Scaling in Geothermal Heat Pump Systems

INTRODUCTION Water quality is a frequently overlooked issue in the application of geothermal heat pump (GHP) systems. When considered at all, is often viewed as a problem unique to open loop systems. In residential open loop applications water is supplied directly to the heat pump’s refrigerant-to-water heat exchanger. If the water has a tendency to be scale forming, fouling of the heat exchanger may occur. This fouling reduces the effectiveness of the heat exchanger and compromises the performance of the heat pump. Water quality is also a consideration for closed loop systems. In the closed loop system, the concern is not the main refrigerant to water heat exchanger but the desuperheater. Water is circulated through the desuperheater and back to the main hot water heater to provide a portion of the domestic hot water heating needs. Again scale formation in this heat exchanger will reduce the contribution of the desuperheater makes to the domestic hot water heating load. In large commercial systems, the groundwater is isolated from the building loop using a plate-andframe heat exchanger. This eliminates the potential for scaling in the heat pump units. In addition, it reduces the maximum temperature to which the groundwater is exposed, thus reducing scaling potential. In most cases, the formation of scale is a slow process occurring over months or years. As a result the impact of the reduced heat pump or desuperheater performance on the utility bill is gradual. This slow erosion of the savings the system would otherwise produce may be imperceptible to the system owner. The object of the work reported here was to identify areas of the country where water quality is such that scale may occur. With this information installers and system owners can plan for the regular maintenance that may be necessary to address the scaling and preserve system performance. In the residential sector, closed loop GHP’s tend to be installed in homes in the upper cost end of the market. Since these homes would likely be equipped with a water softener in hard water areas, the potential for scaling in these instances would be substantially eliminated.