Evaluation of an integrated sewage pipe with ground heat exchanger for long-term efficiency estimation
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Date
2020-02
Authors
Dacquay, Connor
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Abstract
Extracting heat from a sewage pipe through a typical horizontal ground heat exchanger has recently been introduced as a renewable energy alternative to reduce fossil fuel usage. This paper presents a novel design for a ground heat exchanger that extracts heat from the surrounding soil and sewage within the pipe while simultaneously being carried to a wastewater treatment plant. This research focuses on the long-term efficiency of the system under transient conditions in a cold climate. A numerical model using COMSOL Multiphysics was developed to verify the sustainability of the system for over 25 years. The
model used constant inlet fluid temperatures to evaluate heat by conductive and convective heat transfer mechanisms within the pipe and surrounding soils by considering phase change of pore water. The results showed, by adopting the operation strategy proposed in this study, a maximum temperature change in the surrounding soil adjacent to the heat extraction system over 25 years was 0.10°C during the heating season in Winnipeg, Manitoba. The horizontal distance at which the heat extraction system did not show an impact on temperature change of adjacent soil was determined at 4 meters. Critical parameters in this evaluation were system depth, sewage level, and the high-density polyethylene pipe thermal properties. The sustainability of the system was not affected by the system depth due to thermal balancing between climatic, subsurface and sewage heat fluxes. Sustainable behavior was achieved at 50% and 75% of sewage pipe capacity. The effect on thermal performance from the high-density polyethylene pipe thermal properties was deemed insignificant.
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Keywords
Renewable energy, Heat extraction, Geothermal