Uncertainties in the design of ground heat exchangers

Ground source heat pump (GSHP) systems have the potential to reduce the carbon dioxide footprint of buildings by providing them with sustainable heating and cooling energy from the ground at shallow depths. The financial viability of these systems depends on the adequate sizing of the ground heat exchangers (GHEs), which generally contribute the most to the overall capital costs of GSHP systems. Consideration of the uncertainties in the design of GHEs has the potential to improve GHE design by increasing the confidence in sizing GHEs for particular applications. This paper proposes a methodology for the estimation of the uncertainty in the design length of GHEs by considering the uncertainties involved with the selection of design parameters. By using the proposed methodology, a case study is presented where borehole GHEs are sized following a commonly applied design process recommended in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (Ashrae) handbook. The uncertainty in the resulting length of GHEs is estimated, and the sensitivity of this uncertainty to the uncertainties in individual design parameters is discussed. In addition, measures to reduce the length uncertainty are considered.