2022 Volume 44 Issue 4 Pages 155-166
Ground source heat pump systems have become important worldwide since their efficiency is higher than that of conventional heat pump systems, but the cost of installing borehole heat exchangers is one of the major issues in Japan. It is important to improve the performance of vertical borehole heat exchangers, which are the most commonly used type in Japan, in order to shorten the excavation length. Previously, we proposed a lining borehole ground heat exchanger (LBHE) to improve the efficiency of ground heat utilization.
This study aims to examine the heat exchange performance of an LBHE by numerical simulations using a heat transfer model. The simulations will focus on the heat exchange rate per unit length of an LBHE and the coefficient of heat extraction as the performance index for borehole heat exchangers to compare the performance of an LBHE and a double U-tube (DUT), which is a standard borehole heat exchanger.
Three-dimensional heat transfer models of the LBHE and the DUT were developed and validated by comparison with experimental results. As a result, both the LBHE and the DUT heat transfer models were able to reproduce the outlet fluid temperature at the time of heat extraction.
The simulation revealed that the heat exchange rate per unit length of the LBHE increased when a larger drill bit diameter and shorter borehole heat exchanger length were applied to it. There was less than about 20% difference in the heat exchange rate per unit length of the LBHE after the length was increased to 40 m or longer. In addition, the coefficient of heat extraction for the 50 m LBHE was compared with that of the 50 m DUT under various ground and operating conditions. When shorter operating hours per day were applied and higher effective thermal conductivity was applied to the ground conditions, the ratio of the coefficient of heat extraction for the LBHE to that of the DUT increased. The coefficient of heat extraction for the LBHE was 11–29% larger than that of DUT when used for 6–12 hours per day. These results indicate that the LBHE heat exchange performs more efficiently than that of DUT under simulated conditions.
