Coupling ground-source heat pumps with heat storage, borehole heat exchangers

Researchers from China have created an optimization model for medium-depth ground source heat pumps (MD-GSHPs), using an improved chaos particle swarm optimization (CPSO) algorithm. They tested the algorithm in the simulation of both an MD-GSHP system coupled with a heat storage tank and an MD-GSHP with a shallow borehole heat exchanger (BHE).

Particle swarm is an optimization technique inspired by the way birds and fish swarm, adjusting its performance in a way that will be optimal both for one particle and the entire swarm. “In CPSO algorithm, the initialization of particle positions and velocities is performed using a chaotic sequence,” the academics explained.

For the optimization task, the scientists simulated a 3,000 m2 residential building in the Chinese city of Xi'an. The heating temperature was set to 20 C, during the heating season of November 15 to March 15. A building model, including housing parameters, was constructed using the TRNSYS software.

Describing the MD-GSHP with a shallow borehole heat exchanger, the scientists said it contains a medium-depth BHE, followed by a heat pump connected to a heat exchanger, which is, in turn, attached to the shallow BHE. The water moves from the shallow BHE to a second heat pump, spreading heat to the users from there.

“When the system's heat supply exceeds the user's heat load, the heat storage valve opens,” the group explained. “The shallow BHE enters the heat storage mode, where a portion of the hot water generated by HP is directed to the user side, while the remaining portion passes through the heat exchanger to store heat in the shallow BHE. Conversely, when the system's heat supply is lower than the user's heat load, the heat release valve opens. The shallow BHE enters the heat release mode, where all the hot water produced by HP flows to the user side.”

As for the system coupled with a storage tank, it contains a medium-depth BHE, followed by a storage tank that is then connected to the end user. In this case, if capacity exceeds the user's load, the heat is stored in the tank, to be released afterward when the heat is deficient. The modeling of both systems took “comprehensive consideration of all parameters that influence heat transfer performance, including factors like contact thermal resistance and the impact of temperature on the physical properties of water.”

Three different objectives were requested from the optimization model for both systems – one for optimized heating cost, another for the optimized coefficient of performance (COP), and the last one for the geothermal energy utilization coefficient. Furthermore, the system with the heat storage tank had an additional objective, combining both optimal COP and heating costs.

According to the scientists, when the objective function was heating costs, the heating cost per m2 was CNY0.0997 ($0.014). That is a decrease of 10.2% compared to the unoptimized cost, which was CNY0.1108 per m2. Unoptimized, the system's COP was 5.7358, which grew by 10.43% to 6.3341 after optimization. As for the geothermal energy, it grew by 2%, from 0.8334 to 0.8501. “The objective function of minimizing heating cost exhibits a similar trend to the heat load curve,” they emphasized.

Moving to the system coupled with a heat storage tank, when the objective function was heating costs, the heating cost per m2 was CNY0.0996. That is a decrease of 12.82% compared to the unoptimized cost, which was CNY0.1108 per m2. Unoptimized, the system's COP was 5.7358, which grew by 10.35% to 6.3296 after optimization. As for the geothermal energy, it increased by 1.96%, from 0.8334 to 0.8497.

“Comprehensive consideration of heating cost and system COP suggested a favorable weight of 2.8 for continuous system operation,” the researchers said. “In conclusion, when prioritizing heating cost, the system coupled with heat storage tank yields greater economic benefits compared to the system coupled with shallow BHE.”

The results of the study were presented in “Analysis and optimization of a medium-depth ground source heat pump heating systems with heat storage and borehole heat exchangers,” published in Energy Reports. The research was conducted by scientists from China's State Grid Shaanxi Electric Power Research Institute.