ASHRAE Journal - September 2012 - 29

90 Temperature (°F) and Hertz LWT EWT

95 90 85 80 75 70 65 60 55 12a 6a 12p 6p 12a LWT EWT Hz

Temperature (°F)

85

80

75

70 12a

6a

12p Sept. 2, 2010 (Installed 2006)

6p

12a

Sept. 7, 2010 (Installed 2003)

Ground loop entering and leaving water temperature; ENERGY STAR rating = 93; outdoor temperature high = 97°F; southeast Tennessee elementary school, Lbore = 197 ft/ton; tgrn = 61°F.

Entering and leaving water temperature, VSD frequency; ENERGY STAR rating = 83; outdoor air temperature high = 93°F; northwest Georgia elementary school, Lbore = 214 ft/ton; tgrn = 60°F.

Figure 1: Hot day loop temperatures for 70,000 ft2 (6500 m2) Tennessee elementary school. Contributing factors included high capacity ventilation air equipment, low thermal conductivity bore grout, small vertical bore separation distance, and low indoor temperature set points. Although most of the sites had large cooling mode requirements compared to those in heating, no significant increases in long-term temperature rise were noted.

Figure 2: Hot day loop temperatures and VSD speed for 85,000 ft2 (7900 m2) Georgia school. (1000 kW) heat pump system. Complete dimensions for the ground loop were not available but a nearby school built in 2002 with an identical footprint consisted of vertical 1¼ in. (30 mm) U-tubes installed at 214 ft/ton (18.5 m/kW) using an unknown grout. Classrooms are served by heat pumps connected to individual loops while a central loop with two 10 hp (7.5 kW) pumps with VSDs serve other areas of the school. Ventilation air is provided by multiple energy recovery units (ERUs) with a total rated flow of 15,850 cfm (7500 L/s). The ERUs are supplemented by water-to-water heat pumps connected to the central ground loop. Figure 2 indicates the core building ground loop is operating as intended with the leaving temperatures remaining below 83°F (28°C) on a day when the high OAT was 93°F (34°C). The differential temperatures during this near peak load day indicate the pump is near correct size but part-load values suggest the VSD is not operating as intended. This is substantiated by the constant drive speed of 60 Hz shown in Figure 2.

Mild Loops
A GSHP system retrofit was installed in 2006 in a three-story 70,000 ft2 (6500 m2) southeast Tennessee elementary school built in 1929. The ground loop consists of 96 vertical, 1 in. (25 mm) high density polyethylene (HDPE) U-tubes, 300 ft (91 m) in depth with a thermally enhanced cement grout placed in the bore annulus. The loop is connected to a 146 ton (510 kW) heat pump system that is supplemented with a 25 ton (88 kW) energy recovery unit (ERU). A 30 hp (22 kW) pump with a variable speed drive (VSD) provides circulation through the unitary heat pumps and loop field. A 30 hp (22 kW) backup pump is also available. A ground loop previously installed in a nearby middle school experienced elevated ground loop temperatures. Therefore, the ground loop design length for this school was increased by 50%, the bore separation distance was increased from 15 ft to 20 ft (5 m to 6 m), and a fluid cooler was added. Figure 1 indicates this ground loop design was successful since leaving water temperatures (LWTs) are below 80°F (27°C) and entering water temperatures (EWTs) are below 85°F (29°C) on a day that exceeded the design outdoor air temperature (OAT). The 4°F (2°C) differential loop temperature at full load indicates the pump is delivering over twice the optimal flow rate and the pump motor VSD is not properly functioning as differential temperatures are small at part load. The addition of the fluid cooler appears to have been unnecessary since tubes ruptured during the first winter of operation and were only recently repaired. Loop temperatures were recorded for an 85,000 ft2 (7900 2) elementary school constructed in 2003 with a 287 ton m
September 2012

Warm Loops
Ground loop temperatures were recorded in a four-story 78,000 ft2 (7200 m2) senior apartment building. A 125 ton (440 kW) ground source heat pump system is connected to a 130 bore ground loop with 1 in. (25 mm) diameter HDPE U-tubes 320 ft (97 m) in depth. A total of 50 two-bedroom apartments are served by heat pumps located in interior closets placed on platforms above the water heaters. Additional heat pumps serve common areas and two constant speed 25 hp (19 kW) pumps are alternated to provide continuous, constant flow circulation. Figure 3 indicates the 11-year-old system is performing well with 83.5°F to 85.5°F (29°C to 30°C) ground loop LWTs during a day when the high OAT was 96°F (36°C). The low differential loop temperature of 4°F (2.2°C) at full load indicates the pump is delivering over twice optimal flow rate. The local ground temperature is relatively high but extended loop lengths resulted in good loop temperatures and ENERGY STAR rating.
ASHRAE Journal 29
ASHRAE Journal - September 2012

Table of Contents for the Digital Edition of ASHRAE Journal - September 2012
