Blue Power: Turning Tides into Electricity

Abstract

Water has been used by humans as an energy source in the form of tide mills and water wheels for nearly 2,000 years. As a large-scale power source, however, tidal and wave energy is at roughly the same stage of development that wind power was at in the 1980s, numerous observers say, with many concepts but few installations—a situation that reverses as a technology matures. And the field is heating up fast, which is good news given the wealth of human and environmental health effects that follow traditional fossil fuel–fired power plants. Tidal turbines capture the energy of the currents, as well as that of rivers, irrigation canals, dam tailraces, and possibly even ocean currents such as the Gulf Stream, in much the same manner that wind turbines transduce air currents. The diverse taxonomy of wave devices, meanwhile, could convert the ocean’s roiling into grid-ready electrons. Wave and tidal energy, known collectively as “marine energy,” is currently capable of supplying electricity equivalent to 10–25% of today’s world’s production, according to various estimates, or about 2–5% of end-use energy. In the United States, wave energy conversion alone could supply the equivalent of 6.5% of electricity at current consumption rates, according to one fairly conservative estimate by the Electric Power Research Institute (EPRI), the research arm of the electric utility industry. This is equivalent to the electricity generated by all conventional U.S. hydroelectric plants. Tidal power could furnish another 3–3.5% of electricity needs. In the United Kingdom, the Carbon Trust, a government-funded company promoting climate change mitigation, estimates somewhat more optimistically that in the long run, wave and tidal power together could supply 15–20% of British electricity needs. According to the 2006 Carbon Trust report Future Marine Energy, the United Kingdom could be using these technologies to produce two to five U.S. nuclear plants’ worth of electricity by 2020. The zero-emission cleanliness of wave and tidal energy technologies is comparable to that of wind, and marine energy is arguably the least aesthetically disruptive method of producing electricity. Unlike the proposed Cape Wind offshore wind farm, for example, which currently has some legislators in Massachusetts saying “not in my backyard,” wave and tidal technologies are often invisible from shore. For purposes of energy capture, water is similar to wind, except that seawater is more than 800 times denser than air, essentially making it easier to capture energy. Moreover, whereas the wind can come from any direction, in most locations the tides flow only in and out, reducing the complexity of the mechanisms required to harvest that energy. Tidal power is readily predictable, which makes coordinating the flow of electricity in the grid quite manageable. The keys to a strong tidal current are a large rise and fall in the tides and geographical features that funnel the water through a narrow channel. As with wind, the energy available in a tidal current varies as the cube of the current’s speed. Six knots (about 6 mph) is the threshold for economic viability, according to the 2006 EPRI report North America Tidal In-stream Energy Conversion Technology Feasibility Study. But tides this swift are uncommon. Viable wave resources are more widely distributed.