These systems are used for storing energy instead of generating it. They are not-so-distant cousins to pumped hydro (PHS) and compressed air energy storage (CAES). There are two main types of marine energy storage at the moment: underwater compressed air energy storage (UWCAES) and underwater pumped hydro storage (UWPHS). To read in more detail about these devices and how they work take a look at my post.
Underwater Compressed Air Energy Storage
In an UWCAES system air is stored in pliable bags on the seafloor. The depth of the water provides the needed pressure to compress the air. When power is needed an outlet valve is opened and the air is allowed to flow out of the bag and to a turbine generator, located on shore or a surface platform. To charge the system, air is pumped back into the bag.
UWCAES devices work best at depths of 400 – 700 meters underwater; this water depth provides the pressure needed for most turbine compressors where compressed air energy storage is typically used.
This underwater storage method has a big advantage over its land counterpart. In regular CAES air is stored in a reservoir of a fixed volume, as the compressed air is released the pressure inside the vessel will decrease reducing the flow to the turbine. Underwater, the pressure acting on the bag remains pretty much constant providing a steady flow of air to the turbine and boosting efficiency.
Underwater Pumped Hydro Storage
In an UWPHS system seawater is used as the working fluid instead of air. These devices use rigid spheres of steel or concrete. To ‘discharge’ as a energy storage device, the system allows high-pressure seawater to enter through an opening in the sphere by way of a turbine connected to a generator. This high pressure seawater drives the turbine which in turn produces power. To charge the system, the seawater inside the sphere is pumped out, preparing the sphere for the next discharge cycle.
These energy storage devices serve best in grid peak shaving applications. Round-trip efficiencies for UWCAES and UWPHS are on the order of 70-85%, which is commensurate with regular PHS and CAES. Although there are no full scale commercially-sized systems on the grid, developers are claiming that a full scale system would cost approximately $1,500 to $2,500 per kilowatt, an amount that is almost on par with regular pumped hydro storage. The main concerns with these systems are typically the amount of ballast required, how to install them, and performing maintenance.
There are a few company names to keep on your radar that are doing good work on advancing marine energy storage. Hydrostor, a Canadian firm, has a pilot project in Lake Ontario rated at approximately 1 MW which will be tested for the next several years. There is also the Stored Energy in the Sea (StEnSEA) project that is being supported by a consortium of German companies, which is also in the process of a small-scale pilot project over the next couple of years.