Harvesting the Wind - Assessment of Offshore Electricity Storage Systems

Offshore wind energy will comprise a significant share of the future European electricity supply. This, however, comes with the challenge of intermittency of the wind resource, and the threat of energy droughts. Energy Storage Systems (ESSs) are needed to tackle this challenge; integrating storage within offshore wind farms instead of onshore can provide additional benefits to the power system and enable large-scale integration of wind energy. To this end, we investigate a subsea pumped-hydro storage system utilizing the pressure difference between the seabed and the atmosphere to store electricity in the form of potential energy of a working fluid in two reservoirs. To optimize the design of the storage system in terms of energy and power capacity installed, we introduce a mixedinteger linear program (MILP). The system's design and operation is optimized to maximize profits over a period of one month accounting for both investment and operational cashflows. Results show that the system can have high round-trip efficiencies (~70%) comparable to conventional pumped-hydro storage (70-80%) with a storage size of up to 32 MWh for a sea depth of 50 m. In our case study, the system runs up to 4 full cycles per day. When participating in the day-ahead market exclusively, the system becomes profitable for electricity price fluctuations with standard deviations of at least double the 2019 values (in the Netherlands), which are likely to materialize in the future.