Geothermal Heat Pump Presentation

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A Simplified Design Method for PEX Plastic
Downhole Heat Exchangers
by Andrew Chiasson
Geo-Heat Center Oregon Institute of
Technology Klamath Falls, Oregon, USA

GRC 2006 Annual Meeting San Diego, CA
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Presentation Outline

• Introduction, Motivation, Objectives
• Background
• PEX DHE installation and monitoring
• Methodology
• Field testing
• Mathematical modeling
• Results
• Design chart
• Example use
• Concluding summary

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IntroductionWhat 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

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IntroductionWhat are DHEs?

• Some existing applications include
• Space heating of homes
• Space heating of schools
• Snow melting

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Project Motivation Objectives

• Motivation
• DHEs are black iron pipe gt 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

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Project BackgroundDHE Materials
PEX U-tube
PEX compression fitting
PEX tubing 1-in. x 300-ft spools with oxygen
barrier
PEX elbow
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Project BackgroundInstalling the DHE
Steel pipe from old DHE used as installation
guide as well as a convection promoter by
leaving pipe open at the bottom and installing a
tee below the water level
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Project BackgroundPerformance Monitoring
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MethodologyField Testing

• How much heat can actually be extracted from the
  well?
• Heat extraction rate 184,000 Btu/hr or 54 kW

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MethodologySimple 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

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ResultsThe Effective Thermal Conductivity

• keffective 55 Btu/(hr-ft-oF) or 95 W/(m-K)

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ResultsDesign Tool (Heat Output per Unit Length)
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ResultsVerification 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 (TinTout)/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)

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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