Flat plate converters

FLAT-PLATE COLLECTORS
S.RAGUL,S.RAGUL,
ASSISTANT PROFESSOR,ASSISTANT PROFESSOR,
DEPARTMENT OF EEE,DEPARTMENT OF EEE,
KNCETKNCET
kongunadu college of engineering & technologykongunadu college of engineering & technology

1. INTRODUCTION1. INTRODUCTION
Solar collectors are heat exchangers that useSolar collectors are heat exchangers that use
solar radiation to heat a working fluid, usuallysolar radiation to heat a working fluid, usually
liquid or air. They can be classified in threeliquid or air. They can be classified in three
groups:groups:
- Flat-plate collectors,- Flat-plate collectors,
- Evacuated-tube collectors- Evacuated-tube collectors
- Focusing collectors.- Focusing collectors.

 In flat-plate collectors there is no opticalIn flat-plate collectors there is no optical
concentration of sunlight and they are generallyconcentration of sunlight and they are generally
stationary . In addition to this their outlet temperaturestationary . In addition to this their outlet temperature
capability is below 100capability is below 100 °°CC
 However to reach higher temparatures evacuated-tubeHowever to reach higher temparatures evacuated-tube
collectors and focusing collectors are used.collectors and focusing collectors are used.
InIn
evacuated-tube collectors they use vacuun to reduceevacuated-tube collectors they use vacuun to reduce
heat lost and to protect the absorber coating fromheat lost and to protect the absorber coating from
deteration.By this way they can reach temperaturesdeteration.By this way they can reach temperatures
up to 140up to 140 °°C and they can collect both direct andC and they can collect both direct and
diffuse solar radiationdiffuse solar radiation
 And focusing collectors, they are not stable and theyAnd focusing collectors, they are not stable and they
follow the sun to get direct radiation; theycan notfollow the sun to get direct radiation; theycan notkongunadu college of engineering & technologykongunadu college of engineering & technology

2. FLAT-PLATE COLLECTORS2. FLAT-PLATE COLLECTORS
A flat plate collector is basicly a black surface that is placeA flat plate collector is basicly a black surface that is placedd atat
a convenient path of the sun.And a typical flat plate collectora convenient path of the sun.And a typical flat plate collector
is a metal box with a glass or plastic cover (called glazing) onis a metal box with a glass or plastic cover (called glazing) on
top and a dark-colored absorber plate on the bottom. The sidestop and a dark-colored absorber plate on the bottom. The sides
and bottom of the collector are usually insulated to minimizeand bottom of the collector are usually insulated to minimize
heat lossheat loss..
flat-plate collectorsflat-plate collectors

Components of a typical flat plate collector:Components of a typical flat plate collector:
 Absorber plate:Absorber plate:
It is usually made of copper,steel or plastic.TheIt is usually made of copper,steel or plastic.The
surface is covered with a flat black metarial of highsurface is covered with a flat black metarial of high
absorptance.If copper or steel is used it is possible toabsorptance.If copper or steel is used it is possible to
apply a selective coating that maximizes theapply a selective coating that maximizes the
absorptance of solar energy and minimizes theabsorptance of solar energy and minimizes the
radiation emitted by plate.radiation emitted by plate.
 Flow passages:Flow passages:
The flow passages conduct the working fluid throughThe flow passages conduct the working fluid through
the collector. If the working fluid is a liquid , the flowthe collector. If the working fluid is a liquid , the flow
passage is usually a tube that is attached to or is apassage is usually a tube that is attached to or is a
part of absorber plate. If the working fluid is air , thepart of absorber plate. If the working fluid is air , the
flow passage should be below the absorber plate toflow passage should be below the absorber plate to
minimize heat lssos.minimize heat lssos.

 Cover plate:
To reduce convective and radioactive heat losses
from the absorber , one or two transparent covers are
generally placed above the absorber plate. They
usually be made from glass or plastic.
 Insulation:
These are some materials such as fiberglass and they
are placed at the back and sides of the collector to
reduce heat losses.
 Enclosure:
A box that the collector is enclosed in holds the
components together, protect them from weather,
facilitates installation of the collector on a roof or
appropriate frame .

Here in figure 2.2 we can see components of flat plateHere in figure 2.2 we can see components of flat plate
collectors.collectors.
Figure 2.2 CrossFigure 2.2 Cross ssection of aection of a bbasicasic fflat-lat-pplatelate ssolarolar ccollector [ollector [44].].kongunadu college of engineering & technologykongunadu college of engineering & technology

A. Absorber plate & Flow passagesA. Absorber plate & Flow passages
Copper, which has high conductivity and is corrosion-resistant,Copper, which has high conductivity and is corrosion-resistant,
is the material for absorber plates, but because copper isis the material for absorber plates, but because copper is
expensive, steel is also widely used. For a copper plate 0.05expensive, steel is also widely used. For a copper plate 0.05
cm thick with 1.25-cm tubes spaced 15 cm apart in goodcm thick with 1.25-cm tubes spaced 15 cm apart in good
thermal contact with the copper, the fin efficiency is betterthermal contact with the copper, the fin efficiency is better
than 97 percent.than 97 percent.
The surface of the absorber plate determines how much of theThe surface of the absorber plate determines how much of the
incident solar radiation is absorbed and how much is emittedincident solar radiation is absorbed and how much is emitted
at a given temperature. Flat black paint which is widely usedat a given temperature. Flat black paint which is widely used
as a coating has an aas a coating has an abbsorb of about 95 percent for incidentsorb of about 95 percent for incident
shortwave solar radiation. It is durable and easy to applyshortwave solar radiation. It is durable and easy to apply ..

Here a table about matteHere a table about matterrs that absorber plate may be mades that absorber plate may be made
fromfrom
Table 2.1 Characteristics of absorptive coatings [1].Table 2.1 Characteristics of absorptive coatings [1].
MaterialMaterial AbsorptanceAbsorptance
((αα))
EmittanceEmittance
((εε))
Break downBreak down
temparaturetemparature
((°°C)C)
CommentsComments
Black siliconBlack silicon
paintpaint
0.86-0.940.86-0.94 0.83-0.890.83-0.89 350350 SliconeSlicone
binderbinder
Black siliconBlack silicon
paintpaint
0.90.9 0.50.5 Stable atStable at
highhigh
temperaturetemperature
Black copperBlack copper
over copperover copper
0.85-0.90.85-0.9 0.08-0.120.08-0.12 450450 PatinatesPatinates
with moisturewith moisture
BlackBlack
choromechorome
over nickelover nickel
0.92-0.940.92-0.94 0.07-0.120.07-0.12 450450 Stable at highStable at high
temperaturestemperatures

absorber plate and flow passagesabsorber plate and flow passages

B. Cover platesB. Cover plates
A cover plate for a collector should have a high transmittanceA cover plate for a collector should have a high transmittance
for solar radiation and should not detoriate with time. Thefor solar radiation and should not detoriate with time. The
material most commonly used is glass. A 0.32-cm thick sheetmaterial most commonly used is glass. A 0.32-cm thick sheet
of window glass ( iron content, 0.12 percent ) transmits 85of window glass ( iron content, 0.12 percent ) transmits 85
percent of solar energy at normal incidence. And all glass ispercent of solar energy at normal incidence. And all glass is
practically opaque to long-wavelength radiation emitted by thepractically opaque to long-wavelength radiation emitted by the
absorber plate.absorber plate.
Some plastic materials can be used for collector glazing.TheySome plastic materials can be used for collector glazing.They
are cheaper and lighter than glass and, because they can beare cheaper and lighter than glass and, because they can be
used in very thin sheets, they often have higher transmittance.used in very thin sheets, they often have higher transmittance.
However, they are not as durable as glass and they oftenHowever, they are not as durable as glass and they often
degrade with exposure to ultraviolet radiation or highdegrade with exposure to ultraviolet radiation or high
temperatures .temperatures .

Here a table about matters thatHere a table about matters that covercover plate may be made fromplate may be made from
TestTest PolyvinlyPolyvinly
floridefloride
PolyethylenePolyethylene
terephthataletterephthatalet
or polysteror polyster
PolycarbonatePolycarbonate FiberglassFiberglass
rein forcedrein forced
plasticsplastics
SolarSolar
Transmission, %Transmission, %
92-9492-94 8585 82-8982-89 77-9077-90
MaximuMaximu
operatingoperating
temperaturetemperature °° CC
110110 100100 120-135120-135 9595
ThermalThermal
ExpansionExpansion
CoefficientCoefficient
4343 2727 6868 32-4032-40
Thickness,Thickness,
mmmm
0.10.1 0.0250.025 3.23.2 1.01.0
Length ofLength of
life, yearslife, years
In 5 years 95%In 5 years 95%
retainsretains
44 7-207-20

CrossCross sssection of asection of a covercover ppart of aart of a fflat-lat-pplatelate ccollectorollector ..

C. Enclosure / InsulationC. Enclosure / Insulation
The collector enclosure is usually made from steel, aliminiumThe collector enclosure is usually made from steel, aliminium
or fiber glass.or fiber glass.And order to prevent heat from escaping throughAnd order to prevent heat from escaping through
the back of the collector a layer of insulation is placed behindthe back of the collector a layer of insulation is placed behind
the absorber platethe absorber plate..
MaterialMaterial Density Kg/m3Density Kg/m3 ThermalThermal
conductivity atconductivity at
9595 °°C (W/mK)C (W/mK)
TemperatureTemperature
limitslimits °°CC
Fiber glass withFiber glass with
organic binderorganic binder
1111 0.0590.059 175175
““ 1616 0.0500.050 175175
““ 2424 0.0450.045 175175
““ 4848 0.430.43 175175

insulation part.insulation part.
Cross Section of an Insulation Part of a Flat-Plate Collector .Cross Section of an Insulation Part of a Flat-Plate Collector .

3.3. PROPER ORPROPER ORIIENTATENTATIIONON andand
ANGLEANGLE ofof SOLAR COLLECTORSOLAR COLLECTOR
Flat plate collectorts are divided in three mainFlat plate collectorts are divided in three main
groups according to how they are oriented:groups according to how they are oriented:
Flat-plate collectors facing south at fixed tiltFlat-plate collectors facing south at fixed tilt
One-axis tracking flat-plate collectors with axisOne-axis tracking flat-plate collectors with axis
oriented north-southoriented north-south
Two-axis tracking flat-plate collectorsTwo-axis tracking flat-plate collectors

A. Flat-plate collectors facing southA. Flat-plate collectors facing south
at fixed tilt:at fixed tilt:
To optimize performance in the winter, the collector can beTo optimize performance in the winter, the collector can be
tilted 15 ° greater than the latitude; to optimize performance intilted 15 ° greater than the latitude; to optimize performance in
the summer, the collector can be tilted 15 ° less than thethe summer, the collector can be tilted 15 ° less than the
latitudelatitude ..

B. One-axis tracking flat-plate collectorsB. One-axis tracking flat-plate collectors
with axis oriented north-south:with axis oriented north-south:
These trackers pivot on their single axis to track the sun, facingThese trackers pivot on their single axis to track the sun, facing
east in the morning and west in the afternooneast in the morning and west in the afternoon as shown inas shown in
figure.figure.

C. Two-axis tracking flat-plateC. Two-axis tracking flat-plate
collectors:collectors:
Tracking the sun in both azimuth and elevation, theseTracking the sun in both azimuth and elevation, these
collectors keep the sun's rays normal to the collector surface ascollectors keep the sun's rays normal to the collector surface as
shown .shown .

REFERENCESREFERENCES
[1] Jan F. Kreider, Charles J. Hoogendoorn,[1] Jan F. Kreider, Charles J. Hoogendoorn,
Frank Kreith “ Solar Design “ HemisphereFrank Kreith “ Solar Design “ Hemisphere
Publishing Corporation, (1989), pp. 44-55.Publishing Corporation, (1989), pp. 44-55.
[2][2] http://www.flasolar.comhttp://www.flasolar.com
[3][3] http://www.solarnetrix.comhttp://www.solarnetrix.com
[4][4] http://www.solstice.crest.orghttp://www.solstice.crest.org
[5][5] http://www.rredc.nrel.govhttp://www.rredc.nrel.gov

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