Category Archives: BACKGROUND

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.

Electrochromic Devices: Improving the Performance and Color Properties

E. Avendano, A. Azens, J. Backholm, G. Gustavsson*, R. Karmhag*, G. A. Niklasson and C. G. Granqvist

Department of Engineering Sciences, The Angstrom Laboratory, Uppsala University,

P. O. Box 534, SE-75121 Uppsala, Sweden

*Also at Chromogenics Sweden AB, Uppsala University Holding, SE-75183 Uppsala, Sweden

This paper presents a detailed study of the optical properties of a number of electrochromic nickel-oxide-based and iridium-oxide-based films. Chromaticity is analyzed with regard to a set of illuminants pertinent to different natural and artificial lighting conditions. In particular, it is shown that additions of Mg, Al, Si, Zr, Nb, and Ta can improve the transmittance of nickel-oxide-based films, and that Mg, Al, and Ta can have the same effect for iridium-oxide-based films.


Electrochromic materials are able to reversibly change their optical properties upon charge insertion-extraction induced by an external voltage [1-3]. The materials can be integrated in electrochromic devices of several different types and can be all-solid — state constructions as well as polymer laminated ones, with or without self-powering by solar cells [4]. These devices open a number of technologically interesting possibilities to modulate optical transmittance, reflectance, absorptance, and emittance. Recently, special attention has been devoted to designs incorporating electrochromic hydrated nickel oxide films operating in conjunction with electrochromic tungsten oxide; this combination of materials makes it possible to attain a neutral gray color in the dark state. Optical scattering can be essentially nil

[5] . Rigid (usually glass-based) devices [2,3] as well as flexible, polyester-based foil devices [6,7] have been investigated during the last decade.

Among the numerous applications of electrochromism, we note architectural “smart windows”, which are able to combine improved indoor comfort (less glare and thermal stress) with good energy efficiency (especially lowered air conditioning loads in cooled buildings) as apparent from order-of magnitude estimations [6] as well as buildings simulations [8]. The use of “smart windows” has been discussed in detail in literature on innovative architecture [9,10]. Other applications concern non-emissive displays, variable-reflectance mirrors, variable-transmittance eyewear of different kinds, and variable-emittance surfaces for temperature stabilization of space vehicles. For these and other uses, however, there is a long-standing problem with hydrated nickel oxide, which tends to show residual optical absorption in the 400 < A < 500 nm wavelength interval, thereby precluding a fully transparent state.

This paper contains an investigation of the optical properties of hydrated nickel oxide with several different additives introduced with the objective of reducing the

absorption in the visible range without compromising the electrochromic properties. This investigation is complemented by a study of iridium-oxide-based electrochromic materials. The films were made by reactive magnetron sputtering, which is an industrially viable technology with proven upscaling capability.

Passive Cooling

The ambitious limit for the primary energy demand does not allow active cooling for most of the floor space. Therefore various passive cooling strategies have been ap­plied in the demonstration buildings. Common features are moderate glazing propor­tions in the facades, exterior shading systems (total energy transmission < 15%) and low internal loads of less than 190 Wh m-2d-1. Uncovered concrete ceilings serve as mass storage for heat loads during the days, with different elements attached to the ceilings or the walls to compensate unsuitable reverberation properties of the rooms. The heat removal from the ceilings is realised either by night ventilation or by an inte­grated piping system run with ground water (five projects).

Night ventilation can be achieved with a mechanical ventilation system. This guaran­tees a good control of the air mass flow but requires additional electric energy. A low pressure drop along the air path and high temperature differences are advantageous for high cooling efficiencies which lay between 8 (warm nights) and 24 (cold nights) in the Pollmeier building. In the FhG-ISE building the mean temperature level could be lowered by approx. 1.2 K with mechanical night ventilation only during the second half of the night until the early morning.

The mass flow in natural ventilation concepts is determined by the temperature dif­ferences between indoor and outdoor, the difference in elevation of the air inlet and outlet and wind induced pressure differences on the building surface. In the Wagner building an air change rate up to 1.2 h-1 was monitored during the night. Cross venti­lation increases the air change rate; up to 8 h-1 have been measured in hot periods in the FH Bonn-Rhein-Sieg building.

Another component of passive cooling concepts are earth-to-air heat exchangers which take advantage of the heat storage potential of the ground. While playing only a minor role for preheating air in combination with a heat recovery system, the pre­cooling can be essential for achieving comfortable indoor air temperatures. Different types (concrete or plastic tubes) with different diameters and lengths have been used either with mechanical or natural ventilation.

Figure 4 gives an evaluation of the passive cooling strategies of three projects. From the great number of measurements it can be concluded that discomfort can be avoided if the limit of 25 °C is not exceeded by more than 10% of the attendance time. Rooms with two differently oriented glazed facades have to be treated very carefully.


Based on a total primary energy demand of 100 kWh m-2a-1 the electricity demand for lighting accounts to approx. 30%. The monitored projects covered a range between

3.7 and 18 kWh m-2a-1; the differences mainly result from the daylighting supply in the buildings, the applied glare protection/shading system, the electric power of the artifi­cial lighting, the control strategy and the user behaviour. In buildings with a high day­light autonomy the electric power demand shows a clear dependence on the global radiation: in the Lamparter building the daily mean electric power demand decreases below 1 W/m2 (installed power: 12 W/m2) with a global radiation of more than 100 W/m2. Here, sophisticated control systems show only little energy savings. In some of the buildings a rather high consumption (in correspondence with high cooling loads) was measured in corridors.


The funding programme with its realised demonstration buildings is an important step towards an environmental sound and resource-related evaluation of the (total) energy consumption of buildings. A corresponding EC directive on the total energy efficiency of buildings has to be incorporated in national codes within the next two years. The results of the programme show that a primary energy consumption of less than 100 kWh m-2a-1 can be achieved with investment costs that are comparable to con­ventional projects.

While the low energy and passive building standards seem to be transferable to commercial buildings without major problems, the extension of the scope to the sec­tor of electric energy is a real challenge for the planning of HVAC and lighting sys­tems. Passive cooling strategies showed promising results in terms of energy con­sumption and comfort. However the robustness of the concepts has to be improved because no back-up is available when disturbances occur. A better quality assess­ment of the planning and building process as well as of the operation of the building has to be achieved to keep up a maximum of workspace quality. New simulation tools incorporating models of the user behaviour in terms of ventilation, operation of shading systems etc. could improve the quality of decisions.

On the other hand comfort regulations and codes have probably to be revised in or­der to meet the new dynamic indoor climate situations due to passive cooling. Finally,

a number of prices and acknowledgements show that ambitious energy targets can go hand in hand very well with high quality architecture.


The work is funded within the project SolarBau:Monitor by the German Ministry of Economy and Labour (BMWA) under the reference number of 0335007C since 1995 and will end in December 2005. The authors also appreciate the support from the ministry’s project co-ordinator PTJ in Julich.


1. Voss, K.; Lohnert, G.; Wagner, A.: Energieeinsatz in Burogebauden, Bauphysik, part 1: Heft 2, S. 65 72, 2003; part 2: to be published in Heft 5, 2003

2. http://www. solarbau. de

3. Energy and Buildings — Special Issue on Thermal Comfort, volume 34, nr. 6, 2002