Optimizing ground heat exchanger length with GeoperformX pipe
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This document summarizes research on optimizing ground heat exchanger length using GeoperformX pipe. Numerical simulations and thermal response tests showed the GeoperformX pipe can reduce borehole thermal resistance by up to 31% and borehole length by 6-25% compared to conventional pipes. A case study of a campus ground source heat pump system in Grayslake, IL demonstrated a 25 foot borehole length reduction per heat exchanger using GeoperformX pipe. In conclusion, reducing borehole thermal resistance through higher conductivity pipes like GeoperformX is an effective way to optimize heat exchanger length and lower installation costs of ground source heat pump systems.
Optimizing ground heat exchanger length with GeoperformX pipe
- 1. 1 Optimizing ground heat exchanger length with GeoperformX pipe jasmin.raymond@inrs.ca Professor – Ph.D. 2015 IGSHPA Product Showcase
- 2. 2 INRS Overview •A university dedicated to research only •Water, Earth and Environment Center based in Quebec City, Canada •Lab facilities heated and cooled with a ground source heat pump (GSHP) system •Operates a test site with a pilot ground heat exchanger (GHE) and monitoring boreholes
- 3. 3 Collaborations with Versaprofiles to develop GHE pipes • 2015 – Geothermal energy www.geothermal-energy-journal.com/content/3/1/7 • 2011 – ASHRAE Transactions • 2011 – Ground Water • 2011 – GeoConneXion Magazine
- 4. 4 Numerical evaluation of GeoperformX pipe performances 2011 – ASHRAE Transactions •2D and 3D numerical simulations of 1U-pipe GHEs •Evaluated operating temperatures – 0.6 to 1 ºC (1.1 to 1.8ºF) better •Up to 24 % borehole thermal resistance reduction and 9 % bore length decrease
- 5. 5 Verification of the GeoperformX pipe performances with thermal response tests 2011 – Ground Water •TRT-1 : GeoperformX -TC 3.0 W/m K (1.73 Btu/h ft ºF) -Rb 0.065 m K/W (0.112 h ft ºF/Btu) •TRT-2 : Versapipe -TC 3.4 W/m K (1.97 Btu/h ft ºF) -Rb 0.081 m K/W (0.140 h ft ºF/Btu) •20 % less Rb with 1U-pipe GeoperformX • Test performed by Golder Associates (Groleau and Pasquier, 2009)
- 6. 6 Sizing GSHP systems with GeoperformX pipe 2011 – GeoConneXion Magazine •Demonstrated how to size GSHP systems with GeoperformX pipe using commercial design programs (EED, eQUEST, GeoAnalyser, GLD, GLHEPro, GS2000) •Showed 6 to 11 % bore length reduction for three buildings using different sizing approaches (ASHRAE, Sweden) for 1U-pipe configurations
- 7. 7 Designing GHEs to reduce borehole length 150 m • Objective : decrease the installation cost and reduce the pay back period • How : optimize the GHE heat transfer performances to decrease its total length • In most GSHP design programs, the GHE performances are described by the borehole thermal resistance Rb (m K/W – h ft ºF/Btu) • With given subsurface conditions, optimizing the GHE implies reducing Rb
- 8. 8 The borehole thermal resistance • Describes the opposition to the passage of heat between the GHE fluid to the subsurface at the borehole wall • Enclose the thermal resistances caused by fluid flow as well as the properties and configuration of the GHE materials • Most commercial design programs use a 2D approach's to calculate the borehole thermal resistance (GLHEPro, LoopLink, GLD) • A 3D approach including an internal resistance is sometime used (EED)
- 9. 9 The borehole thermal resistance • Varies between 0.05 to 0.35 m K/W (0.09 – 0.61 h ft ºF/Btu) • Can be reduced by: • Increasing the pipe spacing • Increasing the grout thermal conductivity • Reducing the borehole radius • Improving the pipe • Thermal conductivity (TC) • Thickness • Configuration
- 10. 10 Versaprofiles 2nd generation of GeoperformX pipe • Version 2 made with thermally conductive nanoparticles and PE4710 • Can be heat fused with regular HDPE • Meets minimum requirements for geothermal pipes, including IGSHPA guidelines (slow crack growth, PENT, Hydrostatic, etc.) • Available in many diameters (> ½") and dimensions (> SDR-9) • 75 % increase in thermal conductivity • Launched in 2015
- 11. 11 Thermal conductivity of the GeoperformX pipe • Regular HDPE 0.4 W/m K (0.23 Btu/h ft ºF) • GeoperformX 0.7 W/m K (0.40 Btu/h ft ºF) • Was verified on samples with a needle probe RegularHDPE GeoperformXV2
- 12. 12 Borehole thermal resistance of GHEs • To determine the performance of the GeoperformX pipe • Verified various pipe configurations including coaxial • Calculated with the 3D model of EED • Used the multipole (Claesson and Hellström, 2011) and the concentric methods • Accounted for internal heat transfer (Hellström, 1991)
- 13. 13 Assumptions made to calculate the borehole thermal resistance of GHEs • Borehole length 150 m (492 ft) • Grout thermal conductivity 1.7 W/m K (1.0 Btu/h ft ºF) • Subsurface thermal conductivity 2.5 W/m K (1.44 Btu/h ft ºF) • Pipe dimension SDR-11 1¼" except for coaxial GHE • High flow rate to ensure turbulence
- 14. 14 Borehole thermal resistance of GHEs
- 15. 15 Key results to minimize the borehole thermal resistance • When comparing similar configurations, the GeoperformX pipe can reduce Rb by up to 31 % • Highest Rb differences for GHE with conventional and GeoperformX pipes are for coaxial configurations with a thick outer pipe • 2U-pipe with GeoperformX has the lowest Rb • The thermal mass of water, which will affect GHE length, is not taken into account when determining Rb www.versaprofiles.com
- 16. 16 Energy needed to increase the water temperature in the GHE by 1 ºC (1.8 ºF) • 6 to 28 times higher for coaxial GHEs when compared to 1U-pipe • Can damp short-term peak loads and have a positive impact on GHE length reduction
- 17. 17 Sizing calculations to determine GHE length reduction • Calculated with GLHEPro using Rb determined with EED • Thermal short circuiting is taken into account with the 3D approach for Rb with EED (Hellström, 1991) • The g-function used for simulations with GLHEPro considers the thermal mass of water (Xu and Spitler, 2006) • Synthetic cooling dominated building loads were assumed
- 18. 18 Assumptions for sizing calculations • Peak heating : 50 kW (171 kBtu/h - January) • Peak cooling : -150 kW (-512 kBtu/h – July) • Heat carrier fluid is pure water • SDR-11 1¼" pipes except for coax (17 out) • Grout thermal conductivity 1.7 W/m K (1.0 Btu/h ft ºF) • Subsurface thermal conductivity 2.5 W/m K (1.44 Btu/h ft ºF) • Subsurface temperature 10 ºC (50 ºF)
- 19. 19 Assumptions for sizing calculations • Greater depth targeted for 2U-pipe and coaxial GHEs to balance flowrates • Borehole spacing is 10 m (32,8 ft) to minimize thermal interactions • System sized for a maximum operating temperature of 35 ºC (95 ºF) after 10 years • Full GSHP and hybrid systems with a 55 kW (188 kBtu/h) cooling tower were considered
- 20. 20 Sizing calculation results for 1U-pipe GHEs Pipe TC – W/m K 0.4 0.7 0.4 0.7 Total flow rate – L/s 7 7 4.6 4.6 Rb – m K/W 0.0955 0.0777 0.0948 0.0768 GHE grid 4 × 4 4 × 4 2 × 5 2 × 5 Water volume – m3 4.62 4.39 2.65 2.49 Individual GHE length – m 159 151 146 137 Total GHE length – m 2544 2416 1460 1370 GHE length reduction – % --- 5 --- 6 SDR-11 1¼"
- 21. 21 Sizing calculation results for 2U-pipe GHEs Pipe TC – W/m K 0.4 0.7 0.4 0.7 Total flow rate – L/s 7 7 4.6 4.6 Rb – m K/W 0.0563 0.0443 0.0547 0.0442 GHE grid 3 × 4 2 × 5 2 × 4 2 ×3 Water volume – m3 7.89 7.37 4.59 4.37 Individual GHE length – m 181 203 158 199 Total GHE length – m 2172 2030 1264 1194 GHE length reduction – % 15 20 13 18 SDR-11 1¼"
- 22. 22 Sizing calculation results for coaxial GHEs Outer pipe diameter – mm 152 203 152 203 Total flow rate – L/s 9.6 8 6 6 Rb – m K/W 0.0734 0.0630 0.0701 0.0587 GHE grid 3 × 4 2 × 4 2 × 3 2 × 3 Water volume – m3 16.43 27.48 9.40 16.59 Individual GHE length – m 194 246 222 198 Total GHE length – m 2328 1968 1332 1188 GHE length reduction – % 9 23 9 19 The outer pipe TC is 0.7 W/m K for all cases. SDR-11 in SDR-17 out
- 23. 23 Key results to minimize the GHE length • For the given examples, the GeoperformX pipe allowed to reduce GHE length by up to 23 % • Most GHE length reduction is obtained with the coaxial configuration and the GeoperformX for the outer pipe • 2U-pipe with GeoperformX showed similar results, up to 20% GHE length reduction www.geothermalmagazine.eu
- 24. 24 Real case example – Grayslake, IL • A campus-wide GSHP system for the College of Lake County • Variable pumping systems distribute the heat carrier fluid around the campus • Heat exchange is achieved with a common GHE field • Designed by Norbert Repka of Affiliated Engineers Inc.
- 25. 25 First phase – Grayslake, IL GHE field •81 boreholes expendable to 480 •500 ft deep •1U-pipe SDR-9 GeoperformX •Expansion tanks •1500+ tons of heating and cooling capacity with extension •800 to 1000 boreholes needed for the full campus
- 26. 26 TC test results – Grayslake, IL Test carried out by Galen Streich of GRTI GHE Conventional GeoperformX Configuration 1U 1U Borehole diameter – in 7.8 6.6 GHE length - ft 500 503 Pipe SDR ? 9 Subsurface temperature – ºF 53.6 52.3 Grout TC – Btu/h ft °F 1 1 Rb – h ft °F/Btu 0.221 0.174 Subsurface TC – Btu/h ft °F 1.64 1.79
- 27. 27 First phase – Grayslake, IL 25 ft bore length reduction per GHE with GeoperformX
- 28. 28 Conclusions • Reducing Rb is a key to optimize GHE length to decrease installation cost and improve the pay back period of GSHP • The GeoperformX pipe with its 75 % higher thermal conductivity is a unique product to achieve bore length reduction
- 29. 29 Conclusions • Using the GeoperfomX pipe with 1U, 2U and coaxial configurations typically results in 5 to 25 % bore length reduction • Performances have been proven in the lab, in the field and with simulations of systems