having additional expansion valves and evaporators leads to higher losses due to irreversibility in evaporator and condenser. The overall effect is that for 50 °C temperature lift losses for cascade are 3,49 % higher while at 60 °C are 5,08% lower. Even if the efficiency of cascade heat pump is lower than the efficiency of improved multi-stage heat pumps, a simpler and cheaper heat pump with lower maintenance costs can be a good alternative.

Keywords: Heat pump, heat pump cascade, exergy efficiency, COP