Renewable Fuels

Power-to-gas provides long-term storage option for 100% goals

Decarbonizing to meet ambitious renewable energy goals, while minimizing land use, emissions and cost, will require new approaches and ways of thinking. One promising approach is power-to-gas (PtG) technology. 

PtG technology uses excess energy generated from renewable sources (e.g., wind and solar) to produce carbon-neutral fuels, such as synthetic methane or hydrogen. What makes this technology particularly attractive is its ability to cost effectively produce weeks to months of fuel volumes for use in existing thermal units. The combination of stored fuel potential and thermal capacity yields a long-duration energy storage system that acts like a gigantic distributed “battery.” This long-term storage system can be coupled with traditional, shorter-term storage technologies (less than 12 hours of storage) to meet seasonal MWh demand when renewables are variable, and to ensure a reliable and secure supply of electricity during periods of extreme weather.

Power to Gas (Power-to-Methane, PtM)

Power-to-gas (PtG), which here is defined as the process of using excess renewable energy, that would otherwise be curtailed, to produce renewable fuels. The first such fuel to consider is methane, produced through the power-to-methane, or PtM process.  PtM produces carbon-neutral CH4 (methane) via a three-step process.

1. Direct Air Capture (DAC) of CO2 from the atmosphere as a source of carbon
2. Electrolysis of water as a source of hydrogen
3. Mathanation to combine carbon and hydrogen into CH4

The final molecule, CH4 (methane) can be stored and transported in existing natural gas infrastructure and used in households, industries and power plants by any thermal technology that can burn natural gas.  Carbon is recycled from air, so combustion of PtG methane is net-zero, or carbon-neutral, with no increase in atmospheric CO2 levels.

While PtM, or power to gas in general, is not currently used at mass-scale, they are a major avenue for deep decarbonization, particularly in the transportation sector.  The process of electrolysis and methanation are decades old technologies with numerous commercial applications. Direct air capture (DAC) of carbon is the newest technology involved with the PtG process, with several large-scale projects under development.  For example Carbon Recycling International is developing a large DAC facility in China that will produce 180,000 tons per year of liquified natural gas (LNG) and methanol (Carbon Recycling International, 2019). Carbon Engineering is actively developing a 1 million ton per year DAC carbon capture plan in Texas for enhanced oil recovery, where CO2 taken from the air will be pumped into the ground for permanent sequestration and helped to enhance oil production (Rathi, 2019). The California Low Carbon Fuel Standard (LCFS) was amended in 2019 to include DAC, allowing companies to net carbon sequestered from air from the carbon footprint of fuels sold into the California market. 

Power-to-Gas (Power-to-Hydrogen, PtH)

Power-to-hydrogen is an alternate PtG pathway. Power-to-hydrogen requires only electrolysis, where electrolyzers use excess renewable energy to produce hydrogen (from water) for direct use as a fuel.  Hydrogen production with PtH is less expensive than PtM and more efficient as there is no need for carbon DAC or methanation. In addition, hydrogen as a fuel is carbon free. Complexities arise as there is, unlike the existing infrastructure for methane, no comparable hydrogen infrastructure. Thermal power plants designed to burn methane typically cannot burn 100% hydrogen. Existing gas storage facilities, pipelines, compressor stations and distribution lines typically cannot handle 100% hydrogen without expensive upgrades, if not complete replacement.  Still, hydrogen is an efficient and carbon-free alternative to renewable synthetic hydrocarbons and is worth investigating. Power plant technology manufacturers seem to understand this as many of them are in the process of developing technologies that are fueled by 100% hydrogen.