Evaluation of an integrated sewage pipe with ground heat exchanger for long-term efficiency estimation

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 transfer with convective pipe flow and conductive phase change within the soil. The results showed 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 distance at which the heat extraction system did not show an impact on temperature change of adjacent soil was determined at 4 m. 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.