Energy Systems and Technologies for the Coming Century ...

2.3 CAES and AA-CAES power plantsThe two existing CAES power plants in McIntosh, Ohio, and Huntorf, Germany, haveseveral decades of operational experience. As the adiabatic CAES (AA-CAES)technology is expected to reach a higher efficiency of 70% and does not rely on naturalgas as an additional fuel, it is in the focus of this analysis.Figure 5: Function diagram of an adiabatic compressed air energy storage power plantin single-stage configuration (Zunft et al., 2006)Figure 5 displays a diagram of the main components of an AA-CAES facility. The air iscompressed and injected into the underground cavern while the heat is stored in thesurface heat storage. When later producing power, the air passes through the heat storageagain into an expander. Losses of the heat storage are estimated at 2% per day (Radgenet al., 2010). Note that a heat storage corresponding to 1,000 MWh has approximately adiameter of 20 m and a height of 30-40 m. This suits water depths in the North Sea of ca.40 m, so that the heat storage could be used as a gravity foundation for the compressorand expander.Radgen et al. (2010) assume that an onshore AA-CAES facility with a compressor of150MW, 250MW, a storage capacity of 1,000MWh and a round trip efficiency of 70%requires an investment of approx. 180 Mill. Euro.3 Offshore grids and economic considerationsfor storageOffshore wind is a cornerstone of all North Sea riparian countries energy strategies. Firstoffshore wind farms are mainly erected close to shore, but a large number will be built atlarge distances from shore. This requires HVDC connections and involves considerableconnection costs. Several offshore wind farms can be connected via a single HVDC line,as is done e.g. for the German SylWin1 connection. For the future, a meshed offshoregrid might provide a least-cost possibility of combining the connection of offshore windfarms to their countries and building interconnectors for international power trading.Figure 6 gives an illustrative example of a possible topology.Economic considerations for investing in offshore storage technologies can becategorized as following. First, a storage can replace or deter the need for additionaltransmission lines. In the context of offshore wind energy, this seems most probably ifchanges to the original design of an offshore wind farm / cable combination take place –e.g. through the addition or upgrade of wind turbines leading to a larger installedcapacity, or through the addition of wave power.Second, a storage can increase reliability in the operation of a meshed offshore grid.Wind power generation is inherently associated with prediction errors, leading to theeffect that power flows for international trading are scheduled suboptimally on dayaheadelectricity markets. Local offshore energy storage could outbalance theseprediction errors and thus, ensure a more reliable day-ahead scheduling of cables. Thevalue of this option, however, depends largely on assumed rescheduling procedures.Risø International Energy Conference 2011 Proceedings Page 4