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Long-Term Commercial GSHP Performance: Part 3: Loop Temperatures

©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 9, September 2012.

By Steve Kavanaugh, Ph.D., Fellow ASHRAE; and Josh Kavanaugh, Student Member ASHRAE

About the Author
Steve Kavanaugh, Ph.D., is a professor emeritus of mechanical engineering and Josh Kavanaugh is a post-graduate student in the mechanical engineering department at the University of Alabama, Tuscaloosa, Ala.

Ground loop temperatures have a significant impact on system performance. Systems with maximum average ground loop temperatures below 90°F (32°C) had an average ENERGY STAR rating of 92 while those with temperatures above 95°F (35°C) had an average rating of 53. This article is the third in a series that summarizes a data collection and analysis project to identify common characteristics of successful ground source heat pump (GSHP) systems.

In addition to ground loop temperatures, other data include differential loop temperatures, ground loop size, undisturbed ground temperature (tgrn), thermal conductivity (kgrn), pump sizes and variable speed drive (VSD) operation. Several ground loop temperature plots will be presented that will be sorted by temperature range:

  • Mild Loops: Maximum Leaving Water Temperature (LWT) < 85°F (29°C);
  • Warm Loops: 85°F (29°C) > Maximum LWT < 95°F (35°C); and
  • Hot Loops: Maximum LWT > 95°F (35°C).

Results of thermal property tests were rarely available, especially at the older sites. If no tests were performed, the undisturbed deep ground temperature (tgrn) values for tests performed at nearby sites or water well logs were used. Design loop calculations were not available at any of the sites and, in several cases no ground loop design details were included in the drawings. So, the comparison of loop performance results with design intent was not possible.

A few of the newer sites had functioning energy management systems that could provide monitored ground loop temperature data. Ground loop and air temperature measurements at most sites were made with multichannel data loggers. They were installed at each site for three to five week periods during the site visits when building and GSHP system information was collected.

Results indicate the primary reasons for elevated loop temperatures were insufficient heat exchanger bore length. 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.

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 m2) elementary school constructed in 2003 with a 287 ton (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.

 

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.

 

Citation: ASHRAE Journal, vol. 54, no. 9, September 2012

Read the Full Article

 Figure 1Figure 2