Ground heat exchanger performance is a critical factor in ground source heat pump (GSHP) system success. The ground heat exchanger type for all but one of the systems surveyed were vertical high-density polyethylene (HDPE) single U-tubes. Bore length (Lb) is used as a primary indicator, although there are several other factors that affect performance including ground thermal properties (temperature, conductivity, and diffusivity), vertical bore separation, conductivity of the annular grout/fill, integrity of the grout/fill placement, and heat exchanger type. Some scatter in the results is expected since these characteristics varied from site to site and was often not available.
The impact of most of these variables is complex and often uncertain. However, the variation of bore length to approach temperature (difference between the average loop temperature and the ground temperature) is more easily normalized.
Cooling performance is a strong function of ground loop leaving water temperature (LWT) and entering water temperature (EWT). Therefore, the required cooling mode bore length to provide high efficiency in a location with a lower ground temperature will tend to be less than the required length for a warmer location. To better compare optimum ground loop lengths for a variety of locations, the trend between installed bore length and performance is normalized for ground temperature. The adjustment is based on the average ground temperature (tg[avg] = 63 °F [17 °C]) and the average maximum loop temperature ([LWT+EWT]/2 ≈ 90 °F [32 °C]) at the sites in the project survey.
Lb/ton (Normalized) = Lb/ton ×
(90 °F – tg)/[90 °F – tg(avg)]
A ground loop installed at 250 ft/ton (22 m/kWT) of bore would correspond to a normalized length of 185 ft/ton (16 m/kWT) for a ground temperature of 70 °F (21 °C) while 170 ft/ton (15 m/kWT) of bore results in a normalized length of 201 ft/ton (17 m/kWT) for a ground temperature of 58 °F (14 °C). The design bore lengths for the systems monitored during this project were all determined by the cooling load even though some sites had significant heating requirements. Recall the ground loop in cooling must transfer the building load plus the compressor heat, while the heat transfer rate in heating is the heating load minus the compressor heat. If a similar project were conducted in climates where the heating requirement determined bore lengths, normalization based on the winter LWT and EWTs would be more appropriate.
Figure 1 shows the trend for an ENERGY STAR rating to normalized bore length. Systems with bore lengths near 150 ft/ton (13 m/kWT) tend to have an ENERGY STAR rating near 20 while those with normalized bore lengths of 200 ft/ton (17 m/kWT) are more likely to have a rating above 90. A cluster of sites with ENERGY STAR ratings above 90 have normalized bore lengths between 200 and 225 ft/ton (17 to 20 m/kWT). The three sites with the longest bore lengths had ENERGY STAR ratings below 90, indicating that bore length is important, but other characteristics also affect performance results.
Note that the reported values are based on tons of installed capacity rather than building load. The sum of the installed capacity for equipment in each zone is typically 10% to 25% greater than the load the building places on the ground loop due to load diversity and also because equipment is available in capacities of fixed increments that cannot match loads precisely.
As expected, lower building electrical demand for cooling and heating results when bore lengths are increased as shown in Figure 2. The demand vs. bore length slopes are reversed compared to Figure 1 since lower demand tends to reduce energy use and result in a higher ENERGY STAR rating. The cooling data is scattered. A few buildings with lengths in excess of 200 ft/ton (17 m/kWT) had only average cooling demand and one system with a 165 ft/ton (14 m/kWT) length had a low demand. Figure 2 also indicates that 90% of the buildings with normalized bore lengths greater than 200 ft/ton (17 m/kWT) have heating demands less than 4.0 W/ft2 (43 W/m2).
Citation: ASHRAE Journal, vol. 54, no. 7, July 2012