Comparative performance evaluation of cascaded air-source hydronic heat pumps

Abstract The results are reported of an investigation of the effects of cascading air-source heat pumps on performance for hydronic residential systems. Three heat pump systems are modeled as single-stage, single-refrigerant cascaded, and two-refrigerant cascaded. Energy and exergy analyses are performed, and a comparative performance analysis is carried out, considering energy efficiency, refrigerant mass flow rates, evaporator pressure, exergy efficiency, and several other criteria. Three sets of source and supply temperatures, representing different climates and different water sink systems (low, medium and high temperature), are used to provide more comprehensive behavior assessments of the systems. Additionally, the optimum intermediate pressure of the cascaded systems for all working temperature pairs is found for the highest energetic COP and exergetic COP. Compared to a single stage heat pump, cascading improves the overall energy efficiency of the system for low-ambient temperatures, but not for high-ambient temperatures. Although this improvement is minor, the exergetic COP is increased by 67% for the single refrigerant cascaded system and 70% for a two-refrigerant cascaded system, at low ambient temperatures. Using refrigerant R404A in the low-pressure cycle marginally improves the energetic COP of the cascaded heat pump, but increases the evaporator working pressure, making it possible to use smaller compressors. However, the overall refrigerant mass flow rates increase with cascading. The two cascaded systems have higher exergy destructions (by almost four times) compared to the single stage system, mainly due to having more components, including an intermediate heat exchanger. Also, cascading shifts the major exergy destruction centers from the compressors and expansion valves to the evaporators. A comparison of cascaded and single-stage heat pumps shows that the exergy analysis results exhibit a different trend than energy analysis results with source and supply temperatures, highlighting the advantages of exergy methods in determining if cascading is appropriate for a given application.

[1]  E. Groll,et al.  Two-stage air-source heat pump for residential heating and cooling applications in northern U.S. climates , 2008 .

[2]  Ibrahim Dincer,et al.  Exergy and Energy Analyses , 2013 .

[3]  M. J. Moran,et al.  Thermal design and optimization , 1995 .

[4]  J. L. Smith,et al.  Optimum temperature staging of cryogenic refrigeration system , 1994 .

[5]  Wenming Yang,et al.  Advances in heat pump systems: A review , 2010 .

[6]  Ma Guoyuan,et al.  Experimental investigation of air-source heat pump for cold regions , 2003 .

[7]  Eric B. Ratts,et al.  A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method , 2000 .

[8]  Adrian Bejan,et al.  Method of entropy generation minimization, or modeling and optimization based on combined heat transfer and thermodynamics , 1996 .

[9]  Pradeep Bansal,et al.  Thermodynamic analysis of an R744-R717 cascade refrigeration system , 2008 .

[10]  Ibrahim Dincer,et al.  Exergy: Energy, Environment and Sustainable Development , 2007 .

[11]  Min-Soo Kim,et al.  Optimal temperature between high and low stage cycles for R134a/R410A cascade heat pump based water heater system , 2013 .

[12]  Reinhard Radermacher,et al.  Refrigerant injection for heat pumping/air conditioning systems: Literature review and challenges discussions , 2011 .

[13]  J. W. Ramsey,et al.  Thermal Environmental Engineering , 1970 .

[14]  Jahar Sarkar,et al.  CO2-C3H8 cascade refrigeration-heat pump system : Heat exchanger inventory optimization and its numerical verification , 2008 .

[15]  I. Dincer.  Refrigeration Systems and Applications , 2003 .

[16]  Fabio Polonara,et al.  Blends of carbon dioxide and HFCs as working fluids for the low-temperature circuit in cascade refrigerating systems , 2005 .

[17]  Ali Kilicarslan,et al.  An experimental investigation of a different type vapor compression cascade refrigeration system , 2004 .

[18]  T. J. Kotas,et al.  The Exergy Method of Thermal Plant Analysis , 2012 .