The available roof area is the sum of the areas that are suitable to install PV panels. These are the areas where incident solar irradiation is high. Since tall buildings are very rare in Portugal and neighboring buildings are usually far from school buildings their shading effect was neglected. In pavilion schools (flat roofs) the available area is equal to the total roof area because the PV panels

3.1. Radiation transmission/extinction parameters:

The major parameters which determine the optical performance of selective thin films are the spectral transmittance/absorbance (T and A), the transmission/absorption coefficient (т and tX), the skin depth (Of x), the refractive index (n) and the transmittance absorption product О Of), [3,61-67].

These parameters can be classified into two, namely, radiation transmission and radiation attenuation parameters. The radiation transmission parameters are those parameters which have to do with transmission of radiation. These are the transmittance and transmission coefficient.

On the other hand, the extinction coefficient, the refractive index, the absorption coefficient, the reflectance, and the skin depth, which have to do with radiation attenuation or extinction in materials are know as radiation attenuation parameters [3, 62, 63-67].

Of these optical parameters, the most important single parameter which gives an idea of the properties and behaviour of visible transparent window films is the spectral transmittance/absorbance curve in the 0.3 to1.0pm region. This is because both the radiation transmission and radiation attenuation parameters can be derived directly or indirectly from the spectral transmittance/absorbance curve of these films [65-66]. Also, the spectral transmittance/absorbance in the ultra violet-visible-near infrared (UV-VIS-NIR) region gives the behaviour and properties of transparent and semi-transparent selective thin films. Hence this investigation on the optical and spectral behaviour of some transparent and semi­transparent window films which were produced by the solution growth method is therefore carried out to determine the radiation transmission and radiation attenuation optical properties of the films. Also, the possible area of application of these films on the basis of these properties is suggested.

In summer (over +22°C) the functioning of the double facades is aimed at minimizing the greenhouse effect, which occurs in between the two walls, since these can lead to the overheating

of the inter-facade void, with its temperatures exceeding those of the external wall. In order to avoid such a situation it is necessary to [10]:

• use adequate sunshades,

• force intensive air circulation in the inter-facade area,

• cool the building at night through the opening of windows,

• use buffer spaces (such as atriums) and elements with large thermal mass (constructions, water reservoirs).

Sunshades are placed between the two walls of the facade This protects them from the impact of atmospheric conditions, while retaining their efficiency. While in the winter period the circulation of air in the inter-facade void is limited to achieve energy gains, in summer only an intensive circulation of air protects it from overheating. The ventilation ducts in the external wall should be fully opened.

An increase in temperature, which occurs in between the walls causes the speed of the circulating air to increase, so this air is replaced more quickly. However, this increase is not proportional to heat increase [4]. When external air temperatures are high enough, hot air can become trapped inside the inter-facade void, leading to significant overheating of rooms. It then becomes necessary to boost the system with mechanical ventilation and air conditioning.

Double-facade walls allow for night cooling of the building through opening all windows in the internal wall. The external wall protects the interior from the wind and prevents intrusion into the building through open windows. At night, when outside temperature is much lower than during the day, the interiors are able to cool down. The effectiveness of this cooling is greater if the building can store it through internal buffer zones and construction elements, which have a large thermal mass.

Renewables can be introduced with locally installed PhotoVoltaic (PV) cells or solar collectors or by importing ‘green electricity’ e. g. from an off shore wind park. The latter however is not accepted within this project. The precondition we set for ourselves is that the investment for the installation generating renewable energy must stem from the renovation budget.

1.1.1. Application of Photo Voltaic

One option to reach the target is the application of PhotoVoltaic (PV) cells. Starting again from the middle bar in figure 1, the target can be reached by mounting 30-35 m2 of PV-modules of optimum orientation on the roof. However, for technical or architectural reasons this may not always be possible or feasible in renovation. In addition, it may not be very economical, which for Dutch builders is an important criterion. Financing constructions, e. g. leasing the PV cells from an energy supplier or an Energy Service Company (ESC) can help to overcome this barrier.

ECN is cooperating with 15 partners in the European ‘Crystal Clear’ Integrated Project aiming to reduce the cost of PV on a system level down to 3€/Wp, which roughly corresponds to an electricity price of €0.15 — €0.40 per kWh — depending on the location in the EU.

Assuming that any PV electricity not consumed can be fed into the grid, the application of PV does not interfere with other measures. The amount of PV, required to reach our target is therefore used as a measure of the success of other measures. This will be discussed in chapter 5 below.

1.1.2. Increased size of the solar collector system

As mentioned before, main consumers of electricity in a typical Dutch household are appliances such as a washing machine and a dishwasher, that can also be fed with (solar) heat (hot fill). However, solar heat is not always available when needed, especially in wintertime. There are two ways to maximise the contribution of solar heat: 1) to store the solar heat in a storage vessel until the time that it is needed and 2) to shift the moment of heat demand to the moment that solar heat is available. The latter could be achieved using smart control systems that would automatically switch on appliances like a washing machine, when sufficient solar heat is stored in the vessel.

The Dutch practice is to apply (if at all) a rather small solar collector, usually in the order of 3 m2 and a storage vessel of typically 150 l. These are rather modest sizes compared to the practice in e. g. German speaking countries, where collector areas are found of up to 15 m2 and storage vessels of up to 2m3 [5].

It is therefore interesting to see how much a larger solar collector system can contribute to the target of reducing the energy consumption by 75%. The scope of the simulations carried out is broader than just saving on electricity consumption; it includes savings on energy demand for space heating and DHW as well as the solar contribution to hot fill. The results of the simulations depend on the assumptions for the different parameters describing the system. These are briefly discussed in the following chapter.

2.1. Overview

ADS as an application of the non-imaging optics theory [3] for illumination purposes have first been introduced by Courret et al. [4], who have discussed the performance of their “anidolic light — duct” in 1998. This system is shown in Figure 1(left). Direct sunlight and diffuse daylight enters the system through a double glazing tilted towards the sky’s zenithal area. Two anidolic elements (i. e. reflective elements shaped according to the non-imaging optics theory) then redirect the entering daylight flux into the light duct with a minimum of reflexions and a minimum of rejections. At the end of the light duct, the daylight flux is released into the rear of the room and properly distributed by a third anidolic element.

This system’s performance has been thoroughly compared to other ADS by Courret in 1999 [5] and by Courret and Scartezzini in 2002 [2], who have referred to it as the Anidolic Integrated Ceiling (AIC). Its performance under different sky types (i. e. in tropical, subtropical and tempered climates) has been simulated by Wittkopf et al. in 2006 [1]. In 2007, a highly energy-efficient office lighting solution based on the AIC for a Singapore office room has been presented by Linhart and Scartezzini [6].

A successful integration of suspended plastic films into insulating glass units basically depends on the surface properties of the film. Perfect adhesion to the primary sealant is required in order to ensure mechanical stability as well as moisture vapour and gas impermeability and not to compromise the performance of the product during its service life.

Film adhesion to the sealant is in a first step investigated with different sealant materials and adhesion enhancing additives. The results are evaluated by comparison with films commonly used for this application. Single-component films show poor adhesion strength independently of the sealing material used and fail the test, whereas composite films perform very well as expected and are selected for further investigations.

In a second step preliminary accelerated tests on small insulating units (350 x 500 mm) are carried out to meet the long term test requirements for moisture penetration and gas concentration tolerances, according to EN 1279-2 and -3 [8, 9]. Moisture absorption is measured after 3 weeks ageing in a climate chamber at a constant high temperature (58°C) and high humidity (RH >95%) regime. All investigated samples reports results, which are in agreement with the specified normative value. Gas permeability rate is measured after 3, 6 and 12 weeks ageing under the same conditions. Gas losses are still higher than the required 10%, which is supposed to depend on the suboptimal adhesion of the microstructured surface to the sealant, strong enough to ensure mechanical stability, but not efficient enough to guarantee gas tightness. The low reproducibility of the measurements indicates a high sensitivity to the manufacturing process, which still has to be improved.

K. Resch1*, J. Fischer1, A. Weber1 and G. M. Wallner2

1 Polymer Competence Center Leoben GmbH, RoseggerstraBe 12, A-8700 Leoben, Austria
2 Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben, Austria

Corresponding Author, resch@pccl. at

Abstract

In this paper the optical and morphological properties of a thermotropic system with fixed domains were investigated. The optical properties and the switching were determined by UV/Vis/NIR spectrophotometry. The morphology was characterized applying Atomic Force Microscopy (AFM) and Raman microscopy. The thermotropic films exhibited a hemispheric solar transmittance of 85% in the clear state, with a diffuse fraction of 40%. The material underwent a transition from the clear to the scattering state at a temperature of 45°C. Above the switching temperature the hemispheric solar transmittance decreased to a value of 79%, with a diffuse fraction of 64%. In general the thermotropic resin was characterized by a steep and rapid switching process. The comparison of the films switching performance with the additives thermal transition determined by Differential Scanning Calorimetry revealed a good correlation. The significant increase of the diffuse transmittance along with the moderate change in hemispheric transmittance was in good agreement with average scattering particle dimensions of 0.4 to 2.5 pm ascertained by AFM phase imaging and mapping of the chemical constitution of the surface by Raman microscopy.

Keywords: thermotropic resin, UV/Vis/NIR spectroscopy, Atomic Force Microscopy, Raman microscopy

1. Introduction

Thermotropic materials that change their light transmission behaviour from highly transparent to light diffusing upon reaching a certain threshold temperature reversibly can provide overheating protection for solar thermal collectors [1]. Especially thermotropic systems with fixed domains that consist of thermotropic additives dispersed in the matrix of a curable resin possess a high potential for solar thermal applications [2,3,4]. To prevent overheating of an all-polymeric flat plate collector with twin — wall sheet glazing and black absorber thermotropic layers with switching temperatures between 55 and 60°C (thermotropic glazing) or 75 and 80°C (thermotropic absorber) as well as a solar transmittance of 85% in clear state and between 25 to 60% in opaque state are required. The overall objective of this research work is to perform a comprehensive characterization of a thermotropic system with fixed domains and to establish structure-property relationships. Optical properties and the switching characteristics are characterized by UV/Vis/NIR spectrophotometry. The films switching temperature is related to the thermal transition of the additive determined by Differential Scanning

Calorimetry. Furthermore the switching performance is compared to scattering domain size determined by Atomic Force Microscopy (AFM) in phase imaging mode and Raman microscopy.

2. Experimental

The relation between grain size, grain shape and grain distribution and the transmission pattern of solar spectrum of the films can be established by comparing the spectral profiles with the photomicrographs of the films [2, 11-16]. Snl2 and MnBr2 Films: Visual observations of plates 1 and 2 show that Snl2 (plate 1) films are a bit larger but more widely spaces while MnBr2 (plate 2) films are a bit smaller, more compacted and almost continuous throughout.

Wavelength (nm) Fig 1. Spectral Transmittance of VTF (Sn^ and MnBr, )

Fig 3. Spectral Transmittance of IRTSSF (Ag 15 and PbS )

A comparism between the surface microstructure and spectral transmittance of the films show that these films have high UV absorption and NIR reflection with high visible transmittance. The MnBr2 films, which are more closely packed, show higher UV absorption, IR reflection and improved visible transmission than the Snl2 films. This behaviour could also be due to the fact that films that are more oblong and form a closer network tend to absorb UV and reflect IR. On the other hand, the Snl2 films, which are more roundish and seem more, separated from each other, give room for improved transmittance of UV and NIR radiations [2]. In general, Snl2 and MnBr2 Films make transmission from highly absorbing to highly transmitting in the spectral range 320 to 440mm and from highly transmitting to highly reflecting in the range of 700 to 820mm. The spectral pattern therefore shows a gradual rise from zero to maximum absorption/minimum transmission in the ultraviolet and a gradual fall from minimum absorption/rise to maximum transmission in the visible region. This is followed by a gradual fall in transmission/rise in reflectance in the near infrared. Figure 1 [2]

PbBr2 and Pbl2 Films: PbBr2 (plate 3) have more roundish black grains, which are wider apart than Pbl2 (plate 4). Signs of agglomerating particles are shown by the somewhat more visible films of Pbl2 indicating closer and more extended films dotted in between the tiny grains. The generally wide and more uniform spacing of the grains indicates higher reflection of near infrared radiation. Figure 2.

Ag2S and PbS Films: Plates 5 and 6 show that the films have wider shiny (almost whitish) films with smaller tiny darker grains embedded in the shiny background. The grains are not roundish at all but have extensive oblong shapes, which form almost complete continuous network. This collaborated the spectral transmittance pattern (fig. 3), which show high UV and Visible absorption with an improved NIR transmission due to the large grains.

The comparisms between surface structure and spectral properties show that:-

Small oblong extended network-like grains allow high UV absorption, NIR reflection and substantial visible transmission through the tinny spaces in between the grains. This is the case with Snl2 and MnBr2 films.

Medium, darker, more roundish and widely spread gains promote high UV — VIS transmission and high NIR reflection at a wavelength of about 520 to 550mm as in PbBr2 and Pbl2 films.

Larger shinny grains that form almost a network of film favour high UV — VIS absorption and high NIR transmission at a cut of about 850mm as shown for Ag2S and PbS films.

Fig.13 shows monthly heating load and solar energy used to floor heating. Annual heating loads for Case A, B and C were 0.56MWh/year, 1.74MWh/year and 0.77MWh/year, respectively. Annual heating loads for Case B and C increased by 3.1 times and 1.37 times compared with Case A, respectively. Annual solar energy used to floor heating in Case C was 2.96MWh/year. In the dense housing area, heating by active solar space heating system is effective.

Fig.14 shows monthly cooling load. Annual cooling loads for the adjacent houses Cases B and C were about 53% compared with the no adjacent houses Case A. This result is mainly the influence by the difference of incident solar radiations of the east and west surfaces, as Fig.6 showed.

Jan Feb Mar Apr May Jun Jly Aug Sep Oct Nov Dec Month

Fig 13. Monthly heating load and Solar energy
used to floor heating [kWh/month]

Mersin in which there exists a considerable amount of visual pollution caused by solar hot water systems is situated at the Mid Mediterranean region, geographically in Turkey, within the 32 — 35 degrees east longitudes and 37-36 degrees north latitudes.[5] In Mersin as a sea shore city there exist typical Mediterranean climate with typical specialties like hot and humid summer days and rainy and warm winter days. In yearly basis having only 25.3 un-sunny days and being an entirely open and lightly cloudily sky in most of the days of the year and so with a considerable sunny days in this city, solar hot water systems are widely used at the residents for heating the usage water and is amongst the very first lines by system implementation in Turkey. (Figures 4, 5)

The suitable geographical location and climate of Mersin provides flexible possibilities for collector inclination and positioning. Within the context of research project named “System Approaches in Evaluation of Solar Energy Hot Water Systems as an Architectural Component” issued by YTU as it is seen on the data obtained by using the TSOL program (Table 1), the collectors can be used with an inclination range of 0-60° and within the directions of ± 60 ° to the South. [6] In respect to the efficiency — aesthetical optimization, it is clear that wide directive and inclination angle interval will provide flexibilities for system integration to the building.

10th October, Lisbon — Portugal *

The cooling problem of the inner spaces in Mersin and in almost all southern regions of Turkey arises as a more important requirement rather than heating needs. When the energy consumption for cooling in summer and expenditures done taken into consideration, not having adequate solar control facilities on the buildings appear as an important absence in the context of bioclimatic design. Therefore, the usage of solar hot water systems, as solar control component like pergola, on the roofs will provide crucial possibilities in keeping the building envelope cool and for integrating systems with the building. Within the context of the approaches and data indicated above, the possible architectural solution suggestions in respect to solar hot water systems situated at flat roofs in Mersin can be like,

Individual usage — natural circulation systems,

gathering collectors, harmonizing collector areas by considering the form, size, color, pattern properties and /or obstruct the visibility; gathering storages in a collective place that is designed by considering a harmony with the building and its invisibility (Figures 6 a, b)

Figure 6 a)

Individual usage and pumped systems,

Organizing them so as to form a pergola; formation of a suitable area where the tanks are gathered or obstruct them under the collectors, (Figure 7, a, b, c, d)

Figure 7 a)

d)

Central system when the storage is not force to be near the collectors, it is apparent that architectural opportunities would be much more like pergola forms creating usage spaces on the roofs and protection of the roof from sun. (Figure 8 a, b)

It is clear that with innovative design approaches, according to the installation purpose of the systems, the architectural expression, demands of the users, environmental impacts, design approaches, and economy many system solutions specific to that building can be created. According to the fundamental and schematic solutions that are suggested, the visual improvements provided at a multistory residential building in Mersin are shown in Figure 9.

*

2. Conclusion

The collector pollution caused by solar energy hot water systems is a crucial urban problem that needs to be solved urgently in many countries as well as in Turkey. The solution to this problem is possible by regarding the systems not only with their efficiencies but as architectural components having an important role in perception of the buildings and the city.

For a more sustainable future, solar energy in buildings and solar hot water systems has an important role. The usage of these systems mostly has great affects on buildings and their architecture. The goal of making the system usage widespread will determine the tomorrow’s cities way of being. So it is important to use the systems aesthetically and reorganize the improper existing systems for having harmony and integration with the building for our life quality.

For a healthy increase of system applications it is important to understand the dynamics of usage, consumer needs requirements, trends and local conditions. Further more special solutions according to the properties of the place have to be produced. To achieve these goals, it is noteworthy to approach the system usage in a holistic and versatile way of looking with cooperation of different disciplines for the same aim, with clear strategies.

References

[1] Sakinc E. (2006) An approach to evaluate solar active systems as an architectural design component in the context of sustainability, Dr. thesis, YTU, Istanbul.

[2] Sakinc E. & Serefhanoglu Sozen M., Evaluation of solar hot water systems as a design component in the context of sustainability, MIMARIST, 23 (2007). 103-112.

[3] Sakinc E. & Serefhanoglu Sozen M.,. A design approach to evaluate solar active systems as a criterion, GUMMF, 23: (2008) 21-31.

[4] Sakinc E. & Serefhaoglu Sozen, The importance of Building integrated solar hot water systems in the context of architecture and urban design, 8. International HVAC+R technology symposium, (2008)p: 315-324

[5] Mersin Governorship, Environment and Forest director of Province, Report of Mersin province environment, 2005, Mersin

[6] Serefhanoglu Sozen M. & Sakinc E., and others, 2008. Systems approaches on evaluating solar hot water systems as architectural component, ongoing research project of YTU, 26-03-01-03 Istanbul.