Power-to-gas plants use renewable energy to make sustainable fuel
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Power-to-gas plants use renewable energy to make sustainable fuel By Melissae Fellet Feature Editor Christian Bach
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enewable energy provides electricity with significantly less greenhouse gas emissions than fossil-fuel-based power plants. But some sources of renewable energy, such as wind and solar power, produce electricity intermittently. Power produced when electricity demand is low could go unused. Power-to-gas (PtG) plants use excess renewable electricity to electrochemically split water into hydrogen and oxygen. The hydrogen is then used to chemically reduce carbon dioxide into methane that can be stored or used immediately in the existing natural gas infrastructure. In 2013, Audi opened one of the world’s largest industrial scale PtG plants to produce methane as a renewable fuel for some of their natural-gas-powered cars. According to the company’s calculations, methane produced by a PtG plant reduces the lifetime carbon footprint of an Audi compact passenger car enough to be comparable to that of an electric car charged only with renewable electricity.
There are three approaches to water electrolysis for PtG applications: alkaline, polymer electrolyte membrane, and solid-oxide electrolyzers. The challenge for all of these approaches is managing the fluctuating power supply from renewable sources. Most commercial alkaline electrolyzers contain a liquid sodium or potassium hydroxide electrolyte. Common electrode materials include Raney nickel cathodes and nickel–iron–cobalt alloy anodes. Affordable, long-lasting, and durable alkaline electrolyzers have been available commercially for decades. Because this technology is well developed, many pilot PtG projects utilize alkaline electrolysis. However, alkaline cells are slow to respond to power fluctuations, taking up to an hour to start up after shutdown. The relatively slow kinetics of the electrochemical reactions on either side of the cell can reduce the cell’s current density or overall efficiency. The combined thickness of the liquid electrolyte and the diaphragm that separates the two sides of the cell also contribute to low-current density. This distance causes charge carriers to move farther across the cell, essentially increasing the ionic resistivity of the cell. Finally, corrosion from the caustic electrolyte can limit the lifetime of these reactors. Electrolyzers that contain a polymer electrolyte membrane (PEM) can respond within minutes to fluctuating loads. Typically, the membrane is a perfluorosulfonic acid polymer, approximately 100–200 μm thick. Protons travel easily through this membrane to carry charge between the electrodes on either side. The cell design of PEM electrolyzers facilitates high-pressure operation, which generates pressurized hydrogen that can be used directly for methane production. This eliminates energy that would otherwise be needed for gas compression prior to methanation. Hydrogen collected for future use in fuel-cell vehicles would need to be pressurized to 700 bar. To reduce the amount of compression needed for t
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