Low-enthalpy geothermal resource evaluation through simulations of optimal geothermal heat pump systems

Abstract

Simulation models for evaluating utilization of low-enthalpy geothermal resources through the optimally designed geothermal heat pump (GHP) system is suggested. Firstly, a numerical model for the simulation of temperature changes in a borehole heat exchanger (BHE) with fluid circulating through U-tubes is developed. The model can calculate the thermal energy transferred from heat pumps to BHEs while considering the nonlinear relationship between temperature of the circulating fluid and the thermal energy. The use of the developed model enables also the design of a GHP system with the view of pursuing efficiency and financial benefit. The developed model is validated by comparing two measurement datasets with their respective simulation results. In addition, it is used to analyze the sensitivities of design parameters that can affect the performance of the closed-loop GHP system. The most sensitive parameters on the system are the thermal conductivity of the ground and the Darcian groundwater velocity considering acceptable distribution range in the realm of nature. Maximum change of the circulating fluid temperature at the BHE outlet is about 4℃ when thermal conductivity of the ground changes from 2 W/mK to 5 W/mK and the Darcian groundwater velocity changes from 10-8 m/s to 10-6 m/s, respectively. The numerical evaluation of a real GHP system with 28 BHEs and 79 heat pumps involves consideration of the base case and modified cases. In all cases, the temperatures of the circulating fluid at the BHE inlet and outlet, heat pump efficiency, and the heating power and electric power of heat pumps are obtained. The most cost-effective system in this case is for there to be 4, 6, and 6 BHEs on the first, second, and third floors, respectively. The next version of the numerical simulator and grid generator is developed to consider multiple BHEs simultaneously. Thus, massively parallel computing procedures into the simulator are introduced to improve distributing memory requirements and computational efficiency for solving large simulation problems with a great number of grid-blocks. The new grid generator is designed to produce a simulation domain with multiple BHEs. The newly developed simulation model can consider thermal interactions among BHEs when the system is in operation and storing thermal energy in the ground after the operation period of the system. These two mechanisms should be considered in the evaluation of long-term performance of the BHE. The developed simulation model is tested for the performance improvement through parallelization. The computational efficiency of the developed simulation model is considerably increased in direct proportion to the number of the processors. The model is then applied to evaluate the performance of the KIGAM GHP system for a 25-year operation. The temperature of the ground in the vicinity of BHEs is gradually increased with time because of the imbalance of the injected/extracted thermal power to/from the ground during the cooling/heating seasons. It causes the decrease of the efficiency of the system during the cooling seasons for the long-term operation. Finally, a versatile simulation model is developed to simulate not only the vertical closed-loop GHP system, but also the standing column well and open-loop GHP systems. A method to generate an unstructured Voronoi grid for its use in simulations of geothermal heat pump systems is presented. A series of codes is developed to create Voronoi cell center points that are placed at specific positions for well- or pipe-shaped Voronoi grids, to generate a three-dimensional grid from generated Voronoi cell vertices, and to visualize the generated grid and simulation results by ParaView. AMESH program is used to calculate the x- and y-coordinates of the Voronoi cell vertices from the Voronoi cell center points. The developed series of codes can generate the desired form of the grid. The generated grid is tested with confidence through simulations of water production/injection from/to the various kinds of the geothermal wells.