500 M-GRIDDED ESTIMATION OF REQUIRED LENGHS FOR A CLOSED-TYPE BOREHOLE HEAT EXCHANGER IN A GROUND-SOURCE HEAT PUMP SYSTEM

Abstract

 This study demonstrated to estimate required lengths for closed-type borehole heat exchangers in domestic ground-source heat pump systems on a regular 500 m grid over the land of Japan. The heating/cooling loads were assumed for a standard residence (120 m² in total area and 86 m² in heating area) with a constant room temperature. The heat transfer coefficients of the residence were different amon 7 climate areas, but constant within each area. The effective thermal conductivity of the ground was estimated as a probability-weighted average of soil/rock types through the indicator-kriging procedure with adjacent bolrehole data. In each 500-m grid of different climate and thermogeologic conditions, required borehole heat exchaneger lengths were determined at all grids except with no borehole data by three-years system simulations. This study assigned one necessary condition of circulating fluid temperatures and another target condition of seasonal performance factor. The target factor was 4.0, and the value means the efficiency of the ground-source heat pump system relative to conventional systems. As a result, the required length was dependent on the effective thermal conductivity at each location because the heating loads were variable less than 10 % in the same area. When the upscaling to a 10 km grid was carried out, the required lengths indicated a decreasing trend fron North to South according to the heating loads and the ground temperatures. The upscaling lengths were generally shorter than previous lengths, because this study performed the simulation mainly in the grids on low elevaltion plains including borehole data. This study also showed the averaged lengths in classes of climate areas and effective thermal conductivity as references for practical designs of ground-source heat pump systems.