ASHRAE Journal - July 2011 - 34

(6°C) temperature difference between the entering and leaving ground loop temperatures. This building also had a VSD with a central loop for the common areas, which demonstrated water temperature differences similar to those shown in Figure 3.
Temperature (°F)

100 95 90 85 80 75 70 65 9/21 LWT EWT 9/22 9/23 North Georgia (2010) 9/24

Ground Loop Laminar Flow Myth
The myth that laminar flow (Re < 2300) is to always be avoided, even at low part loads in the central loops may result in drastic pump oversizing. Remember that the bad thing about laminar flow, in terms of heat transfer, is the high temperature gradient across the laminar boundary layer. This may be significant in compact heat exchangers with high heat rates, but the heat exchange medium in GHP applications is dirt and plastic pipe. Heat rates are low, especially at low part loads, and the differential temperatures across laminar boundary layers are small compared to those across the plastic pipe, annular grout, and ground. Therefore, designers should carefully weigh the small advantage of non-laminar flow at part load compared to the sizeable increase in pump electrical energy demand, energy use, and added heat rejection to the ground, resulting from misguided efforts to always avoid laminar flow.

Figure 4: Unitary loop on-off pumps. Ground entering and leaving water temperatures at design. Note: outdoor air temperature high 97°F, 94°F and 93°F. • Circulator pump manufacturers should strive to increase the wire-to-water efficiency of small constant speed pumps to at least 50% without expensive speed control technologies.

Conclusions
While certainly GHP systems are not universally well applied and, in some cases, have been abandoned after years of unacceptable performance and litigation, analysis of the preliminary results of this field data indicate two very positive trends. • The simplest GHP pump control methods appear to perform the best in minimizing pump energy consumption (and heat rejection load on the ground loop) in both very warm climates (Texas) and in heating dominated climates (Illinois). • GHP systems with simple pump control can be extremely effective in very warm climates without the assistance of fluid coolers. • While VSD systems on central loops appear to be able to approach the effectiveness of simpler systems, field data (primarily from schools with limited maintenance resources) indicate these systems do not operate as well as intended. • The collected data indicates pump control methods may be a significant factor in ground loop temperature rise. However, more extensive data sets are needed, especially on GHP systems that have not performed well (unlike the majority of those surveyed to date).

References

Recommendations
• Building owners considering GHPs should know that an ENERGY STAR rating of 95 can be expected when systems are properly designed and installed. • As a component of the bid documents, building owners considering GHPs should require architects, engineers and mechanical contractors to submit dossiers that include ENERGY STAR scores and cost of installation for systems they have completed. • Owners and engineers should put emphasis on field data results and measured energy consumption data when selecting design options for GHP systems.
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1. EPA. 2010. ENERGY STAR Certified Buildings and Plants. United States Environmental Protection Agency. http://tinyurl.com/22wspjc (search Leander, Texas). 2. Green, M. 2010. Recent MEP cost for GHP schools. Personal communication with Michael Green, MEP Engineering, Austin, Texas. 3. Ball, L. 2008. Austin Area Closed Loop Geothermal Heat Pump Systems. Personal communication with Lonnie Ball, Ball Drilling, Austin, Texas. 4. Carlson, S. 2001. “Development of Equivalent Full Load Heating and Cooling Hours for GCHPs Applied in Various Building Types and Locations.” ASHRAE TRP-1120, Final Report. Atlanta. 5. 2007 ASHRAE Handbook—HVAC Applications. “Geothermal Energy,” Chapter 32. 6. Kavanaugh, S.P., S. Lambert, and D. Messer. 2002. “Development of Guidelines for the Selection and Design of the Pumping/Piping Subsystem for Ground Coupled Heat Pumps.” ASHRAE TRP-1217, Final Report. 7. Mescher, K. 2009. “Simplified GSHP system.” ASHRAE Journal 51(10). 8. Hinge, A.W. and D.J. Winston. 2009. “Documenting Performance.” High Performance Buildings magazine. Winter 2009. ASHRAE. 9. Kavanaugh, S. 2011. “Long Term Performance of GSHPs.” Interim Project Report to the Electric Power Research Institute, Southern Company and Tennessee Valley Authority. 10. EIA. 2003. Commercial Building Energy Consumption Survey. U.S. Energy Information Administration. http://www.eia.doe.gov/ emeu/cbecs/contents.html. 11. EPA. 2010. ENERGY STAR Certified Buildings and Plants. http://tinyurl.com/22wspjc (search McLean County Unit School District #5). 12. EPA. 2010. ENERGY STAR Certified Buildings and Plants. http:// tinyurl.com/22wspjc (search ASHRAE HQ). ashrae.org July 2011

ASHRAE Journal - July 2011

Table of Contents for the Digital Edition of ASHRAE Journal - July 2011