Hydroelectricity generation exploits the tremendous height differential that occurs naturally at waterfalls or artificially at dams as water flows through the system. Now, efforts are underway to harness a differential of another sort for both energy storage and generation: the pressure under the sea. A Norwegian company called Subhydro is making forays into underwater hydroelectrical power plants, and Canadian company Hydrostor is creating an underwater grid storage system.
Think of water rushing in through the open hatch of a submarine, and you get an idea of the forces at work underwater. Atmospheric pressure and the weight of the water combine to create pressures that compound with increasing depth. At a depth of 400 meters (almost a quarter mile), for example, the pressure is that of 40 atmospheres, one atmosphere being the pressure we experience at sea level. Subhydro envisions installing large concrete tanks at depths of 400-800 meters, and the deeper the better for maximizing energy generation.
When the “hatch” is opened, water is allowed to flow into the tanks through a turbine that drives an electric generator. The more and larger the tanks, the longer the generation can go on. When the tanks are filled, the turbine can be reversed to pump out the water, a process that draws on the power grid and consumes energy. In this way, the pumped storage plant functions like an underwater battery that can be re-charged, much like a hydroelectric plant on dry land pumps water into an upper reservoir after it has passed through a turbine.
According to Subhydro, the efficiency of the underwater plant is about 80 percent, comparable to efficiencies achieved at conventional plants. Integrating the pumped storage plant with wind or solar farms could create a grid storage system that harnesses excess renewable energy generation to pump out the tanks and flood them during peak hours of production.
Another approach to underwater grid storage is in the works at a depth of 80 meters in Lake Ontario, just off shore of Toronto. There, Hydrostor will begin building underwater tanks that will hold compressed air. Surplus energy from renewables (wind, solar) will provide the energy to compress air from the atmosphere and pump it in to the tanks. To put energy back into the grid, the air is allowed to surface, driving generators as it expands back into the atmosphere.
Hydrostor is partnering with Toronto Hydro to build the 1MW/4MWh compressed air energy storage demonstration facility. The system will run at 70 percent efficiency, according to Hydrostor. Earlier this month MaRS Cleantech Fund announced an investment in Hydrostor’s tech.
Clearly, there are still some hurdles to overcome before energy companies everywhere take the plunge. The environmental impact of offshore submerged facilities will need to be considered, as will the building materials themselves. To withstand the underwater pressure, Subhydro is working with research partners to develop thin concrete reinforced with steel fibers, while Hydrostor’s system will use inflatable polyester bags to hold compressed air. Building underwater facilities is itself energy-intensive, so whether the process can be made cost and energy-effective will determine whether cleantech is ready to get its feet wet.
Image via Knut Gangåssæter/Doghouse
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