What is ocean energy
Ocean energy has been developing itself for the last decade as an innovative renewable source of energy. Since water covers nearly 70% of the earth’s surface. So water mass is a larger receiver of solar radiation on earth.
The energy in the ocean waves is a converted form of concentrated solar energy. The energy which comes from the sun is transferred through Complex wind electromagnetic wave interactions. The effect of Earth’s temperature variation due to solar heating combined with a multitude of atmospheric phenomena (variation in temperature and pressure). It is used to power generation or wind current, around the globe.
Ocean wave generation, propagation and direction are directly related to these wind currents. On the other hand, Ocean tide are cycle variation in seawater elevation (tidal height). And flow velocity as a direct result of the earth motion with respect to the moon and the sun. Combined with the interaction of their gravitational force.
A number of phenomena relating to Earth rotational with tilting at rotation Axis. The rate of spinning, and interaction among gravitational and rotational forces cause. The title conditions to vary significantly over time. Tide conditions are more Apparent in coastal areas there constrained channels to augment the water flow and increase the energy density.
The forms of ocean renewable energy sources can be broadly categories into
- Tides energy
- Waves energy
- Tidal current
- Temperature gradient (Temperature difference)
- Salinity gradient (pH difference)
Ocean tidal stream: Potential energy of water which is associated with tides can be exploited by building barrage. Other forms of turbine equipped construction across an estuary.
Ocean waves : Energy associated with Ocean tides wave can be harnessed using modern technologies.
Marine energy : Kinetic energy associated with tidal power/marine current can be honest using modular system.
Temperature gradient : Thermal energy due to temperature gradient between sea surface and deep water can be harnessed. Using different Ocean Thermal Energy conversion (OTEC) processes.
Salinity Gradient : At the mouths of river where fresh water mixes with salt water (estuary area). Energy associated with the salinity gradient can be harnessed using a pressure retarded Reverse Osmosis process and associated conversion Technologies.
Estuary : It is the zone of coastal area where one or more rivers meet the sea.
What temperature difference created between deep of the ocean and surface can be used as source of thermal energy. This temperature difference is developed due to surface heating of ocean with respect to interior by solar radiation.
However the Tides formed by the gravitational pull of the Moon. And waves formed by winds are considered to be good sources of mechanical energy.
Thus ocean energy can be exploited as
- Thermal energy
- Mechanical energy
With advancement of Technology the ocean renewable energy sector. With special emphasis in the field of tidal current and wave energy conversion. Technology have gained significant attention worldwide.
Many of our Technologies are also being explored for energy uses other than electricity generation. Such as producing drinking water through desalination of salt water. Supplying compressed air for aquaculture, and hydrogen production by electrolysis.
Harnessing energy from tides using tidal Barrage or tidal turbine has been far. The longest history of successful generation of electricity from ocean resources. It represents one of the older and mature technologies with a potential for very less environmental impact.
Examples are : In France, the La Rance Barrage has a capacity of 240 megawatt. Whereas in Canada, Nova Scotia power operates a 20 megawatt plant. Other Ocean renewable energy sources such as salinity gradient, temperature gradient and even hydrothermal vents. Offer for their potential for extraction of renewable energy.
The system for harnessing wave and tidal current resources is mostly in the research and development stage. With very few experiencing any kind of pre-commercial deployment. The progress in Ocean energy development is reviewed by the 2006 IES-OES Publication. Review and analysis of ocean energy system development and Supporting policy prepared by AEA technology.
Potential against wind and solar :
Sea water is about 850 times denser than air. Disturbing seas contain lots of energy in comparison to other sources of renewable energy that is wind and solar. When energy is transferred as swell waves across the ocean.
The amount of energy that is lost due to inner friction or viscosity*, is very small. Water waves start losing energy when the depth decreases below one half wavelength. At this time, energy is Dissipated as heat through Friction Against the seabed.
Eventually When the Waves strike against the shore. Some of energy is reflected. Most of the energy is lost as heat in turbulence* and friction. The wave power of ocean is the energy flow or energy flux.
It is expressed as the mean or average power per metre of crest. Length of a wave which is average energy per second. That is passing under the one of wave crests from the surface to the seabed.
*Viscosity is the internal friction of one layer of water moving relative to another layer. Energy lost due to viscosity is very less because most of the energy propagates at surface only.
*Turbulence is related to turbulent flow due to tracking of sea water with sea shore. It is form of random motion in which energy is attenuated as heat due to collision with in molecules.
Wave characteristics:
The theory behind formation of wave is expressed as
Ocean waves are generated by wind current passage over water mass. Wind flowing over Water exerts tangential stress and generates waves. While the turbulent flow of wind near the surface creates pressure fluctuation. These waves Interfere with the above-generated wave and will increase their amplitude.
Since winds on earth surface flow because of temperature difference in different parts of earth. So this energy is part of solar energy. The sun is the ultimate source of energy on earth.
Types of the waves : After generation when the wave reaches at different depths inside the ocean. it forms different types of waves.
- Deep water waves.
- intermediate depth waves.
- Shallow water waves.
The location where the waves are generated in the sea is referred to as Wind sea. Whereas long time data collection for a site. Which may contain the communicative information for many sea states is called a wave climate. When winds change in strength then wave patterns are very complex. These waves travel with minimum loss of energy to produce the swell waves.
Waves are composed of orbit particles of water closer to the surface. Their size is same as the Wave height but the orbits decrease in size as we go deeper below the surface. The energy of ocean waves is closely associated with the size of orbits. Therefore about 90% of the energy of ocean waves lies between the surface and the water depth.
Wave statistics
In order to obtain the statistical picture of the waves in the Ocean at a particular location. Recording of the water state in the sea is performed for a long period. Using this type of data we obtain a scatter diagram in terms of H and T (tidal height and duration).
At the depth of 100m the annual average power per length is approximately 70 W/m. Whereas near shore at death of about 40m, the power may be 50 W/M, (typical case for North Atlantic Ocean). The annual energy per metre of crest length around the world as estimated by the World Energy Council in 2000 TWh. If This much amount of energy is harnessed that extra power demand in the winter season may be managed.
Wave energy devices :
These devices work on the principle of conversion of kinetic energy of moving waves into electrical energy. The rising water of the waves at different position. The wave shape enters into the specially designed chamber and the air already present in the chamber is pushed out.
The pressure created by the air movement is used to turn the turbine. Also used for the motion of piston ( pulling or pushing of piston) and hence the generator associated with them.
General Categories of wave energy converters :
Many systems have been designed to convert the kinetic energy of waves into electrical energy and produce electricity.
On the very basic division the converters are arranged into three categories :
- Wave activated bodies
- Over topping devices
- Oscillating water columns.
Wave activated bodies :
These types of devices the kinetic energy of wave motion of ocean is transferred directly to the motion of the device. The main characteristic of absorber is they are small in the horizontal dimensions compared to the wavelength of the waves.
From which they designed to convert energy. In other words, they take up a relatively small area of the ocean surface. e.g. Swedish hose pump.
Over topping devices :
consist of a ramp or a tapered channel. That force the water of incoming waves to rise up and spill into a pool or reservoir. In this way, since the water surface of the reservoir is elevated relative to the ocean surface.
The energy of the waves has been converted to potential energy. In a manner resembling hydro power plants the water is flown back into the ocean through a turbine.
Oscillating water columns (OWCs) :
Uses the crest of wave or an oscillating pillar of water that pumps air through a turbine. This motion of the water pillar is designed by taking a hallow cylindrical pipe. By placing it partly submerged in the sea.
The waves that roll against the cylinder will make the internal water surface oscillate . This oscillation creates pressure which is used to pump air that drives a turbine. This device may be located onshore, near shore, or offshore. Onshore is exactly at surface.
Near shore are relatively shallow depth and offshore sites have depth. Where the waves are not affected by the sea bed. Most energy can be found onshore. Since the wave has yet to lose energy in friction against the inner layers of the ocean.