Unit-5, Chapter-10 Ocean & Tidal Energy

Tidal Power

• Tidal energy exploits the natural rise and fall of coastal tidal waters caused by the interaction of the gravitational fields of the sun and the moon.

• The ocean level difference caused due to tides contain large amount of potential energy.

• The highest level of tidal water is known as high tide (or flood tide) and the lowest level of tidal water is known as low tide (or ebb). The level difference between the high and low tides is known as the tidal range.

• Tidal range depends on the location. Only tidal range of 5 m and above are suitable for power generation.

Origin and nature of tidal energy • Tides are produced by the gravitational attraction of the

moon and the sun acting upon rotating earth. • Being closer to earth, moon exerts about 70% of the

tide producing force while sun exerts about 30% of the force.

• Surface water is pulled away from the earth on the side facing the moon, and at the same time, the solid earth is pulled away from the water on the opposite side. Thus, the ocean height increases at both the near and far sides of the earth.

Earth Moon

High tide

Low tide

Origin and nature of tidal energy • The solid earth rotates with a period of 24 hours

underneath these two bulges. These bulges are swept westward due to the earth’s rotation, as deep ocean waves with a period of 12 hours 25 minutes.

• The sun’s effect is similar but smaller in magnitude (about 2.2 times less than lunar) and with a period of 12 hours.

• These are thus semi-diurnal changes of ocean level. Due to slight difference in periods, the solar tide moves in and out of phase with the lunar tide.

• When sun, earth and moon are aligned in conjunction, the lunar and solar tides are in phase, producing net tides of maximum range. These are spring tides occurring twice per lunar month at times of both full and new moon.

Origin and nature of tidal energy • When sun-earth and moon-earth directions are

perpendicular, the solar and lunar tides are out of phase producing net tides of minimum range. These are neap tides which again occur twice per month at times of half moon (first and third quarter cycle of the moon).

• Superimposed on these short term variations caused by the sun-moon system, there are many other cycles of small magnitudes with periods ranging from days to years. These are the results of variations in the moon earth distance and complex interactions between the gravitational fields.

•  In open oceans, tidal ranges are generally between 0.6 and 0.9 m. When the tidal waves impinge on coastlines, their range can amplify considerably. Thus, tidal range varies from place to place.

E

Full moon New moon Sun

E

First quarter

Sun

Third quarter

Spring and Neap Tides

Spring Tides

Neap Tides

Tidal variations in a lunar month

Limitations of tidal energy

• Tidal range of 5m or more is required for economic construction of tidal power plant. The plant is therefore site specific.

• Due to mismatch of lunar driven period of 12 hours 25 minutes and day length of 24 hours, the optimum tidal generation is not in phase with demand.

• Changing tidal range in two-week periods produces changing power.

• The turbines are required to operate at variable head. • Requirement of large water volume flow at low head

necessitates parallel operation of many turbines. • Tidal plant disrupts marine life at the location and can

cause potential harm to ecology.

Tidal energy technology

Harnessing tidal energy Single basin system, one way: • The simplest scheme of developing tidal power is the

single basin arrangement, in which a single basin of constant area is provided with sluices (gates), large enough to admit tides with minimum loss of head.

• The level of water in the basin is the same as that of the tide outside.

High tide Low tide

Main valve open

Turbine Valve closed

Main valve closed

Turbine in operation Valve open

Single basin system • When tides are high, water is stored in the basin and

sluice gates are closed. • When tides are falling, sluice gates are opened to allow

water to go through the turbine to generate power. • A head of water is obviously required for the turbine to

generate power. • The turbine continues to generate power until the level

of falling tides coincides with the level of the next rising tide.

• The major disadvantage of the single basin scheme is that it gives intermittent supply of power, varying considerably over the operating period.

•  It is for this reason that the tidal power has not been popular. Also, this scheme utilizes only 50% of tidal energy.

Single basin system

Single basin system, two way: •  In the single basin two way system, the turbine is driven

by the tide in both the directions. • Reversible turbines are used in such systems. • Utilizes 100% of tidal energy.

Tide coming in Tide going out

Single basin, two way system

Sea

Tidal basin

High tide Dam or dyke

Turbine-generator set (Reversible turbines)

Tidal basin

Sea

Low tide

Dam or dyke

Turbine-generator set (Reversible turbines)

High tide

Low tide

Two basin systems

Two basin linked system: •  In the simplest double basin

scheme, there must be a dam between each basin and the sea and also a dam between basins.

• One basin is always maintained at a lower level than the other.

• The lower reservoir empties at low tide, the upper reservoir is replenished at high tide.

•  If the generating capacity is to be large, the reservoirs must be large, implying that long dams are required.

Two basin paired system Two basin paired system: • A paired basin consists essentially of two single basin

schemes. • One scheme generates on high tide and the other on

low tide. • The output is almost continuous.

A tidal turbine

Single basin vs. Two basin tidal systems

Single Basin : •  Intermittent power generation •  Power varies daily

Two Basin : •  Continuous power generation •  Daily variations are less

Tidal current scheme • Tidal current schemes dispense with the dam and its

associated cost. • The energy is extracted from tidal currents using

underwater turbines. • Strong tidal currents, as high as 5 m/s, can be found in

shallow seas, particularly where natural constrictions exist like space between two islands.

• The theory of power from tidal current is similar to wind power with the advantage of predictable velocities of a denser fluid. However, the fluid velocities are much smaller.

Tidal flow: Rance river, France

•  The Rance plant is a single basin two-way system.

•  240 MW plant with 24 x 10 MW turbines operated since 1966

•  Average head is 9 m. •  Area is approximately 22

km2 •  Flow approx, 0.2 billion m3 •  Produces electricity

cheaper than oil, coal, or nuclear plants in France

Wave power •  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. •  The initial solar power level of about 1 kW/m2 is

concentrated to an average wave power level of 70kW/m of crest length.

•  This figure rises to an average of 170 kW/m of crest length during the winter, and to more than 1 MW/m during storms

•  Crest length – length of wave along crest. The Indian Ocean tsunami had a crest length of about 1200 km.

1

2

3

A

B

1 = Direction of propagation 2 = Wave crest 3 = Wave trough

Wave power

•  Wave power refers to the ocean surface waves and capture of that energy to do useful work, including generation of electricity.

•  Wave power is a form of renewable energy

Wave energy technology •  Energy in the waves is harnessed basically in the form

of mechanical energy using wave energy converters. •  A wave energy converter may be placed in the ocean in

various possible situations and locations. •  The fluctuating mechanical energy obtained is modified

to drive a generator. •  Depending on the location of these devices, they can be

classified as follows: •  Off-shore or deep water devices (>40 m depth) •  Shoreline devices

•  They can also be further subdivided as: •  Floating devices •  Submerged devices •  Partly submerged devices

Wave energy converters

Oscillating water Column (OWC):

•  The oscillating water column (OWC) generates electricity in two steps.

•  As wave enters the column, it forces the air in the column past a turbine and increases the pressure within the column.

•  As the wave retracts, the air is drawn back past the turbine due to the reduced air pressure on the ocean side of the turbine.

Wells turbine turns in the same direction irrespective of airflow direction

Incoming air forces air out of OWC

Retreating wave sucks air back into OWC

The LIMPET Oscillating Water Column front and rear, installed on the Isle of Islay, Scotland.

Extraction of wave energy through OWC

Wave energy converters

Pitching type (duck or cam) device:

•  In pitching type devices, the waves strike horizontally on a floating piece causing it to deflect

•  Several cam (duck) shaped pieces are hinged to a common flexible linkage to form a nodding duck assembly.

•  A ratchet and wheel mechanism converts this nodding motion into rotary motion to drive a generator.

Duck rotates with nodding motion as wave passes

Fixed central section

Tapered channel system (TAPCHAN)

•  TAPCHAN system consists of a tapered channel feeding into a reservoir that is constructed on a cliff.

•  The narrowing of the channel causes the waves to increase their amplitude (wave height) as they move towards the cliff face.

•  Eventually the waves spill over the walls of the channel and into the reservoir.

•  The kinetic energy of the moving wave is converted into potential energy as the water is stored in the reservoir.

•  The stored water is then fed through a Kaplan turbine.

Reservoir

Tapered channel

Turbine house

Cliff face

Some other types of wave energy converters

Heaving float device Air turbine operated by wave motion

Advantages and limitations of wave energy

Advantages: •  The energy is free - no fuel needed, no waste produced. •  Most designs are inexpensive to operate and maintain. •  Waves can produce a great deal of energy. •  There are minimal environmental impacts. Limitations: •  Depends on the waves – sometimes lot of energy,

sometimes nothing. •  Needs a suitable site, where waves are consistently

strong. •  Must be able to withstand very rough weather. •  Disturbance or destruction of marine life •  Possible threat to navigation from collisions because the

wave energy devices rise slightly above the water.