An applied research initiated and coordinated by Oklahoma Gas and Electric Company (OG&E) demonstrated that the required borehole depth of vertical closed-loop ground heat exchangers (GHXs), which are predominantly used in ground source heat pump (GSHP) applications, can be reduced by up to 36% using alternative configurations.
In order to identify new technologies and techniques that can reduce the upfront costs of GSHP systems and thus make them more affordable to its customers, OG&E initiated this applied research project in 2011 in collaboration with the International Ground Source Heat Pump Association (IGSHPA), Oak Ridge National Laboratory (ORNL), and other stakeholders of the GSHP industry. A field test was conducted at ten nearly identical homes in a Habitat for Humanity community in Oklahoma City to study the performance of alternative configurations of GHX, which include eight different combinations of new heat exchanger designs, new grouting materials, and various drilling techniques.
ORNL analyzed the performance of the tested GHXs as well as the associated GSHP systems. In addition, ORNL assessed the impacts of these alternative configurations on the required borehole depth through computer simulations. Among the many informative observations derived from this study are the following findings:
- The GSHP systems using the new GHXs successfully maintained the room temperature at the setpoint specified by individual homeowners during the one-year test period (from October, 2011 through September 2012) except in one test home (unit 9), where the building cooling load exceeded the capacity of the installed 2-ton GSHP system.
- The GSHP system efficiency in heating season, valued with Coefficient of Performance (COP), ranged from 3.8 to 4.5 at the ten test homes and the differences in the COP were attributable to individual homeowners' thermostat settings; the system efficiency in cooling season, valued with Energy Efficiency Ratio (EER), varied widely from 9.3 to 18.7 among the test homes. Except in two homes (units 1 and 9), where the GHX was apparently undersized, the system EERs were all above 14.7 during the second hottest summer in the history of Oklahoma City. The discrepancy in system EERs was a result of the wide variation of the leaving fluid temperature from the GHXs during the cooling season.
- The monthly peak electricity demand of the installed 2-ton GSHP systems varied from 1.8 to 2.2 kW during the heating season and it varied from 1.6 to 2.6 kW during the cooling season. The variation in peak electricity demand are attributable to different operating conditions and the variable-speed circulation pump used in the GSHP systems, which were controlled to maintain a constant differential temperature across the GHX. The spike in summer peak electricity demand in unit 9 (2.6 kW) was due to the unexpected high internal heat gain in this particular home.
- The energy consumption of the tested GSHP systems varied significantly among these nearly identical test homes and were affected by many factors, including room temperature (especially in winter, when the room temperature significantly affected the heating load of the test homes), the fluid temperature of the GHXs, and the thermostatic control.
- Both the field tests and the computer simulations concurred that the double U-tube GHX requires 36% less borehole depth compared with a conventional single U-tube GHX while retaining same performance at given building load and ground condition [with a 1.85 Btu/(hr-ft-°F) thermal conductivity]. Other new GHXs being tested also show the potential to reduce the required borehole depth, although the reductions are smaller (20-30%). A larger reduction in required borehole depth can be expected at locations where the ground has better thermal conductivity, as shown in Fig. E1.