In order to utilize shallow geothermal energy resources which can be found anywhere, a vertical U-tube downhole heat exchanger has come to be widely used around the world. This is because this resin U-tube is moderate in price, and it is comparatively easy to insert and to set up in the downhole after drilling a well. In utilizing the U-tube downhole heat exchanger, multiple U-tubes are practically inserted in the downhole for the purpose of extracting more geothermal energy from a stratum. In the present study, a numerical simulation has been performed to clarify the heat extraction characteristics of both vertical double and triple U-tube downhole heat exchangers. The numerical results showed that the heat extraction rates from a stratum for both vertical double and triple U-tube downhole heat exchangers increase with increasing the flow rate of the heat extraction medium of water. Furthermore it was shown that the heat extraction rate for a vertical double U-tube downhole heat exchanger depends on the well depth, in other words, the total length of the U-tube ; and the heat extraction rate increases with increasing well depth.
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