Background/Objectives

In the northeastern United States, ground-source heat exchange (GSHE) systems are a key component of electrification strategies for decarbonization programs.  Many municipalities and universities have ambitious decarbonization goals that benefit from the highly efficient GSHE systems.  Their role in replacing fossil-fuel based heating systems is particularly important. Drilling costs for vertical borehole systems make GSHE capital-intensive; accordingly, system designs must be carefully tailored to the target heating and cooling loads to avoid costly over-design of the GSHE bore-fields.  Bore-field modeling relies on ground thermal properties (derived from site-specific testing) and heating and cooling load profiles, adjusting the GSHE loads to maintain system operating temperatures above freezing but below 95 degrees Fahrenheit (above which heat pump operational efficiency declines significantly).  It is rare that operating data are available from commercial-scale GSHE systems to enable a detailed evaluation of how bore-field modeling predictions compare with operating data.  This study describes the use of operating data to assess the accuracy of model-predicted system operating temperatures over multiple heating and cooling seasons. 

Approach/Activities

Hourly operating data were compiled from a closed-loop GSHE bore-field to estimate heating and cooling loads over years of operation.  These loads and the system operating temperatures were plotted against outside air temperatures to assess seasonal effects on temperatures of the circulating water.  The calculated loads were modeled using standard industry software, and the model-predicted operating temperatures were compared to the actual temperatures derived from a building management system (BMS). 

Results/Lessons Learned

GSHE operating temperatures vary seasonally with heat rejection to the bore-field in the summer, and heat extraction in winter.  Long-term temperature “creep” may occur in systems where the heating-cooling load profile is imbalanced.  A critical design objective is to develop  GSHE loading profiles that maintain acceptable operating temperatures over long-term operation (typically 25 years).  Limited thermal-property data are typically available to support the design due to the expense of test well drilling, and natural variations in geology and formation mineralogy lead to variations in thermal properties that may be detrimental or beneficial to system performance.  The study demonstrates that system operating data can be compared to model-predicted performance, enabling critical review and re-calibration of GSHE bore-field models.  The outcome of this approach is to prevent costly over-design of future GSHE systems at a given location.