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

By
Martin F. Helmke
Martin F. Helmke
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19382, USA
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Jacqueline A. Wilson
Jacqueline A. Wilson
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19382, USA
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Denise C. Gatlin
Denise C. Gatlin
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19382, USA
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Kirsten O. Moore
Kirsten O. Moore
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19382, USA
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Published:
March 01, 2016

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 °C yr−1 between 2011 and 2014, resulting from unbalanced cooling demand from high-occupancy buildings. The greatest recorded daily mean borefield temperature was 34.4 °C, which is significantly greater than ambient ground temperature of 12.8 °C and close to the maximum efficient design temperature of 41 °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−1 °C−1, which is significantly less than the 2.9 W m−1 °C−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.

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GSA Special Papers

Geothermal Energy: An Important Resource

Gordon R. Osinski
Gordon R. Osinski
Centre for Planetary Science and Exploration, Departments of Earth Sciences and Physics and Astronomy, University of Western Ontario, 1151 Richmond St., London, ON N6A 3K7, Canada
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David A. Kring
David A. Kring
Center for Lunar Science and Exploration, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA, and National Aeronautics and Space Administration (NASA) Lunar Science Institute, and NASA Solar System Exploration Research Virtual Institute
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Geological Society of America
Volume
519
ISBN print:
9780813725192
Publication date:
March 01, 2016

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