The wind blows, the sun shines, renewable energy sources produce electricity en masse. But what if that much power is not needed just then? What happens to the surplus energy? How can it be stored for use at a later time when washing machines, computers and televisions are all drawing electricity from the power grid? Experts from four Fraunhofer institutes have come together to investigate these problems. “We are developing decentralized urban hybrid storage systems that store surplus energy in the homes of individual customers, where it can be tapped again as needed,” explains Dr. Christian Doetsch, an engineer at the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT in Oberhausen and head of the consortium. Excess energy will not be stored, then, in a central system but rather in many small storage units located in consumers’ homes – for instance, lithium batteries. Heat accumulators connected to combined heat and power CHP units or heat pumps can also continue to indirectly equalize the power supply system: Centrally-controlled, they either generate or consume electricity, as demand dictates. Many individual, small-scale storage devices with intelligent power generators or heat pumps in the range of 5 to 50 kilowatts together make up a storage system with capacity in the three or four digit kilowatt range.

An example of one such decentralized energy production is the CHP unit in the basement. If the household is cold, it switches on and produces heat that flows through the heat radiators in the home. Electricity is a by-product of this process and is fed into the public energy supply. Until now these units have been temperature controlled, operating only when heat is required. But in the future they could switch on when electricity is needed, too – controlled by a software system called FlexController, designed to control and optimize CHP units, heat pumps, thermal storage devices and household hot water storage tanks in the consumer’s home, allowing the systems to either store energy or produce electricity. All of this goes unnoticed by the consumer – except in the form of an appropriate refund from the power supplier at the end of the year. “We’ll have to wait and see what the ideal model would look like and how remuneration would be organized,” says Doetsch.

Lithium batteries and compact thermal energy storage systems

A further focus of the project is the development of lithium batteries for homes. These batteries are to be safer than conventional lithium batteries and serve as temporary storage for energy produced, for example, by roof-top photovoltaic systems. They can also be controlled by the FlexController software and store energy derived from more general sources such as wind power stations.

A third objective focuses on compact thermal energy storage systems. “We are already working on these systems. Our development to date, however, has focused on their use in refrigeration applications,” says Doetsch. But the principle of thermal storage is the same whether used for heat or cold. The storage medium is an emulsion of paraffin drops in water. If the emulsion is heated, the aggregate state of the wax droplets changes, while the water remains fluid. In the transition between solid and fluid states, the droplets absorb a great amount of energy. If the emulsion is cooled down, the energy is released again. “The advantage of our system is that the emulsion remains in a constant fluid state and thus has the potential for very high charge and discharge capacity,” explains Doetsch.

The status of development in all three areas of the project varies: While the researchers working on thermal energy storage have been at work for some time now, those developing the FlexController are taking on something entirely new. The scientists plan to have all of their development objectives largely met, however, within a time span of three years.