DESIGN, FABRICATION AND TESTING OF

FLAT PLATE SOLAR THERMAL COLLECTOR

PRESENTED BY:BIPIN GUPTA

INTRODUCTION..

SOLAR THERMAL COLLECTOR..

A SOLAR THERMAL COLLECTORS are special kind of heat exchangers that transform solar radiation energy to internal energy of the transport medium.

The major component of any solar system is the solar collector.

SOLAR COLLECTORS:

Collector is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil)flowing through the collector.

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

TYPES OF SOLAR THERMALCOLLECTORS

Solar collectors are either Non-concentrating or Concentrating.

In the Non-concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).In these types the whole solar panel absorbs light.

Concentrating collectors have a bigger interceptor than absorber.

For domestic/ industrial water heating and space heating purposes the solar thermal collectors are classified as..

1. FLAT PLATE SOLAR COLLECTORS.AND

2. EVACUATED TUBE SOLAR COLLECTORS.

FLAT PLATE SOLAR COLLECTORFlat-plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type of solar collector which are widely used for domestic household hot-water heating and space heating, where the demand temperature is low.Flat plate collector is basically a black surface that is placed at a convenient path of a sun.

ETC’s are built to reduce convective and heat conduction

loss (vacuum is a heat insulator).

Each evacuated tube consists of two glass tubes.

The outer tube is made of extremely strong transparent glass

that is able to withstand changing climatic conditions.

The inner tube is also made of glass, but coated with a

special selective coating which features excellent solar heat

absorption and minimal heat reflection properties.

The air is evacuated from the space between the two glass

tubes to form a vacuum.

EVACUATED TUBE SOLAR THERMALCOLLECTOR

COMPONENTS OF SOLAR FLAT PLATE COLLECTOR

They consist of.. A dark flat-plate absorber. A transparent cover that reduces heat losses, called

GLAZING. A heat-transport fluid (air, antifreeze or water) to

remove heat from the absorber. A heat insulating backing Flow passage. Enclosure.

Absorber plate:It is usually made of copper , steel or plastic . The

surface is covered with a flat black material of high absorptance. If copper or steel is used it is possible to apply a selective coating that maximizes the absorptance of solar energy and minimizes the radiation emitted by plate.

Flow passages:The flow passages conduct the working fluid through

the collector. If the working fluid is a liquid , the flow passage is usually a tube that is attached to or is a part of absorber plate. If the working fluid is air , the flow passage should be below the absorber plate to minimize heat losses.

Cover plate:To reduce convective and radiative 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.

A typical diagrams of “ Flat plate solar thermal

collectors”

A A.Absorber 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 [1].

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 [1] .

Here a table about matters that absorber plate may be made from:

Table : Characteristics of absorptive coatings.[1]

Material Absorptance

()

Emittance

()

Break down

temparature

(°C)

Comments

Black silicon

paint

0.86-0.94 0.83-0.89 350 Slicone

binder

Black silicon

paint

0.9 0.5 300 Stable at

high

temperature

Black copper

over copper

0.85-0.9 0.08-0.12 450 Patinates

with moisture

Black

chorome

over nickel

0.92-0.94 0.07-0.12 450 Stable at high

temperatures

Here in figure we can see absorber plate and flow passages

Figure: Cross section of a absorber plate&flow passages of a flatplate solar collector [4] .

B. Cover plates A cover plate for a collector should have a high

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

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

Here a table about matters that cover plate may be made from:

Table: Charactericts of cover plate materials.[1]

Test Polyvinly

floride

Polyethylene

terephthatalet

or polyster

Polycarbonate Fiberglass

rein forced

plastics

Solar

Transmission, %92-94 85 82-89 77-90

Maximu

operating

temperature ° C

110 100 120-135 95

Thermal

Expansion

Coefficient

43 27 68 32-40

Thickness,

mm0.1 0.025 3.2 1.0

Length of

life, years

In 5 years 95%

retains4 4-5 7-20

Here in figure we can see cover part.

Figure: Cross section of a cover part of a flat-plate collector [4].

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

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

Table: Characteristics of insulation materials .[1]

Material Density Kg/m3 Thermal

conductivity at

95 °C (W/mK)

Temperature

limits °C

Fiber glass with

organic binder

11 0.059 175

“ 16 0.050 175

“ 24 0.045 175

“ 48 0.43 175

Here in figure we can see insulation part.

Figure: Cross Section of an Insulation Part of a Flat-Plate Collector [4].

WORKING PRINCIPLE OF FLAT PLATE COLLECTORS

In FLAT PLATE COLLECTORS Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. Thus the heat is transferred to the fluid {liquid (water or oil) or gas(air) } passing through pipes attached to the absorber plate by means of convective heat transfer.

• Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint.

• Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day's hot water use.

Working of flat plate collector

Flat plate collector Solar air heater

COLLECTOR PERFORMANCE 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

And,

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.

So,

QU = AC FR S - AC FR UL (Ti-Ta)

Above Equation is an extremely useful equation and

applies to essentialy all flat-plate collectors.

To improve the performance of solar collector it is necesssary either to reduce the overall energy loss

coefficient or reduce area from which energy is lost.

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

COLLECTOR EFFICIENCY

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

Equation: Energy gained by liquid.

Where;

m’ = Fluid mass flow rate, kg/s

Cp = Fluid specific heat, J/kg°C

)( 0 iPU TTCmQ

The above equation describes the thermal performance of a collector operating under steady conditions, can be rewritten ;

Equation: Useful gain enerrgy equation.

Where,

is a transmittance-absorptance product that is weighted according to the proportions of beam,diffuse, and ground reflected radiation on the collector .

aiLTRcu TTUGFAQ

And finally; instantaneous efficiency can be defined as :

That is;

T

aiLRR

Tc

ui

G

TTUFF

GA

Qn

Tc

ip

iGA

TTCmn

0'

APPLICATIONS

Flat plate collectors are used for both;

A) Domestic applications

B) Commercial applications

Flate plate collectors mainly used in residential buildings where the demand for hot water has a large impact on energy bills. This generally means a situation with a large family, or a situation in which the hot water demand is excessive due to frequent laundry washing[2]

For instance, a family of 4 members consumes on an average 100 litre of hot water a day at 60 ˚C. Hot water of 100 litre capacity at 60 ˚C approximate can be delivered by a single collector system of 2 m² area.[6]

The solar water heating systems are generally provided with auxiliary backup in the insulated hot storage tank for the rainy and heavily overcast cloudy days.

A) Domestic applications

Here we can see solar flat-plate collectors used for heating buildings.

Figure: Flat plate collectors used for heating buildings [7] .

B) Commercial applications

Commercial applications include laundromats, car washes, military laundry facilities and eating establishments. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or with operations such as laundries or kitchens that require large quantities of hot water.

• And unglazed liquid collectors are commonly used to heat water for swimming pools. Because these collectors need not withstand high temperatures, they can use lessexpensive materials such as plastic or rubber. They also do not require freeze-proofing because swimming pools are generally used onlyin warm weather or can be drained easily during cold weather .

Here we can see solar flat-plate collectors used for heating swimming pools.

Figure: Flat-plate collectors used for heating swimming pools [8].

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

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

than other water heating systems.However,they requires

large areas if high energy output is a requirement. Alsosolar energy is free if we do not include the initial

cost for installation and the maintenance.

Finally; bessides these we should remember by using

solar energy we can protect nature