Tides are the result of the interaction of the gravity of the sun, earth, and moon. The rise and fall of the tides in some cases more than 12m creates potential energy. The flows due to flood and ebb currents creates kinetic energy.
Both forms of energy can be harvested by tidal power technologies as renewable energy. Tidal power technologies are not new. Examples were already reported in Roman times and ruins of installations – tidal mills – are found in Europe from around the year 700.
However, new technologies have advanced considerably over the past few years and there are a number of ongoing full-scale demonstration projects.
Reliability
Tidal technologies harvest the potential energy created by the difference in head between ebb tide and flood tide. Such resources exist in locations where due to geological and ecological conditions, large water masses flow into compounded areas or bays and estuaries.
Furthermore, tidal energy is predictable, as the energy production is not influenced by weather conditions, but rather by the cyclical constellations, the gravity of the moon, sun and earth, providing a predictable bi-week, biannual and annual cycle.
Wave and tidal energy technologies have certain advantages over other energy sources. For example, they provide an opportunity to generate energy at a wide range of locations throughout Europe.
Additionally, wave and tidal power produce energy at different times, and more consistently, than other renewable energy sources, such as wind and solar. This will add to the overall stability of Europe’s energy networks.
These new technologies also offer an attractive alternative in areas where the visual impact of electricity generation sources is a concern. Lastly, they can leverage extra value by exploiting synergies with sectors such as offshore oil, gas and offshore wind. Opportunities include using common components and sharing expertise on project development challenges.
The first tidal barrage was completed in the Rance River in north-western France (Brittany) in 1966, but due to plans for greater use of nuclear energy, the further pursuit of tidal energy was abandoned. Between 1966 and 2011, a number of small tidal plants were built in countries such as Canada, China, Iran and Russia, where tidal energy resource is abundant.
The largest and newest tidal barrage in the world is the Sihwa dam in north-eastern South Korea, which was built in 2011 and became operational in 2012.
The Sihwa dam, with a capacity of 254 MW, is an example of a multi-functional tidal barrage, which improves the ecology of a formerly closed sea-arm by creating openings in an existing sea defence and installing hydro-turbines.
This project is a relevant example for a combined tidal range solution, where in the end the priority was placed on ecological water quality improvement
How does Tidal Power work?
Most conventional tidal schemes use bulb turbines, which are comparable to hydropower turbines that are installed in a dam (run of rivers hydro power plant). Tidal technology has a number of options for power generation:
- One way power generation at ebb tide: The reservoir is filled at flood tide through sluice gates or valves that are closed once the tide has reached its highest level. At the ebb tide, the water in the reservoir is released through the turbines and power is generated. With this single cycle, power is generated for only four hours per day. Annapolis in Canada is an ebb generation plant.
- One way power generation at flood tide: At flood tide the sluice gates are kept closed to isolate the reservoir while at its lowest level. When the tide is high, the water from the sea-side flows into the reservoir via the turbines, thus generating power. The disadvantage of this cycle is that it has less capacity and generates less electricity, and it may be ecologically disadvantageous as the water level in the impoundment is kept at a low level for a long time. Sihwa is a flood generation plant.
- Two way power generation: Both incoming and outgoing tides generate power through the turbines. This cycle generates power for four hours twice daily. However, reversible turbines are required. La Rance is an ebb and flood generation plant; bulb turbines can also pump water for optimization.
