Energy Sources

The sun is made up of about

• 80% hydrogen,
• 20% helium
• 0.1% other elements.

Its radiant power comes from nuclear fusion processes, during which the sun loses 4.3 million tonnes of mass each second.

This mass is converted into radiant energy. Each square metre of the sun’s surface emits a radiant power of 63.1 M which means that just a fifth of a square kilometre of the sun’s surface emits an amount of energy equal to the global primary energy demand on earth.

Only a small part of this energy reaches the earth’s surface.

Solar irradiance decreases with the square of the distance to the sun. Since the distance of the earth to the sun changes during the year.

Only a surface that is perpendicular to the incoming sun’s rays receives this level of irradiance. Outside the atmosphere, and therefore not subject to its influence, solar irradiance has only a direct component – all solar radiation is virtually parallel. This irradiance is also called direct normal or beam irradiance.

Under these conditions, a surface that is oriented parallel to the sun’s rays receives no irradiance.

Solar Radiation

It is clean, inexhaustible, abundantly, universally available. Radiant energy in the form of electromagnetic waves from sun referred solar radiations.

The subtend angle is minuscule at the earth’s surface because of large distance, the beam radiation from the sun on earth is almost parallel.

Solar energy is created at the core of the sun when hydrogen nuclei are converted to helium nuclei through a number of intermediates.

For each second of the solar nuclear fusion process, 700 million tons of hydrogen is converted into the heavier atom helium. The solar nuclear process creates immense heat that causes atoms to discharge photons. Temperatures at the core are about 15 million degrees Kelvin.

Depletion of solar energy:

• Scattering - by dust particles and air molecules.
• Reflection and absorption - part of the radiating penetrating in to atmosphere get reflected back to space especially by clouds. Another part is absorbed by clouds, Earth reflects about 30% of radiation falling on it.
• Direct radiation - the radiation not been absorbed or scattered and reaches the ground directly from sun (also called beam radiation) . This radiation produces shadow.
• Diffuse radiation - radiation received after scattering and reflection. Diffuse radiation comes from all the parts of the sky.

Global radiation - sum of direct and diffuse radiation

Solar Energy Conversion

• Helio chemical process: Photosynthesis

• Helio electrical process: photovoltaic cell

• Helio thermal process: solar water heater

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Principles of Solar Collectors:

Flat-plate collectors are the most widely used kind of collectors in the world for domestic water heating systems and solar space heating/cooling.

The first accurate model of flat plate solar collectors was developed by Hottel and Whillier in the 1950's.

• A typical flat-plate collector consists of an : absorber, transparent cover sheets, and an insulated box.
• The absorber is usually a sheet of high-thermal conductivity metal such as copper or aluminium, with tubes either integral or attached.
• Its surface is coated black to maximize radiant energy absorption and to minimize radiant emission.
• The insulated box reduces heat loss from the back or the sides of the collector. The cover sheets, called glazing, allow sunlight to pass through the absorber but also insulate the space above the absorber to prevent cool air to flow into this space.

Concentrating solar collector

A concentrating solar collector is a solar collector that uses reflective surfaces to concentrate sunlight onto a small area, where it is absorbed and converted to heat or, in the case of solar photovoltaic (PV) devices, into electricity. Concentrators can increase the power flux of sunlight hundreds of times.

This class of collector is used for high-temperature applications such as steam production for the generation of electricity and thermal detoxification. Concentrating collectors are best suited to climates that have a high percentage of clear sky days.

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Solar furnace:

• A solar furnace is any device that creates heat by concentrating sunlight through the use of reflectors (mirrors).
• The principle of operation consists of many mirrors pointing at a concentrator. The solar radiation that hit a series of mirrors (called heliostat) will be reflected to a concentrator, where the radiation will be reflected again to a focal point.
• The intensity of the sunlight in that focus will increase many times. This produces very high temperature at the focal point.
• If a crucible is kept at the focal point which is filled with metal then the metal can be melted.
• The Odeillo solar furnace is the world's largest solar furnace. It is situated in Font-Romeu-Odeillo-Via, in the department of Pyrénées-Orientales, in south of France.

Advantages:

1. Heating without contamination.
2. Easy to control temperature.
3. High heat flux.
4. Absence of electromagnetic field

Continuous observation possible.

Disadvantage:

1. limited to sunny days

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Solar desalination

Because of industrialization and the population explosion, demand for fresh water is increasing.

Decrease in rain fall is another cause. Fresh water can be produced with the help of solar stills. This idea was first applied in 1872 at Las Salinas, Chile to supply fresh water for animals in mining areas.

• Solar still is a shallow bottom container which is painted black on the inside; this is filled with saline water.

• The container is covered with a sloping transparent glass sheet as shown. The solar still is exposed to the solar radiation, as the solar radiation enters this glass sheet and reaches the water.

• The temperature in the enclosure increases. Due to this the water evaporates and goes up.

• This vapour will condense under the glass cover and flow down to collect in a channel. This water is free from salt, which is it is distilled water. This water can be used for any purpose to replace fresh water.

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Solar Cell

A solar cell or photovoltaic cell is a device that converts light (solar) to electrical energy.

• It is a sandwich of n-type silicon and p-type silicon.
• It generates electricity by using sunlight to make electrons hop across the junction between the different layers of silicon.
• When sunlight shines on the cell, photons (light particles) bombard the upper surface. The photons carry their energy down through the cell. The photons give up their energy to electrons in the lower, p-type layer.
• The electrons use this energy to jump across the barrier into the upper, n-type layer and escape out into the circuit. Flowing around the circuit, the electrons movement generates electricity which can be stored in a battery as DC current.
• Solar cell is made of a crystalline silicon Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot
• Each cell has a voltage of approximately 0.5 to 0.6 volts.

Major application: space satellites, remote radio communication booster station, marine warning lights

Advantages:

• No moving parts, life of each panel is about 20 years
• Reliable, modular (standard units), durable and maintenance free.

Systems are quite, compatible, instantaneous response.

Disadvantages:

• Expensive

• Low efficiency

• less solar density area- more space, large number of cells needed

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Solar Greenhouse

Solar greenhouses capture the light energy of the sun and convert it into heat energy and store it. The walls and roof of the green house is made of glass or plastic. This will let the short waves of the solar light inside. This is then absorbed by the earth and plants in a greenhouse and converted into heat energy.

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Biomass Energy

It is a renewable energy (plants can be re-grown. It is a indirect form of solar energy obtained by photosynthesis.

It is some form of hydrocarbon. Biomass – plants(including agricultural wastes), Trees, animals waste (dung or manure), fungi and bacteria.

• estimated that, (1/8)th of total biomass produce would be sufficient to meet current energy demand.
• Photosynthesis is primarily responsible for biomass - indirect form of solar energy.
• Direct combustion- solid bio fuel - direct combustion of dried wood, fuel pellets and briquettes or dung cakes.
• Pyrolysis- charcoal (removing moisture and volatile constituents)
• Anaerobic digestion

Fermentation – Ethanol (produced by sugar or starch crops like sugar cane molasses corn or sweet sorghum) and also some non food sources from trees and grasses.

Advantages

• Pollutant emission from biomass is less than fossil fuels. It does not contain sulphur and aromatics. Commercial use of energy from biomass may reduce or avoid disposal wastes.

Disadvantages

• Deforestation is a major concern if wood is the fuel

• Low energy density. (few kJ/kg to MJ/kg)

• labour intensive (expensive)

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Nuclear energy

Nuclear energy is released from radioactive materials due to fission reaction. The fission reaction releases lot of heat energy this is used to heat the water and convert it to super heated steam.

Rest of the electricity is similar to steam turbines.

Advantages

• Small amount of fuel can release large amount of energy. It is a reliable.
• Once installed it is affordable form of energy

Disadvantages

• Installation cost is very high.
• The handling of radioactive fuel is difficult.
• The radiation from nuclear reactor is very harmful and dangerous for human beings. The disposal of nuclear waste should be done very carefully. The people working here should be well trained and should be very careful.

Components of a nuclear power plant:

Fuel - Uranium: U and C mixed homogeneously and used as rods or plates in the reactor heterogeneous reactors

Moderator - Aluminium, stainless steel or zirconium - prevent oxidation.

reduce kinetic energy from 1 MeV (13200 km/s) to 0.25 eV (2200 m/s) for sustainable chain reaction.

Commonly used moderators include: regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types.

Control Road - It starts the nuclear chain reaction and maintains and controls the chain reaction when in steady state condition and to shut down the reaction under emergency condition.

It has high absorption capacity of neutrons. Cadmium, boron or hafnium is used.

Shielding - Neutrons, gamma rays and other radiations are absorbed by a steel plate and concrete wall. It consists of 50 to 60 cm of steel plate surrounding the reactor vessel. It is further surrounded by a thick wall of concrete (few meters). The steel lining will heat when it is absorbing the neutrons and gamma rays. This is cooled by circulating water.

Reactor vessel – it consists of reactor core, shielding and reflector. It houses the control rod. This provides the entrance and exit passage for directing the flow of coolant. The vessel should be strong to withstand very high pressures up to 200 bars.

Heat exchanger – the hot coolant coming out of the reactor vessel transfers its heat energy to another stream of water in another pipe. This water is heated and converted to superheated steam. This steam now runs the steam turbine.

Types of reactor based on neutron energy

• Thermal reactor
  Fuel used - Natural fuel
• Enriched Uranium
  Moderator used - Water
• Heavy water
• Graphite

Coolant used - Water cooled

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Energy from Ocean

• Wave energy

  Caused due to transfer of energy by winds to sea. It is an indirect form of solar energy. Rate energy transfer depends on speed of wind and distance over which it interact. Energy flux is much higher than solar and wind. Near coastline we can extract up to 20, 00,000 MW

• Tidal Energy

  • Tidal energy is because of natural rise and fall of coastal tidal water caused principally by the gravitational fields of sun and moon.
  • The ocean level difference caused due to tides contains large amount of potential energy . The highest level of tidal water is known is high tide. And lowest is low tide.
  • Tidal power facilities harness the energy from the rise and fall of tides. Two types of tidal plant facilities. Tidal barrages. Tidal current turbines. Ideal sites are located at narrow channels and experience high variation in high and low tides. This type of energy is specific to some coasts.

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