Keeping electricity costs low while transitioning toward renewable power will be difficult. Solar power is far more expensive than fossil-fuel power, especially in Germany, where skies are often cloudy. And although wind power is already nearly as cheap as fossil-fuel power—which is why Germany is starting to shift its policies to favor wind—like solar, it is intermittent: even some of the best-situated wind turbines generate electricity only a third of the time.
Ensuring reliable power supplies will therefore require installing high-voltage power lines to get renewable energy from places that happen to be sunny or windy to the places energy is needed. Germany is already struggling with limits to its ability to transmit its existing renewable energy supply, which accounts for about 20 percent of its electricity: according to Siemens, Germany throws away 20 percent of the power its wind turbines produce because it doesn’t have enough transmission capacity.
Renewable energy will require very large-scale energy storage. The most affordable way to store electricity is to use it to pump water up a hill, and then let it flow down again to spin a turbine and generator when electricity is needed. But this only works in places where there are hills and dams, and most of Germany is flat.
The total amount of pumped-water storage in Germany now is about 40 gigawatt-hours—no more than renewable sources could generate in an hour on a sunny and windy day, says Michael Weinhold, Siemens Energy’s chief technology officer. “They were not made for buffering hours or days, or even weeks, of volatility.”
Right now, batteries are far too expensive—and not nearly enough are being made to accommodate the scale required. It would take the battery capacity of millions of electric vehicles to equal the existing pumped-water storage capacity.
Germany does, however, have the potential to store a vast amount of hydrogen, because it’s possible to mix small amounts of hydrogen into existing natural gas pipelines and storage containers. These offer enough capacity to store about two weeks of current renewable energy production in Germany. Salt caverns, some of which are now used to store Germany’s strategic oil reserve, could provide far more storage.
Siemens estimates that generating 85 percent of Germany’s electricity using renewables will require 30,000 gigawatt-hours of storage. The hydrogen needed to supply that much electricity could be stored in a quarter of the space available in underground caverns. The hydrogen could be distributed initially through existing natural gas pipelines, and eventually through dedicated pipelines.
Siemens says its electrolyzers are about 60 percent efficient; 40 percent of the energy generated by a wind turbine would be lost making hydrogen gas. Then at least 40 percent of the energy in the hydrogen would be lost in generating electricity in gas-fired power plants or fuel cells. So only about a third of the original energy would be retained. But Weinhold says the system would make hydrogen from electricity that couldn’t otherwise be used on the grid and therefore would be wasted without such a storage system.
In addition to being inefficient, the system could be expensive. The high cost of fuel cells is a key reason they haven’t been used widely in cars. But Weinhold says Siemens is working to bring down costs. Siemens is conducting pilot demonstrations of the technology this year, and it plans to sell two-megawatt systems by 2015 and to build systems as large as 250 megawatts by 2018. The largest plants could harness the power produced by about 100 wind turbines.