For the evaluation of the heat exchange capacity of horizontal Ground Heat Exchangers (GHEs), field tests were carried out in a Geothermal Heat Pump (GeoHP) system in Fukuoka City, Japan. Slinky-coils were employed as the type of horizontal GHEs considering the limited land space for installing GHEs in most of the GeoHP systems in Japan. Two types of installations of Slinky-coils were examined, namely, horizontal and vertical settings in trenches of 1.5 m deep excavated in the shallow ground. In the cases of horizontal settings, three types of pitches of loops, 0.4 m, 0.6 m and 0.8 m, were tested to investigate the influences of loop pitches on heat exchange performances. The GHEs were connected to a heat pump and a circulation pump to be completed as a GeoHP system. Using the GHEs, thermal response tests and long-term air-conditioning tests were carried out from 2008 to 2010 for collecting detailed operation data and ground temperature data. The obtained field data were interpreted to evaluate the heat exchange capacity of the horizontal GHEs and to compare the heat exchange performances with those of vertical GHEs. The field tests showed that horizontal installation of Slinky-coils results in superior performance to vertical installation in terms of energy efficiency, due to the larger distance to the land surface. Also, the strong influence of the loop pitches on heat exchange capacities was clarified in all tests. Finally, the heat medium temperatures measured in the air-conditioning test showed that horizontally-installed Slinky-coils with 0.4 m pitch have comparable heat exchange capacity to a vertical U-tube GHE drilled in a formation of high thermal conductivity (=2.76 W/(mK)). This suggests that Slinky-coil GHE could be a cost-effective type of GHE considering its low installation costs.
Ground-coupled heat pump (GCHP) systems are considered to be one of the most energy efficient air-conditioning systems. In Japan, however, use of the GCHPs is still limited because of a lack of information on the advantages offered by these systems and because of their high initial costs. The accurate estimation of heat exchange rates, which can minimize the initial cost of the GCHP systems, is therefore of crucial importance to enhance the distribution of the system. The geological condition and groundwater flow strongly affect the heat exchange rates of ground heat exchangers (GHEs). These rates can be maximized and the installation costs of the GCHP systems can be reduced by developing suitability maps based on local hydrogeological information. Such maps were generated for the Fukui Plain, Hokuriku District, using field-survey data and a numerical modeling study. First, a field-wide groundwater and heat transport model was constructed for the area and the results were matched against measured groundwater levels and vertical temperature profiles. Four kinds of thematic maps were prepared using the results from numerical model and hydrogeological information. Finally, suitability maps for GCHP system were developed using index overlay model with thematic maps. Suitability maps indicated that suitabilities for GCHP system in the Fukui Plain are strongly correlated with the depth of groundwater table and the ratio of sand-gravel in the formation.
In order to investigate the optimum locations for the installation of ground-coupled heat pump system, heat exchange rate maps in the Fukui Plain were developed. For the estimation of the heat exchange performance at difference location in the plain, a single ground heat exchanger (GHE) model was constructed. The results of the field-wide groundwater flow-heat transfer modeling by Uchida et al. (2010), such as local hydraulic head and the distribution of subsurface temperature, were used as the boundary and initial conditions of the single GHE model. And then, the heat exchange rates in the cases of the snow melting and the air conditioning were estimated at 13 locations in the plain. The developed ground heat exchange rate maps using the results from the single GHE models were compared with the geological, hydrological and thermal information of the plain. As the result, the geological information (the rate of sand gravel layer thickness) was successfully linked with the ground heat exchange rate maps.
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