PERFORMANCE OF A LARGE GEOEXCHANGE SYSTEM IN FRACTURED GNEISS OF SOUTHEAST PENNSYLVANIA

Geoexchange systems employ subsurface heat storage as a means of heating and cooling buildings. West Chester University, located on the Piedmont of Southeast Pennsylvania, operates a 440-well, closed-loop geoexchange system with plans to expand to 1,400 wells within a decade. The design of the system is unique because it connects residence halls and academic buildings to a central borefield, allowing heat to be shared between structures. This reduces the maximum required capacity (4.6 MW) by 20 percent and provides flexibility to balance district heating and cooling loads.

Critical to the success of any geoexchange system is a sound understanding of the geology. The WCU geoexchange system is comprised of 152-m deep wells installed within the Baltimore Gneiss. Lithologies of the Baltimore Formation include biotite-garnet gneiss, meta-granite, felsic gneiss, mafic gneiss, and meta-diabase of late-Proterozoic age. Groundwater flows predominantly through a surficial 10-m sequence of saprolite and a deeper network of discrete fractures.

Performance of the system was monitored by a large datalogger network and simulated by MODFLOW and MT3DMS to evaluate heat transport coupled with groundwater flow. Thermal tests reveal a ground thermal conductivity of 2.4 W/m K and a volumetric heat capacity of 2.1 MJ/m³ K. Mean ground temperature was 13 °C before the system was installed. During the first 2 years of operation the system supplied two large residence halls, which resulted in a mean ground temperature increase of 3.8 °C/year and annual amplitude of 10 °C. Ground temperature is currently 2.8 °C warmer in the center of the borefield than at the margins.

We conclude that sustainable geoexchange systems must be balanced to operate efficiently on the decades timescale. Adding more heat to the ground than is removed will result in an unsustainable long-term increase in temperature. In the Mid-Atlantic Region, residence halls require more annual cooling than heating due to the heat produced by high occupancy loads. Academic (classroom) buildings require net heating. Groundwater flow mitigates some temperature increase but this effect is temporary. Combining residence and academic buildings at a university provides a unique opportunity to engineer a balanced and sustainable system.