  Citation: Lopez-Ruiz, G.; Castresana-Larrauri, J.; Blanco Ilzarbe, J.M Thermodynamic Analysis of a Regenerative Brayton Cycle Using H 2 , CH 4 and H 2 /CH 4 Blends as Fuel. Energies 2022, 15, 1508. https://doi.org/10.3390/en15041508 Academic Editors: Luis Maria Abadie and Ibon Galarraga Received: 17 January 2022 Accepted: 15 February 2022 Published: 17 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). energies Article Thermodynamic Analysis of a Regenerative Brayton Cycle Using H 2 , CH 4 and H 2 /CH 4 Blends as Fuel Gontzal Lopez-Ruiz 1, * , Joseba Castresana-Larrauri 2 and Jesús María Blanco-Ilzarbe 3 1 Ikerlan Technology Research Centre, Basque Research and Technology Alliance (BRTA), P.º J.M. Arizmendiarrieta, 2, 20500 Arrasate, Spain 2 AZTI, Marine Research, Basque Research and Technology Alliance (BRTA), Txatxarramendi Ugartea, 48395 Sukarrieta, Spain; jcastresana@azti.es 3 Energy Engineering Department, School of Engineering, Building I, University of the Basque Country, UPV/EHU, Plaza Ingeniero Torres Quevedo s/n, 48013 Bilbao, Spain; jesusmaria.blanco@ehu.es * Correspondence: gontzal.lopez@ikerlan.es Abstract: Considering a simple regenerative Brayton cycle, the impact of using different fuel blends containing a variable volumetric percentage of hydrogen in methane was analysed. Due to the potential of hydrogen combustion in gas turbines to reduce the overall CO 2 emissions and the dependency on natural gas, further research is needed to understand the impact on the overall thermodynamic cycle. For that purpose, a qualitative thermodynamic analysis was carried out to assess the exergetic and energetic efficiencies of the cycle as well as the irreversibilities associated to a subsystem. A single step reaction was considered in the hypothesis of complete combustion of a generic H 2 /CH 4 mixture, where the volumetric H 2 percentage was represented by f H 2 , which was varied from 0 to 1, defining the amount of hydrogen in the fuel mixture. Energy and entropy balances were solved through the Engineering Equation Solver (EES) code. Results showed that global exergetic and energetic efficiencies increased by 5% and 2%, respectively, varying f H 2 from 0 to 1. Higher hydrogen percentages resulted in lower exergy destruction in the chamber despite the higher air-excess levels. It was also observed that higher values of f H 2 led to lower fuel mass flow rates in the chamber, showing that hydrogen can still be competitive even though its cost per unit mass is twice that of natural gas. Keywords: hydrogen combustion; Brayton cycle; gas turbines; exergy analysis; renewable energies 1. Introduction New solutions for the efficient generation of carbon-neutral electricity are essential to curb the climate crisis. In the framework of renewable energies, it is well known that the energy supplied by wind or solar renewable power plants presents peaks and valleys due to the weather variability [1]. In order to face up this variable generation and following the so-called “Power to gas” (P2G) concept, hydrogen can be produced by electrolysis of water during the generation peaks [2,3]. Green hydrogen is then stored and distributed for a wide range of end uses (hydrogen to power). Different cost-benefit studies have concluded that green hydrogen will be a feasible and competitive energy carrier in the coming years, optimising the use of natural resources [4–6]. One of the alternatives relies on using the generated green hydrogen for electric gener- ation in electric power plants, as it has already been done in several projects [5,7,8]. The technical report of ENT Global [9] summarises the recent advances in different gas turbine technologies and current capabilities of gas turbines using hydrogen as fuel. However, hydrogen is a very challenging fuel due to its combustion properties and differences with respect to conventional fuels such as natural gas, propane or kerosene. The current litera- ture shows several investigations related to the behaviour of hydrogen flames focused on the flashback phenomena, as well as the thermal NO x formation [10–13]. Energies 2022, 15, 1508. https://doi.org/10.3390/en15041508 https://www.mdpi.com/journal/energies