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The reactor forms the centrepiece of the lime storage facility: scientists
are developing a more efficient method for using lime to store heat.
© DLR (CC-BY 3.0)
Thermochemical energy storage systems
10.12.2015
Lime storage systems: Cheap, efficient and sustainable
At the German Aerospace Centre (DLR), researchers are using lime as a
storage medium in a specially developed heat storage system. This
enables heat to be stored in the form of a reversible reaction. Such
storage systems can help large amounts of energy to be used more
efficiently in industrial processes and households. The new plant has now
been put into operation.
Lime as an effective storage material: Any
amount of lime can be supplied in powder form
from a silo. The lime is then heated while
passing through stainless steel tubes and can
thus store the heat.
© DLR (CC-BY 3.0)
If calcium hydroxide, known as slaked lime, is
heated, this creates calcium oxide at a
temperature of approximately 450 degrees
Celsius, because water is removed from the
calcium hydroxide. During this reaction, around
80 percent of the input energy is stored in the
form of chemical energy.
© DLR (CC-BY 3.0)
Using the lime storage material, the scientists have developed a concept
that can be used in processes with high temperatures. The lime storage
system is a further development of an initial prototype carried out on a
laboratory scale (BINE Information Service reported), and can store
energy even more cost-effectively and efficiently. The storage material –
slaked lime – is first of all heated. At approximately 450 degrees Celsius
an endothermic reaction then begins that creates calcium oxide –
quicklime. About 80 percent of the energy input is stored in the form of
chemical energy – the rest is released as heat.
The chemically stored energy can be stored indefinitely without loss. This
makes the system particularly well suited for extended storage periods.
Only when the energy is required is it released again in the form of heat by
adding water vapour, which causes a strong exothermic reaction.
Lime is ideal for long-term storage
The DLR researchers opted for lime, as it is very cheap to buy. In addition,
its chemical reaction can release over five times more thermal energy per
cubic metre than, for example, water – when cooled from 80 to 30 degrees
Celsius. The energy density of the storage system is therefore extremely high. Dr Marc Linder, Research Area
Manager for Thermochemical Systems in the Thermal Process Technology department at the DLR Institute of
Engineering Thermodynamics, emphasises: “Storing heat thermochemically with lime provides an interesting
alternative to the more developed technologies in the power plant technology and process heat fields. In addition,
however, we also see potential for lime storage systems in the seasonal storage of energy, for example for
supporting the supply of heat to private households with decentrally generated electrical energy.”
Versatile usage thanks to temperature control
In the further developed storage system, the lime is fed through stainless steel tubes past a stationary heat
source. This enables any amount of lime to be burned in the facility. Supplies of lime are provided by silos (left
image, centre) at the beginning and end of the cycle. An additional advantage of the chemical storage lies in the
regulation of the temperature inside the storage system: the new system makes it possible to inject steam at
differently high pressures into the quicklime. If the pressure of the water vapour is increased, the reaction takes
place at a higher temperature. If the pressure is lowered, the temperature of the reaction also reduces.
Linder sees the challenge in the interaction: “With the new lime storage facility, the continuous movement of the
storage material in combination with the heat supply and the water vapour control still needs to be optimised.”
Competence centre and test facility
In the CeraStorE competence centre, both thermal energy storage with liquid salts as well as thermochemical
storage using lime or metal oxides can be investigated for use in power plant processes and industrial
applications.
The German Federal Ministry for Economic Affairs and Energy funded the first test facility for storing energy with
lime. The further development of the reactor was coordinated by the DLR as part of a European research project
and was recently successfully completed.
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