Techno-economic aspects of seasonal underground storage of solar thermal energy in hard crystalline rocks

Research output: ThesisDoctoral ThesisCollection of Articles


Due to the current issue of global climate change, certain actions have been precipitated in the global energy sector to increase the share of renewable, clean energy. One example of renewable energy is solar thermal energy, which can be utilised for domestic heating purposes in solar communities. However, in countries located at high latitudes, such as Finland, solar thermal energy is most abundant in the summer when the heating demand is low, and less abundant in the winter when the heating demand peaks. The solution to this mismatch is Thermal Energy Storage (TES). TES allows the collection of energy during the summer, which is accumulated in a storage medium, stored seasonally, and extracted in the winter to cover the heating demand. Rock and water in the subsurface are perfect storage media, and a selection of Underground TES (UTES) methods exist which could be utilised as long-term energy storage solutions for solar communities. The goal of this research was to develop a numerical modelling approach for the simulation of the Borehole TES (BTES) systems by first determining which UTES method would be best to apply in a solar community in Finland. Furthermore, through this development, a method was devised to numerically simulate hydraulic fracturing in fractured rock. To select the best UTES method for a Finnish solar community, a criteria-based feasibility study was implemented. It revealed that the BTES method is advantageous in terms of its ease in gaining large storage volumes, feasibility at a small scale, its cost-efficiency and adaptability. Two numerical modelling approaches for the simulation of borehole heat exchangers were proposed, validated by an in situ experiment and used to simulate the BTES systems. Numerical modelling revealed that low thermal diffusivity of the rock is essential for maximising the efficiency of seasonal storage. Furthermore, a fracture mechanics-based numerical model was proposed to simulate the interactions between hydraulic and natural fractures in Fractured TES (FTES) systems. The hydraulic fracturing model indicated that pre-existing discontinuities with low dip angles modify the propagating path of sub-horizontal hydraulic fracture potentially hindering the thermal performance of the FTES systems. The three main conclusions address seasonal TES in hard crystalline rocks. The BTES method is suggested as the most optimal method for a Finnish-based solar community. The two proposed thermal numerical modelling approaches of borehole heat exchangers can aid in the design of BTES systems by efficiently simulating their seasonal performance. Lastly, the proposed hydraulic fracturing model can simulate the construction process of FTES systems. The results of this dissertation contribute towards the development of the state-of-the-art of UTES in hard rocks.
Translated title of the contributionTechno-economic aspects of seasonal underground storage of solar thermal energy in hard crystalline rocks
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Rinne, Mikael, Supervising Professor
  • Kukkonen, Ilmo, Thesis Advisor, External person
Print ISBNs978-952-60-8723-8
Electronic ISBNs978-952-60-8724-5
Publication statusPublished - 2019
MoE publication typeG5 Doctoral dissertation (article)


  • borehole thermal energy storage
  • numerical modelling
  • in situ experiment
  • fractured themal energy storage
  • hydraulic fracturing
  • solar community


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