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OPTIMIZATION of

the GEOMETRY & MATERIALof

SOLAR WATER HEATERS.

OPTIMIZATION of

the GEOMETRY & MATERIALof

SOLAR WATER HEATERS.

FLAT PLATE COLLECTORSABSORBER PLATES

OPTIMIZATION OF GEOMETRYSELECTIVE SURFACES

METHODS OF TESTING TO DETERMINE THE THERMAL PERFORMANCE OF FLATE

PLATE COLLECTORSBASIC PERFORMANCE EQUATIONS

TESTING PROCEDURE

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• Flat Plate Collectors are the most common collector

types for residential water-heating.

• The “black” absorber surface transfers the

absorbed energy to the fluid.

•A typical flat-plate collector is an insulated metal box with aglass or plastic cover and a dark-colored absorber plate.•The envelopes transparent to the solar radiation,reduceconvection & radiation losses and back insulation to reduceconduction losses.

Cross Section of a Basic Flat Plate Solar CollectorCross Section of a Basic Flat Plate Solar Collector

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• Optimization of the Geometry & Material of solar water heaters.

• Produce an economic & efficient flat plate solar collector.

• Instead of Cu , Al absorber plate , Fe absorber will be considered.

• Efficiency test and comparison with other collector types.

• Offer a new efficient &cheaper collector.

*Fe is cheaper than the others*Galvanized iron is resistant to humidity & corrosion *Theoretically it is possible to produce cheaper collector absorber plates.

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Four parameters will be changed by using a computer program to find an efficient and

economic collector. Collector area will be kept the same as the

others.Absorber plate & tubes join together by point

welding.

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To galvanize the Fe, the absorber plate will be dipped into the zinc

pool and iron is covered with zinc.

GALVANIZATION PROCESS:

Assuming that materials have been chosen for the pipe and fin, the material cost of the collector

plate depends largely on the thickness of the fin and the spacing between pipes.

CHOICE OF THICKNESS AND PIPE SPACING

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Material costs will obviously be reduced if fins are thinner and spacing between pipes is greater.

However, this also leads to a reduction in the fin efficiency.

Obviously a compromise is required which will minimize the overall cost of a system for a given energy output.

To find the best combination should be maximum

Also collector should be efficient .priceQu /

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Selective Surface has high absorptance (αααα) & low emittance (εεεε) .Examples for selective surfaces :

Black Ni on polished Fe, Black Ni on Al,Black Cr on Cu,CuO on Al,

For Fe absorber plate before application of selective surface, galvanization has to be done.

Absorber surface & tubes will be galvanized together.

By using suitable selective surface the efficiency can be increased. At

least to the same efficiency with the others.

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After these processes, the efficiencies of three collectors having Fe,Cu,Al

absorbers will be compared .

Methods of Testing to Determine the Thermal Performance of Flat Plate

Collectors

ASHRAE Standard• This standard contains methods for

conducting tests outdoors under naturalsolar irradiation and provides test methodsand calculation procedures for determiningsteady state and quasi-steady state thermalperformance, time constants of solarcollectors.

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( )[ ]aiLcRu TTUSAFQ −−=

( )[ ]( ) 2/

2/tanhDWm

DWmF

−−=

δkUm L=

Collector Thermal Efficiency:

η = Qu / IT .Ac

collectorthebyerceptedoruponincidentenergysolarcollectedenergyusefulactual

int=η

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Experimental Determination of The Collector Time Constant:

• The first performance test to be conductedon the solar collector is the determination ofits “time constant”.

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The inlet temperature of the transfer fluid, tf,i , isadjusted to within ±±±±1°°°°C of the ambienttemperature while circulating the transferfluid, water,through the collector at the flow ratespecified, and maintaining steady state or quasy-steady state conditions with an incident solar fluxof greater than 790 W/m2 , the incident solarenergy is then abruptly reduced to zero byshielding the collector from the sun.

Method :

•This may be accomplished mostappropriately by shading with a white,opaque cover.

•The cover should be suspended of thesurface of the collector so that ambientair is allowed to pass over the collector asprior to the beginning of the transient test.

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• The temperature of the transfer fluid atthe inlet, tf,i and outlet, tf,e arecontinuously monitored as a function oftime until the quantity

30.0,,,

,,, <−

−

ifinitialef

ifTef

tt

tt

tf,e,T ; temperature of the transfer fluid leaving thecollector at a specified time.tf,e,initial ;temperature of the transfer fluid leaving thecollector area at the beginning of a specified time period.

Experimental Determination of the Collector Thermal Efficiency:

• The testing of the solar collector todetermine its thermal efficiency isconducted in such a way that a governing“efficiency curve” for near normalincidence is determined for the collectorunder test conditions.

• At least four different values of inlet fluidtemperature are used to obtain the valuesof ∆∆∆∆t/It.

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Experimental Determination of the Collector Thermal Efficiency:

Instantaneous Efficiency

FR (τα)n and FRUL are two parameters that describe how the collector works.

(α τ) :transmittance absorptance product of the absorber-plate couple.IT : incident radiationFR (τα)n : an indication of how energy is absorbedFRUL : an indication of how energy is lostTi : inlet temperatureTa :ambient temperature

T

aiLRnR

Tc

ui I

TTUFF

IAQ )(

)(−−== ατη

( )[ ]aiLcRu TTUSAFQ −−=

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Experimental Collector Efficiency Data Measured for a Liquid Heating Flat- Selective Absorber

Slope : - FRUL

The intercept :FR(τατατατα)n

The Long –term performance of collector can be characterized by the intercept & slope

The efficiency curves will beestablished by data over a time periodequal to the 4 times the time constant .

The integrated value of energyobtained from the collector will bedivided into the integrated value ofincident solar energy to obtain theefficiency value for that test period.

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At least four data points are taken foreach value of tf,i ; two during the timeperiod preceeding solar noon and two inthe period following solar noon, thespecific periods being chosen so that thedata points represents times symmetrical tosolar noon.

This latter requirement is made so that anytransient effects that may be present will not biasthe test results when they are used for designpurposes.

• If the difference between inlet

temperature and ambient temperature

is big then the efficiency will be

small.

• If the difference between inlet and

ambient temperature is small then the

efficiency will be high.

T

aiLRnR

Tc

ui I

TTUFF

IAQ )(

)(−−== ατη

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In this study; Al , Cu and galvanized ironwere compared and it was observed thatthickness of the absorber plate, spacingbetween pipes, diameters of pipes are thedetermining factors for the efficiency of the

collectors.

RESULT :

We will find a good alternative of the flat plate collectorsfor collecting solar energy. It was observed that sincethe optimized efficiency values of galvanized iron is verynear to currently used materials ,cupper andaluminum.That shows the best alternative amongour elements is galvanized iron when its price isconsidered.

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In this way people may use solar energy in their houses by paying less.