Development of Thermal Response Test Device with Automatic Control System

Abstract

In ground source heat pump (GSHP) systems, it is important to carry out thermal response tests (TRTs) and estimate the thermal conductivity of the ground with good accuracy for the optimum design of the system. In conventional TRTs, the circulating medium is heated by heaters of rated output, and the flow rate of the medium is controlled by valves. It is important of maintain a constant heat load in TRTs, but the constant heat load is difficult to achieve it if there is a major change in the ambient temperature, in the flow rate or in the supply voltage. Hence, in this study we developed TRT equipment that automatically controls the heat load on ground heat exchangers to improve the accuracy of TRTs. In the device, we use the PLC (Programmed Logic Controller) for monitoring the heat load and for controlling the system, and adjust the thermal output of the heater to designated values. We carried out two types of TRTs, one with controlled output of the heater with PLC and the other fixed output without controlling, in an experimental borehole of 100 m deep in the campus of Akita University. In the operation tests, we gave the TRT device three types of disturbances, i.e., 1) by increasing the heat loss from the TRT equipment, 2) by decreasing the flow rate of heat medium or 3) by reducing the heat medium temperature using ice. Firstly, we examined the effect of the disturbance and found that the influence was insignificant win the first two cases. On the other hand, the disturbance was clearly observed when the temperature of heat medium was reduced. Then, we examined the performance of the developed TRT equipment by comparing the change of heat load with or without the use of PLC by giving a disturbance on the heat medium temperature. As a result, the output of the heater increased immediately after the occurrence of a disturbance and the heat load recovered in a short time when the PLC was applied. This reduced the loss of heat load by 60% compared to the case without control. The results shows that the developed device is expected improve the accuracy in the estimation of soil thermal properties, which will contribute the optimum design of GSHP systems.