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 concept of Sterner 2009 [3], presented in Fig. 1 can answer to the first and second problem 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. CH4 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 cycle 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 CO2 capture unit and subsequently to dynamic CH4 and CO2 storage. Once the CO2 storage reservoir is charged by means of availability of a cushion and working gas e.g. with CO2
produced by a biogas plant (which result in negative emissions) or from sources which cannot avoid producing CO2
as steel or cement industry (which result in balanced emissions). The CO
2can then be produced from the storage reservoir and used to generate CH4 which again can be stored locally in another storage reservoir as well asproduced and combusted as required. CO2 generated during combustion is then captured and stored underground making it available for the next generation cycle (dark grey shaded area in Fig. 1).