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Fuel Cell Storage for the Home

Fuel cells convert natural gas directly into electrical energy. They are many times more efficient than are combustion engines, such as the car engine. These require an intermediate step. First, they convert chemical energy into thermal energy (heat) and mechanical energy (force). With this force, they drive a generator, which only then generates the electric power. In the process, a large portion of the originally available energy is lost.

"One always speaks of a fuel cell system," says Dr. Matthias Jahn from the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden. "A single cell doesn’t produce enough voltage to obtain a sufficient electrical power. In a fuel cell stack, several cells are connected one to the other. Each of them is about the size of a CD. We call the groups 'stacks'," says Jahn. 

Production of the cell stacks at the Fraunhofer IKTS. Credit: Fraunhofer IKTS

Real-life Test in Private Households 

Together with the heater manufacturer Vaillant, the IKTS has developed a compact, safe and sturdy fuel cell system that generates electricity and heat in private households from natural gas. The researchers were particularly responsible for the construction of the prototype, the design of the overall system, the design of the ceramic components and the development of the reformer and the afterburner. The devices are currently being tested in private households in the Callux practice test.

They are as compact as classical gas heaters that only produce heat. Moreover, they can comfortably be mounted on the wall and easily be maintained. With an output of one kilowatt, they cover the average current consumption for a four-person household. The Federal Ministry of Transport and digital infrastructure BMVI is promoting Callux. Currently, in the European demonstration project ene.field (www.enefield.eu), about 150 further units are being installed in several European countries. In addition, Vaillant started the production of a small-scale series in early 2014. Parallel to the practical test, the two partners are already working on new models. "Now, it’s all about decreasing production costs and increasing the lifetime of the equipment," says Jahn.

The principle of the fuel cell has been known for over 175 years. So far, however, there has not been a market breakthrough. The main reason was the invention of the electric generator. It knocked the more complex fuel cell out of the running. Only in the 1960s was the technology put into practice by NASA in some Apollo moon missions. In the late 1990s, there were other projects in the automotive industry, which have so far not been able to prevail. The reasons are that the fuel cell is too complex, too expensive, and too unreliable.

"In our project with Vaillant, we have made great strides to bring the technology close to the market. Vaillant is already producing a small-scale series, which is sold in funded projects to customers,“ says Jahn. „For the market breakthrough, the costs still have to be decreased significantly."

The miniature power station for home use is based on a solid fuel cell (SOFC). SOFCs operate at a much higher temperature in comparison to competing approaches, such as the proton exchange membrane fuel cell (PEMFC), which is used in cars, for example. While PEMFCs only reach 80 degrees, SOFCs can reach up to 850 degrees. "This allows the SOFCs to be built much more simply and cheaply," says Jahn.

The electrolyte of an SOFC only transfers oxygen ions, not electrons. Otherwise, there would be a short circuit. "Ceramic is particularly well suited as a material for the electrolyte. It has the desired conductivity and can also endure high temperatures," says Jahn. As a result, even without the use of precious metals, all reactions proceed smoothly, which is necessary for the direct conversion of chemical energy into electrical energy: If the fuel cell heater is connected to the gas network, a reformer initially converts the natural gas into a hydrogen-rich gas. This then reacts in the stack with the oxygen of the air in a noiseless "cold combustion," producing power and heat.

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