Water level equalized.
Water is beginning to flow out of basin to ocean, driving turbines.
Water level equalized.
Water is allowed to flow back into the basin, driving turbines.
These are the energy sources which are not very common these days. However, it is expected that these sources will contribute substantially to the energy demand of the future. Some of these are introduced briefly here.
Energy from Tides
One very novel example of obtaining energy from gravitational field is the energy obtained from tides. Gravitational force of the moon gives rise to tides in the sea. The tides raise the water in the sea roughly twice a day. If the water at the high tide is trapped in a basin by constructing a dam, then it is possible to use this as a source of energy. The dam is filled at high tide and water is released in a controlled way at low tide to drive the turbines. At the next high tide the dam is filled again and the in rushing water also drives turbines and generates electricity as shown systematically in the Fig. 4.13.
Fig. 4.13: tidal power plant. Turbines are located inside the dam.
Energy from waves
The tidal movement and the winds blowing across the surface of the ocean produce strong water waves. Their energy can be utilized to generate electricity. A method of harnessing wave energy is to use large floats which move up and down with the valves. One such device invented by Professor Salter is known Salter’s duck (Fig. 4.14). It consists of two parts (i) Duck float.
The wave energy makes duck float move relative to the balance float. The relative motion of the duck float is then used to run electricity generators.
Do You Know?
|The pull of the moon does not only pull the sea up and down. This tidal effect can also distort the continents pulling land up and down by as much as 25cm.|
The Earth receives huge amount of energy directly from the sun each day. Solar energy at normal incidence outside the Earth’s atmosphere is about 1.4 kWm -2 which is referred as solar constant. While passing through the atmosphere, the total energy is reduced due to reflection, scattering and absorption by dust particles, water vapours and other gases. On a clear day at noon, the intensity of solar energy reaching the Earth’s surface is about 1kWm -2 this energy can be used directly to heat water with the help of large solar reflectors and thermal absorbers. It can also be converted to electricity. In one method the flat plate collectors are used for heating water. A typical collector is shown in Fig. 4.15 (a). It has a blackened surface which absorbs energy directly from solar radiation. Cold water passes over the surface and is heated upto about 70ᵒC.
Much higher temperature can be achieved by concentrating solar radiation on to a small surface area by using huge reflectors (mirrors) or lenses to produced steam for running a turbine.
The other method is the direct conversion of sunlight into electricity through the use of semi conductor devices called solar cells also known as photo voltaic cells. Solar cells are thin wafers made from silicon. Electrons in the silicon gain energy from sunlight to create a voltage. The voltage produced by a single voltaic cell is very low. In order to get sufficient high voltage for practical use, a large number fo such cells are connected in series forming a solar cells panel.
For cloudy days or nights, electric energy can be stored during the sun light in nickel cadmium batters by connecting them to solar panels. These batteries can then provide power to electrical appliances at nights or on cloudy days.
Solar cells although, are expensive but last a long time and have low running cost. Solar cells are used to power satellites having large solar panels which are kept facing the sun (Fig. 4.15b). Other examples of the use of solar cells are remote ground based weather stations and rain forest communication systems. Solar calculators and watches are also in use now-a-days.