materials feature | Thermal conductivity
Figure 4 – Borehole thermal resistance for different combinations of thermal conductivity for pipe and backfilling material
Figure 5 – Reduction of the borehole heat exchanger length for different combinations of thermal conductivity for pipe and backfilling material relative to the standard case (thermal conductivity of pipe 0.4 W/(m.K), thermal conductivity of backfilling 0.8 W/(m.K)), assuming a rock thermal conductivity of 3.0 W/(m.K)
conductivity will decrease their contribution to total thermal resistance and consequently increase the contribution of the plastic pipe. it is therefore important to decrease also the thermal resistance of the plastic pipe, since it becomes the limiting factor in the performance of the geothermal system. As shown in section 1, it is possible to double the
through-plane thermal conductivity of plastic pipes by the addition of about 10% of c-Therm 001. in order to estimate the effect of this improvement on the performance and dimensioning of the geother- mal system, the borehole thermal resistance was calcu- lated by geowatt of Zurich, Switzerland, for standard double-u pipe (0.4 w/(m.K)) and for backfilling material (0.8 w/(m.K)) and for different combinations of thermal conductivity for pipe and backfilling material. As shown in Fig. 4, for a pipe diameter of 32 mm and
borehole diameter of 135 mm, the borehole thermal resistance can be reduced by about 50% when using highly conductive backfilling material (5 w/(m.K)), whereas a further reduction of about 20% is achieved using pipes with doubled through-plane thermal conductivity (0.8 w/(m.K)). As an additional consequence, the temperature difference between borehole wall and fluid is de-
References and notes [1] l. chen et al., Experimental Thermal and Fluid Science, Volume 33, issue 5, July 2009, p.922-928 [2] compounding world, october 2010, p.35 [3] Timcal ltd., patent pending [4] compound at 20%w/w loading has too high viscosity to be extruded in real production conditions. [5] H. Kim, c. w. macosko, polymer, Volume 50, 2009, p 3797-3809 [6] g. wypych Handbook of fillers, chem Tec publishing, 2010, Toronto, ontario, canada. [7] F. delaleux et al., Applied Thermal Engineering, Volumes 33-34, February 2012, p.92-99
20 compounding world | February 2012
creased, which has a big advantage in providing lower temperature for cooling purposes. The possible reduction of the borehole heat exchang-
er length relative to the standard case was also determined for a rock with thermal conductivity of 3 w/(m.K), based on a borehole heat exchanger length of 100 m and a flow rate 1 l/s of ethylene glycol (20%). As shown in Fig.5, the borehole heat exchanger
length can be reduced by about 15% when using a backfilling material with 2 w/(m.K) (already commer- cially available) and a plastic pipe with 0.8w/(m.K) (e.g. filled with ~10% of Timrex c-Therm). Shortening the length of the heat exchangers will of course reduce the overall costs of the geothermal system.
Conclusions Timrex c-Therm graphite can be used to produce thermally-conductive compounds suitable for pipe extrusion. At 10% loading, the through-plane thermal conductivity is almost doubled. This allows a significant reduction of the borehole thermal resistivity and consequently of the borehole heat exchanger length. This new generation of thermally-conductive fillers
opens the door to new, cost-efficient technical develop- ments to be applied in geothermal and other applications. ❙
www.timcal.com
Acknowledgements The authors would like to thank mirko luciani (proplast, italy) for compounding, luis Zalamea and detlef Schramm (dow Europe, Switzerland) for pipe extrusion, and ladislaus rybach and Sarah Signorelli (geowatt, Switzerland) for calculations of geothermal systems.
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