Consulting-Specifying Engineer - November 2008 - (Page 35)

Outside air Exhaust air Fan Heat exchanger Fan Exhaust air from spaces Balance damper Return air Heat pump Heat pump Heat pump Supply air VFD (secondary loop) Maximum 4 pipe dia. Differential pressure transducer Supplemental cooling (heating) source Borehole V-1 ventilation mode valve Borehole Borehole VFD (primary loop) Figure 1a Primary/secondary loop configuration with dedicated outdoor air and GSHP ventilation mode. Source: Peter D’Antonio, PCD Engineering Services Figure 1b depicts a piping variation to 1a, where the building loop and ground loop are tied together as a single primary loop. The ventilation mode also exists in this variation and is controlled by valve V-1. Switching to ventilation mode in this scenario will bypass the ground loop, reducing pipe friction loss and pumping energy. Another scenario has been employed in smaller commercial facilities as shown in Figure 2. Water-to-water heat pumps are arranged in a centralized system, and hot and chilled water is fed to buffer tanks. This is termed the “charging loop.” The buffer tanks serve to decouple the primary loop from the distribution loop and accommodate the differing flow rates in the tank charging loop and distribution loop. These systems also are a useful design option when the facility has a mix of heating systems, such as radiant and forced air, and to reduce or prevent short cycling of the central plant equipment/heat pump unit(s) in a system with small water volumes and variable loads. This system is in use at the 8,000-sq-ft Morningstar Learning Center in Big Sky, Mont. Outdoor air for either scenario can be effectively introduced through a dedicated outdoor air system. The outdoor air can be tempered to a neutral temperature of roughly 65 F using any number of sources including a fossil fuel heating system, air conditioning, or heat pump system. Greatest efficiency is gained when the outdoor air is fed into a heat or energy recovery device whereby the exhaust air energy is transferred to the incoming outdoor air as shown in Figure 1a. The ground piping loop, or loopfield, comes in two main forms, open or closed. The three most common geothermal systems are all closed-loop, and include the horizontal (including slinky approach), pond/lake installation, or vertical borehole installation. This article will focus on the vertical as it represents the largest market share of all systems installed, is the predominant choice in commercial construction, and often provides the most efficient use of space. The ground loop can be a single common loop for all heat pumps in the system or can serve each heat pump or small group of heat pumps with its/their own bore(s). The latter is appropriate in single-story buildings with small core areas or when retrofitting an existing building. The former is used for multistory buildings or large core areas to take advantage of load diversity. To heat pumps (see fig 1a) Borehole Borehole Borehole VFD V-1 ventilation mode valve Figure 1b Primary loop configuration. Source: Peter D’Antonio, PCD Engineering Services Consulting-Specifying Engineer • NOVEMBER 2008 35

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Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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