Heat transfer analysis of sewer system and its potential role in thermal energy storage

Wastewater from urban areas contains a large amount of thermal energy. It constantly exchanges this energy with its surrounding. This study analyzes the thermal exchanges between wastewater and its immediate environment. A mathematical model is constructed that allows to predict the level and velocity of the water in the sewer as a function of time. From this information, the model calculates the heat transfer between the wastewater and the surrounding soil. The results show that the soil temperature can be modified over a maximum thickness of 5 to 10 m. Close to the sewer, soil temperature is constantly influenced by the wastewater, while the soil beyond 10 m does not participate to the exchange. Regarding the heat exchange between wastewater and its environment, the results show that at least 90% of the heat exchange takes place with soil through the part of the pipe in contact with the wastewater while only 10% of the exchange takes place through the air contained in the pipe. The simulations also show the interest of carrying out charge/discharge of thermal energy with the ground surrounding a sewer. For a sewer of 1800 m length and a wastewater flow of 65 m3/h during the day and 35 m3/h during the night, one can expect to transfer up to 76 kW during the day and discharge 40 kW during the night. In addition, the flow rate plays an important role in the heat transfer process, especially with a partially filled sewer pipe. A higher flow rate means a larger wet area in the pipe and thus an increase in heat exchange. This preliminary analysis shows that the sewer network can be used as an underground thermal storage system to cope with the variations in heating and cooling demand with the goal of improving urban energy efficiency.