Category: Tidal

Transverse Horizontal Axis Water Turbine | THAWT | Modern Tidal Energy | Most Efficient Wind Turbine

Tidal Energy:

Tidal power is one such developing technology, which harnesses the kinetic and gravitational potential energy in tidal streams. When compared to other renewable sources, tidal streams are a relatively reliable source of energy, as tidal movements can be accurately predicted in terms of direction, timing and magnitude. The rapid development of devices for tidal energy exploitation is being encouraged by government initiatives and by private investment.

01-wind turbine power output -darrieus wind turbine - modern wind turbine

The horizontal axis, axial-flow turbine is the most common design of a tidal stream turbine. A number of variants of this type of device, which incorporate features such as flow-guiding shrouds or specific mounting techniques, have been proposed by different developers, but the underlying hydrodynamics remain similar for these devices. However, a drawback with such designs is that their size cannot be increased significantly, because the limited depth of flow at most sites restricts their diameter. Tidal stream energy is likely to be more expensive than either other renewable resources or combined cycle gas turbines, until at least hundreds of megawatts capacity is installed.

How Does a Free Flow Underwater Turbine Work?

Very simply, it works like a wind turbine, but the blades are moved by a water current instead of by the wind.

01-horizontal axis wind turbine - underwater turbine - THAWT - axial flow turbine

Transverse Horizontal Axis Water Turbine (THAWT):

The Transverse Horizontal Axis Water Turbine (THAWT) has been proposed as a tidal device which can be easily scaled and requires fewer foundations, bearings seals and generators than a more conventional axial-flow device. The THAWT device is a horizontally deployed variant of the Darrieus cross-flow turbine, in which the blades can be oriented into a truss configuration to produce long, stiff multi-bay rotors.

A fluid particle passing through a Darrieus cross-flow turbine encounters two sets of blades. One on the front side of the turbine as the fluid enters, and again on the rear side as it leaves.

This increased stiffness and strength allows longer units to be constructed, and reduces the overall costs of foundations, bearings, seals and generators. A full scale device might have a diameter of 10 – 20 m and would operate in a flow depth of 20 – 50 m.

01-darrieus hydro turbine - modern tidal energy - next generation marine turbine

The THAWT device employs a truss design of blades, which is intended to increase the rigidity of the structure, so that it can be stretched across a channel without significant increases in blade stresses.

The Thawt device is mechanically far less complicated than anything available today, meaning it would cost less to build and maintain. “The manufacturing costs are about 60% lower, the maintenance costs are about 40% lower”.

The size of thawt is not limited by the depth of water in which it is situated, and the need to intersect the largest possible area of current has been incorporated into the design. Power generation of up to 100mw could be achieved by an array of only 10 thawt devices.

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For comparison, if thawt devices were extended across the same area of current as axial flow devices, thawt would require:

  • Less generators,
  • Less primary seals, and
  • Less foundations

and consequently thawt would incur:

  • Lower capital costs
  • Lower maintenance costs, and
  • Lower operational costs

Underwater Tidal Power | Second Generation Tidal Power Plants | Generating Electricity From Ocean Waves

What is Tidal Energy?

Tidal energy is the power of electricity generation achieved by utilization of the variations in sea level caused primarily by the gravitational effects of the moon, combined with the rotation of the Earth by capturing the energy contained in moving water mass due to tides.

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Two types of tidal energy can be extracted:

1. Kinetic Energy: currents between ebbing and surging tides.

2. Potential energy: Difference in height between high and low tides.

In order to be practical for energy production, the height differences needs to be at least 5 meters. Only bays and inlets amplify the height of the tide.

Wave facts:

Waves are caused by a number of forces i.e. wind, gravitational pull from the sun and moon, changes in atmospheric pressure, earthquakes etc. Waves created by wind are the most common waves. Unequal heating of the earth’s surface generates wind and wind blowing over water generates waves.

Types of Tidal Plants

  1. Tidal Fences: Turnstiles built between small islands or between mainland and islands. The turnstiles spin due to tidal currents to generate energy.
  2. Barrage Tidal Plants: Barrage tidal plants are the most common type of tidal plant. Using a dam to trap water in a basin, and when reaches appropriate height due to high tide, release water to flow through turbines that turn an electric generator.
  3. Tidal Turbines: Look like wind turbines, often arrayed in rows but are underwater. Tidal currents spin turbines to create energy.

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First generation Tidal Power Plants:

  • Tidal Fences
  • Barriage style Tidal Power Plants

Second generation Tidal Power Plants:

  • Tidal Underwater Wind turbines
  • Vertical Axis
  • Horizontal Axis
  • THAWT Device

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One site has potential to equal the generating power of three nuclear power plants.

Disadvantages of Second generation Tidal Power Plants:

Presently costly

1. Expensive to build and maintain

2. A 1085 MW facility could cost as much as 1.2 billion dollars to construct and run.

Energy from the Moon:

01-generating electricity from ocean waves

The diagram shows how the gravitational attraction of the moon and sun affect the tides on Earth. The magnitude of this attraction depends on the mass of the object and its distance away. The moon has the greater effect on earth despite having less mass than the sun because it is so much closer. The gravitational force of the moon causes the oceans to bulge along an axis pointing directly at the moon. The rotation of the earth causes the rise and fall of the tides.

When the sun and moon are in line their gravitational attraction on the earth combine and cause a “spring” tide.

When they are as positioned in the first diagram above, 90° from each other, their gravitational attraction each pulls water in different directions, causing a “neap” tide.

The rotational period of the moon is around 4 weeks, while one rotation of the earth takes 24 hours; this results in a tidal cycle of around 12.5 hours. This tidal behaviour is easily predictable and this means that if harnessed, tidal energy could generate power for defined periods of time. These periods of generation could be used to offset generation from other forms such as fossil or nuclear which have environmental consequences. Although this means that supply will never match demand, offsetting harmful forms of generation is an important starting point for renewable energy.

Generating Electricity from the Tide:

Turbines can make electricity when the water turns their blades. The simplest electricity generation system using tides is known as an ebb generating system. It uses a dam, known as a barrage, across an estuary. Sluice gates on the barrage are opened to allow the tide to flow into the estuary on the incoming high tides. They are closed to prevent the water flowing back on the outgoing tide (known as the ebb tide) except through the turbine system.

Two way generation systems, which generate electricity on both the incoming and outgoing tides, are also possible.

Impression of Tidal Turbine Farm:

01-underwater tidal turbine - generating electricity from tides

This form of generation has many advantages over its other tidal energy rivals. The turbines are submerged in the water and are therefore out of sight. They don’t pose a problem for navigation and shipping and require the use of much less material in construction. They are also less harmful to the environment. They function best in areas where the water velocity is 2 – 2.5 m/s. Above this level the turbine experiences heavy structural loads and below this not enough generation takes place.

Types of structures :

  • Monopile,
  • Lattice/gantries,
  • Tripod,
  • Moored

will all have individual responses to loadings Seabed mountings need to be able to withstand applied vertical/horizontal forces and moments.

01-types of structure - underwater tidal turbine