Don't waste the water, use wastewater - excess heat distribution for private households through sewer networks

Private households account for a quarter of the EU's final energy demand. Approximately 80 % of this is used to provide heat and hot water. 84 % of this heat is generated from fossil fuels. The provision of space heating by means of environmentally friendly district heating is regarded as an important lever for the decarbonisation of the building sector. In this respect, one of the EU's targets is to find heat sources for district heating networks that emit no or few GHGs. In this context, industrial excess heat is often discussed as possible supply option. However, if this heat is far away from district heating networks, then the construction of new infrastructure is capital intensive. This can lead to low economic efficiency. For this reason, the existing sewer network offers a solution with an existing infrastructure. Here excess heat can be transferred to the wastewater with heat exchangers, which leads to a rising wastewater temperature. The energy quantity of the wastewater is then taken at another point along the flow direction of the sewer and used as heat source for a heat pump. The heat pump can then be operated more efficiently with the warmer wastewater and the excess heat is thus used. However, the potential for transporting excess heat via sewer networks has not yet been assessed and we are therefore carrying out such an assessment. We use a data set of more than 950 industrial sites and more than 26,000 sewage treatment plants. For a given set of industrial sites with information about excess heat, we identify the nearest sewage treatment plant. After that, we determine the maximum distance and the flow direction of the wastewater and calculate the heat loss in the sewer network. Our results demonstrate that transferring heat via the sewer network offers high theoretical excess heat potentials. In conclusion, in the overall dataset the average heat loss in the sewer network is around 20 %, which is more or less in the range of heat losses in district heating. Further research is needed to quantify the economic and technical potential based on real case studies.