Surface Selectiva

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
Energy is the primary and most universal measure of all kinds of work nature. happens by in human Every the being thing world is and that the

simplest method of

and

most

direct solar

close tounity) and by decreasing the thermal losses. having Surfaces/ selective coatings called Such

harnessing

energy is to convert the incident solar radiation into heat (called solar thermal conversion).

response to solar spectrum are s elective surfaces way of by to surface/coating. offer solar a cost the high increase

expression of flow of energy in one of its forms. If the question is askedas to what energy source will supply the enormous energy demand of the coming years, conventional thinking says, it is fossil fuel. But as the reserve of fossil fuels is gradually depleting, it is becoming a general belief that renewable source is going to be the answer to meet the ever increasing demand of the future energy supply. To-day, industry. the growth services, billions access global,from fossil the fuel for in in of to there The are fi r s t two is for to main meet

Conversion of solar energy into thermal energy
The efficient utilization of solar energy for heating, cooling a n d other plate applications collector or utilizes f l a tconcentrator

effective efficiency collectors

thermal

providing

s o l a r a b s o r p t a n c e (α ) i n t h e visible and nearinfrared s p e c t r u m ( 0 . 3 – 2 . 5µ m ) a n d l o w emittance s p e c t r u m ( 2 – 2 0 µm ) t o r e d u c e thermal losses. The economical and efficient utilization of solar thermal requires almost always the use of an efficient and low cost selective surface/coating. (ε ) in the infrared

systems, which first capture as much as possible of the incoming solar radiation and then deliver a highfraction of the captured energy through a working fluid. The the the conversion absorber system thermal plate is losses e f f i c i e n cy of due the by to and

dependent on the properties of collector limited

challenges for the world energy expected demand exponential energy in the have where not energy. local use of to but will more

Solar absorptance and emittance of selective surfacesFigure between infrared 1a shows the difference the solar visible at radiation and and far at near black

conduction, expressed by

convection the

radiation. The efficiency can be equation (Duffe and Beckman, 1991) as,
Qa FR[( =

particular, countries people do commercial

developing

ultraviolet, body

α )I τ

spectrum at that for does Virtually two

- U

L

( T P - T a)radiation

infrared different behave heat in black exist

The second is to deal with the regional supply will energy more and and 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 energy. Considering these facts, continue time; source and dominate renewable become some

η=

— —

— — — — — — — — —–

spectrum materials optimally conversion nature.

IAc

I

temperatures.Unfortunately, would solar not all

W h e r e , Qu = u s e f u l h e a t g a i n , I = incident AC (απ ) solar = = area radiation of UL the = intensity, collector,

absorptivity–

transmittivity temperature of

factor, the

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

important with time. Among or wind, biomassmost the sea, solar various renewable sources, and the be a are can

hear removal factor. The exponent of the efficiency of the and solar its thermal optical conversion thermal efficiency energy (α
Figure 1a: Spectral distribution of solar terrestrial together and with and the black ideal reflectance curve body (Ti) radiation (Ri) for

non-conventional energy and

geothermal

device is...