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

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School of Engineering | Doctoral thesis (article-based) | Defence date: 2019-10-25
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Date

2019

Department

Rakennustekniikan laitos
Department of Civil Engineering

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Mcode

Degree programme

Language

en

Pages

52 + app. 66

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 169/2019

Abstract

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.

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Supervising professor

Rinne, Mikael, Prof., Aalto University, Department of Civil Engineering, Finland

Thesis advisor

Kukkonen, Ilmo, Prof., University of Helsinki, Finland

Keywords

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

Other note

Parts

  • [Publication 1]: Janiszewski, Mateusz; Kopaly, Antoni; Honkonen, Mikko; Kukkonen, Ilmo; Uotinen, Lauri; Siren, Topias; Rinne, Mikael. Feasibility of underground seasonal storage of solar heat in Finland. In: P.G. Ranjith & Z. Jian (Eds), International Conference on Geo-mechanics, Geo-energy and Georesources: Conference Proceedings, Monash University, Melbourne, Australia, September 28-29, 2016. Pages 959-965.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201710157222
  • [Publication 2]: Janiszewski, Mateusz; Caballero Hernandez, Enrique; Siren, Topias; Uotinen, Lauri; Kukkonen, Ilmo; Rinne, Mikael. In Situ Experiment and Numerical Model Validation of a Borehole Heat Exchanger in Shallow Hard Crystalline Rock. Energies, 2018, volume 11, issue 4, article number 963.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201805222482
    DOI: 10.3390/en11040963 View at publisher
  • [Publication 3]: Janiszewski, Mateusz; Siren, Topias; Uotinen, Lauri; Oosterbaan, Harm; Rinne, Mikael. Effective Modelling of Borehole Solar Energy Storage Concept in High Latitudes. Techno-Press. Geomechanics and Engineering, 2018, volume 16, issue 5, pages 503-512.
    DOI: 10.12989/gae.2018.16.5.503 View at publisher
  • [Publication 4]: Janiszewski, Mateusz; Shen, Baotang; Rinne, Mikael. Simulation of the interactions between hydraulic and natural fractures using a fracture mechanics approach. Journal of Rock Mechanics and GeotechnicalEngineering, 2019, manuscript accepted for publication.

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https://urn.fi/URN:ISBN:978-952-60-8724-5

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