Carnot batteries as energy storage of the future

Test rig for testing working fluids at the University of Bayreuth. Photo: University of Bayreuth / ZET.

A transformation of the energy supply that replaces fossil fuels with renewable energy sources requires new powerful technologies for storing electricity generated by the sun and wind. One promising technology is Carnot batteries, which temporarily store electricity in the form of heat.

A new project of the Chair of Technical Thermodynamics and Transport Processes (LTTT) in the Center of Energy Technology (ZET) at the University of Bayreuth is investigating optimal working fluids for these energy systems. As part of the DFG Priority Programme 2403 “Carnot batteries: Inverse design from markets to molecules,” it will initially be funded for three years with a total of around €298,000.

Carnot batteries are named after the founder of thermodynamics, the French physicist and engineer Nicolas Léonard Sadi Carnot (1796-1832) who calculated the optimal conversion of heat into work. In the future, they can play an important role in technically matching the availability of renewable energies and the demand on the energy market. In principle, a Carnot battery consists of three components connected in series: a high-temperature heat pump, a heat storage unit and a heat engine.

The heat pump converts the electricity generated by sun and wind, but not immediately required, into heat and uses this heat to charge the storage unit. The heat engine is capable of discharging the storage unit according to the demand on the energy market, thereby converting the heat back into electricity. Carnot batteries are therefore also referred to as electricity-heat-electricity systems.

“Initial demonstration plants show that high round trip efficiency can be achieved with Carnot batteries: Up to 70 percent of the excess electricity fed into the grid from renewable energy sources can ultimately be recovered. The storage costs per kilowatt hour are in the range of pumped-storage power plants or electrochemical batteries,” said Florian Heberle, research associate at the LTTT research group and managing director of the ZET at the University of Bayreuth. At the same time, he points out that there is a need for further research and development in order to fully exploit the potential of this form of energy storage.

“In the next few years, the new DFG priority programme will make a significant contribution to further developing and technologically optimizing the interaction of the three components and the integration of Carnot batteries into the overall system of electrical energy supply.”

Functionality of a Carnot battery. Photo: University of Bayreuth / ZET.

The project based at the University of Bayreuth focuses on the challenge of finding optimal working fluids for Carnot batteries. Three criteria are important: Efficiency, operational safety and a low global warming potential (GWP). This means that as few greenhouse gases as possible should be released during operation. The investigations will focus on special mixtures of natural hydrocarbons and unsaturated partially halogenated refrigerants. The aim is to precisely identify thermochemical properties, but also to test the fluids in practice with regard to the very different functions they have to fulfill in the system of a Carnot battery. The latest test benches and measurement technologies are available at the LTTT chair for this purpose.

“The research work in Bayreuth is closely networked with the other projects of the DFG priority programme, which are focussed on other technological aspects of Carnot batteries. The mutual exchange between the participating research locations in Germany and the linking of new findings are particularly important here so that we can make progress in the sustainable transformation of the energy supply. Only with new high-performance storage technologies will it be possible to achieve the decarbonization of energy systems that is essential for climate protection,” said Dieter Brüggemann, holder of the LTTT chair and director of ZET, who is leading the project together with Heberle.

About the new priority programme of the German Research Foundation (DFG)

In the DFG priority programme 2403 “Carnot batteries: Inverse design from markets to molecules,” innovative concepts for the Carnot batteries of the future are to be developed based on the requirements of the energy market. The duration of the first funding period, which began on July 1, 2023, is three years. For this period, the DFG has selected a total of 17 individual projects at 14 German university locations for funding. The total funding amounts to approximately €6.5 million. The coordinator of the priority programme is Burak Atakan from the University of Duisburg-Essen.

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