18 May 2017
The utilization of heat pumps in Hokkaido, where temperature is very low in winter, remains as low as 5% so it is required to develop high-efficiency heat pumps used in the region. In this study, thermosiphons with a new shape have been designed and evaluated to collect heat from surface ground layers. Those thermosiphons utilize underground heat as a heat source for evaporators in order to prevent a reduction in heat pump efficiency. In the experiment, the performance of two types of thermosiphons, a 1-m-long corrugated and a smooth tube, has been evaluated. Moreover, the artery tube is inserted inside the thermosiphon to facilitate the recirculation of the working fluid from the condenser. Four types of artery tubes were used, three of which were showering tubes. Each showering tube is blocked on the bottom, with three holes opened on the side wall. As a result of performance evaluation, the smooth tube showed higher performance than the corrugated tube. This is attributable to the fact that the corrugated tube has a lower heat-transfer coefficient than the smooth tube because of a metal net reinforcement covering the tube. And by inserting the three-direction showering tube, the heat flux in the evaporator increased up to at around 9000 W/m2 and dry out was not observed. Based on the test results, an empirical correlation of heat-transfer coefficients was developed, and then integrated into heat pump simulations. It was found that the use of a three-direction showering tube successfully reduced the number of thermosiphons required.