These are the Non conventional 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. |
Solar Energy
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.
Non conventional energy sources – Energy from biomass
Biomass is a potential source of renewable energy. This includes all the organic materials such as crop residue, natural vegetation, trees, animal dung and sewage. Biomass energy or bio conversion refers to the use of this material as fuel or its conversion into fuels.
There are many methods used for the conversion of biomass into fuels. But the most common are
- Direct combustion
- Fermentation
Direct combustion method is usually applied to get energy from waste products commonly known as solid waste. It will be discussed in the next section.
Biofuel such as ethanol (alcohol) is a replacement of gasoline. It is obtained by fermentation of biomass using enzymes and by decomposition through bacterial action in the absence of air (oxygen).
The rotting of biomass in a closed tank called a digester produces biogas which can be piped out to use for cooking and heating (Fig. 4.16).
For your information |
| The rapid growth of human population has put a strain on our natural resources. A sustainable society minimizes waste and maximizes the benefit from each resource. Minimizing the use of energy is another method of conservation. We can save energy by. (i) Turning of lights and electrical appliances when not in use. (ii) Using fluorescent bulbs instead of incandescent bulbs (iii) Using sunlight in offices, commercial centers and houses during daylight hours (iv) Taking short hot shavers.
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The waste material of the process is a good organic fertilizer. Thus, production of biogas provides us energy source and also solves the problem of organic waste disposal.
Energy from waste products
Waste products like wood waste, crop residue, and particularly municipal solid waste can be used to get energy by direct combustion. It is probably the most commonly used conversion process in which waste material is burnt in a confined container. Heat produced in this way is directly utilized in the boiler to produce steam that can run turbine generator.
| Do you know? |
| Pollution can be reduced if (i) People use mass transportation (ii) Use geothermal, solar, hydroelectrical and wind energy as alternative forms of energy. |
Geothermal energy
This is the heat energy extracted from inside the earth in the form of hot water or steam. Heat within the earth is generated by the following processes.
Radioactive Decay
The energy, heating the rocks, is constantly being released by the decay of radioactive elements.
Residues’ heat of the Earth
At some places hot igneous rocks, usually within 10 km f the Earth’s surface, are in a molten and partly molten state. They conduct heat energy from the Earth’s interior which is still very hot. The temperature of these rocks is about 200ᵒC or more.
Compression of Material
The compression of material deep inside the Earth also causes generation of heat energy.
In some place water beneath the ground is in contact with hot rocks and is raised to high temperature and pressure. It comes to the surface as hot springs, geysers, or steam vents. The steam can be directed to turn turbines of electric generators.
At places water is not present and hot rocks are not very deep, the water is pumped down through them to get steam (Fig. 4.17). The steam then can be used to drive turbines or for direct heating.
An interesting phenomenon of geothermal energy is a geyser. It is a hot spring that discharges steam and hot water, intermittently releasing an explosive column into the air (Fig. 4.18). Most geysers erupt at irregular intervals. They usually occur in volcanic regions. Extraction of geothermal heat energy often occurs closer to geyser sights. This extraction seriously disturbs geyser system by reducing heat flow and aquifer pressure. Aquifer is a layer of rock holding water that allows water to percolate through it with pressure.
