Abstract

A practical method for designing stand-alone and hybrid geothermal heat pump (GHP) systems that use closed-loop, earth heat exchangers installed in vertical boreholes is presented. The design difficulty with hybrid GHP systems is inherently an optimization problem that is best solved with a computer-based system simulation method. Many parameters can be optimized and there is no unique expression of the objective function. In this work the optimization problem is defined as balancing the annual thermal loads on the ground by minimizing the borehole heat exchanger length and supplemental equipment size. The supplemental equipment examined in this research work has been limited to flat plate solar collectors in heating-dominated climates and direct-contact evaporative cooling towers in cooling-dominated climates.

The design method for GHP systems was developed from results of 153 detailed computer simulations. Three dimensionless groups containing key GHP design parameters were identified using the Buckingham Pi Theorem, and correlated with a fitted surface equation. The system simulations utilized an improved model of a vertical borehole earth heat exchanger developed for this work, which employs the finite element method to calculate the one-dimensional, transient heat transfer in the borehole grout material. The U-tube heat exchanger is modeled with an equivalent diameter approximation that was developed in this work, and the transient effects of the thermal mass of the heat exchange fluid are calculated using finite differencing. The improved model results were compared to two analytical solutions and to field-measured data.

With typical design parameters available to a designer, the design method developed here can be used to estimate the total ground loop length for stand-alone GHP systems, along with the quantity of annual energy required to balance the annual ground loads. With additional input parameters also readily available to designers, the area of a solar collector array or the capacity of a cooling tower can be calculated, along with the reduced borehole heat exchanger length. Solar collector array area is calculated using the utilizability method, and cooling tower capacity is calculated using an annual equivalent full load hour concept.