Pitfalls of Rules of Thumb

in

Geothermal Heat Pump Design

Thumbs Up or Thumbs Down?

Andrew Chiasson, Ph.D., P.E.Department of Mechanical and Aerospace Engineering

University of Dayton

ASHRAE Dayton Chapter MeetingFeb. 8, 2010 ASHRAE

Technology for a Better Environment

Presentation Outline

• What is the most prevalent rule of thumb in the geothermal heat pump industry? ft/ton

ft/ton

W/m (in other countries)

• The challenges in defining such a rule of thumb

• Results of a parametric study demonstrating the difficulties of a meaningful rule of thumb

• Concluding summary

Ground Heat Exchanger DesignImportant Parameters

Heat Gains and Losses (Peak Hour and

Annual)

Average

Thermal

Conductivity

Undisturbed

Earth

Temperature

BoreholeThermal Resistance

or

Borehole

Spacing

Is a ft/ton Rule of Thumb Possible?“ft” means feet of What?

• In vertical systems, is the borehole: 4.5, 5, 6 inch diameter?

Grouted with standard bentonite or thermally-enhanced grout?

• What is the borehole-to-borehole spacing?

• What about the u-tube? U-tube spacers specified?

¾, 1, or 1¼ inch u-tube diameter?

Double u-tube?

Concentric tube?

Is a ft/ton Rule of Thumb Possible?“ft” means feet of What?

• Soil or rock thermal properties: Dry volcanic ash vs. moist sand vs. dense rock?

• What is the ground temperature?! Difference between this and the design heat pump entering

fluid strongly affects the loop length

Is a ft/ton Rule of Thumb Possible?“/ton” means tons of What?

• What is the heat pump COP? High, medium, or low efficiency?

The building load is NOT the same as the ground load

• What is the heat pump run time? Primary residence vs. cottage?

Multiple zones: are some zones mostly unoccupied (basement, guest rooms)?

Additional loads of a swimming pool, spa, etc.?

De-superheater?

Two-stage compressor?

Additional runtimes affect ground thermal storage, which affects loop lengths

Is a ft/ton Rule of Thumb Possible?Where did such a rule of thumb originate?

• In the residential sector in Central OK, where some original work in this field was done (late 1970s -1980s)

• The rule of thumb was meant to relate total ground heat exchanger length to the installed capacity of the heat pump (which was usually closely-matched to the cooling load)

• The rule of thumb was adequate for residential buildings in that area where regional geologic conditions are similar

Ground Heat Exchanger (GHX) Design

• Usual formulation: size ground heat exchanger so that it does not exceed design limits on entering fluid temperature to the heat pumps over the life of the system.

• Alternative formulation: adjust loads on ground heat exchanger to reduce required size.

• Optimization for energy performance beyond these two formulations is rare.

Ground Heat Exchanger (GHX) DesignHeating vs. Cooling Constrained

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A Heating-Constrained GHX

Ground Heat Exchanger (GHX) DesignHeating vs. Cooling Constrained

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A Cooling-Constrained GHX

Ground Heat Exchanger (GHX) DesignHeating vs. Cooling Constrained

Previous bldg. with a hybrid cooling tower. Is GHX still cooling-constrained?

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Hybrid Cooling Tower Case Annual Peak Temperatures

Ground Heat Exchanger DesignSome Challenges

• Long time constant of the ground requires two load calculation/design cycles: Daily peak

Annual (extrapolated to multiple years)

• Former used for sizing equipment.

• Both are used for sizing ground heat exchanger.

• Latter is an extra step for most designers of HVAC systems.

• The desire to eliminate the extra step leads to the search for a rule-of-thumb that correlates ground heat exchanger design to peak load.

Ground Heat Exchanger DesignRules of Thumb

• Usually expressed as : Feet of borehole per ton of heat pump capacity,

W peak heat rejection (or extraction) per meter of borehole length, or

Btu/hr peak heat rejection (or extraction) per foot of borehole length

• Neither take into account long-term heat build-up or depletion

• Proving a specific rule-of-thumb is bad: easy

• Proving that no rules-of-thumb are possible: not-so-easy

In Search of a General Rule of Thumb:Parametric Study (Spitler & Cullin, 2008)

• 14 U.S. Locations (in DOE climate zones)

• 3 buildings: office, school, hotel

• Typical design heat pump entering fluid temperature limits Heating-constrained or cooling-constrained

• Experimentally-validated simulation tools

• All have same ground thermal conductivity, same borehole completion

• Building loads and undisturbed ground temperatures depend on location

• Sizes adjusted to meet limits over ten-year period

• Express results in heat rejection or heat extraction per unit length (W/m)

In Search of a General Rule of Thumb:Parametric Study Results

Subset of Results:

Location Bldg. Peak Ht. Rej. Rate (W/m)

Peak Ht. Extr. Rate (W/m)

Chicago, Office 79 36 Illinois School 16 52 Hotel 79 37 Duluth, Office 78 48 Minnesota School 8 38 Hotel 42 26 Houston, Office 67 9 Texas School 46 99 Hotel 26 7

In Search of a General Rule of Thumb:Parametric Study Results

Which rule of thumb would you choose?

Black line represents freezing point of 20% eth.glycol, -7.82C

In Search of a General Rule of Thumb:Parametric Study Results

Which rule of thumb would you choose?

In Search of a General Rule of Thumb:Parametric Study Results

Effects of choosing different design conditions

Peak Heat Extraction Rate (W/m)Chicago Office Building

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-5 0 5 10

Minimum HP EFT ( C)

Pe

ak

Ra

te (

W/m

)

• Notice that the GHX for this building is cooling-constrained if the design minimum EFT were chosen to be 2 C or less

• However, if the EFT were chosen greater than 2 C (or if a shorter amount of GHX {less than 40 W/m} were installed, then the GHX switches to heating constrained

Peak Heat Extraction Rate (W/m)Chicago Office Building

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Grout Conductivity (W/m K)

Pe

ak

Ra

te (

W/m

)

In Search of a General Rule of Thumb:Parametric Study Results

Effects of different borehole designs

In Search of a General Rule of Thumb:Parametric Study Results

Required Ground Heat Exchanger Sizes

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Ranking

W/m

Concluding Summary

• Commercial GSHP systems require two load calculation/design cycles - design day and annual.

• Rules-of-thumb are inherently problematic.

• Alternative:

Annual load calculation

In situ thermal conductivity testing

Simulation-based design

Acknowledgements

• Thanks to Jeffrey D. Spitler, Ph.D., P.E.,C.M. Leonard Professor, Oklahoma State University

• Spitler, J.D., and J. Cullin. 2008. Misconceptions Regarding Design of Ground-source Heat Pump Systems. Proceedings of the World Renewable Energy Congress, July 20-25, Glasgow, Scotland.