Solar collectors nces

Solar COLLECTORS

ByAnu Singla

Associate ProfessorDepartment of EE

Chitkara University, Punjab

Solar COLLECTORS

ByAnu Singla

Associate ProfessorDepartment of EE

Chitkara University, Punjab

Solar Energy CollectorsSolar energy collector is a device which absorbs the incomingsolar radiation, converts it into heat, and transfers this heat to afluid (usually air, water, or oil) flowing through the collector.

The solar energy thus collected is carried from the circulating fluideither directly to the hot water or space conditioning equipment,or to a thermal energy storage tank from which can be drawn foruse at night and/or cloudy days.

Solar energy collector is a device which absorbs the incomingsolar radiation, converts it into heat, and transfers this heat to afluid (usually air, water, or oil) flowing through the collector.

The solar energy thus collected is carried from the circulating fluideither directly to the hot water or space conditioning equipment,or to a thermal energy storage tank from which can be drawn foruse at night and/or cloudy days.

Types of CollectorsSolar collectors can be classified according to their collecting characteristics, theway in which they are mounted and the type of transfer fluid they use.(a)Collecting characteristics: A non-concentrating or flat plate collector is one in

which the absorbing surface for solar radiations is essentially flat with nomeans for concentrating the incoming solar radiation.A concentrating or focusing collector is one, which usually contains reflectorsor employs other optical means to concentrate the energy falling on theaperture onto a heat exchanger of surface area smaller than the aperture.

(b) Mounting: A collector can be mounted to remain stationary, be adjustable asto tilt angle (measured from the horizontal) to follow the change in solardeclination or be designed to track the sun. Tracking is done by employingeither an equatorial mounting or an azimuth mounting, for the purpose ofincreasing the absorption of the daily solar irradiation.

(c) Types of fluid: A collector will usually use either a liquid or a gas as the transferfluid. The most common liquids are water or a water-ethylene glycol solution.The most common gas is air.

Solar collectors can be classified according to their collecting characteristics, theway in which they are mounted and the type of transfer fluid they use.(a)Collecting characteristics: A non-concentrating or flat plate collector is one in

which the absorbing surface for solar radiations is essentially flat with nomeans for concentrating the incoming solar radiation.A concentrating or focusing collector is one, which usually contains reflectorsor employs other optical means to concentrate the energy falling on theaperture onto a heat exchanger of surface area smaller than the aperture.

(b) Mounting: A collector can be mounted to remain stationary, be adjustable asto tilt angle (measured from the horizontal) to follow the change in solardeclination or be designed to track the sun. Tracking is done by employingeither an equatorial mounting or an azimuth mounting, for the purpose ofincreasing the absorption of the daily solar irradiation.

(c) Types of fluid: A collector will usually use either a liquid or a gas as the transferfluid. The most common liquids are water or a water-ethylene glycol solution.The most common gas is air.

Solar collectors are classified basically in two differentcategories:

Flat plate or non-concentrating collectors Concentrating or focusing collectors consisting of

• Cylindrical parabolic collector• Central receiver collector• Compound parabolic collector

Types of Collectors contd.Solar collectors are classified basically in two differentcategories:

Flat plate or non-concentrating collectors Concentrating or focusing collectors consisting of

• Cylindrical parabolic collector• Central receiver collector• Compound parabolic collector

Non concentrating collector has the same areafor intercepting and for absorbing solarradiation, whereasSun-tracking concentrating solar collectorusually has concave reflecting surfaces tointercept and focus the sun’s beam radiation toa smaller receiving area, thereby increasing theradiation flux.

Types of Collectors contd.

Non concentrating collector has the same areafor intercepting and for absorbing solarradiation, whereasSun-tracking concentrating solar collectorusually has concave reflecting surfaces tointercept and focus the sun’s beam radiation toa smaller receiving area, thereby increasing theradiation flux.

Flat Plate Collector (Non-ConcentratingCollector)A simple flat plate collector consists of an absorbersurface (usually a dark, thermally conducting surface); atrap for radiation losses from the absorber surface (suchas glass which transmits shorter wavelength solarradiation, but blocks the longer wavelength radiationfrom the absorber, a heat transfer medium such as air,water etc. and some thermal insulation behind theabsorber surface.Flat plate collectors are used typically for temperaturerequired up to 750C although higher temperature can beobtained from high efficiency collector.

A simple flat plate collector consists of an absorbersurface (usually a dark, thermally conducting surface); atrap for radiation losses from the absorber surface (suchas glass which transmits shorter wavelength solarradiation, but blocks the longer wavelength radiationfrom the absorber, a heat transfer medium such as air,water etc. and some thermal insulation behind theabsorber surface.Flat plate collectors are used typically for temperaturerequired up to 750C although higher temperature can beobtained from high efficiency collector.

Flat Plate Collector (Non-ConcentratingCollector) contd.These flat plate collectors are further classified into twobasic types, based on heat transfer fluid.

Liquid type: Where heat transfer fluid may be water,mixture of water antifreeze oil etc.Air type: Where heat transfer medium is air (used

mainly for drying and space heating requirements).

These flat plate collectors are further classified into twobasic types, based on heat transfer fluid.

Liquid type: Where heat transfer fluid may be water,mixture of water antifreeze oil etc.Air type: Where heat transfer medium is air (used

mainly for drying and space heating requirements).

Liquid-type Flat Plate Collector

In general it consists of:Glazing: One or more covers of transparent material like glass,

plastics, etc. Glazing may be left out for some lowtemperature applications.

Absorber: A plate with tubes or passages attached to it for thepassage of working fluid. The absorber is usuallypainted flat black or electroplated with a selectiveabsorber.

Header or manifolds: To facilitate the flow or heat transfer fluid.Insulation: To minimise heat loss from the back and the sides.Container: box or casing.

In general it consists of:Glazing: One or more covers of transparent material like glass,

plastics, etc. Glazing may be left out for some lowtemperature applications.

Absorber: A plate with tubes or passages attached to it for thepassage of working fluid. The absorber is usuallypainted flat black or electroplated with a selectiveabsorber.

Header or manifolds: To facilitate the flow or heat transfer fluid.Insulation: To minimise heat loss from the back and the sides.Container: box or casing.

Typical Liquid-type Flat Plate Collector

Air Type/ Space Collector

Air type collectors are more commonly used for agricultural dryingand space heating applications.Their basic advantages are low sensitivity to leakages and no needfor an additional heat exchanger for drying and space heatingapplications.However, because of the low heat capacity of the air and the lowconvection heat transfer coefficient between the absorber and theair, a larger heat transfer area and higher flow rates are needed.

Figure shows some common configurations of air heatingcollectors.Common absorber materials include corrugated Al or galvanisedsteel sheets, black metallic screens or simply any black paintedsurface.

Air type collectors are more commonly used for agricultural dryingand space heating applications.Their basic advantages are low sensitivity to leakages and no needfor an additional heat exchanger for drying and space heatingapplications.However, because of the low heat capacity of the air and the lowconvection heat transfer coefficient between the absorber and theair, a larger heat transfer area and higher flow rates are needed.

Figure shows some common configurations of air heatingcollectors.Common absorber materials include corrugated Al or galvanisedsteel sheets, black metallic screens or simply any black paintedsurface.

Some common configurations of airheating collectors

Unglazed, transpired solar air collector offers a low-costopportunity for some applications such as preheating of ventilationair and agricultural drying and curing.

Such collectors consist of perforated absorber sheets that areexposed to sun and through which air is drawn.

The perforated absorber sheets are attached to the vertical walls,which are exposed to the sun.

The most important components, whose properties determine theefficiency of solar thermal collectors, are glazing and absorber.

Air Type/ Space Collector contd.

Unglazed, transpired solar air collector offers a low-costopportunity for some applications such as preheating of ventilationair and agricultural drying and curing.

Such collectors consist of perforated absorber sheets that areexposed to sun and through which air is drawn.

The perforated absorber sheets are attached to the vertical walls,which are exposed to the sun.

The most important components, whose properties determine theefficiency of solar thermal collectors, are glazing and absorber.

Concentrating or Focusing Collectors

Focusing collector is a device to collect solar energy with highintensity of solar radiation on the energy-absorbing surface.A focusing collector is a special form of flat plate collector modifiedby introducing a reflecting (or refracting) surface (concentrator)between the solar radiation and the absorber.Focusing collectors can have radiation increase from low value of1.5 to 2, high values of the order of 10,000.Focusing collector comprises of receiver (absorber) andconcentrator.Concentrators can be reflectors or refractors, can be cylindrical orparabolic and can be continuous or segmented.Receivers can be convex, flat, cylindrical or concave and can becovered with glazing or uncovered.

Focusing collector is a device to collect solar energy with highintensity of solar radiation on the energy-absorbing surface.A focusing collector is a special form of flat plate collector modifiedby introducing a reflecting (or refracting) surface (concentrator)between the solar radiation and the absorber.Focusing collectors can have radiation increase from low value of1.5 to 2, high values of the order of 10,000.Focusing collector comprises of receiver (absorber) andconcentrator.Concentrators can be reflectors or refractors, can be cylindrical orparabolic and can be continuous or segmented.Receivers can be convex, flat, cylindrical or concave and can becovered with glazing or uncovered.

Schematic of concentrating/ focusingcollector

Concentrating or Focusing Collectorscontd.

There are wide variety of means for increasing the flux of radiationon receivers.They can be classified on the basis ofa) Lenses or reflectors.b) The types of mounting and orienting systemsc) By the concentration of the radiation they are able to accomplishd) By materials of construction, or by orientation.

Concentrator is a component used to increase the intensity ofenergy flux on a receiver. Concentration ratio (CR), it is the ratio ofthe quantity = Aa/Ar ,where Aa is the ratio of the area of the concentrator aperture andAr , is the energy absorbing area of the receiver.

It determines the effectiveness of the concentrator.

There are wide variety of means for increasing the flux of radiationon receivers.They can be classified on the basis ofa) Lenses or reflectors.b) The types of mounting and orienting systemsc) By the concentration of the radiation they are able to accomplishd) By materials of construction, or by orientation.

Concentrator is a component used to increase the intensity ofenergy flux on a receiver. Concentration ratio (CR), it is the ratio ofthe quantity = Aa/Ar ,where Aa is the ratio of the area of the concentrator aperture andAr , is the energy absorbing area of the receiver.

It determines the effectiveness of the concentrator.

Because of the apparent movement of the sun across the sky,conventional concentrating collectors must follow the sun’s dailymotion. There are two methods by which the sun’s motion can bereadily tracked.The first is the altazimuth method which requires the tracking

device to turn in both altitude and azimuth, i.e. when performedproperly, this method enables the concentrator to follow the sunexactly. Paraboloidal solar collectors generally use this system.The second one is the one-axis tracking in which the collector

tracks the sun in only one direction either from east to west orfrom north to south.Parabolic trough collectors (PTC) generally use this system.These systems require continuous and accurate adjustment tocompensate for the changes in the sun’s orientation.

Types of Concentrating or FocusingCollectorsBecause of the apparent movement of the sun across the sky,conventional concentrating collectors must follow the sun’s dailymotion. There are two methods by which the sun’s motion can bereadily tracked.The first is the altazimuth method which requires the tracking

device to turn in both altitude and azimuth, i.e. when performedproperly, this method enables the concentrator to follow the sunexactly. Paraboloidal solar collectors generally use this system.The second one is the one-axis tracking in which the collector

tracks the sun in only one direction either from east to west orfrom north to south.Parabolic trough collectors (PTC) generally use this system.These systems require continuous and accurate adjustment tocompensate for the changes in the sun’s orientation.

Types of Concentrating or FocusingCollectorsThere are different types of concentrating or focusingcollector depending upon the concentrator and receivergeometries.These are as follows: Cylindrical parabolic collector. Central receiver collector. Compound parabolic collector

There are different types of concentrating or focusingcollector depending upon the concentrator and receivergeometries.These are as follows: Cylindrical parabolic collector. Central receiver collector. Compound parabolic collector

Cylindrical Parabolic collector

1st Generation PVCrystalline Silicon

It is a system consisting of a paraboloid or a parabolicreflector and having receiver at its focal point.

The concentration ratios are very high and thereforecan be used where high temperatures are required.

In a cylindrical system, the concentration ratio islower than paraboloid counterparts.

In both the cases the receiver is placed at the focus,i.e. along the focal line in cylindrical parabolic systemand at the focus point in paraboloidal system.The parabolic geometry is given by the relationY2 = 4 a XWhere a = Semi Major Axis.


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It is a system consisting of a paraboloid or a parabolicreflector and having receiver at its focal point.

The concentration ratios are very high and thereforecan be used where high temperatures are required.

In a cylindrical system, the concentration ratio islower than paraboloid counterparts.

In both the cases the receiver is placed at the focus,i.e. along the focal line in cylindrical parabolic systemand at the focus point in paraboloidal system.The parabolic geometry is given by the relationY2 = 4 a XWhere a = Semi Major Axis.

Cylindrical Parabolic Collector Diagram

1st Generation PVCrystalline Silicon1st Generation PVCrystalline Silicon



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Basic geometry of paraboloid mirror




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Cylindrical Parabolic Collector




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Cylindrical Parabolic Collector contd.




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Central Receiver Collector


The concept of central receiver collector is simple.

In order to avoid the cost and heat losses in transportinga working fluid to a central location, use of sunlight itselfas the transfer medium is proposed.

To implement the concept, one needs a field of mirrorsprovided with the means of directing reflected sunlightto a central location, or a location at one edge of thefield of mirrors.




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The concept of central receiver collector is simple.

In order to avoid the cost and heat losses in transportinga working fluid to a central location, use of sunlight itselfas the transfer medium is proposed.

To implement the concept, one needs a field of mirrorsprovided with the means of directing reflected sunlightto a central location, or a location at one edge of thefield of mirrors.

Central Receiver Collector contd.


In the typical central receiver, the reflector is composed ofmany smaller mirrors each with its own heliostat to followthe sun.

The heliostats are generally located in the horizontal plane,but when the situation is favourable, can simply follow theexisting terrain.

The basic difference between a single mirror concentratorand the heliostat system is that the heliostat system has adilute mirror. This means that the entire surface within thesystem is not covered with mirror surface.

This diluteness is generally termed as the fill factor. A central receiver with a fill factor of about 40% means that

40% of the land area is covered by mirrors.




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In the typical central receiver, the reflector is composed ofmany smaller mirrors each with its own heliostat to followthe sun.

The heliostats are generally located in the horizontal plane,but when the situation is favourable, can simply follow theexisting terrain.

The basic difference between a single mirror concentratorand the heliostat system is that the heliostat system has adilute mirror. This means that the entire surface within thesystem is not covered with mirror surface.

This diluteness is generally termed as the fill factor. A central receiver with a fill factor of about 40% means that

40% of the land area is covered by mirrors.

Receiver Collector



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Compound Parabolic collector (CPC orWinston collector)It is possible to concentrate solar radiation by a factor of 10without diurnal tracking using this type of collector.A compound parabolic collector is shown in figure (in next slide) .It consists of two parabolic reflectors which funnel the radiationfrom aperture to absorber. The right and the left half belong todifferent parabolas (hence the name compound parabolicconcentrator).The axis of the right branch, for instance, makes an angle θc , withthe collector mid plane, and its focus is at A. At the end points Cand D, the slope is parallel to the collector mid-plane.



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It is possible to concentrate solar radiation by a factor of 10without diurnal tracking using this type of collector.A compound parabolic collector is shown in figure (in next slide) .It consists of two parabolic reflectors which funnel the radiationfrom aperture to absorber. The right and the left half belong todifferent parabolas (hence the name compound parabolicconcentrator).The axis of the right branch, for instance, makes an angle θc , withthe collector mid plane, and its focus is at A. At the end points Cand D, the slope is parallel to the collector mid-plane.

Compound Parabolic collector (CPC orWinston collector) contd.



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Compound Parabolic collector (CPC orWinston collector) contd.The compound parabolic collector developed by Winstonrepresents what may be called as the most ideal collector in thesense that, for a given field of view it achieves the maximumpossible concentration ratio given byCR = W / b =1 / sin θcwhere,W = entrance aperture.b = exit aperture, covered by the absorberθc = field of view (half angle)



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The compound parabolic collector developed by Winstonrepresents what may be called as the most ideal collector in thesense that, for a given field of view it achieves the maximumpossible concentration ratio given byCR = W / b =1 / sin θcwhere,W = entrance aperture.b = exit aperture, covered by the absorberθc = field of view (half angle)

Comparison of Flat Plate and FocusingCollectorsMain advantages of the focusing systems over flat plate type collectors: Reflecting surfaces requires less material and are structurally simpler than

flat-plate collectors. For a concentrator system the cost per unit area ofsolar collecting surface is therefore potentially less than that of the flat-plate collector.

The absorber area of a contractor system is smaller than that of a flat platesystem of same solar energy collection and the insolation intensity istherefore greater.

As it is found that in case of solar energy concentrating collector the energylost to the surrounding is less than that for flat plate collector and theinsolation on the absorber is more concentrated, the working fluid canattain higher temperatures in a concentrating system than that in a flatplate collector of the same solar energy collecting surface.

Little or no antifreeze is required to protect the absorber in concentratorsystems whereas the entire solar energy collection surface requiresantifreeze protection in a flat-plate collector.



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Main advantages of the focusing systems over flat plate type collectors: Reflecting surfaces requires less material and are structurally simpler than

flat-plate collectors. For a concentrator system the cost per unit area ofsolar collecting surface is therefore potentially less than that of the flat-plate collector.

The absorber area of a contractor system is smaller than that of a flat platesystem of same solar energy collection and the insolation intensity istherefore greater.

As it is found that in case of solar energy concentrating collector the energylost to the surrounding is less than that for flat plate collector and theinsolation on the absorber is more concentrated, the working fluid canattain higher temperatures in a concentrating system than that in a flatplate collector of the same solar energy collecting surface.

Little or no antifreeze is required to protect the absorber in concentratorsystems whereas the entire solar energy collection surface requiresantifreeze protection in a flat-plate collector.

The concentrating systems also have some disadvantages as given below:Out of the beam and diffuse solar radiation, components, only beam

component is collected in case of focusing collectors because diffusecomponent cannot be reflected and is thus lost.Additional requirement of maintenance particularly to retain the quality of

reflecting surface against dirt, weather, oxidation etc.Non-uniform flux on the absorber whereas flux in flat-plate collector is

uniform.Additional optical losses such as reflectance loss and the intercept loss, so

they introduce additional factors in energy balances.These disadvantages have restricted the utility of focusing collectors and nolong time practical applications of focusing collectors other than for furnacesare being made.

Disadvantages of Concentrating typeCollectorsThe concentrating systems also have some disadvantages as given below:Out of the beam and diffuse solar radiation, components, only beam

component is collected in case of focusing collectors because diffusecomponent cannot be reflected and is thus lost.Additional requirement of maintenance particularly to retain the quality of

reflecting surface against dirt, weather, oxidation etc.Non-uniform flux on the absorber whereas flux in flat-plate collector is

uniform.Additional optical losses such as reflectance loss and the intercept loss, so

they introduce additional factors in energy balances.These disadvantages have restricted the utility of focusing collectors and nolong time practical applications of focusing collectors other than for furnacesare being made.

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