Consulting-Specifying Engineer - November 2008 - (Page 34)

geothermal heating and cooling system design BY PETER D’ANTONIO, PE, LEED AP, PCD Engineering Services, Longmont, Colo. Commercial G eothermal heating and cooling systems, also known as geoexchange or ground source heat pump (GSHP) systems, provide heating and cooling for buildings. They also may provide domestic water heating to either supplement or replace existing more conventional water heaters, and may even be used for humidity control. GSHP systems work by moving heat, rather than by converting chemical energy to heat as in a combustion process. We all live with heat pumps, the most widely recognized of which is the refrigerator. A refrigerator “pumps” heat from the cold storage area to the room air via a set of heat exchange coils (the condenser coil cage in the back or bottom of the refrigerator). A geothermal heat pump works much in the same way, only its operation is reversible—it can pump heat out of a building to cool it or pump heat into a building to warm it. GSHP technology is not new. Though GSHP systems have existed for decades and the technology has been around for more than 50 years, many improvements have been made to the types of materials used, design and installation methods, and the efficiencies of compressors, pumps, and other equipment. GSHP systems are among the most efficient heating and cooling systems in existence today, and are a powerful tool in the engineer’s arsenal for application in green buildings due to their energy benefit. There are thousands of GSHP system installations around North America, and they generally provide a high degree of satisfaction. When considering a GSHP system, first cost is often the biggest deterrent for owners and a major factor in the slow advance of the technology. Maintenance and long-term performance are also common issues. Primary system types One system approach that has successfully been used in practice is shown in Figure 1a. It is a distributed or decentralized system of water-to-air heat pumps located to serve each zone. Each heat pump is served by a common building piping loop. The building piping loop serves as a heat source (heating mode) or sink (cooling mode) for each heat pump and is connected to the ground loop in a primary-secondary piping arrangement. The pumps serving the building and ground loops can be controlled by variable speed drives to reduce power consumption during non-peak periods. A ventilation mode can operate during mild ambient conditions where the building loop recirculates through the building without a need to run the ground loop. For example, this ventilation mode may run when building supply water temperature is between 40 and 70 F and the return water temperature is between 42 and 68 F, and the ground loop bypass valve is closed. This ventilation mode is shown in Figure 1a with two-position mode valve V-1. Large spaces that cannot be served by one heat pump can use multiple water-to-air heat pumps; alternatively, they can use one or more water-to-water heat pumps serving multispeed air handlers. A variation of this system is in use at the 46,000-sq-ft Salud Health Center in Denver. Design teams must understand the operational, maintenance, and reliability issues associated with geothermal heating and cooling systems. 34 Consulting-Specifying Engineer • NOVEMBER 2008

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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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