The relevance of energy storages for an autarky of electricity supply ...

1 Introduction
The steady increase in consumption of fossil fuels, which are becoming scarcier at
the same time is a major reason for global warming, air pollution and political
instabilities. To cope with these problems, Renewable Energy Ressources can
constitue a clean alternative for producing electricity. These ressources can be
classified into two major groups. The controllable (like biomass, hydrualic electric
plants) can easily be integrated, as they use conventional technologies. Non-
controllable (mainly wind and solar) can cause problems such as haramonics, flicker,
voltage dip, over voltage and phenomeons like islanding. Storage systems, in this
context, can act as power regulator, as the intermittend and unpredictable sources
would consequently lead to power quality and reliability issues. (Goyena, 2009)
Energy storage is defined as the conversion of electric energy into a form in which it
can be stored until converted back to electrical energy. (Nigim, 2009) With the input
of energy or energy carriers, the energy content of energy storage increases (charge).
Afterwards it stores the energy content without many losses over a certain amount of
time to supply energy or energy carriers whenever there is demand for it, leading to a
decreasing energy content within the energy storage (discharge). When fully
charged, the storage contains the maximum energy content, when completely
discharged, the usable energy or mass of the energy carrier is zero. (Rummich, 2009)
Electricity supply has to be matched to the consumption, regardless how it changes.
Reserve capacity or storage systems can be used to guarantee reliable supply. In this
context a large distribution network (grid) as well can handle efficiently the
fluctuation of demand for short interruption events. But storage systems are not only
used to balance fluctuations, peak demand during noon and afternoon can also be
covered by such plants. Peak demand lasts for a few minutes to a couple of hours in
the day. (Nigim, 2009)
Energy storage plants not only act as power generator but also as consumer within
the electricity system, depending on whether there is a lack or oversupply in
electricity production. According to that fact, storags systems provide electricity
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