Category Archives: BACKGROUND

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.

The collector model

Figure 2 represents the studied reactors in this work. They consist of 1 m long and 2 mm thickness steel tubes resisting to the maximum pressure reached by the fluid. The total surface covered is 1 m2 which is composed by a number of cylindrical tubes with an external diameter De, figure 2-a. This number is inferior when finned tubes are used considering a De/2 width fins, figure 2-b. The rectangular reactor has a width of 1 m and a height of H, figure 2-c. All these reactors are isolated from the lateral wall and the back. The tubes are filled by an amount of activated carbon reacting by adsorption with ammonia constituting a porous medium. This medium can be heated or cooled through the metallic wall along a daily cycle.

Figure 2-a. Reactor without fins

Figure 2-. Solar reactor

The thermal and mass transfer calculation in the reactor been made by taking account the next hypotheses; characteristics of the porous medium:

— the porous medium pressure is uniform and to be constant when the reactor is opened.

— the distribution of temperature is bi-dimensional in the rectangular reactor and radial in the case of cylindrical tubes.

— the three phases of the porous medium, in elementary elements of volumes, solid phases made up of coal particle of activated carbon, liquid phases (ammonia adsorbed) and gas phases (ammonia gas) are in thermal, mechanics and chemical local equilibrium.

— the thermal transfer by convection is neglected

— the exchange between the metal wall and the porous medium and the thermal transfer by conduction are characterized respectively by a coefficient of exchange hi and an apparent thermal conductivity Xe [16].

— the physical properties of metal and the activated carbon are constant.

— the condenser and the evaporator are considered to be idea.

ma = PL Vo Exp[-D (T Log ( fs / f ) )n] (1)

where; ma is the total mass of the adsorbed fluid at the pressure P and the temperature T; The product pL Vo is the maximum volume of the pores accessible to the adsorbate. D and n represents the parameters of Dubinin, which depend on the adsorbent / adsorbate couple and which are given experimentally for the activated carbon and ammonia pair [17].

Experimental

For determining the appropriate temperature levels for the acid rain tests, stagnation temperature for an unglazed absorber was calculated iteratively on a hourly basis (Konttinen et al, 2004). Surface samples were immersion subjected to O2-aerated or zero — aerated (with N2) simulated acid rain (Table 1) with pH 3.5 or pH 4.5 and simulated neutral rain with pH 5.5 in temperatures of 60, 80 or 99 °C. Saturating gas was fed at room temperature into the solution. The solution was heated within ±1 K of the required temperature in a flask, which was in a paraffine oil bath (Fig. 1). The oil was typically 6- 10°C (at 60/80°C) or 40-45°C (at 99°C) warmer than the solution. Details of the setup are described in (Konttinen et al, 2004). Samples tagged as “non-aerated” in Figures 3-5 were tested with EIS at similar temperatures and pH. Changes in optical properties of non­aerated EIS-tested samples A-B in Figs. 3-5 are included in this paper for comparison to aerated and zero-aerated test results.

Polarisation measurements and EIS measurements were carried out in a conventional three electrode cell using platinum sheet as a counter electrode (CE) and saturated calomel electrode (SCE) as a reference electrode (RE) (Fig. 2). The methods and measurement system are described in ASTM standard G5 (ASTM, 1987) and in (Lorenz and Mansfeld, 1981), respectively. The RE was connected to the cell with Luggin capillary.

Corresponding author. Tel.: +358-9-4513212; fax: +358-9-4513195; E-mail:

petri. konttinen@hut. fi

The salt bridge filled with 0.1 mol/l Na2SO4. was used between the RE and the Luggin capillary to avoid possible chloride contamination of the test solution. In some polarisation tests 500 mg/l Na2SO4 sulphate was added to increase conductivity (Table 1, b-solutions). The polarisation measurements were conducted in an Avesta cell (Fig. 2a) where a sample was pressed against a round hole on the cell bottom and the system was sealed with a rubber o-ring gasket. The sample area was 0.8 or 1.3 cm2. Long-term immersion EIS tests were conducted in a cell (Fig. 2b) where the samples (working electrode WE) were attached with stainless steel screws to a sample holders made of copper rods. The copper rods and the back sides of samples were insulated with PTFE tape and nolan lacquer. The sample area was 5 cm2.

Before polarisation measurements the corrosion potential was followed for 1h until a stable corrosion potential was achieved. Potential was first changed — 150 mV to cathodic direction and cyclically back to the corrosion potential with a scan rate of 10 mV/min. After cathodic polarisation the potential was changed +150 mV to anodic direction or to potential -200 mV vs. SCE. The EIS measurements were done at the corrosion potential with the amplitude of 10 mV. The EIS system consists of a NF 2000 potentiostat and NF 5050 frequency response analyser and the polarisation measurements system consists of a ACM Instruments Autotafel potentiostat.

Test duration needed to degrade the a and є of the absorber samples according to performance criteria (PC) was determined in each test:

PC = — Дa + 0.25Д^< 0.05

Although not defined for unglazed collectors, Eq. 1 was used as it is typically used to estimate failure limit for solar absorbers inside glazed collectors (Brunold et al., 2000). PC value of 0.05 is generally equivalent to 5 percentage unit decrease of the solar heat gain by the flat-plate solar collector water heating system.

Hemispherical reflectance of the samples was measured before and after each test at room temperature. Solar absorptance, a, was determined between 0.39 — 1.1 pm with a LI-COR LI-1800 type spectroradiometer and a BaSO4 coated integrating sphere. Thermal emittance, e, was determined between 2.5 — 20 pm with a MIDAC Prospect FTIR — spectrometer with a semi-hemispherical integrating device. Spectral reflectance, p^, was analysed for estimating the hydration levels of alumina.

Table 1. Chemical composition of the simulated acid rain. pH 3.5 (a) adapted from (Magaino, 1997). pH 4.5 and 5.5 (a) gained by adjusting the amount of NOj and SO4~ . pH 3.5 (b) and 5.5 (b) gained by adding 500 mg/l Na2SO4 for better conductivity for EIS.

Concentration/mg dm -3

Ion

pH 3.5 (a)

pH 4.5

pH 5.5 (a)

pH 3.5 (b)

pH 5.5 (b)

0.098

0.098

0.098

0.098

0.098

or

1.0

1.0

1.0

1.0

1.0

NOj

7.1

0

0

7.1

0

so2~

9.8

1.5

0.15

348

338

Na+

0.42

0.42

0.42

162

162

K+

0.11

0.11

0.11

0.11

0.11

Ca2+

0.049

0.049

0.049

0.049

0.049

Mg2+

0.051

0.051

0.051

0.051

0.051

Fig. 1. Photograph of the acid rain total-immersion setup including 250 ml three-necked flask, 100 ml simulated acid rain, 23 x 50 mm sample and gas distribution tube (excluding heating system consisting of paraffine oil bath, heater and temperature controller).

Fig. 2. Schematic pictures of the measurement cells in polarisation tests (a) and long-term immersion tests (b).

Results

TRNFOW and Improvements on the Capabilities of TRNSYS 16

M. Hiller 1, S. Holst 1, T. Welfonder 1, A. Weber 2, M. Koschenz 2 TRANSSOLAR Energietechnik GmbH

1 CuriestraKe 2, D — 70563 Stuttgart, tel.: +49 711 / 67976 — 0, fax: +49 711 / 67976-11 welfonder@transsolar. com, http://www. transsolar. com

2 EMPA, Abteilung Energiesysteme/Haustechnik, CH-8600 Dubendorf

In the planning process and evaluation of innovative energy concepts simulation of buildings and systems gets more and more important. With the internationally well known software program TRNSYS [1] those simulations can be accomplished with a very high complexity.

The paper describes the Program TRNFLOW [2] which is a new add-on for TRNSYS for the simulation of natural ventilation. Also the main new features of the TRNSYS Version 16 released in Mai 2004 will be described.

Coupled Airflow Simulation — Current Situation

In order to achieve sustainable buildings new energy systems have been generated using natural effects to renew the air and lead away the heat. Examples are passive night cooling, double facades, solar chimneys, inside courtyards and so on. In these systems the mutual impacts of thermal and air flow behavior are very distinctive. Thus for numerical building simulation programs an integral approach is inevitable.

Already in 1993 in the frame of the IEA project Annex 23 the EMPA has developed a coupling of the multizone air flow model COMIS [3] with the thermal building and system simulation program TRNSYS and this was presented at the TRNSYS Userday 1994. The self-contained program COMIS was modified to TRNSYS Type 157 which can be linked to the thermal building model Type 56 within the TRNSYS-Deck via in — and outputs. The input information of the air flow model are read in by Type 157 from the standard COMIS Input File (CIF). The TRNSYS solver iterates the results of the two models until they match. Meanwhile the coupling has been successfully applied by several projects and simulation tasks. However thereby it was pointed out that the coupling is not very user-friendly and also requires a laborious handling. As the mutual classification of the data is made by hand the inputs tend to be incorrect. This error-proneness is also increased by the redundancies of the two input files. Furthermore it was proven that the TRNSYS solver is not always the perfect solution for such a system. The solver possibly has to be supported by additional convergence promoting Types what makes the handling again more difficult. Yet the need of an integral approach concerning thermal building dynamics and natural air exchange was clearly necessary. Therefor with TRNFLOW an improved version including a deepened integration of the two models (thermal and airflow) has been developed.

Multizone Air Flow Model in TRNFLOW and COMIS

Multizone air flow models idealize the building as a network of nodes and airflow links. A node represents a room volume which a set of state variables can be assigned to. Cracks, window joints and openings, shafts as well as ventilation components like inlets and outlets, ducts and fans represent the links. Boundary conditions and thereby also input factors are: State variables of the air in the zones Local wind pressures

The pressure pZ is a free parameter in the node which is evaluated according to the continuity equation (mass flow balance in the node = 0 ). This results in nZ equations where nZ represents the number of zones.

The relation between mass flow rate m and pressure difference dp and the zone pressure pZ is not linear. Therefor an iterative process is used to solve the system of equations. The mass flow rates per link and all dependent factors such as air exchange rates, air age etc. are calculated of the resulting zone pressures pZi. The calculation is static without an explicit consideration of the timestep. In principle calculating a condition based on a new time is independent of the previous timestep.

Total hemispherical reflectivity

The total hemispherical reflectivity R(A) and transmission T(A) for single-layer on glass samples show a quasi-zero absorption of the films, confirming the previous results from laser reflectometry and spectroscopic ellipsometry methods. The theoretical curves of Figs. 2 and 3 are calculated using the experimental optical constants determined by spectroscopic ellipsometry on single-layer samples. In our model, we suppose homogenous layers and sharp interfaces.

Figure 2 represents the total hemispherical reflectivity of Ti02/Si02 multilayers formed by three and five alternating layers of Ti02/Si02. The thicknesses of the layers are indicated in table 2. We observe a reflectivity peak in the visible range. The peak position determines the color of the multilayer film. The dotted lines indicate the theoretical reflectivity. We observe a good agreement between the experimental and the theoretical values. The reflectivity peak position, its maximum value and its FWHM depend on the layer thicknesses and on the number of layers. In general, at one wavelength the reflectivity peak maximum increases with increasing layer number.

Figure 3 shows the total hemispherical reflectivity of Al203/Si02 multilayers formed by an increasing number of alternating layers having the same thicknesses. The dotted lines indicate the theoretical reflectivity increasing with the layer number. This evolution shows the same tendency as previously presented results obtained by using the simplified model of constant refractive indexes [17].

The thicknesses of the individual layers of Fig. 3 are indicated in table 3. The peak position is relatively constant and its maximum value increases by increasing the number of alternating layers. The disagreement between the experimental and calculated values for

the nine-layers samples can be explained by the long deposition time and an eventual change of the deposition conditions.

Wavelength [nm]

Description of Systems

Within the framework of the IEA Task 27 "Performance of Solar Facades" we have tested a series of different combinations of solar protection and glazings. These systems include internal, integrated and external shading devices. The characteristic data are given in the following section.

External blind systems

• three devices with identical complex lamellae

• white, white perforated and brown lamellae (see Figure 1) external blinds

• 90 mm width, a distance of the lamellae of 80 mm

• combination with a low-e glazing (pos. 2, 16mm Argon), g=48%, U=1.3 W/(m2K)

Internal systems

• white lamella type blinds using slats 25mm white, distance 22mm

• textile roller blind silver (outside) and white (inside)

• combination with a low-e glazing (pos. 2, 16mm Argon), g=48%, U=1.3 W/(m2K)

• combination with a low-e glazing (pos. 2, 16mm Argon), g=35%, U=1.1 W/(m2K)

Integrated systems

• white lamella type blinds using slats 15mm white, distance 13mm

• textile roller blind grey (both sides)

• integrated in low-e glazing (pos. 2, 27mm air), g=47%, U=1.5 W/(m2K)

• integrated in low-e glazing (pos. 2, 27mm air), g=32%, U=1.4 W/(m2K)

Mathematical Model

The heat transfer governing equations for steady state laminar natural convection in cavities are the mass, momentum and energy conservation equations in x, y and z axis for an incompressible fluid [Versteeg, 1995]. These equations can be expressed in conservative form:

Conservation of mass:

where T0 is the reference temperature, and is calculated by the mean temperature distribution of the exterior glass surface and its result is averaged with the temperature of the isothermal wall, so:

Hx, Hy y Hz are the lengths of the edge surfaces of the cubic cavity, Hgx is the thickness of the glass and Tci it is the temperature of the wall 2.

The boundary conditions for the momentum equation are:

u(0,y, z)= v(0,y, z)= w(0,y, z)= 0 u(Hx, y,z)= v(Hx, y,z)= w(Hx, y,z)= 0

u(x,0,z)= v(x,0,z)= w(x,0,z)= 0 (6)

u(x, Hy, z)= v(x, Hy, z)= w(x, Hy, z)= 0 u(x, y,0)= v(x, y,0)= w(x, y,0)= 0

u(x, y,Hz)= v(x, y,Hz)= w(x, y,Hz)= 0

The boundary conditions for the energy equation are:

Wall 1

— k“ (X’0’Z ) = ЧГ3 (X’0’ Z ) (7)

dy

Wall 2

T(0,y, z)= Tci (8)

Wall 3

— ka T(x, Hy, Z ) = ЧГ3 ( Hy, z)

dy

Wall 4

d — dT

— ka (Ях ’У’z)=~kg (Hx, y,z)h qr4(Hx, y, z)h Sa, f

Wall 5

— ka (x’ y ’ Hz ) = qr5 (x y ’ Hz )

dz

Wall 6

дт

— k a —(x’ y,0) = qr6(x ’ y,0)

dz

where qr1(x,0,z), qr2(x, y,0), qr3(x, Hy, z), qr4(Hx, y,z), qr5(x, y,Hz) and qr6(x, y,0) are the energy flux from the radiative exchange between the wall surfaces, Saf is the absorbed energy by the solar control coating and Tg(x, y,z) is the glass temperature for Hx<x< Hx+Hx2, where

d—

Hx is the thickness glass. The temperature gradients (Hx, y,z) in the glazing were

dx

evaluated by using the heat conduction model.

Micro-structured surfaces for solar applications — an overview

Andreas Gombert, Benedikt Blasi, Wolfgang HoRfeld, Volker Kubler, Michael Niggemann, Peter Nitz, Gunther Walze, Fraunhofer-Institut fur Solare Energiesysteme ISE, Heiden — hofstr. 2, 79110 Freiburg, Germany

Jorg Mick, Institut fur Mikrosystemtechnik, Albert-Ludwigs-Universitat, Georges-Kohler — Allee, 79110 Freiburg, Germany

An overview of known methods to modify the optical properties of solar energy ma­terials by using micro-structured surfaces is given. Applications for micro struc­tures in solar energy components are wavelength-selective absorbers, heat mirrors, light traps for PV cells, wavelength-selective concentrators for solar radiation, day­lighting components, antireflective zero-order gratings, and radiation emitters with selective optical properties based on grating resonances. This paper addresses the design and the whole experimental process chain from the microstructure origina­tion on large areas to the replication. The need for cost-effective production tech­nologies and durable structured materials is emphasised.

Introduction

A wide variety of solar energy systems from PV cells to buildings exists. All the very differ­ent systems have in common that they demand sophisticated optical solutions for an effi­cient transport, collection and conversion of the solar radiation. Modifying the optical prop­erties of surfaces or planar devices by coatings and microstructures is often used in order to optimise the radiation power management of solar energy systems. Publications in which diffractive structures were proposed to fine-tune the optical properties date back to the 1970’s [1 — 4].

In fact, advances in diffractive optics like the first approaches to solve the problem of dou — bly-periodic gratings were driven by scientists having solar applications in mind, e. g. Mc Phedran and Maystre [2]. Since the fundamental work of the mentioned authors, diffractive structures in solar energy systems were published for a variety of components in solar en­ergy systems, e. g. [5 — 15]. In Table 1, the quoted publications are classified according to the components for which the diffractive structures were proposed. Additionally, the re­quired optical properties and the proposed structures are listed.

The advantage of considering periodic microstructures is the possibility to model their opti­cal properties rigorously, i. e. by solving the Maxwell equations [16]. Thus, the optical prop­erties of periodically micro-structured surfaces can be simulated with rather high accuracy. This is not the case for aperiodic structures.

The challenge of using microstructures is the requirement on very precise manufacturing technologies. Such technologies exist in the field of microelectronics or microsystems technology but are in general not suitable to structure large areas homogeneously. Thus, many of the published approaches are difficult to realise especially due to the mismatch of dimensions between the microstructures and the areas which have to be micro-structured in solar applications.

From the technical point of view, homogeneous origination of precise microstructures on large areas is the most difficult step in the process chain. Because of its ability to share the high origination cost with a large number of products, microreplication is very promising from the commercial point of view. Microreplication is a suitable process for some of the applications listed in Table 1 but not for all of them.

At Fraunhofer ISE, we have picked up the idea of using periodic microstructures in solar energy applications at the beginning of the 1990s. Since then we have been working on the design and the manufacturing techniques for the following optical components: antireflective surfaces, light traps for PV cells, sun protection systems, and wavelength — selective radiation emitters. By using interference lithography we were able to originate microstructures on areas of up to 4800 cm2.