Deep borehole heat exchangers Henrik Holmberg, Asplan Viak AS and - PowerPoint PPT Presentation

Deep borehole heat exchangers Henrik Holmberg, Asplan Viak AS and NTNU-Department of energy and process engineering. Seminar at KTH, Stockholm, 28.5.2015. Henrik.holmberg@ntnu.no Henrik.holmberg@asplanviak.no

Layout: • Background – motivation • How to - Choice of collector ? • Results from simulations – coaxial BHE - Parametric study - Long term performance • Summary - Conclusions

Background: • Numerical simulation (TRCM-models) of single borehole heat exchangers (within PhD- at NTNU) U-tube BHE Coaxial BHE 0 0 94 min -20 98 min -50 -40 104 min 120 min -60 -100 -80 Depth (m) Depth (m) -100 -150 -120 Exp,t=20.42 h -140 Exp,t=20.92 h Sim,t=19.76 h -200 -160 Sim,t=19.93 h Sim,t=20.1 h -180 Sim,t=20.42 h Sim,t=20.92 h -250 -200 8.5 9 9.5 10 10.5 11 11.5 12 12.5 0 1 2 3 4 5 6 7 8 9 10 Temperature ( o C) Temperature ( o C ) Simulation of thermal responstest Simulation of heat pump – coaxial (tube-in-tube) BHE operation – U-tube BHE Holmberg. H., Acuña. J., Næss. E., Sønju. K. O., Numerical model for non-grouted borehole heat exchanges, part 2-Evaluation. (2014) Accepted for publication, Geothermics

BHE installations in Norway using 500 m boreholes: • Skoger skole, 5 x 500 m • Maudbukta – residental building (Asker), 9 x 500 m These systems use single U-tube collectors, (PEM50) – more on that later.. What is the motivation for these installations? - Scarcity of available land/ construction area - Heating dominated load - Deep soil layers - Increased heat extraction rate/ energy

Temperature measurements in on-shore boreholes in Norway Source: NGU Report 2013.008, Evaluation of the deep Source: Slagstad et al. geothermal potential in Moss area, Østfold County. 2009

How to – choice of collector- Results from simulation based on undisturbed temperature profile from 490 m deep borehole- continuous heat extraction 40 W/m for 50 hours Temperature ( o C) Temperature ( o C) 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 0 Parameter Value T-in -100 -100 T-out k g 3.53 W /m K Depth (m) Depth (m) -200 T-Undis -200 Active length BHE [m] 490 -300 -300 Borehole diameter 140 [mm] -400 -400 Collector (center pipe) 50 x 4.6 -500 -500 [mm] T-in a) U-tube BHE b) Coaxial BHE, inlet through center pipe Collector (outer pipe) 139 x 0.4 T-out [mm] T-Und Temperature ( o C) Temperature ( o C) data4 k c [W /m K] 0.42 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 0 k ins [W /m K] 0.1 Heat carrier Water -100 -100 Depth (m) Depth (m) Mass flow rate [kg / s] 1 -200 -200 Specific thermal load 40 [W /m] -300 -300 -400 -400 -500 -500 c) Coaxial BHE, inlet through annular space d) Coaxial BHE, insulated center pipe T Annular T Centerpipe Undisturbed temperature T Borehole wall a) U-tube BHE. b) Coaxial BHE, inlet through center pipe. c) Coaxial BHE, inlet through annular space. d) Coaxial BHE with lower thermal conductivity of the collector material (0.1 W/ m K)

Choice of collector when extending the borehole depth. • Installation • Economics • Thermal performance • Hydraulic performance Do we need an thermally insulated center pipe? What temperatures can we get? How much energy can we get?

Parametric study – Coaxial pipe-in-pipe BHE, influence of insulation of the center pipe. Thermal Hydraulic • Parametric study with the performance performance overall system performance (COP) as the objective • Varying center pipe wall total ))) thickness and mass flow rate. 1 total / max(COP 0.98 Finding: The system performance Normalized performance (COP (COP) is relatively insensitive to the 0.96 1.5 kg / s 2 kg / s center pipe wall thickness. 2.5 kg / s 0.94 3 kg / s 3.5 kg / s Increases with depth! 4 kg / s 0.92 4.5 kg /s 5 kg /s However.. Maximum 0.9 1 3 5 7 9 11 13 15 17 19 Heat extraction rate, directly Wall thickness (mm) related to mass flow rate!

Temperature profiles in 800 m coaxial BHE 0 t=0.99 h -100 t=1.16 h t=1.33 h -200 t=4.83 h -300 Heat extraction Depth (m) t=32.83 h T-initial -400 q=50W/m -500 center pipe : 90 mm m=4kg /s x ins = 0.51 cm -600 x 5.1 mm -700 -800 6 8 10 12 14 16 18 20 22 24 Temperature ( o C) 0 -100 -200 t=0.99 h Thermal recharge -300 Depth (m) t=1.16 h t=1.33 h -400 t=4.83 h -500 t=32.83 h T-initial -600 q=-50W/m m=4kg /s -700 x ins = 0.51 cm -800 6 8 10 12 14 16 18 20 Temperature ( o C)

Results from simulations – long term performance 16 ℄ center pipe z center pipe wall T inlet T outlet Mass flow rate r b 14 r T g ground T T in T out T in 12 out Mass flow rate (kg/s) o C) 10 Temperature ( T wall 8 outer pipe outer pipe wall 6 4 4 3 2 2 - Constant heat load 1 0 0 - Constant mass flow rate 0 0 10 10 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 100 100 Time (h) The BHE is simulated with a cyclic operation strategy using operation periods of 24 hours and a recovery period of 4 months. Total operation time/ year = 2900 hours ≈ 4 months Holmberg. H., Acuña. J., Næss. E., Sønju. K. O., Deep borehole heat exchangers, application to ground source heat pump systems, Proceedings World Geothermal Congress 2015, Melbourne, Australia. 19 -25 April 2015. - presented by Davide Rolando

Parameters used in the simulation Table 1. Case specific parameters Parameter Value Value Value Active length BHE [m] 600 800 1000 Collector (center pipe) [mm] 75 x 4.3 90 x 5.1 90 x 3.5 Mass flow rate [kg / s] 3.5 4.0 5 Thermal load [W /m] 40 50 60 Pressure drop 1 [bar] 1.0 1.0 1.7 Pump power required 2 [kW] 0.47 0.53 1. 33 1 It is assumed that the annular space is confined within a smooth-walled outer pipe. 2 Assuming η pump =0.75. A relatively high mass flow rate is used – reduces need for thermal insulation The geothermal temperature gradient is constant at 20 K / km

Long term simulation, 10 60 W / m 1000 m, Q=60 kW Yearly energy production 9 50 W /m 800 m, Q=40 kW 40 W / m 600 m, Q=24 kW 8 Depth (m) MWh th / year 600 70 7 Tf mean ( o C) 800 117 1000 175 6 5 4 3 2 0 2 4 6 8 10 12 14 16 18 20 Time (year)

Distribution of specific heat load (W/m) Heat losses in the upper part of the borehole 0 0 0 0 0 100 hours 100 hours 100 hours 100 hours 100 hours -100 1000 hours 1000 hours 1000 hours 1000 hours -100 -100 -100 -100 2000 hours 2000 hours 2000 hours -200 5000 hours 5000 hours -200 -200 -200 -200 Year 5 3% 6% -300 0- 100 m -300 -300 -300 -300 22% Depth (m) 100 - 200 m Depth (m) Depth (m) Depth (m) Depth (m) 9% 200- 300 m -400 -400 -400 -400 -400 300 - 400 m 400 - 500 m -500 500 - 600 m -500 -500 -500 -500 11% 600- 700 m 700 -800 m -600 -600 -600 -600 -600 19% -700 -700 -700 -700 -700 14% -800 -800 -800 -800 -800 -20 0 20 40 60 80 100 16% -20 -20 -20 -20 0 0 0 0 20 20 20 20 40 40 40 40 60 60 60 60 80 80 80 80 100 100 100 100 Specific heat load (W /m) Specific heat load (W /m) Specific heat load (W /m) Specific heat load (W /m) Specific heat load (W /m) ≈ 70 % of the thermal energy from 400 – 800 depth Larger distance required between the lower part of the boreholes

Deep BHEs in combination with shallow BTES.

Ongoing project in Asker municipality

Summary - conclusions • Due to higher temperature level in the borehole the deep BHEs can sustain a higher average specific heat load than conventional BHEs • Best performance with a relatively high mass flow rate – reduces need for thermal insulation • Most energy is extracted in the lower part of the borehole, making deep BHEs insensitive to thermal influence from neighboring BHEs (shallow or deep) in the upper part • The required energy for circulation of the heat carrier fluid in the cases shown is on the order of 1-2 % of the produced thermal energy and can be reduced using a larger borehole diameter • Deep BHEs are, therefore, a viable option for GSHP installations in areas with scarcity of space and negatively balanced loads.