Title: Increasing Confidence In Geothermal Heat Pump Design Methods

Abstract

Sizing the ground heat exchanger is one of the most important tasks in the design of a geothermal heat pump (GHP) system. Undersizing the heat exchanger can result in poor operating efficiency, reduced comfort, and nuisance heat pump lockouts on safety controls, while an oversized heat exchanger increases the installation cost of the system. The cost of ground loop installation may mean the difference between a feasible and an unfeasible project. Thus there are strong incentives to select heat exchanger lengths which allow satisfactory performance under all operating conditions within a feasible project budget. Sizing a ground heat exchanger is not a simple calculation. In the first place, there is usually some uncertainty in the peak block and annual space conditioning loads for the building to be served by the GHPs. The thermal properties of the soil formation may be unknown as well. Drilling logs and core samples can identify the soil type, but handbook values for the thermal properties of soils vary widely. Properly-done short-term on-site tests and data analysis to obtain thermal properties provide more accurate information, but since these tests are expensive they are usually only feasible in large projects. Given the uncertainties inherent in the process,more » if designers were truly working 'close to the edge' - selecting the absolute minimum heat exchanger length required to meet the predicted loads - one would expect to see more examples of undersized heat exchangers. Indeed there have been a few. However, over the past twenty years GHPs have been installed and successfully operated at thousands of locations all over the world. Conversations with customers and facility managers reveal a high degree of satisfaction with the technology, but studies of projects reveal far more cases of generously sized ground heat exchangers than undersized ones. This indicates that the uncertainties in space conditioning loads and soil properties are covered by a factor of safety. These conservative designs increase the installed cost of GHP systems, limiting their use and applicability. Moreover, as ground heat exchanger sizing methods have improved, they have suggested (and field tests are beginning to verify) that standard bore backfill practices lead to unnecessarily large ground heat exchangers. Growing evidence suggests that in many applications use of sand backfill with a grout plug at the surface, or use of bottom-to-top thermally enhanced grout, may provide groundwater protection equal to current practice at far less cost. Site tests of thermal properties provides more accurate information, but since these tests are expensive they are usually only performed in large projects. Even so, because soil properties can vary over a distance as small as a few feet, the value of these tests is limited. One objective of ongoing research at the Oak Ridge National Laboratory (ORNL) is to increase designers confidence in available ground heat exchanger sizing methods that lead to reliable yet cost-effective designs. To this end we have developed research-grade models that address the interactions between buildings, geothermal heat pump systems and ground heat exchangers The first application of these models was at Fort Polk, Louisiana, where the space conditioning systems of over 4,000 homes were replaced with geothermal heat pumps (Shonder and Hughes, 1997; Hughes et. al., 1997). At Fort Polk, the models were calibrated to detailed data from one of the residences. Data on the energy use of the heat pump, combined with inlet and outlet water temperature and flow rate in the ground heat exchangers, allowed us to determine the thermal properties of the soil formation being experienced by the operating GHP system. Outputs from the models provide all the data required by the various commercially-available ground loop sizing programs. Accurate knowledge of both the building loads and the soil properties eliminated the uncertainty normally associated with the design process, and allowed us to compare the predictions of the commercially-available methods.« less

Authors:

Shonder, John A ^[1]; Hughes, Patrick ^[1]

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+ Show Author Affiliations

1. ORNL

Publication Date:

Thu Jan 01 00:00:00 EST 1998

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

978705

DOE Contract Number:  

AC05-00OR22725

Resource Type:

Conference

Resource Relation:

Conference: The Second Stockton International Geothermal Conference, Richard Stockton College, NJ, USA, 19980316, 19980317

Country of Publication:

United States

Language:

English