alternative to salt storage for renewable energy

With the Chesteer research program, the partners of DLR Stuttgart want to test a Carnot battery. This storage system, which runs on salt, makes it possible to store electricity using high-temperature heat pumps.


What is a Carnot battery?

A Carnot battery is a type of thermodynamic device based on the Carnot cycle, a theoretical working process used to assess the maximum efficiency of a heat engine. A Carnot battery is a device used to store thermal energy and convert it into useful work.

The Carnot cycle consists of four thermodynamic processes that occur cyclically:

  1. Heater: The working fluid (eg air or steam) is heated by a heat source to a high temperature.
  2. Expansion: Working fluid expands through an expander, producing useful work.
  3. Cooling: the working fluid is cooled by a low temperature cooling source.
  4. Compression: The working fluid was compressed through a compression valve and returned to its original state.

The Carnot battery is an ideal device which is based on the Carnot cycle and has a theoretical maximum efficiency of 100%. This means that in theory you can fully convert the stored thermal energy into useful work. However, in practice, the Carnot battery is not a usable device due to the impossibility of finding perfect sources of heat and cooling. Additionally, the process of expanding and compressing the working fluid in the Carnot battery is also often inefficient due to friction and other factors.

Carnot battery for renewable energies

The German Aerospace Center (DLR) has commissioned a Carnot battery in Stuttgart to store electricity from the sun and the wind.

The project is part of an EU-funded research project called Chester.

Basically, it is about overcoming dark periods and load peaks in a climate-neutral way. According to the DLR, this is a central issue of the energy transition. With the help of storage systems, excess renewable energy can be used when the wind is not blowing and the sun is not shining.

We are working on optimizing the technology so that it can be used industrially and practically.

Maike Johnson, project manager at DLR’s Institute for Technical Thermodynamics.

The heart of the Carnot battery is a latent heat storage system developed by the DLR’s Institute of Technical Thermodynamics, which is filled with approximately 2 m3 of nitrate salts. A high temperature heat pump heats the salt to 150 degrees Celsius with electricity that will be stored ‘latent’ as the salt melts when heated. Part of the added heat is apparently hidden, that is to say latent, in the detachment of the bonds between the salt crystals. Depending on the salt, latent heat storage can thus absorb about twice as much energy as heat storage without fusion.

Typical storage time for Carnot cells is a few hours to a few days.. To unload the storage unit, a second circuit transfers heat to a heat engine which drives a turbine with an alternator. The electricity produced in this climate-neutral way can be fed back into the grid.

Heat exchangers like snowflakes.

The special feature of the DLR heat storage system is its heat exchangers. Pipes, empirically and mathematically designed by DLR’s Institute of Technical Thermodynamics, pass through the storage tank. They have two channels for the refrigerants: one for charging and another for discharging the heat store. This allows operation with different refrigerants to match different parts of the storage system process.

For efficient energy transfer between the steam circuits and the salt, the heat exchangers have a fin-shaped section that resembles a snowflake. The result is the largest possible contact surface for the salt.

In recent months, researchers have tested each component and process of the storage cycle separately. The difficulty, they say, is finding and establishing the optimal operating parameters.

For a stable heat transfer to take place between the heat pump and the storage tank, and from there to the heat engine, all components must work together in time and with the right performance. How much refrigerant is needed? How fast can salt be heated and cooled? What energy can we get out of the store?

Mike Johnson.

Scientists are testing different load scenarios, heat flow and temperature curves to probe the boundaries of the system. In the case of larger systems, heat losses and different operating states play a particularly important role. With the pilot plant, this is already considered in the study phase.

Carnot batteries have the potential for widespread use in a sustainable energy economy. We anticipate that industrially viable systems will be available on the market within a decade. They will then be designed for longer storage times and powers of several megawatts.

Dr. Andrea Gutiérrez, head of the group specialized in thermal storage with phase change of the Institute of Technical Thermodynamics of the DLR.

Carnot cells for industrial processes.

A big advantage of Carnot cells is that they can supply electricity and heat at the same time. In sector coupling, they can be easily connected to other energy systems. This is particularly interesting for the industry. The stored heat can be used directly in many industrial processes. In combination with seasonal heat storage systems, they can maintain thermal energy for months.

The size, capacity and energy management of the Carnot batteries can be adapted to the respective needs. This makes them, for example, suitable for so-called smart district heating. These are the local electricity and heat networks near the storage facility that supply electricity to housing estates or office parks.


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