論文
地質調査孔を用いたケーブル方式熱応答試験の有効性実証
2023 年 45 巻 2 号 p. 103-119
詳細
2023 年 45 巻 2 号 p. 103-119
In this study, for the purpose of demonstrating the effectiveness of thermal conductivity profiling using heating cable and multi-point temperature sensor (TCP using heating cable) inserted into the geological boreholes in designing ground-source heat pump (GSHP) systems, TCPs using heating cable at a depth of 50 m were conducted at 47 sites in Fukushima Prefecture, Japan. After conduction of TCPs using heating cable at 27 sites, a 50 m-deep borehole heat exchanger (BHE) was installed at the same locations and conventional TRT and TCP using optical fiber sensors inserted into the U-tubes of BHEs (TCP using heat transfer medium) were conducted.
The temperature profiles and apparent thermal conductivity (λ) values of both TCPs were consistent each other at many sites. On the other hand, at a few sites, several range of λ values of the TCP using heat transfer medium were extremely higher than that of the TCP using heating cable. The same section of temperature profiles of the TCP using heat transfer medium showed that the temperature recovered rapidly, suggesting that crossflow may be occurring in the annulus area in the BHEs. Since the TCP using heating cable can be tested in a borehole with a smaller annulus area than the BHEs, it was also confirmed that the risk (e.g., crossflow) of ground modification on the estimation of λ value can be minimized.
The 2,187 λ values by depth for TCPs using heating cable obtained at 47 sites were classified by geology, and their statistics for each geology were obtained. Comparison of the median λ values by geology with representative values of effective thermal conductivity in the literature was consistent for many geologies.
In order to use the λ values estimated from TCP using heating cable for the design of GSHP systems, the averaging method of λ values by depth for TCP using heating cable was investigated: the arithmetic mean calculated after correcting λ value over 4 W/(m·K) to 4, and the arithmetic mean values calculated after excluding λ values above 5 W/(m·K) were consistent with the λ value of the conventional TRT at the same site, respectively. However, since the section of borehole with large λ value has high heat exchange capacity, it is considered more suitable to use the arithmetic mean λ value correcting large λ values for the design rather than excluding the values.
