Waves are a result of wind blowing over the ocean surfaces. The energy transported by the ocean surface waves is known as wave power that can be used for generating electricity, pumping water into reservoirs and water desalination purposes. This energy is extracted by wave energy converter. Wave power varies significantly in different parts of the world. The amount of energy in waves varies based on the height of the waves and time period between successive peaks. Some of the world’s wave power rich areas include Australia, South Africa, northern Canada, northwestern coasts of the US and western Scotland.
Tidal streams, on the other hand, are formed by the rise and fall of the tides. The energy is carried by the water flowing in and out of the estuaries. Tidal power has great potential to generate electricity for future. However, the relatively high cost and limited availability of sufficiently high tidal ranges narrow down its applications. With a number of recent developments in turbine technology and designs like tidal lagoons and dynamic tidal power, the availability of tidal power is likely to increase in the near future.
Wave Energy Technologies
Wave energy technologies are of three types that include the following:
Floats or Pitching Devices
In this, a floating object is mounted on a device placed on the ocean surface or on a floating raft. The pitching action of the object drives the generation of electricity by these devices.
Oscillating Water Columns
These devices generate electricity using rise and fall of water inside a cylindrical shaft. The column drives air in and out of the shaft top to power an air-driven turbine.
Tapered Channel
This can be fixed both onshore and offshore. The channel concentrates the waves and drives them into an elevated reservoir. Water is then released and it falls back into the ocean due to gravity. The energy obtained from the falling water is used to rotate the hydro turbines to generate electricity.
Point Absorber
This is a floating structure capable of moving with respect to the wave action. The relative motion of the structure in turn drives the electromechanical or hydraulic energy converters to produce electricity.
Attenuators
These are long multi-segment floating structures arranged in a direction parallel to the waves. The differing heights of the waves cause flexing in places where the parts of the device are connected, and this flexing, in turn, sets the converters and pumps in motion.
Terminator Device
This is usually placed onshore perpendicularly to the direction of waves to trap or reflect wave energy.
Tidal Energy Technologies
Some key tidal energy technologies are discussed below:
Barrages or Dams
A barrage or dam is installed with turbines and gates. The gates are opened when the tides create the required difference in water level. As a result, water flows via the turbines to activate an electric generator thereby converting tidal energy into electricity. The facility located on the Rance River, France, has a peak generating capacity of 240MW.
Tidal Fences
Tidal fences act like huge turnstiles which cross the channels between small islands or the mainland and an island. Tidal currents make the turnstiles spin for generating electricity. Some of these currents are capable of traveling 5.6 to 9 mph to generate energy as much as the energy produced by high velocity winds.
Tidal Turbines
Tidal turbines are physically more similar to wind turbines. These turbines are arrayed underwater 60-120 ft deep with coastal currents exceeding 5-6 mph such that the turbines are spun through the tidal flow to generate electricity. Tidal turbines are smaller than wind turbines and produce more electricity within a given area owing to the fact that water is denser than air. These turbines works best with currents between 4 and 5.5 mph. The tidal turbine located on the bed of New York's East River has a capacity to generate 35 kW of energy.
Benefits of Wave and Tidal Power
Some of the major advantages of wave power are as follows:
- It provides high energy security by reducing the dependence on fossil fuels.
- It is a renewable resource as wave energy will never run out.
- Wave energy stations can be built along the coastlines without disturbing the natural environment of a neighborhood.
- It does not emit greenhouse gases.
- A wave energy station requires less space and infrastructure when compared to stations of other energy sources.
- Wave energy is steadier and predictable.
- It is abundant and widely available as a number of countries have long coasts and so people can harness wave power for their use.
The key benefits of tidal power are listed below:
- It is renewable and pollution-free.
- It is more predictable as rise and fall of tides is cyclic.
- It has high energy density than that of other renewable energy forms.
- Barrages and dams secure ship ports or nearby places from disastrous tides during storm.
- Tidal turbines used for converting tidal energy into electricity are efficient up to 80%, which is higher than wind or solar energy generators.
- Although initial cost required to build tidal energy plants is high, the cost required for their maintenance is very low.
Sources and Further Reading