THE EFFECT OF GROUNDWATER FLOW ON THE DESIGN OF VERTICAL GEOTHERMAL HEAT PUMP GROUND LOOPS

Geothermal heat pumps (GHP) are one of the cleanest sources of energy for heating and cooling systems used in buildings and residential homes. The increased interest in the use of GHPs warrants an improvement in understanding how the local geological conditions affect the design of these systems. Typical closed loop GHP designs currently do not account for the influence of groundwater on the portions of the system (e.g. boreholes) extending beneath the water table. Our analysis utilizes the semi-analytical solution of the heat flow equation with groundwater flow compared to the commonly used design software (GLHEPRO) that neglects groundwater flow.

The analysis uses numerical integration to compute thermal response from the semi-analytical solutions of Molina-Giraldo et al (2011). The sensitivity of the infinite line source model (assuming a fully penetrating borehole) is evaluated for constant loading rates, as this is the most commonly used model configuration for thermal response tests. The finite line source model (accounting for partially penetrating boreholes) is evaluated for GHP ground loop design with a dynamic loading rate, using the method of convolution. The main parameters of interest in the sensitivity analysis are the subsurface characteristics of thermal conductivity, groundwater flow, and borehole resistance. A dynamic heating (and cooling) load of a typical 2,000 square foot single-family New England residence and the commonly used design constraint of a minimum entering water temperature of 32 degrees Fahrenheit are adopted as a baseline case study. Results of the analysis help to illustrate the effects of uncertainty in subsurface conditions on GHP ground loop design and the potential benefits of site characterization.

The results of these modeling efforts are expected to enhance the understanding of the GHP system performance and lead to a more efficient design for residential systems in New England, where shallow groundwater depths are common.