Tidal power: Harnessing the ocean’s energy
High and low tides are the result of the Earth’s rotation and the moon’s gravitational pull that produce a phenomenon known as tidal force. This force causes our planet as well as its oceans “to bulge out on the side closest to the moon and the side farthest from the moon,” according to the National Oceanic and Atmospheric Administration. “These bulges of water are high tides.” The wealth of energy generated by the tides “could potentially produce between 150 and 800 terawatt hours of renewable energy per year,” write Doug Johnson and Hakai Magazine in their reporting for Scientific American. The ebb and flow of the tides are also as regular as the phases of the moon, “meaning tidal power does not have the major drawbacks of unpredictability associated with wind and solar,” according to the International Energy Forum. But conditions have to be just right.
The Pacific Northwest National Laboratory (PNNL) explains that “[t]idal currents with sufficient energy for harvesting occur when water passes through a constriction, causing the water to move faster.” One location that fits this description is the Pentland Firth, a strait that separates northern Scotland from the Orkney Islands and is “subject to very strong tides with overfalls and tide races at certain locations,” according to Sail North Scotland. Overfalls are areas where the tidal stream is squeezed vertically by underwater features such as reefs or shallow seabeds, while tide races are areas where the tidal stream is squeezed sideways into narrow channels between islands, for example. Both environments create fast flowing water, making the Pentland Firth “one of the world’s best sites” for harvesting tidal energy “and could provide half of Scotland’s electricity demand,” according to reporting by the International Energy Forum.
Tidal energy can also be successfully harvested in places where there is a “sizable swing in sea level between high tide and low tide,” according to Johnson and Hakai Magazine. Ideal locations in this regard include the Bay of Fundy in Canada because it has some of the largest tidal variations on Earth, with differences of up to 50 feet between low and high tides. Other countries with favorable sites for major tidal power projects include China, Russia, India, and the Philippines. But in order to convert this tidal energy into electricity, specially engineered generators must be deployed at such locations.
The world’s largest tidal power station is located at Sihwa Lake on the northwestern coast of South Korea. It opened in 2011 and has 254 megawatts of electricity-generation capacity. The world’s second largest and oldest operating tidal power plant is on the estuary of the Rance River, in Brittany, France. It opened in 1966 and has 240 megawatts of electricity-generation capacity. Both plants use a tidal power generation system known as a barrage.
Structurally similar to dams, barrages are installed across inlets to ocean bays or lagoons to form a tidal basin. “Sluice gates on the barrage control water levels and flow rates to allow the tidal basin to fill on the incoming high tides and to empty through an electricity turbine system on the outgoing ebb tide,” according to the U.S. Energy Information Administration (EIA). However, barrages can change the tidal levels in basins and increase turbidity with unforeseen adverse effects. On the Rance, for example, the barrage increased the levels of silt in the water, unexpectedly resulting in the local extinction of a species of flounder known as plaice.
Tidal turbines can also be deployed outside a barrage. In an article from the Massachusetts Institute of Technology Climate Portal titled “Why don’t we use tidal power more?” Iris Crawford explains that “[t]idal turbines may be installed in water sources ranging from areas with strong ocean currents to tidal streams and estuaries.” When these turbines are installed in connected rows, they are called “an array” or tidal fence. According to U.S. EIA, these fences use “vertical-axis turbines mounted on the seabed.” Jamie Smith of SolarReviews.com explains that, unlike tidal turbines, which spin like a propeller, “tidal fences spin like a turnstile” and create electricity when “energy from tidal currents pushes the turnstile blades, which are connected to a generator.”
Because water is hundreds of times denser than air, tidal barrages, turbines and fences can generate more electricity per unit area than winds. But, like the raging sea itself, the challenges buffeting the technology can sometimes threaten to sink the endeavor. Besides the negative environmental impacts, the International Energy Forum warns that “building and maintaining moving machines in corrosive saltwater, often far from mainland bases is complex and costly.” There is also no supply chain for tidal energy systems, meaning that nearly every component has to be custom-built.
Despite the hurdles, tidal power could still figure into energy resilience efforts suitable to specific locations. The island town of Eastport, Maine, located just off the Bay of Fundy at the easternmost edge of the United States is currently exploring the creation of a microgrid that would incorporate the power of its legendary tidal currents. “Our current is going, and it’s going strong,” says Eastport City Council Member Jeanne Peacock. “So, ideally we would have [it as] a good constant source of energy, which is very exciting.”
Trey Gerfers serves as general manager of the Presidio County Underground Water Conservation District. A San Antonio native, he has lived in Marfa since 2013 and can be reached at tgerfers@pcuwcd.org.