Power to methane to power

Performance analysis of a closed loop energy storage system

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Reducing carbon emissions in the power generation sector can be done by generating energy from renewable sources such as wind and sun. However these sources alone cannot provide a reliable electricity system and therefore an energy storage system is needed. Power-to-gas is a concept in which surplus renewable electricity is used for the production a gas fuel. The gas can be stored and is used for electricity production when there is a deficit in renewable electricity. When CO2 exhaust gases form reactants for new production of gas, the system has no net CO2 emissions. The gas functions as an energy carrier for electrical energy. Methane could be an interesting gas for large scale energy storage as there is much knowledge about methane transport, storage and combustion and the gas is easier to store than hydrogen. Power-to-gas-to-power conversions come with great electricity losses. Therefore it is interesting to investigate what waste heat streams can be extracted from the process to use for external purposes. The aim of this thesis is to map the input and output energy streams of a power-to-methane-to-power system operating in 2030 to find the efficiency of the system and to see how efficiency could be maximized by using waste heat streams for external purposes. Also, a power-to-methane-to-power system requires a lot of gas storage capacity. Therefore it is useful to estimate the capacity of the gas storage facility to find if the system is technically feasible and to see what gas storage does with the efficiency of the system. Last, since the aim is to reduce carbon emissions the (small) CO2 emissions of the system are determined and analyzed. The system is scaled up to a scenario in which it provides a fully renewable electricity grid in the year 2050, to explore the feasibility in terms of carbon emissions and required gas storage capacity. The system is also compared to a power-to-hydrogen-to-power system to find the most feasible solution. The round trip energy efficiency of the system in 2030 is 88.0%, which is the sum of an electrical efficiency of 30.0% and a thermal efficiency of 57.1% The total energy efficiency can only be achieved when streams modeled as usable heat output streams can actually be used. This depends highly on the location of the system. The carbons emissions of a system with a 44MW output in 2030 are 56.3kt and the required gas storage capacity is 5.09 x 10^5 m3 of underground salt caverns. When scaling up the system to provide a fully renewable electricity grid, the total gas storage capacity is 9.34 x 10^7 m3, which is 5.5% of the total potential salt cavern volume in the Netherlands. The carbon emissions are 2.78 kt/y, which is 0.0056% of the current annual carbon emissions caused by the Dutch power generation sector. Compared to a hydrogen system the methane system performs worse in term of electrical efficiency, gas storage capacity and carbon emissions.