Design and Life Cycle Cost of a Vertical Ground Source Heat Exchange System for the Smith College Field House
Committed to becoming a carbon neutral campus by 2030, Smith College is transitioning towards geothermal energy for campus heating and cooling. Energy consultants have been hired to conduct an economic and phasing analysis to prepare a district energy master plan. In conjunction with this planning effort, this undergraduate honors thesis designed and evaluated the life cycle cost of a vertical ground source heat pump system for the Field House as a pilot demonstration project in order to leverage a test bore hole that will be dug this coming summer of 2019. With an E2 Energy to Educate grant from Constellation, an Exelon Corporation, core samples will be taken at depths that have not been scientifically recorded in this region. This undergraduate honors thesis presents a framework to design a vertical geothermal heat pump system in conjunction with an economic analysis of building retrofits in order to guide future capital projects.
A building energy model of the Field House was constructed in Trace 700. A sensitivity analysis identified eight sensitive unknown design parameters, including wall construction, ventilation and infiltration rate, window, wall and floor u-factor and wall height. The model was validated with existing oil usage data. The calibrated model estimates a total annual energy consumption within 4% difference from the oil data.
With this model of building heating load, a ground-source heat pump (GSHP) was designed. The design included the calculation of five key parameters, namely the total and individual borehole flow rate, borehole thermal resistance, total borehole length, number of boreholes and the power of the water and heat pumps. Two methods of borehole length calculation were compared, and a final design was proposed that detailed three boreholes at 600 ft, with a flow rate of 2.4 gpm per well coupled with three heat pumps of 0.6 tons.
A life cycle cost analysis was conducted over a period of thirty years for four design options, including (1) the existing oil-based system, (2) a GSHP system, (3) a GSHP system with medium level building retrofit and (4) a GSHP system with deep level building retrofit. While remaining on oil requires the least cost over the next 30 years, that solution does not meet our carbon neutrality goals and offsets are not being considered as a viable path. As a result, the GSHP only option ranked the least among the three remaining options in terms of the total converted present worth at year 30, $285,000, closely followed by GSHP + Deep, which also reduced the annual heating demand by 28.9%. Economically, it is not worthwhile to retrofit a load bearing masonry building unless a deep retrofit is conducted.