The normal reflectance of prepared and aged samples was measured in the wavelength interval 0.3 to 20 pm. A Perkin-Elmer Lambda 900 spectrophotometer equipped with an integrating sphere of diameter 150 mm, circular beam entrance and sample ports of 19 and 25 mm, respectively was used in the wavelength interval 0.3 to 2.5 pm. The infrared wavelength interval, 2.5 to 20 pm, was covered with a Bomen Michelson 110 FTIR spectrophotometer with an integrating sphere of diameter 4 inches (102.4 mm) and a circular beam entrance and sample port of 11/8 inches (28.8 mm). An evaporated gold mirror was used as a reference mirror for the measurements done with the infrared spectrophotometer. The measurements were combined to create one spectrum and the normal asol and stherm values were calculated using the equations below [8]. Normal solar absorptance, asol, is theoretically defined as a weighted fraction between absorbed radiation and incoming solar radiation. The solar spectrum, Isoi, used here is defined according to the ISO standard 9845-1 (1992) with an air mass of 1.5. Normal thermal emittance, stherm, is as well a weighted fraction but between emitted radiation and the Planck black body distribution, Ip, at 100°C.

4.1

J Isol (A)( 1 — R(A))dA

J03_______________________________

4.1

J Iso, W dA

0.3

20

J Ip (A)(1 — R(X))dA

_ 2.5_______________________________

20

JIp (A)dA

2.5

Results

Absorbing layer

Samples heat treated with 5, 30 and 60 0Cmin-1 up to 580°C were tested for 150 hours. Two samples of each type were subjected to the condensation test. Tests showed that the higher temperature increase rate a sample had been subjected to the better did it perform in the accelerated ageing test. Samples were tested for 150 hours and the samples made with the lowest temperature increase rate showed strong absorption in the infrared wavelength range after the condensation test and consequently failed the performance criterion. The specific bands have not been analyzed but the locations of them indicate that hydroxide compounds are involved. It is most probable that surface alumina had reacted with water and formed some sort of aluminum hydroxide or oxo-hydroxide. The reflectance curve of samples treated with a higher temperature increase rate was not as largely affected. The surface appearance became slightly rough causing the impinging light to scatter more and hence the absorption in the visible wavelength range increased. The transition from low to high reflectance was also shifted towards shorter wavelengths. Samples made with a higher temperature increase rate than 30 0min-1 all passed the PC limit of 0.05.

The absorbing base layer typically attains a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03.

Figure 1a and b. Comparison of absorber samples without an AR layer, before and after 150 hours of an accelerated ageing test. (a) heated with 5 °min1 up to 580°C (b) heated with 60 min1 up to 580oC.

Anti reflection layer

A base layer made of 65 volume percent nickel, heat treated with 50 °min’1 up to 550°C, has been coated onto all samples before the AR coating was applied. The only exception is the base layer for the alumina coated sample which has a 70 % nickel base layer heat treated with 30 °min’1 up to 580°C. The parameters stherm, asoiand PC for the aged samples can be found in table 1. Two samples of each type of coating were subjected to the condensation test. The five different AR materials were: A = alumina, S = silica, HS = hybrid silica (80 mol% TEOS and 20 mol% MTES), ST73 = silica-titania (70 mol% TEOS and 30 mol% TBOT).

The reflectance curve of the alumina coated absorber, sample 1, tested for 80 hours show strong absorption bands in the infrared, see Figure 2a, and consequently the normal thermal emittance value drastically increased. Since the sample already after 80 hours of testing exceeded the limit of the performance criterion, no further testing was needed.

All other AR materials proved to be very resilient, minor or no changes at all to the optical performance were seen, even after 600 hours of testing, see the figures below and Table 1. No difference in reflectance after 300 and 600 hours of testing for the S and HS samples heated to 350°C was observed. Consequently it was concluded that it was enough to test the remaining samples for 300 hours in order to see any trend if there was one.

Samples coated with silica and heated to 350°C showed a small decrease in normal solar absorptance and a small increase in normal thermal emittance. Silica and hybrid silica coated samples heated to 550°C showed an increase in thermal emittance. In general both S and HS samples proved to be very durable and the difference in performance between samples heated to 350 or 550°C was small. However one evident trend was that hybrid silica samples were more resilient than the corresponding silica samples. Samples treated with ST73

revealed as slight increase in normal thermal emittance while the normal solar absorptance value remained constant.

The best selective properties were obtained for samples coated with alumina or silica-titania 70/30 molar %. The normal solar absorptance and the normal thermal emittance values for ST73 were typically 0.91 and 0.03. Alumina coated samples attain the same values but are not that interesting since this material did not withstand the condensation test.

Figure 2a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with alumina, heated to 580°C and tested for 80 hours (b) coated with silica, heated to 350°C and tested for 600 hours.

Figure 3a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with silica, heated to 550°C and tested for 300 hours (b) coated with hybrid silica, heated to 350°C and tested for 600 hours.

Figure 4a and b. Comparison of samples before and after an accelerated ageing test. (a) coated with hybrid silica, heated to 550°C and tested for 300 hours (b) coated with silica(70%)-titania(30%), heated to 500°C and tested for 300 hours.

Discussion

The durability of the absorbing base layer revealed to be very dependent upon the temperature increase rate at which the samples were treated with. It seems like the absorbing layer becomes more durable when it was treated with a high temperature increase rate. Antireflection treated samples coated with silica, hybrid silica, or silica-titania proved to be very resilient. The absolute best test results were found for samples coated with hybrid silica. Hybrid silica seems to be more flexible and less prone to crack, in accordance with the expectations, which make it an excellent protecting layer. The only two layer absorbers which did not pass the condensation test were samples coated with alumina working as the AR layer.

The solution-chemical method investigated has proved to produce coatings with good selective optical properties. The study has however shown that it seems virtually impossible to achieve a durable two layer absorber with more than 0.91 in normal solar absorptance. To be able to compete with commercially available selective absorbers which have a as0i value of about 0.95, a third layer is most probably needed.

Other important factors for the creation of a successful solar selective coating are scratch resistance and adhesion. All samples produced in this study had, after the heat-treatment, excellent adhesion properties and were reasonably tolerable towards scratching. The adhesion ability of the coating solution on the aluminum substrate is very important for the quality of the film. The coating solution will not homogenously stick to a greasy aluminum surface, instead small droplets are formed on the surface and the resulting heat-treated film will appear stained. The pretreatments of substrates in this work were fully adequate. In conclusion the process is simple, utilizes readily available chemicals and does not demand sophisticated equipment, which makes it accessible for not only the industrialized world but also developing countries.

Spin-coating processes are very easy to handle but there is one considerable disadvantage. This technique cannot handle large surfaces. Instead two other wet coating methods could be of practical interest for industrial use, spray — or dip-coating. Spray-coating techniques are quick, easily adaptable to different coating solutions, complex shapes can be coated, suitable for the establishment of an in-line process and there is a minimum of material waste. One shortcoming is that one nozzle can coat only one surface at a time. Dip-coating processes are simpler and coat two sides at the same time but they are slow and the material waste is larger. The number of advantages with a spray-coating method suggests that this is the technique to prefer when up scaling the process.

Acknowledgements

First of all I would like to thank my supervisor Dr. Eva Wackelgard, who came up with the idea that led to this study, for her invaluable support and excellent counseling.

Dr. Gunnar Westin deserves a lot of credit for his guidance and for letting me use his technique even though it has not been patented yet. Further I would like to express my gratitude to Dr. Annika Pohl and Dr. Asa Ekstrand for providing me with help and their never — ending patience. Finally I would like to thank all members of the Solid State Physics group for their backing and encouragement.

The work has been carried out under the auspices of The Energy Systems Program which is financed by the Swedish Energy Agency.

Akos Nemcsics, College of Engineering Budapest, Tavaszmezo str. 17, H-1084 Budapest, Hungary and Research Institute for Technical Physics and Materials Science, P. O.Box 49, H-1525 Budapest, Hungary, e-mail: nemcsics. akos@kvk. bmf. hu

The problem of electrical energy storage possible can be solved with the help of electrochemical solar cell, which is suitable to generate either electrical energy or hydrogen under special condition. The greatest problem of the electrochemical solar cell technology is the find for novel materials with appropriate properties for electrochemical energy conversion. In this work will be presented Cd4GeSe6 which is a novel material for purpose of electrochemical solar cell.

Some Aspects to Electrochemical Solar Cell

Solar cell technology is a very developed area of the microelectronics where are still some researched problems. One of the greatest problems in the solar cell application is the storage of electrical enegy. This problem can possiby be solved with the help of electrochemical solar cells, which are suitable to generate either electrical energy or hydrogen under special conditions [1]. The technology of electrochemical solar cells have some technical and scientific problems. One such problem is the solution of the photo corrosion, which occurs at the electrolyte — semiconductor interface. The photo corrosion ruins the semiconductor electrode during the working of the solar cell. The possible direction of this research is the search for novel materials of appropriate properties for electrochemical applications. One of the important groups of such semiconductor compounds is the chalcogenides such as Cd4GeSe6.

In this work will be presented Cd4GeSe6 which is a novel material for purpose of electrochemical solar cell. The properties of this material are investigated, which has been a scarcely studied material whose properties are not known in detail. Cd4GeSe6 belongs to the agryrodite family where lattice parameters are determined [2]. The band gap and type of band transition was deterimined by absorption and also from — V characteristics determined by photoelectrochemical method [3]. Furthermore were found that this material shows very good resistivity against photocorrosion [4]. The electrical parameters of the Cd4GeSe6- electrolyte junction are very important to know for solar cell application which are also determined in this work. The properties of the Cd4GeSe6 crystal — electrolyte junction are investigated with impedance analysis. The evaluation of the measured data was carried out with the help of a computer program developed by us in pascal language. We used an equalent circuit with physical meaning which is appropriate for the calculation [5].

The multi-zone building model in TRNSYS is known as Type 56. Building input data is defined using a visual interface, TRNBuild. TRNBuild is the next generation of the well — known Prebid interface.

Integration of a 2-band model for solar radiation

Many modern glazing systems have properties depending on the wave-length of the solar radiation. For example, a sun protection glazing might have a transmittance for the entire solar spectrum of Tsol = 38 % whereas the transmittance in the visible band of the spectrum is Tvis = 66 %. Because of those selective properties, a 1-band model can give incorrect results if two or more selective glazing systems are in series, as shown in the following example:

The amount of energy entering the sunspace is 38 % for both models, but differences occur for the room adjacent to the sunspace. The amount of energy entering the room is 14.4 % for the 1-band model. The correct value of 22.3 % is only derived by the 2-band model with a visible and invisible band. The new 2-band model assumes that the solar energy in the undisturbed spectrum is split equally between the visible and invisible. The required glazing properties can be read from the existing window library.

In order to be able to predict expected service life of the component and its materials from the results of accelerated ageing tests, the degradation factors under service conditions need to be assessed by measurements. If only the dose of a particular environmental stress is important then the distribution or frequency function of a degradation factor is of interest.

For measurement of microclimatic variables relevant in the assessment of durability of the static solar materials studied in Task 27, various kinds of climatic data during outdoor ex­posure at different test sites are monitored such as global solar irradiation, UV-radiation, surface temperatures, air humidity, precipitation, time of wetness, wind conditions, and atmospheric corrosivity. Such data will be used to predict expected deterioration in per­formance over time by making use of degradation models developed from results of accel­erated tests. Some results from the measurement of microclimatic data are shown in Table 6 and Figure 5.

Figure 5 Microclimatic data measured during outdoor exposure of solar fagade absorbers at ISE in the IEA Task 27 study. Left diagram: Surface temperature frequency histograms for a black painted and a black chrome absorber; Right diagram: UVA and UVB light doses versus exposure time

To perform the comparison between the numerical Nusselt numbers and the reported ones, we consider a cavity with differentially heated walls; two opposite vertical walls heated at different temperatures and the others were adiabatic. The Rayleigh number of 2.3×106 was considered, which is the same as in the experiment.

Table 5 presents the comparison between the present work and the theoretical and experimental convective, radiative and total Nussetl numbers of Ramesh et al. in 1999 (experimental) and Balaji and Venkateshan en 1994 (numerical). From these results, we can see that for Rayleigh of 2.3×106 the differences between the convective Nusselt number of Ramesh and the present work was 13.2%, meanwhile for the Balaji was 11.45%. For the radiative Nusselt number, the highest difference was for Balaji, but for the experimental one, the difference decreases until 14.61%. That is due to the fact that Balaji considers 2-D model and we considered 3-D model. Nevertheless, the differences between the total Nusselt number and the present work was 1.6% for Ramesh and 5.58% for Balaji.

3. Conclusions

This paper presented the influence of the radiative heat transfer of a three dimensional cavity with a semitransparent wall with solar control coating, considering that the temperature distribution of the test glass is function of the thermal interaction between the interior and the exterior of the cavity. The results indicated that the model can represent very closely the experimental air measurements in the interior of the cavity, thus the model can be considered verified experimentally. From the absorbed thermal energy in the solar control coating, 12.4% was for radiative energy and 11.0% was for convective energy, meaning that the radiative exchange between surfaces plays and important role in the heat transfer process for this cavity.

Additional results were the comparison with reported work. The results indicated that the theoretical model can reproduce the theoretical and experimental total Nusselt number with an approximation less than 6%. However, there is a difference, high 16.9% and

lowest 10.14% when individual convective and radiative Nusselt numbers were

considered.

Films of nickel-based and iridium-based oxides were deposited using standard DC and RF reactive magnetron co-sputtering from 99.95 % pure targets of Mg, Al, Si, Ni, Zr, Nb, Ag, Ta, Ir, Ni(61.5 %)-Al(38.5 %), and Ni(93 %)-V(7 %). The depositions were carried out from 5-cm-diameter targets in a mixture of Ar, O2, and H2, all being 99.998 % pure. The total sputter pressure was approximately 4.7 Pa (30 mTorr). Further details on the preparation and optimization of the nickel-oxide-based films are given elsewhere [11-13]. Unheated glass substrates, pre-coated with ITO having a resistance of 60 Omega/square, were used for optical and electrochemical work, whereas polished carbon substrates were used for compositional analysis. The film thickness was maintained at approximately 200 nm. Elemental compositions were determined by Rutherford Backscattering Spectrometry, and it was verified that atomic ratios according to Mg/Ni = 0.80 and Al/Ni = 0.59 ± 0.03 led to optimized electrochromic properties [13]. It should be noted that these compositions correspond to non-magnetic sputter targets, which is advantageous with regard to manufacturing.

Cyclic voltammograms were recorded by use of a three-electrode electrochemical cell with a working electrode being the oxide-based film, a counter electrode of Pt, and a reference electrode of Ag/AgCl. The voltammograms were obtained in a 1 M KOH solution. After some initial voltammetric cycles to stabilize the electrochromic performance, spectral transmittance T(A) and reflectance R(A) were measured in the 300 < A < 800 nm range with the samples in their fully bleached and colored states. The data were taken at normal incidence using an integrating sphere. The reference for the reflectance measurements was a plate of barium sulfate.

G. M. Wallner1, R. Hausner2, H. Hegedys3, H. Schobermayr4, R. W. Lang1’5 1>1nstitute of Materials Science and Testing of Plastics, University of Leoben, A 2) Arbeitsgemeinschaft ERNEUERBARE ENERGIE, Gleisdorf, A 3> Planungs — und Bauges. m.b. H. HEGEDYS-HAAS, Mitterlassnitzberg, A 4> Dr. Schobermayr Kunststofftechnik, Altenfelden, A 5> Polymer Competence Center Leoben GmbH, A

Abstract

For the application demonstration of Cellulose Triacetate (CTA) polymer film based transparent insulation (TI) structures a technically and ecologically optimized TI facade system was developed and used to equip a south-oriented wall of a solar house meeting passive house standard in Graz, Austria. The demonstration building was euipped with an appropriate data recording system for solar irradiation, temperature, heat flux and humidity. The practical experiences within the heating periods 2002/03 and 2003/04 are reported in this paper. For the optimized TI facade system a solar energy efficiency of about 43% and a U-value of 0.76 W/(m2K) were obtained. Although CTA absorbs a high amount of water no adverse condensation phenomena were observable visually. The reasoning for these findings is explained and related to construction details.

Introduction

At present mainly small-celled capillary and rectangular honeycomb transparent insulation (TI) structures based on PC are available on the market (e. g., www. okalux. de, www. polygal. com,www. les-twd. de). Due to the applied extrusion and cutting processes these small-celled commercially available TI structures contain bulk and surface imperfections which adversely affect solar transmittance. The production process is rather complex and not very flexible concerning the use of different materials or the optimization of the material. In previous years polymer film based TI materials were developed and introduced into the market (www. wacotech. de, www. advancedglazings. com). The processes to produce such structures, described in [1], are relatively simple and allow to process materials which are not easily melt processable (e. g., CA, CTA). However, the realized and commercially available absorber-perpendicular structures are rather large — celled with cell diameters of about 9 mm. In general, the large-celled film based structures have a rather low material fraction, which results in relatively poor infrared absorptance, which is a bottleneck for both, applications of TI materials in solar wall systems and in glazings with high emissivity surfaces.

To develop and optimize a small-celled, polymer film based absorber-perpendicular transparent insulation material especially for solar wall applications a research project was carried out at the former Institute of Polymer Technology (JOANNEUM RESEARCH) and at the Institute of Materials Science and Testing of Plastics at the University of Leoben (Austria) in close cooperation with the Fraunhofer Institute for Solar Energy Systems (Freiburg, Germany). Furthermore, an application demonstration project was carried on in cooperation with the Planungs — und Bauges. m.b. H. HEGEDYS-HAAS (Nestelbach, Austria) and the Arbeitsgemeinschaft ERNEUERBARE ENERGIE (Gleisdorf, Austria) focussing on the manufacture of the small-celled, polymer film based TI structures and their application at the facade of an ultra-low energy solar house. While the results of the

TI structure development and optimization are reported in several papers [e. g., 2, 3, 4], the purpose of this paper is to describe and discuss the results concerning the performance of the novel TI wall system including measurement results during the heating periods 2002/03 and 2003/04.

J. Boudaden1and P. Oelhafen1,
11nstitut fur Physik, University of Basel, Klingelbergstrasse 82, CH-4056 Basel,

Switzerland

Tel: 41-61-267-37-15, Fax: 41-61-267-37-82, jamila. boudaden@unibas. ch
A. Schuler2, C. Roecker2 and J. — L. Scartezzini2
2 Laboratoire d’Energie Solaire et de Physique du Batiment LESO-PB, Ecole
Polytechnique Federale de Lausanne (EPFL), Batiment LE, CH-1015 Lausanne,

Switzerland

Abstract

Our aim is to study the possibility of integrating dielectric multilayer films deposited on glass substrates as a colored glazed cover for thermal solar collectors and building faces. The cover glass should ideally reflect only a narrow band of visible light while transmitting the rest of the sunlight spectrum to minimize energy loss. A compromise between the visible reflectance and the solar transmission has to be found. In our multilayer interference filters, we used two materials having respectively a high and a low refractive index. We studied two cases: Al2O3/SiO2 and TiO2/SiO2. The thin films were deposited by reactive magnetron sputtering. In-situ XPS characterizations were carried out for each film by transferring the sample from the deposition chamber to an ultra-high vacuum analysis chamber without breaking the vacuum. The growth rate of TiO2, SiO2 and A^O3 single layers on Si substrates were monitored by in-situ laser reflectometry. Spectroscopic ellipsometry was used to determine the optical constants and the thicknesses of every individual dielectric layer. Reflectivity measurements of the experimentally realized dielectric multilayers deposited on glass substrates confirmed their transparency and their good accordance with the simulation. The multilayers were also characterized by their solar transmission, visible reflectance and a factor of merit.

Keywords: multilayer, dielectric oxides, thermal solar collectors

1. INTRODUCTION

Transparent oxide films are widely employed as antireflection or high reflection coatings [1,2], band-pass filters [3] and narrow-band-filters [4] in various optical and electronic devices. The performances of these devices are based on interference effects obtained by alternating layers of high and low refractive indices.

Nowadays different deposition methods exist to produce dielectric oxide films. Thin film evaporation underwent rapid development and became a standard method for optical coatings [5]. Afterwards, alternative methods such as chemical vapor deposition [б], dip coating [7], sol gel method [8] and reactive sputtering [9] have been extensively studied.

The latter allows large area coatings and thicknesses uniformity combined with high growth rate deposition [10].

In our case, reflecting multilayers are used as a cover for solar collectors. A large fraction of power from the solar radiation must be transmitted through the coatings. The transparency of the film permits avoiding absorption energy losses within the coating. At the same time, the multilayer films should present a narrow reflection band in the visible range. This selective reflection fixes the color of the reflected light. A combination of different refractive indexes and thicknesses makes it possible to realize a wide range of reflected colors with an acceptable solar transmission [11].

In this work, we report an experimental study for the preparation of optical coatings based on Ti02/Si02 and Al203/Si02 multilayer dielectric films realized by reactive magnetron sputtering by depositing alternating layers of two materials. During the experimental realization, some important requirements must be fulfilled. The deposition technique must allow good control and reproducibility of the optical properties of any individual thin film combined with a high deposition rate. The interface between two layers should be as smooth as possible. To meet the above conditions Ti02, Si02 and Al203 are considered a suitable materials to coversolarthermal collectors.

A possible direction of semiconductor research is the search for novel materials of appropriate properties for different applications. One of the important groups of such semiconductor compounds is the chalcogenides. Well-known examples of binary compounds of this group are CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, etc. They are good photoconductors and have high absorption coefficient; therefore they have practical application in solar cells, in photosensors and in other optoelectronic devices. Material properties can be improved and modified by forming ternary, quaternary etc. compounds of the above.

Let us take for example CdS and CdSe which are unstable in acidic media and corrode easily. However ternary chalcogenid materials such as Cd4GeSe6 and Cd4GeS6

The synthesis of Cd4GeSe6 crystal was carried out from

chalcogenide (CdSe) and dichalcogenide (GeSe2) sources. The powdered source materials were well mixed in a silica ampoule 22 mm of diameter and 150 mm of length. The Cd4GeSe6 material was synthesised during a two day annealing at 1120 K. After the synthesis crystals were produced by chemical vapour transport technique. The transport agent was iodine. The starting material was kept at 830 K. The deposition zone for crystallisation

was 750 K. The crystallising period ranged from two to three weeks. The obtained crystal size was about 10x5x4 mm3. The obtained Cd4GeSe6 pieces are dark grey and are glistening, poly — and monocrystalline. The identification of the crystal was carried out by X-ray diffraction and the obtained data correspond with the literature values. The parameters are the follows: a = 12.937 A, b = 7.402 A, c = 12.852 A and у = 110.9 °. The thermal stability of the crystal was investigated by differential thermal analysis and thermogravimetry measurements. The thermogravimetry investigations showed that the loss of weight was only 1.9 % until 740 K and was no differential thermal analysis effect during this period, too. Therefore we concluded that Cd4GeSe6 is a stable crystal and keeps its stability even at high temperatures under normal atmospheric conditions.

A chilled ceiling model has been integrated in the multi-zone building model (Type 56). The integrated model allows adding a chilled ceiling to a room quickly and easily. This model is similar to the existing model of thermally-active building elements (so-called "active layers" in Type 56).

The model is based on a new resistance model developed by EMPA, Switzerland. Unlike a concrete core heating or cooling system, a chilled ceiling is practically thermally decoupled from the wall. The characteristic constant resistance associated with a particular model of chilled ceiling is determined by data measured under given test conditions (e. g. DIN4715-1). This data is typically supplied by the manufacturer. All other resistances are time-varying and are calculated during the simulation.

The main advantages of the new model are:

• Easy definition as an active layer within the TRNBuild interface (^ no additional external links, etc.)

• Measured data supplied by the manufacturer is used to initialize the model

• Ability to handle different types of chilled ceilings

• Variable flow rates

• Fast simulation

New features of TRNBuild — automatic segmentation of active layers

The thermally-active layer model assumes a linear temperature profile over the pipe length. In general, this requires the use of 3 segments to obtain a good approximation of the real temperature profile and the segments must be coupled to each other. In TRNSYS 15, this had to be done manually. The subdivision and the coupling of wall segments is now done automatically. The figure here below illustrates the approximation of a temperature profile (black line) with one segment (red line, x) and with 4 segments (green line, V).