parameters effecting solar lfpc performance

Parameters effecting solar LFPC performanceG UR UDATH G PR A B HUMT ECH I YEAR R ET

CONTENTS

• LFPC

• CONSTRUCTION

• VARIOUS PARAMETERS

• KEY INVESTIGATIONS

• CONCLUSION

Saturday, April 16, 2016 2

LFPC (Liquid Flat Plate Collector). Most common type of collector.

Simple in design and constructing.

Used for domestic water heating applications.

If the fluid is liquid then it is liquid flat plate collector.

Usually LFPC are stationery and they do not need any tracking.



VARIOUS PARTS OF LFPC AND THEIR CONSTRUCTION

Constructional Features

• Usually rectangular case, open at top side, insulation at sides and bottom.

• Heat transfer fluid flowing through the conduits( headers, tubes) which are attached over or underneath the absorber plate.

• There is transparent cover closing the face of LFPC.

• LFPC casing is usually made up of wood or aluminium.

• Insulation material is glass wool( due to its stability at high temperatures) and it is up to 25mm-100mm thick.

• Mineral wool, rock wool is also used as insulation material.

• Absorber generally is made of copper metal due to its high conductivity and is 0.2mm to 0.7mm thick.


Constructional Features(cont’d)

• aluminium , steel is also used as absorber material.

• Absorber is spray painted black, and heat treated to improve bonding with surface or coated with black metal oxides.(nickel oxide, chromium oxide).

• Conduits are same material that of absorber.

• Tubes are of diameter 10-15mm , they are brazed , soldered or pressure bonded with absorber plate.

• Headers are of 20-25mm diameter.

• Cover is low ferric oxide toughened glass.

• Glass glazing is 2-5mm thick, gap b/w absorber and cover is 15-30mm.


The thermal performance of a collector can be calculated from a first-law energy balance. according to the first law of thermodynamics, for a simple flat-plate collector an instantaneous steady-state energy balance is[1] :

Useful energy = energy absorbed – heat loss to

gain (Qu) by the collector surroundings

• Absorbed energy = AC FR S

• Lost energy = AC FR UL (Ti-Ta)

where ;

AC = Collector area, m2

FR = Heat removal factor, unitless

S = Absorbed solar radiation, J/m2

UL = Heat transfer loss coefficient, J/m2 °C

Ti = The mean absorber plate temperature, °C

Ta = The ambient temperature, °C.



Useful gain energy equation

aiLTRcu TTUGFAQ


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Tc

ui

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TTUFF

GA

Qn

Tc

ip

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0'

THREE LOSS COEFFCIENT

• Top loss coefficient( Covers, upper parts)

• Side loss coefficient( through sides)

• Bottom loss coefficient.(bottom such as insulation)

• TOP HEAT LOSS ACCOUNTS MORE THAN 70% OF TOTAL LOSS AND IS CONTAINS ABSORPTIVITY, EMISSIVITY, MATERIAL VARIANCE, WIND SPEED ETC .


TOP LOSS COEFFCIENT


As the wind loss coefficient increases, more amount of heat is dissipated to atmosphere and consequently lower efficiency can be expected.

BOTTOM LOSS COEFFCIENT


SIDE LOSS COEFFCIENT


PARAMETERS EFFECTING PERFORMANCE

Heat Transfer System

• Heat from the absorber plate is removed from fluid.

• Usually cool fluid enters and gets heated up and is passed.

• Usually thermosiphon principle is used, otherwise external pumping may be adopted.

• It also depends on type of fluid used.

• Some fluids used are: Ethanol, Methanol, water, Acetone etc.

• Medium inside the collector.

• Usually air, sometime inert gases are filled.


Selective Surfaces• Absorber plate surfaces which provides high absorptivity for

incoming solar radiation and low emissivity for outgoing solar radiation is termed as selective surface.

• Solar radiation lies short wavelength band up to 4micro meter, while the absorber plate the long wave radiation of wavelength 8.3 micro meter.

• No natural surface has this selective surface radiation characteristics.

• A successful selective surface can be developed by a thick metallic oxide base on a metal base.

• A good selective surface will have 0.95 absorptivity , 0.1 emissivity.


Selective Surfaces


Table : Characteristics of absorptive coatings

Number of covers

• To minimize the convection and radiation loss the solar collector is covered with a transparent glass sheet over a absorber plate.

• Solar radiation incident on glass passes through it and glass cover.

• Thus the glass sheet cover reduces the heat loss coefficient up to 10W/m^2 K.

• It is seen that with two glass covers the heat loss coefficient was still reduced to 4 W/m^2 K.


Number of covers


Table : Charactericts of cover plate materials

Transmissivity Absorptivity Product


S is absorbed radiation and it is equal to:

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

Equation 4.2 Absorbed solar radiation[6].

)2/cos1(),2/cos1(

Spacing

• Spacing between the absorber plate and the cover or between two covers also influence the performance.

• The operating performance varies with space as well as tilt and service conditions.

• There is no exact way to express spacing.

• The experiments have revealed that 4cm to 8cm will give good results.

• Large spacing reduces collector area requirements.


Glazing

• Low iron clear glass of 5 mm thickness is used as glazing panel.

• The experiment was conduction to check

Optimum glazing covers and space.

• 5 mm , 10 mm, 15mm space with 1, 2, 3

Glazing covers



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Geometric Parameters

• Spacing: Lower the gas conductivity, the smaller the volume required for maximum efficiency.

• Tube thickness: Improvement in efficiency when using 1.0 mm thick tubes instead of 0.5 mm because of the changed Reynolds number.

• Helical pipe collector gives higher temperature in hot water tank.


Materials Used for construction

• Absorber plate material : Low cost galvanized iron as absorber material can give efficiency comparable to copper.

• Tube and header material : Low cost coated stainless steel tube and

aluminium tube gives same performance as copper.

• Absorber coating material : Higher outlet temperature and higher

efficiency for black chrome followed by the matt black and sol

chrome.

• Working fluid : Acetone gives higher efficiency than methanol and ethanol. Water gives higher efficiency than methanol.


Other things

• Use of PCM : The temperature of water in PCM filled FPC is much higher.

• Porous aluminium absorbers : Maximum efficiency for the more porous and thicker absorber at the highest air mass flow rate, but at very low air flow rates the absorber porosity shows a reversed effect.


CONCLUSION

• MAJOR LOSSES OCCUR IN TOP LOSS.

• LOT OF SCOPE FOR REDUCING ABSORBER AREA.

• OPTIMUM GLAZING WILL REDUCE LOSSES.

• BEST INSULATION MATERIAL WILL REDUCE LOSSES.

• SELECTION OF RIGHT MATERIAL WILL GIVE US DRASTIC CHANGE IN EFFICIENCY.

• Solar energy is free if we do not include the initial cost for installation and the maintenance.


• PERFORMANCE STUDIES OF TUBULAR FLAT PLATE COLLECTORS"Md. SakhawatHusain International Centre for Theoretical Physics, Trieste, Italy.INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

• EVALUATION OF HEAT LOSS COEFFICIENTS IN SOLAR FLAT PLATE COLLECTORS Y. Raja Sekhar1, K. V. Sharma1 and M. Basaveswara Rao2

1Centre for Energy Studies, J. N. T. U. H. College of Engineering, Kukatpally, Hyderabad, India 2Department of Mechanical Engineering, S. V. I. T, Secunderabad, India

• RENEWABLE ENERGY SOURCES AND EMERGING TECHNOLOGIESBy D.P. KOTHARI, K. C. SINGAL, RAKESH RANJAN

• Recent investigations in solar flat plate collectors P A Kulkarni1, S P Sabnis2, R Sarangi3Mechanical, DBIT, Mumbai, India

REFERENCE
