Seite - 145 - in Water, Energy, and Environment - A Primer

flowsaroundFlorida in theUSatanaveragespeedof2m/s (4.5 mph). Other areas with high ocean current flows that can be usefully tapped by underwater ‘turbines’ are between islands, aroundheadlands, and entrances to bays and large harbors. While few studies have been carried out to date on this resource’s global potential, in 2000 one study (54) put the number at 450GW.A 2006 study from the USDepartment of the Interior estimates that capturing just 1 part in 1000 of the available kinetic energy of the Gulf Stream would supply one third of Florida’s electrical demand (55). Other countrieswhere studies have been carried out include the UK, Canada, and Japan. One study confirmed that the UK ocean current energy resource was theoretically capable of meeting one-fifth of UK electricity demand. The EU’s JOULE-CENEX study identified many ‘European sites ranging from 2 to 200km2 of sea-bed area, many with power densities above 10MW/km2.’ In addition, ocean current energy can be tapped with little environmental impact and is a reasonably predictable energy resource, lending itself to base-load applications. While ocean currentsmove at lower speeds than air currents passing throughwind turbines, the density of water, more than 800 times that of air, means that them in ½mv2 in the kinetic energy of the moving ocean current is large and compensates for the lower speed. This is reflected in the fact that a water speed of 10mph delivers the same power as an air speed of 42 mph for the same size of turbine system. The Sun is the prime driving force for ocean currents through its creation of winds and temperature differences in the ocean, but other factors like salinity and theEarth’s rotation alsoplay a role. Underwater turbines used to capture this energy can be thought of as smaller-sized, underwater wind turbines designed for ocean environments. Turbine types being developed and tested include horizontal axis turbines, similar towind turbines, vertical axis turbines, and oscillating hydrofoils with flap-like structures. Renewableenergy 145