Energy Systems and Technologies for the Coming Century ...

spinning reserves can be provided by down-regulating the compressor as well as by upregulatingthe generator (and vice versa). This leads to the effect that a least-cost solutionis a case where the generator is permanently producing power with at least 30 MW. Inconclusion, a large benefit of having the unit onshore is its ability to provide spinningreserves.The storage capacity is only rarely used above 50% of its capacity (2,000 MWh). Thus, itseems reasonable that limiting the storage to 1,000 MWh capacity, equivalent to a singlecavern, is beneficial.As Figure 7 illustrates, there is a large number of successive hours when the wind powergeneration is close to its maximum. Therefore, dimensioning the cavern in a way that itwould be able to absorb these long-lasting peaks and allow a smaller cable is notprofitable.Table 1 states the costs of offshore storage and additional interconnector capacityrelative to the benchmark case (onshore storage). System costs are higher in case of anoffshore location, which is partly due to the fact that spinning reserves need to beprovided locally in Northern Germany. This is a competitive advantage for placing anAA-CAES facility onshore, though additional are possible by choosing an interconnectorto the Nordic power system. This, however, cannot support covering the local demandfor spinning reserves. In contrast to total system cost, CO2 emissions are reduced by0.01% if the storage is placed offshore or by 0.03% by an interconnection to Norway.With regard to AA-CAES, this is caused by the losses associated with operating thecompressor and generator simultaneously for spinning reserves.Table 1: Additional costs of different cases relative to onshore AA-CAESInterconnectorOffshore storage Germany‐NorwaySystem cost Mio. Euro 33.8 ‐73.0CO2 cost Mio. Euro ‐2.4 ‐5.1System cost % 0.0464 ‐0.1003CO2 cost % ‐0.0141 ‐0.0306Risø International Energy Conference 2011 Proceedings Page 4