Optimized operation and system design of an energy storage device for post-feed-in-tariff sales of wind energy at the spot market

The relevance of energy storages as important flexibility technology for the prospective energy supply system has been discussed in many projects and studies during the last years. In the context of the project Netzintegrierte Stromspeicher1 Fraunhofer UMSICHT has developed a deterministic optimization model for storage commitment, named GOMES Generic Optimization Model for Energy Storage. It has the purpose to compare and evaluate different storage applications both technical and economical. Due to its generic nature GOMES is applicable to different storage technologies as well as different markets. Moreover it possesses the particularity that a simultaneous participation in different markets is feasible. Its modular design allows an easy and fast modelling of new scenarios. GOMES ap plies a rolling horizon to restrict the perfect foresight to a predetermined timeframe. The optimization determines the most beneficial and economical storage commitment and delivers therefore information about the maximal reachable revenue of the examined storage application. This allows to calculate break-even capital costs and to determine the optimal storage plant design. In this case study GOMES is used to evaluate the profitability and the systemic effect of a hybrid power plant consisting of a wind farm (older than 20 years → post-feed-in-tariff) and an electric energy storage device. Exemplarily a redox flow battery was applied as electric energy storage. Two cases were examined: in the single-market operation mode the hybrid power plant only sells the generated wind energy at the spot market. In the multi-market operation mode the storage device is additionally allowed to trade at the spot market itself. In the single-market operation mode the optimal storage design from an economical point of view tends to be very small (storage power of 0.4 MW which equals 2% of the installed wind farm power, capacity of 6 full load hours). This has the consequence that the systemic effect is also very small. Wind energy is only shifted in 6.7% of the hours of the year. The calculated break-even capital costs for the redox-flow battery lie with 2 200 /kW in the lower range of todays capital costs for redox flow batteries. The multi-market operation mode has several consequences: the storage device performs more cycles and gets a little higher annual revenue. Neverthel ess the break-even capital costs slightly decrease to 2 100 /kW. This results from the shorter lifetime (17.2 instead of 19.2 years because the maximum numbers of cycles of 10 000 is reached earlier) and the therefore shorter amortization period. Simultaneously even less wind energy is shifted from low load to peak load hours. To conclude, selling the generated wind energy with the support of a storage device at the spot market could be an interesting alternative for wind farms that are no longer subject to the feed-in-tariff in the near- to midterm future. A single-market operation mode is both more economic for the operator of the hybrid power plant and more advantageous for the system (however on a low level). Nevertheless future studies should compare the here described strategy to other direct market strategies.