Selective Surface for efficient Solar Thermal Conversion —

D r .

M d .

G o l a m

M o w l a

C h o u d h u r y

Department of Physics, University of Rajshahi, Bangladesh

Introduction

simplest

Energy is the primary and most universal measure of all kinds of work

by

human

nature.

being

Every

happens

in

and

thing

the

that

world

is

the

expression of flow of energy in

method

and of

most

direct

harnessing

solar

response to solar spectrum are called

s elective

Such

surfaces

Conversion of solar energy into thermal energy

energy for heating, cooling a n d

demand

years,

other

applications

conventional thinking says, it is

plate

collector

fossil fuel. But as the reserve of

systems, which first capture as

fossil fuels is gradually depleting,

much as possible of the incoming

it

utilizes

or

f l a t-

concentrator

belief

solar radiation and then deliver

that renewable source is going to

a high fraction of the captured

be the answer to meet the ever

energy through a working fluid.

becoming

a

general

increasing demand of the future energy supply. To-day,

The

conversion

are

two

main

the

absorber

challenges for the world energy

collector

industry.

the

the

The

fi r s t

is

expected

growth

in

services, billions access

for

particular,

developing

people

to

do

regional the

supply

where

fuel

dominate

for

renewable become

will energy

more

use

of

time; source

and

to but will

more

important with time. Among or

various

sea,

biomass

renewable sources,

geothermal

solar

energy

important

and

the

η=

— —

FR[( =

ατ) I

equation

L

( T P - T a)

solar

intensity,

AC

collector,

(απ )

transmittivity

=

radiation

area

=

of

the

absorptivity–

factor,

UL

the

infrared

cost selective surface/coating.

Solar absorptance and emittance of selective surfaces 1a shows the difference the

solar

radiation

visible

and

spectrum

radiation

at

at

and far

two

at

near black

infrared different

temperatures.

Unfortunately,

materials

that

would

optimally

for

conversion nature.

behave

solar

does

not

Virtually

heat

exist all

in

black

=

temperature

of

the

plate,

Ta =

are can

hear removal factor. The exponent of the efficiency of

and

the

the

device is the absorbing surface

be

a

and

solar its

thermal optical

conversion

and

thermal

characteristics.

Sun’s energy can be utilized as

can be increased by increasing

and

in

o v e r a l l h e a t l o s s c o e f f i c i e n t , TP =

major source of power supply. thermal

high

thermal requires almost always

spectrum

I

incident

providing

and efficient utilization of solar

body

W h e r e , Qu = u s e f u l h e a t g a i n , I =

the

thermal

s p e c t r u m ( 2 – 2 0 µm ) t o r e d u c e thermal losses. The economical

ultraviolet,

— — — — — — — — —–

IAc

solar

(ε )

emittance

infrared - U

cost

a m b i e n t t e m p e r a t u r e a n d FR =

non-conventional

wind, most

the

Qa

local

continue

some

and

(Duffe and Beckman, 1991) as,

have

energy.

by

a

increase

s p e c t r u m ( 0 . 3 – 2 . 5µ m ) a n d l o w

between

the

energy. Considering these facts, fossil

to

convection

by

expressed

and and

by

due

offer

s o l a r a b s o r p t a n c e (α ) i n t h e visible and near infrared

in

e n v i r o n m e nt a l i m p a c t r e s u l t i n g from

conduction,

the

limited

collectors

selective

surface/coating. to

of

Figure

The second is to deal with the global,

is

losses

way

efficiency

radiation. The efficiency can be

not

commercial

system

thermal

of

effective

energy

countries

of

meet

exponential

demand

in

to

plate

having

the use of an efficient and low

e f f i c i e n cy

dependent on the properties of

there

Surfaces/

solar thermal conversion).

will supply the enormous energy

is

losses.

solar radiation into heat (called

The efficient utilization of solar

coming

thermal

coatings

asked as to what energy source the

the

energy is to convert the incident

one of its forms. If the question is

of

close to unity) and by decreasing

photovoltaic.

The

the

Bangladesh Renewable Energy Newsletter

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P a g e

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s olar

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1

(α

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Figure 1a: Spectral distribution of solar terrestrial together and

and

with

the

black ideal

t r a ns m i t t a n c e

body (Ti)

transparent heat mirror.

radiation

reflectance curve

(Ri) for

No. 2, Vols. 2 & 3 July 2000 - Dec. 2002

Selective Surface for efficient Solar Thermal Conversion 1b. For comparison, the variation obtained for one of the earliest surface

synthesized

by

Tabor

(1956) is shown in the figure.

1b:

Emissivity/absorptivity

variation for an ideal selective surface and for a typical real surface by Tabor.

materials

have

high

solar

Solar

selective

t h e l a t e s e c o n d h a l f o f t h e 2 0t h century. Tabor (1955) introduced the

infrared

is

surface

manufacture

several

types

s e l e c t i v e m a t e ri a l s w i t h i d e a l o r

surfaces

have

very

since

necessary

to

close

to

properties. should

Thus

ideal

The

it

optical

surface/coating

have

the

following

physical properties (Garg, 1997). 1.

High

absorptance

for

solar

µm

s p e c t r u m r a n g e 0 . 2 -2 . 5 and low emittance spectrum

greater

for

than

2.0

µm. 2.

have

been studied quite thoroughly in

absorptance and also have high emittance.

surfaces

use

of for

spectrally solar

of

then.

and

select ive developed

For

energy

applications, an ideal spectrally selective surface should have an abrupt

transition

between

the

low and high reflectance region around 2 approximately solar

µm, the

which limit of

spectrum.

reflectance

is the

Spectral

behaviour

of

some

selective coatings is shown in Fig

Spect ral transition between

2. An ideal surface is also shown

the

in

region

absorptance emittance

be

of

high

and

low

as

sharp

as

the

figure.

impossible

to

H o w e v e r, prepare

it

such

is an

ideal coating in practice.

The opto-physical properties of the coating must remain stable

under

operation

long

at

temperatures, thermal

of

coating

to

substrate must be good. 5.

Coating

should

be

easily

applicable, and 6.

characteristics

desired

for

a n i d e a l s e l e c t i v e s u r f a c e (α λ = = 1 for for

λ

λ

> 4

< 4

µm

µm )

and

Fig 2: Reflectance selective coatings.

αλ

=

metal

e)

Optical trapping

f)

Multilayer thin films

g)

T ransparent heat reflector / absorber tandem

h)

Transparent

heat reflecting

and conducting coatings. i)

Quantum size effect.

Act ual

selective

utilizes

the

two

or

surface

combined

more

often

action

of

mechanisms

to

obtain high selectivity. Since cost effect i v e n e s s

is

consideration

in

applications,

a

various

an

important

solar

energy

knowledge

properties

helps

of

in

the

c h o i c e o f a co a t i n g f o r a p a r t i c u l a r application.

The

development paints

has

of

recent

semiconductor

provided

a

low

cost

l a r g e a r e a p r o c e s s fo r l a r g e-s c a l e applications systems

in

and

solar

collector

has

received

Methods of preparation selective coatings

of

Solar

be

ελ

ελ

= 0

are shown in Fig

selective

surface

can

1.

Vacuum evaporation

2.

Vacuum sputtering

3.

Ion exchange

4.

Chemical vapour disposition

5.

Chemical oxidation

or

6.

Dipping in chemical baths

Spectral

7.

Electroplating

8.

Spraying

9.

Screen printing, and

behaviour

of

some

Selective surfaces with desired optical

Coating must be economical.

The

/

techniques:

air

ultra -violet

Adherence

Semiconductor

prepared by using the following

repeated

radiation, etc. 4.

d)

term

elevated

cycling,

exposure,

Coating / metal tandem

particular attention.

possible. 3.

c)

selective

collectors

been

Particulate coatings

tandem

Spectrally selective surfaces /coatings Figure

b)

properties

deposited

on

metalized

are

usually

metals

substrates.

selectivity can be achieved in a variety

of

ways.

The

various

types of absorber surfaces are : a)

10. Brass painting method, etc.

Intrinsic absorbers

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No. 2, Vols. 2 & 3 July 2000 - Dec. 2002

Selective Surface for efficient Solar Thermal Conversion Surface and

layer

“nickel

selective

by

copper

surfaces

suitable copper

of

black”

for

layer

chemical

first

found

practical

oxide

oxide

were

to

use.

was

be The

formed

conversion,

by

treating a cleaned and polished copper plate in a hot solution of sodium

hyd roxide

chloride

for

a

and

α

Values of absorptivity and

ε

= 0.17

sodium

specified

time. = 0.89

were obtained.

The “Nickel black” surface was developed

and

commercialized

by Tabor group (1964). Coating

plated

copper

or

steel

of

α

ε

= 0.868 and

Properties

of

some

of

on

galvanized

listed in Table 1.

Using the same method “Black nickel” was deposited on copper and mild steel by Cathro (1975)

α

and values obtained were

ε

0.89 and Cobalt can

oxide

be

=

= 0.09 to 0.15. selective

produced

surface

on

bright

obtained

its and

tandem,

and

composite

which

are

0.11 0.11

Zn / Al Fe Fe, Cu Zn / Fe Fe, Cu

0.88 0.90 0.96 0.94

0.20 0.16 0.07 0.09

0.95

0.09

Zn / Al Ni / Al Zn / Fe

0.95 0.95 0.95

0.12 0.5 0.16

Zn / Fe Ni

0.93 0.92

0.08 0.08

Co 2O

the

simplest

methods energy

incident

and

of

is

to

solar

heat,

most

1.

s t r u c t u r a l p a r a m e t e r s , thus

m ak i n g

the c

selective

o

a

t

i

n

high

a

level

technology.

harnessing

solar

as

K.J.

Christie,

E.A

Selective

Absorbing

Surface,” Meeting on Appl. of

into

Sol. Energy Res. and Dev. in

thermal

thermal

Cathro,

Australia, Melbourne (1975) 2.

conversion

Duffie, W.A:

J. A .

and

Solar

Beckman,

Engineering

of

Thermal

i m m e r s i n g t h e s u b s t r a t e in a n

and

Wiley and Sons, New York

aqueous

characteristics

bath

at

400°C and using the substrate as a cathode. The solar absorptance thermal and

0.87

and

0.92,

of

0.07

e mi t t a n c e

.08

and

of

are

observed

F e -doped

coating

in

CoO

CoO

selective

respectively,

deposited

on nickel-plated steel substrate. One

of

the

selective far

is

been

most

surfaces

“Black

successful

developed

chrome”.

extensively

It

so has

investigated

and recommended even for very high

temperature

application.

T h i s c o a t i n g i s a m e t a l- d i e l e c t r i c composite

consisting

of

a

C r2 O 3

layer over a chromium particle/ C r2 O 3 c o m p o s i t e . I t i s p r e p a r e d by

electroplating

on

a

nickel -

its

optical

absorption and

low

g

technology

a n d R e i d , A . F ; “ N i c k e l bl a c k

convert the

radiation

the solar

on

deposition and

Reference

Conclusion of

dependent

device is the absorbing surface

electrolytic

by

mixed

index

Emissivity

the

is

on

gradient

0.90 0.93

electroplating coating

based

Absorptivity

n i c k e l- p l a t e d s t e e l s u b s t r a t e s b y The

of

for

efficient selective coatings are

Cu Al

conversion. A key component in

techniques.

used

Practical

Substrate

CuO / ZnO CuO Black nickel on bright nickel Black nickel Black chrome on bright nickel

solar

choice

be

s e n s i t i v e l y

Coating

direct

to

s t r u c t u r e

CuO CuO

One

the

techniques

Properties of some important selective

iron

sheet. Values of α = 0.81 and ε = 0.16 to 0.18 were obtained.

the

important selective coatings are

Table 1: coatings

determine preparation.

= 0.088.

was prepared by electroplating method

base.

McDonald (1975) reported values

and such

in

the

thermal as

UV

high

3.

region

thermal emittance in

Processes,

Garg,

H.P

Solar

E ne r g y ,

and

and

John

Prakash,

J;

Fundamental

Applications,

Tata

Mc-

the infrared region. In quest for

graw Hill Pub. Co. Ltd., New-

higher

Delhi

possible

absorptance,

lowest

solar thermal

4.

McDonald,

G.E:

“Spectral

emittance and stability to stay

reflective properties of Balck

at high temperature in air, it is

Chrome

found

Selective

that

produced

surface

that

gives

characteristics conversion energy.

for

of

properties

can

with

be

be

obtained

using

different

Applications,

materials

economics,

inputs

desired

Bangladesh Renewable Energy Newsletter

energy

system

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and

life

would

3

Vol. 1

Tabor, Council

a

Solar Solar

H:

“Selective

of

Research

Israel,

5A

119

(1956).

by

techniques.

as

radiation”, Bulletin

desired

materials and may be prepared by

5.

thermal

adopting different effects of the

use

Coating”,

Energy 17,119 (1975)

desired efficient

solar

Surfaces

can

for

6.

Tabor,

H,

Harris,

J,

Weinberger, H and Doron, B: “Further Studies on Selective Black

Coatings”,

Conf.

on

New

Proc

U.N.

Sources

Energy, 4, 618 (1964).

of

{

No. 2, Vols. 2 & 3 July 2000 - Dec. 2002