Category Archives: BACKGROUND

Comparison between numerical and reported values for 2-D and 3-D

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.

Table 5. Comparison between the Nusselt numbers reported and the present work for

Ra=2.3 x 106

Ramesh,1999

(Experimental)

Balaji, 1994 2-D theoretical

Present work 3-D theoretical

Nucv

10.14 (13.2%)

10.30 (11.45%)

11.48

Nu„

14.61 (11.9%)

15.49 (16.9%)

12.87

Nut

24.75 (1.6%)

25.79 (5.58%)

24.35

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.

Experimental Details

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.

Application demonstration and performance of a cellulose triacetate polymer film based transparent insulation wall heating system

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.

DIELECTRIC INTERFERENCE LAYERS FOR SOLAR. THERMAL COLLECTORS

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.

Appropriate Material for Energy Conversion

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.

New integrated model for chilled ceilings

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).

Service life prediction from results of accelerated testing

Accelerated life testing means to quantitatively assess the sensitivity to the various degra­dation factors on the overall deterioration of the performance of the component and its ma­terials.

Figure 6 Change in thermal emittance observed for some reference solar fagade absorber materials during outdoor testing and during accelerated corrosion testing. The corrosivity dose in terms of metallic mass loss of copper at an exposure time is also given for the different tests to illustrate that outdoor performance of those absorbers can be predicted by making use of the equivalent corrosivity dose approach.

Mathematical models are then set up to characterize the different degradation mecha­nisms identified and from the accelerated life test results the parameters of the assumed model for degradation are determined and the service life then estimated.

In Figure 6 is illustrated how the principle of equivalent corrosivity dose in accelerated cor­rosion testing can nicely be adopted in the prediction of the long-term outdoor performance of some solar fagade absorbers. A prerequisite for this is that the accelerated corrosion test correctly simulates the predominating corrosion mechanism occurring under normal outdoor conditions.

Validation

The best approach in validating an estimated service life from accelerated testing is to make use the results from the accelerated life tests to predict expected change in material properties or component performance versus service time and then by long-term service tests check whether the predicted change in performance with time is actually observed or not.

The results of validation tests therefore can be used to revise a predicted service life and form the starting point also for improving the component tested with respect to environ­mental resistance, if so required. It should be remembered that the main objective of ac­celerated life testing is to try to identify those failures which may lead to an unacceptable short service life of a component. In terms of service life, the main question is most often, whether it is likely or not, that the service life is above a certain critical value.

In the case studies of Task 27 outdoor tests at different test sites are performed for meas­urement of microclimatic variables and for validating predicted loss in outdoor performance from accelerated test results. Tests are performed by CSTB in Grenoble (France), ENEA in Rome (Italy), INETI in Lisbon (Portugal), ISE in Freiburg (Germany), NRELin Colo — rado/Florida/Arizona (USA), SP in Boras (Sweden), SPF-hSr in Rapperswil (Switzerland) and Vattenfall in Alvkarleby (Sweden). In Figure 7 a view of the test site at INETI in Lisbon is shown.

Figure 7View of the outdoor exposure site with facilities for monitoring of climatic data at INETI in Lisbon

Conclusions

The work in IEA Task 27 on durability assessment of static solar energy materials has shown that it is possible to employ a systematic approach in the evaluation of the expected
service life of the materials studied. Based on the work performed recommended test pro­cedures will be worked out for qualification of new materials with respect to durability.

Figure 7 Results from outdoor exposure of antireflective glazing materials performed at SPF-HSR Rapperswil, Switzerland. The decrease in the solar transmittance with time is due to soiling effects, which vary very much with exposure site.

For recommended durability test procedures to be accepted as international standards, it is of utmost importance to demonstrate their relevance for predicting real in-service long­term performance. We think that the work of Task 27 will meet this requirement.

Acknowledgement

The authors sincerely want to thank the colleagues and participants in the work of Task 27 on the static solar materials for contributions to this paper: Michael Kohl and Volker Kubler (Fraunhofer ISE Freiburg), Ole Holk (DTU Copenhagen), Gary Jorgensen (NREL, Golden Colorado), Bjorn Karlsson (Vattenfall Utvecklings AB Alvkarleby), Manuel Lopes Prates (INETI Lisbon), Kenneth Moller (SP Boras), Marie Brogren, Arne Roos, Anna Werner (Uni­versity Uppsala), Michele Zinzi (ENEA Rome), and Michele Ghaleb (CSTB Genoble)

[1]

CHARACTERISATION OF TIO2 LAYERS. FOR THREE DIMENSIONAL SOLAR CELLS

Anca DUTA, Transilvania University of Brasov, Center for Sustainable Development, Romania

Marian NANU, Delft University of Technology, the Netherlands

Introduction: The concept of three dimensional(3D) solar cells lately developed, […], represents a promising alternative to silicon solar cell. This type of solar cell is based on a composite p-n heterojunction mixed on a nanometric scale.

Dense and nanoporouse n-type anatase layers are obtained by Spray Pyrolisys. These films solve the electron conduction in a 3D solar cell with the structure TEC8/dense n-TiO2/nanoporous n-TiO2/ p-CuInS2/Au.

Thin layers of TiO2 were obtained using titanium tetra-isoprpoxyde (TTIP) as precursor. The morphology, crystallinity and conductive properties are discussed in conjunction with the deposition parameters.

The photovoltaic response of the 3D solar cell is presented.

The Three — Dimensional Solar Cell

The development of energy-friendly alternatives to the silicon solar cells, efficient and with short payback time, represents a research topic formulated in the past 15 years. The solar cells involving only solid state compounds, not silicon based, are now-a-days developed: the ETA cells, [2] (where a thin nanoporous n-type and p-type semiconductors) represents an advent of the dye-sensitized (Gratzel type), [3] solar cell.

With the advent of polymer bulk heterojunction and ETA solar cells, a three dimensional (3D) solar cell represents a further development where the n-type nanoporous wide band gap semiconductor thin layer is infiltrated with a p-type, light absorbing nanoporous semiconductor. The large interpenetration nanoporous heterojunction overcomes the drawback of the low conduction in solid state.

In a 3D solar cell the n-type semiconductor is usually anatase TiO2, chosen because its inert chemical behavior in various environment. This condition is necessary to be fulfilled since the p-type semiconductor, CuInS2 (with the band gap of 1.55 eV) is obtained in deposition and reductive annealing conditions that can affect a more reactive oxide (e. g. ZnO considered state-of-art in the ETA cell). For avoiding shunts at the back contact interface, the anatase consists of two layers: a dense thin layer, with low flexibility (100 nm) and the nanoporous matrix (1000 nm) able to be infiltrated with CuInS2.

Literature reports different techniques for obtaining solid state solar cells: The ETA cells with the highest efficiency (2%) are obtained by dipping of microporous anatase TiO2 in liquid precursors of Cd and Te, [4]; CuInS2 for solar application was obtained by vaporization of the metal precursors followed by annealing. Recently, CVD and AlD deposition was reported for CuInS2 in a 3D solar cell, [1].

Spray Pyrolisys Deposition (SPD) is a simple and attractive technique and the aim of developing a 3d solar cell using only this procedure is already formulated.

This paper investigates the possibility of obtaining dense and nanoporous anatase TiO2 layers using SPD. The deposition parameters are discussed in conjunction with the morphology, structure and conduction properties of the obtained layers.

Experimental

Absolute ethanol (EtOH, 99.99% Aldrich) solutions of TTIP (97% Aldrich) were prepared and acetylacetonate (AcAc 99+%, Aldrich) was added in order to obtain regular morphologies.

Dense thin TiO2 films were prepared by spraying, at 350oC, a mixture with a volume ratio TTIP : AcAc : EtOH = 1 : 1.5 : 22.5.

The nanoporous films were prepared by SPD at 450oC, using a mixture with the volume ratio: TTIP : AcAc : EtOH = 1.5 : 1 : 18.33. The substrate was conductive TCO/TEC8 glass (SnO2:F) with a low internal resistance but a rough surface morphology.

A complete ETA cell was developed having a dense n-type anatase layer deposited by SPD, a buffer layer of In2O3 and the absorber — p type CuInS2 both layers being deposited using Atomic Layer Deposition (ALD) as described elsewhere, [5].

The films were characterised as such and after annealing in reductive atmosphere for six hours (hydrogen gas, at 450oC and 1 mbar).

The crystalline structure was investigated by XRD (Bruker D8, CuKal ).

The morphology of the platinum sputtered layers was investigated by Scanning Electron Microscopy (Joel JM 5800 LV). Luminescence spectra were recorded with a home-built set-up (TU Delft) using a Spectra Physics Millennia Nd:YVO4 laser with a wavelength of 532 nm. The recordings were done in a backscattering mode, using a set of notch filters to remove the Rayleigh scattering and a liquid nitrogen flow cooled the CCD camera (Princeton Instruments LN/CCD-1100PB). A Spex 340E monochromator equipped with a 100 grooves/mm grating was used. Corrections for the filters, the sensitivity of the CDD camera and the monochromator are applied. The same set-up, used in a backscattering mode using a set of notch filters and a Spex 340 E monochromator equipped with a 1800 grooves/mm grating, was used for the Raman spectra.

A Solartron 1286 Electrochemical Interface was used for potentiostatic control and to conduct the Mott Schottky and flat band measurements.

For the ETA cell, the current-voltage (I-V) curves are recorded with a DC source Meter (Keithley, Model 2400) in tha dark and under illumination. A calibrated solar simulator, SolarConstant 1200 (K. H. Steuernagel Lichtechnik GmbH) is used as artificial light source.

Electrochromism

Cyclic voltammograms were taken on the various films. In general, the shapes of the voltammograms changed depending on the specific additive, but the main features characteristic of the pure oxide tended to prevail. The charge capacity—and hence the magnitude of the electrochromism—is influenced by the potentiodynamic range, particularly the magnitude of the voltage for full coloration, Ucol. It appeared that similar charge capacities (from 15 to 20 mC/cm2) could be obtained provided that Ucol was varied by 0.05 to 0.1 V when additives were present. This shift is insignificant for electrochromic device applications.

The electrochromism of the films was characterized in terms of a coloration efficiency CE obtained from voltammograms and optical data according to [14]

(1- Rc )2 T

(1- R )2 T,

m

where ДО is the total charge that is exchanged and subscripts b and c refer to bleached and colored states, respectively. The total exchanged charge was

measured for the cathodic bleaching rather than the anodic coloration in order to minimize the effect of oxygen evolution.

Figure 1(a) shows spectral CEs for a nickel-oxide and a nickel-aluminum-oxide film. Both of these films were optimized by an additional introduction of some hydrogen during the deposition. It is noteworthy that the films have CEs that are much larger than those reported in the literature [1]. It should be emphasized that the CEs in nickel oxide and nickel-based oxides are intimately related to the conditions under which the depositions take place, which implies that the results of the enhanced CEs are related to features such as crystallinity, grain size, porosity, and contents of oxygen and hydrogen in the films. The film with the largest effective grain size presents the lowest CE, thus supporting the idea that a large inner surface area of the film is connected with the electrochromic activity, and that the coloration process takes place in the outermost parts of the grains. This fact—together with good crystallinity and high porosity, along with optimized quantities of oxygen and

(a) (b)

Wavelength (nm) Wavelength (nm)

Figure 1. Spectral coloration efficiency (CE) of nickel-oxide-based (a) and iridium-oxide — based films (b) of the shown compositions. NiAlO and IrTaO indicate that Al and Ta are present in the oxides but do not specify the amount.

hydrogen—gives highly efficient electrochromic films [15].

Figure 1(b) shows spectral CEs for two different iridium-oxide-based films. Clearly iridium-tantalum oxide has a lower CE than pure iridium oxide for some wavelengths. However it does not follow that Ir oxide is superior to IrTa oxide in applications, since a more crucial property may be the bleached state transmittance, as we elaborate below.

Optimized TI structure for solar wall applications

Fig. 1 shows a sketch and a fotograph of optimized small-celled TI structures based on polymer films. The structures, which consist of a continuously produced small-celled lamella based on a flat film joined to a corrugated film are either stapled or manufactured in the form of rolled structures. Due to the use of high-quality films structural defects on the surface and within the TI structure were avoided. Based on investigations of more than 20 different polymer film types [2, 3], cellulose acetate films (CA and CTA) were identified as outstanding polymer materials for TI wall applications. Using 30 pm thick cellulose acetate films small-celled lamellae with widths between 100 and 135 mm and a heigth of 5 mm were produced. The material fraction of the lamellae and the stapled or rolled structures thereof were about 1.5 v%. For a 100 mm thick stapled structure a hemispherical solar transmittance of 0.80 and a heat conductance 0.85 W/(m2K) were measured. For 135 mm thick structures, which were used for the application demonstration object, an hemispherical solar transmittance of 0.74 and an heat conductance of 0.74 W/(m2K) were calculated using the program GWERT [5] and polymer film properties as input data [2,3]. The remarkable performance property profile of cellulose based materials for TI applications with maximum service temperatures of about 100°C is related to the high solar transmittance of CA films in combination with a high infrared absorptance due to a high density of functional carbon-oxygene single bonds within the molecular structure of CA. However, this high density of carbon oxygene functional groups results in a high moisture uptake of cellulose based materials. For the fully substituted cellulose triacetate (CTA) the maximum moisture uptake is about 3 m%.

Fig. 1. Stapled and rolled small-celled structures based on polymer films