ENERGY FROM BIOMASS
What is biomass? Biomass is any organic matter from which we can get energy on a renewable basis. It includes agricultural residues, wood, animal excreta, wastes from food processing and municipal wastes. Agricultural residues include straw, hay and husk. Waste from food processing includes bagasse, which is the residue left after extracting the juice from sugar cane. Cow dung (gobar) has been a traditional biomass fuel in our country. Since plants trap solar energy in the form of food, and animals eat plants, the ultimate source of biomass energy is the sun
Wood has been a major source of energy since man discovered fire. It is still used widely as a fuel for cooking and heating. In many Indian homes, food is cooked on stoves that use biomass fuels such as wood. Such a stove is called a chulha.
A traditional chulha operates at a very low efficiency. Hardly 10% of the energy contained in the wood is utilized. Also, it does not burn the wood completely, which causes a lot of smoke. Indian scientists have designed several 'smokeless' chulhas. These produce much less smoke, and the fuel is more efficiently utilized.
Wood can be more efficiently utilized by converting it into charcoal. Charcoal is prepared by burning wood in an insufficient supply of oxygen. The process of burning wood in insufficient supply of oxygen is called destructive distillation of wood. Wood is a mixture of carbon compounds like cellulose (a carbohydrate), which decompose on heating. Most of the products formed escape, and what is left behind is mainly carbon, which is a better fuel than wood. We can prepare charcoal by heating wood shavings in a closed container that has a hole for gases to escape.Charcoal is a better fuel than wood
(a) When the same mass of charcoal and wood are burnt, charcoal produces almost twice the heat produced by wood. So its energy-conversion efficiency is better than that of wood.
(b) Charcoal produces much less smoke than wood.
(c) Charcoal is a compact fuel that is more convenient to handle than wood.
Cattle dung has been traditionally used as a fuel in India. The dung is shaped into flat cakes, dried and used as fuel for cooking. Burning cattle-dung cake produces some heat and a lot of smoke. Scientists have now found a better way of using cattle dung-to produce biogas, which is an excellent fuel. This gas is popularly called gobar gas.
Certain types of bacteria decompose wet biomass in an oxygen-free (anaerobic) environment to produce a combustible mixture of gases, called biogas. The process is called anaerobic digestion. Biogas can contain up to 75% methane, 23% carbon dioxide and 2% other gases, including hydrogen and hydrogen sulphide. Methane is an excellent fuel.
Biogas can be produced in a biogas plant by using cattle dung, sewage, agricultural residues, and so on. It can be used for electricity generation and for cooking. A few lakh biogas plants have been built in our country.
A biogas plant has a digester in which the biomass is decomposed, a mixing tank for mixing the biomass with water, an outflow tank and a means of collecting gas.
The biomass, e.g., cattle dung, is mixed with water in the mixing tank to form slurry. The slurry from this tank flows into the digester, which is a sealed chamber. The biomass decomposes here and expands, and the gas produced presses down the slurry. This causes the spent slurry to overflow into the outflow tank. The spent slurry is used as manure. The upper part of the digester has an outlet for gas. To prevent excessive pressure build up, the gas is removed from time to time.
Advantages of biogas
(a) A biogas plant is quite simple and can be easily built in rural areas.
(b) Biogas is an excellent, clean fuel that burns without producing ash and smoke.
(c) The spent slurry is good manure.
(d) Biogas plants are a safe and useful way of waste disposal.
(e) Use of biogas in rural areas leads to saving of firewood, and reduces deforestation.
The earth receives a huge amount of energy from the sun. Each square metre of the earth's upper atmosphere receives about 1.36 kilojoules (kJ) of solar energy per second. In other words, solar energy is incident at the rate of 1.36 Kw/m2.
The entire energy incident on the upper atmosphere of the earth does not reach its surface.
About 40% of this energy is reflected by the atmosphere into space. Then some of it is absorbed by the gases and particles present in the atmosphere. Finally, only about 47% of the incident energy reaches the earth's surface.
Solar energy sustains life processes on the earth. Plants use it to make food. Traditionally we have been using solar energy for drying clothes and grains, making salt from sea water, etc. Newer uses include solar heating devices and solar cells. Solar heating devices such as solar cookers and solar water heaters use the heat energy that comes with sunlight. Solar cells convert solar energy into electricity.
Advantages of solar energy
(a) It is available everywhere.
(b) It is available in plenty (renewable), and it is free.
(c) It does not cause pollution.
Limitations of solar energy
(a) It cannot be used at night.
(b) Devices based on solar energy do not work well on cloudy days.
Solar Heating Devices
A solar heating device uses solar energy to heat things like water and food. To make a solar heating device effective, we have to ensure that the device absorbs a lot of heat from sunlight and does not lose too much of the collected heat. To achieve these, the following measures are taken.
Increasing the absorption of heat Everything on which sunlight falls absorbs heat from it. However, black and dark-coloured surfaces absorb heat better than do light-coloured and white surfaces. That is why we feel hotter than usual when we wear dark clothes in summer.
In certain types of solar heating devices, the surfaces meant for collecting sunlight are painted black to increase the absorption of heat.
Reducing the loss of heat A hot object loses heat to it surroundings by conduction, convection and radiation. Different methods are used to reduce the loss of heat in heating devices. In a box-type device, to reduce heat loss by conduction, the walls are made of an insulator, i.e., a material that is a poor conductor of heat. The box is made airtight so that the air inside cannot take the heat out of the box. Thus, convection loss is also reduced. Radiation loss is reduced by covering the top of the box with glass. Sunlight passes through the glass and heats the materials inside the box. The heated materials radiate heat, but these get reflected back into the box by the glass. This traps the heat (in the same way it is trapped in a greenhouse).
Increasing the amount of sunlight collected A reflector such as a plane mirror is used to reflect sunlight into the heating device so that more sunlight enters it.
Box-type solar cooker
A solar cooker is a device that uses solar energy for cooking. In its simplest form it consists of a special box in which the food to be cooked is placed. The box is made of an insulating material such as plastic or wood. The box may be double-walled, with an insulating material (even air) between the outer and inner walls. This reduces heat loss due to conduction more effectively. The inner walls are painted black to increase heat absorption. The box is covered with a glass sheet. This reduces heat loss by radiation and traps heat, which increases the temperature inside the box. The covered box is made airtight to reduce heat loss by convection. To increase the amount of sunlight going into the box, a plane mirror is hinged at an angle at the top of the box.
The food to be cooked is taken in a vessel and placed inside the cooker. The temperature inside the cooker reaches 100-140°C in about 2-3 hours in summer. So, we can cook food that requires slow heating like dal, vegetables, rice, and so on.
Advantages Solar cookers are inexpensive to own and operate. We can put food in several vessels inside the cooker so that they get cooked simultaneously. These cookers reduce firewood and LPG consumption. And they do not produce any smoke.
Disadvantages A solar cooker can be used only during daytime. They can be used effectively only in regions with warm climate. They take a long time to cook food in winters and on cloudy days. We cannot use box-type cookers to fry food or make roti.
Solar water heater
Solar energy can be used to heat water. In a solar water heater, sunlight is allowed to fall on a box made of a poor conductor of heat. The glass top of the box lets in sunlight and traps heat. Water enters a tube that is painted black to increase the absorption of heat. It is bent several times to increase its length inside the box. This allows the water flowing through it sufficient time to absorb heat. Hot water collects in the tank of the heater for use.
A device which converts solar energy directly into electricity is called a solar cell or a photovoltaic (PV) cell. Some substances like silicon are semiconductors. Their ability to conduct electricity is much less than that of conductors. However, when certain substances are added to semiconductors, they are able to conduct electricity much better. Solar cells are made from such altered semiconductors.
When sunlight falls on a solar cell, a potential difference of about 0.5-1 V develops across it. The power of a solar cell depends on its surface area. The greater the surface area of the solar cell, the greater is the solar energy collected by it, and hence, the greater is the power generated. Small solar cells are, therefore suitable for use in only those electronic devices that require very little power. Such devices include calculators and watches. A typical solar cell delivers much less power than a common dry cell.
Although' a solar cell provides very little power, a large number of connected solar cells, spread over a large area, can provide sufficient power for many applications. Such an arrangement of solar cells is called a solar panel. The solar cells in a solar panel are connected in such a way that the total potential difference and the total capacity to provide electric current become large.
Uses of solar panels The advantage of solar panels is more in areas where the usual energy sources are not available. That is why they are used as the source of electric power in satellites. Solar panels have also been used in unmanned aircraft that fly at high altitudes for long periods, conducting scientific experiments. Experimental solar-powered cars have also been made. In many parts of India, solar panels are being used to charge rechargeable batteries during the day. At night, these batteries provide electric power for lighting, etc. They are also being used for operating traffic lights, water pumps, telephones, TV sets and radio receivers.
Advantages of solar cells
(a) Solar cells are suitable for use in remote areas where electrical power lines have not reached.
(b) Solar cells require little maintenance and last for a long time.
(c) After installation, no further cost is involved in generating electricity directly from solar cells.
(d) Solar cells are environment friendly, as they do not cause any pollution.
Limitations of generating electricity from solar cells
(a) Solar panels are expensive. Although silicon is the second-most abundant element on earth, the cost of purifying silicon for making semiconductors used in solar cells is very high. This increases the cost of solar cells.
(b) The solar cells in a solar panel are connected using silver, to keep the resistance as low as possible. This also increases the cost.
(c) Solar cells provide direct current (D.C.), while most household appliances work on A.C. Special devices (inverters) are needed to convert the D.C. to A.C.
(d) Since the power output of solar cells is low, a large number of solar panels, spread over a large area, are required, even to meet the normal electrical power requirement of a household.
(e) Like all solar devices, solar cells work only during the day, and their effectiveness is reduced in winters and on cloudy days. For the power requirement at night, storage batteries are charged by solar cells during the day. The batteries further add to the cost.
ELECTRICITY FROM THE OCEAN
Electricity can be generated from the ocean by utilizing the following facts.
Tides cause the level of sea water to rise and fall periodically. This is mainly due to the gravitational pull exerted by the moon, and to some extent by the sun. The water level is maximum at high tide and minimum at low tide. The difference in water levels can be utilized to generate electricity.
Waves in the ocean involve a lot of kinetic energy, which can be converted to electricity .
The water at the surface of the ocean is warmer than the water deep below. Their temperature difference can be utilized to generate electricity.
Generating Electricity from Tides
A tidal power plant can be located at the mouth of a narrow bay, where tides cause regular and appreciable rise in the level of water. A dam with gates is constructed at the mouth of the bay. At high tide, the level of the water in the bay rises, and it is allowed to flow in and collect behind the dam. When the tide ebbs, the water level in the sea starts falling. The dam gates are then closed. As a result, the level of water behind the dam remains higher than that in the bay. When the difference in the levels is sufficient, the gates of the dam are opened, and water is allowed to fall into the bay. This falling water is used to drive turbines connected to electric generators. (In some power plants, the water flowing in at high tide is also used to generate a part of the power.)
For a tidal power plant, the difference between the water levels at high tide and low tide should be about
5 metres. Not many places have this. For this reason, tidal energy cannot be a major source of electrical power.
Generating Electricity from Ocean Waves
We can convert the energy of ocean waves into electricity at places where there are strong waves. Figure shows schematically one setup for generating electricity from waves. A large air-filled object, called a surface follower, floats in the sea and moves up and down with the waves. It is linked to a shaft at the bottom of the sea in such a way that its up-and-down motion is converted into the rotational motion of the shaft. The rotation of the shaft is used to turn the shaft of an electric generator.
Ocean waves are caused by winds. Since winds arise due to the heating of the earth's surface by the sun, the ultimate source of energy of ocean waves is the sun.
Ocean Thermal Energy Conversion (OTEC)
Solar energy falling on the surface of the ocean warms it.
The water at the surface of the ocean is warmer than the water deep below. Generally, the difference in temperature is about 20°C between the surface water and the water at a depth of 2 km. This temperature difference can be used to operate an ocean thermal energy conversion (OTEC) plant. Clearly, the ultimate source of the stored thermal energy of the ocean is the sun.
In one system for OTEC, a fluid with low boiling point such as ammonia or chlorofluorocarbon (CFC) is used as the 'working fluid'. Warm sea water is used to vaporize liquid ammonia in an evaporator. The expanding vapours of ammonia turn a turbine connected to a generator. Then the vapours go to a condenser. There, cold sea water, pumped up from the deep, is used to liquefy the ammonia. This ammonia is reused, and the cycle goes on.