The role of excess heat in reducing the environmental impacts of district heating systems.

The European Union (EU) aims to reduce its greenhouse gas (GHG) emissions 55 % from 1990 levels by 2030 and achieve climate neutrality by 2050. One strategy to lower emissions in the energy sector is to utilize the excess of waste heat to substitute primary energy. District heating (DH) is essential in this context, as it allows for the large-scale utilisation of excess heat. An increasing volume of literature aimed at tackling this is appearing, but from a variety of methods. This review article critically examines 39 academic articles on the use of excess heat in district heating systems (DHS) and the estimation of its environmental impacts. The analysis identifies four primary areas for future research. Firstly, there is a notable lack of standardized definitions for excess heat, with existing definitions varying significantly across studies. This inconsistency underscores the need for a universally accepted definition to facilitate comparative research. Secondly, the scope and system boundaries of life cycle assessments (LCAs) vary widely, with some studies focusing on a broad range of impact categories while others only consider climate change impacts. This variability highlights the need for standardization in LCA scope to ensure comprehensive environmental assessments and avoid burden shifting. Thirdly, environmental considerations are inconsistent, particularly regarding the allocation of burdens between suppliers and users of excess heat. This issue risks double counting GHG-reductions and other environmental impacts, potentially undermining genuine reductions and enabling exploitation of environmental benefits without substantial changes. Lastly, the time aspect is often overlooked in LCAs, despite its importance in understanding the dynamic environmental impacts of excess heat utilisation. The mismatch between the non-seasonal supply of excess heat and the seasonal demand for district heating complicates the assessment of environmental impacts. Only one study incorporates the time aspect by differentiating between summer and winter seasons. Addressing these gaps, particularly the time aspect, is crucial for accurate and effective environmental impact assessments of excess heat in DHS.