Deep borehole heat exchangers

Transcription

1 Deep borehole heat exchangers Henrik Holmberg, Asplan Viak AS and NTNU-Department of energy and process engineering. Seminar at KTH, Stockholm,

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

3 Background: Numerical simulation (TRCM-models) of single borehole heat exchangers (within PhD- at NTNU) -5 U-tube BHE 94 min 98 min 14 min 12 min Coaxial BHE Temperature ( o C ) Simulation of heat pump operation U-tube BHE Exp,t=2.42 h -14 Exp,t=2.92 h Sim,t=19.76 h -16 Sim,t=19.93 h Sim,t=2.1 h -18 Sim,t=2.42 h Sim,t=2.92 h Simulation of thermal responstest coaxial (tube-in-tube) BHE Holmberg. H., Acuña. J., Næss. E., Sønju. K. O., Numerical model for non-grouted borehole heat exchanges, part 2-Evaluation. (214) Accepted for publication, Geothermics

4 BHE installations in Norway using 5 m boreholes: Skoger skole, 5 x 5 m Maudbukta residental building (Asker), 9 x 5 m These systems use single U-tube collectors, (PEM5) 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

5 Temperature measurements in on-shore boreholes in Norway Source: NGU Report 213.8, Evaluation of the deep geothermal potential in Moss area, Østfold County. Source: Slagstad et al. 29

6 How to choice -1 of collector- Results from simulation based on undisturbed temperature profile -2 from 49 m deep borehole- continuous heat extraction 4 W/m for 5 Parameter k g hours Value Active length BHE [m] 49 Borehole 2 4 diameter [mm] Collector (center pipe) 5 x 4.6 [mm] Collector (outer pipe) [mm] k c [W /m K].42 k ins [W /m K].1 Heat carrier 3.53 W /m K 139 x.4 Water T-in T-out -2 Temperature ( o T-Undisturbed C) data Mass flow T Annular rate [kg / s] T 1 Centerpipe Specific thermal load 4 Undisturbed -2temperature T Borehole wall [W /m] T-in a) U-tube BHE b) Coaxial BHE, inlet through center pipe T-out Temperature ( o T-Undist C) data 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 (.1 W/ m K)

7 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?

8 Parametric study Coaxial pipe-in-pipe BHE, influence of insulation of the center pipe. Parametric study with the overall system performance (COP) as the objective Varying center pipe wall thickness and mass flow rate. 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! Hydraulic performance Normalized performance (COP total / max(cop total ))) 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 Thermal performance Wall thickness (mm)

9 Temperature profiles in 8 m coaxial BHE Heat extraction t=.99 h t=1.16 h t=1.33 h t=4.83 h t=32.83 h T-initial q=5w/m m=4kg /s x ins =.51 cm center pipe : 9 mm x 5.1 mm Thermal recharge t=.99 h t=1.16 h t=1.33 h t=4.83 h t=32.83 h T-initial q=-5w/m m=4kg /s x ins =.51 cm

10 Results from simulations long term performance z center pipe center pipe wall r b r T inlet T outlet Mass flow rate T out T out T in T in T g T wall outer pipe outer pipe wall ground - Constant heat load - Constant mass flow rate Time (h) Mass flow rate (kg/s) 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 = 29 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 215, Melbourne, Australia April presented by Davide Rolando

11 Parameters used in the simulation Table 1. Case specific parameters Parameter Value Value Value Active length BHE [m] Collector (center pipe) [mm] 75 x x x 3.5 Mass flow rate [kg / s] Thermal load [W /m] Pressure drop 1 [bar] Pump power required 2 [kw] It is assumed that the annular space is confined within a smooth-walled outer pipe. 2 Assuming ηpump=.75. A relatively high mass flow rate is used reduces need for thermal insulation The geothermal temperature gradient is constant at 2 K / km

12 Long term simulation, W / m 1 m, Q=6 kw 5 W /m 8 m, Q=4 kw 4 W / m 6 m, Q=24 kw Yearly energy production MWhth/ year Tf mean ( o C) Time (year)

13 Distribution of specific heat load (W/m) Heat losses in the upper part of the borehole (m) 1 hours 1 hours hours 2 hours -2 5 hours -2 Year % % % Specific heat 4 load (W /m) Specific heat load (W /m) 6% 3% 16% 22% 19% - 1 m 1-2 m 2-3 m 3-4 m 4-5 m 5-6 m 6-7 m 7-8 m 7 % of the thermal energy from 4 8 depth Larger distance required between the lower part of the boreholes

14 Deep BHEs in combination with shallow BTES.

15 Ongoing project in Asker municipality

16 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.