Thermogeologic performance of a large-scale, district geoexchange system in southeast Pennsylvania

Geoexchange systems utilize the heat capacity of the ground to provide efficient heating and cooling of buildings. West Chester University is developing a 16 MW district geoexchange system in southeast Pennsylvania. The system currently includes 529 borehole heat exchangers installed in fractured gneiss, with an anticipated 1400 borehole heat exchangers when complete. Borefield temperature, heat flux, and electric demand were recorded at 5 min intervals. Mean annual borefield temperature increased 2.1 degrees C yr (super -1) between 2011 and 2014, resulting from unbalanced cooling demand from high-occupancy buildings. The greatest recorded daily mean borefield temperature was 34.4 degrees C, which is significantly greater than ambient ground temperature of 12.8 degrees C and close to the maximum efficient design temperature of 41 degrees C for geothermal heat pumps. West Chester University is mitigating this unsustainable increase in temperature by installing a 738 kW cooling tower and heated sidewalks for snow removal. Inverse modeling of borefield temperature revealed an effective thermal conductivity between 1.3 and 1.4 W m (super -1) degrees C (super -1) , which is significantly less than the 2.9 W m (super -1) degrees C (super -1) of the formation, indicating that heat exchange is limited by borehole construction. Despite this performance issue, the system is operating with a coefficient of performance of 3.4 and 4.9 for heating and cooling, respectively. We conclude that thermogeologic investigation of borefield response could lead to significant improvements in efficiency and reduction in the number of wells required to maintain system performance.