Category Archives: BACKGROUND

New links with external programs: EES[5], Excel or Matlab

New Types that link TRNSYS with EES, Excel or Matlab are now available in TRNSYS 16. To allow for such models to be included in TRNSYS simulations it was necessary to create components in the TRNSYS code that explicitly link to the external programs, execute the models and provide some method of data transfer between the programs. These components work by having TRNSYS call the external program at each iteration. Especially the link to Excel is really a powerful feature cause it allows to communicate in a TRNSYS- Simulation quite easy with real system components like a controller over the Excel API — Interface like it was shown in [6].

Improved numerical precision and "time definition"

All variables in TRNSYS16 are now double precision numbers. This leads to an improved numerical accuracy, which is measurable for example by the very small time steps achievable in TRNSYS 16 (down to 1/100th of a second). This allows to simulate the real behavior of controllers Programming new Types is also easier thanks to the consistent use of double precision.

Enhancements to the TRNSYS solver

A new numerical solver has been added to TRNSYS. The new solver is intended to replace the default solver (successive substitution) in special cases with a strong feedback without a sufficient capacity in the system. A typical case is natural ventilation simulation, where a strong coupling exists between temperature and airflows. The new TRNSYS solver is hence similar to the solver implemented by EMPA in TRNFLOW, the extended Type 56 with integrated airflow modeling.

Other usability enhancements

The equations can now be declared in any order in a TRNSYS input file. New functions have also been added to TRNSYS equation processor The TRACE command can be used for equations

Equation solving can now be done to within a TOLERANCE instead of being absolute, aiding significantly in convergence.

A timer routine is available to identify the routines where TRNSYS spends most of the simulation time

Conclusions

TRNFLOW is a tool which allows an easy calculation of natural ventilation, passive night cooling, double facades and exhaust air shafts quite fast.

No new input file structure or user interface has to be learned. Using PREBID 5 simply the known user-friendly user interface has been expanded.

The newly-developed internal solver is featured with high stability. Due to integrated automatically optimization of convergence the user is no longer bothered with numerical questions.

In TRNSYS version 16, the capability of TRNSYS to adapt to new types of simulation problems has been pushed beyond new limits: Writing a component in any programming, copying the DLL provided by the model author onto the hard disk is sufficient to run it! Several new Types have been added, including a type for the link between TRNSYS and Matlab as well as a link to EXCEL. The window model of the building has been improved and treats now visible and nonvisible sunlight according to the spectral data of the window. A model for chilled ceilings has been added to the building.

Characterisation of the photoelectrochromic devices

To characterise the kinetics of the colouring and bleaching processes during illumination and bleaching in the dark, we needed to have an experimental set-up which measures the transmittance in the same way in both cases. For this purpose, we developed the set-up illustrated in fig. 2. A halogen lamp illuminates the photoelectrochromic sample, and the intensity of the transmitted light is detected by a silicon photodiode. The light intensity of the halogen lamp on the surface of the photoelectrochromic cell corresponds to 1 sun (1000W/m2), taking into account the mismatch factor of dye solar cells. The two electrodes of the photoelectrochromic device are connected via a variable shunt resistance and a switch. For all configurations, the TiO2 layer is always directed towards the lamp, so that the colouring of the Wo3 does not alter the light intensity on the TiO2.

Fig. 2: Experimental set-up to characterise the kinetics of the colouring and bleaching process. T: transmittance, U1: corresponds to the current with switch closed, U2 corresponds to voltage with switch open. The TiO2 layer faces toward the lamp. The filter is either in position 1 (cell illuminated) or in position 2 (cell in the dark).

The shunt resistance was chosen to be 10 Q, which is similar to the resistance of the TCO layer. With this construction, the voltage in the open circuit state and the current in the short circuit state were measured. For the dark state, an optical filter was placed between the lamp and the sample. This filter absorbs all the light with wavelengths below 715 nm. Above this wavelength, the dye is not sensitive and no electrons are excited, as we demonstrated by spectral response measurements. It was necessary to install several collimators in order to suppress scattered light from the environment. For the illuminated state, the filter was removed and placed between the sample and the photodiode detector. In this way, the optical signal is the same for both filter positions, and the transmittance signals for the dark and illuminated states are equivalent.

The measured value of the transmittance is a convolution of the spectra of the halogen lamp, the filter and the photodiode. In order to calculate the visible (or solar) transmittance from the measured value of the transmittance, the set-up was calibrated in the following way: A special PEC device was coloured to different extents by applying different voltages (0V, 0.3V, 0.4V, 0.5V, 0.6V, 0.7V) for 30 min, so that an equilibrium state was achieved. While holding this voltage with the potentiostat, the transmittance was measured with our set-up and immediately afterwards with the spectrometer which recorded the spectrum between 320 and 2000 nm. From these spectra, the visible (and solar) transmittance and
optical density were calculated. The visible (solar) optical density plotted versus the optical density determined with our set-up shows a linear dependence to a good approximation. A linear fit gave a simple equation to calculate the visible (solar) optical density from the values determined with our set-up.

These calibration measurements were made with a special PEC cell which was made without the dye. Coloured by an external voltage, this special cell has the same transmittance spectrum as the normal PEC cell coloured by illumination, because the amount of dye in the normal cell is very low. Using this special cell for the calibration had the advantage that the measured transmittance is not influenced by the different lighting conditions inside the spectrometer and in our set-up.

The condition for this calibration is that the form of the transmittance spectrum is independent of the depth of coloration. This condition is fulfilled because the dependence of the optical density on the charge is linear: The coloration efficiency is independent of the intercalation degree x [8]. On the other hand, the optical density OD represents the intercalation degree x (x: ratio of electrons and Li+ cations per W-atom in the Wo3):

Long-duration transmittance measurements of self-bleaching in the dark under open circuit conditions were made with a light-emitting diode with an intensity maximum at a wavelength of 655 nm without any filters. This light-emitting diode was switched on once every minute for a short measurement so that it did not influence the coloration of the photoelectrochromic device.

Transmittance spectra were measured with a Perkin-Elmer 330 Spectrometer.

Comparison of Heat and Mass Transfer. in Three Adsorption Solar Reactors

AROUDAM El Hassan

Department of Physics,

Faculty of Sciences B. P. 2121 — Tetouan — Morocco
Fax: (212) 39 99 45 00, E-mail: Aroudam_hass@Hotmail. com

Abstract

The performance evaluation of 1 m2 area of three type of adsorption solar reactors used in refrigeration are presented. The numerical computation results are obtained relatively for four typical sunshine days characterized by a maximal solar radiation measured locally and shows that both the cycled mass of refrigerant and the thermal performance coefficient of each machine are functions of the height of the porous medium and they are very depending on the meteorological data.

The effect of fins, the offered volume in the porous medium and the meteorological conditions are analysed and shows that the performance of machine is important for the month of April corresponding to some diurnal and nocturnal temperature and a solar radiation favourable for an application. A two-dimensional distribution of temperature and concentration of fluid inside the rectangular configuration is also presented along a daily cycle. The physical model adopted use the mass and heat balance equations and the Dubinin — Radushkevich formula for the determination of fluid desorbed fraction in each element of the discretised medium of the porous medium at a constant pressure.

Key Words: Adsorption — cooling machine — Solar reactor — Ammonia — Activated carbon

1. Introduction

The product of cooling energy by applying solar energy in the isolate site favoured by solar radiation present mainly a great interest. These are very important if the solar system used is very sample and not need a mechanical organ. An application suitable use the systems of adsorption phenomena between a solid and gas for ice production or refrigeration. They differ to other machine by the functioning mode of the collector, which represents the motor organ of the machine. The principle element which represents the thermal motor functioning by adsorption / desorption mode between a solid and a vapour under temperature — pressure variation, allowing to circulate a refrigerant fluid through tubes connecting different elements constituting the machine in the course of a daily cycle. The performance deduction of this machines type is very important by report to the case of absorption cooling machines. The efficiency depends strongly on the expressions of the produced energy quantity and that offered to the system. In the order to improve the performance, many researchers are developed and studies rigorously each component of the solar machines in the aims to enhanced the efficiency. Several experimental and theoretical works have been presented, they focus the objects on the heat transfer, thermal regeneration, the couples thermo physical characteristics, the effect on the environment, recourse to the new mixture, etc [1-12].

The interest of adsorption refrigerator application are increased when new other adsorbent are used such the active carbon fiber (ACF); substitute for activated carbon [13,15]. In this work, we focus on the numerical study of the daily evolution
of temperature, fluid mass and the pressure inside a solar reactor, submitted to real solar radiation data recorded locally. We propose to compute and compare the performances of three solar cooling machines considering rectangular and cylindrical reactors, with and without fins, in order to characterized the obtained cycles under real weather conditions.

We describe solar adsorption cooling machine using ammonia / activated carbon couples and the Dubinin-Raduskevich formula [16] for determination the adsorbed mass of ammonia, and to present a numerical distribution of temperature and adsorbed mass, versus time, inside a reactor heated by variable solar flow. A good design of elements of the compatible machine with the ambient medium, the heat source and the functioning mode allow to improve the efficiency of the refrigerator.

Chromaticity Coordinates

The perceived color is a very important property for architectural windows, including “smart” ones [9]. Quantitative assessments can be performed in several different ways; here we employ the CIE Colorimetric System [20,21]. The purpose of colorimetric analysis is to give color specifications for observers with normal vision in terms of tristimulus values or chromaticity coordinates. The ideal observer’s color matching functions are denoted x, y, and z, and represent red, green, and blue primaries, respectively. One can describe any color as an additive mixture of these. The y curve is chosen so that it coincides with the luminous efficiency of the light — adapted eye. The CIE 1964 tristimulus values corresponding to a certain color stimulus Ф(А) are obtained from

j" x (A W (A )dA *aE = j 7 (A)S (A )dA

and analogously for Vcie and Zcie. The color stimulus of present interest is

V(A) = S(A)T(A) (4)

where S(A) is the relative spectral irradiance function. Chromaticity coordinates, denoted x, y, and z, are then obtained from

X =——————— ^CIE——————— ^ (5)

XCIE + VCIE + ZCIE

and correspondingly for y and z. These formulas lead to the chromaticity diagram shown in the lower parts of the various panels of Figs. 3 and 4. Any color can be represented as a point within the shown boundary. The chromaticity coordinates for a colorless (achromatic) material are x = y = z = 0.333.

The chromaticity coordinates were calculated for four different standard illuminants representing a chosen set of light sources. Illuminant D65 signifies the average north sky daylight at 6500 K, illuminant A represents a tungsten halogen incandescent light

source at 2856 K (typical home or store accent lighting), illuminant F11 pertains to a commercial rare-earth-phosphor narrow-band fluorescent light source at 4000 K (used in Europe and the Pacific Rim for typical office or store lighting), and illuminant F2 represents a commercial wide-band-fluorescent cool white light source at 4150 K (typical office or store lighting in the U. S.A.).

Figure 3. CIE chromaticity diagrams representing the color of optimized electrochromic nickel-oxide-based films in fully bleached and colored states under daylight illumination (CIE D65; panel a), incandescent illumination (CIE A; panel b), narrow band fluorescent illumination (CIE F11; panel c), and cool white fluorescent illumination (CIE F2; panel d). Coordinates signifying the illuminant as well as colorlessness (i. e., the achromatic point) are plotted as a reference. The upper part of each panel is a magnification of the central region of the lower part. The designation NiXO (with X being Mg, Al, Si, V, Zr, Nb, Ag, or Ta) indicates that X is present in the oxide but does not specify the amount.

Figure 4. CIE chromaticity diagrams representing the color of optimized electrochromic iridium-oxide-based films in fully bleached and colored states under daylight illumination (CIE D65; panel a), incandescent illumination (CIE A; panel b), narrow band fluorescent illumination (CIE F11; panel c), and cool white fluorescent illumination (CIE F2; panel d). Coordinates signifying the illuminant as well as colorlessness (i. e., the achromatic point) are plotted as a reference. The upper region of each panel is a magnification of the central part of the lower part. The designation IrXO (with X being Mg, Al, Zr, or Ta) indicates that X is present in the oxide but does not specify the amount.

Figure 3 shows chromaticity coordinates for the optimized nickel-oxide-based thin films reported on in Fig. 2(a) under CIE standard illuminants D65 (panel a), A (panel b), F11 (panel c), and F2 (panel d), respectively. In panel (a), the data pertaining to the bleached state lie very close to the point representing colorlessness for illuminant D65. The data for the colored state are rather scattered, with the chromaticity depending on the specific additive. Also the trajectories in color space between the bleached and colored states are different for each of the additives. Another characteristic of the bleached state is that the dominant wavelengths are short so that the eye has less sensitivity. The chromaticity coordinates for the incandescent light, reported on in panel (b), indicate a light yellow color in the bleached state and a yellow-green color in the dark state. The data points for the bleached state appear less dispersed than the corresponding points for D65. For fluorescent lights, represented in panels (c) and (d) for narrow-band fluorescent and cool white fluorescent light sources, respectively, the data are similar and dispersed as regards the dark states. The films show a green-yellow color in the bleached state.

Figure 4 shows analogous chromaticity coordinates for the iridium-oxide-based films. From panel (a) it can be seen that the films, in both colored and bleached state, have chromaticity coordinates close to the achromatic point, and they are therefore almost colorless. Their dominant wavelength changes significantly from greenish blue to green, but the chromaticity remains close to the achromatic point. With the incandescent illuminant, for which panel (b) is appropriate, the color coordinates indicate that the films appear yellowish-orange to orange, and the color is more evident than for the daylight illuminant. In fluorescent light, panels (c) and (d) show that the films appear close to yellow. On the whole, it can be stated that the chromaticity coordinates for the iridium-based oxide films do not change to the same extent as the coordinates for the nickel-based oxide films. It can also be noticed that the iridium-based film that changes the most is the one containing Mg, while the film with the least change in color is comprised of pure iridium oxide.

Figure 5 shows Vqie, which corresponds to the luminous transmittance. The results in panel (a) are entirely consistent with those in Fig. 2(a). Films of nickel oxide mixed with Mg and Al show as much as 85 % luminous transmittance, whereas films containing Si and Zr yield up to ~83 %. For additives of Nb or Ta, the transmittance can be ~80 %. The case of the vanadium admixture is different, though, and shows a luminous transmittance not exceeding 75 %, i. e., lying ~4 % below the value for pure nickel oxide. Generally speaking, the luminous transmittance shows a rather weak dependence on the specific illuminant.

Panel (b) in Fig. 5 shows luminous transmittance of iridium oxide films mixed with Al, Mg, Ta, or Zr. Excepting the Zr-containing film, the additives lead to an improved luminous transmittance in the bleached state. Al and Mg rises the luminous transmittance to ~83 to 84 % from the initial 79 % of pure iridium oxide, while the Ta — containing film has a luminous transmittance of ~82 %. Zr, on the other hand, lowers the luminous transmittance by ~4 % due to a higher reflectance; thus such a film unsuited for applications requiring high transmittance.

Conclusions

We have shown that electrochromic nickel-oxide-based films can display enhanced short-wavelength transmittance in the bleached state when Mg, Al, Si, Zr, Nb, or Ta was added. However, admixtures of transition metals such as V and Ag did not improve the optical properties of the films. The colored state was not strongly affected by any of these additions. Larger admixtures are likely to change color, though, as one may infer from an analogy with mixed tungsten-based oxides [1,22]. Iridium-oxide-based films show improved transmittance in their bleached state when mixed with Al, Mg, or Ta, but not when mixed with Zr.

Acknowledgements

This work was supported by the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden. E. Avendano gratefully acknowledges a scholarship from the University of Costa Rica to complete his Ph. D. work at Uppsala University.

Performance of the TI wall heating system

During the heating periods 2002/03 and 2003/04 excellent results were obtained for the realized TI facade. The maximum temperatures of the absorber and of the indoor wall surface were about 75 and 30°C, respectively. During a cold and sunny week in January 2002 heat gain fluxes of up to 50 W/m2 were measured. The evaluation of the optimized TI wall according to van Dijk et al. [6] is illustrated in Fig. 5, showing performance charts, on a weekly and 4-weekly (monthly) basis, obtained from August 2002 until April 2003 (2002/03) and from August 2003 until March 2004 (2003/04). The equivalent U-value (Ueq), which is the ratio of the net heat flux through the wall divided by the temperature difference between indoor and outdoor is plotted versus the degree-day related irradiation, which is the daily sum of irradiation on the wall divided by the degree-day. Only in December 2002 a small monthly heat loss was obtained. Due to relatively high solar irradiation in December 2003, for all months of the heating period 2003/04 monthly heat gains were obtained. The linear regression of the data points yielded a solar energy efficiency of about 43% and a U-value ranging from 0.74 to 0.79 W/(m2K), which are outstanding values compared to data of various TI walls given in [7]. Comparing the TI wall performance of the two successive heating periods it can be concluded, that the air-openings of the TI elements work properly and that no adverse pollution of the TI facade or material

2003/04 (right)

Special attention was given to the evaluation of the hygro-thermal performance of the TI wall system. Although CTA absorbs a high amount of water no adverse condensation phenomena within the TI facade was observed visually during the investigated heating periods. These findings may be attributed to the diffusion-open construction of the TI facade based on timber profiles with minimized material fraction, to the large airgap between glass pane and TI structure and to the fact, that the pre-painted wall was in use two years without transparent insulation, which enabled the wall to get dry.

Fig. 6 shows the weekly averaged values of the ambient and room temperature (top layer), the calculated weekly maximum values of the relative humidity at the glass pane (middle

layer) and the weekly averaged values of the ambient and air gap water vapour pressure (bottom

layer). The maximum weekly averaged values of the room temperature (28°C) were observed in August 2003. These findings are related to the fact, that efficient shading of the TI facade is provided from May to July. In August the shading effect is relatively poor. Furthermore, August 2003 was an unusual warm and sunny month. The calculated weekly maximum values of the relative humidity at the glass pane clearly indicate, that during the whole measuring period never 92% were exceeded. Thus, no adverse condensation was observeable. A detailled analysis showed, that the maximum values of the relative humidity at the glass pane (above 90%) mainly occur during clear and cold nights at the end of cloudy periods with very poor irradiation in winter time (e. g., December 2002). The plot of the weekly averaged ambient water vapour pressure and the water vapour pressure in the air gap of the diffusion-open TI facade indicates, that the moisture exchange with the environment is quite good. Furthermore, it can be seen, that the maximum values of ambient water vapour pressure exceed the vapour pressure in the air gap. This phenomenon can be related to the fact, that the testing field is positioned in the middle of the TI facade element. Humidity entering the TI element at the bottom is absorbed by the cellulose acetate TI structure at the bottom. Whereas in sunny periods the desorption of water vapour from the TI structure is rather quick, in periods without irradiation the water vapour adsorption takes a long time.

RESULTS AND DISCUSSION

3.1 LaserReflectometry

The reflectivity data at 532 nm were monitored as a function of time during the deposition of TiO2, SiO2and Al2O3 films on silicon substrates using the laser light polarization parallel to the incidence plane.

The obtained curves can be interpreted in a qualitative way. The experimental curves show oscillations with constant amplitude, indicating the transparency of the film. Hence, the extinction coefficient k at 532 nm is zero as expected. A quantitative determination of the optical constants is done by numerical fitting of the experimental data using the reflectivity formula of a one layer on substrate model. In a straightforward way, in-situ laser reflectometry fits provide the growth rate vd, the refractive and extinction indexes at one wavelength (532 nm) as summarized in table 1.

n

vd [nm/min]

TiO2

2.2

1.2

SiO2

1.47

7.6

A^O3

1.5

25

Table 1. Optical constants and deposition rate of TiO2, SiO2 and Al2O3 thin films using laser reflectometry.

OPTIMIZATION

The cell parameter we attempted to optimize in this work is the active layer thickness considering other material properties as they have been earlier described.

A computer program has been developed according to the mathematical analysis, which implements the 3D model previously described. This program calculates the external quantum efficiency of the cells under consideration for wavelength range from 0.4pm to 1.1pm considering material properties such as effective recombination velocity, eq.11. The simulated illumination corresponds to the standard AM1.5 spectrum with power density of 1000 W/m2.

Initially the user must enter the data on the cell manually e. g., data on the front layer and the substrate thickness on Nd, Sf etc. These data are then used as the starting point for optimization.

The presence of any AR coating, the BSF, the grid structure of the cell (grid spacing and coverage), is inserted in the PC via the modelling procedure. The user has to define the lower and upper bounds of parameters to be optimized (epilayer thickness and doping). The program is then started in batch mode and the simulation is performed according to the input parameters. The optimization parameters are varied simultaneously in order to respect their interaction.

We obtained optimization of the cell during a procedure in which the considered parameters are adjusted through a systematic method, getting optimum design parameters. In this way, useful results have been extracted.

After it has finished, the output results are stored in a file and shown in the screen (e. g., calculation of the efficiency) for further examination.

A similar calculation is also performed in order to calculate the short circuit current, which is evaluated through numerical integration for the corresponding spectrum. As a next step, we can also investigate the dark current, the open circuit voltage, and the fill factor of the cells.

3. RESULTS

type solar cells

1.5 spectral conditions. The experimental values assigned to the model parameters are p=1Q. cm, Ln=66pm. The measured

photocurrent density for these cells is Jsc=24.07 mA/cm2 and their maximum efficiency is 8.86%.

The figure shows the influence of grain boundary recombination velocity on the photocurrent for different values of grain size.

The optimal photocurrent seems to be heavily affected from recombination in the grain boundaries for small grain sizes (50 and 100 pm).

Since such grain sizes, are smaller or comparable to the base diffusion length, it can be concluded that a significant amount of the photogenerated carriers recombine in the grain boundaries.

For high grain boundary recombination velocities (higher than 500 cm/sec) the smaller the grains the lower the photocurrent.

On the contrary, for grain sizes 250 and 500 pm the influence of grain boundary recombination velocity is not so significant.

In any case for grain boundary recombination velocities lower than 200 cm/sec the

photocurrent density becomes independent of grain size.

Figure 3 illustrates efficiency as a function of grain boundary recombination velocity for four different grain sizes, which is calculated from the optimization program for such base thickness that the efficiency is maximized for the given

parameters.

It can be observed that for grain size 50 and 100 pm the efficiency is heavily affected from grain boundary recombination, where a rapid decrease appears in the values of n for recombination velocities of greater than 103 cm/sec.

On the contrary there is no significant effect of the

recombination velocity for grain sizes 250 and 500 pm, resulting to almost constant efficiency for recombination velocities lower than l03 cm/sec.

As far as the epilayer thickness is concerned, Fig. 4 shows that the quality and the solar cell efficiency is high enough if Sgb reaches 10[39] cm/sec but little is gained if Sgb is lower.

The optimal layer thickness increases when Sgb decreases (better quality, higher Ln). We observe a broad maximum in the efficiency for low grain boundary recombination velocity leading to an optimal base thickness near the value of Ln (66 pm). In this case grain boundary recombination is low and efficiency is limited from

……………. Sgb=10[40] cm/s

——— Sgb=103 cm/s

……………. Sgb=10[41] cm/s

——— Sgb=10[42] cm/s

——— Sgb=106 cm/s

11.6

11.5

11.4 11.3 11.2 11.1 11.0 10.9 10.8

10.7

10.6

bulk recombination, directly related to Ln. For large Sgb this maximum efficiency shifts to lower thickness, near 35 pm proving for such value of epilayer thickness the effect of a strong BSF and increased grain boundary recombination, confirming the ratio Ln/d2 ~2 as the optimal choice as it is already proved [2].

The external quantum efficiency curves of figure 5 shows a comparison between the 1D model presented in [3] and the 3D equivalent for a small grain cell.

We observe in Fig. 5 that the quantum efficiency of the 3D model is lower compared to the 1D equivalent for wavelengths greater than 0.5pm (visible and infrared solar spectrum) corresponding to the contribution of carriers generated in the p (base) region, due to the influence of grain boundaries.

For shorter wavelengths the blue response (contribution of n+ front region) is still low, indicating that the emitter can be improved and seems to be independent of the grain boundary recombination velocity. Therefore, the differences between 1D and 3D are

negligible. That can be explained due to emitter heavy doping which reduces the grain boundary barrier height [6] and recombination is unaffected from the grain boundaries.

4. CONCLUSIONS

We have developed a three-dimensional model for n+pp+ polycrystalline Si solar cells and found the grain size and grain boundary recombination velocity influence on photocurrent and solar cell efficiency under AM 1.5 irradiance conditions. The

calculated results show that the optimal photocurrent and conversion efficiency for small grains is heavily affected from the recombination velocity. Concerning the optimal values of epilayer thickness it is obvious that its value is lower, leading also to lower efficiency values, for higher values of grain boundary recombination velocities, confirming the ratio Ln/d2 =2 as the optimal choice for cells of such material quality. Further elaboration is needed in order to investigate other characterizing parameters such as the open circuit voltage, and the fill factor.

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.