Consulting-Specifying Engineer - November 2008 - (Page 41)

Purge port (at system high point) Ground-Coupled Heat Pump Systems, available from the IGSHPA (www.igshpa.okstate.edu). Since thermal conductivity (Btu/h-ft-°F) and thermal diffusivity are the most important design variables to consider before attempting to design a commercial geothermal system, an in-situ thermal conductivity test may be warranted for a project. The test runs typically 48 hours and requires a test bore with a unicoil that has been backfilled with grout. The test can typically begin five to seven days after the test bore has been completed and should follow the recommended procedures in the ASHRAE Handbook, HVAC Applications. A detailed drill log must be kept to assist in the proper estimating of drill costs as well as conductivity heat transfer. The test will provide information about drilling conditions, the types of soil present, thermal properties of the soil, and deep-earth temperatures. Generally, soil conductivity tests are conducted on systems larger than 20 tons, as design variations in loop sizing assumptions do not have a significant impact on installed costs for smaller systems. Typical costs for the test per borehole are $6,000 to $11,000, and the borehole can be reused for the installed system. Importance of commissioning A system is only as good as its design and installation, and performance testing must be mandated in any commercial GSHP system. Good commissioning practice will verify proper water and air flow through each heat pump, the heat pump capacity and efficiency, and the ground loop installation. All ground installations must be flushed to remove construction debris, purged to remove air, and pressure tested to 100 psi, or the requirement of the authority having jurisdiction, before backfilling or grouting. Figure 3 shows locations of purge ports required to effectively purge air from a geothermal loop. Operating efficiencies should be verified using the following equations: Cooling Efficiency (Imperial Units) • Heat from electrical energy (Btu) = V x amp x Power Factor x 3.412 Btu/W (single-phase electrical power) • Heat rejected from loop (Btu) = GPM x Delta T x 485 (water/antifreeze loop) • Net cooling capacity (Btu) = heat rejected from loop (Btu) – heat from electrical energy (Btu) • Total cooling capacity (Btu) = heat rejected from loop (Btu) + heat from electrical energy (Btu) • EER = Net cooling capacity (Btu)/W Heat pump Air vent (optional) Expansion tank (optional) Purge port Borehole field Cold water make-up Figure 3 Purge port configuration. Source: Peter D’Antonio, PCD Engineering Services Heating Efficiency (Imperial Units) • Heat from electrical energy (Btu) = V x amp x Power Factor x 3.412 Btu/W (single phase) • Heat absorbed from loop (Btu) = GPM x Delta T x 485 (water/ antifreeze loop) • Total heating capacity (Btu) = heat absorbed from loop (Btu) + heat from electrical energy (Btu) • COP = Total heating capacity (Btu)/heat from electrical energy (Btu) GSHP systems have enjoyed a surge in recent years thanks to developments within the industry and the push toward more energy- and environmentally efficient buildings. However, there is still uncertainty about first cost, lifecycle cost, operation and maintenance, and system long-term reliability. In order to achieve broader acceptance, mechanical design teams must have a better understanding of not only the economic, environmental, and comfort benefits that these systems can provide, but also the long-term operational, maintenance, and reliability issues that they can expect to face over the life of the system. D’Antonio is president of PCD Engineering Services, a provider of energy and environmental efficiency in buildings. He is a member of Consulting-Specifying Engineer’s editorial advisory board and a 2008 “40 Under Online resources 40” honoree. Here is a partial list of resources related to the design of geothermal systems: References 1. Kavanaugh, S. P., et al. 1997. Ground-source • www.igshpa.okstate.edu/publication heat pumps: Design of geothermal systems /publication.htm for commercial and institutional buildings. • www.geoexchangebc.ca/pdf/ahsrae ASHRAE. _april292005.pdf 2. Shonder, John A. and Patrick J. Hughes, PE. • www.geoexchangebc.ca/products Increasing confidence in geothermal heat pump design methods. Oak Ridge National Laboratory, .aspx Oak Ridge, Tenn. 1998. • www.nyserda.org/programs/ 3. ASHRAE. 2002. Commissioning, preventative geothermal/geothermal_manual.pdf maintenance, and troubleshooting guide for • www.nyc.gov/html/ddc/downloads/pdf commercial ground-source heat pump systems. /geotherm.pdf ASHRAE SP-94, 2002. Consulting-Specifying Engineer • NOVEMBER 2008 41 http://www.igshpa.okstate.edu http://www.igshpa.okstate.edu/publication/publication.htm http://www.igshpa.okstate.edu/publication/publication.htm http://www.geoexchangebc.ca/pdf/ahsrae_april292005.pdf http://www.geoexchangebc.ca/pdf/ahsrae_april292005.pdf http://www.geoexchangebc.ca/products.aspx http://www.geoexchangebc.ca/products.aspx http://www.nyserda.org/programs/geothermal/geothermal_manual.pdf http://www.nyserda.org/programs/geothermal/geothermal_manual.pdf http://www.nyc.gov/html/ddc/downloads/pdf/geotherm.pdf http://www.nyc.gov/html/ddc/downloads/pdf/geotherm.pdf

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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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