Citation: Prigmore, S.; Okon-Akan, O.A.; Egharevba, I.P.; Ogbaga, C.C.; Okoye, P.U.; Epelle, E.; Okolie, J.A. Cushion Gas Consideration for Underground Hydrogen Storage. Encyclopedia 2024, 4, 847–863. https://doi.org/ 10.3390/encyclopedia4020054 Academic Editors: Massimiliano Lo Faro and Raffaele Barretta Received: 26 March 2024 Revised: 7 May 2024 Accepted: 11 May 2024 Published: 14 May 2024 Copyright: © 2024 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/). Review Cushion Gas Consideration for Underground Hydrogen Storage Sadie Prigmore 1 , Omolabake Abiodun Okon-Akan 2,3 , Imuentinyan P. Egharevba 4 , Chukwuma C. Ogbaga 5,6 , Patrick U. Okoye 7 , Emmanuel Epelle 8 and Jude A. Okolie 1, * 1 Gallogly College of Engineering, University of Oklahoma, Norman, OK 73019, USA; sadie.m.prigmore-1@ou.edu 2 Wood and Paper Technology Department, Federal College of Forestry Jericho, Ibadan PMB 5087, Nigeria; okon-akan.o@frin.gov.ng 3 Forestry Research Institute of Nigeria, Ibadan PMB 5054, Nigeria 4 Department of Chemical Engineering, Faculty of Engineering, University of Benin, Benin City PMB 1154, Nigeria; imuetinyan.egharevba@uniben.edu 5 Departments of Biotechnology, Microbiology, and Biochemistry, Philomath University, Kuje 903101, Nigeria; chukwumaogbaga@gmail.com 6 Department of Biological Sciences, Coal City University, Enugu 400104, Nigeria 7 Instituto de Energías Renovables (IER-UNAM), Privada Xochicalco S/n Col. Centro. Temixco, Morelos 62580, Mexico; ugopaok@ier.unam.mx 8 Institute for Infrastructure and Environment, School of Engineering, The University of Edinburgh, Robert Stevenson Road, Edinburgh EH9 3FB, UK; eepelle@ed.ac.uk * Correspondence: jude.okolie@ou.edu Abstract: Due to the increasing world population and environmental considerations, there has been a tremendous interest in alternative energy sources. Hydrogen plays a major role as an energy carrier due to its environmentally benign nature. The combustion of hydrogen releases water vapor while it also has a vast industrial application in aerospace, pharmaceutical, and metallurgical industries. Although promising, hydrogen faces storage challenges. Underground hydrogen storage (UHS) presents a promising method of safely storing hydrogen. The selection of the appropriate cushion gas for UHS is a critical aspect of ensuring the safety, efficiency, and reliability of the storage system. Cushion gas plays a pivotal role in maintaining the necessary pressure within the storage reservoir, thereby enabling consistent injection and withdrawal rates of hydrogen. One of the key functions of the cushion gas is to act as a buffer, ensuring that the storage pressure remains within the desired range despite fluctuations in hydrogen demand or supply. This is achieved by alternately expanding and compressing the cushion gas during the injection and withdrawal cycles, thereby effectively regulating the overall pressure dynamics within the storage facility. Furthermore, the choice of cushion gas can have significant implications on the performance and long-term stability of the UHS system. Factors such as compatibility with hydrogen, cost-effectiveness, availability, and environmental impact must be carefully considered when selecting the most suitable cushion gas. The present study provides a comprehensive review of different types of cushion gases commonly used in UHS, including nitrogen, methane, and carbon dioxide. By examining the advantages, limitations, and practical considerations associated with each option, the study aims to offer valuable insights into optimizing the performance and reliability of UHS systems. Ultimately, the successful implementation of UHS hinges not only on technological innovation but also on strategic decisions regarding cushion gas selection and management. By addressing these challenges proactively, stakeholders can unlock the full potential of hydrogen as a clean and sustainable energy carrier, thereby contributing to the global transition towards a low-carbon future. Keywords: hydrogen; cushion gas; reservoir; sustainability; aquifer; natural gas 1. Introduction Fossil fuels have come under increased scrutiny in recent years due to various factors such as high greenhouse gas emissions, fluctuating prices, and a rise in energy needs [1]. Encyclopedia 2024, 4, 847–863. https://doi.org/10.3390/encyclopedia4020054 https://www.mdpi.com/journal/encyclopedia