Synthetic Methane as a renewable fuel

As utility systems attain greater renewable penetrations, seasonal trends or drought impacts on renewable energy throughput have greater impacts on the reliability and affordability of the power system. 100% carbon neutral/free systems must install enough capacity (with the right capabilities) to meet energy needs in worst-case scenarios. At a minimum, to assure reliability and avoid blackouts, utility system planners and policy makers need to account for seasonal trends in availability of renewable resources. Energy storage systems designed for daily shifting with less than 12 hour duration are not cost optimal for long-term storage and energy time-shifting in high renewable power systems.

One form of long-term energy storage is generation of synthetic methane using renewable energy. Synthetic methane is created in a multi-step process. Electricity powers hydrolysis to extract Hydrogen from water. A separate process uses electricity for direct-air carbon capture (DACC), a process by which Carbon is pulled from the air. A third process (called methanation) combines hydrogen and carbon to form renewable methane, CH4. Synthetic methane in this context is renewable insofar as the electricity powering the process is provided by renewable (wind, solar primarily) or other carbon-free sources (such as hydro or nuclear). The ingredients (water and air) are also renewable, their consumption does not alter the mass balance of either on a global basis. The final fuel is carbon-neutral. All carbon in the fuel is sequestered from the atmosphere, therefore any release of CO2 upon combustion is net-zero. The process results in no net increase in CO2 emissions, and is therefore carbon-neutral, or in effect carbon-free.

One benefit of renewable methane is that it can be stored and transferred using already existing natural gas infrastructure to generate power on demand using gas fired generators utilities already own. The intent is not to power the system entirely with carbon-neutral methane, but rather to store the fuel for long-term time shifting, meeting MWh needs in months when wind, solar or hydro output are minimized.

For this work power-to-gas is characterized as follows: Direct-Air-Capture of CO2 is coupled with hydrolysis (for Hydrogen) and a methanation process to generate synthetic, renewable methane, CH4. The process is collectively called “Power-to-Gas”, or PtG, and is assumed to be 60% efficient, powered entirely by renewable or carbon-free energy; that is, for every MWh of energy put into the PtG process, 0.6 MWh of useable CH4 is produced for long-term storage and use. Simulation of PtG was enabled through use of the PLEXOSTM “Gas Module”, used for Long-term capacity expansion simulations. Forward pricing curves for PtG (Fig 3.6-1) were developed in consultation with Lappeenranta University of Technology in Finland, with pricing curves corresponding with published values in the literature.