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A NOVEL DESIGN FOR HIGH-TEMPERATURE SOLAR

SELECTIVE COATING

Dawei Ding, Weimin Cai

School of Environmental science and Engineering

Shanghai Jiao Tong University

Shanghai 200240, China

Davidding1@sjtu.edu.cn

ABSTRACT

This paper reviews the most recent development in

high-temperature solar selective absorbing coating, and

presents a novel design of coating surface. Through

fundamental research and computer simulation, a double

anti-reflection layer surface for solar selective coating has

been developed with better solar efficiency than that using

single AR layer. The AR materials used in this research are

combination of SiO

/Al

, SiO

/AlON or SiO

/CuO,

depends on dielectric function of metal-ceramic materials

of the sub-layer absorber. It significantly lowers the

reflectance of solar coating surface in the visible region

while has little impact on the reflection in the infrared

region. The optical performance of this new design was

optimized for Mo, W and Ru based cermets by applying

computer program, Bruggeman effective medium theory

was used to calculate dielectric function of the composite,

where an optimization calculation yields a solar

absorptance of 0.93~0.95, while normal emittance at

temperature of 350ć was controlled in the range of

0.03~0.05.

1. INTRODUCTION

For a superb solar selective coating, it should have the

properties of absorbing maximally at solar main irradiation

region while minimally at the IR region avoiding large heat

loss due to Kirchhoff’s law. Thus, the selective surface

combining perfect absorber at Visible-Near IR(0.3~

2.5μm ) region and reflector at the IR region (λ>2.5) with a

sharp reflectance cut-edge could meet this need.

Of various structures for the solar selective coating, the

two-cermet-layer absorbing coating has been proved to be

the most suitable and practical design

[1]. This structure,

from surface to substrate, usually consists of an

antireflection (AR) dielectric layer, an absorbing layer

composed of two homogeneous cermet layers, a low metal

volume fraction (LMVF) cermet layer on a high metal

volume fraction (HMVF) cermet layer deposited on metal

reflector. Since the nanometer-sized metal in the sub-layer

is found to be the main contributor to convert solar energy

to heat, it’s proved that increasing the volume of metal

fraction and layer depth could improve absorptance, which

also leads to tremendous emittance due to absorption in IR

region. There’s large room to improve the absorber’s

solar-thermal conversion efficiency by lowering its

reflectance at the visible range of solar irradiation without

heat loss.

The objective of this study is to optimize the solar coating’s

selectivity and to improve its efficiency using a new film

structure. The solar thermal performance of this coating of

various material combinations were calculated by computer

program. The simulated materials combinations were

selected with respect to their use in high-temperature

(T>350 ) solar thermal energy conversion. Great ć

consideration is given to the black body radiation at given

temperature while setting the best cut-edge wavelength.

Optimized solar absorptance of 0.93-0.95 and emittance of

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

506

0.03-0.05 at 350ć has been achieved for different

materials.

2. CALCULATION

2.1 Absorptance and Emittance Calculation

Solar absorption and thermal emittance of the coatings can

be calculated from the reflectance spectrum which is

defined as:

() ()

()

1 d

λγλλ

λλ

Φ∗−

∫

(1)

() ()

()

bT d

bTd

λγλλ

λλ

∗−

∫

(2)

= (3)

Where, γ (λ) is spectral reflectivity, Ф (λ) is intensity of

solar radiation, b (λ, T) is intensity of black body radiation

and f is defined as spectra selectivity. The absorptance was

calculated using the calculated reflectivity spectrum in the

solar radiation region 0.3~2.5

μm and the AM1.5 spectrum.

The thermal emittance was calculated from the calculated

spectrum in the IR region, 1.0~20

m and the plank

blackbody spectra distribution for 350 .ć

2.2 Dielectric Function of Cermets

The optical properties of inhomogeneous materials can be

described by the so-called effective dielectric functions if

the wavelength of light is much larger than the typical sizes

of the inhomogeneities of the system. The most widely-

used physical models for the dielectric function of a

composite have been proposed by Bruggeman [2]. The

average dielectric function of a composite in the

Bruggeman (BR) is given by the following formula:

()

εε

εε ε ε

−

+− =

−+

(4)

Where ε

is the average dielectric function of a composite

in Bruggeman (BR) approximations, ε and ε

are the

dielectric functions of metal and ceramic, respectively. The

filling factor f represents the volume fraction occupied by

the metal spheres.

The method used in this article is a computer program

called CODE (Coating designer)[3], which could calculate

dielectric function of inhomogeneous system by applying

different physical models including Bruggeman using

dielectric function of each material from the database

attached to CODE. The optimization of the optical

performance of designed layers is conducted through fit

parameters, where the depth and metal volume fraction are

free to modify.

3. RESULTS

Through the computer calculation, one can easily

understand contribution of each sub-layer to the solar

absorptance and thermal emittance. The following shows a

series of selective surfaces with different cermets, including

Mo, W and Ru cermets. The calculations of absorptance

and emittance are optimized so as to evaluate the optical

performance of the new designed with double-AR layers

and those with single AR layer.

3.1 Mo-Al

Cermet Solar Coating with Double AR

Layers

Figure 1 shows two calculated reflectance spectra for

Mo-Al

cermets, SiAlO-Mo corresponding to a 60nm

cermet layer of f

=0.2 on a 80nm cermet layer of f

=0.4,

with a 70nm SiO

AR layer on a 20nm Al

AR layer, and

AlO-Mo corresponding to the same two cermet layers with

a 50nm Al

anti-reflector layer, both are deposited on Al

reflector. The data for the complex refractive index of SiO

, Mo and Al in the wavelength range of 0.3~2.5 μm

are from the Palik book contained in the database, and

those from 2.5~2.0

μm are extrapolated accordingly.

The AR layer contributes to the reflectance reduction in the

wavelength range 0.3~1.2

μm while hardly changes the

reflectance in the range 3.0~25

μm . Since, in most cases,

the metal and dielectric were co-sputtered for the formation

of absorber sub-layer on which the same dielectric material

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

507

was used for AR. The refractive index of dielectric must be

close to that of air to reduce the surface reflectance, at the

same time, it’s required to be near the optical constant of

metal to increase the absorptance in the solar region

[4]

. The

refractive index of Mo (n=3~3.7) in the visible region

renders it more suitable for the alumina (n=1.76) than

quartz (n=1.55) to be selected as dielectric, while the

surface reflectance is relatively much higher in the solar

region (as shown in Figure 1).

Fig. 1: Two calculated reflectance spectra for the solar

selective film SiAlO-Mo (solid curve) and AlO-Mo

(dashed curve).

By applying a quartz layer on the surface of alumina,

reflectivity in the region of 0.5~1.0um was reduced by

2.3%, thus has a higher α (=0.951) while thermal emittance

was hardly increased.

3.2 W-AlON Cermet Solar Coating with Double AR Layers

The value of n and k of W (n=3~3.5) in the range of

0.3~2.5um are close to those of Mo, the optical properties

of W cermets will be nearly the same as that of Mo cermets.

The AlON-W or AlN-W cermets solar coating with two

cermets layers was firstly introduced by Zhang

[5]. Figure 2

gives the calculation result of new designed SiAlON-W

cermet and the original AlON-W cermets. SiAlO-W

corresponds to a 60nm cermets layer of f

= 0.15 on 70nm

cermets layer of f

=0.5 on the Al reflector, with 70nm

SiO

anti-reflector layer instead of AlON

used in the

original W-AlON, the value of α was raised from 0.915 to

0.948 with little increase of ε. The optimization process

shows that in this case, AlON is a better dielectric than

quartz for the W based cermet, while a worse AR material.

The application of AlON for AR layer could result in high

absorption loss (about 4%) in the wavelength range of

0.3

~ 1.0μm, replacing AlON with quartz rather than

overlying it with quartz could bring better optical

properties.

Fig. 2: Two calculated reflectance spectra for the solar

selective film SiAlON-W (solid curve) and

AlON-W (dashed curve).

3.3 Ru-Al

Cermet Solar Coating

Ruthenium oxide film as high temperature solar selective

film was first predicted by J.H. Schön[6], deposition of the

films upon metallic substrates by either spraying or dipping

method made at laboratory exhibits high absorptance

(α=0.98) as well as high emittance (ε=0.8)[7]. In stead of

using Ru-RuO

composites, we apply Ru-Al

for the

solar selective films and calculate their optical

performances in both film structures-SiAlO-Ru and

AlO-Ru. SiAlO-Ru corresponds to a LMVF (f

=0.2)

cermet layer on a HMVF (f

=0.65) deposited on Al

reflector, with a double-layer AR of 70nm SiO

and 10nm

on top. AlO-Ru corresponds to a LMVF (f

=0.25)

cermet layer on a HMVF (f

=0.6) cermet layer deposited

on Al reflector, with 50nm Al2O3 AR layer on top. The

optimization of the film structure was aimed at obtaining

both high solar absorptance and selectivity f. The

calculation results of both films are shown in Fig 3. These

two films feature nearly the same low thermal emittance,

ε=0.037~0.039, while SiAlO-Ru has a higher α (=0.945)

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

508

than AlO-Ru (α=0.934) due to its nearly zero reflectivity in

the solar region.

The values of n and k of Ruthenium in the wavelength of

0.3~2.5um are those of S. W. Choi[8] and in the range of

2.5~20um come from extrapolation.

Fig. 3: Two calculated reflectance spectra for the solar

selective film SiAlO-Ru (solid curve) and AlO-Ru

(dashed curve).

3.4 Ru-CuO Cermet Solar Coating with Double AR Layers

The nano-crystalline CuO applied for solar selective

absorber was discussed by Yu. P. Sukhorukov

[9]

. It shows

that the absorption edge of nano-sized CuO could be

shifted to 0.6eV without any change to the refractive index.

Since the value of n of CuO (n=2.3~2.9) in visible region is

close to that of Ru (n=3~5), and the extinction coefficient

(k) of CuO is rising from zero in the IR region to a degree

close to its refractive index (n) in the visible region, both

prominent interference effect and high internal absorption

are expected.

Fig. 4 shows the calculated reflectance of a Ru-CuO

composite solar film. The film structure is composed of a

Ru-CuO cermet layer deposited on Mo reflector, with two

AR layers of SiO

and CuO on top. Unlike the

two-cermet-layer absorber structure used above, this film

actually consists only one single cermet absorbing layer

(Ru-CuO composite) with homogenous metal volume

fraction, which is more easily deposited for practical

consideration. The data for the complex refractive index,

n+ik of CuO and Mo are from the CODE database.

For this structure, high solar absorptance (α=0.943) and

low thermal emmitance (ε=0.047) at 350ć was achieved.

It should be pointed out that in this case, CuO layer, which

is between the top AR layer and Ru-CuO cermet layer, not

only plays the role of reducing incident reflection, but also

the role of absorbing the incoming light by both internal

absorption and interface destruction.

Fig. 4: The calculated reflectance spectra for the solar

selective film Ru-CuO.

4. CONCLUSIONS

In previous attempt to improve solar performance of

selective coating, the two-cermet-absorber structure was

used to achieve high solar selectivity. To obtain higher

solar absoptance, one should increase either the metal

volume in absorber layer or layer depth to move the

absorption edge to longer wavelength, which also lowers

down the reflectance at IR region, thus increases the

emmitance, and the thermal loss at high working

temperature could be prominent. However, there is a

large room to increase absorptance by lowering

reflectance to a minimum degree in the visible region

without influencing reflectance curve in the IR region. In

this article, a new film structure of two AR layers has

been developed to maximize the photo-thermal

conversion for Mo, W and Ru based cermet layer applied

for high-temperature selective films. Calculation results

show that this design can lower the normal reflectance of

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

509

film remarkably in the solar region while maintaining

high reflectance in the IR region, of which high solar

absorptance (α=0.95) and low thermal emittance

(ε=0.037) at 350 have bć een achieved. The films were

supposed to be thermally stable at high temperature and

experimental deposition is expected.

5. REFERENCES

(1) Q. C. Zhang, “Recent progress in high-temperature

solar selective coatings”, Solar Energy Materials &

Solar Cells, 2000.

(2) D.A.G.Bruggeman, “Berechnung verschiedener phy-

sikalischer Konstanten von heterogenen Substanzen”,

Ann. Physik, 1935.

(3)

M. Theiss, Hard and Software for Optical Spectroscopy,

Dr.-Bernhard-Klein-Str. 110, D-52078 Aachen,

Germany, 2002

(4)

M. Farooq, “Computations of the optical properties of

metal/insulator-composites for solar selective

absorbers”, Renewable Energy, 2003.

(5) Q. C. Zhang, “High performance W–AlN cermet solar

coatings designed by modelling calculations and

deposited by DC magnetron sputtering, Solar Energy

Materials & Solar Cells, 2004.

(6) J.H. Schön, “Computer modeling of the performance of

some metal/dielectric multilayers for high- temperature

solar selective absorbers”, Solar Energy Materials and

Solar Cells, 1996.

(7) Morales-Ortiz, U, “Ruthenium oxide films for selective

coatings”, Solar Energy Materials & Solar Cells, 2006.

(8) W. S. Choi, “Dielectric constants of Ir, Ru, Pt and IrO2:

Contributions from bound charges”, Physical Review

B-Condensed Matter and Materials Physics, 2006.

(9) Yu. P. Sukhorukov, “Nanocrystalline Copper Oxide for

Selective Solar Energy Absorbers”, Technical Physics

Letters, 2006.

A NOVEL DESIGN OF THE COLLECTOR’S COVER OF

SOLAR INDUCED CONVECTION POWER PLANT

Li Ping, Mei Danhua, Li Zaoyang, Chen Weixiong,

Zhang Yongzhi

School of Energy and Power Engineering

Xi'an Jiaotong University

P.O. Box 2264,

Xi 'an 710049, China

Zhao Liang

State Key Laboratory of Multiphase Flow

in Power Engineering

Xi'an Jiaotong University, Xi 'an 710049, China

lzhao@mail.xjtu.edu.cn

ABSTRACT

Solar Induced Convection Power Plant is a technology of

new energy using. It has a very broad perspective. However,

the efficiency of the technology not so acceptable. This

paper focuses on improving the efficiency of the

technology. The thermal physical properties of common

glass, polycarbonate and polycarbonate with CO

ware

compared in this paper to study the superiority and

scientificity of polycarbonate with CO

. The results show

that, the heat transfer coefficient of polycarbonate with CO

is the smallest so that it has the best effect of warm-keeping.

and it can improve the efficiency of the system to large

extent. According to the study, the polycarbonate with CO

is the best one for solar radiation to go through and it is not

easy for Far Infrared ray to do that. Base on the results and

other advantages of the polycarbonate with CO

, we

conclude that polycarbonate with CO

is the best choice for

the material of the slope solar collector cover.

1. INTRODUCTION

The concept of solar induced convection power plant has

developed and studied. This system consists of three

important parts, collector, chimney, and the turbine. The

working process of the system is as follows: The air and the

soil under the collector are heated while sunlight beats

down on the collector. The heated air makes temperature of

air higher and density of air lower. The burst of ascendant

air current is made, which drives the turbine to work. At the

same time, the cold air around the collector accessorizes

into it to make the continuous air circulation.[1-6]

As a basic part of solar induced convection power plant, the

collector plays an important role in the efficiency of the

whole system. Based on many materials which have been

consulted, the collector, which is the major part of the

system, is a large air heater, and it’s energy degradation

makes up more than 40% of the system.[7] The

transmissivity of the material affects the strength of

illumination and spectral component, and it also affects the

penetrating power of the solar radiant energy, the efficiency

of shielding off surface radiation. While the heat-transfer

character of the material affects the insulated capacity of

the collector, thusly affects the temperature increment of

the air in the collector. The effectiveness of the collector

totally depends on the factors mentioned above.

Consequently, a material with favorable transmissivity and

heat-insulating property is badly in need.

Nowadays, common glass and medi-empty glass are used

as the material of collector. However, the density of glass is

so large that it asks the framework to have larger bearing

capacity, and glass itself is easily broken. At the same time,

medi-empty glass is sometimes explosible as negative

pressure is formed when it is evacuated. While

polycarbonate is much lighter, whose transmissivity and

heat-insulating property are better than common glass and

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

511

medi-empty. Considering the greenhouse effect of CO

the

thermal physical properties of common glass,

polycarbonate and polycarbonate with CO

were compared

in this paper to study the superiority and scientificity of

polycarbonate with CO

2 .

So it is very necessary to improve the efficiency of the

collector.

2. MATERIAL CHOOSING AND DESINING

2.1 Choosing of Experimental Material

In consideration of the analysis above, this paper intends

to improve the system through meliorating the material

and framework the collector, reinforcing the sunlight-

absorption capacity of the collector and decreasing heat

dissipating capacity of the long-wave radiation from the

heat accumulator layer. Investigations indicate that

ordinary glass and insulating glass are commonly used as

the materials of collector. However, the density of glass

is so large that it requires the framework to have larger

bearing capacity, and glass itself is easily broken.

Besides, insulating glass is sometimes explosible as

negative pressure is formed when it is evacuated.

Although it has no explosion danger to human pumping

inert gas into the hermetic double-deck glass cavity, it

has unsatisfied insulated capacity as the coefficient of

heat conductivity of inert gas is 5 times larger than that

of the air, moreover it costs much more. So it is an

important way of improving the efficiency of the system

looking for a new material for the collector. While

polycarbonate hollow slab is a kind of thermoplasticity

resin, compared with other materials, it has much more

advantages. Its transmissivity is just a little lager that of

glass, but the coefficient of heat conductivity is much

smaller than that of glass. It can work in the situation

when the temperature is between -233K and 293K. And

the heat dissipating can be cut down, because large-scale

air circulation in the cavity of polycarbonate hollow slab

can be avoided depending on many separated columns

laid equidistantly between the two surfaces of

polycarbonate hollow slab.

What’s more, considering the construction, the strength of

resisting impact of polycarbonate hollow slab is 80 times

as large as that of glass, while the weight is about 1/12ˉ

1/15 as large as that the glass with the same thickness.

And it can be bended in both hot and cold conditions.

Low price and long life will make it more economical to

build a large integrated solar induced convection power

plant. Therefore, the analysis above suggests

polycarbonate hollow slab will play a satisfied role in

improving the efficiency of the system.

2.2 Experimental Ideas

Considering the greenhouse effect of CO

, a novel design

of the collector’s cover of solar induced convection power

plant was developed in this paper, CO

was charged into a

polycarbonate hollow slab and sealed up. Then an

experimental investigation was conducted to compared the

optical and thermal properties of ordinary glass,

polycarbonate hollow slab and polycarbonate hollow slab

charged with CO

. This paper divides the experimental

investigation into three parts. The first one is research on

heat-conducting property of these three materials, the

second one is research on transmissivity of ultraviolet-

visible light- near infrared, the last one is research on

transmissivity and reflectivity of far-infrared.

3. EXPERIMENTAL RESULT AND ANALYSIS

The result of research on heat-conducting property

indicates, as can been seen in Fig. 1, the coefficient of heat

conductivity of polycarbonate hollow slab decreases by

about 85% over that of ordinary glass, while the coefficient

of heat conductivity of polycarbonate hollow slab charged

with CO

decreases by about 16% over that of that of

polycarbonate hollow slab. The mathematical expression of

Fourier heat conduction principal Eq.(1) manifests that

when other situations don’t change, heat-conducting

property is inversely proportional to the coefficient of heat

conductivity. So polycarbonate hollow slab charged with

has the best heat-conducting property.

q gradt

λλ

∂

=− =−

∂

n (1)

Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2

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