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 Meeting Feb. 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 Design Important Parameters Heat Gains and Losses (Peak Hour and Annual) Average Thermal Conductivity Undisturbed Earth Temperature Borehole Spacing BoreholeThermal Resistance or
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.
Peak Heat Pump Entering Fluid Temperature ( o C) Min. Heat Pump Entering Fluid Temperature ( o F) Ground Heat Exchanger (GHX) Design Heating vs. Cooling Constrained A Heating-Constrained GHX 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 86 82 79 75 72 68 64 61 57 54 50 46 43 39 36 32 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (years)
Max. Heat Pump Entering Fluid Temperature ( o C) Max. Heat Pump Entering Fluid Temperature ( o F) Ground Heat Exchanger (GHX) Design Heating vs. Cooling Constrained A Cooling-Constrained GHX 33 91 31 88 29 84 27 81 25 77 23 73 21 70 19 66 17 63 15 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (years) 59
Peak Heat Pump Entering Fluid Temperature ( o C) Peak Heat Pump Entering Fluid Temperature ( o F) Ground Heat Exchanger (GHX) Design Heating vs. Cooling Constrained Previous bldg. with a hybrid cooling tower. Is GHX still cooling-constrained? 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 93 90 86 82 79 75 72 68 64 61 57 54 50 46 43 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (years) Hybrid Cooling Tower Case Annual Peak Temperatures
Ground Heat Exchanger Design Some 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 Design Rules 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: notso-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?
Peak Rate (W/m) In Search of a General Rule of Thumb: Parametric Study Results Effects of choosing different design conditions Notice that the GHX for this building is cooling-constrained if the design minimum EFT were chosen to be 2 C or less Peak Heat Extraction Rate (W/m) Chicago Office Building 50 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 40 30 20 10 0-5 0 5 10 Minimum HP EFT ( C)
Peak Rate (W/m) In Search of a General Rule of Thumb: Parametric Study Results Effects of different borehole designs Peak Heat Extraction Rate (W/m) Chicago Office Building 70 60 50 40 0.5 1 1.5 2 2.5 Grout Conductivity (W/m K)
W/m In Search of a General Rule of Thumb: Parametric Study Results Required Ground Heat Exchanger Sizes 45 40 35 30 25 20 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 Ranking
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.