Offshore Hydrokinetic Energy Conversion for Onshore Power Generation

Design comparisons have been performed for a number of different tidal energy systems, including a fully submerged, horizontal-axis electro-turbine system, similar to Verdant Tidal Turbines in New York's East River, a platform-based Marine Current Turbine, now operating in Northern Ireland's Strangford Narrows, and the Rotech Lunar Energy system, to be installed off the South Korean Coast. A fourth type of tidal energy system studied is a novel JPL/Caltech hydraulic energy transfer system that uses submerged turbine blades which are mechanically attached to adjacent high-pressure pumps, instead of to adjacent electrical turbines. The generated high-pressure water streams are combined and transferred to an onshore hydroelectric plant by means of a closed-cycle pipeline. The hydraulic energy transfer system was found to be cost competitive, and it allows all electronics to be placed onshore, thus greatly reducing maintenance costs and corrosion problems. It also eliminates the expenses of conditioning and transferring multiple offshore power lines and of building offshore platforms embedded in the sea floor. For time-dependent tidal energy, the pressurized hydraulic energy can be stored in an elevated onshore reservoir that can be used as per consumer energy demand, rather than as per tidal energy supply. This technology is a spinoff of a miniature ocean hydraulic energy transfer system that JPL is developing for the Office of Naval Research (ONR). The ONR device uses ocean temperature differences to provide pressurized hydraulic energy which supplies all electrical power for small submersibles. This type of hydraulic energy device is expected to allow submersibles to stay submerged for years. A three-month ocean endurance test is scheduled for late 2009. Similar types of hydraulic energy transfer systems are potentially applicable to all types of hydrokinetic energy, including free-flowing rivers, ocean wave energy, and energy from ocean currents, such as the Gulf Stream. In each case, the corrosion-prone, submerged electrical turbines are replaced by all-mechanical water pumps without any electrical components, and the energy is hydraulically transferred to remote onshore hydroelectric plants by inexpensive pipes. The submerged mechanical turbine blade/pump assemblies can be attached by long, small-diameter, flexible pressurized lines to the larger submerged, stationary pipe lines, thus allowing the submerged blade/pump assemblies to be lifted to the surface and serviced by boat. Check valves in the flexible lines allow damaged turbine blade/pump assemblies to be automatically taken off-line and later repaired or replaced as required.