– How to store “Green Energy” Power to Gas

Power to Gas – How to store “Green Energy”
Today renewable energy is growing more and more important, especially wind and solar
power. The problem is that those energies have a high fluctuation rate because of different
wind speeds and radiation intensities. For a steady power supply system it is necessary to
keep the demanded and offered energy in balance, so surplus energy must be stored to be
available when it is needed.
Pumped storage hydro power stations have proofed to be a highly efficient and sustainable
technology, but there are some problems, e.g. a huge demanded area, limited storage
capacity and high investment costs. We need some alternatives.
There already are some new ideas how to store surplus energy, like “Power to Gas”, where
you produce synthetic natural gasoline. Thereby you use a technology called “Methanation”
to transform hydrogen and carbon dioxide to methane and water. This happens in two steps:
1. Surplus energy, produced by renewable energy, is used to divide water to oxygen
and hydrogen by electrolysis. Just up to 5% by volume of this oxygen can be fed
directly into the existing gas distribution system, otherwise turbines or other gas
installations may be damaged.
2. Carbon dioxide (CO2) and four molecules hydrogen (H2) are fused under high
pressure, increased temperature and a nickel catalyzer to methane (CH4), which is
the most important component of natural gasoline. Methane can be fed directly into
the distribution system without the risk of damaging a machine. Later, if you need
more energy than is provided you can burn the gas in conventional power stations to
produce electricity again.
Figure 11: Electrolysis
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The next figure shows how Power to Gas will be integrated in the existing energy network:
Figure 22: Integration of the new technology into existing networks
One big advantage of this technology is that this storage technology is highly capable of
being integrated into the existing energy system because the distribution structure already
exists. E.g. you could supply whole Germany for two to three months with electricity with a
storage capacity of 200TWh. This would relativize building expensive new electricity lines
from north to south. The following graph shows the capacity of different storage technologies:
Figure 33: Storage technologies in comparison
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Another good argument for Power to Gas: If you use carbon dioxide produced in bioethanol
production plants, sewage treatment plants and breweries this technology is CO2 neutral.
The research institute ZSW (Centre for Solar Energy and Hydrogen Research) and SolarFuel
(a Company from Stuttgart, Germany) already demonstrated in 2009 that it is also possible to
absorb CO2 from the ambient air. If that is taken into account you will be completely
independent of fossil energy sources.
SolarFuel, based on sooner achievements of ZSW and Fraunhofer IWES (Institute for Wind
Energy and Energy System Technology), has built a 25 kilowatt station in 2010. They aim
was to prove that Power to Gas can be included into a decentralized energy supply system.
Two years later in 2012 they finished a second Power-to-Gas research plant with 250
kilowatt connected electrical load. It produces up to 300 cubic metres methane per day and
is ten times more powerful than the test plant constructed before. During operation the
optimisation of the technology will be continued to scale up Power to Gas plants into the
relevant range for the industry.
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Figure 4 : Electrolysis at the 250 kW research plant
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Figure 55: Gas mixing and Methanisation at the ZSW
The “EnergieGenossenschaft-Flensburg” (Energy Cooperative Flensburg) plans to build
another Power to Gas station in one or two years in Flensburg, Germany. At present the
windmills in North Friesland are stopped during grid congestions. Power to Gas offers a
highly interesting solution:
If there is surplus wind energy provided it will be used in Flensburg to produce hydrogen –
this can be used for fuel cells in cars – and methane. The needed CO2 is coming from the
local sewage treatment plant. The methane will be fed into the existing long-distant heating
grid. Flensburg already gets much more electricity through wind energy than is needed and it
is intended to triple the production to 2020 – so Power to Gas is a very good perspective.
There has been built some more pilot plants by now to do research about this new
technology and it is planned that in near future Power to Gas stations will be fully functional
as well as affordable. So it will be no longer necessary to stop renewable power stations
during grid congestions.
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