Increased deployment of renewable energy systems has coincided with expanded options for energy storage. However, economic factors can limit the use of grid-scale battery banks, and pumped hydro is primarily feasible only in mountainous areas. A lower cost storage system that can serve coastal areas or islands without mountains is proposed by an international research team: Buoyancy Energy Storage Technology (BEST).

The gravitational energy storage concept based on buoyancy can be used in locations with deep sea floors Schematic of the proposed BEST system. Source: Julian David Hunt et al.and applied to both the storage of offshore wind power and compressed hydrogen. Stored renewable electricity is harnessed to power a motor that lowers a compressed gas recipient and then generates electricity by releasing the compressed gas recipient to rise back to the surface through the water.

The BEST design starts with a platform secured to the sea floor with weighted anchors and connected via cables to a 100 m² square array of high-density polyethylene pipes, each filled with a compressed gas. Electricity transmitted from the surface via power cables drives powerful electric motors, which pull the buoyant tubes down toward the sea floor to store the energy. When stored energy is needed, the tubes are released, enabling their buoyancy to pull the motors in reverse, turning it into a generator and feeding power back into the grid.

The system described in the Journal of Energy Storage can operate at a maximum depth of around 10,000 m and pressure of 1,000 bars and a minimum depth of around 3,000 m and pressure of 300 bars. When combined with offshore wind, the proposed system can store electricity at an investment cost of $50/kWh to $100/kWh.

Simulations indicate that the deeper the system, the less the volume of compression gases varies with depth and the more energy the system stores. While installing the system at greater depth increases overall cost, use of a BEST system to store energy incurs a lower per MWh cost compared to that of using conventional battery systems.

Researchers from International Institute for Applied Systems Analysis (Austria), Federal University of Rio Grande do Sul (Brazil), Aalborg University (Denmark), AccuraSys (Brazil), Hamburg University of Applied Sciences and Núcleo Interdisciplinar de Planejamento Energético (Brazil) contributed to this study.