Category Archives: BACKGROUND

Energy Consumption of the Monitored Office Buildings

At the beginning of 2003, twenty demonstration buildings were registered in the fund­ing programme. Six projects are already completed, eleven buildings are monitored and three are within the planning and building process. On the SolarBau homepage (see reference 2) a detailed description is given for each project.

Figure 2 summarises the results from projects which are used primarily as office buildings and for which data from at least one year were available at the beginning of 2003. Five of the nine buildings have a primary energy consumption of less than the limit of 100 kWh m-2a-1 for HVACL. The LEO building, an older low energy office which was completed in 1996, exceeds the limit together with the other four buildings. However it is satisfying to see that the consumption of all monitored buildings is much lower than in comparative buildings. A primary energy consumption between 300 and 600 kWh m-2a-1 is reported in different other studies for office and administration buildings in Germany and Switzerland. The limit of 100 kWh m-2a-1 was exceeded mainly because of an unexpectedly high heating energy consumption (DB, FH BRS) or a high electricity consumption for lighting (FH BRS, ECOTEC) and heating/cooling via reversible heat pumps (ECOTEC). Some of the reasons can clearly be referred to the planning concept, but most of them could have been avoided by better operation management. It is evident that with a high heating energy consumption the band for electricity consumption is rather small if the overall limit shall not to be exceeded.

Even on the level of end energy, the Lamparter and Wagner buildings fulfilled the expectations of Germany’s first non-residential passive buildings. Despite an unex­pectedly high heating energy consumption during the first year of 73 kWh m-2a-1, the Pollmeier building avoided high consumption values for primary energy by burning wood chips from its own sawmill. Also the KfW bank (not listed in fig. 2) is heated with wood pellets. Co-generation plants benefit from a primary energy credit (Wag­ner, ISE) as well as PV plants (ECOTEC, ISE, LAMPARTER). PV systems are the favoured solar applications because of the small hot water demand in most projects. In the Lamparter building the PV system covers 37% of the whole electric energy consumption (primary energy); for the Energon project, also a passive building (not listed in fig. 2), a coverage of 65% is expected with a PV generator of 1300 m2 on the close-by garage.

For the Solvis factory (not listed in fig. 2) a zero-CO2-emission concept was realised on the basis of a co-generation plant fuelled by rape oil and solar systems (PV and thermal collectors with a large storage). A large thermal collector system has been installed on the Wagner building together with a seasonal storage.

As the funding only included additional planning costs and the monitoring, the total investment costs were only determined by the available budget of the building owner or investor. Besides the economic situation in the building sector and site-specific conditions (e. g. foundations, underground garages), the total construction costs, i. e. the costs of "building construction" and "building services technology", are strongly determined by the costs of indoor architecture. Thus, the comparative costs in the building cost index cover a wide range, where most of the demonstration projects can be found within. A general experience of the projects is that expenses associated with energy savings and use of solar energy affect the building costs much less than

the general standard of interior architecture. It is interesting to see that the Lamparter building, which shows an extraordinarily low energy consumption, is also the one with the lowest total construction costs.

Sample preparation

The absorber was produced out of two layers. The substrate was first coated with a solution containing nickel and aluminum ions [1] which after a heat treatment formed the absorbing layer. AR coatings made of varioius oxides was added on top. The substrate was optically smooth highly specularly reflecting aluminum with a surface roughness rms (root mean square) value of 0.02 pm [5]. Cutting aluminum plates into 55 x 55 mm squares produced suitably sized substrates. The substrates had to be cleaned before being coated, since the coating solution has poor adhesive abilities onto a contaminated aluminum surface.

The sol-gel route used to produce the silica and hybrid silica solutions originates from a paper by Tadanaga et al [3]. Tetraethoxysilane, TEOS, and methyltrietoxysilane, MTES, were used as starting material. If only TEOS is used the resulting film will consist of 100% silica. Mixing TEOS and MTES will generate a hybrid silica film, the higher the proportion of MTES in relation to TEOS is the more flexible will the resulting coating be. TEOS was mixed with ethanol before H2O containing 0.06 wt% HCl was added to the solution. The resulting mixture was stirred for one hour at room temperature in order to hydrolyze TEOS. A proper molar ratio of MTES was then poured into the solution. After being stirred for 24 hours in a closed container, to ensure full hydrolysis, the obtained solution was used for coating. The molar ratios of ethanol and H2O to the total alkoxide (TEOS + MTES) were 5 and 4, respectively.

The silica-titania mixtures were produced with a sol-gel technique originating from Dawnay et al [6]. TEOS, ethanol, H2O and HCl were mixed and stirred for 30 minutes. Ethanol was added to dilute the solution to a suitable concentration. Lastly acetylacetone, Acac, and tetrabutylorthotitanate, TBOT, were added and the resulting solution was stirred for 6 hours before being used. TBOT had to be added in drops in order to get a homogenous solution. The amounts of TEOS and TBOT were varied to get solutions with 70/30 and 50/50 Si/Ti molar ratios, respectively. The molar ratios of TEOS:EtOH:H2O:HCl were 1:1:2:0.1 and TBOT to Acac was 1:1.

There exist several different methods to coat a surface with a liquid medium. Some methods worth mentioning are spin-, flow-, spray — and dip-coating. The most suitable and used process for these laboratory experiments turned out to be spin-coating. Spin-coating methods utilize a spinner, where the number of revolutions per minute can be chosen. A spin coater of make Chemat Technology and model KW-4A was used. A syringe with approximately 0.35 ml of coating solution was employed to eject the liquid on top of the center of the substrate. In a fraction of a second the substrate was fully covered with coating solution and a completely homogenous and even film was acquired. Further evaporation of solvents and as a result an increase in coating stability was acquired by letting the spinning process continue for about 30 seconds after the solution was ejected. By changing the spin rate it was easy to vary the film thickness. Rates between 1200 to 6000 rpm were used. The total metal ion concentration of the solution is the second most important parameter that determines how thick the resulting film becomes. The higher the total metal ion concentration of the solution is, the thicker the film on the substrate becomes. The metal concentration can be increased through
evaporation of the solution solvents. A parameter that additionally can influence the film thickness and/or film properties is the substrate material.

After the substrate was coated it was heat treated inside a glass tube of 60 mm in diameter inserted in a split able oven of type ESTF 50/14-S and make Entech. Two parameters could be varied during the heat treatment process, the temperature increase rate and the final temperature, Ts. If the final temperature was set too low, residual organic groups would not be completely removed and a poor coating quality would be the result. The standard heat treatment for the absorbing layer was performed according to the following procedure; starting from room temperature the temperature was increased with varying speeds up to the final temperature at 550-580°C. The heat treatment for the AR layer varied in final temperature from 350 to 580°C but the temperature increase rate was always 50°/min"1.

Lanthanide oxides doped titania as photocatalysts in solar hydrogen generation

Marek Laniecki, Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6,

60-780 Poznan, Poland

Maciej Zalas, Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6,

60-780 Poznan, Poland

Industrial methods of hydrogen generation based on fossil fuels are well known since the end of XIX and beginning of XX century. Recent reports of the big oil companies clearly indicate that estimated reserves of natural gas and oil will last at least for 50 years. The constant and increasing depletion of fossil fuels, especially natural gas, applied in the steam-methane reforming (SMR), requires the research and implementation of new methods of hydrogen production based on renewable energy resources. It seems that one of the very promising routes of hydrogen generation directly from water is photocatalysis. Different “clean” methods of hydrogen generation are currently considered, however, only two of them, namely biophotolysis and photocatalysis seems to be the only simple ways of transformation of solar energy into chemical energy stored in hydrogen. Although these methods are still in the “juvenile” period of research, it is belived that after improvement of the yields of photogenerated hydrogen they can become one of the very important way of solar energy storage.

This paper is an attempt to clarify the doping effect of platinized titania applied as photocatalyst in hydrogen generation from water.

General remarks

The early studies [1,2] of water photocatalysis showed that this method, applying titania as photocatalysts, gives rather low yields of hydrogen and requires illumination mainly in the region of ultraviolet light. Later works disclosed that different semiconductors such as sulfides, perovskites and many other combinations of transition metal oxides can operate as the efficient photocatalysts as well [3,4]. Unfortunately the most efficient photocatalyst — CdS, operating in the region of visible light, due to the environmental reasons and very rapid photocorrosion has been eliminated from further application considerations. Since the early seventies of the XX century different laboratories tried to improve both the hydrogen yield and photostability of the applied photocatalysts. Recently such materials like indium titanate, indium tantalate [5] as well as copper(I) managanate(III) [6] were tested under the visible light in water splitting. However, the yields of photocatalytically generated hydrogen were still far beyond the expectations.

This situation prompted many research groups to improve the particular semiconducting systems by modifying the surface of the photocatalyst. To date four different methods of modification have been studied:

— modification of the semiconductor surface with metal (e. g. Pt, Au, Ag, Ni )

— generation of composite semiconductors (e. g. CdS-TiO2 systems [7])

— surface sensitization (e. g. titania with chemisorbed Ru(bpy)32+complexes)

— transition metal doping.

The benefit of transition metal doping species is better capability of trapping electrons to inhibit electron-hole recombination during illumination. The concentration of the beneficial transition metal oxides dopants is very small whereas large concentrations are detrimental. Literature data survey indicate that transition metal oxides such as iron (III)[8] or copper (II) [8,9] actually inhibit electron-hole recombination. Other authors claim that doping titania with e. g. Cr3+ ions [10] leads to the decrease of photoactivity due to the very fast
recombination in electron-hole process. It is belived that these transition metals create acceptor and donor centers where direct recombination occurs.

The different opinions about the inhibition of electron-hole recombination indicate that this field of research requires further studies which in consequence can lead to the significant improvement of the photocatalytical splitting of water.

The research in our group since few years was concentrated on the doping of titania with yttria and oxides of the lanthanide group [11-13], and the present paper is the contribution to a better understanding of this phenomenon during photocatalytic generation of hydrogen.

Experimental

Two series of platinized titania photocatalysts were prepared by hydrolysis of titanium chloride containing appropriate amounts of lanthanides nitrates. Lanthanide oxides after dissolution in nitric acid were placed in 1 dm3 of distilled water and next titanium chloride was added dropwise at room temperature. The hydrolysis process was completed after adding ammonia till pH = 9. The obtained precipitate after removal of chloride and nitride ions was dried and calcined at 675 K for two hours.

Powdered samples containing 0.1 or 0.5 mol % of lanthanides were impregnated by the incipient wetness method with chloroplatinic acid. The concentration of the impregnating solution was adjusted to obtain 0.3 wt. % of supported platinum. The supported H2PtCl6 was decomposed by calcining of the samples at 675 K. The reduction of platinum oxide to the supported platinum particles was performed in situ in photocatalytic reactor at the initial stage of reaction.

Photocatalytic experiments were performed in Pyrex glass round bottom flask (250 cc) applying four Ultra-Vitalux bulbs (300 watts each — Osram ). The applied source of light indicate very similar spectrum of light as the natural solar irradiation. In order to mimic natural conditions the temperature of the irradiated reactor was close to 355 K. The evolved hydrogen in the argon carrier gas was directed towards gas-sampling loop and further directed onto TCD of Varian 3800 gas chromatograph.

In all cases 0.1 gram of photocatalyst was applied and the mixture of water and methanol (50:1) was used as the reaction medium. Methanol was applied as the sacrificial agent. Occasionally other higher alcohols were applied as the sacrificial reagents.

The applied catalysts were characterized with XRD, BET surface measurements, UV-VIS reflectance spectroscopy, temperature programmed reduction whereas dispersion of platinum was measured with hydrogen chemisorption.

Results

The results presenting the amount of the evolved hydrogen after 100 minutes of the photoreaction are shown on Figure 1. As it was shown in our earlier studies [11] the best performance for the samples containing only 0.1 mole % of lanthanide were found for yttria doped catalysts. Relatively low yields of hydrogen obtained for this series of catalysts (low lanthanide oxide content) indicate that doping of titania with lanthanides oxides, depending on the element applied, is very complicated process and still requires much efforts to understand this phenomenon. No simple relationship between the amount of electrons located on f-orbital of the dopant and catalytic photoactivity in water splitting reaction can be established for the samples containing 0.1 mole % of the lanthanides oxides. Certainly this amount of the dopant probably speeds up the electron-hole recombination, however, other explanations can not be excluded. It is also possible that platinum dispersion, which is strongly related with the rate of chloroplatinic acid decomposition can influence the yield

of hydrogen. Photoreduction of platinum is the multi step process what is visible for all studied catalysts as the hydrogen evolution rate at the initial stage of reaction. Figure 2 presents the rate of hydrogen evolution for three selected catalysts, but similar shapes were always observed for all studied samples. The characteristic steps visible during first hour of reaction clearly shows that independently of the applied lanthanide oxide and its concentration, the stabilization is always attained after first hour of reaction.

Figure 1B (concentration of lanthanide oxide — 0.5 mole %) which shows the amounts of hydrogen evolved after 100 minutes of reaction, indicate in certain cases (Gd, Eu, Sm, Ho) hydrogen yields higher than for pure platinized titania (1.66 mmol/100 min.). It is worth to notice that in the absence of metallic (Pt) photocatode the yields of hydrogen were on the level of detection. The analysis of the hydrogen yields for samples containing 0.5 mol% of such ions like Ce4+, Tb4+, Pr4+ indicate that these ions can protect Ti4+ ions against their photoreduction towards Ti3+and in consequence to very rapid recombination of electrons. In contrast, those samples containing samarium, europium gadolinium or holmium indicate

that ions of these elements incorporated within the anatase (XRD measurements) structure protect the excited electrons from rapid recombination.

However, the temperature programmed reduction (see Figure 3) in the case of cerium indicated that reduction of cerium doped titania occurs within the same region of tempera­tures like pure non-doped titania (850-1100 K). Moreover, the presence of 0.5 mol % of ceria in catalysts containing 0.3 wt.% of platinum significantly lowers the temperature reduction of supported platinum particles. The presence of two maxima at « 480 K and 620 K suggests formation of two different platinum species. This specifically low temperature of platinum particles reduction requires further studies. It is also unclear why certain surface segregation of platinum particles occurs.

The application of sacrificial compounds, other than methanol, indicate that in the case of water splitting other donors of electrons (such organic compounds like ethanol, glycol, glycerine or glucose) can fulfill their role as well. However, the best results were always obtained for methanol or ethanol and the worse for glucose or fructose. This results indicate that in the future experiments also certain wastes from food industry can be applied as the source of electrons for photoreduction of water.

An attempt to clarify the role of lanthanide oxide as a dopant of titania has been made on the basis of electronic spectra of synthesized non-platinized photocatalysts. Spectra presented on Figure 4 show typical shapes, band positions and adsorption intensities for the studied catalysts. The comparison of these spectra within 200-400 nm region indicate that the ratio between the electronic absorption band at 385 nm and the bands at 270 and 250 nm can serve as the indicator of better or worse activity in photocatalytic water

Figure 4.

UV-VIS reflection spectra of selected non-platinized photocatalysts containing 0.5 mol % of lanthanide oxide.

splitting. The activity in hydrogen photoevolution in the case of samples containing 0.5 mol % of lanthanide oxide can be related with standard oxidation potentials versus hydrogen electrode. It was established that those ions which indicate high potential of tetravalent strongly acidic aqua ions eg. Gd (7.9 V) Eu (6.4 V), Yb (7.1 V) or Ho (6.0 V) can be responsible for relatively high hydrogen yield in photocatalytic splitting of water.

The preliminary experiments under natural irradiation and comparison of these results with those performed under the laboratory conditions with the same catalysts indicate that both methods with same catalysts leads to very similar results. Those catalysts which indicated good performance in laboratory were also good photocatalysts under natural irradiation.

References

1. A. Fujishima, K. Honda, Nature, 238(1972)37

2. J. Augustynski, J. Electrochim. Acta, 38(1993)43

3. A. L. Linsebigler, G. Lu, J. T. Yates Jr, Chem. Rev. 95(1995)735

4. A. Mills, S. La Hunte, J. Photochem. Photobiol., A:Chem. 108(1997)1

5. Z. Zhou, J. Ye, K. Sayama, H. Arakawa, Nature, 414(2001)625

6. Y. Baasekouad, M. Trari, J. P. Doumerc, Int. J. Hydr. Energy, 28(2003)43

7. K. R. Gopidas, M. Bohoroquez, P. V. Kamat, J. Phys. Chem.,94(1990)6435

8. E. C. Butler, A. P. Davis, J. Photochem. Photobiol., A: Chem. 70(1993)273

9. M. Fujihara, Y. Satoh, T. Osa, Bull. Chem. Soc. Japan, 55(1982)666

10. J-M. Herrmann, J. Disdier, P. Pichat, Chem. Phys. Letters 108(1984)618

11. M. Laniecki, M. Zalas, S. Manas, C. Richter, Proceed. 14 WhEc Conference, June 9-13, 2002, Montreal, CD-ROM edition

12. M. Laniecki, M. Zalas, IHP Programme Materials, Access Camapaigne 2002, Almeria 2003, Spain, pp.75-80 (2003)

13. M. Zalas, M. Laniecki, Proceed. 15 WHEC, June 27-July 3, 2004, Yokohama, CD — ROM edition

Duct Network System in TRNFLOW

Beneath the room nodes (thermal zones) the air flow model needs additional auxiliary nodes for the ventilation duct network as junctions of the individual parts of the duct system. In order to keep the thermal model small these nodes are not modeled as thermal zones in Type 56.

The temperatures of the auxiliary nodes are directly calculated from the temperatures of the joining air flows. Thermal capacitance or other heat gains or losses of these nodes are not considered. In the thermal model air flows of the auxiliary nodes into the space zones are interpreted as ventilations.

To model an air heater or cooler, resp. a humidifier or dehumidifier, temperature as well as humidity can be defined for an auxiliary node by means of a constant, input or schedule variable. The power necessary to reach this set point for the air can be obtained by an output.

Figure 6 shows an example for the illustration of a duct network. The reference temperature of 20°C is defined for the auxiliary node AN6. For Room3 results a ventilation of 33kg/s with 17.9°C. Rooml and Room2 have zero ventilation.

Resulting temperature of aux. nodes

Aux. nodes Temperature [°C]

AN1 18.8

AN2 17.9

AN3 17.9

AN4 18.0

AN5 15.0

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

properties

properties

capability and long-term per-

formance

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.

A.

B1

Degradation of protective coating on reflector

Insufficient coating of reflective

layer at production

A4

A5

A1

A2

A3

D1

D2

Soiling

Erosion

Ageing with

Loss in

Loss in

Loss in

Degradation

material

protective

adhesion

adhesion of

of substrate

decomposition

capability

to

reflective

and loss in

due to

reflective

layer to

barrier

mechanic

layer

substrate

properties

al damage

Increase

Increase

C1

of

of surface

Corrosion of reflective layer

absorp-

rough-

tion and

ness

scatter-

ing

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

80

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

40

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

64

A4 Surface soiling

56

A5 Surface erosion

50

B1 Insufficient coating of reflective layer at production

70

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

112

D1 Loss of adhesion of reflector from substrate

70

D2 Degradation of the substrate

32

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

0.70

0.60

0.

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

shading

inc. angle

Ventilation

[l/min*m]

g

[-]

U2 g

[W/(m2K)] [-]

U

[W/(m2K)]

Ipasol

0

0

0.219

0.86

0

60

0.229

1.37

0

24

0.221

1.06 0.218

1.06

60

24

0.19

1.06 0.184

1.06

Silverstar

0

0

0.284

0.94

0

60

0.297

1.46

0

30

0.291

1.20 0.305

1.21

60

30

0.245

0.258

1.21

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

10.5

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

[42]gb.

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

2.3×106

и

и

0.10m

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

1.00

Thermal conductivity of glass

1.4 W/m K

Reflectance of glass

0.00

Specific heat of glass

750 J/kg K

Absorptance of glass

0.00

Density of glass

2500 kg/m3

Absorptance of the solar control coating

0.50

Thermal conductivity of air

0.0263 W/m K

Exterior emittance of wall 2, (glass)

0.86

Specific heat of air

1006 J/kg K

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

0.90

Density of air

1.22 kg/m3

Interior emittance of wall 2

1.00

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

Heating

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.

Ventilation

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.