Category Archives: BACKGROUND

Numerical results

In Fig. 3 the trend of S varying with d is shown, for the two cases of copper wall (CW) and brick wall (BW). For d<0.045 m the flow is laminar with Reynolds numbers inferior to 2500; for d>0.060 m the flow is turbulent with Reynolds numbers superior to 3500: the laminar — turbulent transition zone is pointed out in dashed outline. In the case of turbulent flow the roughness value of the ventilation duct has been assumed to be equal to 0.005 m.

The brick outer slab turns out to be more convenient, from an energy point of view, than the copper one.

The following figures all refer to a ventilation duct being 0.04 m in thickness.

In Fig. 4 the trend of S varying with G for the two examined walls, CW and BW, is reported. Two values have been considered for the indoor air temperature: T=24°C and T=26°C. The percentage energy saving S distinctly increases as G and the indoor air temperature Ti rise. In Fig. 5 the trend of S varying with G is shown for the wall BW. The following values have been considered for the friction factors on the inlet and outlet sections: Xin=0.5 and Xou=1; Xin=1 and Xou=2; Xin=2 and Xou=4; Xin=4 and Xou=8. The graphs clearly show the convenience to reduce, as much as possible, the head losses occurring on the inlet and outlet sections.

Figure 3 — Variation of S with d (m) for the two walls CW (solid line) and BW (dashed line). The laminar-turbulent transition zone is pointed out in dashed outline.

Figure 4 — Variation of S with G for two values of the indoor air temperature: Tj=24°C (solid line) and Ti=26°C (dashed line).


The trend of the mean heat flux Q coming into the room through the ASW varying with G is reported in Fig. 6, for the two examined walls, CW and BW. The two cases of T=24 and T=26 °C have been considered. The trend of the mean heat flux Q0 (obviously the same for the two walls) coming into the room when the ventilated duct is closed (dotted line) is also reported for comparison. The difference (Q0-Q) and, therefore, the reduction in summer thermal loads, achievable by using a ventilated wall, increases as G and Ti rise.

In Fig. 7 the trend of Q varying with the sol-air temperature Te, for the wall BW, is reported for three values of the air temperature in the shade: T0=24°C, 26°C and 28°C. The trend of Q0 (dotted line) is reported for comparison. Obviously, it results that Q=Q0 for Te=T0 (without solar radiation). The ventilation convenience increases as Te rises as well as it increases, for a given value of Te, as T0 decreases.

The Fig. 8 refers to winter and the two examined walls. In this figure the trend of Q varying with G for two outdoor air temperatures in the shade is shown: T0=0°C and 7°C. The trend of Q0 (dotted line) is reported for comparison. Notice that, in this case, the wall showing less heat losses is the copper one; it happens as a consequence of the fact that the thermal resistance RB of the wall CW, with copper outer slab, is higher than the resistance RB of the wall BW, with brick outer slab (see Tab. 1). The graphs clearly show that, in winter, the ventilation always determines a rise in heat losses.

Conclusions

The ASW can meet, if well designed, the aesthetic and formal requirements of contemporary architecture, and also contribute to reduce energy consumption in buildings. The examined graphs clearly show that the use of ASW can determine a remarkable reduction in summer thermal loads; the duct is, obviously, required to be, as much as possible, free from any obstacle and the head losses to be reduced on the inlet and outlet sections for the above-stated reduction in summer thermal loads to occur. Hence the necessity of an accurate design of the inlet and outlet openings.

The energy saving achievable using the ASW distinctly increases as insulation increases; for a given value of the insulation and of the outdoor air temperature in the shade, the
reduction in the summer thermal load increases sensibly as the temperature provided for the indoor environment increases. In the examined situations the brick-faced wall (BW) has turned out to be more convenient than the copper-faced one (CW), from an energy point of view. In any case, it seems to be not convenient to consider air duct thicknesses inferior to 4-5 cm.

In winter, remarkable rises in heat losses can occur, leaving the duct open, especially connected with remarkable values of G. This leads to advise closing the duct in winter. But, considering that in winter the values of G are usually moderate, it would be advisable reducing the ventilation, e. g. with self-regulating dampers at the duct inlet and outlet sections, in order to drain the humidity due to possible infiltrations or condensation phenomena.

Acknoledgements

This research was supported by Italian Ministry of Education, University and Scientific Research (MIUR) and by University of Pisa within the National Relevant Interest Project (PRIN 2003-2005): "Energy and environmental diagnosis on existing buildings: research methodologies, determination of qualification parameters and technico-economic assessments”.

1.

The mathematical formula

The solar radiation daily variation corresponding to the typical clear days characterized by a sunshine fraction a > 0,9 and a nebulosity index Kd < 0,2 [18], The variation of temperature, pressure and the total are obtained by establishing a mass and a thermal balance of the volume elements of the porous medium discretised on equal thickness and to evaluate the equations of heat and mass transfer in each slices separately.

In each slice, the transfer of heat is obtained by applying the first principle of thermodynamic for an open system by taking into account the fractions corresponding to the adsorbed fluid, the gas and the solid:

d(^U) + ^ qs hs — ^ qe he = Ф + E (2)

Su = Vc [(1 — s)psUs + (є — a)pgUg + a paUa] (3)

The combined of equations (3) and (4), enable us to obtain the general equation of heat and mass transfer in a layer, equation (5), these equations are written in the case of :

— cylindrical elements:

rn 4 4 , idT P d[(s-a)pg]

TOC o "1-5" h z [(1 -S)pscs + (є — a)pgcg +apaca] —

dt pg St

d(apa) P d(apa) d2T 1 d)T

-AHads (T, P) =2e ( +

dt pa dt dr2 r dr

These equations in the porous medium are completed by the initial and boundary conditions:

— Initial condition:

— T(r,0) = Ta ( r = 0,…, R ) (7)

— T(i, j,0) = Ta ( i = 1,., n ) and ( j = 1,., m ) (8)

Ta is the ambient temperature before the sunrise All the reactor is a constant temperature.

— Boundary conditions:

The boundary conditions to the center of the porous medium is a condition of symmetry;

(ffr) _ 0 , (ffr) _ (ffT) _ 0 (9)

(~&r ) r=°_0 (Ж) x=i, у — ~оу) x, y=i — 0 w

The thermal balance of the metallic wall is given by the following equations;

— Cylindrical tube without fins

pac VacCac — TvUacPsDe —UlDe(Tac—Ta) — hinDi(Tac — T) (10)

— Cylindrical tube with fins

Caofitc Vc— = Tv Oca Ps (De + 2 ШУ — U (-Dev + 2 Qi)(Tac-Ta ) — k 7t D, (Tac~T)

TOC o "1-5" h z dt 2

In this equation we take account the efficiency of the fins into consideration [19]

Q =tanh (m £) (19) and m = VUl/2acs (11)

m t

I is the wide of the fin

— Rectangular tube

Qj m

pac Vac Cac— = Tvaac Psu — Ul Sr (Tac ~ Ta) ~ 4 ^ hi AY (Tac ~ Tnj) —

» ‘ (12)

2 hi AY (Tac — Tnl) — 4 £ hi AX (Tac — Tim) — 2 hi AX (Tac — T 1m)

i

The obtained equations from a system of non linear differential equations that are solved by the implicit finite difference method [20].

The efficiency of the machine is characterized by the thermal coefficient of performance; COP and a solar performance coefficient COPs, deduced from the characteristic points of the obtained cycle using the following relations;

Tmax

Qc ^ mi Cpi dT + Qdes

Index i relates to ammonia, the activated carbon and the metal tube.

Qdes is the quantity of energy necessary to the desorption of the quantity Am [20];

5. Results:

The numerical simulation of the modelled reactor, under ambient temperature and solar radiation recorded in Tetouan, enable to describe aspects of heat and mass transfer inside the porous medium. The results gives the characteristic parameters of the functionning machine.

The numerical results obtained under real conditions of ambient temperature and solar irradiation relative to typical clear days of each season, allow the evaluation of the considered reactors performances from the cycle characterising points. The adsorption temperature is equal to the ambient one, the evaporation temperature is zero and the condensation temperature corresponds to the ambient temperature related with the beginning of ammonia desorption inside the condenser.

Figure 3 shows the variation of the thermal performance coefficient COP versus the normal and finned tubes diameter for the studied typical days. We observe that for each case there is a maximum value corresponding to a given tube diameter representing the optimum values.

Hence, under the applied functioning conditions the optimum COP value (diameter) are variable and depends strongly on solar radiation and on the ambient temperature. The same remarks are observed for the variation of the daily cycled mass versus the diameter figure 4-a, considering a collector of a 1 m2 of surface composed a number of equal tubes. The total cycled mass corresponds to the sum of the desorbed quantities by each cylindrical tube. We note that the optimum values are higher for the rectangular reactors compared to the cylindrical ones figure 4-b corresponding to the amount of the activated carbon used and thus to the offered volume to the reacting mixture.

The high values of the COP in April and October can be explained considering the fact that ammonia adsorption takes place before the sun rise in a uniform temperature porous medium, equal to the ambient temperature but less than that in July. So, the adsorption is very important, the choice of typical clear days characterised with high solar radiation allows to heat to the maximum values the reactors and thus the COP is a function of the considering temperature and that the maximum heating of the absorbent permits an important heat adsorption.

The variation of the maximum temperature at the center of the porous medium is a decreasing function of its width, it is had has the thermal conductivity of the porous medium and the thermal capacity of the whole of the reactor figure 5. We notice that the finned tubes improves the thermal exchanges between the metallic walls and the porous medium, consequently the maximum temperature attained is greater allowing an important desorption for the finned reactor with regard the same diameter normal tube, figure 5-a. Figure 6 shows the evolution of temperature at the center of the porous medium versus the time for the three optimum widths reactors. For the cylindrical tube, we compare the temperature variation inside the tubes with a similarly diameter for the finned and normal reactor. The studied cycle begin the morning where all the reactor is at the ambient temperature and finish at midnight marking the start of a new cycle relatively to the temperature recorded at LT.

The rectangular reactor heating duration is higher, owing to the important volume of the fixed bed containing the mixture, than the cylindrical cases.

Figure 7 shows the pressure evolution inside the reactors versus time, causes by the temperature variation. The temperature elevation during the heating phase of the closed reactors causes an increases in the gas pressure until it becomes just larger than the condensation pressure which corresponds to the saturation pressure at the temperature condensation, then the desorption of ammonia into the condenser starts at a constant pressure and the heating of the fixed bed continues until the temperature reaching the maximum value. The reactors are closed and both temperature and pressure decrease.

At the pressure value of 4,2 bar the reactors are opened and the adsorption phenomena of ammonia vapour start with a cooling product quantity.

These evolution of temperature and pressure is represented in a Clapeyron diagrams, corresponding to the variation of Log P versus the temperature figure 8, and shows the daily thermodynamic cycle characterised by two isosters and two isobars representing four phases relatively to the heating or the cooling of the reactors.

In figure 9, we show the daily evolution of ammonia total mass, both adsorbed and gaseous, inside two cylindrical tubes having the same diameter in the two optimal cases. At the beginning the temperature is the same inside both of the tubes implying that their respective ammonia masses are also the same. During the heating of the closed reactor, condensation pressure inside the finned reactor is reached before the tube without fins, causing the opening of valve V1 and hence ammonia desorption. This desorption is important considering the temperature elevation and the values of 2,42 kg and 1,55 kg are collected for the unit area respectively for the normal and finned reactor.

Inside a 1m2 surface captor, 1 m long and 1 m wide, equals to the multiplication of the number of tubes by their external diameter. The total desorbed mass represents the sum of all the desorbed amounts in each tube. The non desorbed mass is the total fluid mass inside the reactor which the variation during a cycle is showed in figure 10.

We gives in table I the values of the computed amounts and those of the parameters under which the reactors functioning for the typical clear days of July, of which can be compared the three reactors. The obtained optimal geometry of each reactor presents an evaluation of the parameters that characterises the functioning conditions, the efficiency of the machine and the computed both provided and useful energy.

6. Conclusion

In this work, the aims is to present a model and an optimisation of solar adsorption cooling machine using ammonia / activated carbon couples, that allows a design according to the real functioning condition. The prediction of the performance of the solar refrigerator require the knowledge of various parameters, which characterise the daily thermodynamic cycle. The optimisation is based on heat and mass transfer in the porous medium consider the collected mass, the thermal and solar performance coefficient, allow to give an idea of the transitory evolution of temperature, pressure and ammonia concentration inside the reactors. The efficiency of each reactors are enhanced and the preferential adsorber depends on the desired role to generate (the useful cooling quantity).

A presentation of temperature and adsorption ammonia quantity inside the reactor that develop solar radiation is carried out in this paper. Thus, the simulation has been performed using some assumption will be applied to an experimental test.

Table II. Comparison of the operating parameters and results of each reactors

Height optimum (diameter) (cm)

rectangular

8

without fins 7.29

with fins 6.94

Tads (K)

297,2

297,2

297,2

Tcond (K)

299,37

299,05

298,79

Tmax (K)

360,3

356,7

369,4

Pcond (bars)

10,41

10,31

10,23

Mass used AC (kg)

40

27,134

13,244

Desorbed mass

(kg)

5,8

2,42

1,55

Total desorbed mass fraction (kg/kg)

0.145

0.089

0.117

Time of beginning condensation (LT)

10 h 24 min

9 h 48 min

9 h 18 min

Time of end condensation (LT)

16 h 12 min

14 h 36 min

14 h 30 min

Quantity of cooling product at the evaporator (kj )

4443,8

2759,7

1777,2

Quantity of heat the reactor (kj)

10371,2

7538,8

4293,9

Thermal COP

0,43

0,366

0,414

Solar COP

0,17

0,105

0,068

SHAPE * MERGEFORMAT

Figure 4. Total daily condensed mass versus the tube dimensionless -—- January April July -0- October

SHAPE * MERGEFORMAT

Changes in solar absorptance and thermal emittance

Absorptance changes were generally larger and occurred faster at lower pH values (Fig. 3). Changes in emittance were mainly the opposite, i. e. larger at lower pH values (Fig 4). The resulting PC values (Eq. 1.) were almost in all cases within the acceptable limit at pH

3.5, distributed both side of the limit at pH 4.5, and were generally above the acceptable limit at pH 5.5 (Fig. 5).

The majority of the samples exhibited neither specific temperature-depending nor gasification type/rate — depending behaviour. In addition, there is no clear difference in degradation between the O2, N2 or non-aeration or the rate of aeration at any pH level. It seems that the pH level is the major determinant regarding to the degradation rate. Unfortunately, there was large deviation especially in the absorptance results at pH 5.5 exposure times between 0.5h and 4h.

In previous condensation tests for similar samples with de-ionized water according to draft proposal ISO/CD 12592,2 (Brunold et al., 2000) all the samples exhibited Arrhenius-type temperature- and time-dependent degradation (Konttinen and Lund, 2003). Complexity of the simulated acid rain test method including multiple variables makes it difficult to determine the reasons for non-Arrhenius type behaviour. The most likely reason is uncontrolled movement of the acid rain solution causing irregular chemical reactions. Futhermore, the primary assumption of the combined effect of gas feeding and natural convection being sufficient for moving the solution seems to be inadequate. The amount of reactants in the solution is quite small (Table 1). Therefore small variations in solution composition can have caused different results as well.

New Coupling Concept in TRNFLOW

At one side the indoor temperatures are important boundary conditions for the multi zone air flow model and should therefore not be defined on the basis of a user’s guess. On the other side the indoor air temperatures calculated by the thermal model strongly depends on the exchange of air between the zones as well as the outside. To link the two models and mutually use the results is the obvious consequence. In TRNFLOW the multi zone air flow model of COMIS is completely integrated into the thermal model of Type 56. This means that the exchange of data between the thermal and the air flow model is made internally and no longer by inputs and outputs. The proper classification of air flows (infiltration, ventilation, couplings) and temperatures to the air flow node resp. the thermal zones and the appropriate other model is automatically carried out by the program.

The input files of both models are kept in the existing formats (BUI, CIF) but are created by only one user interface witch is a TRNFLOW Version of PREBID. Air flow model data depending on time, like wind velocity or window opening factors are defined as inputs or schedules. Outputs like air flows or zone pressures are declared as outputs by means of
new NTYPES and can be written into an output file using a printer type or processed otherwise. The standard COMIS Output File (COF) is optionally also available.

Global performances

In order to take account of the solar spectrum, a multilayer sample is characterized by its solar transmission Tsol, its solar reflectivity Rsoi defined respectively by the following relations:

_ JT(A) Iso, (A) dA

J Iso. (A) dA

We note here Isol the intensity of the solar spectrum AM1.5. The integration range is given by the limits of the solar spectrum. The visible reflectance Rvis is determined from the photopic luminous efficiency function V(l), the standard illumination D6s(A) and the hemispherical reflectivity R(A):

R _ fR(A) • Р65(Л) • V(A)dA vis f 065(Л) • V(Л)dЛ

For the theoretical case of a delta-distribution-shaped reflectivity, Schuler et al. [18] introduced a merit factor M defined as the ratio of the visible reflectance Rvis and the solar reflectivity Rsoi. M is then large for a high visible reflectance or low solar energy losses and consequently describes the energy efficiency of the visual perception.

Numerical simulations allow optimizing the reflectivity and transmission of the multilayer films as a function of the film thicknesses, the refractive indexes and the number of alternating layer. They show a correlation between the difference of the refractive index of the two materials. For example, a lower refractive index difference increases the optimal thicknesses of the individual layers and the layer number, but the solar transmission is high. The simulation optimization results based on the experimental optical constants of single layers will be published elsewhere [19].

Table 2 shows the solar transmission, the solar reflectivity, the relative visible reflectance and the merit factor M = Rvis/Rsoi in the case of the Ti02/Si02 multilayers. We indicated the experimental and calculated values. We see that for a given number of alternating layers, it is always possible to obtain either a high solar transmission or a high relative visible reflectance by adapting the thicknesses of both oxide materials. In order to obtain the best compromise between the energy losses by reflectivity and the visual effect, both parameters have to been optimized. Samples a and c show that the merit factor is not a sufficient indicator and one has to take into account the absolute Rsol. In fact, in these examples, the solar transmission is low and results in a uselessly high visible reflectance.

dTi02

[nm]

dSi02

[nm]

Tsol

exp theo

Rsol

exp theo

Rvis

exp theo

Rvis/Rsol

exp theo

2L

27

195

88.1

87.6

12

12.4

19.8

20.1

1.65

1.62

3L

a

30

122

77.8

77.2

22.1

22.8

39.4

39.1

1.78

1.71

63

73

66

67.3

33.7

32.7

64.1

58.7

1.90

1.80

b

18

160

85.8

84.7

14.3

15.3

24.2

25.2

1.70

1.65

5L

c

35

148

70.7

69.9

29.2

30.1

60.5

61.1

2.10

2.00

d

14

155

85.5

85.9

14.2

14.1

23.3

24.8

1.83

1.75

e

19

130

82.9

82.9

16.9

17.1

23.3

22.2

1.37

1.30

Table 2. Measured parameters (thicknesses, solar transmission and reflectivity, visible reflectance and merit factor) of TiO2/SiO2 multilayers combined with the same theoretical parameters

Table 3 shows the solar transmission, the solar reflectivity, the relative visible reflectance and the merit factor M in the case of the Al203/Si02 multilayers. The solar transmission is slightly decreasing by increasing layer number, but stays at a high level superior to 89%, which is comparable to the solar transmission of uncoated glass (92 %). As mentioned above, this is due to the small refractive index difference between Si02 and Al203. The relative visible reflectance and hence the factor M increases.

The result shows that the prepared coatings can meet the requirements for obtaining different reflected colors. More efforts are needed to improve at the same time the solar transmission and the visible reflectance by considering other oxides and by optimizing the layer thicknesses.

dAl2O3

[nm]

dSiO2

[nm]

Tsol

exp theo

Rsol

exp theo

Rvis

exp theo

Rvis/Rsol

exp theo

3L

a

83

95

90.5

90

9.8

10

12.7

13.5

1.3

1.34

5L

b

83

92

89.9

89.6

10.2

10.4

15.2

16.4

1.5

1.58

7L

c

80

91

89.7

89.1

10.3

10.9

16.7

20

1.63

1.84

9L

d

80

90

89.4

88.8

10.7

11.2

18.7

21.7

1.74

1.93

Table 3. Measured parameters (thicknesses, solar transmission and reflectivity, visible reflectance and merit factor) of Al2O3/SiO2 multilayers combined with the same theoretical parameters

4. CONCLUSION

In this work, colored glass to cover solar collectors has been obtained by alternative deposition of dielectric layers with high and low refractive indices. The stoichiometry was first checked by XPS. The deposition rate has been controlled by in-situ laser reflectometry and confirmed by ex-situ ellipsometry for complex systems with several layers. The optical properties of individual oxides of titanium, silicon and aluminium have been determined. A Cauchy dispersion model is adequate for extracting the refractive and extinction index in the case of reactive magnetron sputtering deposition.

The reflectivity and the solar transmission depend on the thicknesses and the number of the alternative dielectric layers. The fabricated multilayers fulfilled the fixed requirements: quasi-zero absorption, reflectivity peak in the visible, solar transmission above 85% up to 89% and an acceptable visible reflectance.

More effort will be directed to study the lifetime of the multilayer coatings by aging tests in orderto investigate theirapplicabilityfor architectural integration in buildings.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. M. Ley for helpful discussions and R. Steiner for the technical support. This work is supported by the Swiss Federal Office of Energy and the Swiss National Science Foundation.

Solar calorimetric testing of glazings and shading devices

Within the European project ALTSET (Angular-dependent Light and Total Solar Energy Transmittance) the angular properties of so-called complex glazings (i. e. glazings with special optical properties) and shading devices have been investigated. The scope of the project was

the comparison between direct calorimetric measurements and modeling from layer properties

— the important experimental factors for good interlaboratory comparison of results

— error analysis and evaluation procedure

— conclusions for and development of a testing procedure.

The results showed that using different approaches for apparatus design one can reach uniform results for well-defined reference conditions and the project team has developed a test procedure for this purpose. Solar calorimetry could be shown to be an indispensible methodology for all complex glazings like diffusing glazings, transparent insulation,

shading lamellae and switchable glazings and can be used for validation of optical-thermal glazing models. But also for conventional glazings the accuracy is comparable with standard testing techniques. [ 4]

In a German national project REGES this approach has been extended for shading devices (internal and external devices), and recommendations for testing have been given. [ 6]

The principle and advantage of solar calorimetric testing is that glazing plus eventually shading devices, the complete fagade structure, can be tested in a "black box” approach, i. e. no additional information on the layer properties or the internal heat transport (ventilation) is needed. There is no modelling involved which is always a problem for more complex fagade constructions. Only a general parameter model is being used which is parameterized using the information from the experiment and parallel optical measurements.

The second important advantage is that angular dependence can be measured with these test procedures.

Figure 2: View of the solar calorimetric test Figure 3: Test frame for internal solar

cabin of Fraunhofer ISE, with an external shading devices shading device mounted in front of the calorimetric plate

Radiative Model

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

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

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

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

Aj

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

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

Regimes of periodic microstructures and their optical functions

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

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

geometrical optics (ray tracing)

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

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

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

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

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

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

Towards lean building concepts

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

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

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

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

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

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

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

Novel durable solution-chemically derived spectrally selective absorbers

T. Bostrom, Division of Solid State Physics, Department of Engineering Sciences,

Uppsala University, Sweden.

E. Wackelgard, Division of Solid State Physics, Department of Engineering Sciences,

Uppsala University, Sweden.

G. Westin, Division of Inorganic chemistry, Department of Materials Chemistry, Uppsala University, Uppsala, Sweden.

A promising novel solution-chemistry method to fabricate spectrally selective solar absorber coatings has been developed. The objective is to create highly efficient, flexible, inexpensive and durable absorbers for solar thermal applications using simple techniques. The selectively absorbing film consists of a composite with nickel nano­particles embedded in a dielectric matrix of alumina.

The AR material should have the following properties: the proper refractive index, low thermal emittance, dense, flexible and long term stable. The AR materials tested were silica, alumina and mixtures of silica-titania. The refractive indexes of the above mentioned materials range from 1.4 (silica) to about 2.1 (50/50 molar ratio silica/titania mixture). Besides increasing the normal solar absorptance, asoi, it is equally important that the AR layer is long term stable in order to create a successful solar selective coating. The AR coatings were synthesized using different solution-chemical methods and deposited on the absorber surface by spin coating. Prepared samples were subjected to an accelerated lifetime test. In the test procedure the temperature of the environment was set to 40°C and the relative humidity to 95 %. Samples made with alumina as AR coatings failed the ageing test. The other materials, silica and silica — titania mixtures proved to be very resilient. Samples that were coated with these AR materials showed no visible degradation of the sample surface even after 600 hours of testing.

Absorbers without an AR layer typically attain a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03. Of the samples made with durable films a 70/30 silica/titania mixture showed the greatest increase of the asol value, 0.91, while the thermal emittance remained unaltered.

Introduction

The most efficient thermal solar collectors for hot water production use a spectrally selective surface that absorb and convert solar radiation into heat. There are already high performing selective surfaces but there are a few difficulties with some of them, such as the long-term durability, moisture resistance, adhesion, scratch resistance, cost and complicated production techniques. In order to make thermal solar collectors more accepted and widespread, the price per unit has to decrease. The most costly component of a thermal solar collector is the spectrally selective surface.

The main aim was to investigate the durability of spectrally selective absorbers produced by a newly invented solution-chemical method. This work is a continuation of a preceding study
where spectrally selective absorbers were produced using a novel solution-chemical technique [1]. Advantages with this technique are that it is simple and easy to control, the coating can be manufactured under ambient pressure conditions, the chemicals involved are environmentally friendly and it is low in material consumption. Furthermore there exist several methods like spin-, flow-, spray — and dip-coating to coat a surface with a liquid medium. The method seems promising and could hopefully reduce production costs for absorbers and hence make them less expensive and more available. The focus in this part of the thesis has been set on the durability properties of anti reflection treated absorbers. The optical characteristics of produced samples before and after the accelerated ageing testing were investigated.

The used absorber belongs to a group of absorbers called metal-dielectric composite/metal tandem, which normally consists of metal embedded in a dielectric matrix applied on a metal surface [2]. The absorbing layer, spin coated on an aluminum substrate, consists of nickel particles embedded in an aluminum oxide matrix. The composition of the absorbing layer is 65 volume percent nickel and 35 volume percent alumina and the thickness is about 100 nm. The metal particles are between 5 — 10 nm in size. A major advantage with a composite is that it offers a high degree of flexibility. By varying the choice of particle, particle size, particle orientation and shape, film thickness and particle concentration in the film, innumerable combinations can be created. Thus spectral selectivity can easily be achieved. By applying the coating on a poor thermal emitter, in this case aluminum, a low thermal emittance value, stherm, is obtained. The normal solar absorptance value, asoi, for the absorbing layer is about 0.80 and the normal thermal emittance value 0.03.

The AR material should have the following properties: the proper refractive index, poor thermal emitter, dense, flexible and durable. A correct refractive index is required in order to obtain as high solar absorptance as possible. At the same time the AR material should be a poor thermal emitter not to increase the stherm value. Lastly the material has to withstand accelerated ageing tests in order to be successful and should therefore be dense and elastic.

Five different AR coatings were studied, alumina, silica, hybrid silica, and two compositions of silica — titania. Silica is well known to be a very resilient but static material. In order to make silica more flexible an organic compound can be incorporated into the structure and then the resulting material is called hybrid silica [3]. A flexible material is more likely to perform well in accelerated ageing tests since it is less prone to crack when heated or cooled. Alumina has a higher refractive index in the visible wavelength range than silica, 1.6 compared to 1.4. Pure titania has a refractive index of 2.7. Thus refractive indices between 1.4 and 2.7 can be obtained by mixing silica and titania.

The materials structure of a thin coating is not completely permanent with time. Factors like high temperature, high air humidity, airborne pollutants and sun radiation can cause the coating to deteriorate and hence affect the optical selectivity of the surface [4]. High temperatures can speed up oxidation processes and high levels of humidity may create hydrolytic reactions i. e. electrochemical corrosion. Airborne pollutants might also accelerate electrochemical corrosion processes and solar radiation can initiate photochemical redox reactions. A combination of these processes can be devastating for a large number of materials, including solar selective coatings. The most accurate method to test the durability of a solar absorber is to assess it under normal working conditions. These so called in-situ tests are though very hard to carry through because of the great time length required to get
satisfying results. Instead of exposing the absorber surface to its natural working conditions for many years, inexpensive laboratory tests can be done in a climate chamber, where temperature and humidity can be controlled. The temperature and/or the humidity in the chamber can be elevated above normal levels in order to accelerate ageing processes.