Assessment of power-to-methane. Applicability to Spain

As a result of the continuous penetration of renewable energy from wind and sunlight in the electricity systems, recent focus has been put on the development of renewable gases to store surplus power. As wind and photovoltaic are intermittent and fluctuating sources, the produced energy needs to be balanced for power grid stability reasons in times when production exceeds demand. A common practice is reducing the power output below the maximum availability. This reduction of renewables production is called curtailment. In order to evaluate these surpluses and avoid the mismatch between supply and demand, several strategic solutions are developed, named Electric Energy Storage (EES). A wide range of technologies for EES such as chemical or electrochemical, mechanical, electromagnetic, or thermal storage exist with this purpose. However, these technologies remain limited compared with the potentiality of Power-to­ Gas (PtG) in general and Power-to-Methane (PtM) in particular, standing out between other storage solutions. This concept has been claimed as a long­ term solution for large-scale energy storage. The PtM process links the power and natural gas grids by the conversion of electricity into another energy carrier: substitute natural gas (SNG). SNG production is accomplished in two steps: electrolysis and methanation. The electrolysis allows the conversion between electric energy into The different C02 sources for PtM implementation are flue gas from combustion processes, emissions from cement and metallurgical industry, biomass gasification and biogas. However, considering C02 emissions, it would be interesting to consider only biogenic C02 emissions in which biogas is a potential source. Biogas is formed from the anaerobic digestion of biomass and it is mainly formed by methane and carbon dioxide. An interesting and feasible solution would be to implementa biogas anda PtMplant. However, nowadays most of the Substitute Natural Gas comes from biogas feedstock, which is subsequently cleaned with an upgrading step increasing the CH4 fraction in biogas, called biomethane, meeting the specifications of commercial natural gas. The economic viability of SNG production from renewable sources depends largely on the investment costs of the electrolyzer and the electricity price to compete with the price of natural gas. However, it is expected in the near future the growth of carbon tax, which could considerably contribute to the levelized cost of energy (LCOE) of the SNG produced, as well as another valorization incentives from the reacted products such as oxygen selling for medical and industrial purposes, heat integration from catalytic methanation, and water reuse. The environmental impact of this technology is strongly determined by the use of renewable electricity for electrolysis and methanation in order to consider the production of 100% green hydrogen which is an extremely valuable resource. Therefore, if the source of C02 is biogenic, such as biogas, it contributes to reducing GHG emissions. The interest of the in-depth analysis of this technology in all the mentioned aspects is derived from the academic stay at the French university IMT Atlantique within the Erasmus program during the university course focused on "Energy and Environmental Transition". The objective of evaluating the PtM technology and potentiality for Spain to utilize these fluctuating renewable energy surpluses is based on the growing penetration of wind and photovoltaic energy in the Spanish power generation panorama projected for the years 2025 and 2030. These predictions are included in the Integrated National Plan of Energy and Climate roadmap established by the Ministry of Ecological Transition in January 2020, which must be implemented by all European countries. The case of Spain is especially interesting because is the European country with the second largest installed wind power capacity in Europe, after Germany. The difficulties in increasing electrical connections with other neighboring countries are limited by our geographical location on the Peninsula, so the need to make the most of these energy surpluses is considered important. Through an analysis of the Spanish electricity mix for these years and a consequent estimation of renewable energy surpluses, it is concluded that the target scenario (the one which envisages decarbonization by 2050) provides an ideal scenario for the implementation of Power-to-Methane technology, which competes with other energy storage techniques. To arrive at this result, the penetration ratio of wind and photovoltaic energies has to be estimated, as well as the expected curtailment ratio of these renewable energies with respect to total generation. As a principle for meeting the target scenario and accelerating the development of the necessary infrastructure, it is essential to maximize the implementation of installed power in the wind and photovoltaic generator farm, a 57% increase respect the business-as-usual scenario by 2030, and thus make the greatest possible use of the existing gas network as a method of energy storage that serves at the same time to make the electricity grid more flexible, as so much unmanageable renewable energy is produced as generation increases. Finally, the future lines of research can harness the potential of this technology by defining regulatory and operational frameworks, improving its economy and efficiency to overcome the actual barriers of this promising technology