SIEMENS

What a waste! In northern Germany, the wind is blowing — but many rotors in nearby wind farms are motionless. “Up to 20 percent of the time, wind systems on the North Sea coast have to be switched off; otherwise they’d produce more power than needed at a given moment,” says Erik Wolf, a technology strategist for Siemens’ Solar & Hydro Division. “This indicates a central challenge associated with renewable energies — production fluctuates as weather conditions change. In other words, supply isn’t based on demand, as is the case with conventional power plants.” Indeed, Germany’s wind energy trade association estimates that the German power grid was unable to accommodate 150 gigawatt-hours of electrical energy in 2010 simply because it was already operating at full load.

This explains why wind turbines often remain inactive during a storm and why older, coal-fired power plants with high carbon dioxide emissions are reconnected to the grid on calm days — circumstances that are becoming more pronounced as Germany produces a larger share of its power from the wind and sun. According to the German Federal Government, the country expects to meet about 50 percent of its total demand for power with renewable energies by 2030, and to achieve 80 percent from such sources by 2050.

These targets cannot be met without massive energy storage systems — systems capable of capturing excess energy when winds are intense and feeding it back into the grid later when demand is high. “To meet the future challenges of an energy system based on renewable energies, we’ll need a variety of storage technologies suitable for everything from periods of seconds or hours to long-term periods of days or weeks,” says KatherinaReiche, Parliamentary State Secretary in the German Federal Ministry for the Environment.

And Germany is certainly not alone. Many other countries that are now moving toward increased use of renewable energy sources will also need to augment their power grids with storage systems. “We’re involved in detailed discussions at the moment in a number of places — for example, in Denmark and the United States,” adds Wolf.

And when it comes to storing the power produced by excess electricity, electrolysis is set to play a key role. Here, water is decomposed into oxygen and hydrogen gas by means of an electrical current. At a pressure of 200 bars, the energy density of the hydrogen gas is comparable to that of a lithium-ion battery.

Large quantities of the gas could thus be stored in the underground caverns of salt domes of the sort used by natural gas suppliers as reservoirs, or in the existing natural gas grid, which can accommodate up to five percent hydrogen without difficulty. In purely mathematical terms, the latter could store 130 terawatt-hours of electrical energy in the form of hydrogen, which represents almost a quarter of German power consumption per year.

Underground Storage. On calm or cloudy days, the hydrogen gas could then be retrieved from caverns and, for example, burned in a combined-cycle power plant that drives an electric generator to produce electricity. At the moment, of course, there are no turbines that can burn pure hydrogen — but by 2014, Siemens hopes to present a prototype. Although approximately half of the energy produced by wind would be lost during electrolysis and subsequent combustion in a gas turbine, windmills would no longer have to be shut off because of overcapacity.

What’s more, the problem of fluctuating power production would be solved. “In Germany, depending on the nature of future power consumption, we will need a maximum of 400 cavern reservoirs for hydrogen with a volume of about 500,000 cubic meters each. At present, 200 such reservoirs for natural gas could also be used,” says Wolf. “The maximum 60 terawatt-hours of energy that could be stored in these facilities corresponds to about ten percent of annual demand in Germany. That would be enough to tide consumers over during relatively long periods of low wind or solar power production.”

Two small hydrogen caverns in the UK and U.S. have been in operation for years. These facilities have demonstrated that this form of storage is safe. Experts expect that a typical hydrogen storage facility will cost between €10 and €30 million. Utilities must also invest in gas-fired plants that typically require an investment of between €50 million and €700 million depending on plant output.

Power companies see great potential in hydrogen technology. “We want to sharply reduce CO₂ emissions. So we’re building and developing new, efficient power plant technologies and operating more and more wind farms,” says Dr. Sebastian Bohnes from the research department of Germany’s RWE Power. “These days, wind turbine speeds are throttled, mostly because of bottlenecks in the power grid. Efforts to expand the use of renewable energies could lead to a rapid increase in overcapacities. Electrolysis offers an interesting way to store — in the form of hydrogen gas — electricity that can not be used immediately.” This presupposes that the electrolyzers that produce the energy-rich gas from electricity have the ability to react quickly to the fluctuating supply of electrical power. So far, the systems, which have a reaction time of a few minutes, have been too slow.

Flexible Hydrogen Factory. For years, researchers from Siemens Corporate Technology have therefore been refining an alternative electrolysis technology that is much more flexible. In their electrolyzer, a proton exchange membrane (PEM) separates the two electrodes at which oxygen and hydrogen are formed — in contrast to conventional alkaline electrolysis technology. “Our PEM electrolyzer reacts within milliseconds and can easily handle three times its nominal power rating for a while. In other words, even if there’s a sharp increase in power generation, it can make use of the excess power without any difficulty,” says Roland Käppner, head of the Hydrogen Solutions business unit in Siemens’ Industry Sector.

Siemens’ PEM technology is now mature enough to move out of the lab and into practical applications. Building on results from a research electrolyzer with a nominal power rating of ten kilowatts, Käppner’s team is now working on a new electrolyzer that will have a nominal power rating of 0.1 megawatts and a peak rating of 0.3 megawatts. It will produce two to six kilograms of hydrogen per hour and is scheduled to be operational by the end of 2012. “We’ve optimized the design and all the peripherals, such as the control system and the power supply,” says Käppner, describing the efforts that brought the system out of the lab and into the field. “We’re also working on reducing costs considerably with innovative materials and structural features.”

Hydrogen production via electrolysis still costs upwards of €10,000 per kilowatt of installed load. But thanks to further refinements in design, Käppner hopes to lower costs to under €1,000 per kilowatt by 2018, at the latest. By then, the third generation of Siemens electrolyzers is expected to be able to accommodate up to 100 megawatts, thus converting excess wind-generated electricity into hydrogen in large quantities. A 60 to 90-megawatt electrolyzer would suffice to convert the surplus energy of a large wind farm.