by Katherine Marquis
The global effort to reduce fossil fuels and limit greenhouse gases has emboldened interest in renewable energy sources. While wind and solar energy often claim the ‘renewable energy spotlight’, there exists another prospective contender: tidal power.
Tidal energy is a form of hydropower produced by the rise and fall of the tides. The flux of tides is caused by the combined effects of gravitational forces (influenced by the Moon and Sun) and the Earth’s rotation. For any given locale, systematic predictions can define tidal times and amplitude. As tides are calculable and stable, they offer the potential to be a reliable source of energy.
To harness the energy within moving masses of water, specialized generators are employed to extract and convert energy. Tremendous potential energy can be harvested from the ocean with estimates of up to 1 terawatt of exploitable power in the surging tides (that’s enough energy to power over 15 billion standard 60W light bulbs!). To-date, few commercial-sized tidal power plants exist worldwide, and therefore reports of energy production remain low. Obstacles in obtaining investors and navigating underwater land ownership have limited progress even within the United States.
Nevertheless, several systems have been developed to harness the energy of tides:
Tidal Stream Turbines
One method of harnessing tidal energy involves establishing turbines in tidal streams. Turbines produce power using a rotor outfitted with vanes that revolves in response to a rapid flow of water or air. Due in part to the higher density of water compared with air, turbines exploiting tides capture more energy than their wind counterparts.
However, tidal turbines require sturdier materials, making them more expensive to produce. Finding an appropriate location for tidal turbines also poses challenges. Introducing a turbine to a site should not influence the presiding tide or be a detriment to the surrounding ecosystem. Generally, turbines are constructed in shallow water where the ebb and flow move fastest and the energy potential is highest; the slow revolution of the blades makes for limited encounters with marine life and able avoidance by ships.
An example of this method was the trial tidal power station constructed in 2007 at Strangford Lough in Northern Ireland, which generated near 1.2MW between 18-20 hours per day. Although the original system was recently decommissioned, two smaller experimental underwater turbines have been installed.
A dam-like structure called a barrage is another system used to capture tidal energy. Spanning across the entrance of a tidal inlet, it creates an enclosed reservoir in which water flow can be pointedly regulated. Instead of restricting water to one side like a conventional dam, a barrage allows water to move using underwater tunnels with sluice gates. Turbines placed at the sluice gates capture energy as the water flows in and out.
Scientists are still exploring ways to minimize environmental impacts of this system. Marine life can be constrained within the reservoir, and changes to water levels and salt concentration can dramatically affect multiple facets of a marine ecosystem.
An example of a barrage system used to harness tidal power is the Rance River estuary in France, which was built in 1966 and still functions today. While it provides an annual output of approximately 600GWh, it has caused progressive biological issues to the river environ.
An emerging hydropower method involves the generation of tidal lagoons. A tidal lagoon operates like a barrage system, capturing a large volume of water behind a natural or manufactured structure which can then be systematically released to drive turbines and generate electricity. While a barrage structure spans an entire river estuary, a tidal lagoon encloses just an area of coastline with a high tidal range behind a low sea wall, or breakwater.
Encouragingly, this proposed system is designed to impose minimal environmental impact and ongoing research is being conducted to assess the threat turbines pose to marine life. Ultimately, the tidal energy setup is currently assessed to be low-impact – especially when compared to other coastal energy projects like offshore drilling. The risk of injured animals is estimated to be minor, as the blades move too slowly to be lethal to marine mammals. The lingering concern surrounds the influence noisy machinery and churning water could potentiate – would modified water flow disrupt migration patterns? Would sediment circulation affect seafloor animals? Would machinery sound upset ecosystem conditions?
Race to Save the Planet
With an increasing urgency to reduce fossil fuel usage and expand renewable energies, tidal power may surface as an important corroborative energy source. Ongoing efforts to assess and improve tidal systems are being spearheaded by scientists worldwide. Tidal power may fill the void in renewable energies – and we’re hoping it will make a splash in the near future.