ABSTRACT A practical method for designing 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 appropriately sizing supplemental equipment. The supplemental equipment examined in this work has been limited to direct-contact evaporative cooling towers. The design method for GHP systems was developed from results of 91 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. With typical design parameters available to a practitioner@ 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 capacity of a cooling tower can be calculated@ along with the corresponding reduced borehole heat exchanger length. Cooling tower capacity is calculated using an annual equivalent full load hour concept.