Green energy Physics Unit 5
Classification • Renewable/ non conventional • Non renewable/ conventional
How much solar energy? The surface receives about 47% of the total solar energy that reaches the Earth. Only this amount is usable.
Direct Conversion into Electricity • Photovoltaic cells are capable of directly converting sunlight into electricity. • A simple wafer of silicon with wires attached to the layers. Current is produced based on types of silicon (n- and p-types) used for the layers. Each cell=0.5 volts. • Battery needed as storage • No moving partsdo no wear out, but because they are exposed to the weather, their lifespan is about 20 years.
A proper metal contacts are made on the n-type and p- type side of the semiconductor for electrical connection • Working: • When a solar panel exposed to sunlight , the light energies are absorbed by a semiconduction materials. • Due to this absorded enrgy, the electrons are libereted and produce the external DC current. • The DC current is converted into 240-volt AC current using an inverter for different applications. PH 0101 Unit-5 Lecture-2
Mechanism: • First, the sunlight is absorbed by a solar cell in a solar panel. • The absorbed light causes electrons in the material to increase in energy. At the same time making them free to move around in the material. • However, the electrons remain at this higher energy for only a short time before returning to their original lower energy position. • Therefore, to collect the carriers before they lose the energy gained from the light, a PN junction is typically used. PH 0101 Unit-5 Lecture-2
A PN junction consists of two different regions of a semiconductor material (usually silicon), with one side called the p type region and the other the n-type region. • During the incident of light energy, in p-type material, electrons can gain energy and move into the n-type region. • Then they can no longer go back to their original low energy position and remain at a higher energy. • The process of moving a light- generated carrier from p-type region to n-type region is called collection. • These collections of carriers (electrons) can be either extracted from the device to give a current, or it can remain in the device and gives rise to a voltage. PH 0101 Unit-5 Lecture-2
The electrons that leave the solar cell as current give up their energy to whatever is connected to the solar cell, and then re-enter the solar cell. Once back in the solar cell, the process begins again: PH 0101 Unit-5 Lecture-2
Conduction band High density Valence band Low density E The mechanism of electricity production- Different stages The above diagram shows the formation of p-n junction in a solar cell. The valence band is a low-density band and conduction band is high-density band. PH 0101 Unit-5 Lecture-2
Conduction band High density Valence band Low density E Stage-1 When light falls on the semiconductor surface, the electron from valence band promoted to conduction band. Therefore, the hole (vacancy position left by the electron in the valence band) is generates. Hence, there is a formation of electron-hole pair on the sides of p-n junction. PH 0101 Unit-5 Lecture-2
Conduction band High density Valence band Low density junction E Stage-2 In the stage 2, the electron and holes are diffuse across the p-n junction and there is a formation ofelectron-hole pair. PH 0101 Unit-5 Lecture-2
Conduction band High density Valence band Low density junction E Stage-3 In the stage 3, As electron continuous to diffuse, the negative charge build on emitter side and positive charge build on the base side. PH 0101 Unit-5 Lecture-2
Conduction band High density Valence band Low density junction E Power Stage-4 When the PN junction is connected with external circuit, the current flows. PH 0101 Unit-5 Lecture-2
A solar panel (or) Solar array • Single solar cell • The single solar cell constitute the n-typelayer sandwiched with p-type layer. • The most commonly known solar cell is configured as a large-area p-n junction made from silicon wafer. • A single cell can produce only very tiny amounts of electricity • It can be used only to light up a small light bulb or power a calculator. • Single photovoltaic cells are used in many small electronic appliances such as watches and calculators PH 0101 Unit-5 Lecture-2
N-type P-type Single Solar cell PH 0101 Unit-5 Lecture-2
Solar panel (or) solar array (or) Solar module • The solar panel (or) solar array is the interconnection of number of solar module to get efficient power. • A solar module consists of number of interconnected solar cells. • These interconnected cells embedded between two glass plate to protect from the bad whether. • Since absorption area of module is high, more energy can be produced. PH 0101 Unit-5 Lecture-2
Types of Solar cell • Based on the types of crystal used, soar cells can be classified as, • Monocrystalline silicon cells • Polycrystalline silicon cells • Amorphous silicon cells • The Monocrystalline silicon cell is produced from pure silicon (single crystal). Since the Monocrystalline silicon is pure and defect free, the efficiency of cell will be higher. • In polycrystalline solar cell, liquid silicon is used as raw material and polycrystalline silicon was obtained followed by solidification process. The materials contain various crystalline sizes. Hence, the efficiency of this type of cell is less than Monocrystalline cell. PH 0101 Unit-5 Lecture-2
Amorphous silicon was obtained by depositing silicon film on the substrate like glass plate. • The layer thickness amounts to less than 1µm – the thickness of a human hair for comparison is 50-100 µm. • The efficiency of amorphous cells is much lower than that of the other two cell types. • As a result, they are used mainly in low power equipment, such as watches and pocket calculators, or as facade elements. PH 0101 Unit-5 Lecture-2
Comparison of Types of solar cell PH 0101 Unit-5 Lecture-2
Advantage, disadvantage and application of Solar cell • Advantage • It is clean and non-polluting • It is a renewable energy • Solar cells do not produce noise and they are totally silent. • They require very little maintenance • They are long lasting sources of energy which can be used almost anywhere • They have long life time • There are no fuel costs or fuel supply problems PH 0101 Unit-5 Lecture-2
Disadvantage • Solar power can’t be obtained in night time • Solar cells (or) solar panels are very expensive • Energy has not be stored in batteries • Air pollution and whether can affect the production of electricity • They need large are of land to produce more efficient power supply PH 0101 Unit-5 Lecture-2
WIND POWER • What is it? • How does it work? • Efficiency
WIND POWER - What is it? • All renewable energy (except tidal and geothermal power), ultimately comes from the sun • The earth receives 2 x 1017 watts of power (per hour) from the sun • About 2 percent of this energy is converted to wind energy • Differential heating of the earth’s surface and atmosphere induces vertical and horizontal air currents that are affected by the earth’s rotation and contours of the land WIND. ~ e.g.: Land Sea Breeze Cycle
Wind is slowed by the surface roughness and obstacles. • A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. • The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed. • The kinetic energy of a moving body is proportional to its weight. In other words, the "heavier" the air, the more energy is received by the turbine.
Wind Turbines LARGE TURBINES: • Able to deliver electricity at lower cost than smaller turbines, because foundation costs, planning costs, etc. are independent of size. • Well-suited for offshore wind plants. • In areas where it is difficult to find sites, one large turbine on a tall tower uses the wind extremely efficiently.
SMALL TURBINES: • Local electrical grids may not be able to handle the large electrical output from a large turbine, so smaller turbines may be more suitable. • High costs for foundations for large turbines may not be economical in some areas. • Landscape considerations
Wind Turbines: Number of Blades • Most common design is the three-bladed turbine. The most important reason is the stability of the turbine. A rotor with an odd number of rotor blades (and at least three blades) can be considered to be similar to a disc when calculating the dynamic properties of the machine. • A rotor with an even number of blades will give stability problems for a machine with a stiff structure.
Wind Turbine Generators • Wind power generators convert wind energy (mechanical energy) to electrical energy. • The generator is attached at one end to the wind turbine, which provides the mechanical energy. • At the other end, the generator is connected to the electrical grid. • The generator needs to have a cooling system to make sure there is no overheating.
Power of Wind *No other factor is more important to the amount of power available in the wind than the speed of the wind The power in wind is proportional to the cubic wind speed ( v^3 ). 20% increase in wind speed means 73% more power Doubling wind speed means 8 times more power WHY? ~ Kinetic energy of an air mass is proportional to v^2 ~ Amount of air mass moving past a given point is proportional to wind velocity (v)
Calculation of Wind Power • Power in the wind Effect of air density, • Effect of swept area, A • Effect of wind speed, V Power in the Wind = ½ρAV3 R Swept Area: A = πR2 Area of the circle swept by the rotor (m2).
Environmental benefits • No emissions • No fuel needed • Distributed power • Remote locations
Limitations of Wind Power • Power density is very low. • Needs a very large number of wind mills to produce modest amounts of power. • Cost. • Environmental costs. • material and maintenance costs. • Noise, birds and appearance. • Cannot meet large scale and transportation energy needs.
The Future of Wind Energy • Future of wind energy can be bright if government policies subsidize and encourage its use. • Technology improvements unlikely to have a major impact. • Can become cost competitive for electricity generation if fossil energy costs skyrocket.
Ocean Energy • Thermal energy-OTEC(Ocean Thermal Electric Conversion) • Mechanical energy From waves From tides
Wave Facts: Mechanical energy-From waves • Waves are caused by a number of forces, i.e. wind, gravitational pull from the sun and moon, changes in atmospheric pressure, earthquakes etc. Waves created by wind are the most common waves. Unequal heating of the Earth’s surface generates wind, and wind blowing over water generates waves. • Wave energy is an irregular and oscillating low-frequency energy source that must be converted to a 50-Hertz frequency before it can be added to the electric utility grid.
Three Basic Kinds of Systems • Offshore (so your dealing with swell energy not breaking waves) • Near Shore (maximum wave amplitude) • Embedded devices (built into shoreline to receive breaking wave – but energy loss is occurring while the wave is breaking)
3 basic systems for ocean wave energy devices • 1. Channel systems that funnel waves into reservoirs • 2. Float systems that drive hydraulic pumps • 3. Oscillating water column systems that use waves to compress air within a container • mechanical power either directly activates a generator, or transfers to a working fluid, water or air, which then drives a turbine/generator
Wave Power Designs • Wave Surge or Focusing Devices-Channel SystemThese shoreline devices, also called "tapered channel" systems, rely on a shore-mounted structure to channel and concentrate the waves, driving them into an elevated reservoir. These focusing surge devices are sizable barriers that channel large waves to increase wave height for redirection into elevated reservoirs.
Floats or Pitching DevicesThese devices generate electricity from the bobbing or pitching action of a floating object. The object can be mounted to a floating raft or to a device fixed on the ocean floor.
Oscillating Water Columns (OWC)These devices generate electricity from the wave-driven rise and fall of water in a cylindrical shaft. The rising and falling water column drives air into and out of the top of the shaft, powering an air-driven turbine. 17-42
-Advantages and Disadvantages- • Advantages • The energy is free – no fuel needed, no waste produced • Not expensive to operate and maintain • Can produce a great deal of energy • Disadvantages • Depends on the waves – sometimes you’ll get loads of energy, sometimes almost nothing • Needs a suitable site, where waves are consistently strong • Some designs are noisy. But then again, so are waves, so any noise is unlikely to be a problem • Must be able to withstand
Tidal Power • Tidal power generators derive their energy from movement of the tides. • Has potential for generation of very large amounts of electricity, or can be used in smaller scale.
Tides • The interaction of the Moon and the Earth results in the oceans bulging out towards the Moon (Lunar Tide). The sun’s gravitational field pulls as well (Solar Tide) • As the Sun and Moon are not in fixed positions in the celestial sphere, but change position with respect to each other, their influence on the tidal range (difference between low and high tide) is also effected. • If the Moon and the Sun are in the same plane as the Earth, the tidal range is the superposition of the range due to the lunar and solar tides. This results in the maximum tidal range (spring tides). If they are at right angles to each other, lower tidal differences are experienced resulting in neap tides.
How do tides changing = Electricity? • As usual, the electricity is provided by spinning turbines. • Two types of tidal energy can be extracted: • kinetic energy of currents between ebbing (tide going out) and surging tides(tide coming in) • and • potential energy from the difference in height (or head) between high and low tides. • The potential energy contained in a volume of water is E = xMg where x is the height of the tide, M is the mass of water and g is the acceleration due to gravity.
1.) Tidal Barrage Two types: Single basin system Double-basin system Utilize potential energy Tidal barrages are typically dams built across an estuary or bay. consist of turbines, sluice gates, embankments, and ship locks. Basin
Single basin system- Ebb generation: During flood tide basin is filled and sluice gates are closed , trapping water. Gates are kept closed until the tide has ebbed sufficiently and thus turbines start spinning and generating electricity. Flood generation: The basin is filled through the turbine which generate at flood tide. Two way generation:Sluice gates and turbines are closed until near the end of the flood tide when water is allowed to flow through the turbines into the basin creating electricity. At the point where the hydrostatic head is insufficient for power generation the sluice gates are opened and kept open until high tide when they are closed. When the tide outside the barrage has dropped sufficiently water is allowed to flow out of the basin through the turbines again creating electricity.
Double-basin system • There are two basins, but it operates similar to en ebb generation, single-basin system. The only difference is a proportion of the electricity is used to pump water into the second basin allowing storage.