Relationship between Building Thermal Properties and Heat Exchange Performance in Geothermal Heat Pump Systems

摘要

The heat balance of the ground is important for the geothermal heat pump (GeoHP) systems to operate effectively over a long term except when the thermal property of the ground is greatly affected by the ground water flow. On the other hand, the cooling and heating load of the systems depends on the thermal property of buildings. Because of this, we calculated how the thermal property of buildings can affect the heat balance of the ground using dynamic simulation model. The simulation model is composed of a GeoHP system model and a building model. The GeoHP system model consists of vertical ground heat exchangers and a heat pump. In the GeoHP model, only heat conduction is considered to model the heat transfer in the ground. On the other hand, an isolated house for residential purposes is simulated in the building model. The building model is based on an equation of heat balance of room air and the house components. The effect of solar radiation and nocturnal radiation is considered in the building model. In the simulation, we used the weather data measured in two different locations to compare the influence of buildings thermal property between different regions: One is measured in Fukuoka, located in the southern part of Japan; the other is measured in Sapporo, located in the northern part of Japan. The ground heat exchanger model is verified by comparing measured and calculated temperature at outlet of U-tube during the thermal response test. We simulated 9 different types of houses with combinations of 3 different thicknesses of glass-wool insulation (50,150 and 250 mm) and 3 different structures of windows (single float glass, double glass and low emissive double glass).The results of simulation revealed that the heat balance of the ground in the GeoHP systems depends on the thermal property of buildings and that there is a suitable thermal property of building for keeping the heat balance of the ground. But, the suitablethermal property varies by regions. In addition, as the thermal property of building is improved, the cooling and heating load becomes smaller. This suggests that an effectual thermal design of buildings is helpful to reduce the length of ground heat exchangers in the GeoHP systems. The reduction of the length of ground heat exchangers means less initial investment on the GeoHP systems. Therefore, an ingenious thermal design of buildings can realize low-cost and sustainable GeoHP systems.