Energy

 Energy is the capacity to do work. A plenty of energy is needed to sustain industrial growth and agricultural production.

CLASSIFICATION OF ENERGY
It is broadly classified into

1. Conventional energy: is in practice for long duration of time and well established technology is available to tap and use them. e.g. Coal, oil, natural gas, hydro power, nuclear power etc.
2. Non-conventional energy:  source can be used with advantage for power generation as well as other applications in a large number of locations and situations. These energy sources cannot be easily stored and used conveniently. e.g. Solar, wind, tidal and geothermal etc.

Based upon nature, energy sources are classified as

1. Renewable energy sources are inexhaustible and are renewed by nature itself. Solar, wind, tidal, hydro and biomass are few examples.
2. Non-renewable energy sources are exhaustible within a definite period of time depending upon its usage. Fossil fuels (coal, oil, gas) and nuclear fuels are few examples.

SOLAR ENERGY
The surface of the earth receives about 1014 kW from sun in the form of solar energy which is approximately five orders of magnitude greater than that currently being consumed from all resources. It is evident that sun will last for 1011 years.
There are two obvious obstacles to harnessing solar energy. Firstly it is not constantly available on earth. Thus some form of storage is needed to sustain solar energy through the night and during rainy season. Secondly the solar energy is diffused. Although the total amount of energy is enormous, the collection and conservation of solar energy into useful forms must be carried out over a large area which entails large capital investments.

SOLAR WATER HEATING
By using solar radiation, water or any fluid can be heated by using a solar collector. Such systems can provide hot water for different applications in industries directly or as boiler feed and also in hostels, hotels and canteens.
There are two types of solar collectors in use:

1. Flat plate collector
2. parabolic or concentrating collector

Flat plate collector:
The absorber plate is metallic. It is usually coated black to absorb more heat energy. Tubes, passages or channels integral with the collector carry water or other working fluid. Insulation should be provided at the back and at the sides to minimize the heat losses. Usually glass wool is used as insulation material. A transparent cover (glass) will be provided at the top to permit the radiation from the sun to the metal plate.
          

Parabolic or concentrating collector
Highly polished metallic surfaces are used as the reflector. The reflector will have a parabolic shape so that the sun rays striking the profile will be reflected on its focal point. If a tube carrying a fluid is kept along the focal line, the fluid will be heated to a very high temperature.

 

SOLAR THERMAL CONVERSION

1. Low temperature system
2. High temperature system

Low temperature system
This system uses a flat plate collector, which can heat water up to about 800C only. Water is circulated through the flat plate collector. Heat exchanger is used to vaporize a low boiling fluid (butane) and the butane vapor will run the turbine, butane vapor is condensed and reused.

 

           

 

High temperature system
Solar power tower is employed for high temperature system. A receiver is mounted on the tower which is 100 to 500m high and designed to withstand the weight of the receiver. Here reflecting mirrors are known as heliostat. The heliostats are arranged in such a way that they direct the solar energy on the receiver throughout the day. This is achieved by a microprocessor based tracking system, which turns the heliostats depending on the position of sun. The receiver is designed and positioned in such a way to intercept, absorb and transfer maximum heat to working fluid. The temperature achieved is 1500°C to 1700°C.  Steam generated in the receiver runs the turbine, condensed to water and recycled.

 

 

 

ADVANTAGES

1. Renewable source of energy
2. Pollution free
3. After the capital cost, the cost of power generation is quite low
4. Wide range of applications, powering street lights to satellites

DISADVANTAGES

1. Capital cost is very high
2. Large area of land is required
3. Large number of solar panels are required
4. Affected by seasons.

 

WIND ENERGY
The electrical energy can be generated by wind energy by utilizing the kinetic energy of wind.  The wind energy which is an indirect source of energy can be used to run a wind mill which in turn drives a generator to produce electricity.

WIND MILL:
Wind mills are classified into two types

1. Horizontal axis wind turbine
2. Vertical axis wind turbine

HORIZONTAL AXIS WIND TURBINE
Horizontal axis wind turbines have the main rotor shaft running horizontally.

Fig shows a schematic arrangement of a horizontal axis machine.  This system consists of a tower mounted two bladed or multi bladed rotor facing the wind, rotating around a horizontal axis and turning an electrical generator.
The Blades are generally made of composite material, usually fibre reinforced plastic (FRP) because of its high strength and light weight. Wind mills are manufactured with a capacity from a few kilowatts to several megawatts in Europe, the USA, and other parts of the world including India.

 

VERTICAL AXIS WIND TURBINE
Vertical axis wind turbines have the main rotor shaft running vertically.  The tower construction is simple here because the generator and gear box can be placed at the bottom, near the ground.
Vertical axis wind turbine can be classified into two types

1. Darrieus type
2. Savonius type

Darrieus type rotor
This wind mill needs much less surface area. It is shaped like an egg beater and has two or three blades shaped like aero foils.

Savonius type rotor
Savonius turbine is S-shaped if viewed from top. This turbine turns relatively slow, but yields high torque. It is used for grinding grains and for pumping water.

 

ADVANTAGES

1. Wind is Renewable and free of cost
2. Pollution free
3. Can be installed in remote villages, thus reducing costly transmission lines

DISADVANTAGES

1. Capital cost is very high
2. Large area of land is required
3. Maintenance cost is very high

 

TIDAL ENERGY
The periodic rise and fall of water level of sea which are carried by the action of the sun and moon on water of the earth is called “tide”. The large scale up and down movement of sea water represents an unlimited source of energy.
The main feature of the tidal cycle is the difference in water surface elevations at the high tide and at the low tide. If the differential head could be utilized in operating a hydraulic turbine, the tidal energy could be converted into electrical energy by means of an attached generator.

 

TIDAL POWER PLANT
A Tidal power plant mainly consists of the following:
1. A barrage with gates and sluices
2. One or more basins
3. A power house
A barrage is a barrier constructed across the sea to create a basin for storing water. The barrage has to withstand the pressure exerted by the water head and also should resist the shock of the waves.
A basin is the area where water is retained by the barrage. Low head reversible water turbine are installed in the barrage separating the sea from the basin.-

 

During high tide, water will flow from sea to tidal basin through turbine, thus producing electricity. During low tide, water will flow from tidal basin to sea through turbine producing electricity.

ADVANTAGES

1. It is inexhaustible source of energy
2. No problem of pollution
3. The cost of power generation is quite low
4. High output can be obtained compared to solar or wind energy

DISADVANTAGES

1. Capital cost is very high
2. As the head is not constant, variable output is obtained
3. As the head is low, large amount of water is necessary for the turbine
4. It will not operate when the available head is less than 0.5m

GEOTHERMAL ENERGY
Geothermal power plants derive energy from the heat of the earth’s interior. The average increase in temperature with depth of the earth is 10C for every 30-40m. At a depth of 10-15km, the earth’s interior is as hot as 1000-12000C. In certain areas of our planet, the underground heat has raised the temperature of water to over 2000C which bursts out as hot steam through the cracks in the earth’s crust. These are called thermal springs. This steam can be utilized for power production.

GEOTHERMAL SOURCES
The following five general categories of geothermal sources have been identified:
1. Hydrothermal convective systems
(i) Vapor dominated or dry steam fields
(ii) Liquid dominated or wet steam fields
(iii) Hot water fields
2. Geo-pressure resources
3. Petrothermal or hot dry rocks
4. Magma resources
5. Volcanoes
The hydro thermal convective systems are best resources for geothermal energy exploitation at present. Hot dry rock is also being considered.

GEOTHERMAL POWER PLANTS
Geothermal wells are drilled at suitable locations. Water vaporized into steam comes out of the earth’s surface in a dry condition at around 200°C and 8 bar.  The moisture is removed by a centrifugal separator and this steam will run the turbine coupled with a generator. Steam is condensed in a condenser and re injected back into the ground by a rejection well.    

 

 

ADVANTAGES

1. Geothermal energy is cheaper
2. Used as space heating for buildings
3. Used as industrial process heat
4. Geothermal energy is inexhaustible

DISADVANTAGES

1. Low overall power production efficiency (about 15%)
2. Large areas are needed foe exploitation of geothermal energy

 

OCEAN THERMAL ENERGY CONVERSION
OTEC uses the temperature difference of the sea water at different depths to generate electricity
OTEC utilizes the temperature difference that exists between the surface waters heated by the sun and the colder deep (up to 1000m) waters to run a heat engine. This source and sink provides a temperature difference of 20°C in ocean areas within 20 of the equator. These conditions exist in tropical coastal areas, roughly between the tropic of Capricorn and the tropic of cancer. Such a small temperature difference makes energy extraction difficult and expensive. Hence, typically OTEC systems have an overall efficiency of only 1 to 3%. The OTEC is shown in fig.

 

 

 

SHORT ANSWER QUESTIONS

1. Differentiate between renewable and non-renewable source of energy.
2. Differentiate between flood and ebb tide.
3. Write short note on parabolic collector.
4. What are the types of wind mill?
5. State the advantages and disadvantages of geothermal energy.
6. Differentiate between active and passive solar heating.

 

LONG ANSWER QUESTIONS

1. Briefly explain the tower concept of solar power generation.
2. Explain the various types of wind mills with a neat sketch.
3. Explain power generation using geothermal energy.
4. Write short notes on tidal power plant.
5. With neat sketch, explain the ocean thermal energy conversion.