Development of a spiral type heat exchanger for ground source heat pump system

In Japan, net Zero Energy Buildings are crucial for reducing energy use and environmental load to realize a sustainable society. Ground-source heat pump systems are a key technology for reducing energy consumption by air conditioning systems. There are two types of ground-source heat pump systems: "closed loop type" and "open loop type". In general, open loop type ground-source heat pump systems have better relative performance than closed loop type systems. However, pumping up underground water is prohibited in urban areas of Japan to prevent the ground surface level from sinking. Therefore, closed loop type systems are used more extensively in Japan. The typical and conventional heat exchangers used for closed loop type heat pump systems are of the "U tube" or "double U tube" types. However, neither type has a high heat exchange capacity per unit length. Thus, in this study, a spiral-type heat exchanger for a ground-source heat pump system is developed. The aim of the heat exchanger is to perform intensive heat-exchange in the aquifer layer near the ground surface (ten to twenty meters in depth). To use the underground water flow in order to facilitate intensive heat exchange, the length of the heat exchanger is planned to be inserted between ten and twenty meters below the surface into the upper part of the aquifer. The diameter of the spiral-type heat exchanger is determined such that a borehole machine for piles can be used for settlement of heat exchangers to reduce the construction cost. The performance of the heat exchanger is simulated under various flow rates and soil conditions using the numerical simulator "TOUGH2/EOS1." Based on the simulation and construction cost study, the cost-effectiveness of the spiral-type exchanger is made clear. (C) 2016 The Authors. Published by Elsevier Ltd.