Solar Radiation Collectors

1. INTRODUCTION1. INTRODUCTION

Solar collectors are heat exchangers Solar collectors are heat exchangers that use solar radiation to heat a that use solar radiation to heat a working fluid, usually liquid or air. working fluid, usually liquid or air. They can be classified in three groups:They can be classified in three groups:

- - Flat-plate collectors, Flat-plate collectors, - Evacuated-tube collectors- Evacuated-tube collectors - Focusing collectors.- Focusing collectors.

Fixed Vs TrackingFixed Vs Tracking  

A A tracking collectors tracking collectors are controlled to follow the sun throughout are controlled to follow the sun throughout the day.the day.  A tacking system is rather complicated and A tacking system is rather complicated and generally only used generally only used for special high-temperature applicationsfor special high-temperature applications..  Fixed collectors are much simpler Fixed collectors are much simpler - their position or orientation, - their position or orientation, however, however, may be adjusted on a seasonal basismay be adjusted on a seasonal basis. They remain fixed . They remain fixed over a day’s timeover a day’s time  Fixed collector are less efficient than tracking collectorsFixed collector are less efficient than tracking collectors; ; nevertheless they are generally preferred as they are less costly to nevertheless they are generally preferred as they are less costly to buy and maintain.buy and maintain.  

In flat-plate collectors In flat-plate collectors there is no optical there is no optical concentration of sunlight and they are generally concentration of sunlight and they are generally stationary. In addition to this their outlet temperature stationary. In addition to this their outlet temperature capability is below 100 capability is below 100 °°CC

However to reach higher temparatures However to reach higher temparatures evacuated-tube evacuated-tube collectors and focusing collectorscollectors and focusing collectors are used. are used. In evacuated-tube In evacuated-tube collectors they use vacuun to reduce heat lost and to collectors they use vacuun to reduce heat lost and to protect the absorber coating from deteration.By this protect the absorber coating from deteration.By this way they can reach temperatures up to 140 way they can reach temperatures up to 140 °°C and C and they can collect both direct and diffuse solar radiationthey can collect both direct and diffuse solar radiation

And And focusing collectorsfocusing collectors, they are not stable and they , they are not stable and they follow the sun to get direct radiation; theycan not utilize follow the sun to get direct radiation; theycan not utilize diffuse radiation. And they are also capable of producing diffuse radiation. And they are also capable of producing high temperatures.high temperatures.

Flat-plate and ConcentratingFlat-plate and Concentrating

Concentrating collectors uses mirrored Concentrating collectors uses mirrored surfaces or lenses to focus the collected surfaces or lenses to focus the collected solar energy on smaller areas to obtain solar energy on smaller areas to obtain higher working temperatures. higher working temperatures. 

Flat-plate collectors may be used for water Flat-plate collectors may be used for water heating and most space-heating applications.heating and most space-heating applications.

Flat Plate Solar CollectorFlat Plate Solar Collector

A flat plate collector is basically a A flat plate collector is basically a black surfaceblack surface that is placethat is placed d at a convenient path of the sun. at a convenient path of the sun. And a typical flat plate collector And a typical flat plate collector is a metal box is a metal box with a glass or plastic cover (called glazing) on with a glass or plastic cover (called glazing) on top and a top and a dark-colored absorber plate on the dark-colored absorber plate on the bottombottom. The sides and bottom of the collector are . The sides and bottom of the collector are usually usually insulated insulated to minimize heat lossto minimize heat loss

Flat Plate Solar CollectorFlat Plate Solar Collector

Outer Glass Cover

Insulation Fluid Flow Tubes

Absorber Plate

Inner Glass Cover

Flat Plate Collector

Incident Solar Radiation ( tG )

Consists of an Consists of an absorber plateabsorber plate, , cover glasscover glass, , insulationinsulation and and housinghousing..

  

Flat Plate Solar Collector’s Flat Plate Solar Collector’s componentscomponents

• The The absorber plateabsorber plate is usually made of is usually made of copper & steel or plastic. To increase the copper & steel or plastic. To increase the absorption of solar radiation, the surface is absorption of solar radiation, the surface is covered with a flat black material of high covered with a flat black material of high absorptance and minimizes the radiation absorptance and minimizes the radiation emitted by plate.emitted by plate.

• The The cover glass or glassescover glass or glasses are used to are used to reduce convection and re-radiation losses reduce convection and re-radiation losses from the absorber.from the absorber.

• InsulationInsulation is used on the back edges of the is used on the back edges of the absorber plate to reduce conduction heat absorber plate to reduce conduction heat losses (material such as fiberglass)losses (material such as fiberglass)

• TheThe housing housing holds the absorber with holds the absorber with insulation on the back and edges, and cover insulation on the back and edges, and cover plates. plates. 

• Enclosure: Enclosure: A box that the collector is A box that the collector is enclosed in holds the components together, enclosed in holds the components together, protect them from weather,protect them from weather, facilitates facilitates installation of the collector on a roof or installation of the collector on a roof or appropriate frameappropriate frame

• The The working fluid working fluid (water, ethylene glycol, air (water, ethylene glycol, air etc.) is circulated in a serpentine fashion etc.) is circulated in a serpentine fashion through the absorber plate & carry the solar through the absorber plate & carry the solar energy to its point of use. energy to its point of use.

Outer Glass Cover

Insulation Fluid Flow Tubes

Absorber Plate

Inner Glass Cover

Flat Plate Collector

Incident Solar Radiation ( tG )

Cross Cross ssection of a ection of a bbasic asic fflat-lat-pplate late ssolar olar ccollector ollector

A view of flat plate collector

Flat Plate Collector

Flat Plate Collector components:

Plate with tubing Insulation Glazing

Characteristics of Flat Plate Characteristics of Flat Plate CollectorCollector

• Used for moderate temperature up to 100 CUsed for moderate temperature up to 100 C

• Uses both direct and diffuse radiationUses both direct and diffuse radiation

• Normally do not need tracking of sunNormally do not need tracking of sun

• Use: water heating, building heating and air-Use: water heating, building heating and air- conditioning, industrial process heating. conditioning, industrial process heating.

Absorber plate & Flow passagesAbsorber plate & Flow passages

Copper, which has high conductivity and is corrosion-resistant, is the material for absorber plates, but because copper is expensive, steel is also widely used. For a copper plate 0.05 cm thick with 1.25-cm tubes spaced 15 cm apart in good thermal contact with the copper, the fin efficiency is better than 97 percent.

The surface of the absorber plate determines how much of the incident solar radiation is absorbed and how much is emitted at a given temperature. Flat black paint which is widely used as a coating has an absorptance of about 95 percent for incident shortwave solar radiation. It is durable and easy to apply .

Characteristics of Characteristics of AAbsorptive coatingsbsorptive coatings

Material Material Absorptance Absorptance

(())

EmittanceEmittance

(() )

Break down Break down

temparature temparature ((°°C)C) CommentsComments

Black siliconBlack silicon

paint paint 0.86-0.94 0.86-0.94 0.83-0.89 0.83-0.89 350 350 Slicone binder Slicone binder

Black copper Black copper

over copperover copper 0.85-0.90.85-0.9 0.08-0.120.08-0.12 450450 --

Black chromBlack chromee over nickelover nickel

0.92-0.940.92-0.94 0.07-0.120.07-0.12 450450 Stable at high Stable at high

temperaturestemperatures

Cross Cross ssection of a ection of a absorber plateabsorber plate && flow passagesflow passages ofof a a fflatlat p plate late ccollectorollector

Cover platesCover plates

A cover plate for a collector should have a high A cover plate for a collector should have a high transmittance for solar radiation and should not transmittance for solar radiation and should not detoriate with time. The material most commonly used detoriate with time. The material most commonly used is glass. A 0.32-cm thick sheet of window glass (iron is glass. A 0.32-cm thick sheet of window glass (iron content, 0.12 percent ) transmits 85 percent of solar content, 0.12 percent ) transmits 85 percent of solar energy at normal incidence. And all glass is practically energy at normal incidence. And all glass is practically opaque to long-wavelength radiation emitted by the opaque to long-wavelength radiation emitted by the absorber plate.absorber plate.

Some plastic materials can be used for collector glazing.Some plastic materials can be used for collector glazing. They are cheaper and lighter than glass and, because They are cheaper and lighter than glass and, because they can be used in very thin sheets, they often have they can be used in very thin sheets, they often have higher transmittance. However, they are not as durable higher transmittance. However, they are not as durable as glass and they often degrade with exposure to as glass and they often degrade with exposure to ultraviolet radiation or high temperatures.ultraviolet radiation or high temperatures.

Characteristics of cover plate materials Characteristics of cover plate materials

Test Test PolyvinlyPolyvinly

floride floride

PolyethylenePolyethylene

terephthatalet terephthatalet or polysteror polyster

PolycarbonatePolycarbonate Fiberglass Fiberglass reressin forced in forced

plasticsplastics

Solar Solar Transmission, %Transmission, %

92-94 92-94 8585 82-8982-89 77-9077-90

MaximuMaximumm operating operating temperaturetemperature°° C C

110110 100100 120-135120-135 9595

Thermal Thermal Expansion Expansion CoefficientCoefficient

4343 2727 6868 32-4032-40

Thickness, mmThickness, mm 0.10.1 0.0250.025 3.23.2 1.01.0

Length of life, Length of life, years years

In 5 years In 5 years 95% retains95% retains

44 7-207-20

Enclosure / InsulationEnclosure / Insulation The collector enclosure is usually made from steel, aliminium or The collector enclosure is usually made from steel, aliminium or

fiber glass.fiber glass. And order to prevent heat from escaping through And order to prevent heat from escaping through the back of the collector, a layer of insulation is placed behind the back of the collector, a layer of insulation is placed behind the absorber plate.the absorber plate.

MaterialMaterial Density Kg/mDensity Kg/m33 Thermal Thermal

conductivity atconductivity at

95 95 °°C (W/mK)C (W/mK)

TemperatureTemperature

limits limits °°C C

Fiber glass with Fiber glass with organic binder organic binder

1111 0.0590.059 175175

“ “ 1616 0.0500.050 175175

“ “ 2424 0.0450.045 175175

““ 4848 0.430.43 175175

Characteristics of insulation Characteristics of insulation materials materials

Cross Section of an Insulation Part of a Flat-Plate CollectorCross Section of an Insulation Part of a Flat-Plate Collector

PROPER ORPROPER ORIIENTATENTATIIONON & ANGLE OF& ANGLE OF SOLAR COLLECTOR SOLAR COLLECTOR

Flat plate collectorts are divided in three Flat plate collectorts are divided in three mainmain groups according to how they are groups according to how they are oriented:oriented:

Flat-plate collectors facing south at Flat-plate collectors facing south at fixed tiltfixed tilt

One-axis tracking flat-plate collectors One-axis tracking flat-plate collectors with axis oriented north-southwith axis oriented north-south

Two-axis tracking flat-plate collectorsTwo-axis tracking flat-plate collectors

Flat-plate collectors facing south Flat-plate collectors facing south at fixed tilt :at fixed tilt :

To optimize performance in the winter, the collector To optimize performance in the winter, the collector can be tilted 15 ° greater than the latitude; to can be tilted 15 ° greater than the latitude; to optimize performance in the summer, the collector optimize performance in the summer, the collector can be tilted 15 ° less than the latitudecan be tilted 15 ° less than the latitude..

Flat-plate collector at fixed tilt Flat-plate collector at fixed tilt

One-axis tracking flat-plate One-axis tracking flat-plate collectors with axis oriented collectors with axis oriented

north-south:north-south: These trackers pivot on their single axis to track These trackers pivot on their single axis to track

the sun, facing east in the morning and west in the sun, facing east in the morning and west in the afternoon.the afternoon.

Flat-plate collector one axis trackingFlat-plate collector one axis tracking

Two-axis tracking flat-plate Two-axis tracking flat-plate collectors:collectors:

Tracking the sun in both azimuth and elevation, Tracking the sun in both azimuth and elevation, these collectors keep the sun's rays normal to the these collectors keep the sun's rays normal to the collector surface. collector surface.

Flat-plate collector with two axis trackingFlat-plate collector with two axis tracking

Collector PerformanceCollector Performance

x

y

x

T

)T(x

Temperature Distribution in the Absorber Plate

• The temperature of the working fluid in a flat-The temperature of the working fluid in a flat-plate collector may range from 30 to 90plate collector may range from 30 to 90ooC, C, depending on the type of collector and the depending on the type of collector and the application. application.

• The amount of solar irradiation reaching the The amount of solar irradiation reaching the top of the outside glazing will depend on the top of the outside glazing will depend on the location, orientation, and the tilt of the location, orientation, and the tilt of the collector.collector.

• Temperature of the absorber plate varies Temperature of the absorber plate varies along the plate with peak at the mid section along the plate with peak at the mid section

• Absorbed heat diffuses along the length Absorbed heat diffuses along the length towards the tube with and transferred to the towards the tube with and transferred to the circulating fluid. circulating fluid. 

Collector PerformanceCollector Performance• The collector efficiency of flat-plate The collector efficiency of flat-plate

collectors varies with design orientation, collectors varies with design orientation, time of day, and the temperature of the time of day, and the temperature of the working fluid.working fluid.

• The amount of useful energy collected will The amount of useful energy collected will also depend onalso depend on

- The optical properties (transmissivity - The optical properties (transmissivity and reflectivity) of cover glassesand reflectivity) of cover glasses

- The properties of the absorber plate - The properties of the absorber plate (absorptivity and emissivity) and(absorptivity and emissivity) and

- Losses by conduction, convection and - Losses by conduction, convection and radiation. radiation.

  

Outer Glass Cover

Insulation Fluid Flow Tubes

Absorber Plate

Inner Glass Cover

Flat Plate Collector

Incident Solar Radiation ( tG )

EEnergy diagram of typical flat plate collectornergy diagram of typical flat plate collector is shown is shown belowbelow. 92. 92%% of the total sunshine reaches to the copper absorber.of the total sunshine reaches to the copper absorber. 88%% of the total of the total sunshine is reflected from glass.sunshine is reflected from glass. 55%% of the sunshine is emitted from of the sunshine is emitted from the panel, 12the panel, 12%% is lost through convection and conduction. is lost through convection and conduction.

Energy diagram for typical flat plate collector Energy diagram for typical flat plate collector

COLLECTOR COLLECTOR PERFORMANCEPERFORMANCE

The thermal performance of a collector can be The thermal performance of a collector can be calculated from a first-law energy balance. according to calculated from a first-law energy balance. according to the first law of thermodynamics, for a simple flat-plate the first law of thermodynamics, for a simple flat-plate collector an instantaneous steady-state energy balance collector an instantaneous steady-state energy balance is: is:

Useful energy Useful energy gain (Qgain (QUU) ) = = EEnergy absorbednergy absorbed – – HHeat loss eat loss to to surroundingssurroundings

by the collectorby the collector

Absorbed Absorbed EEnergy = Anergy = ACC F FRR S S Lost Lost EEnergy = Anergy = ACC F FRR U ULL (Ti-Ta)(Ti-Ta)

QQUU = = AACC F FRR S S A ACC F FRR U ULL (Ti-Ta) (Ti-Ta) ………..(1)………..(1)

(It is useful gain energy equation)(It is useful gain energy equation)wwherehere: :

AACC = Collector area, = Collector area, mm2 2 ; ; FFRR = Heat removal factor, unit = Heat removal factor, unit lessless

S S = Absorbed solar radiation, J/= Absorbed solar radiation, J/mm2 2 ;; UULL = Heat transfer loss coefficient, = Heat transfer loss coefficient, J/J/mm22°°CC

Ti Ti = = MMean absorber plate tempean absorber plate temp.. °°CC ; ; Ta = Ta = AAmbient temperature, mbient temperature, °°C. C.

Absorbed radiation (S):Absorbed radiation (S):

In equation In equation ((11)) S is absorbed radiation and it is equal S is absorbed radiation and it is equal to:to:

……………………(2)(2)

In equation In equation ((22) ) are are the view factors from the collector to the sky and the view factors from the collector to the sky and from the collector to the ground, respectively. from the collector to the ground, respectively. The subscripts b,d, and g represent beam, The subscripts b,d, and g represent beam, diffuse, diffuse, and ground respectively.and ground respectively. i is transmittance s transmittance and absorptance product.and absorptance product. RRbb is the is the ratio of beam ratio of beam radiation on the tilted surface to that on a radiation on the tilted surface to that on a horizantal surface at any time.horizantal surface at any time.

)2/cos1(),2/cos1(

Collector heat removal factor (FCollector heat removal factor (FRR):):In equation In equation ((11)) F FRR is collector heat removal factor is collector heat removal factor ; ; a a quantity that relates the actual useful energy gain of quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector a collector to the useful gain if the whole collector surfaces were at the fluid inlet temperature. And it is surfaces were at the fluid inlet temperature. And it is given by equation given by equation ((33))..

wwherehere:: m’ = Fluid mass flow rate, kg/sm’ = Fluid mass flow rate, kg/s Cp = Fluid specific heat, J/kg Cp = Fluid specific heat, J/kg °°CC

The maximum The maximum possible useful energy gain (heat possible useful energy gain (heat transfer) in a solar collector occurs when the all transfer) in a solar collector occurs when the all whole collector is at the inlet fluid temperature; heat whole collector is at the inlet fluid temperature; heat losses to the surroudings are than at a minimum .losses to the surroudings are than at a minimum .

Overall heat loss coefficient (UOverall heat loss coefficient (ULL):):

In equation In equation ((11)) U ULL is the collector overall is the collector overall loss coefficient and it is equal to the sum loss coefficient and it is equal to the sum of the top, bottomof the top, bottom and edge loss and edge loss coefficientscoefficients..

UULL=U=Utoptop+U+Ubottombottom+U+Uedgeedge,, W/mW/m²²KK ……….(4)……….(4)

COLLECTOR COLLECTOR EFFICIENCYEFFICIENCY

The basic method of measuring collector The basic method of measuring collector performance is to expose the operating performance is to expose the operating collector to solar radiation and measure the collector to solar radiation and measure the fluid inlet and outlet temperatures and the fluid inlet and outlet temperatures and the fluid flow rate.fluid flow rate. The useful gain is The useful gain is ::

wwherehere::

m’ = Fluid mass flow rate, kg/sm’ = Fluid mass flow rate, kg/s

CCpp = Fluid specific heat, J/kg = Fluid specific heat, J/kg°°CC

)( 0 iPU TTCmQ

TThe thermal performance of a collector he thermal performance of a collector operating under steady conditions, can be operating under steady conditions, can be rewrittenrewritten as : as :

wwhere is a transmittance-absorptance here is a transmittance-absorptance product that is weighted according to the product that is weighted according to the proportions of beam, diffuse, and ground proportions of beam, diffuse, and ground reflected radiationreflected radiation (total incident radiation, (total incident radiation, IITT) ) on the collector on the collector..

aiLTRcu TTUIFAQ

And finallyAnd finally::

IInstantaneous efficiency can be defined asnstantaneous efficiency can be defined as::

That is;That is;

T

aiLRR

Tc

ui G

TTUFF

IA

Qn

Tc

ipi IA

TTCmn

0'

Efficiency vs. Temperature Efficiency vs. Temperature DifferentialDifferential

TTo improve theo improve the performance of solar performance of solar collector it is necesssary collector it is necesssary ““EEither to ither to reduce the overall energy loss coefficient reduce the overall energy loss coefficient or reduce area from which energy is or reduce area from which energy is lost.lost.””

TThe maximum possible useful energy he maximum possible useful energy gain (heat transfer) in a solar collector gain (heat transfer) in a solar collector occurs when occurs when ““TThe whole collector is at he whole collector is at the inlet fluid temperature; heat losses to the inlet fluid temperature; heat losses to the surroundings are then at a the surroundings are then at a minimum.minimum.””

FLATE PLATE FLATE PLATE COLLECTOR COLLECTOR

APPLICATIONSAPPLICATIONS

ApplicationsApplications

Water Heating (for domestic use and for swimming

pools)

Space Heating (Heating & Cooling of buildings)

Crop Drying

Food Cooking

Solar Distillation

Electricity generation

solar water heater

Laboratory model

Multi-residential application

Flat plate collectors used for heating buildingsFlat plate collectors used for heating buildings

Flat-plate collectors used for heating swimming pools Flat-plate collectors used for heating swimming pools

Solar Pool HeaterSolar Pool Heater

Working of low temperature Electricity GenerationWorking of low temperature Electricity Generation

CONCLUSIONCONCLUSION Flat-plate collectors which are used for water heating, Flat-plate collectors which are used for water heating,

are long lasting, and also in long term they are cheaper are long lasting, and also in long term they are cheaper

than other water heating systems.than other water heating systems. However,they requires However,they requires

large areas if high energy output is a requirement.large areas if high energy output is a requirement.

Than solar energy is free if we do not include the initial Than solar energy is free if we do not include the initial

cost for installation and the maintenance. cost for installation and the maintenance.

FinallyFinally,, bes besiides these we should remember by using des these we should remember by using

solar energy we can protect nature.solar energy we can protect nature.