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Presentation Outline. . Introduction, Motivation, ObjectivesBackgroundPEX DHE installation and monitoringMethodologyField testingMathematical modelingResultsDesign chartExample useConcluding summary. Introduction: What are DHEs?. . Closed-loop immersed in water wellsKnown installations in U.S., Turkey, and New ZealandLess common/experimental installations in Iceland, Hungary, Russia, Italy, Greece, JapanIn U.S., most concentrated uses are in Klamath Falls, OR and Reno, NVOver 500 DHE installations exist in Klamath Falls, OR.
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2. Presentation Outline Introduction, Motivation, Objectives
Background
PEX DHE installation and monitoring
Methodology
Field testing
Mathematical modeling
Results
Design chart
Example use
Concluding summary
3. Introduction:What are DHEs? Closed-loop immersed in water wells
Known installations in U.S., Turkey, and New Zealand
Less common/experimental installations in Iceland, Hungary, Russia, Italy, Greece, Japan
In U.S., most concentrated uses are in Klamath Falls, OR and Reno, NV
Over 500 DHE installations exist in Klamath Falls, OR
4. Some existing applications include:
Space heating of homes
Space heating of schools
Snow melting
Introduction:What are DHEs?
5. Project Motivation & Objectives Motivation:
DHEs are black iron pipe => subject to unpredictable corrosion rates (2 15 years)
Chiasson et al. (2005) reported on installation and monitoring of a cross-linked polyethylene (PEX) plastic DHE
Monitoring demonstrated that the PEX DHE was significantly over-designed
Current design methods are rules of thumb
Objectives:
Develop a design method for PEX DHEs that is simple and reliable
6. Project Background:DHE Materials
7. Project Background:Installing the DHE
8. Project Background:Performance Monitoring
9. Methodology:Field Testing How much heat can actually be extracted from the well?
Heat extraction rate = 184,000 Btu/hr or 54 kW
10. Methodology:Simple Mathematical Model How can the design length be calculated?
Kelvins classic Line Source Model (1882)
Applied to earth heat transfer by Ingersoll and Plass (1948)
A pure conduction heat transfer model
All effects of groundwater flow are lumped together to give an effective thermal conductivity
11. Results:The Effective Thermal Conductivity keffective = 55 Btu/(hr-ft-oF) or 95 W/(m-K)
12. Results:Design Tool (Heat Output per Unit Length)
13. Results:Verification and Example Use Determine peak heating load (98,000 Btu/hr or 29 kW)
Determine the geothermal resource temperature (202oF or 94.4oC)
Determine the average DHE loop temperature at design conditions (Tin+Tout)/2
Tout = heating coil or radiator design temperature
Tin = above temperature design temperature drop
(175oF + 145oF)/2 = 160oF or 71oC
x-axis value: Estimate or measure the effective thermal conductivity
y-axis value: DT = 42oF or 23oC
Predicted Design Output = 550 Btu/hr/ft (7% lower than actual)
14. Concluding Summary A prototype DHE constructed of PEX plastic has been installed in a retrofit application in Klamath Falls, OR
A simple design method to determine required PEX DHE length has been developed
The method produces favorable results, provided the user has a reasonable estimate of the rock effective thermal conductivity
Future work will attempt to check the design method at other locations with differing geothermal conditions