2nd Mid Sem Part1 solar

8/10/2019 2nd Mid Sem Part1 solar

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SOLAR ENERGY

It is radiant light and heat from the sun harnessed using a range of ever-evolving technologiessuch as solar heating, solar photovoltaics, solar thermal electricity, solar architecture andartificial photosynthesis.

Applications of solar technology: Architecture and urban planning. Agriculture and horticulture. Transport. Solar thermal

o Water heating.o Heating, cooling and ventilation.o Water treatment.o Process heat.o Cooking.

Electricity production : There are two methods toconvert the solar energy into electrical energy.

1. Concentrated solar power.2. Photovoltaic cell.

A typical example of the concentrated solar powerschematic has been shown in the figure.

Concentrating Solar Power (CSP) plants

Solar thermal power plants generally use reflectors toconcentrate sunlight into a heat absorber. Such power

plants are known as Concentrating Solar Power(CSP) plants.

Concentrating solar power plants produce electric power by converting the sun's energy into high-temperature heat using various mirror configurations.The heat is then channeled through a conventionalgenerator. The plants consist of two parts, one thatcollects solar energy and converts it to heat, andanother that converts heat energy to electricity.



Types of CSP plants:

1. Parabolic Trough Systems: The sun's energy isconcentrated by parabolic curved, trough-shapedreflectors onto a receiver pipe running along theinside of the curved surface. This energy heats oilflowing through the pipe and the heat energy isthen used to generate electricity in a conventionalsteam generator.

2. Solar Power tower: A power tower is a largetower surrounded by tracking mirrors calledheliostats . These mirrors align themselves andfocus sunlight on the receiver at the top of tower,collected heat is transferred to a power station

below. This design reaches very hightemperatures. High temperatures are suitable forelectricity generation using conventional methodslike steam turbine or a direct high temperaturechemical reaction such as liquid salt. Byconcentrating sunlight current systems can get

better efficiency than simple solar cells. A largerarea can be covered by using relativelyinexpensive mirrors rather than using expensivesolar cells. Concentrated light can be redirected toa suitable location via optical fiber cable for suchuses as illuminating buildings.

3. Parabolic dish systems: Parabolic dish systemsconsist of a parabolic shaped point focusedconcentrator in the form of a dish that reflectssolar radiation onto a receiver mounted at thefocal point. These concentrators are mounted on astructure with a two-axis tracking system tofollow the sun. The collected heat is typicallyutilized directly by a heat engine mounted on thereceiver moving with the dish structure.

Photovoltaic cell: A photovoltaic cell is the basic device that converts solar radiation intoelectricity. A solar cell is basically an electrical current source, driven by a flux of radiation.



Construction overview

Circuit diagram

Working : It consists of a very thick n-type crystal covered by a thin n-type layer exposed to thesun light as shown in the figure. If a load is connected across the p-n terminals which areexposed to sunlight as shown, a photon generated current will flow through this external circuit.This current will be proportional to the number of electron-hole pairs generated, which in turndepends on the intensity of illumination. Thus, an illuminated p-n junction becomes a photo

voltaic cell with a positive terminal on the p side.A PV cell can be either circular in construction or square . Cells are arranged in a frame toform a module. Modules put together form a panel. Panels form an array. Photocells may haveefficiency in the range of 10 – 20% and can produce electrical energy of 1 – 2 kWh per sq. m perday in ordinary sunshine. Typically it produces a potential difference of about 0.5V and a currentdensity of about 200A per sq. m of cell area in full radiation of 1kW per sq.m. A typicalcommercial cell of 100sq.cm area, thus produces a current of 2A. It has a lifespan in excess ofabout 20 years.Series and Parallel combination of cells

Cells in Series: When

two identical cells areconnected in series,the short circuitcurrent of the systemwould remain same

but the open circuitvoltage would betwice.

Cells in parallel: When two cells are connected in parallel as shown in the following figure, theopen circuit voltage of the system would remain same as an open circuit voltage of a single cell,

but the short circuit current of the system would be twice as much as of a single cell.



Applications: The major uses of photovoltaic cell have been in space satellites, remote radio-communication booster stations and marine warning lights. These are also used for lighting,water pumping and medical refrigeration in remote areas. Solar powered vehicles and batterycharging are some of the recent applications of solar PV power.

Disadvantages :

1. Cost of solar cell is very high.2. Efficiency of solar cell is low. As solar radiation density is also low, a large area of solar

cell modules is required to generate sufficient useful power.3. Not a fully dependent system.

WIND ENERGY

Wind is a form of solar energy . Winds are caused by the uneven heating of the atmosphere bythe sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are

modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motionenergy, when "harvested" by modern wind turbines , can be used to generate electricity .

Mathematical Expression of Available Power The wind power is generated due to the movement of wind. The energy associated with suchmovement is the kinetic energy and is given by the following expression: Energy = KE = v

2, where ,m = Air mass in kg = Volume (m 3) × Density (kg/m 3) = Q × ρ,Q = Dischargev = Velocity of air mass in m/s

Hence, the expression for power can be derived as follows:

Power =



Here, = Rate of discharge (m 3/s) = A (m 2) • v (m/s), where

A = Area of cross section of blade movement

Use: - Wind power is the conversion of wind energy into a useful form of energy, such as usingwind turbines to produce electrical power, windmills for mechanical power, wind pumps forwater pumping or drainage, or sails to propel ships.Advantage: - Wind is a clean source of renewable energy that produces no air or water pollutionand since the wind is free, operational costs are nearly zero once a turbine is erected.

Classification of Wind-mills

Wind turbines are classified into two general types: Horizontal axis and Vertical axis. Ahorizontal axis machine has its blades rotating on an axis parallel to the ground. A vertical axismachine has its blades rotating on an axis perpendicular to the ground. There are a number ofavailable designs for both and each type has certain advantages and disadvantages. However,compared with the horizontal axis type, very few vertical axis machines are availablecommercially. Horizontal Axis : This is the most common wind turbine design. In addition to being parallel tothe ground, the axis of blade rotation is parallel to the wind flow. A tail vane is usually used tokeep the blades facing into the wind. Some very large wind turbines use a motor-drivenmechanism that turns the machine in response to a wind direction sensor mounted on the tower.Commonly found horizontal axis wind mills are aero-turbine mill with 35% efficiency and farmmills with 15% efficiency.Vertical Axis: Although vertical axiswind turbines have existed for centuries,they are not as common as theirhorizontal counterparts. The main reasonfor this is that they do not takeadvantage of the higher wind speeds athigher elevations above the ground aswell as horizontal axis turbines. The

basic vertical axis designs are the

Darrieus, which has curved blades andefficiency of 35%, the Giromill, whichhas straight blades, and efficiency of35%, and the Savonius, which usesscoops to catch the wind and theefficiency of 30%. A vertical axismachine need not be oriented with



respect to wind direction. Because theshaft is vertical, the transmission andgenerator can be mounted at groundlevel allowing easier servicing and alighter weight, lower cost tower.

Main Components of a wind-mill

Rotor and Blades: The portion of the wind turbine that collects energy from the wind is calledthe rotor. The rotor usually consists of two or more wooden, fiberglass or metal blades whichrotate about an axis (horizontal or vertical) at a rate determined by the wind speed.

Hub: The central solid portion of the rotor wheel is known as hub. All blades are attached to thehub. The mechanism of pitch angle control is also provided inside the hub.

Nacelle: The term nacelle is derived from the name for housing containing the engines of anaircraft. The rotor is attached to the nacelle, and mounted at the top of a tower. It contains rotor

brake, high speed shaft, gear box, controller and generator. Brakes are used to stop the rotor when power generation is not desired. It can also be

used for maintenance purpose.

Gear box is used to step up the shaft speed (RPM) to desired speed. Controller is used to provide necessary control action. Generator is used to generate electrical energy.

Yaw Control Mechanism: The mechanism to adjust the nacelle around the vertical axis to keepit facing the wind is provided at the base of the nacelle. Yaw motor and Yaw drive are used forthis purpose.



Tower: The tower on which a wind turbine is mounted is not just a support structure. It alsoraises the wind turbine so that its blades safely clear the ground and so it can reach the strongerwinds at higher elevations. Maximum tower height is optional in most cases, except wherezoning restrictions apply. The decision of what height tower to use will be based on the cost oftaller towers versus the value of the increase in energy production resulting from their use.Studies have shown that the added cost of increasing tower height is often justified by the added

power generated from the stronger winds.

Operating Characteristics of wind mills

All wind machines share certain operating characteristics, such as cut-in, rated and cut-out windspeeds.

Cut-in Speed: Cut-in speed is the minimum wind speed at which the blades will turn andgenerate usable power. This wind speed is typically between 10 and 16 km/h.

Rated Speed: The rated speed is the minimum wind speed at which the wind turbine willgenerate its designated rated power. Rated speed for most machines is in the range of 40 to 55km/h. At wind speeds between cut- in and rated, the power output from a wind turbine increasesas the wind increases.



Cut-out Speed: At very high wind speeds, typically between 72 and 128 km/h, most windturbines cease power generation and shut down. The wind speed at which shut down occurs iscalled the cut-out speed. Having a cut-out speed is a safety feature which protects the windturbine from damage. Shut down may occur in one of several ways.

Betz Limit : It is the flow of air over the blades and through the rotor area that makes a windturbine function. The wind turbine extracts energy by slowing the wind down. The theoreticalmaximum amount of energy in the wind that can be collected by a rotor is approximately 59%.This value is known as the Betz limit. If the blades were 100% efficient, a wind turbine wouldnot work because the air, having given up all its energy, would entirely stop. In practice, thecollection efficiency of a rotor is not as high as 59%.

TIDAL ENERGY

Introduction : The rise and fall of tides nearly twice a day which is associated with the rotationof earth every 24 hours in relation to both Sun and Moon forms the basis of power generation

from tides. In large portion of world tidal ranges are very low (say 1m or so) and, therefore, notsuitable for electric power generation. The tide amplitude may be considerable 18m to 21m have been noted near Magellan Strait and the shores of USA respectively are very useful for the purpose.

Site selection : The use of tides for electric power generationis practical favourable situated in sites where the geographyof an inlet or bay favours the construction of a large scalehydro electric power plant. For this a dam would have to be

built across the mouth of the bay. It will have large gates init and also low water head turbines installed in it.

Working with non reversible turbine : For storing water inthe storage basin, the gates are opened at the time of hightide and after storing water the gates are closed. After thetide has receded , there is a working water head between the

basin water and open sea and the water is allowed to flowback to the ocean through water turbines installed in thedam. Thus the tidal energy stored at height can be utilized to

drive a turbine coupled to an electric generator and thusgenerate electricity. It uses low head bulb type units. Thisarrangement is simple and economical but it is not usefuldue to variations in the output (the electric power generationis not continuous).



Working with reversibleturbine : By using reversiblewater turbine, the turbinecan run continuously bothduring high tide and lowtide. This system is costlier

but provides greaterflexibility and continuous

power output. This system isvery popular in France,Germany, UK, Canada andUSA.

Advantages of tidal power :

1. It is free from the problems of uprooting the people and disturbing the ecology balance.2. It is everlasting and not influenced by the changing mood of the nature such as failure of

the monsoon.3. No extra submerging of the land is involved.

Disadvantages of tidal power :

1. It is free from the pollution problem.2. Tidal power have uneven and discontinuous operation.

3. High initial capital required.4. Long construction period.

The drawback of uneven operation of tidal power plants however, can be overcome by operatingthe tidal power plant in conjunction with pumped storage plant or run of river power plant havinga storage reservoir. This system is however costly.

Potential in India : In India the potential for tidal energy exists mainly in Gulf of Kutch(Gujarat), Bhavnagar and Sunderban area of West Bengal. Total tidal power potential in India isestimated to be around 9000 MW. 900 MW Kutch tidal power project is being set up by the CEA(Central Electricity Authority) in Gujarat.

2nd Mid Sem Part1 solar

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