Category Archives: EuroSun2008-4

DEVELOPMENT AND IMPLEMENTATION OF A FLEXIBLE. DESIGN FOR LOW ENERGY BUILDINGS

I. Visa*, A. Duta, S. Fota, C. Nastac

Transilvania University of Brasov, Centre: Product Design for Sustainable Development, Eroilor 29,

500036 Brasov, Romania

* Corresponding Author, visaion@unitbv. ro

Abstract

The concept of low energy building is mostly addressed to as an answer to the requirements for clean energy and environment protection. The use of renewable energy systems, implemented in the design of different residential applications must be correlated with the availability of the input energy and requires, in certain climatic conditions, supplementary co-generation systems. The integrated use of solar energy, for solar-thermal and solar PV conversion, along with a heating pump system, can offer an integrated solution for inner space and water heating/cooling and, partially, for the power consumption. The design of such a system must be adapted to the beneficiaries’ demands but, must follow a suitable, flexible algorithm. The paper presents the results obtained in the design of a combined system based on solar-thermal and heating pump system associated with a PV array, implemented in the building of the research department Product Design for Sustainable Development, in the Transilvania University of Brasov, Romania.

Keywords: Low energy building, photovoltaic, solar-thermal, heating pump

1. Introduction

The climatic changes due to the greenhouse gases (GHG) represent a major concern for our sustainable future. The eight Millennium Development Goals are the core of the meeting agenda in the next UN Session from 25 September 2008, when the world leaders are expected to renew their commitment and to set out concrete plans and actions, by 2015, for further sustainable development, including environmental sustainability. The concept answering to these needs involves sustainable energy: energy efficiency, energy saving and renewable energy systems, jointly approached both in economy sectors and at social/societal level.

European Community had clearly moved towards action for climate change mitigation, through reduction of GHG, sustainable development, security of supply and the development of a knowledge based economy, by developing the Directive on the promotion of the use of energy from renewable energy sources, [1]. The directive, known as 20/20/20 sets a mandatory target of a 20% share of renewable energies in overall Community energy consumption by 2020. The directive actually re­formulates the values of the targets previously set for each EU country thus long term plans can already be found, as it is the Dutch Future Energy System, [2] that considers a complex of actions for energy efficiency in industry, CO2 neutral feedstock for transportation, renewable energy and highly

insulated buildings in the built environment. The expected result is an 80% reduction of the GHG emissions (comparing with 1990), till 2050.

The long term strategies require an integrated approach, while resource management becomes a key issue; thus there must be considered the quantification of all the energy and materials needed in the consumption, both for construction/product development (embodied energy and materials) and for usage; exergetic life-cycle assessment models have been developed for approximating the overall needs in industrial but mainly in residential applications, [3].

Concluding, a reduction of the GHG emissions arising from the built environment can be achieved by the development of new building materials (with low thermal transfer coefficient) and the adoption of the building integrated renewable energy conversion technologies.

Monitoring Results 2008

The analysis of the data for the cold production in summer 2007 has shown significant shortcomings according the absorption chiller itself. Especially in the second half of the year the machine has reached neither the capacity nor the efficiency written in the technical documents of the manufacturer. Therefore, the manufacturer carried out a maintenance of the machine in early May 2008.

Afterwards, a series of measurements were conducted. The intention was to test whether the technical characteristics would be fulfilled better. Therefore the system was partly controlled manually to provide constant conditions for the absorption chiller.

To reproduce the characteristic curve of the machine different temperatures for the hot and cold — water circuit were set up. Each condition was hold for 45 minutes to reach steady-state conditions.

Подпись: Fig. 7. Measured data in early May 2008.

The temperatures in all three circuits of the machine — heat circuit, cold-water circuit and re­cooling circuit — where measured with three different sensors. The data recorded by the data acquisition system during the fifth series of the measurements are shown as an example in figure 7.

The required capacity is reached in some of the five series, but the COP does not reach the specification in any of them. The output deficit, relative to the desired/planned COP, is between 23 and 30 %.

For the performance of the machine this means a significant additional consumption of heat and electric power. The primary energy coefficient increases from 0.26 to 0.38 (+46 %) and the emission of carbon dioxide per produced kilowatt hour cold rises from 39 to 63 g/kWh (+62 %).

Because of the not satisfying results the absorption chiller was optimized again in early June 2008 and another measurement was done, showing that the required capacity of 40 kW is slightly exceeded with 41.7 kW. The COP averages 0.64. It falls below the specified COP by 2 %. Hence, the performance now meets the demands. To ensure that this is a stable situation another check is planned in autumn 2008.

5. Conclusions

In 2007, the absorption chiller used in the Museum Ritter yielded no advantage with reference to either primary energy or CO2 emissions compared to a good conventional compression chiller. The main reason was, besides the poor thermal COP, the high consumption of electric energy.

But even with an ideal COP of 0.7 the energy consumption worsens the primary energy factor from 0.28 to 0.44 and the specific CO2 emissions from 17 to 51 gCO2/kWhcold. For a constant performance with a high COP the operating parameters have to be controlled more precisely as for a compression chiller. By increasing the re-cooling temperature by 1 K, the absorption chiller looses between 10 to 20 % efficiency whereas the reduction of the actuation temperature from 86° C to 82° C lessens the efficiency by 20 %. A decrease of the cold-water inlet temperature by 1 K induces a loss of efficiency of about 10 %.

A compression chiller however can be run over a broad range with constant efficiency (e. g. from 10 to 21° C cold-water inlet temperature and 20 to 45° C re-cooling-water inlet temperature), although with changing COP. Therefore, the heat delivery and re-cooling must ensure preferably constant conditions. This is difficult due to the complicated hydraulics of the technical systems in the Ritter Museum.

Due to the actuation by heat, the higher demand on re-cooling and the lower re-cooling temperature an absorption chiller induces a higher consumption of auxiliary energy. That includes electricity for the pumps and fans used for heat generation and re-cooling. Also the water conditioning of the back feed of the cooling tower has to be taken into account. In the Museum for example six pumps are installed for this purpose. Therefore it is important to plan these auxiliary energy consumers in a very energy saving way. This includes short circuit pipes for heating and re­cooling, generously calculated cross section diameters to achieve small pressure losses and high efficient pumps. The electric auxiliary energy consumption is usually not accounted in the usual technical documentations.

Absorption chillers are not operated in full load all the time. Therefore a standard test series with different operation modes would be useful to compare units of different manufactures. This could result in more reliable specification for planning and comparison of absorption chillers.

Overall, the results show that the further development and improvement of small and medium sized absorption chillers is necessary. Also it is evident, that the monitoring of these systems is very important in order to detect performance failures in real (transient) operating modes which are normally not tested in test facilities..

The monitoring of the Ritter Museum will be continued in 2008. Hence, the optimization will be verified.

Acknowledgement

This work was funded by the German Ministry of Economy and Technology (BMWi) within the research programme "Energy optimized buildings (EnOB)"; reference number 0329084D. The authors also like to thank the project coordinator (PTJ) and the Museum Titter staff for their support.

Estimation of the PV area

From the available roof area, the total suitable PV area can be estimated. Two scenarios are considered: a PV Fraction=30% and a PV Fraction=20%. Where PV fraction is the percentage of occupation of suitable roof area by PV.

A fraction superior to 30% will be difficult to implement. The 20% option is taken as a conservative scenario in order to account for difficulties in the installation due to the wide type of devices that can be located on the roofs (such as the air handling units required by the new regulation [2,3,4]). The total area of PV (for the 172 schools) will range between 115 000 m2 and 172 000 m2, for a PV fraction of 20% and 30% respectively.

Optical parameters deduced from transmittance curve

Transparent and semi transparent thin films of some metal halides, were deposited on glass slides using a chemical solution bath containing either a metallic sulphate, nitrate or higher valance halide; an alkali halide and an alkali hydroxide as the complexing agent (ligand). The films grown by this method are iron halides, tin halides, and manganese halides.

The spectral transmittance/absorbance curve of the films in the UV-VIS-NIR region were obtained using a Pye Unicam spectro photometer sp 100 and 650. These spectral curves were used to obtain the absorbance and percentage transmittance values. Also, the absorption coefficient and transmission coefficient were computed from the values of the absorbance and transmittance. The other parameters were then derived from the absorption coefficient using relation between the absorption coefficient and other optical parameters [3, 63]. For example, the relation between the absorption coefficient and the extinction coefficient and refractive index are given [3, 63] as:

k = and

a k/4n

(1)

n =

k/x

(2),

For films whose k and n could not be obtain from literature.

Where k is the extinction coefficient, a is the absorption coefficient which is obtained from the absorbance curve, n is the refractive index, к is the wave length of radiation which is obtained from the curve as well, and x is the thickness of the film which was obtained by the gravimetric (weight deferential) method and confirmed with a talystep instrument.

X — ray diffraction and analysis showed that the films produced were good quality polycrystalline films of Sn, Mn and Fe chlorides, bromides and iodides; and Ag2S,,when compared with standard values from ASTM X-ray diffraction files [3].

Double-facade walls and energy saving

Assuming that through the use of double-facade walls it is possible to decrease the building’s demand for air conditioning and mechanical ventilation, as well as to have net energy gains from solar energy for its heating, the overall energy balance of the building should improve. Few published studies on the subject, however, fail to wholly support this thesis.

K. Daniels published a study which compares the energy cost of an office building with a given set of parameters describing its internal physical environment, dependent on the chosen model of facade [2]. Altogether, 5 options were taken under consideration:

• Double facade with a box structure and insulating glass in the interior wall — no.1

• Double facade with a box structure, insulating glass and a thermal filling in the interior wall — no.2

• Single facade with insulating glass and a thermal filling in the interior wall — no.3

• Single facade with a glass Luxguard Natural — no.4

• Single facade with a glass Climasol Neutral — no.5

The presented cumulative diagram (Fig.6) sums the different elements of energy cost (heating, cooling, lighting etc.) and shows that the difference between two best results for a double facade

(2) and a single facade (5) is not more than 8% (sunshades were not taken into consideration).

D. Richards published a study on comparative cumulated energy expenditures of buildings with different types of external walls, including double facades [9]. In comparison to a single facade with single glazing and without external insulation (under today’s thermal insulation standards — discontinued), energy savings from using double facades reach 75%. When single glazing was replaced with low emission insulation, these gains fall to 50%. Should an additional external system of protection from the sun’s radiation be used, same levels of energy consumption are recorded (the difference is within 2%). In the case of buildings which use external shading systems (low buildings), much less expensive single facades can be a serious competitor for double facades. Comparisons should then be made in other areas, such as protection from the noise or the aesthetics of both solutions.

4. Conclusions

The analysis of utility value of double facades is related to their ability to take advantage of natural weather conditions (sun and wind). It points to the following advantages and shortcomings of their application in buildings (tab.1):

Tab. 1 Advantages and shortcomings of double facades

advantages

shortcomings

■ ability to take advantage of energy gains from sun radiation in cold and intermediate periods;

■ levelling of temperatures between external and internal walls during cold and intermediate periods;

■ ability to open windows and introduce natural ventilation in buildings, in which this was not possible before;

■ ability to use inter-facade void as an area which participates in the transversal ventilation of the building;

■ danger of overheating interiors during hot days

■ problems related to cleaning, maintenance and repair of tcomplex facade systems;

This enumeration allows us to conclude that double facades largely meet the requirements of interactive walls. Such was the dominant view emerging from publications dating from mid 1990s, when such solutions were at the top of their popularity. However, the experience from exploitation of these double-facade buildings over the next 10 years do not allow for such one-sided opinions.

The main problem is that the advantages stipulated above can be attained only with a proper design process, which requires much higher inputs of knowledge, experience and cost that in the case of other types of walls. It also necessitates interdisciplinary research involving simulation, testing and comparison of different types of solutions to choose optimal options. In the case of many constructed buildings, these were deliberately omitted [11], but other advantages came to the fore: acoustic properties of double facades (good insulation from external noise even when windows are opened) and their architectural value. These buildings are characterized by above-the-average energy consumption and offer a number of discomforts for their users, including overheating in summer. Buildings that took advantage of interactive double facade walls (such as the Commerzbank head office in Frankfort, RWE office building in Essen, Post-Office in Bonn) have better exploitation characteristics, even though these are slightly worse than planned. Therefore, even a careful process of designing is prone to error. The functioning of double facades is dependent on many elements and feedbacks with other elements of the building (buffer zones, installation systems), which are hard to precisely assess beforehand. It also requires important outlays for cleaning, maintenance and repair. Their effectiveness also depends on the conscientiousness of their users and rationality of their behaviour. Furthermore, each building should be treated separately and optimized for its shape and location on site.

Perhaps technological development will allow for the design process to be both more simple and effective. However, it may also be that progress in combining glass with nanomilimeter layers, which improves its utility value, will allow to construct a wall, in which control over internal environment of the building will take place within the microscale of the glass pane. The sense of creating multilayer, complex structures, such as double facades, could then be seriously questioned.

Refercences

[1] Compagno, „Intelligent Glass Facades“, Birkhauser Publishers, Basel-Bostn-Berlin1997.

[2] K. Daniels, „Low-Tech Light-Tech High-Tech Building in the Information Age“, Birkhauser Publishers Basel-Boston-Berlin 1999.

[3] K. Daniels, „The technology of Ecological Building”, Birkhauser Verlag Basel-Boston-Berlin 1997.

[4] Double-Skin Walls”, collective research of Ecole Polytechnique Federate de Lausanne, Institut de Technique du Batiment, Departement d’Architecture 1999.

[5] G. Gogolok,„Sciany oslonowe dwupowlokowe” [Double-layer facade walls], conference materials Nowoczesne lekkie sciany oslonowe i okna” [Modern light walls and windows], Mrqgowo 1999.

[6] V. Lerum „High Performance Building”, John Wiley&Sons, New Jersey 2008.

[7] R. D.Lieb, E. Oesterle, “’Das Dusseldorfer Stadttor’ Exercising an Integrated Approach to Double Facade Planning”, 1999.

[8] Oesterle, Lieb, Lutz, Heusler, „Double Skin Facades, Integrated Planning”, Prestel Munich-London-New York 2001.

[9] D. Richards, “An Integrated Approach to Sustainability”, “Dimensions of Sustainability”, E&FN SPON 1998.

[10] K. Zielonko-Jung “Podwojne elewacje szklane we wspolczesnej architekturze” [Double glass facades in contemporary architecture], Ph. D. thesis, Faculty of Architecture, Warsaw Polytechnic, Warszawa 2003.

[11] K. Zielonko-Jung „Uwarunkowania technologiczno-ekologiczne stosowania scian dwupowlokowych — tendencje rozwojowe” [Technological and environmental determinants of double-facade walls — development trends], unpublished research, Faculty of Architecture, Warsaw Polytechnic, Warszawa 2005.

i Published research grant financed (2007-2009) by the Ministry of Science and Higher Education, Fusion of traditional and advanced technologies as a new environmental orientation in contemporary architecture”.

Model of the solar collector and storage vessel

The solar collector and the storage vessel were modelled in Matlab. A collector heats the water in the primary loop, shown on the left in figure 2, allowing a temperature stratification within the storage vessel. Water at the top of the vessel is extracted for space heating, DHW and hot fill and simultaneously cold water (10°C) is fed to the bottom of the vessel. If the temperature of the extracted water is too low, the boiler will heat it to the desired level: 40 °C for space heating, 55°C for DHW and 60°C for hot fill. The latter values are relatively high in order to prevent Legionella contamination. The water at 55°C is mixed with cold water to obtain the desired temperature level of the DHW tapings.

The type of collector is a vacuum collector[5] of varying size, oriented towards the south with a tilt of 45 degrees. The storage vessel is modelled as a cylindrical vessel of varying size which is thermally insulated with a U-value of 0.1-0.2 W/m2K. The temperature stratification in the vessel is modelled as 8 isothermal segments of water.

Подпись: Fig. 2 Schematic of the model of solar collector integrated in the heating system. The shower is the symbol for all DHW demand including hot fill of dishwasher and washing machine.

In the base case scenario, the DHW pattern is modelled after that in the Dutch Energy Performance Norm EPN [6], which entails approx. 20 short and longer tapings of a particular temperature level including a long taping of 50 l every morning and every evening for showering. The pattern for the hot fill of dishwasher is a daily taping of 14 l at 60°C and that for the washing machine two tapings of 13 l on Saturday and Sunday each and another on Monday. This pattern and a number of variants (see chapter 3.2.5) were simulated and the contribution to the saving of primary energy was calculated.

System at LESO solar experimental building

The AIC shown in Figure 1(left) is particularly interesting for applications with comparably deep office rooms due to its potential to lead daylight deeply into the room through the highly reflective light-duct (“core-daylighting”). The office rooms within the LESO-SEB are, however, comparably short (i. e. only 4 m of room depth). The AIC has therefore been adapted to these office rooms’ geometry. The new solution is shown in Figure 1(right): the system has basically been reduced to

one single anidolic collector element, located underneath the tilted double glazing. This collector element reflects the entering daylight flux onto the room’s diffuse white ceiling, from where it is distributed throughout the entire office. Altherr and Gay have compared the performance of this ADS to that of a normal double glazing [7]. They showed that this system has two great advantages compared to a normal vertical window:

1) The system blocks out large parts of the direct daylight flux that would reach the room’s window section through a normal vertical window. It therefore reduces the illuminance levels in this area. This corresponds to massive glare reduction and improved visual comfort for occupants working next to the windows.

2) The blocked daylight is not rejected but rather reflected towards the room’s diffuse white ceiling. From there, it is distributed comparably evenly within the room. The results are higher daylight factors in the centre and rear sections of the office room compared to the “normal vertical window”-case.

Optical and thermal modeling

As a result of our first suitability tests, UV-stabilized composite films are selected for prototyping a prismatic structure as light-redirecting device with seasonal shading features.

On the basis of the experimental material data, we carry out ray-tracing simulations with different glazing designs integrating the prototype film to determine the solar optical properties of the complete system. Centre-of-glass performance indices (solar heat gain coefficient — SHGC — and thermal transmittance — U-value-) of the corresponding glazings are then calculated taking into account the infrared optical properties and thermal conductivities of the single components.

Figure 4 shows the incident angle dependent SHGC of a triple glazing with 2 highly transmittive low-e coatings: values variable from 0.58 at low solar altitude to less than 0.10 at high solar altitude can be achieved, demonstrating the dynamic properties of the prototype

To evaluate the impact of this angular selectivity on energy savings, solar energy gain for a south facing window is simulated over a year and compared to the performance of conventional multiple glazings, whose angular dependent SHGC are calculated with the software WINDOW 5.2 [10]. Hourly simulations are based on beam and diffuse irradiance data of the German Test Reference Year, Region 3 [11]. The results are shown in Fig.5 and confirm the potential of the prismatic glazing, which performs similar to the triple glazing during the winter months and similar to the

Подпись: T3 1) ъ j3 О cft Подпись: 80 70 60 50 40 30 20 10 0 image242

best solar-control double glazing during the summer period, thus providing both heating and cooling energy savings.

Fig.4. Simulated angular

Fig.5. Calculated monthly solar energy gain of the prismatic prototype compared to
commercial double and triple glazings (conventional glazings are labelled
with light transmittance and SHGC evaluated at normal incident angle)

For the use as transparent insulating material, where high energy gain is required, the performance
of the glazing can be further improved by substituting clear with white glass and by applying

antireflective coatings. According to our simulations SHGC up to 0.64 can be achieved, even higher than a common double glazing.

For the use in a glass facade, where visual appearance and comfort play an important role, the implementing of a light scattering glass pane as inner layer is currently investigated. Depending on the optical properties of this pane, it is possible to adjust the visible and solar transmission as well as the solar heat gain coefficient over a wide range in order to meet the application-specific requirements [12]. Detailed results of our investigations will be published at a later date.

Concerning the insulating properties of the glazing, we calculate night-time thermal transmittances between 0.6 and 0.8 W/m2K, depending on gap-width (10-12mm) and gas-fill (Argon or Krypton). These values are comparable to those of common triple glazed systems and demonstrate the negligible effect of partial infrared transparency of plastic films if combined with low emissivity coatings.

Material preparation

A thermotropic layer with an additive concentration of 5 m% was prepared. For the production of the functional material, a dissolution of the thermotropic additive (wax) in the resin was filled into an intervening space located between two glass panes, which were sealed around the edge. Afterwards the mixture was cured by UV-radiation (Ultra Vitalux, Osram GmbH, Mtinchen, GER) for 5 minutes. The film thickness was 800 pm.

2.1. Material Characterization

Solar transmittance in clear and opaque state, switching temperature and switching process of the thermotropic layer was determined by UV/Vis/NIR spectrophotometry. A double beam UV/Vis/NIR spectrophotometer (Lambda 950, Perkin Elmer Instruments GmbH, Uberlingen, GER) equipped with an Ulbricht-sphere (diameter 150 mm) was adapted by a heating stage to adjust the sample temperature. For the given measurement apparatus the radiation passing through the specimen outside a cone of approximately 5° relative to the incident beam direction is defined as diffuse (scattered) component. Hemispheric and diffuse transmittance were recorded at normal incidence in the spectral region between 250 and 2500 nm in steps of 5 K from 25°C to 100°C. Prior to the measurements the sample was maintained at the selected temperature for 10 minutes. The integral solar transmittance was determined by weighting the recorded spectral data in steps of 5 nm by the AM 1.5 global solar irradiance source function, given in Bird and Hulstrom [5]. The data were corrected for radiative losses due to the measurement setup according to Resch et al. [3].

The switching behaviour of the thermotropic layer is determined by thermal transitions of the additives. Thus a comprehensive thermal characterization of the additive was done by Differential Scanning Calorimetry. A thermogram was recorded under static air on a Mettler Toledo DSC823e (Schwarzenbach, CH) in a temperature range between 0°C and 130°C applying a heating rate of 10 K/min. The sample mass was 10 mg.

Information as to the morphology of the polymer films was obtained by Atomic Force Microscopy (AFM). AFM was performed in tapping mode (frequency: ~80 kHz) with a MFP 3D (Asylum Research, Santa Barbara, CA, USA). Specific focus was given to the generation of high resolution phase images. Due to the fact that an AFM phase image represents the phase lag between the agitating oscillation and the detected cantilever oscillation, the differentiation between hard (i. e., resin) and soft (i. e., additive) sample areas is possible. Sharpened Si NCH tips with a tip radius <10 nm and an opening angle of 18° were used.

The chemical constitution of the thermotropic layer surface was analysed by Raman spectroscopy using a LabRam HR800 confocal-Raman spectrometer (HORIBA Jobin Yvon GmbH, Bensheim, D). Raman Spectra were obtained by illuminating the specimen through a 100x objective with 514.5 nm excitation from an Ar+ laser. To obtain a spectral resolution of about 1 cm-1 a diffraction grating of 1800 grooves/mm was used. Mapping studies were carried out over the spectral range from 2750 to 3100 cm-1 at 0.5 pm intervals across the sample surface. Raman spectra were averaged over 5 scans,

and accumulation times of 10 s were employed. The Raman map was generated by integrating the relevant additive (wax) peak.

3. Results and Discussion

Подпись: 70 (D en 60

In Fig. 1 the hemispheric and diffuse transmittance of the thermotropic layer is plotted as a function of temperature. Furthermore the switching characteristic of the thermotropic layer is compared to the thermal transition of the additive used. In clear state the material exhibits an excellent hemispheric solar transmittance of 85% with a diffuse fraction of 40%. At a temperature of 45°C the hemispheric transmittance starts to decrease associated by an increase of the diffuse transmittance. This indicates that the layer undergoes a transition from a clear to a scattering state. In general the thermotropic layer shows a rapid and steep switching process within a temperature range of 15 K. Above the switching threshold the hemispheric transmittance decreases by 6% to a value 79%, with a diffuse fraction of 64%. The significant increase of the diffuse transmittance causes a loss of view-through behaviour. According to this switching range and switching temperatures the layer does not provide adequate overheating protection for a solar collector.

Подпись:50

Fig. 1. Thermogram of the additive (top) and hemispheric (middle) and diffuse solar transmittance (bottom) of

the thermotropic layer as a function of temperature.

The comparison of the thermotropic layers switching performance with the additives thermal transitions reveals a good correlation. At the onset of the additives melting peak between 45 and 50°C the hemispheric transmittance decreases and the diffuse transmittance increases significantly.

Subsequently the switching proceeds with the melting of the additive. The thermotropic layer reaches its final scattering state once the additive is in a fully melted state. Besides the melting peak of the additive an endothermic effect at lower temperatures (25-35°C) is observable. This peak can be attributed to a phase transition in the crystalline structure. This endothermic effect leads to a slight change of the solar transmittance. The excellent correlation shows that the switching temperature and process of the thermotropic layer can be tailored by selecting adequate additives.

Подпись: Fig. 2. AFM phase images of the produced thermotropic layer (left: 20x20pm2; right: 2x2 pm2); dark regions indicate less phase shift.
In Fig. 2 AFM phase images of the thermotropic material are depicted. Dark colours represent a smaller phase lag and bright colours a larger phase shift. According to this, the darker areas represent the softer additive particles and the resin matrix appears bright. This is confirmed by the higher magnification of a dark area shown in the right image, which reveals a superstructure characteristic for the additive used. It is observable that the additive particle dimensions and thus scattering domain sizes range between 0.4 and 2.5 pm.

Figure 3 illustrates a map of the chemical constitution of the surface determined by Raman microscopy along with spectra in the wavenumber range between 2750 and 3100 cm-1. The two presented spectra are representative for the whole surface area. While the top right spectrum characterizes the resin the distinct peak at a wavenumber of 2883 cm-1 of the bottom right spectrum can be attributed to an asymmetric stretching vibration of CH2 groups of the additive. According to the spectra bright colours of the map represent the additive. Additive domain sizes of about 1 pm are detectable. Hence, Raman microscopy confirms the morphological features determined by AFM phase imaging. However, due to the limit lateral resolution of the Raman microscope the domains are less distinct than in the AFM phase image.

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The particle size of the thermotropic layer between 0.4 and 2.5 pm is in good agreement with its moderate reduction of the hemispheric transmittance and the significant increase of the diffuse transmittance. For optimum back scattering efficiency domain diamteres between 200 and 400 nm are required [6]. Larger particles lead to increased diffuse scattering above the switching threshold. Thus to enhance the light shielding performance of the thermotropic layer a significant reduction of scattering domain size is required.

Fig. 3. Raman map (left: 20x20pm2) showing the distribution of additive on the layer surface together with
Raman spectra of a pure resin (top right) and a resin containing the additive (bottom right).

4. Summary and Conclusions

Within the present investigation a thermotropic resin was characterized as to light-shielding efficiency, switching temperature and switching performance. In the clear state the thermotropic layer showed a solar transmittance of 85%. Above the switching temperature of 45°C the hemispheric transmittance amounted to 79%. The diffuse transmittance increased significantly from 40% in the clear state to 64% in the scattering state. The increase of the diffuse transmittance along with the moderate change in hemispheric transmittance was found to be in good agreement with average additive particle dimensions and thus scattering domain sizes of 0.4 to 2.5 pm ascertained by AFM phase imaging and by Raman microscopy. Thus to enhance the film’s reflectance efficiency a significant reduction of scattering particle diameters is required. An excellent correlation between the thermotropic layers’ switching performance and the thermal transition of the additive was ascertained. This enables the production of thermotropic materials exhibiting tailor-made properties by selecting adequate additives.

References

[1] G. M. Wallner, K. Resch, R. Hausner, Solar Energy Materials and Solar Cells, 92 (2008) 614-620.

[2] K. Resch, G. M. Wallner (2007). In Proceedings of ISES SWC 2007, Beijing, China, pp. 541-545.

[3] K. Resch, G. M. Wallner, R. W. Lang, Macromolecular Symposia, 265 (2008) 49-60.

[4] K. Resch, G. M. Wallner (2008). Submitted to Solar Energy Materials and Solar Cells.

[5] R. E. Bird, R. L. Hulstrom. Solar Energy, 30 (1983) 563-573.

[6] P. Nitz, H. R. Wilson (2008). In Proceedings of 2nd Leobner Symposium Polymeric Solar Materials, Leoben, Austria, pp. XIn-1-XIn-6.

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Acknowledgements

Specialties (Drogenbos, BEL) and Sasol Germany GmbH (Hamburg, GER) for providing the materials.

Grain Size Parameters and Porosity Factor — Spectral Relations

The computed values of (i) ASTM and (ii) IOS grain size numbers (Gn and Gm), (iii) average grain

Area (iv) average grain diameter d, (vi) grain size distribution (or uniformity) (Gd) and porosity factor (PT) for the films (Table 1) show the followings for the different films.

Snl2 and MnBr2 Films: NA = Gn and Gm values are higher for MnBr2 than for Snl2 indicating that

MnBr2 contain more grains than Snl2, as explained in the section above. A and dvalues are larger for Snl2 than for MnBr2, indicating that Snl2 films have larger grains. L value is larger for MnBr2 than for Snl2 showing that MnBr2 films have more oblong grains, while Snl2 grains tend to be more roundish or circular. Gd for MnBr2 is larger than for Snl2 showing that Snl2 have more uniform grains than MnBr2. PT for Snl2 is larger than that for MnBr2 indicating that Snl2 is more porous than MnBr2. These also explain why MnBr2 tends to have is higher absorption of UV radiation that Snl2. This is because when a film with tinny grains is porous, there is little or no absorption of UV — VIS radiation by the grains, hence the radiation finds it easier to pass through as in Snl2. But when a tinny grained film is dense, UV absorption tends to be higher with a substantial NIR reflection as well. The only wavelength of transmission for such films would therefore be the visible radiation as shown in fig 1.

PbBr2 and Pbl2 films: Gn and Gm are about the same for both films although they are a little bit

higher for Pbl2 than for PbBr2 indicating somewhat more grains for Pbl2. A and d are a bit larger for PbBr2 than Pbl2 showing somewhat larger grains for PbBr2. Lv is higher for PbI2 then for PbBr2 indicating that PbI2 have more oblong grains than PbBr2. Gd PbBr2 is larger than Pbl2 showing that Pbl2 grains are more evenly distributed than PbBr2. PT is higher for PbBr2 than for Pbl2 showing that PbBr2 is more porous than Pbl2. Generally, these films are highly porous, hence they will transmit highly in the UV but have improved late visible and infrared reflection from about a cut off of 450 to 550mn.

Ag2S and PbS films:Gn and Gm are higher for Ag22 than PbS, hence Ag22 have more grains than

PbS. A and d for PbS are higher than those for Ag22, indicating that PbS grains are larger than Ag2s grains. Lv for PbS is higher indicating that their grains are more oblong than those of Ag2S Gd for PbS is higher showing that PbS is less evenly distributed. RT is higher for Ag2S showing that it is more porous. The very large nature of the grains of these films combines with moderate porosity and high void make them highly absorbing in the UV and Visible but highly transmitting in the Near Infrared. High voids and vacancies through out the film would allow infrared radiation to go through easily whereas the visible ultraviolet radiations would be easily intercepted (absorbed) by the large grains.

3. Conclusion

From the above, film microstructures (i. e. the geometric exterior and grain size parameters) were observed to be related to the spectral characteristics of films [2]. Films with smaller grains, which tend towards continuous, show high UV absorption, NIR reflection and Visible transmission as in Snl2 and MnBr2 and Pbl2 films. Films with moderate grains, which are more widely and uniformly spaced show high UV-VIS transmission and improved NIR reflection like the PbBr2 and Pbl2 films. On the other hand, films with larger grains which tend towards continuous absorb UV-VIS radiation completely but transmit in the infrared region as in the Ag2S and PbS films [2].

The Snl2 and MnBr2 films can be used as window coatings for warm climates to prevent UV and IR radiation from entering buildings but allow only visible radiations for “day lighting” [2, 15-16]. Hence natural air conditioning can be created by using these films.

Therefore Snl2 and MnBr2 films can be used as visible transparent window coating. (VTWC) otherwise known as “Cooling Windows” to block and prevent UV (high energy) AND IR (high heat) radiations from entering but allow visible radiation only into the buildings if the glass doors and windows of such buildings are coated with these films. This can create natural air-conditioning (cooling) of the building interior [12, 15, 16].

Pbl2, PbBr2, PbS and Ag2S films can be used to split the solar spectrum into two broad bands, which can be used for different applications. The lead halide films, which favour UV-VIS transmission and IR reflection can be used to reflect infrared radiation and prevent it from getting to the surface of photovoltaic cells if the films are used to coat solar cell surfaces [2, 15-16]. This is because solar cells work better in the absence of infrared radiation. Such films are called Visibly transmitting spectral splitters (VTSS) and behave like “Cold Mirror”. On the other hand, PbS and Ag2S films can be used to coat glasses, which can be used to absorb UV-VIS radiation, while NIR radiation is transmitted. These glass systems called infrared transmitting spectral splitters (IRTSS) and behave like “Heat or Dark Mirrors” they can be used to regulate solar and thermal energy for Conventional Green House Agriculture (CGHA) and for Control Environmental Agriculture

(CEA), in which various parameters that affect yield and quality of crops are optimized and

regulated [15, 18-20].

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