A multi-step multicriteria approach to evaluate geothermal energy tunnels in urban contexts

Cities need profound change to meet energy transition goals and pursue sustainable urban development. The prevailing energy systems are based on the use of finite resources and produce considerable environmental degradation. Consequently, there is an imperative to diversify energy sources, to mitigate climate change and enhance urban resilience. In this context, the reuse and transformation of elements already present in the city and little exploited, such as underground tunnels, can be the starting point for creating new energy systems based on renewable energy sources, such as geothermal (Barla et al. 2025). The development of this technology requires a two-fold analysis: firstly, a spatial analysis to assess the most suitable areas, taking into account multidimensional and spatial criteria; and secondly, a physicaltechnical tunnel analysis to identify priority tunnels. Specifically, a spatial multi-criteria analysis (S-MCA) (Malczewski, 1999) is employed to assess the suitability of territories, according to seven dimensions (social, urban, economic, energy, environmental, cultural, and morphological), resulting in a suitability map for territory intervention. After that, a multicriteria analysis (MCA) (Greco et al. 2016) is, then, used to evaluate existing tunnels with respect to geometric-structural and property characteristics, usability, accessibility, and management. The results of the two analyses are aggregated through a system of weighing the different factors and dimensions to obtain a final overall categorization to identify priorities for intervention. The work is part of the European call Driving Urban Transitions (DUT) and, specifically, the REgENEraTE project, coordinated by the Polytechnic University of Turin, and focused on the study and development of innovative solutions for the redevelopment of tunnels (operational or abandoned) and existing underground spaces in urban areas for thermal energy production and storage (REgENEraTE, 2025). The methodological proposal is applied, in the first instance, to the case study of Turin (Italy), which is characterized by a significant presence of underground tunnels, particularly air raid shelters. However, the proposed methodology will be replicated in other contexts, not only to assess the feasibility of reusing individual tunnels, but also to analyze the spatial suitability of the areas where these structures are located.