ASHRAE Journal - June 2012 - 50

GSHP Series Overview
The Long-Term Commercial GSHP Performance series will summarize the results of a project that collected data from buildings heated and cooled by ground source heat pump systems. The buildings were primarily commercial or institutional and the ground heat exchangers were almost all closed-loop vertical designs. The age of the systems ranged from three to 23 years of operation and installation cost information for the newer buildings was included. Project Overview and Loop Circuit Type: This article will provide a description of the project and present a summary of energy performance of all buildings and the function of different types of loop circuits. Loop circuits are categorized by 1) central building loops, ground loops and pumps, 2) central one-pipe building loops, ground loops and pumps with individual pumps on each heat pump, 3) individual loops and pumps for each heat pump, and 4) common building and ground loops with individual pumps for each heat pump. The ENERGY STAR rating is used as the primary indicator of performance since it is based on measured source energy use and is normalized for building type, climate, primary loads, occupancy, schedules and other appropriate characteristics. Ground Loops, Pumps, Ventilation Air, and Controls: This article will present a summary of energy and demand performance of GSHPs as a function of several important characteristics. These include vertical ground heat exchanger dimensions, relative building and ground loop pump size, specified flow rate of the ventilation air system, and the general type of building control system. Ground Loop Temperatures: This article will provide a sum-

mary of energy and demand performance of GSHPs as a function of ground loop temperatures. The trends of temperatures to ground loop dimensions and differential temperature as a function of pump size will be shown. Data will be presented to address the issue of long-term ground loop temperature changes in applications with imbalanced cooling and heating loads. GSHP System Installation Costs: This article will provide a list of the installation costs for newer systems. Costs will be itemized by interior HVAC costs (74% of the total) and ground loop cost (26% of the total). The sampling is weighted toward one-pipe and individual loops since little information was made available for central and common loops. Occupant and Operator Satisfaction: This article will provide a summary of satisfaction levels of building occupants and the personnel that maintain and operate the systems. Occupants were provided rating surveys for their satisfaction levels with indoor temperature, air quality, acoustics, lighting, access to controls, and maintenance. Some of the many comments will be listed. Maintenance personnel were provided surveys and the opportunity to provide comments and recommendations. The rating results are insightful and comments were abundant. Characteristics of Quality GSHPs: This article will summarize the results of the project and highlight characteristics that tend to optimize energy use, installation cost, and occupant/operator satisfaction. A suggested portfolio format will be presented that is intended for engineering firms to follow that can demonstrate the quality and success of previous projects.

indicates GSHPs, in some cases, have been poorly designed and installed. Figure 2 (Page 49) indicates a general trend of improved ENERGY STAR ratings for newer GSHP systems. All of the surveyed sites installed in the last six years have ratings above 80 and most of them are above 90. However, the highest rated building has been operating for 10 years and three buildings with 15 years of operation have ENERGY STAR ratings above 90. The longest operating system (23 years) obtained an ENERGY STAR rating of 78 in spite of the fact that it has operated in a southern climate with vertical bore spacing less than 15 ft. (5 m). The lowest rated buildings have GSHP systems that have operated nine, 16, 18 and 12 years.

ENERGY STAR Ratings by Loop Circuit Type
This article summarizes ENERGY STAR ratings categorized by loop circuit type. The most common type of ground loop is a central loop with central pumps as shown in the upper left portion of Figure 3. Almost all of the systems were connected inside the building to individual heat pumps and most had variable speed drives. Two of the central systems incorporated reversible central chillers and air-handling units with VAV (variable air volume) terminals rather than heat pumps. Another central ground loop type is the one-pipe system shown in the lower left portion of Figure 3. A single central
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pipe loop inside the building is connected to a central ground loop. Central pumps provide continuous flow, which is typically regulated by ground loop return temperature. As each heat pump is activated, a secondary pump circulates water to the heat pump and returns the discharge downstream into the common one-pipe loop. The third category is unitary single loops connected to individual heat pumps with low head, on-off circulator pumps as shown in the lower right section of Figure 3. These resemble residential systems and in some cases these individual loops serve the classrooms and offices while the common areas are served by air-cooled equipment or a central loop GSHP. The fourth category is a common building with a ground loop with individual on-off circulator pumps on each heat pump as shown in the upper right section of Figure 3. Note that a check valve is installed on the pump discharge to prevent reverse circulation when the heat pump is not operating. These pumps typically only provide 25 to 30 ft of head (75 to 90 kPa). They are not able to circulate through large central loops unless the header diameters are increased above standard practice or two pumps are placed in series on each unit. One elementary school in the survey had a single common loop while the others had multiple common loops. Figure 4 is a graph of ENERGY STAR ratings for the central GSHP systems. Eight of the 20 systems (40%) achieved
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ASHRAE Journal - June 2012

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