energies Article Selection of Underground Hydrogen Storage Risk Assessment Techniques Barbara Uliasz-Misiak * , Joanna Lewandowska- ´ Smierzchalska and Rafal Matula   Citation: Uliasz-Misiak, B.; Lewandowska- ´ Smierzchalska, J.; Matula, R. Selection of Underground Hydrogen Storage Risk Assessment Techniques. Energies 2021, 14, 8049. https://doi.org/10.3390/en14238049 Academic Editor: Ruud Weijermars Received: 30 October 2021 Accepted: 29 November 2021 Published: 1 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Faculty of Drilling, AGH University of Science and Technology, Oil and Gas, Mickiewicza Av. 30, 30-059 Krakow, Poland; joannal@agh.edu.pl (J.L.- ´ S.); matula@agh.edu.pl (R.M.) * Correspondence: uliasz@agh.edu.pl Abstract: The article proposes the use of the analytic hierarchy process (AHP) method to select a risk assessment technique associated with underground hydrogen storage. The initial choosing and ranking of risk assessment techniques can be considered as a multi-criteria decision problem. The usage of a decision model based on six criteria is proposed. A ranking of methods for estimating the risks associated with underground hydrogen storage is presented. The obtained results show that the application of the AHP-based approach may be a useful tool for selecting the UHS risk assessment technique. The proposed method makes it possible to make an objective decision of the most satisfactory approach, from the point of view of all the adopted decision criteria, regarding the selection of the best risk assessment technique. Keywords: underground hydrogen storage; risk assessment; analytical hierarchy method 1. Introduction Hydrogen is a very good energy carrier, a “clean” fuel (its combustion produces only energy and water) and can be an energy store. The use of hydrogen as an energy carrier can play a major role in a carbon neutral economy. This can be hydrogen produced from renewable energy by electrolysis (“green hydrogen”), natural gas or carbon with CO 2 capture (“blue hydrogen”) [1]. Hydrogen will also allow the decarbonisation of economic sectors where reducing carbon dioxide emissions is difficult to achieve. This element can replace natural gas in the chemical, metallurgical and transport industries, and in the long term also in the aviation and maritime sectors. It is predicted that in the near future, energy is to be obtained mainly from renewable sources (RES) (wind and solar power plants). The introduction of the hydrogen economy will allow an increase of the share of renewable energy in the total energy balance thanks to the possibility of storing energy of multi-megawatt power, e.g., by conversion to hydrogen. 1.1. Hydrogen Storage Hydrogen can be stored in various ways in metal tanks on and below the ground surface, in gas networks as a mixture of H 2 with natural gas, in materials or underground in deep geological structures. Underground hydrogen storage facilities should distinguished by high storage capacity, technological simplicity, low costs and safe operation [1–3]. Gaseous hydrogen can be stored in tanks on or below the ground surface. In the case of large volumes, storage in above-ground pressure vessels entails high investment costs. On the other hand, the storage of hydrogen in underground metal tanks requires close supervision due to corrosion, which is difficult to control underground. Hydrogen from renewable sources can also be stored in gas networks as a mixture of hydrogen and natural gas (Power-To-Gas technology). However, due to the properties of the gas mixture (after adding hydrogen to it) being significantly different from the properties of natural Energies 2021, 14, 8049. https://doi.org/10.3390/en14238049 https://www.mdpi.com/journal/energies