11.4: Energy Storage Technologies

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There are several types of devices that can be used to store energy. In practice, the input may be either electrical energy (EE), or heat (Q) = flow of thermal energy (TE). The same applies to the output. Then, there are different forms of energy contained in the “storage reservoir”. Electrical energy cannot be stored “as it is”. It must be converted to a different energy form. The “storable” forms are thermal energy, potential energy due to gravity (PEG) or due to compression of the storing medium (PEC), kinetic energy of spinning bodies (KESB), or – last, but definitely not least – chemical energy (ChE). There is a large variety of possible combinations of input energy, stored energy and output energy. Here, we list those that are of practical importance, using a scheme: input → stored energy → output:

EE → ChE → EE: it is how electric batteries work, and currently it is the fastest growing storage technology (further on, battery storage is discussed as a separate topic). Another method is to produce and to store a “green” fuel, i.e., hydrogen, and later on convert it to electricity or heat.
EE → PEG → EE: this is the scheme according to which pumpedstorage power plants work; currently, considering the total storage capacity of all such plants worldwide, this is the “Number One” storage technology (however, storage in batteries may outweigh it some time from now).
H → ThE → EE: this method is currently used primarily in Concentrated Solar Power technology (see Chapter 7).EE → PEC → EE: here compressed air is used as the “carrier” of the stored energy.
EE → ThE → EE: there are several variants of this scheme, now only in the early stage of development.

All the methods listed will be discussed in closer detail in the following subSections and sub-sub-Sections.