Study On The Combination Use Of Downhole Heat Exchanger And Heat Pump In Space Heating

As a new energy resource, the geothermal energy has such merits as wide distributed, low cost, clean and so on. It can also be exploited and utilized directly. Nowadays, the ground source and the underground water source heat pump systems are the two popular options (respectively shortened by GSHPS and GWHPS) for geothermal energy uses. GSHPS has the feature that absorbing or releasing heat to the soil through heat conduction. Since the heat conduction in soil has a low heat transfer rate, the amount of heat extracted by a single borehole heat exchanger is very limited. GWHPS could have a high heat transfer rate, however, a reinjection well is needed (which is called as doublet) in order to have a sustainable production That the reinjection is smooth or not depends much on the hydrodynamic properties of the aquifers. That is to say, if the production rate is large, and without a reinjection, the original balance of the geothermal water in the aquifers could be destroyed that the under ground water level might drop and even the surface subside. The two options above both have their own limitations, that exploiting a new sort of option has become necessary for popularizing the geothermal utilization. The combination use of downhole heat exchanger(DHE) with heat pump is, therefore, proposed to be investigated in the paper. DHE is a geothermal exploited device extracting just heat but not water. It is the fact that our country is abundant in low to intermediate temperature geothermal resources. Therefore, the combination use of downhole heat exchanger and heat pump in space heating is expected to be a new tendency of geothermal utilization in the future.Firstly, a heat conduction model is built for analyzing the lowest heat transfer rate of a downhole heat exchanger. In this model the horizontal section surrounding the U-tube is divided into two parts, i.e., the region of pure fluid inside the well casing and the region of porous formation around the well casing. By solving a set of ordinary differential equations, the temperature profiles along the two legs of U-tube are obtained. The effects of mass flow rate and inlet temperature of circulating water on the total heat output are analyzed.Secondly, a thermodynamic analysis for a space heating system with both DHEs and heat pumps are performed, in which have four cases classified by using different terminal heat radiators and with or without loading a heat pump system. Optimum design parameters could be suggested if the building, DHE, geothermal reservoir, and environment information are known in advance.A real DHE-heat pump combination space heating system was established in Shacheng city, Hebei province, in which a data acquisition system was also installed. By analyzing the test data collected in the last heating season and some experimental data, the heat transfer performance of the DHE was obtained. It can be concluded that the natural convention did occur inside the DHE, which plays the main contribution to the total extracted heat.