Energy storage will play a key role in the energy transition and as such also in offshore energy systems. The value of storage in combination with marine energy technologies can be twofold. Either the utilisation of cables is improved by reducing curtailments and providing a higher baseload. Or a system is designed for a fully autonomous energy supply for off-grid applications. For both scenarios either storage technologies or conversion to other energy carriers (such as H2 and ammonia) can be considered.
Different storage technologies include for example batteries, pressure storage, mechanical storage and thermal storage as well as the conversion to green hydrogen by electrolysis. Different storage types can provide flexibility at different speeds, for different time frames and for various total capacities.
Batteries is a collective term for a large range of different technologies all based on chemical energy stored in individual cells. From extremely dynamic lithium ion batteries, which can serve for frequency control on the grid, to flow batteries which are providing a continuous base load and serve for storage of energy for time frames typically beyond 6-8 hours. Small scale batteries are already being used in offshore applications for example to provide continuous power to ROVs in combination with wave energy devices.
Compressed Air Energy Storage
Compressed Air Energy Storage (CAES) involves compressing air and storing it in underground reservoirs, and then releasing it to generate electricity. Offshore, this could be executed using underwater caverns or dedicated pressure vessels. CAES can offer large-scale, long-duration energy storage, with a long operational life cycle, integrate seamlessly with renewable energy sources and provide grid reliability. While traditional CAES systems have been used onshore, adapting them for offshore use is still in the developmental and testing phase.
Hydro-Pneumatic Energy Storage
Hydro-Pneumatic Energy Storage, sometimes referred to as underwater compressed air energy storage, harnesses renewable energy to compress air and store it in underwater accumulators. The system uses the pressure difference between the stored air and the surrounding water to power a turbine, generating electricity when it's required. Hydro-Pneumatic Energy Storage enables efficient, constant pressure, long duration, scalable storage. Its proximity to offshore renewable sources offers direct energy storage, reducing transmission needs, while the submerged nature benefits from natural seawater cooling.
Hydro Pumped Storage
Hydro Pumped Storage stands as a prominent technique for energy storage, especially in the context of offshore renewable energy systems. It works by using surplus electricity to pump water to a reservoir. When electricity demand spikes, the stored water is released, activating turbines to produce electricity. A major advantage of hydro pumped storage is its ability for large-scale storage, providing power for extended durations.
Power to X
Offshore green hydrogen
Hydrogen can be produced in three different ways, which are conveniently colour labeled. Grey hydrogen is won from hydrocarbons such as natural gas, which at high temperatures are split into H2 and CO2 (called steam reformation). The blue hydrogen process follows the same principle, however with the extra step of capturing the CO2 emission of the reformation process. Green hydrogen is produced by electrolysis of water with the use of green electricity, only producing oxygen emissions. As an energy dense chemical energy carrier, hydrogen can be used for faster energy transport and fast fueling processes. While in many cases existing pipelines can be used for hydrogen transport, its low molecular size and low ignition energy can make the process less efficient and more risky than for other chemicals.
Ammonia (NH3) is a potential solution for offshore renewable energy storage, particularly as "green" ammonia produced using renewables. It has a higher energy density than hydrogen, making storage and transport more efficient. Already used globally as a fertilizer, its infrastructure might be repurposed for energy storage. Ammonia can power certain equipment like gas turbines and fuel cells directly or serve as a hydrogen carrier, addressing hydrogen's storage and transport challenges.