Novel durable solution-chemically derived spectrally selective absorbers

T. Bostrom, Division of Solid State Physics, Department of Engineering Sciences,

Uppsala University, Sweden.

E. Wackelgard, Division of Solid State Physics, Department of Engineering Sciences,

Uppsala University, Sweden.

G. Westin, Division of Inorganic chemistry, Department of Materials Chemistry, Uppsala University, Uppsala, Sweden.

A promising novel solution-chemistry method to fabricate spectrally selective solar absorber coatings has been developed. The objective is to create highly efficient, flexible, inexpensive and durable absorbers for solar thermal applications using simple techniques. The selectively absorbing film consists of a composite with nickel nano­particles embedded in a dielectric matrix of alumina.

The AR material should have the following properties: the proper refractive index, low thermal emittance, dense, flexible and long term stable. The AR materials tested were silica, alumina and mixtures of silica-titania. The refractive indexes of the above mentioned materials range from 1.4 (silica) to about 2.1 (50/50 molar ratio silica/titania mixture). Besides increasing the normal solar absorptance, asoi, it is equally important that the AR layer is long term stable in order to create a successful solar selective coating. The AR coatings were synthesized using different solution-chemical methods and deposited on the absorber surface by spin coating. Prepared samples were subjected to an accelerated lifetime test. In the test procedure the temperature of the environment was set to 40°C and the relative humidity to 95 %. Samples made with alumina as AR coatings failed the ageing test. The other materials, silica and silica — titania mixtures proved to be very resilient. Samples that were coated with these AR materials showed no visible degradation of the sample surface even after 600 hours of testing.

Absorbers without an AR layer typically attain a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03. Of the samples made with durable films a 70/30 silica/titania mixture showed the greatest increase of the asol value, 0.91, while the thermal emittance remained unaltered.

Introduction

The most efficient thermal solar collectors for hot water production use a spectrally selective surface that absorb and convert solar radiation into heat. There are already high performing selective surfaces but there are a few difficulties with some of them, such as the long-term durability, moisture resistance, adhesion, scratch resistance, cost and complicated production techniques. In order to make thermal solar collectors more accepted and widespread, the price per unit has to decrease. The most costly component of a thermal solar collector is the spectrally selective surface.

The main aim was to investigate the durability of spectrally selective absorbers produced by a newly invented solution-chemical method. This work is a continuation of a preceding study
where spectrally selective absorbers were produced using a novel solution-chemical technique [1]. Advantages with this technique are that it is simple and easy to control, the coating can be manufactured under ambient pressure conditions, the chemicals involved are environmentally friendly and it is low in material consumption. Furthermore there exist several methods like spin-, flow-, spray — and dip-coating to coat a surface with a liquid medium. The method seems promising and could hopefully reduce production costs for absorbers and hence make them less expensive and more available. The focus in this part of the thesis has been set on the durability properties of anti reflection treated absorbers. The optical characteristics of produced samples before and after the accelerated ageing testing were investigated.

The used absorber belongs to a group of absorbers called metal-dielectric composite/metal tandem, which normally consists of metal embedded in a dielectric matrix applied on a metal surface [2]. The absorbing layer, spin coated on an aluminum substrate, consists of nickel particles embedded in an aluminum oxide matrix. The composition of the absorbing layer is 65 volume percent nickel and 35 volume percent alumina and the thickness is about 100 nm. The metal particles are between 5 — 10 nm in size. A major advantage with a composite is that it offers a high degree of flexibility. By varying the choice of particle, particle size, particle orientation and shape, film thickness and particle concentration in the film, innumerable combinations can be created. Thus spectral selectivity can easily be achieved. By applying the coating on a poor thermal emitter, in this case aluminum, a low thermal emittance value, stherm, is obtained. The normal solar absorptance value, asoi, for the absorbing layer is about 0.80 and the normal thermal emittance value 0.03.

The AR material should have the following properties: the proper refractive index, poor thermal emitter, dense, flexible and durable. A correct refractive index is required in order to obtain as high solar absorptance as possible. At the same time the AR material should be a poor thermal emitter not to increase the stherm value. Lastly the material has to withstand accelerated ageing tests in order to be successful and should therefore be dense and elastic.

Five different AR coatings were studied, alumina, silica, hybrid silica, and two compositions of silica — titania. Silica is well known to be a very resilient but static material. In order to make silica more flexible an organic compound can be incorporated into the structure and then the resulting material is called hybrid silica [3]. A flexible material is more likely to perform well in accelerated ageing tests since it is less prone to crack when heated or cooled. Alumina has a higher refractive index in the visible wavelength range than silica, 1.6 compared to 1.4. Pure titania has a refractive index of 2.7. Thus refractive indices between 1.4 and 2.7 can be obtained by mixing silica and titania.

The materials structure of a thin coating is not completely permanent with time. Factors like high temperature, high air humidity, airborne pollutants and sun radiation can cause the coating to deteriorate and hence affect the optical selectivity of the surface [4]. High temperatures can speed up oxidation processes and high levels of humidity may create hydrolytic reactions i. e. electrochemical corrosion. Airborne pollutants might also accelerate electrochemical corrosion processes and solar radiation can initiate photochemical redox reactions. A combination of these processes can be devastating for a large number of materials, including solar selective coatings. The most accurate method to test the durability of a solar absorber is to assess it under normal working conditions. These so called in-situ tests are though very hard to carry through because of the great time length required to get
satisfying results. Instead of exposing the absorber surface to its natural working conditions for many years, inexpensive laboratory tests can be done in a climate chamber, where temperature and humidity can be controlled. The temperature and/or the humidity in the chamber can be elevated above normal levels in order to accelerate ageing processes.

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