22 Oct 2015
Posted: Oct 21, 2015
Sun and wind are important sources of renewable energy, but they suffer from natural fluctuations: In stormy weather or bright sunshine electricity produced exceeds demand, whereas clouds or a lull in the wind inevitably cause a power shortage.
For continuity in electricity supply and stable power grids, energy storage devices will become essential.
So-called redox-flow batteries are the most promising technology to solve this problem. However, they still have one crucial disadvantage: They require expensive materials and aggressive acids.
A team of researchers at the Friedrich Schiller University Jena (FSU Jena), in the Center for Energy and Environmental Chemistry (CEEC Jena) and the JenaBatteries GmbH (a spin-off of the University Jena), made a decisive step towards a redox-flow battery which is simple to handle, safe and economical at the same time: They developed a system on the basis of organic polymers and a harmless saline solution.
“What’s new and innovative about our battery is that it can be produced at much less cost, while nearly reaching the capacity of traditional metal and acid containing systems,” Dr. Martin Hager says.
CAPTION Jena research team and its innovative battery (from left to right) are: Prof. Dr. Ulrich S. Schubert, Tobias Janoschka und Dr. Martin Hager.
The scientists present their battery technology in the current edition of the renowned scientific journal Nature (“An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials”).
In contrast to conventional batteries, the electrodes of a redox-flow battery are not made of solid materials (e.g., metals or metal salts) but they come in a dissolved form: The electrolyte solutions are stored in two tanks, which form the positive and negative terminal of the battery.
With the help of pumps the polymer solutions are transferred to an electrochemical cell, in which the polymers are electrochemically reduced or oxidized, thereby charging or discharging the battery. To prevent the electrolytes from intermixing, the cell is divided into two compartments by a membrane.
“In these systems the amount of energy stored as well as the power rating can be individually adjusted. Moreover, hardly any self-discharge occurs,” Martin Hager explains.
Traditional redox-flow systems mostly use the heavy metal vanadium, dissolved in sulphuric acid as electrolyte.
“This is not only extremely expensive, but the solution is highly corrosive, so that a specific membrane has to be used and the life-span of the battery is limited,” Hager points out.
In the redox-flow battery of the Jena scientists, on the other hand, novel synthetic materials are used: In their core structure they resemble Plexiglas and Styrofoam (polystyrene), but functional groups have been added enabling the material to accept or donate electrons. No aggressive acids are necessary anymore; the polymers rather ‘swim’ in an aqueous solution.
“Thus we are able to use a simple and low-cost cellulose membrane and avoid poisonous and expensive materials”, Tobias Janoschka, first author of the new study, explains. “This polymer-based redox-flow battery is ideally suited as energy storage for large wind farms and photovoltaic power stations,” Prof. Dr. Ulrich S. Schubert says. He is chair for Organic and Macromolecular Chemistry at the FSU Jena and director of the CEEC Jena, a unique energy research center run in collaboration with the Fraunhofer Institute for Ceramic Technologies and Systems Hermsdorf/Dresden (IKTS).
In first tests the redox-flow battery from Jena could withstand up to 10.000 charging cycles without losing a crucial amount of capacity. The energy density of the system presented in the study is ten watt-hours per liter. Yet, the scientists are already working on larger, more efficient systems. In addition to the fundamental research at the University, the chemists develop their system, within the framework of the start-up company JenaBatteries GmbH, towards marketable products.
Source: Friedrich-Schiller-Universitaet Jena
Tesla is launching the home battery business partly because it’s already making vehicle batteries—and as a result it can benefit from the economies of scale that come from making both. Another reason is that the market for storage is expected to grow in concert with the use of solar power. Tesla needs both electric vehicles and solar power to boom if it hopes to fulfill the projected output from a vast $5 billion battery “gigafactory” it’s building in Nevada.
“The obvious problem with solar power is that the sun does not shine at night,” Tesla CEO Elon Musk said at the unveiling of the new batteries at the company’s design studio in Hawthorne, California, yesterday. “We need to store the energy that is generated during the day so you can use it at night.”
A number of solar companies now offer batteries to accompany their solar panels (see “Solar Power, and Somewhere to Store It”). Although just a tenth of a percent of U.S. homes now get power from rooftop solar panels combined with energy storage, such systems could account for 3 percent of homes by 2018, according to Greentech Media Research.
Tesla’s residential battery, called Powerwall, will be available in several months and will come in two sizes, a seven-kilowatt-hour battery system that costs $3,000 and a slightly larger 10-kilowatt-hour system for $3,500. The larger battery would keep an average-sized home running for a day. It is unclear what the cost of installation would be.
Tesla expects that many sales will come from commercial customers who pay a variable rate of electricity over the course of a day based on demand. Such customers already see significant reductions in their energy bills by drawing on stored electricity during periods of peak energy demand.
In the near term, the market for home energy storage will depend on how states regulate homeowners’ ability to buy and sell electricity. Net metering, currently available in 43 states, allows residential customers to sell excess generation back to their utility company at retail rates. The policies are being challenged by utility companies that say it undermines their ability to recoup grid infrastructure costs. But as long as net metering continues, consumers will have little need to buy an energy storage system because they can sell the excess solar power they generate rather than store it, says Jay Stein, an analyst with energy consulting company E Source. “I don’t see any financial payoff for them to buy batteries,” he says.
Most utilities that offer net metering, however, also allow residential customers to buy and sell electricity at rates that vary throughout the day based on demand. Battery storage would allow such people to maximize the value of the electricity they sell back to the utility.
“There are some arbitrage values emerging,” says Karl Rábago, executive director of the Pace Energy and Climate Center in White Plains, New York. “If I could export selectively, using a storage device, I might beget higher value for my generation.”
Home energy storage will make more sense in the years to come. Residential and commercial solar-plus-storage systems will offer a clear cost advantage over electricity from the grid throughout the United States by 2030, according to a recent report by the Rocky Mountain Institute, an energy research and consulting group.
Tesla’s Nevada gigafactory, which it’s building with Panasonic, will have an annual production capacity of 35 gigawatt-hours by 2020, more than all the lithium-ion batteries produced globally in 2013.
Such a large investment in what is still a niche market is risky, but Tesla claims that the new factory will cut battery costs by 30 percent when it begins operations, as early as 2016. Tesla’s biggest challenge will likely be filling enough orders for the output. By 2020, the plant will be able to produce enough batteries for half a million electric vehicles per year. Last year, Tesla sold around 20,000 cars.