Hydropower

Lecture 2

INTRODUCTION

i. Availability of water with sufficient head, and a suitable site

ii. The amount of power that can be developed depends on

the quantity of water available, the rate at which it is

available, the head, etc.

iii. Hydro-electric projects involve a large amount of civil

engineering construction work, Often the project is

multipurpose embracing irrigation, flood control & power

navigation.

HYDROLOGY

When considering the possibility of a hydro-electric project,the first requirement is to obtain data regarding the streamflow of water and to predict the yearly possible flow from thedata.

The water cycle in general consists of evaporation fromseas of oceans and/or other water surfaces on the earth dueto heat from the sun, the formation of moist air and cloudsthe circulation of air currents, the condensation of vapourand precipitation in the form of rain, hail or snow.

Determination of the amount of stream flow and itsvariation involves a study of hydrography (or hydrology)which deals with the occurrence and distribution of waterover and under the earth's surface.

STREAM FLOW: HYDROGRAPHS: FLOW DURATION CURVESAnalysis of long period data is desirable

The maximum flow helps in:-

• Estimating the floods and in designing the spillway.

• Estimating the stream flow helps in determining the capacity

of storage reservoir for equalizing the flow to a given

minimum,

• Knowing the maximum stream flow conditions, it is possible

to estimate the necessary capacity of a flood-control

reservoir for limiting the discharge to a pre-determined

maximum under all conditions except extreme floods.

For stream flow estimation it is necessary to:-

• The depth of the river

• Measurement of discharge in open channels

• The cross-sectional areas are found by measuring the widthand depth at suitably spaced points to show the shape ofthe river bed,

• Velocities measured by float or current meters

• The other method is

Q = CLh3/2

Where L = length of the weir in meters; h is the head inmeters and c = constant for a sharp crested weir = 1.85approximately.

STREAM FLOW: HYDROGRAPHS: FLOW DURATION CURVES (contd.)

Hydrographs

When a river discharge has been measured, a curve can beplotted showing discharge in m3/s against time in hours. Thecurve is known as a hydrograph.

• Hydrographs help in noting the extremes of flow morereadily than the inspection of tabular values of thedischarge. Hydrographs are essential in studies of the effectof storage on flow.

• A hydrograph shows the variation of flow with time. It willalso indicate the power available from the stream atdifferent times of the day or year

Flow duration curve

The flow duration curve is a very convenient form ofhydrograph for determining the available power at the site.

• Low water flow decides the primary power that can be

developed. The flow duration curve also shows the possibleheavy flood flow and the time during which it may occur.

• The data are useful in designing a spillway on the dam toallow flood water to escape from the reservoir, It is easy toconvert a hydro graph into a flow duration curve.

Example 1

Hydrograph

Flow rates available during different length of time

Flow duration curve

In practice, the flow data for a number of years are required

when selecting a suitable site for a hydro-electric project.

Hydrograph and flow duration curve for the various periods

are therefore determined.

If the head at which the discharge of water is available is

known, the possible power that can be developed from the

water can be determined. Thus, the flow duration curve can

be converted to a power duration curve with different scales.

The power P, is given by

P = (0.736/75) * Q x w x h x η; kilowatts

Where Q = discharge rate m3/s

W = density of water, 1000kg/m3

h = head, m and η = efficiency of the turbine and generator

Example The flow duration curve is shown in Figure above, the head is100 m, find the power that can be developed per m3/s if theefficiency of the set-hydraulic turbine and electric generator is90%.

Solution:-

The power per m3/s at a head of 100 m and an efficiency of 90% is P = 9.82 x 100 x 0.9 = 883.8 kW.

The scale for the flow duration curve is:

1 graph unit = 200 m3/sec

The scale for the power duration curve is:

1 graph unit = 200 x 883.8 /1000 = 176.76 MW

The average flow during the period is 575 m3/sec, and the corresponding average power is 575 x 883.8 kW or 508 MW.

Mass curve: storage

• Reservoirs are required to store the river water;

• Storage is useful for, power generation, irrigation, fishery and flood control.

EXAMPLE

The data for weekly flow at a particular site are as given onnext slide. Assume that the site is suitable for building thedam necessary to provide storage in the reservoir.

(a) Draw the mass curve for the period

(b) Calculate the size of reservoir

(c) Calculate the possible rate at which flow would be available after the reservoir had been built.

Solution:

To obtain the mass curve, the cumulative volume of waterthat can be stored week after week must be found. The rateof now is given in m3/sec. If the mean flow in the week isavailable at that rate, it will have a volume of 7 x day xnumber of m3/sec, or 7 day-second-meters. If the summationof the volume of water in m3 or day-second-meters weekafter week is plotted against weeks, the mass curve isobtained.

• The summation of the volume of water that can be stored isplotted against the number of weeks shown in next slide.This is the mass curve.

Capacity of reservoir = 55 x 103 day second meters.

• Drawing a tangent at the points of inflexions, A and B, the slope of the line AB is found.

In the diagram it is: HG/FG = 87.5 x 1000/4 x 7 = 3120 m3/s

• If the rate at which water is drawn is higher than this, thereservoir level goes down; if the rate is less, the reservoirbeing full, there will be an overflow.

Effect of storage on flow duration curve

INVESTIGATION OF SITE• Preliminary investigations are made when selecting the site

for a dam.

• The dam should be as close to the turbines as possible, andshould have maximum size of poundage and the shortestlength of conduit.

• The ideal site will be one where the dam will have the largestcatchment area to store water at a high head, and yet beeconomical in construction.

• Different surveys and analysis made to select the suitablesite.

When land is submerged, it may be necessary to relocate railways and

roads. Sufficient information should be available to enable the cost of the

dam and project to be estimated. After the general location has been

chosen. the exact positions or the various structures are fixed by

considering the following factors:-

1. Requirements of head, flow demands and storage capacity

2. Details of foundation conditions.

3. Availability and character of construction material.

4. Arrangement and, type of damn, intake, conduits, surge tank, power

house, tail-race location, construction equipment, camps, coffer dam,

construction of roads and railways.

5. Transport facilities and accessibility of site.

6. Cost of the project.

Hydropower plants Examples

Hydropower

Lecture 3

Number of Dams for Power Project

• An ideal power system on a river is one whichwould give the maximum uniform powertheoretically possible with the head and wateravailable. Of two systems each comprising a , Inumber of dams, the system with the smallernumber of dams is theoretically the better, for, if

• It is practical for each system to use all theavailable water, the system with fewer darns maywork at a greater uniform energy rate.

Definitions

Heel contact with the ground on the upstream side

Toe contact on the downstream side

Abutment Sides of the valley on which the structure of the dam rest

Galleries small rooms like structure left within the dam for checking

operations

Diversion tunnel Tunnels are constructed for diverting water before the

construction of dam This helps in keeping the river bed dry

Spillways It is the arrangement near the top to release the excess water of

the reservoir to downstream side

Sluice way An opening in the dam near the ground level, which is used to

clear the silt accumulation in the reservoir side

Head and Power

• The available gross head at a hydro-electric power plant is the

difference between the water level in the pond or reservoir behind the

dam and the water level in the river where the tailrace is located.

• When the level at the tailrace rises more rapidly than that in the pond

above the dam as the discharge increases, a backwater effect takes

place, and the net available head is reduced.

• For determination of power, the average head is used. The available

power is known from the area under the flow duration curve converted

to a power duration curve.

• The net or effective head on a hydro-electric plant is the gross head

minus all the losses above the entrance of the scroll case and below the

exit from the draft tube. It is the total net height of the water column

effective on the turbine runner when generating power.

TYPES OF DAMS

ACCORDING to the SIZE of the DAM

1. Large (Big) dam

2. Small dam

• International Commision on Large Dams, (ICOLD) assumes a dam as

big when its height is bigger than 15m.

• If the height of the dam is between 10m and 15m and matches the

following criteria, then ICOLD accepts the dam as big:

• If the crest length is bigger than 500m

• If the reservoir capacity is larger than 1 million m3

• If the flood discharge is more than 2000 m3/s

• If there are some difficulties in the construction of foundation

ACCORDING to HEIGHT of DAM

• High Dam or Large Dam• If the height of the dam is bigger than 100m

• Medium Dam• If the height of the dam is between 50m and 100m

• Low Dam or Small Dam• If the height of the dam is lower than 50m

ACCORDING to the STATICAL DESIGN of DAM BODY

• Gravity Dams

• Arch Dams

• Butress Dams

• Embankment Dams

• Composite Dams

GRAVITY DAMS

Gravity Dams use their triangular shape and the sheer

weight of their rock and concrete structure to hold back the

water in the reservoir.

e.g. Warsak, Gomal Zam

Dam and spillways

Type of dam Gravity

Impounds Kabul River

Height 76.2 m (250 ft)

Length 140.2 m (460 ft)

Spillways 9 floodgates

Spillway type Service, controlled

Spillway capacity 540 m3/s (19,070 cu ft/s)

Reservoir

Active capacity31,207,090 m3 (25,300 acre·ft) (design, now silted)

Surface area 10.3 km2 (4 sq mi)

Power station

Commission date 1960, 1981

Hydraulic head 144 m (472 ft)

Turbines4 x 40 MW, 2 x 41.5 MW Francis-type

Installed capacity 243 MW

Annual generation 1100 GWh

ARCH DAMS

Arch Dams utilize the strength of an arch to displace the load

of water behind it onto the rock walls that it is built into.

BUTRESS DAMS

Buttress Dams use multiple reinforced columns to support a

dam that has a relatively thin structure. Because of this,

these dams often use half as much concrete as gravity

dams

EMBANKMENT DAMS (Rock Fill or Earth Fill Dams)

• They are mostly composed of natural materials such as, clay, sand, gravel etc...

• Impervious core is placed in the middle of the embankment body

• Generally riprap is used to control erosion

COMPOSITE DAMS

• Composite dams are combinations of one or more dam types. Most often a large section of a dam will be either an embankment or gravity dam, with the section responsible for power generation being a buttress or arch. NJHPP

Gravity & Rock Fill

TYPES OF TURBINE AND THEIR CHARACTERISTICS

• Hydraulic turbines are simple in construction, efficient andeasily controllable;

• they have the ability to work as either peak-load or standbyplant; they can start from cold and pick up full load in a veryshort time;

• they can work on load variations and can run for weekswithout much attention.

• These advantages make hydraulic turbines very useful asprime movers in power stations. The main types of turbineused in hydro-electric power stations are,

i. reaction andii. impulse .

• In the reaction turbine, the water passages are completelyfilled with water, the water acting on the wheel vanes is underpressure greater than atmospheric pressure, the water entersall round the periphery of the wheel, and the energy in the formof both pressure and kinetic energy, is utilized by the wheel.

• In the impulse or tangential wheel, the wheel passages arenot completely filled, the water acting on the wheels is atatmospheric pressure and is supplied at a few points at theperiphery of the wheel (one, two or four) and kinetic energy issupplied to the wheel. With reference to the direction of flow ofwater, reaction turbines are divided into the following types:

Radial flow inward:

• The runner receives water under pressure in a radially inwarddirection and discharges it in a substantially axial direction.These are known as Francis turbines. A typical section andplan is shown in Figure 2

Axial flow:

• The runner vanes of axial-flow turbines are either fixed oradjustable. These are propeller-type turbines and those withadjustable blades are known as Kaplan turbines.

• The fixed vane type is preferred when the head and flow aresubstantially constant and where base-load operation ispossible.

• The adjustable vane-type is preferred where the head andflow vary over a very wide range and the station is subjected tovariable load operation.

• The impulse wheel or turbine is generally a Pelton wheel. Therunner consists of a single casting containing the central discand the buckets. The runner is of cast steel. In some cases thebuckets are cast separately and attached by bolts and lugs tothe wheel hub. For runners, sometimes stainless steel with12% to 14% chromium with or without 1% nickel is used. Thespear and nozzle arc made of stainless steel or specialbronze, depending on the head and the quality of the water.The spear tip and nozzle control the jet of water impinging onthe buckets.

• The jet deflector sometimes used for control is made up ofstainless steel.

Characteristics of Turbines

Head at the turbine

• Reaction turbines of various types are used for heads up to500 m, the Francis type for heads of 70 to 500 m, and thepropeller type for heads less than 70 m.

• For fairly constant load operation, fixed-vane propeller turbinesare used.

• For heads lower than 30 m and for variable load operation,Kaplan or movable-vane turbines are used.

• For heads above 500 m up to about 1700 m, Pelton wheels with a single or multi-jet arrangement are used.

• The design head of a turbine is the head for which the runneris designed to run at the best speed and to operate at thehighest efficiency.

• Where the head on the turbine varies considerable from thenominal head, the lower head should be used: and wherethere is a small difference between the maximum andminimum heads not exceeding 10% above or below thenominal head, the higher head should be used.

• The maximum head under which a Francis turbine operatesshould not exceed 125% of the design head, and the minimumhead should not be less than 65% of the design head, in orderto facilitate stable operation and avoid excessive cavitation.

• The maximum head under which a fixed-blade propeller wheeloperates should not exceed 110% of the design head, and theminimum head should not be less than 90% of the designhead.

• For the adjustable-blade, or Kaplan turbine the rang ofoperation is from 50% to 150% of the design head. Generallythe design head is selected as the head, above and belowwhich the average annual generations of power areapproximately equal.

• This is thus, the point of best efficiency at the weightedaverage head. The rated head is that at which the full gateoutput of the turbine will produce the rated capacity of thegenerator in kilowatts. For run-off river plants havingapproximately constant heads, the design and rated heads areusually the same.

Efficiency of turbines at various loads

• Francis turbines are suitable for operation of plant from 75%load to full load.

• Propeller turbines with fixed-blade construction are used atfairly constant load between 75% and 100% of theircapacity, and if the variation of head is very mall, the units canbe operated at a point near their best efficiency.

• The Kaplan turbine efficiency curve shows that it is useful forfrequent part load operation. It is possible to obtainconsiderably better efficiency at part-gate openings withadjustable-blade turbines than with fixed-blade turbines.

An adjustable-blade unit is smaller than a fixed-blade unit ofthe same rating, and it will operate at a higher speed. Anadjustable-blade unit requires a smaller power house than afixed blade unit.

• The maximum efficiencies of Francis, fixed-blade propellerand Kaplan turbines are approximately 92% to 93%.

• The adjustable-blade Kaplan turbine is used efficiently atloads from 40% to 80% or higher, the efficiency curve withinthis range being practically flat.

Specific speed of turbine• The basis for comparison of the characteristics of hydraulic turbines is the

specific speed ns, or the speed at which a turbine would run if the runnerwere reduced to a size which would develop 1 metric. horsepower (h.p.)under 1 m head. This speed is proportional to the square root of the horsepower and inversely proportional to the 5/4 power of the head, i.e.

• In general, the higher the specific speed for a given head andhorsepower output, the lower is the cost of the installation asa whole.

• There are certain limitations of ns of a runner for a given headand horsepower. Too high a specific speed would reduce thedimensions of the turbine to such a size that the dischargevelocity of the water into the throat of the draft tube would berelatively high. This is undesirable because the energy of thedischarging water can be only partially regained in the drafttube. In the extreme case, a vacuum may be created.

• Too high specific speed with a high head would increase thecost of the turbine disproportionately, on account of the highmechanical strength required.

• Too Iow specific speed with a low head would increase thecost of the generator installation owing to the low turbinespeed.

• An increase in specific speed is usually accompanied by alower maximum efficiency and a greater depth of excavation forthe draft tube. When choosing a turbine, the incremental cost ofexcavation of the foundation should be considered in additionto the efficiency. Maximum efficiency may not be desirable; theweighted efficiency over the anticipated operating range ofoutput is usually a more important criterion. For an impulsewheel, the specific speed may be found for a single-jet turbine.If a greater output is required, a multi-jet turbine may be used.For a multi-jet turbine:-

The specific speeds of different types of turbine are different.

• For high-head plants, where impulse and Pelton wheelswould be used, the specific speed is low.

• For reaction turbines, the specific speed is higher. dependingon the head, and is higher in the order: Francis, fixed-blade andKaplan.

• The first step in the selection of a turbine for a particular plant isto choose a specific speed depending on the head available atthe plant and the power to be developed. With regard tostrength and cavitations considerations, certain specific speedsare suitable for certain heads. In a first rough estimate thefollowing empirical formulae are used:-

The speed of a turbine has to match the generator speed, which, to

give the correct frequency, is fixed by the number of pairs of poles. To

that extent, the specific speed as estimated may have to be modified.

Turbine setting

• Reaction turbines have to be set correctly with reference to the

tailrace water level so as to avoid cavitations in the draft-tube

portion. Thus, a Francis turbine runner should be placed at a level

very near or below the lowest tailrace level. Francis turbines adjust

themselves better to varying water levels in the storage reservoir,

and, therefore, use the storage to better advantage and with

improved mean efficiency.

• Fixed-blade propeller turbines have higher specific speeds than

Francis turbines and are therefore, set at a lower level.

• Adjustable blade turbines have still higher specific speeds, and

so, for the same head, the adjustable-blade runner would be set

considerably lower than the fixed-blade runner.

• A Pelton wheel is always set at a higher level than the highesttailrace level. This results in loss of head compared to theFrancis turbine, which makes use of the total available headon account of the draft tube. A Pelton wheel is set at least 2 mabove the highest tailrace water level.

• If the Francis and Pelton wheel types are compared for thesame head (common range if specific speed and head permit),the speed of the Francis turbine will always be higher than thatof the Pelton wheel, with corresponding reduction in the cost ofgenerators and dimensions of units, the size of the machinehall and power house equipment.

Speed and pressure regulation

• When the load on the generator coupled to theturbine changes suddenly, the governor regulate theinflow of water into the turbine, but this takes sometime and depends on the speed at which the governoracts.

• With a sudden closing of the gates there is apressure rise in the penstock. Which is counter-balanced by a suitable choice of surge tank installedat the end of the penstock line near the power station.

• Pressure-release openings or valves aresometimes provided on the pipelines at suitableplaces.

• The normal maximum pressure rise permissible with Francisturbines is of the order of 30%;

• with Pelton wheels it is between 10% and 20%. The governortime is generally 2 to 3 seconds, but in modern machines withautomatic operation this has now been reduced considerably.With the pressure regulator, the permissible range of pressurerise changes and is improved. The following are approximatepermissible pressure rises in different cases:

Runway Speed

• When a turbine is connected to a generator and the generatoris fully loaded, the speed of the turbine will initially fall, andowing to governor action the gates will open wide to admitmore water to suit the load condition and to bring the speed upto normal.

• In the event of the load suddenly being thrown off and thegovernor action failing to close the gates suddenly, to keep thespeed of the turbine constant, there is a tendency of theturbine to race.

• The maximum possible speed at which a turbine will run underthe worst conditions of operation with all gates open to allowall possible water inflow under maximum head is known as therunaway speed.

Runway Speed:

• The runaway speed of a water turbine is its speed at full flow, and no shaft load.

• The practical values of run away speeds for various turbines with respect to their rated speed N are as follows:

Pelton Wheel …1.8 to 1.9N

Francis turbine (mixed flow) …2.0 to 2.2N

Kaplan turbine (axial flow) …2.5 to 3.0N

• Specific Speed.

• Rotational Speed.

• Efficiency.

• Part load operation.

• Cavitation.

• Disposition of turbine Shaft.

• Head.

Selection of turbine

DESIGN OF THE MAIN DIMENSIONS OF TURBINES

• With reaction turbines, both Francis andpropeller types, the determination of the maindimensions of the turbines involves finding thenominal diameter, the outlet diameter or draft tubeinlet diameter, and the width of the distributorpertaining to the main runner. This has to beaccompanied by a determination of the preliminarydimensions of the draft tube and scroll case andthe setting of the turbine runner with reference tothe tailrace water level

Reversible Francis pump-turbine

In times of reduced energy demand, excesselectrical capacity in the grid (e.g. from wind turbines) may be used to pump water, previously used to generate power, back into an upper reservoir.This water will then be used to generate electricity when needed. This can be done by a reversible pump-turbine and an electrical generator-motor.

CAVITATION

• Cavitation occurs especially at spots where the pressure is low.

• In the case of a Kaplan turbine, the inlet of the runner is quite susceptible to it.

• At parts with a high water flow velocity cavitation might also arise.

• The major design criteria for blades is : Avoid Cavitation.

• First it decreases the efficiency and causes crackling noises.

• The main problem is the wear or rather the damage of the turbine’s parts such as the blades.

• Cavitation does not just destroy the parts, chemical properties are also lost.

(i) Runner/turbine may be kept under water.

(ii) Cavitation free runner may be designed.

(iii) By selecting materials that can resist better the cavitation effect.

(iv) By polishing the surfaces.

(v) By selecting a runner of proper specific speed for given load.

Methods to avoid cavitations:

The suction head

• The suction head Hs is the head where the turbine is installed;

• if the suction head is positive, the mean line of turbine is located above the trail water;

• if it is negative, the mean line of turbine is located under the trail water.

• To avoid cavitations, the range of the suction head is limited.

• The maximum allowed suction head can be calculated using the following equation:

netdevapatm

s Hg

V

g

ppH

2

2

net

des

gH

VN

25241.1

246.1

Lecture 4

Solar Energy

Two Main Categories:

Solar Thermal Solar Photovoltaic (PV)

Water heating and cooking Electricity production

Producing Electricity using Solar Energy

Solar Energy can be used to generate electricity in 2 ways:

Photovoltaic Solar Energy:

Using solar energy for the direct generation of

electricity using photovoltaic phenomenon.

Thermal Solar Energy:

Using solar energy for heating fluids

which can be used as a heat source or

to run turbines to generate electricity.

Photovoltaic Electricity

Photovoltaic comes from the words photo, meaning light, and volt,

a measurement of electricity.

Photovoltaic Electricity is obtained by

using photovoltaic system.

A basic photovoltaic system consists of four

components: Solar Panel, Battery, Regulator

and the load.

Solar Panel

Solar Panel is an indispensable component of this system.

Solar Panel is responsible to collect solar radiations and transform

it into electrical energy.

Solar Panel is an array of several solar cells (Photovoltaic cells).

The arrays can be formed by connecting them in parallel or series

connection depending upon the energy required.

Solar Panel Manufacturing Technologies

The most common solar technology is crystalline Si. Its two types

are: Mono- Si and Poly- Si.

Mono-Si: Crystal Lattice of entire

Sample is continuous.

Poly-Si: Composed of many crystallites

of varying size and orientation.

Solar Panel Manufacturing Technologies

Mono-Si Solar Panels:

Mono-Si is manufactured by Czochralski Process.

Since they are cut from single crystal, they gives the module a uniform

appearance.

Advantages:

Highest efficient module till now with efficiency

between 13 to 21%.

Commonly available in the market.

Greater heat resistance.

Acquire small area where ever placed.

Disadvantages:

More expensive to produce.

High amount of Silicon.

High embodied energy (total energy required to produce).

Si boule for the

production of wafers.

Solar Panel Manufacturing Technologies

Poly-Si Solar Panels:

Polycrystalline (or multicrystalline) modules are composed of a number of different

crystals, fused together to make a single cell.

Poly-Si solar panels have a non-uniform texture due to visible crystal grain present due to

manufacturing process.

Advantages:

Good efficiency between 14 to 16%.

Cost effective manufacture.

Commonly Available in the market.

Visible crystal grain in poly-Si

Solar Panel Manufacturing Technologies

Disadvantages:

Not as efficient as Mono-Si.

Large amount of Si.

High Embodied Energy.

Visible difference between Mono-Si and Poly-Si Panels:

Mono-Si solar cells are of dark color and the corners of the cells

are usually missing whereas poly-Si panels are of dark or

light blue color. The difference between the structure is only

due to their manufacturing process.

Mono-Si Panel

Poly-Si Panel

Solar Panel Manufacturing Technologies

Solar Panel Manufacturing Technologies

Thin Film Solar Panels:

Made by depositing one or more thin layers (thin film) of photovoltaic material on a

substrate.

Thin Film technology depend upon the type of material

used to dope the substrate.

Cadmium telluride (CdTe), copper indium gallium

selenide (CIGS) and amorphous silicon (A-Si) are three

thin-film technologies often used as outdoor photovoltaic

solar power production.

Amorphous-Si Panels:

Non-crystalline allotrope of Si with no definite arrangement

of atoms.

Advantages:

Partially shade tolerant

More effective in hotter climate

Uses less silicon - low embodied energy

No aluminum frame - low embodied energy

Disadvantages:

Less efficient with efficiency between 6 to 12% .

Less popular - harder to replace.

Takes up more space for same output .

New technology - less proven reliability.

Solar Panel Manufacturing Technologies

1/25/2013 Submitted by: Gourav Kumar

Comparison of Si on the basis of crystallinity

Comparison of Mono-Si, Poly-Si and Thin film Panels

Mono-Si Panels Poly-Si Panels Thin Film Panels

1. Most efficient with max.

efficiency of 21%.

1. Less efficient with

efficiency of 16% (max.)

1. Least efficient with max.

efficiency of 12%.

2. Manufactured from single

Si crystal.

2. Manufactured by fusing

different crystals of Si.

2. Manufactured by

depositing 1 or more layers

of PV material on substrate.

3. Performance best at

standard temperature.

3. Performance best at

moderately high temperature.

3. Performance best at high

temperatures.

4. Requires least area for a

given power.

4. Requires less area for a

given power.

4. Requires large area for a

given power.

5. Large amount of Si hence,

high embodied energy.

5. Large amount of Si hence,

high Embodied energy.

4. Low amount of Si used

hence, low embodied energy.

6. Performance degrades in

low-sunlight conditions.

6. Performance degrades in

low-sunlight conditions.

5. Performance less affected

by low-sunlight conditions.

7. Cost/watt: 1.589 USD 7. 1.418 USD 7. 0.67 USD

8. Largest Manufacturer:

Sunpower (USA)

8. Suntech (China) 8. First Solar (USA)

What is a Solar Cell?

• A structure that converts solar energy directly to DC electric energy.

It supplies a voltage and a current to a resistive load

(light, battery, motor).

Power = Current x Voltage=Current2 x R= Voltage2/R

• It is like a battery because it supplies DC power.

• It is not like a battery because the voltage

supplied by the cell changes with changes in the

resistance of the load.

Principle p-n Junction Diode:

The operation of a photovoltaic (PV) cell requires 3 basic attributes:

The absorption of light, generating either electron-hole pairs or

excitons.

The separation of charge carriers of opposite types.

The separate extraction of those carriers to an external circuit.

Silicon Solar cell

Ref. Soft Condensed Matter physics group in

univ. of Queenland

An individual PV cell typically produces 0.6 watts and are

joined in an array to produce the required power.

How a panel is created?

Panel wiring diagram connecting cells

Blocking Diodes When the sun shines, as long as the voltage produced by the panels is greater

than that of the battery, charging will take place.

However, in the dark, when no voltage is being produced by the panels, the

voltage of the battery would cause a current to flow in the opposite direction

through the panels, which can lead to the discharging of battery. Hence a

blocking diode is used in series with the panels and battery in reverse biasing.

Normal p-n junction diodes can be used as blocking diodes.

To select a blocking diode, following parameters should

be kept in mind:

i) The maximum current provided by the panels.

ii) The voltage ratings of the diode.

iii) The reverse breakdown voltage of the diode.

Hot- Spot and Bypass Diodes Hot Spot phenomenon happens when one or more cells of the panel is shaded while the

others are illuminated.

The shaded cells/panels starts behaving as a diode polarized in reverse direction and

generates reverse power. The other cells generate a

current that flows through the shaded cell and the load.

Any solar cell has its own critical power dissipation Pc

that must not be exceeded and depends on its cooling

and material structures, its area, its maximum operating

temperature and ambient temperature.

A shaded cell may be destroyed when its reverse

dissipation exceeds Pc. This is the hot spot.

To eliminate the hot-spot phenomenon, a bypass diode is

parallely connected to the module or group of cells in reverse polarity

which provides another path to the extra current.

Bypass Diodes working

When part of a PV module is shaded, the shaded cells will not be able to produce as much

current as the unshaded cells. Since all cells are connected in series, the same amount of

current must flow through every cell. The unshaded cells will force the shaded cells to pass

more current through it. The only way the shaded cells can operate at a current higher than

their short circuit current is to operate in a region of negative volt age i.e. to cause a net

voltage loss to the system.

The voltage across the shaded or low current

solar cell becomes greater than the forward bias

voltage of the other series cells which share the

same bypass diode plus the voltage of the bypass

diode thus making the diode to work in forward

bias and hence allowing extra current to pass

through it, preventing hot-spot. Bypass diode working phases

Bypass Diodes working For an efficient operation, there are two conditions to fulfill:

1. Bypass diode has to conduct when one cell is shadowed.

2. The shadowed cell voltage Vs must stay under its breakdown voltage (Vc).

Ideally, a bypass diode should have a forward voltage (VF) and a leakage current (IR) as

low as possible.

Two types of diodes are available as bypass diodes in solar panels and arrays:

1. p-n junction silicon diode

2. Schottky barrier diode

To select a bypass diode, following parameters should be

checked:

1. The forward voltage and current ratings of the diode.

2. The reverse breakdown voltage of the diode.

3. The reverse leakage current.

4. Junction Temperature Range

Solar Panel specifications

Mechanical Specifications:

1. Solar Cell Type: Defines the type of module or cell used in the module.

e.g.- Mono-Si, Poly-Si or Thin Film.

Design Implication: This determines the class of conversion efficiency of the module.

2. Cell Dimension (in inches/mm.): Defines the size of cell used in the module.

e.g.- 125(l) 125 mm(b) (5 inches).

Design Implication: This determines the output power of a single solar cell.

3. Module Dimension (in inches/mm.): Defines the size of the

panel. e.g.- 1580 (l) 808 (b) 35 (h) mm.

Design Implication: Determines the number of cells

Accommodated in the module.

Across length: 1580/125 = 12.64 ~ 12 [least integer].

Across breadth: 808/125 = 6.4 ~ 6.

This means number of cell be 72 (6*12).

Solar Panel specifications

Mechanical Specifications:

4. Module Weight (in kgs./lbs.): Defines the weight of the module.

e.g.- 15.5 kgs. (34.1 lbs.)

Design Implication: Determines the maximum number of panels which can be installed.

5. Glazing or front Glass: Defines the type and width of the front glass used.

e.g.- 3.2 mm (0.13 inches) tempered glass.

Design Implication: Width determines the strength of the covering. The type of glass

used depends upon thermal insulation requirements or strength requirement.

6. Frame: Defines the type of frame used in the module.

e.g.- Anodized aluminium alloy

Design Implication: Frame material is chosen so that it can

Withstand the environmental effects such as corrosion,

hard Impact, etc.

Solar Panel specifications

Mechanical Specifications:

7. Output Cables: Defines the type of cables and sometimes their dimensions provided at

output to connect with connector specifications.

e.g.- H+S RADOX® SMART cable 4.0 mm2 of length 1000 mm (39.4 inches) with

RADOX® SOLAR integrated twist locking connectors.

Design Implication: The rating of the cable is as per rating of the PV module and of

optimum length generally required by the customers.

8. Junction Box: Defines the protection level of electrical casing at the back of panel. Also

includes the no. of bypass diodes (if used).

e.g.- IP67 rated with 3 bypass diodes.

Solar Panel specifications

Electrical specifications:

1. Peak Power (W): Defines the maximum power of the panel.

e.g.- P: 195 W

Design Implication:

2. Optimum operating Voltage: Defines the highest operating voltage of panel at the

maximum power at STC.

e.g.- Vmp: 36.6V

Design Implication: Determines the number of panels required in series.

3. Optimum operating current: Defines the highest operating current of panel at the

maximum power at STC.

e.g.- Imp: 5.33A

Design Implication: Determines the wire gauge.

Used to calculate the voltage drops across the modules or cells.

Solar Panel specificationsElectrical Specs:

4. Open Circuit Voltage: Defines the output voltage when no load is connected under

STC.

e.g.- Voc : 45.4V

Design Implication: Determines the maximum possible voltage.

Determines the maximum number of modules in series.

5. Short Circuit Current: Defines the protection level of electrical casing at the back of

panel. Also includes the no. of bypass diodes (if used).

e.g.- Isc: 5.69A

Design Implication: Determines the current rating of fuse which is to be used for

protection.

Determines the conductor size.

General I-V curve

Solar Panel specifications

Electrical Specifications:

6. I-V Characteristics: Defines the current and voltage variation for the panel. Also shows

I-V characteristics for different irradiance.

e.g.-

Variation in I-V characteristics with Irradiance

Design Implication: This parameter determines the module current and voltage for a

particular value of irradiance.

This can be used to obtain the output voltage at the lowest irradiance for a region.

Solar Panel specifications Electrical Specifications:

7. Module Efficiency: Defines the conversion efficiency given by a given module (which is

generally lesser than the single solar cell used in the module).

e.g.- 15.3%

Design Implication: This parameter helps in solving the problem of choosing a module.

8. Operating Temperature: Defines the range of temperature for which the module can

function.

e.g.- -40 C to 85 C

Design Implication: Determines the temperature range for the environment in which the

panel can be kept.

9. Max. Series Fuse Rating: Defines the max. current which can be handled by the

module without damage.

e.g.- 15 A

Design Implication: This defines the rating of fuse to be used with the module.

Solar Panel specifications Electrical Specifications:

10.Power Tolerance: Defines the range of power deviation from its stated power ratings

due to change in its operating condition. It is defined in %.

e.g.- 0/+5 %

Design Implication: This parameter determines the upper limit for power of a module.

11. Parameters defined under NOCT: These parameters are same as defined under STC

conditions with different values.

Difference between STC and NOCT:

STC (Standard Test Conditions):

Irradiance 1000 W/m2, Module temperature 25 C, Air Mass=1.5

NOCT(Nominal Operating Cell Temperature):

Irradiance 800 W/m2, Ambient temperature 20 C, Wind speed 1 m/s

Solar Panel specifications Electrical Specifications:

12. Temperature Coefficients: These coefficients are defined to show the possible rate of

change of values under varying module temperature and irradiance.

Design Implication: These parameters can be

used to calculate the power, current and

voltage of the module.

Temperature Coefficient of Voc can also be

used to determine the maximum panel voltage

at the lowest expected temperature.

Packing Configuration:

Pieces per pallet: Number of modules per box.

Pallet per container: Number of boxes per container.

Pieces per container: Number of modules per container.

e.g.- Pieces per pallet (26) X Pallets per container (12)= Pieces per container (312)

How to choose a solar panel?

Critical parameters to be considered for solar panel evaluation:

1. Selecting the right technology : The selection of solar panel technology generally

depends on space available for installation and the overall cost of the system.

3. Selecting the right manufacturer for better warranty.

4. Check operating specifications beyond STC ratings

5. Negative Tolerance can lead to a lower system

performance and reduced capacity

6. Solar Panel efficiency under different conditions

and over time.

How to design a PV Off-grid system?1. Collect some data viz. Latitude of the location, and solar irradiance (one for every

month).

2. Calculation of total solar energy.

3. Estimate the required electrical energy on a monthly/weekly basis (in kwh):

Required Energy= Equipment Wattage X Usage Time.

4. Calculate the system size using the data from ‘worst month’ which can be as

follows:

a) The current requirement will decide the number of panels required.

b) The days of autonomy decides the storage

c) capacity of the system

i.e. the number of batteries required.

Batteries are often used in PV systems for the purpose of storing energy

produced by the PV array during the day, and to supply it to electrical loads as

needed (during the night and periods of cloudy weather).

Other reasons batteries are used in PV systems are to operate the PV array

near its maximum power point, to power electrical loads at stable voltages, and

to supply surge currents to electrical loads and inverters.

In most cases, a battery charge controller is used in these systems to protect

the battery from overcharge and over discharge.

In many stand-alone PV systems,

batteries are used for energy storage.

Figure shows a diagram of a typical

stand-alone PV system powering DC

and AC loads

Diagram of stand-alone PV system

with battery storage powering DC and AC loads.

Why Are Batteries Used in Some PV Systems?

How Are Photovoltaic Systems Classified?

Photovoltaic power systems are generally

classified according to:

• functional and operational requirements,

•component configurations,

• how the equipment is connected to other

power sources and electrical loads.

The two principle classifications are

grid-connected or utility-interactive

systems stand-alone systems.

Photovoltaic systems can be designed to

provide DC and/or AC power service, can

operate interconnected with or independent of

the utility grid, and can be connected with other

energy sources and energy storage

systems.1.7.1 Grid-Connected (Utility-

Interactive) PV Systems.

Diagram of grid-connected photovoltaic system

Types of PV Systems

Stand-alone PV systems

are designed to operate

independent of the electric

utility grid, and are

generally designed and

sized to supply certain DC

and/or AC electrical loads.

These types of systems

may be powered by a PV

array only, or may use

wind, an engine-generator

or utility power as an

auxiliary power source in

what is called a PV-hybrid

system.

photovoltaic hybrid system

The simplest type of stand-alone PV system is a direct-coupled system,

where the DC output of a PV module or array is directly connected to a DC load

Since there is no electrical energy storage (batteries) in direct-coupled systems,

the load only operates during sunlight hours, making these designs suitable for

common applications such as ventilation fans, water pumps, and small

circulation pumps for solar thermal water heating systems.

Matching the impedance of the electrical load to the maximum power output of

the PV array is a critical part of designing well-performing direct-coupled

system.

For certain loads such as positive-displacement water pumps, a type of

electronic DC-DC converter, called a maximum power point tracker (MPPT) is

used between the array and load to help better utilize the available array

maximum power output.

Direct-coupled PV system.