Long-term efficiency of horizontal closed-loop geothermal heat exchangers for stabilization of permafrost beneath a Subarctic lagoon
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Date
2021
Authors
Fatollahzadeh Gheisari, Amir
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Abstract
Wastewater treatment lagoons are practical and cost-effective systems for small municipalities to reduce nutrient and oxygen release into the environment. However, as they disrupt the natural soil temperatures, they initiate permafrost degradation and cause foundation instability and safety concerns in subarctic regions. In this thesis, the long-term effects of closed-loop horizontal geothermal heat exchangers (GHEs) on the stabilization of permafrost below a wastewater lagoon in northern Canada were studied. This research examined three different geometrical and operational parameters including pipe spacing, heat carrier inlet velocity, and temperature which have the potential to impact the GHE performance in preserving ice-rich permafrost. Thaw settlement was addressed in this context. Also, a machine-learning algorithm was employed to predict unavailable future lagoon temperature based on the currently available weather data. The thesis concludes that the GHE with high-density polyethylene pipes can effectively mitigate and postpone the predicted permafrost thawing under a lagoon. However, under the projected climatic scenario, the GHE system even with every different selected operational parameter fails to eliminate thawing over its lifetime of 50 years. The heat exchangers’ operational parameters substantially affect the permafrost thaw depth. Among all three studied parameters, the heat exchanger fluid temperature is the most influential parameter while the fluid inlet velocity only makes small differences in the thaw depth and thaw settlement.
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Keywords
Sustainability, Permafrost, Geothermal heat exchangers, Wastewater lagoon