Heat transfer experiment of a Deep Downhole Heat Exchanger (DDHE) was carried out in winter in Tianjin. The DDHE was installed in a 2070 m deep well, the hot water was pumped out through the insulated inner tube and returned through the outer annular to the geothermal reservoir. The test result shows that the average thermal output is 270 kW. A heat conduction model was established and the parameters affecting the thermal output of DDHE were studied. The results show that the temperature gradient has radial heat conduction is the main influencing factor, and the opening at the bottom of the inner tube is beneficial for increasing the thermal output. The results show that the temperature gradient has a large effect on the heat output, and the heat output increase linearly when the temperature increases from 0.02℃/m to 0.04℃/m; the lower inlet temperature can obtain a larger heat output.

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