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Energy Systems and Technologies for the Coming Century ...

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where large amounts of offshore wind energy are to be installed – namely in Germany,Denmark <strong>and</strong> <strong>the</strong> Ne<strong>the</strong>rl<strong>and</strong>s. These countries share <strong>the</strong> common characteristics that<strong>the</strong>ir coastal regions are flat, i.e. that pumped hydro cannot be realised regionally.Building new pumped hydro storages in Sou<strong>the</strong>rn Germany faces environmentalconcerns as a central obstacle <strong>and</strong> requires new transmission capacity over severalhundred kilometres. The only large-scale storage option that is currently operational inNor<strong>the</strong>rn Germany is a diabatic Cavern Air <strong>Energy</strong> Storage (CAES) plant. Whenelectricity prices are low, air is compressed into an underground salt cavern. Forgeneration, this pressurized air is injected into a gas turbine <strong>and</strong> thus, raises <strong>the</strong> turbine’sefficiency. Adiabatic CAES, which is independent of gas supply <strong>and</strong> achieves a higherefficiency through a heat storage, has lately been <strong>the</strong> subject of a research project <strong>and</strong> isgenerally considered feasible (see also Crotogino/Donadei, 2011 or Gatzen, 2008). Inseveral scenarios of a 100% renewable energy study <strong>for</strong> Germany, CAES plays a majorrole (German Advisory Council on <strong>the</strong> Environment, 2011). As technical feasibility is inprinciple also given <strong>for</strong> offshore locations, this paper analyses possible offshoreapplications. It is structured as following: first, technological issues are addressed.Second, economic effects in offshore grids are presented qualitatively be<strong>for</strong>e turningtowards <strong>the</strong> model <strong>and</strong> data description. Onshore storage, offshore storage <strong>and</strong> anincreased interconnector capacity between Germany <strong>and</strong> Norway are compared be<strong>for</strong>emoving to <strong>the</strong> discussion <strong>and</strong> conclusion of this first estimation on <strong>the</strong> subject.2 Technology2.1 CavernsNatural gas has been stored successfully in large quantities in underground salt caverns<strong>for</strong> many years worldwide in facilities like <strong>the</strong> Ll.Torup Energinet Storage Facility inNorth Jutl<strong>and</strong>, Denmark, see Fig. 1.Salt caverns are artificially constructed cavities in salt deposits created by solutionmining. Typical dimensions of salt caverns are a geometrical volume of 500 000 m³ atdepths of over 1 000 m, <strong>and</strong> pressure ranges of 60 to 180 bar.Unlike surface storages, salt caverns enable extremely large amounts to be stored at lowspecific costs. The storages are almost maintenance-free <strong>and</strong> <strong>the</strong>y boast negligibleleakage rates. This is why salt caverns are also highly suitable <strong>for</strong> <strong>the</strong> storage ofcompressed air <strong>and</strong> hydrogen.A precondition <strong>for</strong> <strong>the</strong> installationof salt caverns to storecompressed air is <strong>the</strong> availabilityof suitable salt <strong>for</strong>mations with <strong>the</strong>necessary thickness <strong>and</strong> <strong>the</strong>appropriate depth range of around700 to 1 300 m. There also have tobe adequate quantities of freshwater available <strong>for</strong> solutionmining <strong>the</strong> caverns, as well asenvironmentally-compatibleoptions <strong>for</strong> disposing of <strong>the</strong> largevolumes of brine produced during<strong>the</strong> solution mining process.Figure 1: Figure 1: The 7 gas caverns in Ll. Torup(Energinet)A borehole with a cementedcasing gives access to <strong>the</strong> cavern –an additional production stringthat can be pulled out of <strong>the</strong>cavern is also installed <strong>for</strong> safetyreasons. In <strong>the</strong> special case ofRisø International <strong>Energy</strong> Conference 2011 Proceedings Page 4

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