Intermittent Renewables: will 'Power-to-Gas' be the Solution?






The German government has committed the country to an 'Energiewende', in which at least 80% of electricity production and 60% of primary energy needs are to be supplied by solar, wind, and other renewable energy sources by 2050. A big open question is how the intermittency of renewable energy sources like wind and sunshine can be reconciled with the need to reliably supply energy whenever and wherever it's needed, whether to heat homes, fuel trucks and trains, or power electrical equipment.

'Power-to-gas' and 'power-to-liquids' could be the answer, according to engineers and researchers who spoke to a packed hall at the third annual conference of the Power to Gas Association in Berlin on Wednesday (2.7.2014), hosted by the German Energy Agency (DENA).
Michael Sterner, a professor at East Bavarian Technical University in Regensburg, says the technology is crucial to the success of the Energiewende. "Power-to-gas is absolutely necessary for a 100 percent renewable energy power supply and for the decarbonization of the transportation and chemical industry," he told DW.

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In principle, the technology proposition is straightforward. Germany, like most developed countries, already has a well-developed network of pipelines and storage tanks for natural gas. Natural gas, a fossil fuel, is used to heat homes and generate electricity in gas turbines. Methane, the main component of natural gas, also serves as a basic feedstock for the petrochemical industry, which makes everything from plastics to pharmaceuticals.
It turns out that the existing gas storage and distribution network could be used to solve the country's energy storage problem. That's because fossil fuel reservoirs (natural gas wells) are not the only available source of methane. Professor Sterner explained that methane can also be synthesized in chemical factories from three simple and common ingredients: carbon dioxide, water, and electricity.
Using a long-established process called 'electrolysis', chemical engineers can tear apart water and carbon dioxide molecules (H2O and CO2), and then recombine the pieces into any number of new molecules - starting with methane, CH4.

The methane synthesis process requires electricity as an input, which is why it's called 'power-to-gas'. If the electricity comes from a renewable energy source like a wind turbine or a solar array, the resulting synthetic methane is called 'renewable gas', or sometimes by special names like 'wind-gas' or 'solar gas', depending on the source of the primary energy input.
Among many other things, engineers can take synthetic methane and further process it to make synthetic liquid fuels like methanol or butanol - which can be used to fuel diesel or gasoline engines - or kerosene, which is the main constituent of jet fuel. The relevant terms of art are 'power-to-liquids' or 'renewable liquid fuels'.
So far so good - but while the technology is quite straightforward, the business case isn't. It doesn't cost very much to sink a well and tap natural gas from a geological formation, or to send it to distant markets by pipeline. Natural gas is cheap. Synthetic methane is much more expensive. Read on...

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