Innovative combined heat and power generation for off-grid energy supply

By Petra Wiesmayer for Innovation Origins
Image: In the “BiFlow” project, the STAGE76 student residence in Bruchsal is equipped with an innovative energy storage system that supplies the residents with heat and electricity.

Together with its partners, the Karlsruhe Institute of Technology (KIT) in Germany is developing a new kind of hybrid storage system consisting of a combination of lithium-ion and redox flow battery.

One of the major hurdles to the expansion of renewable energies has so far concerned the ability to store energy that has been generated. This is why various alternative options to lithium-ion technology are being researched so that larger amounts of energy can be stored. All this also with a view to the potential scarcity of resources in the future.

Consequently, the Karlsruhe Institute of Technology (KIT) in Germany is now developing a new type of hybrid storage system together with its partners in the ‘BiFlow’ project. This system combines the special advantages of a lithium-ion battery and the Redox Flow Battery – and simultaneously serves as storage for heat.

World premiere

To date, electricity from renewable energy sources has been stored in home storage systems using lithium-ion batteries for the most part. The advantages of this method are that these batteries are relatively inexpensive to purchase and at the same time extremely compact. Redox flow technology has not yet caught on, but it does have its advantages.

“Output and energy in redox flow batteries can be scaled as needed. Moreover, they have a long service life and cycle stability as well as an exceptionally high operational reliability,” explains Nina Munzke from the KIT Battery Technical Center and project manager in the BiFlow research project. This is the reason why she and her colleagues are now planning to combine the two battery types. This would allow their specific advantages to be brought together and their disadvantages to be offset. “What’s more, we want to use the electrolyte cells in the redox flow battery as thermal storage and thereby increase the overall efficiency of the system. This form of combined heat and power generation is a world premiere.”

STAGE 76 Student Residence Field Test Project

The first field test with the new technology is already underway. Project partner Storion Energy GmbH (SEG) has installed a redox flow battery in STAGE76, a student residence in Bruchsal. By developing special Stacks (the energy converter of this battery type) Storion is able to provide particularly high performance levels, the scientists say. The overall integration and intelligent control of the entire storage system are being handled by KIT.

Another project partner, the Fraunhofer Institute for Chemical Technology (ICT, also in Germany) is working on an optimized electrolyte composition for the redox flow battery. This is a prerequisite for hearting applications. The researchers point out that the electrolyte used in the battery does not have to be recycled, but can even be reused after the end of the battery system’s service life of about 20 years since it is not depleted during use.

STAGE76 is largely independent of the power grid thanks to this storage system. Solar energy surpluses that are generated and also exceed the electrical capacity of the hybrid storage system are used for other purposes. They are used to supply heat to the building, as are the heat losses when the redox flow battery is charged and discharged. According to the KIT scientists, a large-scale application of measurement instrumentation guarantees a sound analysis and visualization of the system’s operation. “We are delighted with the trial system,” says Matthias Holoch, the student residence manager. “It will not only provide important data for the energy transition, but also supply the students with electricity and heat in real-time conditions.”

Intelligent energy management for a complex complete system

The hybrid storage system will also incorporate a charging infrastructure for electric cars featuring three charging points with an output of 22 kilowatts each. These charge points will also be used to optimize the vehicle’s internal energy consumption and will be regulated based on usage patterns: “Our goal is to create an overall system that is as economical as it possibly can be,” says Dr. Christian Kupper from the Battery Technology Center, who is also involved in the research project. “To achieve this, we are developing not only an optimized storage management system but also a superior intelligent energy management system. In terms of sector coupling, BiFlow combines the power supply with heating and mobility into a highly complex complete system.”

The ‘BiFlow’ project is funded by the German Federal Ministry of Economics and Energy (BMWi) to the tune of over €1.3 million.

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