Heat pump is considered to be the most effective way to provide space heating and cooling compared to electrical heater or fire heater. It normally counts for more than half of the total energy usage of each house or building. However, the economical and environmental burden of heat pump has become larger and larger due to the growing awareness of the greenhouse effect and the increasing price of electricity. A number of modifications have been added to heat pumps to increase its performance and reduce the operation costs. Solar assisted ground source heat pump system (SAGSHP) which has been developed recently could provide a sustainable way to modify traditional heat pumps. In this paper, the thermodynamic principle, description and working procedures of SAGSHP will be introduced as well as the discussion of the advantages, disadvantages and future status of such heat pump system.
Coefficient of performance (COP) (which is defined as the ratio of the heat transfered to the work input) is the major way to examine the performance of a heat pump system. For the pumps that are at the same power, the higher COP could indicate higher performance. From temperature point of view, the COP can be defined as, by equation, COP=Th(Th-Tc) in heating mode and COP=Tc(Th-Tc) in cooling mode. Therefore, theoretically, the COP can be increased by increasing the temperature in cold region in heating mode, and reducing the temperature in hot region in cooling mode. This is the basic thermodynamic principle of SAGSHP system.
The whole system can be described as a heat pump in conjunction with an external heat source. The heat source can increase the temperature at cold region in heating mode and can reduce the temperature in hot region in cooling mode. The energy in the heat source is obtained from solar energy and the energy stored in the ground. The figure below could well illustrate the configuration of the SAGSHP system.
Figure 1: Schematic diagram of a solar assisted ground source heat pump system The system consists of three major circuits which are 1) ground coupling circuit; 2) refrigerant circuit and 3) fan-coil circuit. The area that connected by ground coupling circuit and refrigerant circuit is called cold region in heating mode and hot region in cooling mode.
* Ground coupling circuit
The ground coupling circuit has a closed ground loop heat exchanger (CGHE), solar thermal collector, 4 valves and a circulate pump. The working fluid inside this circuit is water. In extremely cold weather, anti-freeze substances are often added into the water.
There are three configurations of the CGHE, which are horizontal loop, vertical loop and spiral loop. The choice of the CGHE depends on the availability of land, the local ground condition and the excavation cost (Younis, Bolisetti & Ting 2010). The horizontal loop is suitable for the land that has cheap price and large area while the vertical loop and spiral loop are more suitable for the limited land area. The vertical loop requires drilling equipments and drilling costs while the spiral loop requires more total pipe length than the other two configurations (Omer 2006).
The solar thermal collector is connected in series with a closed ground loop heat exchanger. Typically, according to Ozgener and Hepbasli in 2004 that flat-plate solar collector would be selected due to it could utilize both the diffuse and direct solar radiation. The solar thermal collector is placed on the top of roof to take the most use of the solar energy.
* Refrigerant circuit
The refrigerant circuit is a heat pump system that consists of compressor, reversible expansion valve, and two heat exchangers. However, compared to traditional heat pump, the outdoor heat exchanger of this SAGSHP system is connected to the ground coupling circuit instead of air. The thermodynamic principle of this system, which is mentioned at beginning,...