ASHRAE Journal - September 2012 - 45

ground in heating far exceeds the heat rejected. In cooling dominant applications, the long-term temperature rise is mitigated by the evaporative cooling effect from reductions in moisture content when ground temperature rises in the loop field. The heat rejection required to affect a 1% reduction in ground moisture is approximately the same amount needed to raise the ground temperature 30°F (17°C).2 The moisture content is likely to be restored to its natural condition via groundwater movement and rainwater percolation. However, in cold climates the latent heat available from freezing of soil moisture is significantly less than from evaporation, moisture migration is obviously minimal, and little is known regarding the thermal and physical properties of conventional grouts at low temperatures. Therefore, no projections of longterm temperature decline can be offered.

• Although several of the sites in this study were in climates with annual heating loads greater than cooling loads, the vertical ground loop design lengths were determined by the maximum hourly cooling load.

Conclusions
• Ground heat exchanger size is the dominant influence on maximum loopto-ground approach temperatures and undersized loops are the primary cause of hot loops. • The thermal conductivity of the ground and bore grout/backfill were not available for many older sites, but low conductivity and bore separation may also contribute to hot loops. • Variable speed pump drives were largely inoperable and likely saved little energy. • This data set does not reveal any significant increases in long-term ground loop temperature rise. • Additional GSHP system data collection and analysis efforts are needed to more thoroughly access the prevalence of long-term temperature change. This is needed in both cooling dominant and heating dominant climates. • The thermal performance of ground and ground moisture is complex and models that do not address phase change phenomena are likely to over-predict ground temperature changes.

Summary
• GSHP Systems with maximum average ground loop temperatures below 90°F (32°C) had an average ENERGY STAR rating of 92 and systems with maximum average ground loop temperatures above 95°F (35°C) had an average ENERGY STAR rating of 53. • The three sites with the largest increases in loop temperature have been operating for less than 10 years and have bore lengths of less than 120 ft/ton (10.4 m/kW). • Sites with large ground loop temperature increases, but with long bore lengths, have low conductivity grout, small bore separation, and low indoor temperatures. • Trend lines indicate systems that have been operating for a greater number of years actually have lower ground loop temperature increases. These systems also tend to have longer ground loops but smaller vertical bore separation and lower thermal conductivity backfill/grout. • The limited data set indicates bore separation distances near 15 ft (5 m) experienced high ground loop temperatures during extended periods of drought and high outdoor air temperatures. • Measured differential loop temperatures indicate most central loop pumps were significantly oversized and variable speed drives did not reduce flow at part load.
September 2012

Acknowledgments
The project was made possible with a tailored collaboration through the Electric Power Research Institute (EPRI), with the Southern Company (SoCo) and the Tennessee Valley Authority (TVA) providing the funding. Project direction and collaboration were provided by Ron Domitrovic (EPRI), David Dinse (TVA), and Chris Gray (SoCo).

References
1. EPRI. 2012. “Long-Term Performance of Commercial Ground Source Heat Pumps.” Final Report Draft. EP-P40851/EP-P40852. Electric Power Research Institute. 2. EIS. 2009. “Ground heat exchanger model uncertainty.” Instructional Manual— GshpCalc 5.0 GSHP Design Software, Energy Information Services.

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ASHRAE Journal

ASHRAE Journal - September 2012

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