Tidal Power
Using Tidal Power as an Ocean Energy Resource
Tidal Power, also known as tidal energy, is an ocean-based technology with the high potential of harnessing the power of ocean tides to provide us with clean and free energy for the future. Typically, tidal power involves leveraging the kinetic energy stored in the movement of the incoming and outgoing tides, as well as the daily differences between the high tide and the low tide at a given location.
Harnessing tidal energy, one of the oldest methods involves building a tidal barrage or dam across a suitable bay or estuary that has large differences in elevation between high and low tides. Today, many tidal power plants exist worldwide, using tidal barrages, dams, oscillating hydrofoils, tidal turbines, and tidal kites for small scale electricity generation within the shallow and deeper waters around different coastal areas.
There are many different types and varieties of renewable energy systems, but tidal power, being an ocean-based technology, is one of the few sustainable sources that can be accurately predicted over many years. The ebb and flow of the tides, or tidal energy, rely on the tidal forces exerted by the gravitational movement of the sun and moon.
Since the movement of the tides, the primary source of tidal energy, around a coastline does not occur at the same time, but is staggered around the coast, full tidal power generation will be available at one tidal location when there is no tidal power available at another location around the coastline, thus allowing power generation from multiple locations over a period of time.
As a marine renewable technology, tidal power plants or generating machines can be located underwater and beneath the waves in under-utilised locations. This gives a significant advantage over other marine-based systems as the tidal turbines cannot be seen, unlike offshore wind farms or wave energy devices.
Tidal Power Devices
There are many different types of tidal energy technologies and machine designs used around the world, but there are basically two methods of generating electricity from the movement of the tides, making up the core of tidal energy systems:
- Tidal Range Devices: these take advantage of the potential energy stored between the high and low tide levels, a fundamental principle of tidal energy.
- Tidal Stream Devices: these harness the kinetic energy of tidal flow in tidal currents, a form of tidal energy, to generate electricity.
A Tidal range device, a key component of tidal energy systems, leverages the vertical difference in the water level between a high tide and a low tide. This is achieved by trapping sea water within a tidal lagoon or a flooded basin behind a large tidal barrage. The water is then released back to the sea via turbines. By manipulating sluice gates, sea water is allowed to enter the basin or estuary before being trapped on one side, creating a static head of water across it due to the cyclic movement of the tides.
When the head of sea water is sufficiently large, the sluice gates are re-opened, allowing the impounded water to flow back to the sea. This process, a key part of a tidal power plant, uses the force of gravity across horizontal axis turbine blades for electricity generation. By strategically designing the turbine enclosure, the sea water can be concentrated and accelerated over the turbines blades, thereby increasing the efficiency of generation.
We can deploy tidal range devices in various tidal power plant schemes for electrical generation such as: flood generation, ebb generation and two-way electrical energy generation. The choice of scheme depends on the strength of the tide and water requirements.
Tidal Stream Devices
Tidal stream devices a form of tidal generator, are typically designed for deep water operation where it’s too deep to mount tidal turbines directly to the seabed. These tidal technologies use large turbines to extract the tidal energy from the sea, operating in a manner similar to wind turbines. Like their wind turbine counterparts, tidal turbines use axial shaped turbine blades that operate according to the principles of aerodynamic lift.
As the dense seawater flows over the turbine blades, it generates a rotational force that turns the blades, producing electricity. This process, a key aspect of harnessing tidal energy, compensates for the slower velocity of tidal currents compared to wind, allowing tidal stream devices to generate similar levels of electrical energy to conventional three-bladed wind turbines.
Also, unlike wind power, tidal energy systems do not experience sudden or extreme changes in the speed of underwater tidal currents that could potentially damage the tidal stream devices. Additionally, storms or severe weather conditions above the surface cannot damage the tidal stream devices or force them to shut down.
Although tidal energy devices like tidal stream turbines do not have significant impacts on water levels, unlike tidal fence devices with their dams and tidal barrages, they can impact on the surrounding water quality. This is due to the reduction in both the upstream and downstream tidal current velocities caused by the extraction of energy, which allows for sediment concentrations to build up.
This could affect both the erosion and deposition of the sea bed a considerable distance away from the location of the tidal energy array. Also, large rotating marine energy devices such as tidal turbines can have many other unseen impacts on the surrounding environment. These include underwater noise pollution, the generation of electromagnetic fields around the generators and electrical cables, as well as the striking of fish and marine animals by the turbines rotor blades or other moving parts.
Oscillating Tidal Energy Devices
Oscillating hydrofoil devices, a type of tidal energy technology, extract energy from the tidal current in a similar manner to a rotating tidal turbine but oscillate in an up and down motion. An oscillating hydrofoil developed by the University of Strathclyde, consists of a large hydrofoil wing attached to a long lever arm that is allowed to move up and down.
Due to its wing-like shape, as the tidal energy current flows over the hydrofoil wing, it generates a vertical lift which causes the attached lever to move upwards. At the peak of the movement, the hydrofoil’s angle of attack relative to the approaching tidal currents changes, so that now the lift is being generated on the underside of the hyd