Studies of a Heat Pump Using Dual Heat Sources. of Solar Heat and Ambient Air

Sadasuke Ito, Toshiyuki Matsubayashi and Naokatsu Miura
Department of System Design Engineering
Atsugi, 243-0292 Japan
Tel/Fax +81-462-91-3091
Email: ito@sd. kanagawa-it. ac. jp

1. Introduction

Increased use of renewable energy such as solar energy is important to prevent global warming. The aim of the demand of solar heat in 2010 is 19.1 Mm3 in equivalent oil, which is about 4 times of that in 1997, to achieve the goal proposed in the Kyoto Protocol. However, the demand of solar heat has not increased since that time. Much more effort is necessary technically and politically to increase the demand of solar heat. Solar heat for hot water supply is usually collected by a solar collector though which a heat transfer medium of air, water, or antifreeze liquid flows. Heat loss from the collector increases as the temperature of the heat transfer medium increases. Therefore, useful heat can not be obtained when the solar radiation becomes too small.

Solar collectors can be used also as evaporators of heat pumps for raising the evaporation temperature of the refrigerant by solar heat to increase the thermal performance of the heat pumps. Charter and Taylor0 reported on such a system using flat-plate collector without a cover. A heat pump can be operated efficiently even when there is no or small solar radiation if the collectors have fins to absorb heat easily from the ambient air.

Ito et al2) demonstrated that solar collectors with photovoltaic modules on the flat surfaces can be used for generating electric power and collecting solar heat at the same time. These collectors, which are called by PV/T panels or panels, can give high efficiency of conversion of solar radiation to useful energy. Heat is absorbed from the ambient air also when the refrigerant in the panels evaporates at the temperature less than the ambient air temperature. If the panels are set on the roofs of houses, it is difficult for the panels to absorb heat from the back sides. In order to operate the heat pump efficiently when there is no or small solar radiation, we propose to use an air-refrigerant heat exchanger together with PV/T panels as the evaporators.

Roll-bond type collectors without photovoltaic modules on the surfaces are
used in the present studies. Experimental and analytical works were done previously on the performance of a heat pump with these panels3). In the present study, the thermal performance of a heat pump using the panels and an air-refrigerant heat exchanger is investigated and the effectiveness of using the heat exchanger is discussed.

2. Experiment

Fig.1 Experimental apparatus.

Fig.1 shows the experimental apparatus. The heat pump system is mainly composed of panels (evaporator), an air-refrigerant heat exchanger (heat exchanger, evaporator), an electrically driven rotary compressor with rated capacity of 250 W, a cylindrical condenser made of two copper tubes soldered together, a C-charged type thermostatic expansion valve at each inlet of the evaporators. Refrigerant 22 circulates these components. The panels are an aluminum roll-bond type. Three panels with the total area of 1.91 m2 are connected in series. The panels are mounted on plywood and installed at 50° tilt angle facing south on the roof of a house. The heat exchanger is made of a

copper tube of 10 mm outside diameter with many aluminum fins with thickness of 0.1 mm. The height, width and depth of the heat exchanger is 200 mm, 230 mm and 45 mm, respectively. A fan with a capacity of 8 W is used to draw the ambient air though the heat exchanger. Either one of the evaporators or both arranged in parallel are used in an operation of the heat pump.

In an actual system, a thermal storage tank would be used. In the present study, a constant temperature bath is used instead so that the thermal performance of the heat pump can be investigated under the condition of a constant temperature of the water flow at the inlet of the condenser. The flow rate of water is set to 3.0 l/min. The temperatures and pressures at various points are measured by using thermocouples and Bourdon tube pressure gauges, respectively.

3. Analysis