Illustration Energiespeicher

DLR researchers have built a pilot system in Stuttgart to optimise the technology of the Carnot battery for industrial use.

One of the big challenges of energy transition is developing sufficient, cost-effective storage capacity to compensate for fluctuations in renewable energy generation. Since 2014, the Institute of Engineering Thermodynamics at the German Aerospace Centre (DLR) has been working on a concept called the Carnot battery, which stores electricity in the form of heat for prolonged periods in nitrate salts.

Now, in the CHESTER project, the DLR researchers have built a pilot system in Stuttgart to optimise the technology for industrial use. At the heart of the prototype is a latent heat storage unit that’s filled with around 2 of thermal salts. Electricity (from renewable sources) powers a high-temperature pump to heat the salt to 150 °C. The melting salt in a latent state can absorb twice as much energy compared to just heating the crystals.

The DLR’s proprietary battery consists of pipes that criss-cross through the storage tank on two coolant circuits – one for charging and one for discharging. The result is a heat exchanger unit that resembles a snowflake in cross-section, thus providing a large contact surface for energy transfer to take place.

The researchers have been testing different load scenarios, heat flows and temperature curves to determine the system’s limits. “The main challenge is to find and set the optimal operating parameters. For a stable heat transfer between the heat pump and the storage tank and then to the heat engine, all components have to work together in harmony and at the necessary level of performance,” explains DLR’s Maike Johnson in a press release.

Carnot batteries are interesting for sector coupling as they can store high temperature heat (up to c. 600°C) that can be used for processing, as well as generating electricity. And they could be the missing link in renewable district heating networks, buffering thermal energy for weeks or months to bridge periods of reduced sunlight and peak loads.