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)

8.5 9 9.5 10 10.5 11 11.5 12 12.5

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Temperature (oC) Depth (m)

Exp,t=20.42 h Exp,t=20.92 h Sim,t=19.76 h Sim,t=19.93 h Sim,t=20.1 h Sim,t=20.42 h Sim,t=20.92 h

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Depth (m) Temperature ( oC )

94 min 98 min 104 min 120 min

Simulation of heat pump

• peration – U-tube BHE

Simulation of thermal responstest – coaxial (tube-in-tube) BHE U-tube BHE Coaxial 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

Source: NGU Report 2013.008, Evaluation of the deep geothermal potential in Moss area, Østfold County.

Temperature measurements in on-shore boreholes in Norway

Source: Slagstad et al. 2009

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Depth (m) Temperature (oC)

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Depth (m) Temperature (oC)

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Depth (m) Temperature (oC)

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Depth (m) Temperature (oC)

T-in T-out T-Und data4 T Annular T Centerpipe Undisturbed temperature T Borehole wall T-in T-out T-Undis a) U-tube BHE b) Coaxial BHE, inlet through center pipe c) Coaxial BHE, inlet through annular space d) Coaxial BHE, insulated center pipe

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

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)

Parameter Value kg 3.53 W /m K Active length BHE [m] 490 Borehole diameter [mm] 140 Collector (center pipe) [mm] 50 x 4.6 Collector (outer pipe) [mm] 139 x 0.4 kc [W /m K] 0.42 kins [W /m K] 0.1 Heat carrier Water Mass flow rate [kg / s] 1 Specific thermal load [W /m] 40

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.

• Parametric study with the
• verall system performance

(COP) as the objective

• Varying center pipe wall

thickness and mass flow rate.

Thermal performance Hydraulic performance

Finding: The system performance (COP) is relatively insensitive to the center pipe wall thickness. Increases with depth! However.. Heat extraction rate, directly related to mass flow rate!

1 3 5 7 9 11 13 15 17 19 0.9 0.92 0.94 0.96 0.98 1 Normalized performance (COP

total / max(COP total)))

Wall thickness (mm)

1.5 kg / s 2 kg / s 2.5 kg / s 3 kg / s 3.5 kg / s 4 kg / s 4.5 kg /s 5 kg /s Maximum

Temperature profiles in 800 m coaxial BHE

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Temperature (oC) Depth (m)

t=0.99 h t=1.16 h t=1.33 h t=4.83 h t=32.83 h T-initial

q=50W/m m=4kg /s xins= 0.51 cm

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Temperature (oC) Depth (m) t=0.99 h t=1.16 h t=1.33 h t=4.83 h t=32.83 h T-initial

q=-50W/m m=4kg /s xins= 0.51 cm

Heat extraction Thermal recharge center pipe : 90 mm x 5.1 mm

Results from simulations – long term performance

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

• Constant heat load
• Constant mass flow rate

z r rb Tg ground center pipe

• uter pipe

Twall T

• ut

Tin

℄

center pipe wall

• uter pipe wall

Tin Tout

• 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

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Time (h)

Temperature (

• C)

10 20 30 40 50 60 70 80 90 100 1 2 3 4

Mass flow rate (kg/s)

Tinlet Toutlet Mass flow rate

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 drop1 [bar] 1.0 1.0 1.7 Pump power required2 [kW] 0.47 0.53

• 1. 33

1It is assumed that the annular space is confined within a smooth-walled outer pipe. 2Assuming ηpump=0.75.

The geothermal temperature gradient is constant at 20 K / km A relatively high mass flow rate is used – reduces need for thermal insulation

Long term simulation,

2 4 6 8 10 12 14 16 18 20 2 3 4 5 6 7 8 9 10 Time (year) Tfmean (oC) 60 W / m 1000 m, Q=60 kW 50 W /m 800 m, Q=40 kW 40 W / m 600 m, Q=24 kW

Depth (m) MWhth/ year 600 70 800 117 1000 175

Yearly energy production

Distribution of specific heat load (W/m)

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Specific heat load (W /m) Depth (m) 100 hours

Heat losses in the upper part of the borehole

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Specific heat load (W /m) Depth (m) 100 hours 1000 hours

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Specific heat load (W /m) Depth (m) 100 hours 1000 hours 2000 hours

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Specific heat load (W /m) Depth (m) 100 hours 1000 hours 2000 hours 5000 hours

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Specific heat load (W /m) Depth (m) 100 hours 1000 hours 2000 hours 5000 hours Year 5 3% 6% 9% 11% 14% 16% 19% 22%

0- 100 m 100 - 200 m 200- 300 m 300 - 400 m 400 - 500 m 500 - 600 m 600- 700 m 700 -800 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

• 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
• f space and negatively balanced loads.

Summary - conclusions