Powered by the moon, tidal and wave energy has been captured since the Middle Ages when “tide mills” were used to grind grain. The first tidal power station was the Rance tidal power plant built between 1960 and 1966 in La Rance, France. But the technology has never really caught on, given its high capital cost and the limited availability of sites with sufficient tidal flows. However, innovations in turbine technology and design have begun to lower those costs and make deployment possible in more areas.
Still, the technology is decades behind other forms of renewables and will likely need significant government support and research to become a true clean energy player.
Think of windmills installed on a seabed or riverbed and you essentially have a tidal turbine. The underwater current produced by the ebb and flow of the tides drives the turbine blades, which then drive the generators to create electricity. One big advantage over their wind-driven siblings comes from water’s high density, which means an underwater turbine can turn much slower than a wind turbine and still produce the same amount of power. Further, while wind is unpredictable, you can set your clock by the tides, which means you can accurately predict how much power a given turbine is going to produce.
The U.S. Department of Energy’s latest assessment identified up to 1,400 terawatt hours of potential tidal energy generation per year—one terawatt-hour of electricity is enough to power about 85,000 homes. Developing just a small fraction of the available wave and tidal energy available in the U.S. could power millions of American homes.
“Wave and tidal energy represent a large, untapped resource for the United States and responsible development of this clean, renewable energy source is an important part of our all-of-the-above energy strategy,” said Assistant Secretary for Energy Efficiency and Renewable Energy David Danielson in a statement last year. The U.S. is currently underwriting development of 17 tidal and wave energy demonstration projects with $16 million.
Yale’s environment360 blog writes:
A recurring theme among wave power experts is that wave energy is where wind energy was three decades ago. At that time, engineers had not settled on the optimal design for wind turbines, but decades of ensuing research have resulted in highly sophisticated turbine designs. With wave power, some research occurred after the Arab oil embargo of the 1970s, but since then government and commercial research and development into wave power has paled compared to wind and solar energy.
There are several types of tidal energy technologies. The Department of Energy describes them like this:
Barrages or Dams
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which activate a generator. Gates and turbines are installed along the dam. When the tides produce an adequate difference in the level of water on opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at five to eight knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as in some wind farms. The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
IHS Emerging Energy Research estimated that more than 1.8GW of ocean projects in 16 countries were in development as of 2011.
The Electric Power Research Institute reports that ocean energy could eventually supply 10 percent of U.S. energy, with 50,000 MW off of the Northwest coast, or enough to offset the use of 50 nuclear power plants.
UK Leading the Pack
The UK is “now set to lead the world into the next phase by installing the first wave and tidal energy farms, known as arrays – a complex engineering challenge,” according to the UK government. “While the current installed capacity is fairly modest at almost 9 megawatts (MW), the industry is on track to deliver over 120MW by 2020 – making a meaningful contribution to the UK’s energy mix.”
Scotland is among the most active proving grounds in the UK. “The Pentland Firth, the narrow run of water between the north-east top of Scotland and the Orkney islands, is possibly the best place in the world to urn racing tides into electricity,” according to Carbon Commentary.
“Some U.K. locations have significant tidal head ranges,” Mark Baker, a marine renewables business manager at GE Power Conversion, told GE Reports. “They offer a tantalizing energy generation potential.” GE is reading the testing of an array tidal turbine system planned for the Pentland Firth.
Energy produced by tidal projects at Pentland Firth could supply nearly half all Scotland’s electrical needs, according to engineers from Edinburgh and Oxford universities.
“Perhaps the reason why this technology has only recently begun to take off and attract serious financial support is that, not only is there growing recognition that we urgently need large scale sources of clean energy to substitute for fossil fuels, but also engineering challenges which were almost insuperable even 25 years ago are becoming solvable today,” writes Peter Fraenkel, chief technology officer for Marine Current Turbines in his white paper Tidal Current Energy Technologies. “In short, marine renewables are needed and they are becoming technically possible; they should represent a major industrial growth area in the years to come.”
Brock N Meeks is Editor-in-Chief of Ideas Lab.