Category Archives: EuroSun2008-4

Window products

There are a large number of coated and uncoated glass panes available on the market, and all of these can be combined into numerous different window combinations. To help customers select suitable windows among this almost infinite number of glazing combinations, the International Glazing Data Base (IGDB) has been set up by the Lawrence Berkeley National Laboratory in California [12]. The database contains glazing products manufactured by most major glass manufacturers in the world, and their main optical properties are provided, usually together with reflectance and transmittance spectra. It is a complex task to choose the most suitable window for a certain building and location out of all these products. The important parameters for the function of the window vary within wide limits, and if we for example consider the U-value and the g-value, we can see in Fig. 1, that windows with many different values of these parameters are available. Each point in the diagram represents a window made up from two panes found in the IGDB. The figure only includes a small selection of double glazed configurations air filled insulated glazing units. Triple glazed configurations with argon-fill would extend the graph down to U-values of around 0.6 W/m2K. Depending on the function of the window and the type of building, orientation and climate, different combinations of high or low U — and g-values would be the best choice. For optimum performance in a certain situation we may want to look for a window close to one of the corners in the graph. Often a compromise has to be selected since summer and winter may require quite different properties for optimum performance.





image15310 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9


Fig. 1. U-value vs. g-value for over two thousand Fig. 2. Visible transmittance vs. g-value for twelve double pane windows. double pane windows.

These considerations lead to the conclusion that we often would want windows with variable optical and thermal properties depending on the time of year, time of day or weather. The electrochromic coatings mentioned in the introduction bring us a few steps towards this situation. Existing products on the market or prototypes made up in the laboratory show that we can identify window products with variable optical properties. A few examples are shown in Fig. 2 where the product specifications have been plotted in a graph with tv versus g-value. Table 1 gives a brief description of the double glazed windows presented in this graph. This type of graph is mainly useful for solar control glazing, for which both a low g-value is desired to prevent over-heating and a low Tv-value is desired to prevent glare. The problem is that we also want high tv for the day­lighting and visual contact with the surroundings. Thus, the switchable electrochromic glazing could be the ideal solution. In the graph we can see that the shown optical properties can be varied within quite wide limits, which makes it possible to optimize the performance for different weather conditions and according to the needs of the occupants. The non-physical area in the graph is due to the fact that around 50% of the solar radiation is visible light.

Table 1. Description of pane configuration used for double pane windows in Fig. 2

Hard Low-E

SnO2 low-e coating on surface three combined with an outer floatglass pane.

Soft Low-E

Single silver coating on surface three combined with an outer float glass pane

Soft SC

Double silver coating on surface two combined with an inner float glass pane.

Abs. SC

Absorbing outer pane combined with an inner float glass pane

SS clear

Solid state electrochromic film in its clear state on surface two combined with an inner float glass pane.

SS dark

Solid state electrochromic film in its dark state on surface two combined with an inner float glass pane.

Foil/float C

Electrochromic plastic foil in its clear state on the inside of an outer float glass pane combined with an inner float glass pane.

Foil/float D

Electrochromic plastic foil in its dark state on the inside of an outer float glass pane combined with an inner float glass pane.

Foil/Low-E C

Electrochromic plastic foil in its clear state on the inside of an outer float glass pane combined with an inner low-e pane.

Foil/Low-E D

Electrochromic plastic foil in its dark state on the inside of an outer float glass pane combined with an inner low-e pane.

MH trans.

Metalhydride electrochromic film in its clear state on surface two combined with an inner float glass pane.

MH refl.

Metalhydride electrochromic film in its reflective state on surface two combined with an inner float glass pane

3. Method

3.2. Energy simulations in WinSel

The energy simulations were performed using WinSel, a simulation tool developed at Uppsala University [7,8]. The program was designed as a window selection and energy rating tool, hence the acronym WinSel. Hourly climate data of direct and diffuse radiation and temperature are used for hour-by-hour calculations of the annual energy balance. Window in-data are the total solar energy transmittance (g-value), the thermal transmittance (U-value), and parameters controlling the

angular dependence of the g-value. The building input data are limited to the thermal mass dependant time-constant and the balance temperature. The balance temperature is defined as the outdoor temperature when neither cooling nor heating is required to maintain the set indoor temperature. Consequently, the heating season is defined as all hours of the year when the outside temperature is lower than the balance temperature. The cooling season is defined with a temperature swing allowing the indoor temperature to increase to a value above the set indoor temperature before any cooling is considered.

Exergetic analysis of the EACS

The notion of exergy, also called availability, was introduced at the end 19th century by Georges Gouy. This concept merge both the first and second law of thermodynamics and then allow to take into account the amount of energy transfer during a process but also the quality of this energy transfer. The exergy could be physically defined as the maximum mechanical work that a system could provide during a transformation between a thermodynamic state and a reference state. This means if a system is in thermodynamic non-equilibrium compared to the ambiance, the maximum work it could provide to reach back ambient conditions is the exergy; the difference between the achieved work and the exergy between the two states being the losses or irreversibilities. In this case, the exergy is positive; a negative exergy meaning that work should be provided to the system to reach back the reference state. Exergy is a state function and is defined as

e = (h — h°)-T0 (s — s0) (9

de = dh — T0ds

the indexes "0" referring to the reference state. For driving cycles, the reference state is conventionally the standard conditions, T0 = 15°C, P0 = 1bar. In addition, in the case of vapor evolving in a closed system, the only equilibrium condition compared to the ambient is the temperature condition. Indeed at equilibrium the system will reach the same temperature than the

ambient. However the equilibrium pressure will be the saturation pressure. For such system the reference state is T0 = 15°C, P0 = Psat (T0 ) . For driving cycle this definition is convenient since

this reference temperature is closed to the cold source temperature of the cycle, providing a very small or almost zero exergy loss at the condenser. Indeed, even though a large amount of energy is lost to the ambient, the quality of this energy is quite poor.

In our trithermal air-conditioning cycle, the hot source is the hot water previously heated up by solar collectors. The useful effect occurs at the cold source e. g. the water to cool down (for room air-conditioning) while the sum of those two previous heat quantity is discharged to the ambient through the condenser which is the medium temperature source of the cycle. Similarly to what happens at the condenser for a driving cycle, this significant amount of energy has a poor quality in regards to the targeted effect (cool down compared to ambient). Consequently, an almost zero exergy loss should be requested at this component. In addition the useful effect at evaporator occurs close to 15°C (tow — tiw «12 -18) providing an almost zero exergy variation for cooled

water and almost 100% of losses at the evaporator. All these conditions show that the usual reference state for driving cycles is not suited to air-conditioning cycles. Consequently in this study the reference temperature will be the ambient temperature Tciw at the condenser side. For the refrigeration cycle, the reference pressure will be the saturation pressure at T0 for propane and for secondary cycles (generator, condenser, evaporator) it is the atmospheric pressure (P0 = 1bar).

From an energy point of view the quality of a compression air-conditioning cycle is determined by the global COP as

COP = Qe (10)

comp, global p

1 comp, a

For an ejector based air-conditioning cycle this global COP would then be:

COP, c„b,, = P &+ Q (II)

pump, a sol

With subscript “a” for “absorbed” and Qsol the useful thermal power received by collectors.

However it could be convenient to define cycle based COP in order to eliminate mechanical efficiencies of components which may change significantly according the technology, power, etc…, and also to eliminate the source of heat supply. Indeed, in the proposed cycle solar collector are used to heat up the water but other low grade energy sources could be used to provide this energy; and in this case the way of calculating the energy transfer between the water and a source would be different or it would use very different efficiencies. Consequently to be more general, the concept of cycle based COP is used in this analysis:

COPcomp = PQ^ (12)





e P




-Qw. g + Qw


These purely energy criteria are not satisfactory to make a relevant comparison between both systems. Indeed for the compression system the energy input is purely mechanical and then this energy is actually a pure exergy, while the ejector system uses primarily thermal energy which does not have the same exergetic value than a mechanical energy. Therefore, in order to compare

Подпись: ew,g = (Qw,g + Qw,sup) I 1 Подпись: T 0 T Подпись: (14)

both systems on the same objective basis, these COP should be turned into exergetic COP. For the compression system it is obvious because the compression power is purely exergy while for the ejector system Qw should be replaced by its exergy equivalent. An easy way of calculating this exergetic value would be to consider the secondary water as a perfect source delivering a thermal power Qwg at a certain temperature T and using the Carnot factor:

This method may also be used for condenser or evaporator and was used in literature [3]. However this provides not accurate results since the water temperature varies and then it is not a perfect source. In this way the real exergy flux through the generator is used since all secondary cycles (generator, evaporator, condenser) are fully computed in this work:

ew, g = mgw (ew,0 — ew,, )g +amgw (ew,0 — ew,, LP (15)

In addition as proposed in literature an exergetic value of Qe should be used in exergetic COP.

image221 Подпись: (16) (17)

This definition is not used in the current analysis because the useful effect is not mechanical such as in driving cycles/machines but thermal. Such definitions are not relevant since the exergetic value of Qe is very low giving even lower COP and not relevant to compare with classical COP. Consequently it is proposed to compare COP for a same thermal effect at evaporator:

Results are presented in the table 2. It is seen that in an energetic point of view, the ratio between the COP obtained for a conventional compression system and the EACS is very high (18.42 to 29.9). But the ratio of exergetic COP is noticeably lower, with values of around 3.4 to 5.7. It is thus important to highlight that both kind of systems are more comparable in an exergetic point of view, and this definition points out the valorization of the low grade energy. For Tciw=28.4°C, COPej and

COPe undergo the same increase of 6.6 % when the subcooler is used. For Tciw=29°C, COPej

and COPe decrease both of 29%. Note that a comparison between two cases at different Tciw is not

possible in an exergetic point of view, because the chosen reference T0 used for the exergetic analysis is equal to Tciw and thus varies with the considered case.

Table 2: COP and COPex comparison of the EACS and vapor compression cycle, for a=0.13, with and without the use of the subcooler. * denotes a working with regenerator using (P>0)

T (°C)










comp ej


comp ej

with subcooler















without subcooler















3. Conclusion

A detailed rating modeling of the EACS was presented. It was seen that the subcooler can have either beneficial or harmful effect on performances. The advantage of using of a regulator is clearly demonstrated. Moreover, reducing the system performances to an optimization of the ejector entrainment ratio is not judicious. The whole cycle must be taken into account. Finally, a first step of the exergy analysis of the EACS emphasized the more relevant comparison with a conventional vapor compression system. A more detailed exergetic study is in progress to determine the distribution of irreversibilities in the cycle and its variation with different parameters.


[1] W. Pridasawas and P. Lundqvist, A year-round dynamic simulation of a solar-driven ejector refrigeration system with iso-butane as a refrigerant, International Journal of Refrigeration, Vol. 30, 2007, 840-850.

[2] G. K. Alexis, E. K. Karayiannis, A solar ejector cooling system using refrigerant r134a in the Athens area, Renewable Energy, Vol. 30, 2005,1457-1469.

[3] W. Pridasawas and P. Lundqvist, An exergy analysis of a solar-driven ejector refrigeration system, Solar Energy, Vol. 76, 2003, 369-379.

[4] A. Hemidi, Y. Bartosiewicz, J. M. Seynhaeve, Ejector air-conditioning system: cycle modeling, and two — phase aspects, Heat 2008 conference, Vol. 2, 421-428.

[5] A. Hemidi, Y. Bartosiewicz, J. M. Seynhaeve, Modeling of an ejector air-conditioning system: sizing and rating tools, IIR conference, 2008.

Review of the Results

The main problem with regard to the performance of the absorption chiller is that it does not reach the projected COP. The following reasons can be found:

• The required temperature for driving the chiller is not provided constantly (see Error! Reference source not found.).

• The re-cooling temperature is mostly at about 28° C and not at 27° C as planed (see Error! Reference source not found.). According to the manufacturer’s characteristics for the machine this causes a performance loss of 10 to 20 %.

• In simultaneous operation of the compression chiller the required flow rate in the cold-water circuit is not achieved.

• The cold-water temperature is lower than planned.

Подпись: Stunden im Jahr Fig. 6. Integrated frequency curves of the temperatures of the driving and re-cooling circuit.

The integrated frequency curves for the temperatures in the driving and re-cooling circuits are shown in figure 6. The minimum required temperature of 75° C in the supply line of the driving circuit is reached for about 3,600 hours of almost 5,000 hours of total operation. That’s nearly 75 % of the time. Only for 2,000 hours a temperature of over 80° C is achieved, less than half of the operation time. The temperature for the supply line of the re-cooling circuit is always higher than 27° C, the temperature is quasi-constant at 28° C.

The parasitic energy consumption of the solvent pump and the refrigerant pump is independent from the refrigerating capacity. Therefore, the total COP declines and the primary energy coefficient and the carbon dioxide emission of the chiller rise disproportional.

The demand of electric energy of the installed absorption chiller is very high compared to other products. Comparable products from other manufactures only need a tenth of the electric connected load. Solely the reduction of the electric energy consumption of the absorption chiller itself to this level would reduce the primary energy coefficient to the level of the compression chiller. The emission of carbon dioxide would fall far below the level of the compression chiller.

4. Optimisation

To improve the conditions for the operation of the absorption chiller several changes where developed and partly realized. For a more constant heat supply additional storage capacity was installed. This storage is reserved for the wood pellet combustion unit. So the existing storage capacity can be used only for the solar collectors. At the same time three bigger units replaced the four wood pellet boilers. The total output of the boilers rises from 128 to 168 kW.

The cold-water outlet temperature will be adjusted according to the demand. Because of the strong power loss of the absorption chiller when the cold-water outlet temperature falls below 9° C, the temperature will be raised to 10° C in times where no de-humidification is needed. This temperature is sufficient for sensible cooling of the air. The cold-water outlet temperature is lowered to 6 to 8° C only in times with high external humidity loads.

To supply the required re-cooling temperature minor improvements to the hydraulics where implemented in 2006 already. However, these improvements did not show any effects in 2007. Further investigations where undertaken to find the reasons. The result was that the control strategy for the re-cooling pumps had to be optimized.

Furthermore, there is the consideration to run the absorption chiller solely by solar heat. The very high consumption of electric energy of the absorption chiller could be only reduced by a complete replacement with another machine.

Estimation of the available roof area

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

Optical performance of selective thin film

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.

Thermal characteristics in summer

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.

Application of renewables

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.

Anidolic Daylighting Systems (ADS)

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].

Glazing integration

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.

Overheating protection with thermotropic resin systems: Effect of. material structure and morphology on light-shielding efficiency

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


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