ASHRAE Journal - May 2012 - 84

emerging technologies
Benefits Belie Lack of Popularity

commercial gshPs
By Alissa cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

round source heat pump (GSHP) technology has limited adoption in the commercial buildings sector, although it provides significant

energy savings benefits. In 2010, commercial buildings consumed 18.35 quads (19.35 EJ) of primary energy. 13.7% of this supplied space heating and 10.1% space cooling.1 Compared to air source heat pump

(ASHP) technologies, GSHP technologies can provide space conditioning primary energy savings between 20% and 50%.2 During the 1990s, many GSHPs were installed in schools and other commercial buildings and are still running efficiently. The complexity of GSHP system design and high initial cost are factors limiting adoption of this technology.
overview of technology
A commercial GSHP is comprised of three main components, the ground loop (ground heat exchanger [GHX]), the heat pump, and the air handling unit (AHU). A GSHP operates using the earth as an external reservoir, rejecting heat to it in the cooling mode, and taking heat from it in the heating mode. Similar to systems designed for residential applications, commercial scale GSHPs use several different ground loop configurations. The ground loops fall into three categories: closed-loop, open-loop, or direct exchange. The type of loop used depends on system size, available land area, soil quality, and groundwater or surface water quality and availability. Table 1 outlines possible closed loop system configurations.3 In closed loop systems the heat transfer fluid in the loop is separated from the ground, and a pump circulates the heat transfer fluid from the ground to the
84 ASHRAE Journal

heat pump. The benefit of a closed loop is that the heat pump is protected from potential mineral scale build up found in systems which use groundwater directly. Horizontal loops are most commonly used for residential and small commercial applications, while vertical boreholes are often used commercially. Of the closed loop systems, boreholes provide the most benefit if a body of water is not readily available to support an open loop configuration. Boreholes relative to horizontal configurations require less land area, less piping, less pump energy, and are least subject to ground temperature variations because of their depth.3 The use of thermally enhanced grout in grouted boreholes increases the conductivity between the HDPE (high density polyethylene) U-tube pipe and the ground. It also helps to prevent unwanted groundwater migration.4 A main downside of vertical boreholes is the
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thermal energy buildup that develops when boreholes are inadequately spaced. Open loop systems use ground- or surface water as their heat transfer fluid through the use of created wells or existing bodies of water. Well configurations can include a production and injection well or only a production well. Standing column configurations return the water used to the production well only. Configurations which return the water back to the ground eliminate the issue of aquifer depletion, while those which return water to surface bodies of water do not.5 Pond loops are often used commercially. The pond can either be present in the topography of the site or manmade.3 Groundwater GSHP systems can be limited by state or federal environmental regulations. Additionally, the heat exchanger in the heat pump is at risk for scale buildup and fouling, as it is in direct contact with groundwater. These systems are most appropriate for areas with access to a groundwater supply of high quality. Systems require up to 3 gpm/ton (0.054 L/s·kW) of cooling. While potentially economical, open loop systems can have higher pump energy requirements than other system types.3 Open loop systems are advantageous because they are simpler to design. Also, they have the thermodynamic advantage of using groundwater at ground temperature, instead of a heat transfer fluid with a temperature difference between itself and the ground. In addition, they can be less expensive to install than closed loops. The third type of GHX system is the direct expansion ground loop. This loop circulates the refrigerant directly to the ground heat exchanger, using copper tubes instead
May 2012

ASHRAE Journal - May 2012

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