Experiment on single well groundwater heat pump systems in different distances between pumping and injection screens

Abstract: As the global energy crises and environmental problems become more and more serious, ground source heat pump (GSHP) systems are perhaps the most widely used green HVAC system, with an estimated 1.1 million ground source heat pumps installed worldwide. These systems have become an important energy-saving and environment protection technology for use in residential and commercial buildings in China. These applications included two types of systems: closed-loop (ground-coupled) and open-loop. As one kind of semi-closed-loop systems, single well groundwater heat pump (SWGWHP) systems have become increasingly popular for use because of their economic advantages from lower installation cost, lower operating cost, and improved overall performance in regions with suitable geological conditions. In general, SWGWHP systems included three different variations, i.e. standing column well (SCW) system, pumping & recharging well (PRW) system, and pumping & recharging well filled with gravel (PRWFG) system. Compared with SCW system, the well pipe in PRW system and PRWFG system are divided into three parts by clapboards, i.e. production zone, seals zone and injection zone. In recent decades, considerable research efforts have been spent on SWGWHP systems, especially on SCW system. However, little attention has been focused on the PRW system and PRWFG system. For GSHP systems, sand tank experiment is one of the important methods of laboratory investigation for discussing the performance of geothermal heat exchanger, due to many parameters can be set and adjusted more easily and economically than that in in-situ experiments. In order to provide a framework for discussing the influence on distances between pumping and injection screens (DPI) in SWGWHP systems, a sand tank experiment system was designed and set up in Harbin Institute of Technology, China. In this study, we tested the temperature of outlet and inlet water, the aquifer temperature, and flow rate of outlet water. From the reasonable results obtained in these tests, the absorption and rejection quantities of SWGWHP systems were calculated. The results pointed out that with increasing the DPI, the outlet water temperature and heat transfer quantities of these three kinds of thermal wells can be improved significantly. When the conditions permit, the DPI should be increased greatly. Additionally, when the DPI increases the same value, the improvements of outlet water temperature are as follows: the SCW system is the best, followed by PRWFG system, and PRW system. It can be concluded that increasing the DPI is more beneficial to the thermal well with serious thermal breakthrough. Moreover, when the DPI is short, increasing the DPI can improve the temperature difference between outlet and inlet water and heat absorption quantities of PRW system significantly. When the DPI increases to a certain value, this improvement of thermal breakthrough is not significant. However, the PRWFG and SCW systems are different, i.e. increasing the DPI that benefits are always obvious. With the distance between the measuring point and thermal well increasing, the influence of thermal well on the measuring point is weaker. With the DPI increasing, the influence of the cold inlet water on the radial extent of the aquifer is weaker. The thermal influence scope of SCW system is the smallest one among the three types of thermal wells. Although there are some differences between the laboratory model tests and complexity of the practical geological conditions, the rules obtained from the experiments for the guidance of practical project still have great significance.