A geothermal heat exchanger (GHE) uses the geothermal energy for heating or cooling of residential places during winter or summer. Two different designs of GHEs, the straight-pipe and coiled-pipe designs, are evaluated in this study, and the effects of operating conditions including the inlet fluid Reynolds number and the inlet fluid temperature as well as the effects of ceramic nanoparticle additives to the working fluid of water are studied. For this purpose, a mathematical model is developed and used to predict the temperature gain, heat gain, exergy gain, and pressure loss of the working fluid for different concentration of additive ceramic nanoparticles (Al2O3­­ and MgO) into the working fluid. The developed mathematical model takes both laminar and turbulent flow regimes and the nanofluids into account. The validation of the model is performance for Reynolds numbers less than 7,500. It is shown that the coiled-pipe design has better performance compared to the straight-pipe design for GHEs. It also shown that how the temperature, heat gain and exergy gain change with increasing the additive nanoparticles into the base-fluid (water). The summary of remarkable results of this study are as follows: The coiled-pipe GHE design has higher temperature gain compared to the straight-pipe GHE design, while the pressure loss for both designs are low. The temperature gain of water as the working fluid for the coiled-pipe design for the Reynold number of 20 and the inlet fluid temperature of 12°C and 25°C reaches about 10°C and 6°C, respectively, while the pressure loss is low. Hence, the density difference between the downward and upward pipes of the GHE may be enough for natural circulation of water in the GHE. However, the flow rate corresponding to this low Reynolds number may not be enough for a GHE. The temperature gain significantly increases with increasing the volume fraction of additive nanoparticles to water, while the pressure loss will not increase too much. At low Reynolds number of 10, for the volume fraction of 2% of ceramic MgO and Al2O3 nanoparticles the temperature gain increases 1.62 and 1.56 times, respectively, compared to using only water without any additive nanoparticles to the working fluid. This will help to increase the flow rate for the natural circulation of fluid and the GHE may not need a circulating pump to run at low flow rates. The nanofluids can also significantly improve the heat and exergy gains. At the Reynolds number of 10, for the nanoparticles volume fraction of 2%, the heat gain of the nanofluid with MgO is about 6% greater than the nanofluid with Al2O3, and it is about 70% greater than the base-fluid (water). At the same conditions, the exergy gain ratio is about 1.72 and 1.62, respectively. As remarkable results, the nanofluid with MgO nanoparticles shows a better heat and exergy gains compared to the nanofluid with Al2O3 nanoparticles; and both heat and exergy gains of nanofluids increase by increasing the Reynolds number of working fluid.

Geothermal Heat Exchanger Performance With Nanofluids Containing Ceramic MgO and Al(2)O(3) Particles

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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