GEOTHERMAL SUSTAINABILITY AT WCU: PERFORMANCE OF A 350-WELL, DISTRICT GEOEXCHANGE SYSTEM IN SOUTHEASTERN PENNSYLVANIA

Geoexchange, or “geothermal”, systems employ subsurface heat storage as a means of heating and cooling buildings. West Chester University, located on the Piedmont of Southeastern Pennsylvania, operates a 350-well, closed-loop geoexchange system with plans to expand to 1,400 wells in the near future. The design of the system is unique because it connects 2 residence halls and 4 academic buildings to a centralized wellfield, which allows heat to be shared between structures. This reduces the maximum required capacity (4.6 MW; 1,300 tons) 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 composed of 500-ft deep wells installed within the Baltimore Gneiss. Thermal tests reveal a ground thermal conductivity of 1.37 W/m K and a volumetric heat capacity of 2.1 MJ/m³K. 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 an annual amplitude of 10 °C. Ground temperature is currently 2.8 °C warmer in the center of the wellfield than at the margins. The addition of four academic buildings in October 2012 resulted in a decrease in mean ground temperature, which may stabilize the system.

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 a long-term increase in temperature, which is unsustainable. 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. Combining residence and academic buildings at a university presents a unique opportunity to engineer a balanced and sustainable system.