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Simulation of Borehole Heat Exchangers using MODFLOW-USG and USG-Transport

Simulation of Borehole Heat Exchangers using MODFLOW-USG and USG-Transport. Sorab Panday, spanday@gsienv.com Thomas Conzen, t.conzen@geoconsulting.de. Introduction.

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Simulation of Borehole Heat Exchangers using MODFLOW-USG and USG-Transport

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  1. Simulation of Borehole Heat Exchangers using MODFLOW-USG and USG-Transport Sorab Panday, spanday@gsienv.com Thomas Conzen, t.conzen@geoconsulting.de

  2. Introduction • There is a push in the US and EU to reduce use of fossil fuels for heating and cooling buildings. Water Framework Directive (2000/60/EC) for groundwater quantity and Groundwater Directive (2006/118/EC) for groundwater quality • As electric power for the operation of heat pumps is increasingly generated from renewable energies, CO2 emissions can be reduced at least to less than 40%. • Borehole heat exchangers (BHEs) are effective solutions for controlling the temperature of buildings in several locations

  3. BHE Concept and Designs Schematicdrawingsshowingthearrangementsofdifferent systems. Single U-pipe BHE. (b) Double U-pipe BHE. (c) Coaxial BHE with internal return. (d) Coaxial BHE with external return red=flow, blue =return (withrespecttoheat pump). (Graphic: Sassand Mielke, 2012.) • A BHE is a long U-shaped tube encased in grout, emplaced 10s to 100s of meters below the ground • Heat is transferred from groundwater through the grouting material to the fluid in the tube which is used for temperature control of the building • The relatively stable groundwater temperatures help to heat buildings in winter and cool them in summer

  4. There is a competing use of the thermal resource due to the steady growth of large geothermal plants, especially in the inner city area. Major cities have observed increases in groundwater temperatures over the past 30 years Simulations can help understand and manage the growth and efficiency of BHE systems, and the thermal groundwater quality of complex settings Importance of Comprehensive Simulations for BHE Operations Mutual influence of geothermal well systems for heating and cooling in the city of Bonn near the river Rheine, Germany (Conzen, 2011) Geothermal plants Stockholm, Sweden

  5. MODFLOW-USG Transport – capabilities for comprehensive analyses of BHE systems and their impacts

  6. MODFLOW-USG Horizontal Grid Flexibility Nested grids capture small-scale details Quadtree grids capture resolution at streams/wells Voronoi grids conform to basin geometry and provide resolution at streams/wells Key Point: Domain complexity and required resolution can be accommodated for large complex aquifer systems

  7. MODFLOW-USG Vertical Grid Flexibility • Each layer can have different nesting structure • Sub-layering can be performed • Cross-layer connections and layer pinching are allowed Key Point: Complex geology can be accommodated Snapshot from Groundwater Desktop (www.sspa.com)

  8. Connected Linear Network – Karst, Rivers, Wells, BHE U-Tubes Courtesy of Alex Mikszewski and Neven Kresic Courtesy of Geary Schindel, Edwards Aquifer Authority and Neven Kresic Key Point: Flexible features of code accommodate simulating BHE U-Tubes as well as interactions with rivers and karstic features which can play a significant role in energy transport.

  9. CLN discretization is independent of GWF cells – scale and resolution considerations Many CLN cells within one GWF cell One CLN cell can span multiple GWF cells Analytical solution reduces GW discretization effects Includes well efficiency; skin effects Linear Features – CLN – GWF Interaction Key Point: Flexibility in discretization with minimal loss of solution accuracy.

  10. Heat Transport Capability • USG-Transport solute capability modified to accommodate heat transport • Divide energy balance equation by • Temperature equivalent to Concentration • Storage of heat equivalent to storage of solute in water • Storage of heat on soil is related to adsorption as • Dispersion and thermal diffusivity are related as • Heat conductivity of medium is obtained as • CLN-GW interaction expressed as • Conversions and complexity already accommodated in USG-Transport

  11. Simple BHE Simulation Setup 100 m 50 m Quadpatch refinement 7 levels 16 layers, U-tube through layer 12 CLN flow = 2 cu-m/hr @ 273.15 K Groundwater Temperature = 286 to 283 K Horizontal and vertical groundwater temperature gradients

  12. Groundwater Temperature Regime After 1 year 286 Cross-section through return pipe location 7 m 278 Areal Groundwater Temperature Distribution Layer 2 Areal Groundwater Temperature Distribution Layer 12 Key Point: Groundwater temperature regime is complex even in a homogeneous setting with just one borehole

  13. Borehole Exit Temperature for Warming Buildings Entry temperature is 273.15 K • Temperature jumps up immediately responding to warmer groundwater at system startup then cools with surrounding groundwater. • Temperature stabilizes by 50 days at 1.7 degrees above inflow temperature Key Point: Borehole exit temperatures are complex even with a steady ambient water quality regime

  14. Next step: one model to simulate them all • Design of large, multiple BHE systems in flowing groundwater has been possible only to a limited extent • There can be a very strong influence of Darcy velocity on performance specially with large fluctuations in flow field – influence on cities built near rivers can be large • 50 to 100 % changes in efficiency compared to conventional designs without considering flow • Quantify the effects of hundreds or thousands of plants on each other and the aquifer to sustainably and economically use the geothermal energy resource especially in urban centers • Establish zones and corridors for cities where heat or cold is stored in groundwater 3 km Velocity during a flood event of the river Rhein, Cologne, Conzen 2017

  15. Accurate simulation of BHEs can assist with Design evaluations of different capacity systems in different groundwater conditions Evaluation of operations through heating and cooling cycles Understanding complex interactions of multiple BHE systems Estimating the long term and short term impact of BHEs on the groundwater temperature regime MODFLOW USG-Transport provides effective simulations of BHE systems for local and regional evaluations Can capture complexities at the groundwater and U-tube scale of the geothermal energy transport simulation Includes heterogeneity, fissured rock evaluations (CLNs), multi-scale capability, complex transient boundary conditions Open source, public domain; https://www.gsi-net.com/en/software/free-software/modflow-usg.html Conclusions

  16. Questions • Sorab Panday, spanday@gsienv.com • Thomas Conzen, t.conzen@geoconsulting.de ??

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