Martin Streibel et al. / Energy Procedia 40 ( 2013 ) 202 – 211 203 An energy system which is based on renewable energy sources needs to account for some limitations:Short term shortage of electricity: Due to the volatile nature of the electricity production by wind and sun large fluctuations in the power supply can occur. Hence, the system needs to be stabilised by a quick responding reserve in order to maintain a high quality supply during the day.
Longer term shortage of electricity: These periods of up to two weeks with little wind and sunlight can occur during autumn and winter. Therefore, storage and generation capacity must be available to bridge potential electricity gaps [2].
In order to accommodate the feed-in of a large number REU distributed all over Germany the power
grid needs extensive development at all levels.
Fig. 1. Mass flow of the extended Wind-to-Gas concept
The extended Wind-to-Gas concept, based on the conc
ept of Sterner 2009 [3], presented in Fig. 1 can
answer to the first and second prob
lem considering the requirements of
the future energy system. In this
study, we focus on the first issue. The concept is based on converting excess electricity by REU into
hydrogen and subsequently into methane. CH
4
is distributed and stored in the existing natural gas
network. It is combusted at times of limited availability of REU electricity in a combined cycle plant
(light grey shaded area in Fig. 1). Combined cy
cle plants are able to quickly change generation from
partial to full load. The extension to the Wind-to-Gas
system presented here is coupling the plant to a CO
2
capture unit and subsequently to dynamic CH
4
and CO
2
storage. Once the CO
2
storage reservoir is charged
by means of availability of a cushion and working gas e.g. with CO
2
produced by a biogas plant (which
result in negative emissions) or from sources which cannot avoid producing CO
2
as steel or cement
industry (which result in balanced emissions). The CO
2
can then be produced fr
om the storage reservoir
and used to generate CH
4
which again can be stored locally in another storage reservoir as well as
produced and combusted as required. CO
2
generated during combustion is then captured and stored
underground making it available for the next gener
ation cycle (dark grey shaded area in Fig. 1).