For Sweden, a country with one of the most developed district heating sectors in the world, the continued decarbonisation of central heat supplies and the contemporary development towards lower heat distribution temperatures is generating increased interest in large-scale seasonal thermal energy storage technologies. By virtue, not only of energy storage, but also of loadshifting and peak shaving capabilities, seasonal storages are recognised today as key components in highly integrated and smart energy systems but have so far seen very limited application in Sweden. Given that Denmark, neighbour to Sweden and with a district heating development on par, is a world-leader in the application of pit thermal energy storages (PTES), this paper aims to initiate investigations into the possibilities for a similar application in Sweden. A spatial analysis sequence, akin to indicator modelling, identifies suitable areas for the construction of PTES by elimination of non-suitable areas based on a set of input data parameters and associated selection criteria, all within cost-effective heat transmission distances of aggregated district heating areas. The national potential in terms of available suitable land, expressed and discussed here by two model extremes scenarios, is generally high, but with local variations and with a characteristic latitudinal difference. ©The Author(s).2025 This is an Open Access article published by Aalborg University Open Publishing under a CC-BY-NC-ND 4.0 license.

This paper is the second of three accounts which describes a Swedish study aiming to derive a first order assessment of the national potential for large-scale solar thermal heat production with pit thermal energy storage's (PTES) connected to existing district heating systems (DHS). Whereas the first paper presented project objectives, outset parameters, and an updated Swedish district heating database – and the third is planned to report on the final project results and conclusions – this paper focuses on the assembled study data and the associated selection criteria applied to these data categories under the objective to distinguish suitable (and non-suitable) land areas within cost-efficient heat transmission distances from the existing DHS. The approach centres around a principal spatial analysis with superposition of study data and elimination of non-suitable land areas according to the used selections criteria but also entails a wide periphery of related activities, such as literature reviews, gathering of technology preferences, meetings with sector experts, data management etc. Apart from technical specifications for solar heat production and seasonal storage, key data categories for the spatial analysis consist of geological data (e.g. soil types, soil depth, bedrock etc.), hydrological data (lakes, rivers, wells, soil moisture, ground water levels etc.), geographical data (elevation, built-up areas, administrative units etc.), and thematic data (energy statistics, building heat demands, district heat deliveries etc.). Selection criteria for the relevant data categories have been defined iteratively during e.g. expert consultancy, for example minimum soil depth, preferred soil types, maximum feasible transmission distance to existing DHS etc. By application of the selection criteria, raw input data are converted to processed data extracts to be used in the final analysis. Study data categories are illustrated and summarised (raw and processed) together with a listing and discussion of the used selection criteria.

Sweden was among the first countries to install solar thermal plants for district heating (DH) as early as in 1970s. However, in recent years, the focus on solar DH installations has shifted primarily to Denmark and Germany, with only one recent installation reported in Sweden. Nonetheless, due to changes in the overall heating market, the use of large-scale storage (both with and without solar heat) is becoming increasingly important. Despite significant advancements in adopting DH systems, the combination of solar DH with PTES is not well studied from Swedish context. The economic and geological prerequisites for the deployment of PTES remain largely unexplored. This paper explores the integration of large-scale solar thermal systems into DH networks in Sweden, particularly highlighting the feasibility and potential of pit thermal energy storage (PTES) systems. Through findings from a national project, this paper assesses the techno-economic-geological viability of PTES alongside solar thermal collectors, providing insights into the project’s methodological approach and initial findings.