Heat pump water heaters

During the lasts months the CTTC laboratory has extended its infrastructure on vapor compression systems with an experimental facility to study and analyze an air-source heat pump water heater, to be used as an alternative heating system for some of the domestic heating necessities currently still covered by electric resistances.
Meanwhile, the efforts have also focused on the extension and improvement of the numerical analysis tools of vapor compression refrigeration cycles. The data obtained on the experimental unit will be fundamental for validation of this numerical tool in the near future.

Experimental facility

The heat pump water heater constructed is composed by a rotary compressor, a brazed heat exchanger as a condenser, a capillary tube or a thermostatic valve as expansion devices, and an aluminum finned tube brazed as evaporator.
The facility is highly instrumented. All inlet and outlet temperatures of the system elements are measured by thermocouples (TCK). Two absolute pressure sensors are placed at the inlet and outlet of the compressor. The pressure drop in the evaporator and the condenser are obtained with the corresponding differential pressure sensor. The mass flow of the refrigerant is evaluated with a Coriolis mass flow meter.

Fig. 1-2: Heat pump water heater experimental facility.

In the water circuit the inlet and outlet temperature of the condenser is measured with two thermoresistances (PT100) and two thermocouples to get correctly the transient response. The water flow is also obtained by a Coriolis flow meter. The air inlet and outlet temperature are evaluated using two thermo resistances (PT100). The drop pressure is measured using a differential pressure sensor, and the volumetric low is estimated using a turbine flowmeter.

Experimental results

The heat pump water heater is experimentally studied in order to evaluate the impact of the refrigerant charge, to investigate the heat pump with two different expansion devices, and also to evaluate the efficiency of the system under different air and water temperature levels and refrigerant types.
As an example of the results obtained, the evolution of COP with an increasing water inlet temperature is depicted in the following figure, for two different expansion devices and for two different refrigerant charges. Further work is in progress to confirm the observed trends, by changing the position of the refrigerant flow meter and by reducing the observed leakage level of such an instrumented system.

Fig. 3-4: Heat pump water heater experimental results: COP vs. T water inlet for different expansion devices (left) and refrigerant charge (right).