Category Archives: BACKGROUND

Radiative Model

The radiative exchange between the walls surfaces is calculated by using the net radiation method [Siegel, 1992], in which the net radiative heat transfer is:

qrk (x, y, z) = Jj (x, y, z)- qk (x, y, z) for k & j (12)

where Jj is the radiosity and qk is the irradiation on the wall surface given by:

qk(x, y,z)= J Jj(x, y,z)dFk-J k&j and k & 2

Aj

Jj (x, y, z) = £k&Tk {x y>z) + (l -£k ))k (x, y >z) for k & J’

k and j are the wall numbers, in which the heat flux was calculated. dFk-j is the differential view factor that indicates the fraction of energy that leaves from a wall k and strikes wall j. For walls 1, 3, 4, 5 y 6 the thermal emittance is ek&1 and for wall 2, ek=1.

Regimes of periodic microstructures and their optical functions

The optical properties of the micro-structured surface and also the theoretical models to describe them depend very much on the relation between the period Л of the grating and the wavelength X of the incident radiation. Therefore, a classification of gratings defined by the period-to-wavelength relation is very helpful (Fig. 1).

effective medium theories rigorous diffraction theories rigorous diffraction theories photonic band structure calculation rigorous diffraction theories extended sea ar diffraction theories

geometrical optics (ray tracing)

If Л << X then the microstructured regions can by regarded as an effective media. They lead to a modification of reflection and transmission at the boundary air to material but not to a modification of the propagation direction of the radiation (Fig. 1). Only the zero-order diffracted wave is propagating and the wavelength dependence of the optical properties is small in this case. The effective refractive indices of this effective media depend on the refractive indices of the two media in the structured region and on the volume fractions of each of the media. Such subwavelength gratings can be used for antireflective surfaces or for polarisation sensitive devices.

If Л = X then resonance effects dominate and result in a strong wavelength dependence of the optical properties. It is possible to achieve high diffraction efficiencies in a specific dif­fraction order just due to the fact that only few diffracted waves propagate. The optical properties of such gratings have in general to be modelled by using rigorous diffraction theory, i. e. by solving Maxwell equations numerically [16]. Gratings in the resonance re­gion have mainly been used for spectral filtering but also for radiation deflection due to the high diffraction efficiencies which can be achieved.

If Л >> X then many diffracted orders propagate. The distribution of the diffraction efficien­cies depends very much on the structure profile. For very large ratios Л/X the optical properties of the surface-relief grating can derived by means of geometrical optics be­cause this is an approximation which holds for X ^ 0. The grating then represents an array of prisms, lenses, etc. which can be modelled to some extend by using ray tracing meth­ods. For ratios Л/X even as large as 100, the ray tracing method is not sufficiently accurate in many cases. Then, extended scalar approaches or rigorous diffraction theory has to be applied. Surface-relief gratings with a large ratio Л/X are particularly suited for light re­directing elements.

Energy Efficiency in Commercial Buildings — Experi­ences AND RESULTS FROM THE GERMAN FUNDING PRO­GRAM SolarBau

A. Wagner, Building Physics and Technical Building Services, University of Karlsruhe; S. Herkel, Fraunhofer ISE, Freiburg; G. Lohnert, sol id ar Planungswerk — statt, Berlin; K. Voss, Building Physics and Technical Building Services, University of Wuppertal.

Within the funding programme1 "Solar optimised building — SolarBau" projects of commercial buildings are subsidised, if the predicted primary energy de­mand for all technical building services does not exceed 100 kWh m-2a-1. Main objective of the demonstration buildings is the combination of high workspace quality with a low energy consumption. An accompanying research programme is evaluating the buildings with a two-year data acquisition campaign. The pa­per summarises some of the results and experiences.

Towards lean building concepts

In contrast to the established low energy and passive house standards in the sector of domestic dwellings there is only little consciousness of the energy consumption of commercial buildings — neither by their planners nor by owners and users. Numerous office buildings of the eighties and nineties show a very high energy consumption due to the fact that they have been designed without any respect to the interdepend­ence between outdoor and indoor climate. As a result the thermal and visual comfort in office rooms can only be guaranteed by extensive technical building services for heating, ventilation, air-conditioning and lighting (HVACL). High investment costs and a space demand of about 20 — 30 % of the building volume for HVACL equipment characterise a large amount of commercial buildings. The electricity consumption is dominated by HVACL facilities and not by office equipment.

Despite internal heat gains caused by the electric energy consumption, there is still a high demand of heating energy due to the high proportion of glazing and high air ex-

German Ministry for Economy and Labour / Deutsches Bundesministerium fur Wirtschaft und Arbeit

change rates. The left graph in figure 1 qualitatively shows the energy consumption of a normal office building as a function of the ambient temperature. The base load is caused by office equipment and the idling consumption of building services facilities. The waste heat associated with this base load affects the position of the balance temperature.

Figure 1: Qualitative end energy consumption of a conventional office building (left) and a lean office building (right). The dependence of the total consumption (HVACL and office equipment) on the ambient temperature is shown based on daily average values.

A higher common consciousness of resources, an increasing awareness of operation costs of buildings and the preference of users towards individual control of the indoor climate have led to a new trend in architecture: buildings with moderately glazed fa­cades, a high amount of daylight at the workspaces and the option of natural ventila­tion through windows that can be opened. However, a combination of integrated measures for "passive cooling" is a pre-requisite to ensure summer comfort without actively cooling or dehumidifying the inlet air.

Due to the reduced HVAC equipment these "lean" building concepts show a different performance (figure 1, right graph). Energy efficient office equipment, lower air change rates and a higher daylight autonomy reduce the base load and better insula­tion results in a lower balance temperature. Above this temperature the indoor condi­tions remain within the comfort range only by passive cooling measures. Although the indoor climate will vary more than in a completely air-conditioned building, this does not necessarily affect the perceived comfort negatively. Only extreme outdoor condi­tions may lead to short periods of discomfort.

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.

Electrochemical tests

Figs. 6-7 show polarisation curves of C/Al2O3/Al absorption surfaces and aluminium substrate at solutions pH 3.5(a), 5.5(a) and 5.5(b) (see Table 1). The conductivity of the solution pH 5.5(a) was too low for polarisation measurements without sulphate addition. The anodic current densities of the C/Al2O3/Al absorption surfaces and the aluminium substrate show passive behaviour in acid rainwater (pH 3.5a) in wide potential area up to -200 mV (Fig. 6a). The anodic current density of the C/Al2O3/Al surface is ca. decade lower than that of aluminium substrate probably indicating lower corrosion rate of the C/Al2O3/Al absorber surface. The current densities of the C/Al2O3/Al surface and aluminium substrate are higher in acidic rainwater at pH 3.5(a, b) than at neutral rainwater at pH 5.5(b). This indicates higher expected general corrosion rate of the C/Al2O3/Al surface and the aluminium substrate at pH 3.5 than at pH 5.5. The current densities of aluminium substrates are slightly higher than those of the C/Al2O3/Al surfaces in neutral rainwater at pH 5.5 and in acid rainwater at pH 3.5, thus indicating possibly better corrosion resistance of graphite coated aluminium compared to an aluminium substrate.

Aging time [h]

Fig. 3. Changes in solar absorptance for samples exposed to simulated acid rain immersion tests. Samples analyzed with EIS denoted as A and B.

0 20 40 60 80 100 120 140

Aging time [h]

Fig. 5. PC values for samples exposed to simulated acid rain immersion tests. Samples analyzed with EIS denoted as A and B.

Figs. 8 — 9 show the EIS impedance spectra of an C/Al2O3/Al absorption surface and an aluminium substrate at the beginning of the test, after 1 and 5 days at pH 3.5(a) and after 1 and 14 days at pH 5.5(a), respectively. The passive metal in a passive state is characterised by a wide capacitive area, i. e. linear plot in the Bode impedance diagram. The faradaic charge transfer reaction associated with corrosion gives rise to a finite resistive element at low frequencies. Therefore the impedance at low frequency area decreases as a result of the initiated corrosion. Indications of material degradation are also a decrease of the resistance element in the high frequency region and the change from a largely capacitive to a resistive behaviour. Generally, the constant impedance value in low frequency area i. e. the polarisation resistance is inversely proportional to the corrosion rate of the material.

In neutral rainwater (pH 5.5(a), Fig. 8) an C/Al2O3/Al absorber surface exhibited mostly capacitive behaviour during the 14 days of immersion. The corrosion resistance of an aluminium substrate seemed to be lower than that of the C/Al2O3/Al surface. Reason for this is unclear, but possible causes are minor failures in the nolan lacquer insulation of the samples. Generally, the corrosion probability of aluminium is lower at pH 5.5 than at pH

3.5 (Pourbaix, 1966).

At higher potentials the impedance is probably distorted due to the low conductivity of the solution (a) (Table 1). The C/Al2O3/Al samples were lightened after immersion. The impedance of an C/Al2O3/Al absorber surface between 1 and 1000 Hz (Fig. 9) decreased during the immersion test high frequency area indicating possible decrease in graphite coating quality. However, this sample did not exhibit typical optical degradation (Sample B in Figs. 3-5).

In acidic rainwater (Fig. 9) the impedance of the C/Al2O3/Al surface and the aluminium substrate in low frequency area are relatively low and therefore expected corrosion rate is higher than at pH 5.5(a). The increase in the impedance values after 1 days indicates some kind of passivation of the surfaces. However, after 5 days the impedance at low

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al., 2003a) on the surface is oxidated through chemical reactions forming CO, CO2 and other compounds (Hihara and Latanision, 1994). The revealed A^O3 layer subsequently probably follows typical alumina-aluminium corrosion mechanisms. We used FTIR — spectroscopy for determining the hydration level of the absorber substrate, consisting of thicker than naturally formed heterogeneous alumina layer on 0.5 mm thick aluminium substrate of 99.5 % purity.

The combined results are shown in Figs. 3-5. All samples inside dotted boxes contain identified Al2O3 hydroxides i. e. pseudoboehmite and/or boehmite, possible other forms of hydroxides as well (e. g. bayerite and gibbsite). Detailed analyses of the results show that hydroxides are identified in all samples exposed to pH 5.5 except one sample. The most degraded samples (PC > 0.18) have absorption bands related to both pseudoboehmite and boehmite (see Fig. 5 in (Konttinen et al, 2004)), whereas the rest of the hydrated samples do not have the characteristic absorption band of pseudoboehmite.

Discussion

Total-immersion test results were compared to standard condensation test results (Konttinen and Lund, 2003) in order to determine correlation between the two methods. Samples degraded to PC=0.05 in total-immersion tests exhibit generally similar p*. as the samples degraded to PC=0.05 in standard condensation tests (for details, see (Konttinen et al, 2004)). Still, when comparing all the optical results (Figs. 3-5) we can conclude that the total-immersion method (Fig. 1) without controlled solution movement turned out not being optimal as the samples did not exhibit clearly detectable temperature and time dependencies. This method was chosen instead of e. g. rotating disk method (Magaino, 1997) because impedance spectroscopy (EIS) tests required the use of a liquid electrolyte in contact with the sample. We assumed that the gas feed and natural convection would rotate the solution inside the flask sufficiently enough for reproducible test results. It seems that this was not the case at least at pH 5.5. A controlled solution or sample movement combined with controlled temperature and gas feeding rate should be implemented in the future tests.

Semi-integrating device attached to the FTIR-spectrometer increases interference thus disturbing the exact determination of precise absorption peaks. Therefore we had to make rougher analyses of absorption bands.

Hihara and Latanision (Hihara and Latanision, 1994, pp. 251-252) noted in their study about corrosion of C/Al2O3/Al metal matrix composites (MMC) that proton reduction on graphite will polarise aluminium to noble potentials, explaining the negligible galvanic corrosion rates in de-aerated solutions for MMC. Their results are not directly comparable to ours as the MMC materials differ in structure and purpose of use from the C/Al2O3/Al absorber surfaces. Although our results for samples A-B do not generally match to those for MMC (Hihara and Latanision, 1994) with regard to aeration, it is possible that similar polarising phenomena occurs on the absorber surfaces as well under similar conditions, and may corrode the C/Al2O3/Al absorber surfaces during long-term natural exposure.

As a reference we exposed three C/Al2O3/Al absorber samples to non-aerated total — immersion at the room temperature, one at each pH. The samples exposed to pH 5.5 and pH 4.5 were hydrated approximately to the level of PC « 0.5-0.6 after 30 and 80 days, respectively. Sample exposed to pH 3.5 did not show any degradation after 160 days by visual inspection.

Conclusions

Tests reported in this paper have provided with a general picture on the degradation effect of acid rain (pH 3.5 or 4.5) and neutral rain (pH 5.5) rain on the rough C/Al2O3/Al solar absorber surfaces at 60, 80 and 99°C. In order to provide more accurate temperature — and time-dependent results the total-immersion test method used needs to be further
developed to include controlled movement of solution or sample. Similar to standard tests (draft proposal ISO/CD 12592,2) the main degradation mechanism has been found to be hydration of aluminium oxide (especially at pH 5.5). It is possible that pH 3.5 is too acidic for aluminium hydroxides to be formed, thus preventing further corrosion of the C/Al2O3/Al surface.

For aluminium/aluminium oxide containing samples FTIR-spectroscopy can be used for determining stages of hydration. In our tests we have observed similar degradation for another type commercial aluminium substrate based absorber surface as well.

According to the electrochemical measurements the corrosion rate of aluminium substrate is faster at pH 3.5 than at pH 5.5. At pH 3.5 the anodic current density of the C/Al2O3/Al surface is smaller compared to aluminium substrate. Electrochemical measurement results for aluminium substrate do not deviate significantly from the results for the C/Al2O3/Al surfaces, indicating that the electrochemical measurements measure corrosion characteristics of aluminium to a large extent. Optical degradation of C/Al2O3/Al surface is mainly due to hydration of aluminium oxide, and this phenomenon was not clearly detectable in these electrochemical measurements. With another type of EIS test system setup, it may be possible to obtain more coherent results.

All these results indicate that unglazed solar absorber surfaces based on aluminium substrate need to be well protected against rain diffusion onto the substrate in order to prevent degradation caused by hydration of aluminium oxide.

Acknowledgements

We wish to thank Mr. Mikko Mikkola for setup of the gas distribution system and Mr. Iwao Nitta for translating reference (Takahashi et al., 1987) into English.

References

ASTM, 1987. Standard reference method for making potentiostatic and potentiodynamic anodic polarisation measurements, G5. 1987, Annual book of ASTM Standards.

ASTM, 1989. Standard practice for calculation of corrosion rates and related information from electrochemical measurements, G102, 1989, Annual book of ASTM Standards. Brunold, S., Frei, U., Carlsson, B., Moller K., Kohl, M., 2000a. Accelerated life testing of solar absorber coatings: Testing procedure and results. Sol. Energy 68, 313 — 323. Carlsson, B., Frei, U., Kohl M., Moller, K., 1994. Accelerated Life Testing of Solar Energy Materials — Case study of some selective materials for DHW-systems. IEA SHCP Task X. Hihara, L. H., 1997. Corrosion of aluminium-matrix composites. Corros. Rev. 15, 361-386. Hihara, L. H., Latanision, R. M., 1994. Corrosion of metal matrix composites. Int. Mater. Rev. 39, 245-264.

Howells, G., 1990. Acid rain and acid waters. Ellis Horwood Limited, Chichester, p. 14. Konttinen, P., Kilpi, R. Lund, P. D., 2003a. Microstructural analysis of selective C/Al2O3/Al solar absorber surfaces. Thin Solid Films 425, 24-30.

Konttinen, P., Lund, P. D., 2003. Thermal stability and moisture resistance of C/Al2O3/Al solar absorber surfaces. Sol. Energy Mater. Sol. Cells, in press.

Konttinen, P., Lund, P. D., Kilpi, R. J, 2003b. Mechanically manufactured selective solar absorber surfaces. Sol. Energy Mater. Sol. Cells 79, 273-283.

Konttinen, P., Salo, T., Lund, P. D., 2004, Corrosion of unglazed rough graphite-aluminium solar absorber surfaces in simulated acid and neutral rain, submitted to Solar Energy Lorenz, W. and Mansfeld, F., 1981. Determination of corrosion rates by electrochemical DC and AC methods. Corrosion Science 21, 647 — 672.

Magaino, S., 1997. Corrosion rate of copper rotating-disk-electrode in simulated acid rain. Electrochim. Acta 42, 377-382.

Magaino, S., 1999. Zinc corrosion in simulated acid rain. Electrochim. Acta 44, 4307-4312.

Shreir, L. L., Jarman, R. A., Burstein, G. T., 1994. Corrosion. Butterworth Heinemann, Oxford, pp. 19:18-19:20.

Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon, New York, 1966.

Takahashi, H., Yamagami, M., Furuichi, R., Nagayama, M., 1987. FTIR Analysis of Hydroxide Films on Aluminum. J. Surface Sci. Soc. Japan 8, 279-281 (in Japanese). Tanemura, S., Yoshimura, K., Taga, K., Odaira, R., Ishida, M., Tsuboi, S., Yoshikawa, M., 1990. Accelerated aging testing of Ni pigmented anodized Al under heat and/or high humidity loads and degradation mechanism. In: Horigome, T., Kimura, K., Takakura, T., Nishino, T., Fujii, I., (Eds.), Proceedings of the 1989 Congress of the International Solar Energy Society, 4-8 September 1989, Kobe City, Japan. Pergamon Press.

Internal Solver and User Interface for TRNFLOW

Due to the integration of the air flow model into Type 56 the TRNSYS Solver can no longer be used for the iteration process between the two models. A into Type56 integrated solver is used to reach convergence. Depending on the boundary conditions the coupling of the two models forms a recursive flow of information with negative feedback. E. g. at cold outdoor air an increasing indoor temperature imposes an increasing exchange of air. In turn this counters another increase of the indoor temperature so that a balance will appear (Fig. 4). If the negative feed back gets too strong there will be the risk of getting an unstable system. A solver algorithm with a successive substitution method (TRNSYS solver 0) then also becomes unstable. Therefor the new solver integrated into Type 56 automatically dampens individually every recursive flow of information according to the iteration process so that an optimum of stability and convergence is achieved.

For TRNFLOW the functionality of PREBID has been enlarged. Beneath traditional data of the thermal model also the required input information of the air flow model can be entered into the new PREBID 5. This data is also stored in the BUI file and can be read in from it again. Analog to the Standard Type 56 files (TRN, BLD) the new PREBID 5 also creates a COMIS Input File (CIF) which is read by TrNfLOW. The CIF created this way is — with few additions — completely equivalent to the format described in the COMIS 3.1 User’s Guide [4] Unlike to former simulations with COMIS the COMIS Input File (CIF) is now checked to be consistent with the thermal model (bui-file).

Assessment of durability and service lifetime of some static solar energy materials

Bo Carlsson, SP Swedish National Testing and Research Institute, Sweden Stefan Brunold, Institut fur Solartechnik SPF Hochschule Rapperswil, Switzerland Andreas Gombert, Fraunhofer Institut fur Solare Energiesysteme, Germany Markus Heck, Fraunhofer Institut fur Solare Energiesysteme, Germany

To achieve successful commercialisation of new advanced windows and solar fa­cade components for buildings, the durability of these need to be demonstrated prior to installation by use of reliable and well-accepted test methods.

In Task 27 of the International Energy Agency Solar Heating and Cooling Pro­gramme, a general methodology for durability test procedures and service lifetime prediction (SLP) methods therefore has been developed that should be adaptable to the wide variety of advanced optical materials and components used in energy efficient solar thermal and buildings applications. The general durability assess­ment methodology is now adopted to some static solar materials to allow prediction of service lifetime.

Introduction

The IEA Solar Heating and Cooling Programme, Task 27 on the Performance of Solar Fa­cade Components started at the beginning of year 2000 with the objectives of developing and applying appropriate methods for assessment of durability, reliability and environ­mental impact of advanced components for solar building facades [1].

For the work on durability there are two main objectives. The first is to develop a general framework for durability test procedures and service lifetime prediction (SLP) methods that are applicable to a wide variety of advanced optical materials and components used in energy efficient solar thermal and buildings applications. The second is to apply the ap­propriate durability test tools to specific materials/components to allow prediction of service lifetime and to generate proposals for international standards.

As the result of this work, a general methodology has been developed [2], which is now adopted to some static solar materials. The work is performed in three case studies on anti-reflective glazing materials, reflectors and solar facade absorbers. Anti-reflective ma­terials that are studied include sol-gel coated and etched AR glasses. Reflectors that are studied include aluminium alloy based mirrors; some protected by clear coats, and glass mirror reflectors. Solar Fagade Absorbers that are studied include coloured sputtered se­lective solar absorber coatings, absorber coatings made with sol-gel technology and thick­ness insensitive spectrally selective paints.

Comparison of Measurements and Modelling

Modelling

For the modelling of the lamella and roller type blinds in combination with glazing two different tools were used. The first one is the well-known European Window Information System WIS (version 2.0b) which has been developed further within the European WINDAT project (see http://www. windat. org). The algorithm used for the blinds is the standard algorithm using a radiosity method, splitting up the individual blinds in 10 flat sections reflecting completely diffuse (5 on the upper part and 5 on the lower part). Mirror — type blinds cannot be modelled which such an algorithm). A second simplification within WIS is the treatment of the slats as completely flat and without extension. Thus lamellae like the ones in Figure 1 pose a problem for this algorithm when radiation is passing nearly parallel through the slats. The model underestimates the possibility to hit a lamella and overestimates the transmission. WIS does estimated the convective heat flow through
devices based on a plug-flow model using the temperatures of the layers. This model is described in the ISO/FDIS 15099 standard.

Because of the optical simplification a second simple radiosity model (using only one section of the slat — one facing upward and one downward) based on view factors has been programmed. The approach is similar to the one documented in prEN13363-2 [ 2], however with two important extensions: firstly the lamella might be curved with a radius given, and secondly a direct transmission part is taken into account. Using the solar transmittance and reflectance calculated with such an algorithm, the total solar energy transmittance of the shading device in combination with a glazing (either inside or outside) is calculated using a simple resistance model. Convective and radiative surface coefficients from the glazing surface to the blinds, and through the blinds to the environment are estimated based on the "openness” of the blinds. The convective part is always constant. This model is called the “ISE model” in this paper but should not be confused with another more refined inhouse model utilising raytracing. [ 5]

So two simplified models were used, each having deficiencies in some areas. The models are both quick and can be used also with spectral information on the optical components (glass, slats). They should give a clue how good certain approximations and simplifications are for the estimation of total g-values.

Conductive Model of the Semitransparent Wall

The 3-D conduction equation of the glazing considers constant thermophysical properties and is given by:

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where F(x) = 0<Є Sg(x x, Sg is the extinction coefficient of the glazing and Hx is the length of the edge sides of the cubic cavity. The interior surface boundary condition is calculated by applying the following energy balance:

qabs (Hx, y,z) = qcd-g (Hx, y,z) + qcd-a(Hx, y,z) +qr4 (Hx, y,z) (14)

where qabs(Hx, y,z) is the thermal energy that is absorbed by the solar control coating of the glazing and is given by the heat flux that is transported by conduction in the glass, qcd. g(Hx, y,z), the heat flux that is transported to the interior air by the solar control coating, qCd — a(Hx, y,z) and the net radiative exchange from the glazing to the interior air, qr4(Hx, y,z).

The exterior boundary conditions used are the ones measured and reported in [Flores and Alvarez, 2002]. Figure 2 shows the temperature distribution on the exterior of the glazing that was taken as boundary condition for the mathematical model.

Tg (Hx2,y, z)= Texo(Hx2,y, z)

The boundary conditions for the edge sides of the glazing were adiabatic:

dT, . dT dTg . . dTg. .

-(xAz) = 0 , -x, Hy, z)= 0 , -(x, y,°)= 0 y -((y, Hz )= 0

dy dy dz dz

for Hx < x < Hx+Hx2.

Structure origination and replication

The origination of master microstructures on large areas is still not very well established. Within the field of mechanical, electrical and optical microsystems origination techniques such as e-beam writing, laser writing, focused ion beam etching, photo or x-ray lithography are widely used and mature. Unfortunately, many of them are not suited to originate well defined continuous surface-relief profiles and are especially not suited to originate the mi­crostructures on large areas homogeneously. So far, mainly ultra-precision machining and interference lithography are used as origination techniques for homogeneous large-area master structures.

Ultra-precision machining is a technique where the classical machining techniques such as turning, drilling, milling, and cutting are performed by using ultra-precision machines, dia­monds as tools and metals as material. The typical dimensions of microstructures which are made by ultra-precision machining are in the range of 10 pm to 500 pm.

Interference lithography makes use of the interference pattern which is formed when two or more coherent light waves are superposed. In a typical optical set-up, a laser is used as a source for ultra-violet (UV) radiation. The laser beam is split into two beams. Each of the beams is directed by mirrors towards a substrate coated with photoresist where the beams are superposed after being expanded. In Fig. 2, a photo of one of the interference lithog­raphy laboratories at Fraunhofer ISE is shown. When the process is sufficiently controlled also very demanding surface-relief structures can be originated by single or multiple expo­sures (Fig. 3). Of course, origination of such structures on large areas is still a technologi­cal challenge and not every exposure gives the required result.

The master structures cannot be used as embossing tools directly in the case of photore­sist and are not often used for cost reasons in the case of machined metal masters. The standard process chain includes therefore the replication of the master structures by elec­troforming into nickel. When photoresist master structures are used, a thin conducting layer is deposited by evaporation, sputtering or by the wet chemical reduction of silver firstly. Then, nickel is grown with a thickness in the range of 50 pm to 3 mm by using nickel sulphamate solutions on top of the master structures. This first nickel replica is then sepa­rated from the orginal. After passivation, the first nickel replica is copied by electroforming again. By applying the process repeatedly, several generations of so-called nickel shims can be produced without too much loss in the structural details.

The daughter generations of the nickel shims are used for replicating the micro structures. For polymers a large variety of mature replication techniques exists, e. g. hot compression molding, injection molding, and reactive processes including radiation curing. The latter are especially suited for high-volume large-area applications.