wave energy and its electrical conversion systems

P SRINIVAS RAO

B110907EE

Contents

Brief introduction to waves

Wave characteristics

▪ Wave power and global distribution

▪ Techniques to harness power

▪ Various systems used for electrical power generation

▪ Advantages and disadvantages

▪ Conclusion

▪ reference

wave energy origin

▪ Differential warming of the earth causes pressure differences in the atmosphere, which generate winds

▪ As winds move across the surface of open bodies of water, due to the pressure differences, gravity and friction they transfer some of their energy to the water and create waves

The amount of energy transferred and the size of the resulting wave depend on

▪ the wind speed

▪ the length of time for which the wind blows

▪ the distance over which the wind blows, or fetch

Wave characteristics

• Wavelength - distance between two consecutive crests or troughs

• Height H – the vertical distance between from crest to trough

• Wave period T – time in seconds needed for the wave to travel the wavelength

• Frequency f – indicates the number of waves appear in a given position

• Wave speed – the ratio of wavelength and time period

l

Tv

crest

crest

trough

H

Power associated with waves

The power associated with a wave of wavelength l and height H and a front b is given by

Power per metre (b = 1 m) of front wave is approximately

l

H

b

r

bgHP 2

2

1

20.5 P H T W m

Power associated with waves

▪ Example: Consider moderate ocean swells, in deep water, a few km off a coastline, with a wave height of 3 m and a wave energy period of 8 seconds. Using the formula to solve for power, we get

▪ meaning there are 36 kilowatts of power potential per meter of wave crest.

▪ In major storms, the largest waves offshore are about 15 meters high and have a period of about 15 seconds. According to the above formula, such waves carry about 1.7 MW of power across each metre of wavefront.

Global distribution of wave power

How to harness this wave power??

▪ The sea wave’s motion can be converted into mechanical energy by using proper mechanisms

▪ Waves retain energy differently depending on water depth

▪ Lose energy slowly in deep water

▪ Lose energy quickly as water becomes shallower because of friction between the moving water particles and the sea bed

▪ Wave energy conversion devices are designed for optimal operation at a particular depth range

Harnessing techniques

Therefore, devices can be characterized in terms of their placement or location.

▪ At the shoreline

▪ Near the shoreline

▪ Off-shore

At shore line, the mostly implemented technique is

Oscillating Water Column systems

Oscillating Water Column• An Oscillating Water Column (OWC) consists of a partially submerged

structure that opens to the ocean below the water surface. This structure is called a wave collector.

• This design creates a water column in the central chamber of the collector, with a volume of air trapped above it.

air turbine/generator

airflow

breakwater

Oscillating Water Column

▪ As a wave enters the collector, the surface of the water column rises and compresses the volume of air above it.

▪ The compressed air is forced into an aperture at the top of the chamber, moving past a turbine.

▪ As the wave retreats, the air is drawn back through the turbine due to the reduced pressure in the chamber.

• The turning of the turbine drives a generator, producing electricity

• The type of turbine used is a key element to the conversion efficiency of an OWC.

• Traditional turbines function by gas or liquid flowing in one direction and at a constant velocity. When the flow is not always from the same direction or at a constant velocity – such as in the OWC – traditional turbines become ineffective.

• Different types of turbines have been developed for the OWC to address this problem.

• The technologies have been demonstrated to work in a number of locations, with varying degrees of efficiency.

▪ Wavegen’s LIMPET

▪ Energetech’s Australia Wave Energy System

converter

Wells turbine

double fed wound rotor induction generator

~

grid

wind

turbine Wells

asynchronous generator

Generator/rectifier air turbine group

Wave

Structure

Parallel valveGenerator Turbine

Series valve

Air output

LIMPET

▪ Pictured here is the LIMPET (Land Installed Marine Powered Energy Transformer), an Oscillating Water Column located on the Isle of Islay, Scotland, and designed by Wavegen

LIMPET

To overcome the problems of traditional turbines, LIMPET employs a Wells turbine that turns in the same direction irrespective of the airflow direction.

• a wells turbine is a bidirectional turbine

▪ The collector is tilted such that the resonance of the internal water column coincides with the peak energy period of the waves, easing passage of water into the water column

▪ The collector was divided into 3 chambers, with large holes at the top of each dividing wall to allow the air above the 3 water columns to combine to feed the turbine-generation system

▪ This design optimized performance for annual average wave intensities of 15 – 25 kW/m

▪ The system contains a pair of Wells turbines, each of which is connected to a 250 kW induction generator

▪ LIMPET has a generation capacity of 500 kW

▪ Designed to supply power into the Islay grid

Energetech’s Australia Wave Energy System

▪ Located 200 meters from the Port Kembla Harbour Breakwater

▪ The system uses a variable pitch turbine called a Denniss-Auld turbine, potentially with a higher conversion efficiency than the Wells turbine

▪ The turbine drives an induction generator

▪ System components are computer controlled

▪ The computer uses a sensor system with a pressure transducer to measure the pressure exerted on the ocean floor by each wave as it approaches the collector

▪ The transducer sends a signal proportional to that pressure to a Programmable Logic Controller which adjusts various parameters ▪ Optimizes conversion for the particular conditions and energy

content of the wave▪ Protects system components and ensures safety

▪ The device employs a parabolic wall to focus the wave energy into the collector

▪ The ends of the wave plane are reflected by the parabolic wall and converge on the focus of the parabola

▪ At the focus, the water will rise and fall with an amplitude of approximately 3 times that of the incoming waves

▪ The center of the collector sits at the focus of the parabolic wall

▪ The plant also includes a small desalination unit that can produce nearly 2000 liters of fresh drinkable water per day using nothing but the seawater and wave energy.

Near shore mechanisms

near shore installations are a few hundreds of meters away from the shore, one such system is

1) Wave dragon system

Wave dragon system

▪ The Wave Dragon system was the world’s first nearshore wave energy converter producing power to the grid

▪ Wave Dragon System is a floating slack-moored energy converter of the overtopping type that can be displayed in a single unit or in arrays

▪ Groups of 200 Wave Dragon units result in a wave power park with a capacity comparable to a traditional fossil fuel based power plant.

Working method

▪ The basic idea of this system consists of two large "arms" that focus waves up a ramp into a reservoir.

▪ The water returns to the ocean by the force of gravity via a low head hydro turbine which drives an electric generator.

▪ Wave Dragon is a very simple construction and only the turbines are the moving parts.

▪ This is essential for any device bound for operating nearshore where the extreme forces seriously affect any moving parts.

▪ In comparison with traditional hydroelectric power stations, this new technology is competitive.

Offshore mechanisms▪ Offshore meaning certain kilometers away from the shore, that is the

system installed is in middle of sea where the wave power per meter is high enough.

Offshore mechanisms involves the following

1) Pelamis converter

2) Bristol cylinder

Of all the wave energy conversion mechanisms proposed till date, Pelamis converter is the most successful in terms of power extraction.

Pelamis converter construction▪ The Pelamis Wave Energy Converter is a technology that uses the motion

of ocean surface waves to create electricity.● a Scottish invention, consists of 4 articulated cylinders of 3.5 m in

diameter and 30 m in length (floaters) connected to 3 power modules of 3m diameter and 5 m in length.

● This articulated structure with 140 m in a total length is placed 2/3 semi-submerged offshore in deep waters.

● Each of the machine weigh about 700 tones of carbon steel.● The structure is secured by flexile cables fitted to the seabed. in such way

that the float axis is oriented in the predominant wave direction.

Operation

● Due to the waves, this structure up and down and side to side as a sea snake (Pelamis in Greek).

● The wave-induced motion of these joints is resisted by hydraulic rams. ● As waves moves the joints up and down and side to side, hydraulic ram

moves according to it which pumps high pressure oil to the hydraulic motor.

● The hydraulic motors drive electrical generators to produce electricity.● The generated electricity is transmitted through power sea cables. ● Several devices can be connected together and linked to shore through a

single seabed cable, called umbilical cable.

Hydraulic arm

High pressure container

Motor/generator set

Distributor

Reservoir

Vertical articulated axis

Horizontal articulated axis

Electric generator

Gas under pressure

HP

Valve 1

Hydraulic motor

Hydraulic arm 1

Hydraulic arm 2

Fluid under pressure

Reservoir

LP

force

force

Valve 2

High pressure

Low pressureHydraulic arms

Wave direction

Pelamis

Seabed

Anchorage

Pelamis anchorage to the seabed

Flexible cable

Wave farm of Aguçadora of 2.25 MW

Association of Pelamis units in a total of 30 MW

2100 m

600 mof

Pelamis wave energy converter of 750 kW

Bristol cylinder▪ The Bristol Cylinder consists of a floating cylinder that collects the

motion of waves.

▪ The cylinder is mechanically connected to the energy unit by flexible joints and rods.

▪ The rods are moving slowly with cylinder and the reciprocating motion is transferred to the axels in converter unit.

▪ This converter unit, called Escone, after his inventor Esko Raikano, is the heart of the system and converts the reciprocating motion to a rotating shaft connected direclty to a generator for generating electrical energy with high efficiency.

▪ For the energy unit a suitable slow speed generator will be needed.

▪ Two or more Bristol cylinders could be connected in parallel.

▪ This method of collector wave energy is in the process of pending patents in Finland.

Wave power in india

▪ A prototype 150kw wave power plant is installed in Vizhinjam near thiruvantapuram, which converts sea wave energy to electricity and is given to the local grid.

▪ This plant is based on the oscillating water column(OWC) principle. A caisson (watertight retaining structure) was constructed in December 1990 at Vizhinjam and two generations of power modules have so far been tested.

Advantages of wave power

Wave energy is not expensive to operate and maintain.

no fuel is needed and no waste is produced.

minimal environmental impact when properly placed.

▪ Low operation and maintenance costs after construction.

▪ No emissions during operation.

disadvantages

▪ Improperly placed wave power plants can damage the marine ecosystem.

▪ Efficiency drops significantly in rough weather due to safety mechanisms.

▪ Limited locations where waves are strong enough to produce electricity without damaging equipment.

▪ Power only produced near oceans making transmission to inland customers difficult.

▪ Winds (and thus waves) can be unpredictable and far from reliable. Can’t produce electricity at all times.

conclusion

Wave energy is not expensive to operate and maintain.

no fuel is needed and no waste is produced.

However, it depends on the intensity of the waves and needs a suitable site where waves are consistently strong.

The infrastructure must be able to withstand very rough weather.

Wave power lies not in huge plants but in a combination of on-shore generation and near-shore generation (using a different technology) focused on meeting local or regional needs.

conclusion• The Pelamis Wave Energy Converter is a revolutionary concept resulting

from many years of engineering development. It was the world’s first commercial scale machine to generate electrical energy into the grid from offshore wave energy and the first to be used in commercial wave park projects.

• The technical challenges are solvable with proper funding and encouragement

▪ The problems lie in facilitating the testing and development of the technology to make it more affordable

▪ Need federal funding

▪ Need a regulatory process conducive for rapid deployment of prototypes and research equipment

References

▪ http://www.bfi.org/Trimtab/summer01/oceanWave.htm

▪ http://www.oceanpd.com/

▪ http://www.newenergy.org.cn/english/ocean/overview/status.htm

▪ http://www.energy.org.uk/EFWave.htm

▪ http://www.wikipedia.org

▪ http://www.alternative-energy-news.info

Thank you