Category Archives: BACKGROUND

General methodology for durability assessment

The methodology adopted by Task 27 includes three steps: a) initial risk analysis of poten­tial failure modes, b) screening testing/analysis for service life prediction and microclimate characterisation, and c) service life prediction involving mathematical modelling and life testing.

Initial risk analysis

The initial risk analysis is performed with the aim of obtaining (a) a checklist of potential failure modes of the component and associated with those risks and critical component and material properties, degradation processes and stress factors, (b) a framework for the selection of test methods to verify performance and service life requirements, (c) a frame­work for describing previous test results for a specific component and its materials or a similar component and materials used in the component and classifying their relevance to

the actual application, and (d) a framework for compiling and integrating all data on avail­able component and material properties.

The programme of work in the initial step of service life assessment is structured into the following activities: a) Specify from an end-user point of view the expected function of the component and its materials, its performance and its service life requirement, and the in­tended in-use environments; b) Identify important functional properties defining the per­formance of the component and its materials, relevant test methods and requirements for qualification of the component with respect to performance; c) Identify potential failure modes and degradation mechanisms, relevant durability or life tests and requirements for qualification of the component and its materials as regards durability.

Table 2 Specification of critical functional properties of booster reflectors and requirements set up by the IEA SHCP Task 27 group

Critical functional

Test method for determining functional

Requirement for functional



capability and long-term per-


Reflectance (specu­lar, A pspec, and dif­fuse, Pdif)

ASTM E903-96 „Standard test method for solar Absorptance, Reflectance, and Trans­mittance of Materials Using Integrating Spheres“

PC = 0.35-A pspec +(0.1/C)-Apdif < 0.05

with concentration ratio C=1.5

Adhesion between coating and sub­strate

Visual assessment

ISO 4624:2002 „Pull-off test for adhesion“ ISO 2409:1992 „Paints and varnishes — Cross cut test“

No blistering Adhesion > 1 MPa Degree 0 or 1

The first activity specifies in general terms the function of the component and service life requirement from an end-user and product point of view, and from that identifies the most important functional properties of the component and its materials. In Table 1 and Table 2 results are shown from the analysis made by the Task 27 group on booster reflectors. How important the function of the component is from an end-user and product point of view needs to be taken into consideration when formulating the performance requirements in terms of those functional properties. If the performance requirements are not fulfilled, the

particular component is regarded as having failed. Performance requirements can be for­mulated on the basis of optical properties, mechanical strength, aesthetic values or other criteria related to the performance of the component and its materials.

Potential failure modes and important degradation processes should be identified after failures have been defined in terms of minimum performance levels. In general, there exist many kind of failure modes for a particular component and even the different parts of the component and the different damage mechanisms, which may lead to the same kind of failure, may sometimes be quite numerous. In Table 3 an example from the Task 27 work on booster reflectors is presented.

Fault tree analysis is a tool, which provides a logical structure relating failure to various damage modes and underlying chemical or physical changes. It has been used for the static solar materials studied in Task 27 to better understand observed loss in performance and associated degradations mechanisms of the different materials studied. In Figure 1 and Figure 2 are shown examples on how the different failure modes and associated deg­
radation mechanisms can be represented for booster reflectors and antireflective glazing materials.



Degradation of protective coating on reflector

Insufficient coating of reflective

layer at production










Ageing with

Loss in

Loss in

Loss in





adhesion of

of substrate





and loss in

due to


layer to






al damage





of surface

Corrosion of reflective layer



tion and




Loss of reflector performance

Figure 1 Representation of failure modes and associated degradation mechanisms for booster reflectors from the IEA SHCP Task 27 study

The risk associated with each potential failure/damage is taken as the point of departure to judge whether a particular failure mode needs to be further evaluated or not. Risks may be estimated jointly by an expert group adopting the methodology of FMEA (Failure Modes and Efffect Analysis) [2,3]. In Table 4 the result of a risk analysis made by the Task 27 group on booster reflectors is presented.

Failure/Damage mode / Degradation process

Estmated risk number associated with damage mode (based on FMEA)

A1 Degradation of the protective layer — Ageing with material decomposition


A2 Degradation of the protective layer — Loss in protec­tive capability due to mechanical damage


A3 Degradation of the protective layer — Loss in adhe­sion to reflective layer


A4 Surface soiling


A5 Surface erosion


B1 Insufficient coating of reflective layer at production


C1 Corrosion of the reflecting layer (Result of mecha­nisms A1-A3, B1)


D1 Loss of adhesion of reflector from substrate


D2 Degradation of the substrate


Table 4 Risk assessment on different damage modes of booster reflectors made by the IEA SHCP group using the methodology of FMEA [2,3]______________________________

Transmittance through Venetian blinds

In a first step we compared an interior Venetian blind measurement using a large integrating sphere [Platzer, 19XX] with the solar transmittance calculated with the two radiosity models. The measurement of the exterior blinds shown in Figure 1 was deemed to be to complicated as the port aperture of the sphere is close to one period of the shading device. For the interior blinds with smaller period several measurements with laterally displaced blinds were averaged.

Figure 4: Comparison of optical measurements for Venetian blinds (25mm white) for different tilt angles are compared to modeled data using WIS and the ISE model.

From the comparison of experiment with model data one can conclude that both methods reproduce quite well the optical transmittance in the main parts of the angular incidence intervall. As predicted WIS overestimates direct maximum transmittance to some extent — that would be even more extreme with dark slats (one has to take into account that due to the 10 degree calculation intervall the maximum value of 100% is not sampled in most cases).

The ISE extended view factor model gives a better approximation for the maximum transmittance, however, for large negative incidence angles (reflections from the ground) this model seems to underpredict the transmittance.

In order to see the effects for the more complex lamella shapes of Figure 1, we have to have a look at the calorimetric measurements.

Total solar energy transmittance through external blinds

Three variants of this blinds have been measured in combination with a glazing coated on position 2, namely white, white-perforated and brown lamellas. For this lamella geometry the differences between the two models are more pronounced. In the maximum transmittance region, but very similar in others (see Figure 5 and Figure 6). The differences are in line with the ones observed with the integrating sphere measurements, however modulated to some extent by the glazing behind the blinds.

For internal blinds the glazing should have an even more important influence on the angular function. This can be seen in the next paragraph.

Total solar energy transmittance through internal blinds




Internal Venetian white blinds have been tested and modeled with two different solar control glazings. Both comparisons with experimental data show a very good correspondence of the results. The small differences between the models are due to different treatment of the glazing data. The angular dependence of a float glass pane was used for the the glazing g-value in the simplified ISE model. This is obviously not completely consistent with the measured data, especially for the glazing Ipasol 6634 in Figure 7. However, this has no big influence on the overall result.

Internal roller blinds

Table 1: Modeled and experimental g — and U-values for glazings with internal grey roller


inc. angle





U2 g

[W/(m2K)] [-]















1.06 0.218





1.06 0.184














1.20 0.305







8The specific cost of the whole solar-field investment with allocation of the overhead expense would be 139 €/m2 at a size of 300 000 m2

[1] Including Biomass boiler

[2] Starting point were 150 €/m2 (solar field size 437’639 m2) for a »third« plant. Using the cost structure of the Solarmundo collector the specific investment resulted in 130 €/m2 due to systematic collector optimization (e. g. more mirrors for one absorber tube). The formula leads so reduced specific costs for large units.

[3] Currently the raising of this tariff is being discussed in Spain.

[4] In previous examinations it was found out that a more sophisticated power cycle does not lead to lower LEC due to 1.) higher specific costs, 2.) elevated heat losses in the solar field and 3.) higher suiting losses due to the fourth collector section for intermediate superheating.

[5] Other operating strategies are conceivable as well.

[6]The aperture width is defined as the net primary mirror-field Wap = N x B

[7]levelized electricity cost

[8]The sunshape is assumed to be a distribution with a circumsolar-ratio of CSR=7.5%

[9] If one considers not the local distribution but the effective distribution over a sufficient absorber length

[10]The effective relative radiance Xeff is referred to the incident beam radiation Ib

[11]It is calculated with the optimal configuration for an optical error of a — = 4.56 mrad

[12] Wagon moved above the appropriate cover, PEM deactivated and lowered, then acti­vated again to retrieve the cover by lifting it up;

[13] assessment of error at each intermediate stage of calibration and processing, a final error being deduced;

[14] profilo climatico dell’Italia, ENEA, 1999

[15] “Manual de Arquitectura Bioclimatica”, Guillermo Gonzalo, Tucuman 1998

[16] The solar system has been design together with Antonio Bee from Costruzioni Solari s. r.l.

[17] The air solar system Solarwall has been design together with Rolando Malaguti from Solarwall Italia

[18] Binz, A. (Projektleitung): MINERGIE und Passivhaus: Zwei Gebaudestandards im Vergleich, Schlussbericht. Ausgearbeitet durch Zentrum fur Energie und Nachhaltigkeit im Bauwesen, im Auftrag des Bundesamtes fur Energie (BFE), Marz 2002. (Vertrieb: EMPA ZEN, CH-8600 Dubendorf, www. empa. ch/ren )

[19] Kleiven, T. (2003) Natural Ventilation in Buildings. Architectural concepts, consequences and possibilities. PhD thesis at Department of Architectural Design, History and Technology, NTNU.

[20] Lapithis, P. Solar Architecture in Cyprus. PhD theses, University of Wales, UK, 2002

[21] Ibid

[22] Ibid

[23] Ibid

[24] Kolokotroni, M., The Thermal Performance of Housing in Greece: a Study of the Environmental response to Climate, MSc, Bartlett School of Architecture, UCL, 1985.

[25] Ibid

[26] Sergides, D. “Zero Energy for The Cyprus House", The Architectural Association, 1991

Solar Heating and Cooling Implementing Agreement

Energy Conservation in Building and Community Systems Implementing Agreement

12 P. Nitz et al, "Sonnenschutz und Lichtlenkung durch mikrostrukturierte Oberflachen",

Tagungsband 9. Sympos. Innovative Lichttechnik in Gebauden, Staffelstein, 23./24.1.2003, S. 103-108

[30] C. Buhler „Mikrostrukturen zur Steuerung von Tageslichtstrbmen“ PhD thesis, A.-L.-Universitat Freiburg, Germany (2003)

[31] V. Wittwer, A. Georg, W. Graf, J. Ell, Casochromic Windows, ISES Solar World Congress 2003, Gbteborg, Juni 2003

[32] T. J. Richardson, J. L. Slack, R. D. Armitage, R. Kostecki, B. Farangis, M. D. Rubin, Switchable mirrors based on Nickel-Magnesium Films, Applied Physics letters 78 no. 20 (2001) 3047-3049

[33] DDC „direct digital control”

[34] PCU: „processor controlled unit“

[35] PT100: Platinum resistor temperature sensor, 0°C, 100,000 i increases each above 1 °C approx.. 0,4 i

[36] up to now, there is no signal from this

[37] D/A-converter: converts analogue (electrical) in digital bits.

‘Data allocated by BASF. Shear-velocity from 50 to 450 і


20 30 40 50 60 70 80

Epitaxial Layer Thickness (mic.)

[40] V. Perraki, Thesis (Paris 1988).

Figure 4. Efficiency graph versus of base thickness (pm) for polycrystalline Si solar cell under AM 1.5 irradiance conditions, calculated for grain size 250 pm and variable


Heat Transfer Coefficients

Hx is the length of the edge sides of the cubic cavity and Tci is the cold wall temperature.

The radiative Nusselt number used by Yucell et al. in 1989 and some other authors such as Behnia et al. in 1990 is given by:

qrad(Hx, y,z) is interior glazing surface the heat flux.

3.1 Solution Procedure

The conservation equations (1)-(5) and its boundary conditions (6)-(12) were solved using the well known finite-volume method described in detail by Patankar 1980. The SIMPLE algorithm was used in order to couple the velocity and pressure fields. A 21x21x21 no uniform grid was used and near the wall surfaces a finer grid was used. All equations were coupled and solved using a iterative procedure. The iterative solution procedure was terminated when the residuals of all grid points, normalized by suitable reference quantities, felt below 0.01 percent. Knowing the temperature distributions and the heat flux through the glazing, the solar heat gain coefficient can be calculated by [ASHRAE, 2001]:

where G is the solar radiation that strikes the exterior surface of the glazing, т is the transmissivity of the glazing and q, is the total thermal heat flux through the glazing.

2. Results

Table 1 shows the input parameters that were used in the numerical code from the mathematical model.

Table 1. Input parameters for the simulation.

Ambient conditions

Geometry of the cavity

Rayleigh number





Initial temperature

25 °C

Thickness of the glass

0.002 m

Normal incident radiation

1000 W/m2

Reference temperature T0

35.4 oC

Temperature of cold wall

25.0 oC

Temperature of hot wall

Texo(y, z)

Optical properties

Thermophysical properties at T0

Transmittance of glass


Thermal conductivity of glass

1.4 W/m K

Reflectance of glass


Specific heat of glass

750 J/kg K

Absorptance of glass


Density of glass

2500 kg/m3

Absorptance of the solar control coating


Thermal conductivity of air

0.0263 W/m K

Exterior emittance of wall 2, (glass)


Specific heat of air

1006 J/kg K

Interior emittance of wall 1,3, 4, 5 y 6


Density of air

1.22 kg/m3

Interior emittance of wall 2


Viscosity of air

0.0000155 m2/s

Antireflective surfaces as an example

For an ideal antireflective effect, the surface of the transparent cover should have a gradi­ent of the index of refraction ranging from 1.0 for air to the index of refraction of the cover material. For the required very small indices of refraction no materials exist in nature. A solution is given by subwavelength surface-relief gratings with a continuous profile which form an effective refractive index gradient. This type of anti-reflective surface-relief grating is called "moth-eye” structure according to the example found in nature on the cornea of night-flying moths [17]. In Fig. 4 an artificial "moth-eye” structure made in photoresist with periods of 250 nm is shown. By replicating such types of antireflective surfaces into poly­mers or sol-gel films on glass, the transmittance of films or sheets can be increased sig­nificantly in the region of the solar spectrum (Fig. 5). While replication in polymers is a well established process, the replication in sol-gel films resulting in purely inorganic micro­structures is still under development. The sol-gel technology is especially interesting in the case of antireflective surfaces which are exposed directly to the incident solar radiation where polymer microstructures are not sufficiently durable.

Fig. 5: Transmittance spectra of a micro-structured and an unstructured PET film with a thickness of 125pm. The interference modulation of the unstructured film is due to the mismatch of the refractive indices of the PET film and the acrylic coatings in which the moth-eye structure was replicated.

Micro-structuring surfaces allows are very variable modification of the optical properties of the surfaces. Many solutions for optical components in solar applications are given by this approach in principle. The major difficulty in realisation is the big mismatch between the dimension of the functional structure and the area which has to be homogeneously struc­tured. Thus, the challenges is still to develop suitable structure origination and replication techniques. It has been shown that interference lithography is a quite versatile tool to originate the required structures on areas of up to half a square meter so far. Further in­crease of the homogeneously structured area is nevertheless still necessary for some of the applications.

Building Performance and Experiences


Five of the buildings shown in figure 2 show a heating energy consumption (end en­ergy) of less than 40 kWh m-2a-1 which is mostly due to the high insulation standard. The mean U-values of these buildings are between 0.21 and 0.43 W m-2K-1. Four of all funded buildings (3 offices, 1 factory) reach the passive house standard with a maximum heat load of 10 W/m2 and air heating systems. In buildings with a high in­sulation standard and a heat recovery system, almost the complete heating energy is required below ambient temperatures of 13 °C. The mean room temperatures in the offices lie around 22 °C which corresponds to experiences from domestic dwellings. Passive solar gains play a minor role in most of the buildings because combined ex­terior glare and sun protection systems are used frequently. Also the favouring of south-orientated glazed facades for office rooms often leads to a functionally and economically unfavourable design solution for the whole building.


Since ventilation losses become dominant in large compact buildings, most of the funded buildings have a heat recovery system. Air change rates in the winter are around 1 h-1. Due to different systems the efficiencies (ratio between heat supply and electric energy demand) vary between 5.7 kWhth/kWhei (LAMPARTER, flat plate heat exchanger with a heat recovery efficiency of 80%) and 3.3 kWhth/kWhel (POLL — MEIER, exhaust air system with heat pump). Air leakage rates are very low for almost all buildings (n50 << 1.2 h-1). In a lot of buildings atria are included into the ventilation concept, serving either for collecting exhaust air (figure 3) or distributing fresh air.

Figure 3: Ventilation concept of the KfW building in Frankfurt

Accelerated ageing test

To perform the accelerated ageing tests a Votsch Industrietechnik climate chamber of type VC 4033 MH was used. In the chamber it is possible to run tests within a temperature range of 25 to 90oC while regulating the relative humidity between 10 to 95 %. The sample holder was water cooled to make sure that condense was formed on the samples. A thermostat bath of type Techne Tempunit TU-16D controlled that the temperature of the cooling water was about five degrees lower than the temperature of the chamber.

Condensation, high temperature and aggressive airborne pollutant tests are recommended for solar absorber coatings. The condensation test is however the most important test to perform on the type of absorbers investigated in this work because previous experience shows that alumina can be sensitive to moisture [7]. To be able to compare accelerated ageing test results from different laboratories some kind of test procedure has to be defined. There exists no European standardized procedure to test the durability of a solar absorber surface, but some assessment methods have been proposed by the IEA Task X Working Group, designated ISO/DIN 12952.

One assessment method is a condensation test where the temperature of the environment is set to 40°C and the relative humidity to 95 % [4]. The sample temperature should be a few degrees colder than the surroundings to ensure that condensation occurres on the sample surface. Tested samples should be assessed after 80, 150, 300 and 600 hours according to the following performance criterion:

PC = — Aasol + Q.25Astherm < 0.05

where Aasoi is the difference in normal solar absorptance before and after the test and Astherm is the difference in normal thermal emittance. The 0.25 factor reduces the importance of a change in thermal emittance compared to a change in solar absorptance. The testing cycle should continue up to 600 hours as long as the PC value is less than 0.05, if not, it is terminated. A PC value of more than 0.05 indicates that the surface is not condense proof and has poor resistance towards moisture and hence there is no use in continuing the test. Note that it is possible to get a negative PC value, which indicates an actual improvement of the optical selective properties of the surface.


M. Sanchez, D. Leinen, J. R. Ramos-Barrado, F. Martin.

Laboratorio de Materiales y Superficie. Departamentos de Ffsica Aplicada & Ingenieri’a
Qui’mica. Universidad de Malaga. 29071 Malaga. Spain


The radiation emitted from the sun peaks from 300 to 2000 nm. A warm object emits most energy in the infrared region. Selective surfaces make use of this spectral separation to maximise the energy absorbed in the solar spectral range (300-2000 nm) and minimise the energy emitted in the infrared spectral range (beyond 2000 nm). The ideal characteristic of a photothermal converter can be approximated by an absorber-reflector tandem. A practicable solar selective absorber is obtained when a metal of high infrared reflectance is coated with a thin film of high solar absorptance. The thin film must be highly absorbing over the solar spectrum and transparent in the infrared, to allow the metallic reflector to transmit through this region and to determine the thermal emittance. The solar absorptance (as) represents the proportion of absorbed radiation in the solar region, and the thermal emittance (et) represents the proportion of heat radiation emitted in the infrared. Therefore, as should be as close to 1.0 as possible, whereas et should be as close to zero as possible. To enhance the solar absorptance of the coated metal, the solar spectral region should have the lowest possible reflectance and, to suppress the thermal emittance, the infrared spectral region should have the highest possible reflectance.

Black paints have been often applied as absorbing films, with high solar absorptances but also with high emittances. On the other hand, various single or multi-component transition metal oxides exhibiting a black colour, have been studied with the aim of obtaining solar absorber coatings like Co2O3, CuO, MnO2 [1-4]. Within this context, we have tested the possibility to obtain molibdenum and molybdenum/tin oxide films by chemical spray pyrolysis on copper sheets, with the purpose to use them as selective surfaces. Molybdenum oxide (MoO3), is an important material for electrochromic devices. In addition to electrochromic properties, this material also demonstrates photochromic and thermochromic properties, due to the formation of colour centres by light irradiation and temperatures effects, respectively [5]. Tin oxide has been used as infrared mirror, usually doped with F, in smart windows [6]. Also, we have tested the possibility to use thin films of copper and zinc sulphide. ZnS is a high refractive-index material with low absorption from 400 to 14000nm [7-8], and copper sulphide have been used in air-glass tubular solar collectors as absorber coating, in photodetector and photovoltaic applications. First, we have used copper sulphide with the intention to grow a buffer layer between copper and the oxide absorbent film.

In spray pyrolysis, we use aqueous solutions of the metal precursor which are sprayed by an air stream onto the heated substrate surface, where pyrolysis take place and the oxide film grows. A problem is the need to heat the copper sheet in order to allow the precursor to decompose and to form the metallic oxide film, and at the same time, to avoid the oxidation of the copper substrate. We assume that at our conditions for spray pyrolysis, the presence of some amount of copper oxide at the interface is practically unavoidable. We have tried to minimize this problem using low temperatures of deposition.


Compressed air was used to atomise the solution through a spray nozzle over the heated substrate. Air is directly compressed from the atmosphere, using filters to remove water and oil waste in order to avoid contamination. Aqueous solutions of (NH4)6Mo7O24- 4H2O (10’2 M) and SnCl2- H2O (5- 10’3 M), have been used as precursors of Mo and Sn respectively. The solutions were pumped into the air stream by means of a syringe pump at a rate of 50 cm3/h. A stream of 20 l/min of air, measured under atmospheric conditions, was used to atomise the solution. A 5 cm x 5 cm of copper substrate, was uniformly coated. The time of deposition was changed to obtain different film thickness. Thermal treatments were carried out with a 800 W halogen lamp.

Aqueous solutions of CuCl2, Zn chloride or Zinc acetate, and thiourea in a molar ratio from 1:2 to 1:4 were used as precursor solutions to obtain the copper sulphide and zinc sulphide films. The concentrations of the copper and zinc precursor solutions were 10-2 M. The substrate temperature was in the range of 225-250 °C, and the time of deposition of 3 to 7 min.

Measurements of the near-normal hemispherical reflectance were performed on a Perkin-Elmer lambda 19 UV-Vis-NIR spectrometer equipped with an integrating sphere coated with BaSO4 (0.3-2.5 pm range), and with a Bruker IF66/S spectrometer equipped with a diffuse gold coated sphere for the MIR — FIR range.

The solar absorptance as was calculated by weighted integration of the spectral reflectance with the hemispherical solar spectrum AM1.5. The thermal emittance et was calculated by weighted integration of the spectral reflectance with the Planck Black Body radiation distribution at 373 K.

The XRD spectrums were recorded with a SIEMENS D-501 diffractometer with CuKa radiation. Scanning electron microscopy (SEM) pictures were obtained with a JEOL JSM 5300 apparatus. Surface and in-depth composition, films were studied by X-ray Photoelectron Spectroscopy (XPS) using a PHI 5700 spectrometer with Mg Ka (1253.6 eV) and Al Ka radiation as excitation sources. The energy scale of the spectrometer has been calibrated using Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7 eV, 368.3 eV and 84.0 eV respectively. PHI ACCESS ESCA-V6.0 F software package was used for data acquisition and analysis. Atomic concentrations were determined from the photoelectron peak areas using Shirley background subtraction and sensitivity factors provided by the instrument manufacturer. Spectra were referenced to the C1s line of the adventitious carbon at 284.8 eV.

Example: Double Facade Simulation with TRNFLOW

The development of TRNFLOW was finished in March 2003. Since that date it has already proved it’s strength in various projects like in the simulation of cross ventilation of offices, natural ventilation of double facades and atria’s, the pressure difference at doors of high-rise buildings or pollutant concentration in a building. The Figures 7 and 8 show a multi story building with double facade as an example.

TRNFLOW has demonstrated its suitability for large building models by an example with more than 60 thermal zones and lots of auxiliary nodes. No numerical or other difficulties have been obtained in those runs.

Fig. 5: Air flow network model of a multi story building with double facade

Screening testing/analysis for service life prediction

Screening testing is thereafter conducted with the purpose of qualitatively assessing the importance of the different degradation mechanisms and degradation factors identified in the initial risk analysis of potential life-limiting processes.

When selecting the most suitable test methods for screening testing, it is important to se­lect those with test conditions representing the most critical combination of degradation factors.

Using artificially aged samples from the screening testing, changes in the key functional properties or the selected degradation indicators are analysed with respect to associated material changes. This is made in order to identify the predominant degradation mecha­nisms of the materials in the component. When the predominant degradation mechanisms have been identified also the predominant degradation factors and the critical service con­ditions determining the service life will be known.

Screening testing and analysis of material change associated with deterioration in per­formance during ageing should therefore be performed in parallel. Suitable techniques for analysis of material changes due to ageing may vary considerably.

On the static solar materials of Task 27, a number of accelerated screening have been performed including simulation of possible degradation in performance under the influence of high temperature, high humidity/condensation, UV, and corrosion loads; either single or combined loads; see Table 5.

In Figure 3 the results from a series of screening tests on pure aluminium, used as refer­ence reflector material, are shown as an example of result from the Task 27 study. Degra­dation in optical performance is observed mainly, as expected, in the corrosion tests. In Figure 4 the result from the testing of a number of antireflective glazing materials at 80 °C and 95 %RH is given. The cause of degradation in optical performance is in this case not understood and the degradation therefore needs to further analysed. To identify degrada­tion mechanisms for the tested materials various analytical techniques are presently em­ployed.

Comparison between numerical and experimental values

To verify the mathematical model, the experimental results reported by [Flores and Alvarez, 2002] were used. The verification consisted in comparing interior air temperatures measured with the same ones calculated by the theoretical model. Table 2 presents twelve experimental temperatures, the numbers in the parenthesis are the coordinates of each measured point. Table 3 shows their corresponding theoretical air temperatures and Table 4 presents their corresponding percentage differences. From this table, we can see that the maximum percentage difference was 6.04% (30.34°C experimental, 28.51 °C calculated, maximum difference 1.83°C,) and the minimum percentage was 0.11%. The average percentage difference was 1.87%, which corresponds to an average difference of

0. 66°C. The uncertainty of the experiment was ±0.5°C, thus the theoretical model can represent very closely the experimental air measurements in the interior of the cavity.

Table 2. Experimental air temperatures and its x, y, z-coordinate

31.75 (0.2,9,5)

40.63 (2.5,9,5)

42.35 (7.5,9,5)

49.29 (9.8,9,5)

30.34 (0.2,5,5)

35.55 (2.5,5,5)

35.72 (7.5,5,5)

43.69 (9.8,5,5)

26.50 (0.2,1,5)

30.49 (2.5,1,5)

31.71 (7.5,1,5)

41.76 (9.8,1,5)

Table 3. Theoretical air temperatures calculated and its x, y, z-coordinate

31.14 (0.2,9,5)

40.59 (2.5,9,5)

41.81 (7.5,9,5)

49.01 (9.8,9,5)

28.51 (0.2,5,5)

34.71 (2.5,5,5)

34.77 (7.5,5,5)

44.71 (9.8,5,5)

26.42 (0.2,1,5)

30.04 (2.5,1,5)

32.01 (7.5,1,5)

41.08 (9.8,1,5)

Table 4. Percentage differences between the experimental and theoretical interior air ________________________________ temperatures in the cavity._______________________________













Knowing the air temperatures distribution from the numerical solution of the governing equations, the interior convective, radiative and total Nusselt numbers were calculated for an irradiation of 1000 W/m2. From the results, assuming that the absorption of energy of the glass is zero, the absorbed thermal energy by the solar control coating was 500 W/m2, in which, 81.1 W/m2 is transferred to the interior by thermal radiation; 72.4 W/m2 is by convection to the interior. Thus the energy that is transferred to the interior is 653.5 W/m2 and 346.5 W/m2 is going to the exterior. Therefore, we can see that percentage of radiative energy that goes to the interior is 12.41% and the percentage convective energy is 11.01%.

Using equations (17) and (18) the convective and radiative numbers were calculated. The convective Nusselt number was 11.48 and the radiative one was 12.8. Thus, the order of magnitude of the contribution of the radiative energy is almost the same as the convective one, meaning that the radiative exchange between surfaces plays and important role in the heat transfer process for this cavity. The solar heat gain coefficient calculated by using equation (19) was 0.65 and imply that 65% of the energy goes into the interior.