Modelling, design, and control of hybrid ground source heat pump system coupled with district heating in an educational building complex

Ground source heat pumps (GSHPs) are an efficient solution for the decarbonization of building heating and cooling systems. They utilize borehole fields to extract heat from the ground for heating. The borehole fields can also supply free cooling energy for buildings in cold climates. Since buildings are heating-dominated in cold climates, conventional GSHPs face significant underground thermal imbalances. This often requires larger and more costly borehole fields to sustain high operating performance. An effective way to improve operational viability and financial sustainability is to integrate GSHPs with backup heating sources, such as district heating (DH), creating a hybrid GSHP system. This thesis investigated the modeling, design, and control of a hybrid GSHP system coupled with DH in an educational building complex based on simulation studies. The simplified modeling of an asymmetric-layout borehole field was validated using onsite brine temperature measurements. High-performing design methods for the hybrid GSHP system were explored by adjusting key design parameters, such as air handling unit (AHU) cooling water temperature level, indoor air temperature heating and cooling setpoints, GSHP design heating power, borehole number, and borehole depth. Regarding control, a cost-effective control strategy was developed to reduce system energy costs. The effects of power limitations, the input coefficient of performance (COP) value, and the control time horizon within the control algorithm were analyzed. Furthermore, a demand response control strategy was implemented to utilize the thermal storage of building thermal mass, investigating various DR control algorithms and parameters. Results indicate the simplified-geometry borehole field models can predict average inlet and outlet brine temperatures within a deviation of 1 °C from measured data and can also reduce computational time by up to 72% compared to the detailed models. Compared to the reference hybrid GSHP system design, increasing AHU cooling water temperature level and using lower indoor air temperature heating and cooling setpoints can raise the minimum outlet brine temperature from –6 °C to –3 °C over a 25-year lifetime, while also slightly improving the average heat pump COP in the last heating season. To ensure non-freezing boreholes, it is still necessary to increase the overall borehole length or reduce the GSHP design heating power. Compared to a GSHP-prioritized control strategy, the cost-effective control strategy, which incorporates power limitations and optimizes input COP values for control, achieves a 6.4% reduction in annual energy costs with hourly electricity and DH pricing. Implementing the DR control strategy to space heating enhances energy flexibilities for both electricity and DH networks without compromising indoor thermal comfort. Among studied DR control algorithms, the dual-price DR algorithm yields the highest cost savings. When it is combined with the cost-effective control, the hybrid GSHP system can achieve by up to a 10.8% reduction in annual energy costs with hourly electricity and DH pricing.