Tidal energy can be harnessed in two ways.

1) Impound the water at high tide and release it through a turbine at low tide.

2) Harness the tidal current with the underwater equivalent of wind turbines

Underwater turbines use tidal flows.

Atlantis Resources Corporation was founded in Singapore but developed its technology in Australia where it has been operating its own dedicated tidal power test facility in San Remo, Victoria, since 2006.

Biostream

"BioSTREAM^TM employs a patented oscillating hydrofoil system to extract energy from .. current speed of 2.5m/s or greater, .

An onboard computer continually adjusts the angle of the hydrofoil (fin) relative to the oncoming flow such that the tail and fin system develops a swimming motion. The energy transferred by this side-to-side motion is converted to electricity by O-Drive^TM modules installed on the BioSTREAM^TM."  

Biostream website     Watch video

Underwater kites

Deep Green produces electricity from underwater kites.

1. The water current creates a lifting force on the wing pushing the kite forward .
2. The kite is steered in an 8-shaped trajectory by a rudder and reaches a speed ten times the water current speed.
3. As the kite moves, water flows through the turbine producing electricity.
4. The electricity is transmitted through a cable to the tether.
5. Then electricity continues to the shore.

Deep Green claims the following advantages:

• The only known power plant that cost-effectively produces electricity at sites with velocities between 1.2-2.5 m/s and depths between 60-120 meters.
• High Efficiency.
• Due to the unique characteristic of increasing the relative flow Deep Green is the most cost effective power plant on the market today.
• Small in size and lightweight. The plant weighs only seven tonnes which is 10-25 times less per MW than competing technologies which are designed for tidal flows above 2.4m/s.
• Low-cost offshore operations. Small boats and equipment are used for installation, service and maintenance. The offshore operations are minimised since only attachment and detachment has to done offshore.
• Efficient manufacturing. Size, weight and modularity of Deep Green offers the possibility of lean manufacturing.
• Utilisation of ocean currents. The ability to be efficient at low velocities makes Minesto´s Deep Green the only technology to be cost-efficient in both tidal and ocean currents.
• Predictable electricity production. Tides are generated by the relative motion of the Earth, Sun and Moon, which can be calculated with almost 100% accuracy. Ocean currents are nearly constant.
• Minimal visual and environment impact. Deep Green operates completely submerged at least 15 meter below the water surface.

Tidal barrage

These work rather like a hydro-electric scheme, except that the dam is much bigger.

A huge dam (called a "barrage") is built across a river estuary. When the tide goes in and out, the water flows through tunnels in the dam.

The ebb and flow of the tides can be used to turn a turbine, or it can be used to push air through a pipe, which then turns a turbine. Large lock gates, like the ones used on canals, allow ships to pass.

If one was built across the Severn Estuary, the tides at Weston-super-Mare would not go out nearly as far - there'd be water to play in for most of the time.

But the Severn Estuary carries sewage and other wastes from many places (e.g. Bristol & Gloucester) out to sea. A tidal barrage would mean that this stuff would hang around Weston-super-Mare an awful lot longer!

Also, if you're one of the 80,000+ birds that feeds on the exposed mud flats when the tide goes out, then you have a problem, because the tide won't be going out properly any more.

Ref

Top: the the Dam for the tidal power plant on the estuary of the Rance River, Bretagne, France

Bottom: Artists impression of typical tidal barrage.

Tidal barages are not new. England had at least one in Roman times. At one time there were 750 tide mills on the shores of the Atlantic, in England, Wales, Ireland, France, and USA. The diagram below is of a mill in 787 AD.

Top right:

       Moulin maree brehat

Bottom right:

       Eling tide mill is open to the public. It was mentioned in the Domesday Book in 1086 AD.

Tidal power for Darwin

A plan to power Darwin with tidal energy – and to turn the Northern Territory into a tropical tidal energy hub – has come one step closer to being realised this week, after the signing of an Memorandum of Understanding to build a 2MW pilot plant and research centre in Clarence Strait, off the Territory’s coast.

Tenax Energy has identified three locations in Australia that meet these requirements. They are:

Banks Strait, Tasmania
Port Phillip Heads, Victoria
Clarence Strait, Northern Territory

Tidal power in UK

From tidal barrages you can reasonably expect you can get 15% of UK electricity needs, that’s a very solid number,” co-author Dr Nicholas Yates from the National Oceanography Centre told BBC News. “On top of that there is a 5% tidal stream figure, and with future technological development that is likely to be an underestimate in my view.

Source

            Artist impression of barrages on the Severn UK Source

This proposal has been refused on environmental grounds.

Tidal lagoons

The damming of a bay can cause too many environmental changes, so it may be better to make an artificial lagoon in shallow water.

Tidal lagoons are created by building a ring-shaped sand-core breakwater . Turbines mounted are mounted within the wall. When the tide moves in and out, the wall holds water back, and once it reaches a certain level, gates are opened and the water flows through the turbines, producing electricity.

The project would need subsidies of £156 per MWh (15.6 p/kWh ) - similar to the subsidy on nuclear power.

http://www.tidallagoonswanseabay.com/  The Guardian    Subseaworld

Tidal lagoon planned for Swansea Wales.

Video of the Swansea tidal lagoon

An Idea with pumped hydro

It would be more profitable to wait for low and high tide, then buy power from the grid to pump water in or out of the impounded water.

For example, if the tide is 2 metres, then the pumps may be able to pump in another 0.5 metres at high tide, or reduce the level by that much at low tide. Then at the opposite tide there is 2.5 M of fall to generate electricity. Five times the energy less losses.

To be self contained the lagoon could be run with a pumped hydro.

Excess power can be stored by pumping water up a nearby hill into a storage dam. This can then generate power on demand, including when it is needed to pump water in or out during the turn of the tide.

The water flow may even be able to turn the turbine/pumps directly.

The maths to calculate the optimum conditions should be lots of fun.