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A Simplified Design Method for PEX Plastic Downhole Heat Exchangers

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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A Simplified Design Method for PEX Plastic Downhole Heat Exchangers

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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? Kelvin’s 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

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