Effects of Variable Borehole Thermal Resistance and Heat Pump Behavior on Sizing and Energy Consumption of a Swedish Residential Ground-Source Heat Pump System
Ground-source heat pump systems are commonly used to provide space heating and domestic hot water in Swedish residential buildings. Design and energy analysis of these systems are commonly done assuming constant borehole thermal resistance and fixed heat pump COP, ignoring the impact of backup electric resistance heating. Since most Swedish boreholes are groundwater filled, the borehole thermal resistance tends to increase when the groundwater in the borehole approaches the maximum density point of water, about 4°C. Under cold conditions, the heat carrier fluid inside the U-tube may also drop into laminar flow, increasing the thermal resistance of the boreholes. Modern water-source heat pumps with variable speed compressors and circulating pumps may have COP and, hence, heat extraction rates that vary significantly over the year. Backup electric resistance heating may also affect sizing of the borehole heat exchanger. It is therefore expected that borehole sizing, heat pump energy consumption, and economics may be affected by the level of detail used in modeling the system. This paper describes improvement in borehole heat exchanger and heat pump modeling and examines the effects on sizing, prediction of energy use, and economics, for a house in Sweden.