A team of researchers from the University of Stavanger in Norway, in a recent study, explored the large-scale energy storage potential of the ocean to support intermittent renewable energy sources like wind and solar and stabilize the grid.

The research published in the Journal of Energy Storage focuses on a concept called Subsea Pumped Hydro Storage (SPHS), which utilizes the high pressures of the deep ocean to store energy in underwater compressed air tanks.

The concept involves placing a sturdy tank on the ocean floor, where the pressure difference between the tank’s interior and the surrounding ocean creates potential energy. The tank is initially filled with air at a lower pressure than the ocean’s hydrostatic pressure. By opening valves connected to the ocean, water rushes into the tank, thereby spinning the turbines to generate electrical power for the grid while compressing the air inside.

To charge the system, pumps remove the water, causing the gas to expand and the pressure to decrease until it reaches its initial level. The tank’s total volume is composed of both air and water, with their pressures assumed to be equal.

Source: University of Stavanger, Norway

The concept differs from traditional pumped hydro storage, where fresh water is pumped from a lower reservoir to a higher reservoir during excess power generation or low electricity demand. Water is then released back into the lower reservoir through turbines to generate electricity during times of high demand.

The study developed theoretical models to analyze key parameters like the energy density, state of charge, and power output of SPHS systems. The results found that suitable conditions for SPHS exist in many regions worldwide, with energy densities comparable to other leading storage technologies.

The researchers also evaluated the efficiency of traditional hydropower turbines, like Pelton turbines, for energy recovery in SPHS systems. The analysis found that with certain operational limits, Pelton turbines could achieve theoretical efficiencies of more than 90% during discharge cycles and could be used for the extraction of power in the SPHS system.

While more empirical testing is still needed, the authors conclude that SPHS shows promise as a viable large-scale energy storage solution, especially for offshore renewables. Co-locating underwater storage with offshore wind farms, like in the North Sea, could reduce transmission costs and make renewables more grid competitive. Co-location could also increase revenue streams for operators by storing energy behind the meter, allowing operators to participate in day-ahead or intraday markets.

However, the authors also emphasized the need for additional research and field testing to develop the concept further.

In June last year, India’s government introduced several initiatives to expedite the commissioning of pumped storage projects. Earlier that year, the Ministry of Power had also proposed that pumped storage projects be supported through concessional climate finance.

In a study published by the International Institute for Applied Systems Analysis, it was revealed that the use of vertical shafts in decommissioned underground mines for energy storage using gravity could provide a viable alternative to battery energy storage and underground pumped hydro storage systems.