The Role of Exergy and Emissions in Municipal Heat Planning: Insights from Hamburg

Abstract

As part of municipal heat planning, large cities such as Hamburg are required to develop a heat plan by the end of June 2026. A central challenge is the substitution of heat supply in urban areas currently connected to the gas network. For a sustainable heat supply, district heating networks mainly compete with decentralized heat supply using air-to-water heat pumps. The decision to implement a district heating network often relies on linear heat density as a key criterion. This study examines heat planning in three Hamburg districts, comparing current primary energy, greenhouse gas emission, and resource exergy factors for different heat supply options. The results suggest that the primary energy factor is influenced by political frameworks, while the greenhouse gas emission factor depends on the applied accounting methodology. The resource exergy factor provides a physically consistent evaluation metric for assessing the efficiency of local heat supply options and is the only factor investigated in this study that can distinguish among renewable energy sources. Considering the current electricity mix and the assumptions made, combined heat and power plants and waste heat utilization in district heating networks outperform air-to-water heat pumps. The study also reveals a prevailing tendency toward decentralized heat supply as a substitute for gas-connected districts, which poses significant lock-in risks. These risks stem primarily from the recent abolition of the so-called 65 % renewable energy obligation, which may prolong the use of fossil-fuel-based heating systems. At the same time, large-scale deployment of decentralized heat pumps may substantially increase electricity demand, so that both developments may adversely affect the energy transition. The findings suggest that linear heat density alone is insufficient and underscore the necessity of a holistic, locally differentiated assessment framework. Integrating exergy analysis and life cycle assessment into future municipal heat planning to assess efficiency and material consumption is proposed as a holistic approach to effectively achieve climate-neutral heat supply in Hamburg.