Category Archives: BACKGROUND

Influence of the Radiative exchange in the Heat Transfer. of a Cubic Cavity with Semitransparent Wall using. Room temperature Conditions

J. J. Flores’ and G. Alvarez +

CENIDET, Mechanical Engineering Department. Prolong. Av. Palmira s/n. Col. Palmira.
Cuernavaca, 62490, Morelos, Mexico. Tel./Fax: +52 777 312-7613. Emails:
jasson@cenidet. edu. mx, gabv@cenidet. edu. mx.

+CIEMAT. Renewable Energy Department. Av. Complutense No. 22. Madrid, Spain.

28040. Tel/Fax: 91 346 6344.

Abstract

This paper presents the influence of the radiative exchange of a three dimensional cavity with a semitransparent wall with solar control coating, considering that the temperature distribution of the test glass is function of the thermal interaction between the interior and the exterior of the cavity. The theoretical heat transfer model was compared with experimental measurements. The amount of the radiative and convective terms are quantified and their results are that the radiative term influences the heat transfer as much as the convective term in 12.4%. Also the present results of theoretical heat transfer model were compared with theoretical and experimental results reported in the literature. These results show that the maximum difference was of 5.58% be total Nusselt number and for the individual radiative and convective Nusselt numbers the difference increases.

1. Introduction

The evaluation of solar heat gains or losses through windows implies to calculate the amount of solar energy that is able to cross through the glass towards the interior of a room with respect to the one that strikes on the outside of the glass. From the solar energy that goes into the glass, some of that energy is transmitted, another is reflected and the rest is absorbed. The absorbed solar energy in the glass is transported by conduction towards the interior of the room and is transferred by convection and radiation from both sides of the glass to the interior and exterior air. Therefore, to quantify the amount of solar energy transported to the interior, it is necessary to evaluate the direct solar energy transmitted and the fraction of energy that is transferred in the form of thermal heat by convection and radiation.

To make the evaluation of the fraction of solar energy that is transferred by the glass towards the air in the room, some thermal parameters have been defined, such as: the Shading Coefficient (SC) [AsHRAE, 1997], the Solar Heat Gain Coefficient (SHGC) [ASHRAE, 2001] and the Solar Rejection Factor (SRF) [Alvarez, 1994]. To calculate these parameters, it is necessary to evaluate the overall heat loss coefficient of the glazing that depends on the convective and radiative heat transfer coefficients. The evaluation of these heat transfer coefficients can be carried out by idealized experimental or theoretical models. Hollans et al. in 1976, ElSherbiny et al. in 1982, Pepper and Hollands in 2002, ISO-9050 in 2001, ASHRAE in 2001 among other authors determined several theoretical
correlations for those heat transfer coefficients. Janssen and Henkes in 1995, Leong et al. in 1998 and Leong et al. in 1999 among other authors determined several experimental correlations. Most reported theoretical studies of the transport of heat in rooms consider modeling a room as a closed cavity heated differentially on the vertical walls, considering just heat transfer by natural convection. Very few studies in cavities consider the three modes of heat transfer: convection, conduction and radiation; and even lesser studies consider semi-transparent walls or windows. Among the studies that consider semitransparent walls, most of them are in two dimensions and only Alvarez in 1994 and 2000 consider a solar control coating on the glass.

In this paper we present the influence of the radiative heat transfer of a three dimensional cavity with a semitransparent wall with solar control coating, considering that the temperature distribution of the test glass is function of the thermal interaction between the interior and the exterior of the cavity.

Optical properties

Fig. 3: Samples of photoelectrochromic devices with solid electrolyte in the bleached state (left, short circuit) and coloured by illumination equivalent to one sun (right, open circuit).

Fig.3 shows a sample with solid electrolyte in the bleached state (left, short cicuit) and the coloured state (right, after illumination in a solar simulator). The corresponding transmittance spectra are shown in fig. 4. The transmittance in the coloured state depends strongly on the thickness of the WO3 layer. It should be noted that the thickness of the Pt

layer and the amount of the dye are small enough to allow a transmittance of 62% in the bleached state for the photopic response spectrum, and 41% for the solar spectrum. The main losses of transmittance are due to the TCO layers (especially in the infra-red range) and the redox electrolyte, which can be made thinner.

Transparent nanoporous TiO2 and WO3 layers were prepared using sol-gel techniques described in [9]. Ormosilane was used as a binder in WO3 and TiO2 sols. TCO-coated
(F:SnO2) glass plates from Pilkington were covered by dip-coating first with WO3, then with TiO2. The thickness of the TiO2 layers was about 150 nm, and of the WO3 layers about 500 nm. The diameter of the particles in the WO3 layers is around 20 to 30 nm, in the TiO2 10 nm, as displayed by SEM (Scanning Electron Microscopy) measurements (fig. 5a). The thin Pt layers were sputtered. The dye (Ru 535 bis-TBA from Solaronix) was deposited by soaking the TCO/WO3/TiO2 layers in a solution of the dye in ethanol.

We investigated the WO3-TiO2 layers with high-resolution transmittance electron spectroscopy (HRTEM), IR spectroscopy, Auger electron spectroscopy and energy dispersive X-ray spectroscopy (EDXS) [10]. The result was that the WO3 particles consist of a crystalline monoclinic core (m-WO3), which is surrounded by an amorphous phase (a — WO3, fig. 5b). Because of the preparation process, TiO2 and SiO2 are left inside the WO3 layer, mainly situated in the amorphous phase. The content of TiO2 increases inside the amorphous phase from the inner to the outer parts of the WO3 grain.

Description of the adsorption cooling machine

The adsorption cooling machine under solar radiation is represented in figure 1. It consists of the following elements:

— A solar collector containing the adsorption reactor, it is heated under the action of solar radiation, and desorbs a gas amount which is a function of the maximum temperature.

— A condenser inside which the desorbed cooler condenses.

— A tank where the liquid cooler is stored.

— An evaporator, in contact with the cold source, in which takes place the evaporation of the liquid fluid at the pressure of evaporation Pev to produce cold.

— Automatic valves working under the basis of the pressure.

All these elements are connected by tubes, necessary to move the cooling fluid along a closed circuit made of, the adsorber, the condenser, the evaporator and the adsorber.

At the beginning, the porous medium temperature is uniform at ambient one and at evaporating pressure. The reactor is heated by solar radiation, which causes an increases in temperature and pressure. Thus an in-homogeneity in temperature derive a temperature gradient, while the pressure remains uniform in the reactive beds. Then a first phase is characterised by an elevation of pressure until the condensing value following an isoster. When the pressure reaches the condensing value corresponding to saturated pressure of ammonia at ambient temperature, the valve Vc linking the adsorber and a condenser is opened, permitting the circulation of ammonia vapour to the condenser through a tube. At this moment, the condensing phase start at constant pressure. The heating reactor and desorption of ammonia continues, until the temperature at the centre of the reactor reaches a maximum value. Then valve Vc closes automatically and the cooling of the reactor starts following an isoster. The valve Ve is closed and a new phase begins, characterized by a fall of the pressure to the evaporating pressure. Then the valve Ve is opened which permit to entering the fluid in the adsorber, induced as a result of the adsorption of ammonia on the activated carbon.

Towards corrosion testing of unglazed solar absorber surfaces in simulated acid rain

Petri Konttinena, Teppo Salob, Peter Lund3 aAdvanced Energy Systems Laboratory, P. O. Box 2200, Helsinki University of Technology, FIN-02015 HUT, Finland

bLaboratory of Corrosion and Material Chemistry, P. O. Box 6200, Helsinki University of Technology, FIN-02015 HUT, Finland

Introduction

Evaluating and testing of acid rain induced corrosion on non-glazed solar absorber surfaces is to a large extent an unexplored problem. This paper communicates the first methods used for acid rain tests and results for a rough graphite-alumina-aluminium (C/Al2O3/Al) absorber surface. The surface consists of a carbon and alumina containing heterogeneous matrix structure on an aluminium substrate sheet (Konttinen et al., 2003a, Konttinen et al., 2003b). A relatively low-cost total immersion acid rainwater test method was developed with rate and type of aeration (non, O2 or N2), temperature (60/80/99°C) and pH levels (3.5/4.5/5.5) as parameters. The test method was mainly based on recommendations in (Shreir et al, 1994). Absorber samples were analysed by UV-Vis.-NIR and FTIR spectroscopy. Electrochemical properties were investigated by electrochemical impedance spectroscopy (EIS) and polarisation curves. pH levels chosen for the tests were based on the global acid rain measurements (Howells, 1990). Metal corrosion studies in simulated acid rain (Magaino, 1997, Magaino, 1999) and corrosion studies of carbon fiber (graphite) — aluminium metal-matrix composites (Hihara, 1997, Hihara and Latanision, 1994) were used as first guidelines for designing the tests and analysing the corrosion behaviour of C/Al2O3/Al solar surfaces in simulated acid rain. In standard condensation tests for glazed absorbers according to a draft proposal ISO/CD 12592,2 (Brunold et al, 2000) the main degradation mechanism of the C/Al2O3/Al solar surfaces has been found to be hydration of aluminium oxide (Konttinen and Lund, 2003).

Summary and conclusions

In the paper the application demonstration and performance of a novel polymer film based TI structure was described. The TI facade system based on optimized cellulose triacetate polymer film TI structures yielded excellent performance properties. Although cellulose triacetate absorbs a high amount of water no adverse condensation phenomena within the TI facade were observeable. These findings are mainly related to the diffusion-open
construction of the TI facade based on timber profiles with minimized material fraction. During the investigated measuring period a successful operation and no adverse pollution of the TI facade were obtained.

Due to the promising results of the development and application demonstration project further activities are planned. In cooperation with the Arbeitsgemeinschaft ERNEUERBARE ENERGIE we intend to disseminate the developed TI structure by a do — it-yourself assembly-system to further 3 to 5 demonstration objects focussing also on building renovation. Furthermore, concepts for an industrial production of the novel TI structures will be elaborated together with partners from the polymer processing industry.

Acknowledgements

The authors wish to express their acknowledgements to the Fraunhofer Institute for Solar Energy Systems (Freiburg, Germany) for the cooperation in the course of this study, especially for making the modelling software and some of the laboratory equipment available for investigations.

Parts of the research work of this paper were performed within project S.14 at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within framework of the Kplus-Program of the Austrian Ministry of Traffic, Innovation and Technology with the contributions by the University of Leoben, Graz University of Technology, Johannes Kepler University Linz, Joanneum Research ForschungsgesmbH and Upper Austrian Research GmbH. The PCCL is funded by the Austrian Governments of Styria and Upper Austria.

Ellipsometry of individual layers

The ellipsometric data consist of the / and A spectra in the range 300 to 850 nm for different incident angles between 40° and 70°. Consequently, we have made a systematic study of the optical properties of individual dielectric layer for Ti02, Si02 and Al203. The model consists of a single uniform film on silicon substrate. These data were fitted with a widely used Cauchy dispersion formula for Ti02, Si02 and Al203i where the refractive index n and extinction coefficients kare given by:

n(A)= n0 + + C^

(1)

k(A) = k0 + + C1k4.

(2)

Пі, ki, Ci are constants and я is the wavelength in nm. We take C0 =102 and C1 =107, which are mostly used, to avoid large values of ni, ki, n2 and k2.

The films were modeled as homogeneous dielectric layers on a semi-infinite silicon substrate. A native silicon dioxide interlayer was included in the model. Surface roughness was neglected.

The experimental / and A for deposited Ti02, Si02 and Al203 on silicon substrate combined with the best theoretical fits using the Cauchy dispersion model, permits to determine the optical properties of individual films. A good agreement between the fit and the experimental data is observed between 250 and 850 nm for Si02 and Al203, and above 350 nm in the case of Ti02 thin film, which confirms our results previously obtained for the same deposition conditions [11].

Figure 1 shows the refractive index n as a function of the wavelength in the UV-Vis for Ti02, Si02 and Al203 on silicon substrate. The results of the fit parameters confirm that no absorption occurs in the films. It should be noted here that the thicknesses of dielectric layers are in a good agreement and are within 5% of the determined one by the laser reflectometry.

Measurement of angle-dependent properties of different solar protection devices

Thomas Knauer, Tilmann Kuhn, Werner J. Platzer

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110 Freiburg,

Fax: +49-(0)761/4588-9000, werner. platzer@ise. fraunhofer. de

Introduction

In the sixties of the last century solar calorimetric outdoor experiments have been performed on the combinations of solar shading devices and single and double glazed window units. From these experiments typical reduction factors for the effective total solar energy transmittance have been derived and published. In standards such as EN 832 or DIN 4108 part 6 tables with typical reduction factors of the main shading device types are given. However, for a more exact derivation of the total solar energy transmittance of a fagade including solar shading device, other means are needed.

Within IEA Task 27 we try to develop and check methodologies to characterize solar shading devices experimentally, and compare with different theoretical calculations. The experimental characterisation is based on a new generation of solar calorimetric testing devices developed within completed European and national projects. In this paper We want to compare experimental results using these devices with calculated results using some new and unvalidated standards for different combinations of solar protection devices and glazings.

State of the art

Solar calorimetric measurements are suitable for determining the total solar energy transmittance of a facade consisting of glazing plus solar protection devices. The methodology and measurement procedures have been developed in the European project ALTSET (Angular Light and Total Solar Energy Transmittance) [ 4] as well as the German project rEgES [ 6]. However, only few results have been published, for example in a comparison of the experimental results with a model based on raytracing and an adapted resistance network [ 5].

The European standardisation tried to improve methodologies to calculate the shading reduction factors in an approximate way using relatively simple algorithms, and finally issued a very rough calculation method in the standard EN 13363 part 1 [ 1] and a more detailed reference method has passed the formal vote recently (part 2 [ 2]). However, there are several restrictions and simplifications in both documents which do not allow to characterize solar shading devices using lamellae in a sufficient way.

Also the parallel activity of ISO, trying to develop a consistent standard for solar and thermal performance calculations of glazings, windows and shading devices does not solve this problem completely [ 3].

Similarly the European computer tool for glazings and shading devices WIS (Window Information System) has been developed further in a European network called WINDAT (www. windat. org). The modelling of solar shading in this tools is implementing the international standard ISO/FDIS 15099. All these algorithms based on standards are rather restricted in the scope as they can be applied only to special situations and describe only idealised systems (flat slats). Moreover, the modelling has not been validated by measurements. This gap should be closed with this paper.

Physical Model

Figure 1. Schematic diagram of the cavity.

As solar radiation strikes the glazing, a percentage of the solar radiation is reflected, some percentage is absorbed and the rest is transmitted to the interior of the cavity. The solar radiation reflected to the outside by the glass and the solar control coating is considered that it does not contribute in the processes of heat transfer inside the cavity. The solar radiation absorbed by the glass implies heating the glass and the solar control coating. The solar radiation that is transmitted by the glass and solar control coating goes to the wall 2 and it is absorbed, without causing heating of this wall, since it is considered at constant temperature.

As consequence of the temperature difference between the external surface of the glass and the air of the environment, the heat is transferred by convection (qc) and radiation (qr) to the ambient air. In a similar way, the interior solar control coating surface transfers heat from the glazing to the interior air and to the interior surfaces of the cavity. Inside the glass, the heat is transferred by conduction.

Kinetic Properties

The photoelectrochromic device, as it is described in this paper, allows various switching modes (fig.6). The device colours on illumination with open circuit and it bleaches in the dark with short circuit within about 10 minutes with an solid ion conductor. It is possible to adjust the electrolyte such that the device bleaches with short circuit under illumination or that it retains its colour. Slow bleaching occurs with open circuit conditions in the dark (about 10 hours for liquid electrolyte, up to 100 hours for solid electrolyte).

Fig. 6: Various switching modes under illumination (sun) or in the dark (cloud) with open circuit and short circuit. The top three modes take about 10 minutes, whereas the last one (dark, open circuit) takes about 10 to 100 hours.

Thus, the only impossible process seems to be a colouring in the dark, but this is usually not needed. However, if needed, the device still acts as an electrochromic device, i. e. it can be coloured and bleached by applying an external voltage, independent of the conditions of illumination.

The dominating kinetic processes were investigated in detail, with a focus on liquid electrolytes as a model system. The results will be published soon. For solid ion conductors, the colouring and bleaching is shown in fig. 7. For open circuit, one can measure the voltage of the device, which reaches about 0.5V. For short circuit, one can measure the current density with respect to the area of the device, and integrate to get the

charge. The charge is proportional to the optical density, the coefficient of proportionality being the coloration efficiency.

Fig. 8: Bleaching in open circuit in the dark. (liquid electrolyte)

The curves displayed in fig. 8 and 9 were measured applying a liquid electrolyte. Fig.8 shows the bleaching in open circuit in the dark due to loss reactions, which are mainly electron transfers from the WO3 to the I3- in the electrolyte. For solid ion conductors, an even longer time (100 hours instead of 10 hours) of self bleaching can be achieved. Fig. 9 demonstrates the switching during constant illumination by switching between open and short circuit conditions, with open circuit voltage and short circuit current density, respectively.

Fig. 9: Colouring in open circuit and bleaching in short circuit with constant illumination (1 sun). (liquid electrolyte)

Conclusions

A photoelectrochromic device has been presented, which combines an electrochromic layer of WO3 with a dye solar cell. The layers show a complex nanostructure. The high porosity allows the electrolyte to penetrate into the layers of WO3 and TiO2, even for polymer ion conductors. The device can be switched under illumination as well as in the dark. For a cell with solid electrolyte, the visible (solar) transmittance changes from 62% (41%) to 2% (1%) in roughly 10 minutes.

Acknowledgements:

This work was supported financially by the University of Freiburg, Germany and by the German Ministry of Education and Research BMBF.

Solution procedure

For the radiative thermal resistance between the two slabs of the duct the following relation has been considered:

Г = (Ve1 +1/ є2 — 1)/4oT3

with о Stefan-Boltzmann constant, T = (T1 + T2)/2, є1 and є2 emissivity of the duct’s inner faces, respectively of the slabs A and B.

A detailed description of the used calculation procedure is reported in [8].

In the calculations the following reference dimensions have been considered: L=15 m; h=10 m; d=0.04 m. Assuming Pr=0.72, D~2d=0.08 m, the Eq. (1) is fully satisfied for all values of the Reynolds number (Re<2500) being peculiar to laminar flow.

For a standard situation the following reference climatic conditions have been assumed: Ti=24 °C, T0=28 °C in summer and Ti=20 °C, T0=0 °C in winter; all the graphs reported hereinafter should be meant to refer, unless it is indicated otherwise, to such values.

An iterative calculation procedure has been used to evaluate the quantities defined by the Eq. (2), to take into account the dependence of Г on the temperatures T1 and T2, and also to consider the variability of the air density and viscosity with the temperature T. All calculations have been developed in Maple programming software. For the friction factors Xin and Xou the following values have been considered as reference values: Xin=2 and Xou=4.

As reference values for the thermal resistances re, ri, and Rcd, (see Tab. I) the ones recommended by the technical rule EN ISO 6946 [11] have been chosen. For the considered values of d, Rcd=0.18 m2KW-1 has been assumed.

In the studied cases, the outer slab is supposed to have been realized so that the air infiltrations through the joints and the permeability to air of the used material could be disregarded.