Optimized use of ground collector thermal energy storage in a heat pump system with different ambient heat sources
The seasonal performance of heat pump systems can be increased by using ground-based heat sources like geothermal wells, horizontal ground collectors, or groundwater wells. For sites with restrictions on groundbased heat sources or limited property areas, a heat pump system with building-integrated heat sources is used to achieve high energy efficiency. A solar absorber integrated into the façade of a building regenerates a horizontal ground collector thermal energy storage located beneath the building, which serves as a heat source for the heat pump. The ground collector's area is limited by the size of the building's ground floor and therefore used as medium-term storage. To complement the contribution of the two building-integrated heat sources, an ambient air heat exchanger is integrated in the system. A simulation model of the system is used to optimize the use of available ambient heat sources throughout the entire heating season. Due to its position underneath the building, freezing of the underground heat storage must be avoided. Therefore, a variable-speed heat pump is used to operate with reduced heat extraction, enabling a deeper discharge of the storage while keeping the brine loop outlet temperature above freezing. Control strategy setups for optimal utilization of the heat sources, including a night setback of the heat supply, are developed. The proportion of the underground heat storage relative to the other ambient heat sources increases from 40 % for a single-speed heat pump to 52 % for a variable-speed heat pump and to 61 % when the night setback is applied. Through the increased utilization, less heat extraction from ambient air is required on cold winter days, resulting in an increase of the seasonal performance from 3.99 to 4.63 and 4.91, respectively. The heat pump system is compared with an air-source heat pump with a seasonal performance of 4.29.