Study On The Use Of Downhole Heat Exchanger For Space Heating

As a kind of renewable energy resources, geothermal energy has been given more and more attentions in China. The ground source and the underground water source heat pump systems are the two popular options (respectively shortened by GSHPS and GWHPS) for shallow geothermal energy. However, both of these systems have the shortages: GSHPS realizing the heat transfer with the surrounding soil by heat conduction has a low heat transfer rate, in addition, they are not easy to be maintained for the buried looped pipe underground. For the GWHPS, the 100% reinjection of used water back to the shallow aquifer is a challenge. In a long-term recharging process, the impurities in water will fill up with the formation void, around reinjection well, so that the reinjectivity will decline fast with time. If the geothermal water in the aquifers can not recharged rationally, the under ground water level might drop and even the ground subsidence might occur. This paper shows an experiment study of downhole heat exchanger (DHE), and especially for the combination use of downhole heat exchanger (DHE) with a heat pump.Numerous tests have been conducted for the DHE by varying the inlet water temperature and/or the flowrate circulating the closed looped DHE. Some interesting phenomenon have been observed, which are helpful for us to understand the heat transfer mechanism of DHE and improve the heat transfer intensity. Two sets of DHE have been manufactured, which are made of PPR and copper, respectively. It is found that while the inlet water temperature remains almost the same, the outlet temperature from the DHE increases sharply at first, followed by a stable increase reaching the maximum, and then decreases slightly and followed by quick drawing down to a stable constant temperature. A correlation of heat transfer for the outside of DHE in a form of Nusselt number with Rayleigh number is proposed according to the experimental data.A one dimensional heat conduction model for DHE is established in the paper, and the Lattice Boltzmann Method is adopted for solving the transient heat conduction problem with a heat source varying with time and location. The comparison between the measured and simulated outlet temperatures of DHE shows that they are in general in agreement. Their differences may be probably caused by the natural convection occurrence around the DHE.The conclusions show that the amount of heat taken from the geothermal well depends on the inlet temperature, circulating flowrate and the material of the heat exchanger. The lower the inlet temperature, or the greater the circulating flowrate, the smaller the thermal resistance of DHE, and the greater the amount of heat taken from the geothermal well. The intensity of natural convection outside of DHE is crucial important for its design and application.