Assessment of a polygeneration system for district heating and cooling using biogas from wastewater treatment plants

Biogas, as a product of anaerobic biodegradation processes of organic matter, has significant potential to be integrated into district heating networks to improve their efficiency. Wastewater treatment plants can be transformed into dual-purpose centres for sustainable resource generation and pollution reduction while also providing sludge management. This work investigates the role of biogas generated in wastewater treatment plants in fulfilling the demand requirements for heating, cooling, and domestic hot water in urban areas with district heating and cooling systems. The analysis estimates available biogas potential, evaluates total thermal energy demand, selects equipment for a polygeneration plant, and conducts techno-economic optimisation. Three urban locations in southern Europe with different climatic conditions were selected as case studies to establish energy demand profiles. Specific configurations were proposed for each plant, aiming to meet local thermal energy demands with an optimised cost. With overall efficiencies and equivalent electric efficiencies in the plants of up to 84 % and 79 %, respectively, integrating biogas into the proposed polygeneration systems as a support for primary energy achieves a reduction in primary energy consumption by 21 % to 39 % of total demand and economic savings from 40 % to 50 % of annual cost. It shows the interest and the technical and economic feasibility of partially replacing natural gas with biogas, supporting system decarbonisation and contributing to long-term climate neutrality goals.