Heat transfer analysis of vertical U-tube heat exchangers in a multiple borehole field for ground source heat pump systems.

摘要

In the United States, a large amount of energy is used for space conditioning, which accounts for about 30% of annual energy consumption. Ground source heat pump (GSHP) systems are one of the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available today.; Vertical U-tube heat exchangers are the most widely used heat exchangers for large GSHP systems. U-tube heat exchangers experience thermal interference (or thermal short circuiting) between the two adjacent legs due to asymmetric boundary conditions in a non-homogeneous medium. Large GSHP systems may use dozens of U-tube heat exchangers in a multiple borehole field where thermal interference from adjacent boreholes affects long-term performance.; A finite element numerical model was used to analyze various borehole configurations, field geometric configurations and to make parametric analyses with source temperature, nonhomogeneous properties, borehole size, soil thermal conductivity, and separation distance between boreholes.; In-situ experiments were conducted to determine down hole soil properties, specifically effective thermal conductivity. Numerical models were used to simulate the ground heat transfer around the U-tube heat exchangers for both continuous and cyclical operations. Comparisons were made between the theoretical predictions and the experimental results. Numerical simulations were also performed to evaluate the ground heat buildup for long-term operation and the effect of an imbalance between heating and. cooling loads.; It was determined that the equivalent diameter of a single U-tube heat exchanger, useful for cylindrical source models, varies with pipe spacing. An insulating lamina between the two legs of the U-tube in the borehole had minimal impact on thermal short-circuiting. The thermal conductivity of the fill material in the borehole annulus made as much as 20% difference in heat transfer rate. Boreholes in a field could be characterized by their exposure to the far field conditions, grouped as either corner, edge, or interior boreholes. The thermal performance for both steady and cyclical operating conditions could be accurately modeled using the finite element software.