Thermodynamic Performance Analysis of High Thermal Conductivity Materials in Borehole Heat Exchangers in the European Climate
Abstract
:1. Introduction
2. Exergy Analysis for Thermodynamic Performance Analysis of GSHPs
3. Methodology
3.1. Scope of Work and Simulation Approach
3.2. Scenarios Definition
- Baseline: This reference scenario is representative of the material and performance achievable using state of the art materials for BHE construction. Specifically, this includes the BHE pipe and grout. Badenes et al. [5] document the developments in pipe and grouting material achieved in the GEOCOND project. The baseline corresponds to the PE100 pipe (thermal conductivity of 0.42 W/mK) and a grout with thermal conductivity of 2 W/mK.
- Semi-GOECOND: The pipe used is the standard PE100 but with the improved GEOCOND grout (thermal conductivity of 3 W/mK).
- GEOCOND: In this scenario, both the pipe and grouting used GEOCOND materials, with thermal conductivity values of 1 W/mK and 3 W/mK, respectively.
Stockholm 2 × 107 m | |
---|---|
Ground | |
Thermal conductivity | 3.5 W/(m·K) |
Heat capacity | 2.16 MJ/(m3·K) |
Surface temperature | 7.6 °C |
Geothermal heat flux | 0.05 W/m2 |
Borehole | |
Configuration | 1 × 2 line |
Depth | 107 m |
Borehole Spacing | 10 |
Borehole Installation | Single U |
Borehole Diameter | 120 mm |
U-pipe diameter | 32 mm |
U-pipe thickness | 3 mm |
U-pipe thermal conductivity | 0.42 W/(m·K) |
U-pipe shank spacing | 60 mm |
Filling thermal conductivity | 2 W/(m·K) |
Contact resistance pipe/filling | 0 m·K/W |
Thermal Resistances | |
Number of multipoles | 10 |
Heat Carrier Fluid | |
Thermal conductivity | 0.48 W/(m·K) |
Specific heat capacity | 3795 J/(Kg·K) |
Density | 1052 Kg/m3 |
Viscosity | 0.0052 Kg/(m·s) |
Freezing point | −14 °C |
Flow rate per borehole | 2 L/s |
3.3. The Modelled Building
Exergetic Indicators for Performance Analysis
4. Results and Discussion
Energy Demand | Exergy Demand | BHE Exergy Extracted | BHE Exergy Extracted Improvement | BHE Exergy Proportion | Electricity Demand | GSHP Exergy Efficiency | |
---|---|---|---|---|---|---|---|
kWh | kWh | kWh | % | % of total | kWh | % | |
Stockholm 2 × 107 m | |||||||
Baseline | 17,833 | 1166 | 235 | 0 | 20.15 | 4596 | 25.37 |
Semi-GEOCOND | 17,833 | 1166 | 247 | 5.11 | 21.18 | 4554 | 25.6 |
GOCOND | 17,833 | 1166 | 265 | 12.77 | 22.73 | 4497 | 25.93 |
Stockholm 1 × 190 m | |||||||
Baseline | 17,833 | 1166 | 333 | 0 | 28.56 | 4786 | 24.36 |
Semi-GEOCOND | 17,833 | 1166 | 349 | 4.8 | 29.93 | 4763 | 24.48 |
GOCOND | 17,833 | 1166 | 376 | 12.91 | 32.25 | 4727 | 24.67 |
Valencia 2 × 80 m | |||||||
Baseline | 14,366 | 378.22 | 229 | 0 | 60.55 | 3655 | 10.35 |
Semi-GEOCOND | 14,382 | 384.9 | 219 | −4.37 | 56.9 | 3591 | 10.72 |
GOCOND | 14,398 | 407.7 | 209.8 | −8.38 | 51.46 | 3513 | 11.61 |
5. Conclusions and Recommendations
- Enhanced BHE Exergy Extraction: The adoption of pipe and grout materials boasting thermal conductivities of 1 W/mK and 3 W/mK, respectively, can usher in notable improvements in the exergy extraction capabilities of the borehole heat exchanger (BHE). This assertion is substantiated by the observed 13% enhancement for the Stockholm scenario. This uptick subsequently curtails the GSHP’s electricity requisites by approximately 99 kWh/year, a factor with direct economic ramifications spanning the entire use phase of the life cycle.
- Regional Variations in Exergetic Performance: For Valencia, the exergetic gains were found to be rather marginal. Additionally, the average exergy efficiency in Valencia trailed at roughly 11%, as opposed to Stockholm’s more robust 25%. A pivotal takeaway from our analysis underscores the dependency of exergy efficiency—and the scope for thermodynamic augmentation via high-conductivity materials—on external factors like the outdoor dry bulb temperature and its deviation from the indoor setpoint.
- Advantages of Increased BHE Depth: A deeper BHE (1 × 190 m in contrast to 2 × 107 m) results in more potent exergy extraction. This depth increment not only results in exergy efficiency that rivals the shallower BHE but also achieves this with a 24 m reduction in total BHE length. It is imperative to note, however, that this study didn’t encompass potential variations in electricity demands due to pumping, which could be influenced by the increased depth.
- Exergy Loss Due to Energy Quality Degradation: Within the GSHP energy paradigm, the most pronounced exergy drain stems from the inevitable quality degradation as electricity morphs into low-grade heat. This accounted for a significant 77% loss for Stockholm and an even steeper 89% for Valencia. These figures accentuate the importance of energy quality alignment between supply and demand mechanisms.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Khattak, S.; Badenes, B.; Urchueguia, J.; Sanner, B. Thermodynamic Performance Analysis of High Thermal Conductivity Materials in Borehole Heat Exchangers in the European Climate. Buildings 2023, 13, 2276. https://doi.org/10.3390/buildings13092276
Khattak S, Badenes B, Urchueguia J, Sanner B. Thermodynamic Performance Analysis of High Thermal Conductivity Materials in Borehole Heat Exchangers in the European Climate. Buildings. 2023; 13(9):2276. https://doi.org/10.3390/buildings13092276
Chicago/Turabian StyleKhattak, Sanober, Borja Badenes, Javier Urchueguia, and Burkhard Sanner. 2023. "Thermodynamic Performance Analysis of High Thermal Conductivity Materials in Borehole Heat Exchangers in the European Climate" Buildings 13, no. 9: 2276. https://doi.org/10.3390/buildings13092276