Experimental study of a heat pump with high subcooling in the condenser for sanitary hot water production

Abstract

The use of heat pumps in order to produce sanitary hot water have been demonstrated as a very efficient alternative to traditional boilers. Nevertheless, the high water temperature lift (usually from 10°C to 60°C) involved in this application has conditioned the type of used solutions. In order to overcome it, transcritical cycles have been considered as the most suitable solution. The current article analyzes a new heat pump prototype able to enhance the heat pump efficiency using a subcritical cycle. The proposed prototype is able to control the system subcooling and make it capable to work at different subcoolings in the condenser. That kind of mechanism has demonstrated its capability to increase the efficiency of the heat pump. The obtained results have shown that coefficient of performance depends strongly on subcooling. In nominal condition (inlet/outlet water temperature at evaporator is 20°C/15°C and the water inlet/outlet temperature in the heat sink is 10°C and 60°C), the optimal subcooling is 42 K with a heating coefficient of performance of 5.35, which is about 25% higher than the same cycle working without subcooling.

Nomenclature
BPHE	=	brazed plate heat exchanger
COP	=	coefficient of performance, [−]
Cp	=	specific heat capacity [kJ kg−1 K−1]
EU	=	European Union
EV	=	expansion valve
LR	=	liquid receiver
HX	=	heat exchanger
	=	mass flow rate [kg s−1]
	=	capacity [kW]
Sc	=	subcooling [K]
Sh	=	superheat [K]
SHW	=	sanitary hot water
SPF	=	seasonal performance factor
T	=	temperature [°C]

Subscripts

condensing	=	part of the condenser where condensing takes place
desuperheat	=	part of the condenser where desuperheat takes place
h	=	heating
liq	=	liquid
ref	=	refrigerant
sat,evap	=	saturation at evaporator
w	=	Water
w,ci	=	Water condenser inlet
w,co	=	water condenser outlet
w,cond	=	water through condenser
w,ei	=	water evaporator inlet
w,sub	=	water through subcooler