Sources of Energy

Energy is not destroyed, but the usable form spreads into the surroundings. The heat and light of a candle cannot be gathered back into wax. Once hot water has cooled, the lost heat cannot be collected to make it hot again. So a source, after it has done work, is not available again in that same form.

A good source does a large amount of work per unit volume or mass, is easy to reach, is easy to store and transport, and is economical. For a fuel ask how much heat it gives, how much smoke it makes, and whether it is available.

Coal and petroleum were formed over millions of years. The reserves are limited, so they are non-renewable. Burning them makes oxides of carbon, nitrogen and sulphur. These are acidic oxides, and acid rain harms water and soil. Carbon dioxide adds to the greenhouse effect. Raising the efficiency of burning, and holding back ash and harmful gases, cuts some pollution. It does not end it.

In a thermal plant the fuel heats water, the steam turns a turbine and the turbine turns a generator. Sending electricity over the same distance is more efficient than hauling coal or oil, so many plants are built near coal or oil fields.

Hydroelectricity turns the potential energy of falling water into electricity. High dams stop a river and fill a reservoir. Rain fills it again, so this source is renewable. About a quarter of India’s requirement is linked to hydro power. Big dams cannot be built everywhere. Fields and homes are submerged. Submerged plants rot without oxygen and release methane, a greenhouse gas. The Tehri dam is on the Ganga and Sardar Sarovar is on the Narmada. Rehabilitating displaced people is a separate problem.

If wood burns in a limited supply of oxygen, charcoal is left. It burns with almost no flame and less smoke. Cow-dung, crop residue and sewage, broken down with no oxygen, give biogas. The dome is made of bricks. Slurry goes from the mixing tank into the digester. In a few days methane, carbon dioxide, hydrogen and hydrogen sulphide form. Biogas contains up to 75 percent methane. There is no smoke and no ash. The leftover slurry is a manure of nitrogen and phosphorus. A windmill turns a turbine. One mill’s output is small, so a farm is built. The speed must be above 15 km/h. About 2 hectares are needed for 1 MW. A large farm near Kanyakumari produces 380 MW. India ranks fifth in wind electricity. Denmark takes more than 25 percent of its electricity from the wind. Germany leads in total output.

As machines increased, the demand for an outside source increased. Grandparents walked or cycled to school, water came from a well and entertainment used less electricity. The same jobs today take more energy from outside. Going back to living as our ancestors did is not an easy fix. Making a known source more efficient and looking for a new source are both needed.

The Sun has been giving energy at about this rate for nearly 5 billion years and will do so for about 5 billion years more. The atmosphere absorbs about half. Under a clear sky India receives about 4 to 7 kWh per square metre in a day. The solar constant is about 1.4 kW per square metre. A black surface absorbs more heat than a white or a reflecting surface.

A glass plate on a solar cooker keeps heat inside by the greenhouse effect. A concave mirror is the right one for gathering the rays, not a plane or a convex mirror. An insulated box and a black inner surface increase absorption. A cooker and a water-heater are useful only for some hours of the day. A cloudy day does not give hot water.

A solar cell turns sunlight into electricity. A typical cell gives 0.5 to 1 V and about 0.7 W. Many cells together make a panel. There is no moving part, maintenance is low and a focusing device is not required. A panel is useful in a remote hamlet where laying a wire is costly. Silicon is abundant in nature, but the solar grade is limited. The silver that joins the cells in a panel adds to the cost. Satellites, traffic lights, calculators and toys run on cells. Wide use at home is still costly.

A tide comes mainly from the Moon’s pull. A dam on a narrow opening runs a turbine. Such sites are few. The kinetic energy of a wave is useful only where the waves are strong. An ocean-thermal plant runs when the gap between the surface and water at a depth of about 2 km is 20°C or more. Warm water boils a volatile liquid such as ammonia. The vapour turns a turbine. Cold water from the deep turns the vapour back into liquid.

Geothermal energy comes from a hot spot of hot rock. Underground water becomes steam and a hot spring can reach the surface. Such plants run in New Zealand and the United States of America. This energy does not come from the Sun. In nuclear fission, slow neutrons split a heavy nucleus of uranium, plutonium or thorium. One uranium atom gives about ten million times the energy of one carbon atom of coal. In a reactor the chain stays controlled and the heat makes steam. In fusion, light nuclei join, as when deuterium makes helium. The Sun burns by this. Commercial reactors are still the fission kind.

Every source disturbs the environment in some way. The choice rests on four things: how easy it is to extract the energy, what it costs, how efficient the technology is, and what the damage will be. Calling CNG clean is incomplete. The accurate sentence is that one source is cleaner than another. A solar cell makes no smoke while it runs, but damage was already done while the cell was made.

A source that will run out one day is exhaustible or non-renewable. If firewood is cut and the same number of trees is planted again, biomass continues at a rate. Such a source is renewable. An estimate of about 200 years for coal does not end the worry; the Sun’s about 5 billion years is not counted as exhaustible on a human time scale.