The optimization process proceeds to obtain the maximum efficiency of the MIS/IL solar cell by optimizing the cell parameters. The optimization process depends on the number of iterations needed to achieve the maximum efficiency of the cell. After 40 iterations (Fig.6), the efficiency reaches a maximum steady state value of 22.95 %. Table 1. shows the parameters required to achieve the this efficiency (data(1)). The table also includes the results of the optimization process by using aluminum as a contact metal ^m = 4.1). In this case, only five parameters are used for optimization (data(2)). The results of the optimization process when using aluminum as metal contact and with no external back bias (V=0) (is shown in data

(3) ). The optimization results are compared with a theoretical optimization based on an analysis of structure parameters [7]. The comparison has shown good agreement of the genetic algorithm optimization with the other mentioned method as shown in Table 2.

SHAPE * MERGEFORMAT

Deplation region

Fig.2. Energy band diagram of MIS/IL Solar cell

SHAPE * MERGEFORMAT

S. Bartha1, I. Farkas2, D. I. Teodoreanu1 and V. Ursu1.

1I. C. P. E.-New Energy Sources Laboratory (NESL)

Bucharest, Splaiul Unirii 313 ROMANIA

Tel.: +40213467236, Fax: +40213467268 Email: sbartha@planet. ro, danteo@icpe. ro

Department Physics and Process Control,

2Szent Istvan University
Godollo Pater K. u. 1. H-2103 HUNGARY
Tel.: +36 28 522055, Fax: +36 28 410804 Email: Farkas. Istvan@gek. szie. hu

Abstract

Use of renewable for rural remote households, this is the principal way to energy produced what can be applied in rural electrification. A large part of the Romanian population, approx. 45 % is living in rural areas. Despite that the Romanian national power grid covers the whole territory, still there are some 70. 000 rural not electrified dispersed households. Several tens of thousands from this cant are connected to the public grid in the near future due to the large distances to the grid, implying prohibitive cost.

In the development of energy sources in rural regions in Romania at the brink of the 21st century, it is necessary to view the use of solar and wind energy in all applications as one of the most promising new and renewable energy sources. This paper presents a study, sizing model and design of a complete photovoltaic system for providing the electrical loads in a family house according to their energy requirements. A several computer programs are used to achieve this design and to determine the specifications of PV system components. The system configuration is tested in two different meteorological areas one in the mountains, in Carpathen and the second near Black Sea coast. The load profile of the both applications is estimated for 480Wh/-day energy consumption. The produced energy is storage in a battery bank that offers 20-30 day autonomy in functioning.

The monitoring equipment used for evaluation of the system parameters is made by ENERPAC and presents the principal functioning parameters of the applications. All numerical data are recorded by a data logging system at intervals one of hour with averages of parameters in each 10 minute. The monitoring data are presented for a two — year period. The simulation parameters of the system were evaluated with using the Solar Design Studio 4. 0, the NSol 4.2 and the Hommer software’s packages. In finally the test results includes the energy system parameters and also the energy performance from the application. Payback time estimations were also presented.

1. Introduction

In the region of East and Central Europe there are rural villages which are still not
fully electrificated. For that reason, it is a real case to design their energy supply
system using renewable. Before a PV system is built, system planers and installers

should simulate using a computer program. In this case the installation companies can also demonstrate to the consumers the productivity of the PV system. The design of the PV and/or PV-wind hybrid system would involve the determination of optimum values for the rated capacity of the PV modules and wind turbine, the capacity of batteries for storage that would meet the required reliability conditions for the system. When different system configuration are under developed each system configuration can be quickly simulated in order to determine the optimum from an energetic, economic, and ecological stand point. For this risen we developed a PV system for stand alone application what can be installed in different region and this PV application was tested on mountain and on sea meteorological conditions. Before the system installation begging the system planners should make a preliminary layout scheme and estimate the energy yield. In this way poor system designs can be avoided and the simulation results can be critically evaluated. In this paper initial simulation results of a small household PV-wind system is presented. The studied applications are suited in the mountain region near in Anghelus, Romania and in Agigea on the Black Sea coast. The load of system is designed for special remote shelter used by researchers making some special measurements in this area. The studied system comprises of a PV unit, charger and battery bank, inverter and several other necessary control and safety components. Monitoring equipment was also installed in order to measure and record the main working parameters of the system. The monitoring program and equipment made by Enerpac. The meteorological data used for the design and simulation of this system were obtained from the softwares and from climate database.

The process control, as well as the acquisition, processing, storing and reporting of all data, is achieved through a virtual instrument developed with the help of the graphic programming software LabVIEW. The data are stored in the computer hard disk in a universal format, allowing their subsequent processing with LabVIEW or any other known electronic sheet.

The following Virtual Instruments were implemented:

a. VI for monitoring the solar radiation and ambient temperature.

The measurements of the solar radiation and ambient temperature are made using a pyranometer SP-Litte, of the firm Kipp&Zonen, and a termistor respectively. This VI allows the achievement of the following tasks:

■ Signal capture at a rate of 1 dat/s, and subsequently storage of the mean value calculated over 30 sec.

■ Display of numerical data and graphics of the daily variation (between 5 a. m. and 7 p. m.) of the irradiance and temperature on a computer screen. Fig. 2 depicts the front panel of the VI developed, where the daily variation of the solar radiation and ambient temperature of a typical summer day in Bogota is shown.

07

■ The data is simultaneously stored in Excel format and processed to provide information concerning the mean (daily, monthly, yearly) irradiance and ambient temperature as well as the number of hours of standard irradiance.

b. VI for monitoring the system variables.

This VI allows achieving the following tasks:

■ Measure, store and report the numerical data of current and voltage supplied to both, DC and AC loads.

■ Determination of the efficiency of the PV system and of the inverter, as well as the DC and AC energy through calculations carried out with LabVIEW. Numerical data of these variables are also displayed on the computer screen.

■ Harmonic analysis of the AC signal generated by the inverter through Fourier analysis carried out whit the help of a tool of the LabVIEW package.

In this case the DC voltage is measured directly with the FP-AI — 100 module, whereas the current (DC and AC) are measured using as transducer a DC (or AC) current clamp. The AC current clamp provides an output DC-signal of 10mV/A (in the range used) and the DC-current clamp has a sensitivity of 1mV/A. The AC voltage was measured using a transformer as transducer with a transformation ratio of 110 to 5.

c. VI for measuring the I-V characteristic This VI allows achieving the following tasks:

■ Linearization of the I-V scan through a program in LabVIEW

■ Capture of V and I data and instantaneous display of them in the computer screen until the entire I-V curve is completed.

■ Processing of the data of the I-V curve to get graphics of P vs V and values of the parameters characterizing the performance of the PV-generator (Isc, Voc, Pmax, FF, n). The P vs V curve and the numerical data of the parameters are also displayed in the computer screen. Fig. 3 depicts the front panel of the VI developed. The reported I-V and P-V curves correspond to a PV array of 5 modules (each of 75Wp) connected in parallel and illuminated with an irradiance of 289 W/m2.

Test of the Monitoring System

The operation of the monitoring system started on January 2003, and since then, it has been continuously tested. The equipment has been used to monitoring the performance of a PV-solar plant installed in Bogota and for measure and evaluate the solar irradiance and ambient temperature measured close to the PV-solar plant.

Until now, all the units of the equipment have functioned very well and the related measurements have shown to be reliable. Fig. 4 shows the yearly profile of the mean daily solar radiation and ambient temperature measured in Bogota during the year 2003.

CONCLUSIONS

An automatic data acquisition system for monitoring PV solar plants has been developed using a non conventional procedure based on Virtual Instrumentation. It measures and displays graphics of solar radiation, ambient temperature and numerical values of the more usual system variables. The equipment also includes a special unit for measure the I-V characteristic of the PV plant at a high scanning speed. Graphics of I vs V, P vs V, as well as the values of the electrical output parameters of the PV plant can be displayed in the computer screen.

The system allows the collection of data during long periods of time, without human intervention. Tests carried out during one year have indicated that the equipment developed is highly reliable and suitable to monitor and evaluate the performance of PV — solar plants.

ACKNOWLEDGEMENTS. This Work was supported by COLCIENCIAS and Universidad Nacional de Colombia.

[1]

This PV grid-connected system shall be interconnected to the Metropolitan Electricity Authority (MEA) distribution line of 24 kV under the regulation on the Synchronization of Generators to the Power Utility System of MEA there is no need for storage batteries. It operates in parallel with the main electrical grid and stores the produced electricity to the grid. The building roof area is 14,000 m2; with 16 degree azimuth, 3 degrees face East and 3 degree face West pitch (Fig. 1). The PV generators shall be installed on top of the roof (Alu-zinc roof sheet), which proper direction facing in order to achieve the maximum electricity production and to be possible for engineering installation and be architecturally pleasant. Special attention was paid not only to the technical characteristics of the pV generators, such as high performance and reliability, but also to their color appearance after their interaction with solar radiation. The PV system is connected in a three-phase mode to the electricity grid and consists of at least 350 kWp PV generators, sinusoidal interconnected inverters and a control board for system parameter control and monitoring via a data recorder. The recorder has the ability to store data produced from various measurements. The block diagram of the PV grid-connected system shall be installed as shown in Fig. 2.

a) Two-layered systems

From the field of anti-reflection coatings, two-layered coating designs such as the V — and the W-design are known [24]. These systems owe their name to the shape of the reflection minimum. The effect of anti-reflection extends over a certain wavelength range, but aside from this region considerable reflectance peaks occur, which can be used to produce a colored reflection. The region of antireflection enhances the solar transmission. Creating a region of higher reflectance at short wavelengths, blue and green colors of reflection can be easily achieved in combination with a good solar transmission.

As an example we show the W- design. Both sides of the substrate are coated identically, as it occurs often in sol-gel dip coating. Our calculation takes into account the multiple reflections between the two sides. The optical model has the structure

air // L 2H // glass /2HL // air, where the letters "L” and "H” mean quarterwave layers of low and high refractive index, respectively. The corresponding layer thicknesses t(L) and t(H) have thus been chosen to n x t(H) = A.0/2 and

n x t(L) = A.0/4, where the

so-called "design wavelength A. o” indicates the center of the region of anti-reflection (here: Xo = 800 nm). A refractive index of 1.52 has been assumed for the glass substrate. The resulting reflectance spectra are displayed in Fig.1. For the shown examples, the resulting colors are in the region of bluish green. Within a rather large region the reflectance is lower than the one of an uncoated substrate (approx. 8%). Due to the partial antireflection, the achieved color saturation and as well the solar transmission are remarkable. A survey of the characteristic figures, the color coordinates x and y, the visible reflectance Rvis, the solar transmission Tsoi and the figure of merit M = Rvis/Rsoi, is given in TABLE I.

TABLE i:

CIE color coordinates x and y, the visible reflectance RVIS, the solar transmission Tsol and the figure of merit M = RVIS/Rsol, as computed for the curves displayed in Fig. 1.

b) Three-layered system

Starting from a classical V-design, we have studied the influence of adding a third layer. We consider a glass substrate with a refractive index of 1.52 being coated on one side by a stack of three layers, the first layer being 30 nm thick with a refractive index of 2.2, the second layer 140 nm thick with n = 1.46, and a third layer of variable thickness with n = 2.2. By variation of the thickness t3 of the topmost layer from 0 nm to 50 nm, we obtain the reflectance spectra displayed in Fig.2. The thin black line illustrates the reflectance for the two-layered V-design (thickness

t3 = 0 nm). Adding the additional layer results in an increase of the colored reflection. The spectrum corresponding to the case of a 30 nm thick top layer exhibits a strong enhancement of the reflection peak and still a region of anti-reflection at a wavelength of 1000 nm. For the top layer being 50 nm thick, the region of anti­reflection is already less pronounced. Also from the point of view of the solar transmission Tsol(%), the region between 10 nm and 40 nm appears most interesting. TABLE II shows the numerical results for the color coordinates x and y, the visible reflectance RVIS, the solar reflectance reflectance Rsol, the solar transmission Tsol and the figure of merit M = RVIS/Rsol, in dependence on the thickness t3 of the third layer.

TABLE II:

CIE color coordinates x and y, the visible reflectance RVIS, the solar reflectance Rsol, the solar transmission Tsol and the figure of merit M = RVIS/Rsol, as computed for the curves displayed in Fig. 2.

c) Maxima of higher order

Towards shorter wavelengths, the reflectance spectra for dielectric thin film stacks exhibit rapid oscillations between the maxima and minima of higher order. By using high enough layer thicknesses, the maxima can be placed in the visible spectral region. Fig.3 illustrates an example showing the oscillations in the high wavelength region which are used for the coloration.

For e. g. a color of pink two peaks, corresponding to blue and red contributions, are necessary. The calculation is based on literature data for the refractive indices of SiO2 and TiO2 [27-29]. For

simplicity, coatings have been assumed to be transparent within the solar spectral region. Layer thicknesses of 213 nm and 258 nm are assumed for the TiO2 and the SiO2 layers, respectively.

Color coordinates of x = 0.39 and y = 0.24 have been found, accompanied by a visible reflectance of 9.8 %, while the solar transmission amounts to 84.6 %. Some more examples are listed in TABLE III. The selection comprises two, three and four layered systems. Layer 1 is the next to the glass. A refractive index of 1.52 has been assumed for the glass, the coating is supposed to be only on one side of the substrate.

Color coordinates x and y, the visible reflectance RVIS, the solar reflectance Rsol, the solar transmission Tsol and the figure of merit M = RvIS/Rsol, as computed for a variety of two, three and four layered systems.

d) Quarterwave stacks

In quarterwave stacks, all individual layers are of the optical film thickness n ■ t = 20/4 , where X0 is called the design wavelength. Usually layers of a high index material (H) alternate with layers of a low refractive index material (L), resulting in a stack of the form HLHLHL… . Often, these filters are employed as high reflectivity mirrors, exhibiting a nearly perfect reflectance over a large frequency band. The larger the difference in the refractive indices, the larger is the spectral region of high reflection. We are interested in the opposite, a narrow reflection peak, which can in principle be created by employing a large number of layers (e. g. forty layers for a linewidth in the order of 20 nm [30]). Here we consider a system, where both sides of the glass are coated each by a stack of five individual layers. Consequently, the structure of the optical model is air//HLHLH//glass//HLHLH//air.

The refractive indices n(H) = 1.65 and n(L) = 1.47 have been

chosen (in combination with a refractive index of the glass substrate of 1.52) . Our

calculation has been performed for various angles of incidence, rising from normal incidence in steps of 20°. A graphical representation is given in Fig.4, the figures are summarized in Table IV. For normal incidence, the FWHM amounts to 167 nm, the visible reflectance to 34 %.

The solar transmission (84%) is acceptable; the energy loss compared to an uncoated glass amounts only to 8 % . The angular dependence of the reflectance is nicely illustrated in Fig. 4. For an angle of incidence of 20° , the curve does not change significantly; the position of the maximum shifts slightly from 550 nm to 534 nm. For 40° , the shift continues to 499 nm. For 60 °, a blueshift in the order of magnitude of the peak width is accompanied by a rise of the background level. For 80°, the background level rises further, the peak is barely distinguishable, and the properties are quite close to those of uncoated glass (representation in Fig. 4 omitted for clarity). With increasing angle of incidence, the difference between the transmission of uncoated glass Tglass and the transmission Tsol of the coated system decreases, while the figure of merit M converges to unity. In TABLE IV a survey of the characteristic numbers is given.

TABLE IV:

Position of the reflectance maximum, color coordinates x and y, the visible reflectance RVIS, the solar transmission Tsoi, the transmission of an uncoated glass Tglass, the difference Tglass — Tsol between the latter and the solar transmission, and the figure of merit M = Rvis/Rsoi, as computed for the curves displayed in Fig. 4. The values for the angle of incidence of 80° are added.

By adapting film thicknesses according to the relation n ■ t = 20/4, the peak position X0 can be shifted easily. For design wavelengths X0 of 450 nm and 650 nm, colors of blue (x = 0.20 , y = 0.24 ) and orange (x = 0.44 , y = 0.36 ) are attained. The solar transmission increases slightly or changes barely ( 6 % and 84 %, respectively), while the visible reflectance decreases ( 17 % and 29 %, respectively).

Discussion

Several design types have been proposed to achieve an energy-efficient coloration of solar collector glazing. Known anti-reflection coatings such as the two-layered V — and W-design can be modified to create a colorful reflection in the visible spectral region and a region of antireflection e. g. in the infrared. This approach is especially suitable to create blue and green colors in combination with a high solar transmission. Addition of a third layer to a V-design leads to a strong enhancement of the reflectance peak. Already with such a three-layered coating design of moderate total coating thickness, considerable peak heights and thus a strong visible reflectance can be achieved. Therefore this design is attractive for large area coating processes where production costs scale with the number of individual layers and the total coating thickness. Another way to create a colored reflection is to make use of the maxima of higher order appearing towards short wavelengths. In this regime the shapes of the reflectance spectra become rather complex, but can be used to produce colors where more than one reflection peak is needed. A systematic approach to the problem of achieving a single, isolated reflectance peak is the one of quarterwave stacks. Already with stacks of five layers and a suitable choice of refractive indices, sufficiently narrow reflectance peaks can be produced. Interference colors are in general angle-dependent. This could both increase (nice effect, high-tech image) or decrease acceptance of the proposed collector covers, which is a subject of discussion with architects, manufacturers and end-users. However, for the shown example of a five-layered quarterwave stack, the peak shift at 60° angle of reflection is in the same order of magnitude as the half peak width. Additionally, if the multilayered coating is only applied at the inner side of the collector, the angular dependence can be reduced by diffusing elements, such as rough surfaces/interfaces or a diffusing interlayer. Common thin film deposition processes are magnetron sputtering, plasma enhanced chemical vapor deposition, vacuum evaporation, or SolGel dip coating. Transparent oxides such as silicon dioxide (n ~ 1.47), aluminum oxide (n ~ 1.65) , or titanium dioxide (n ~ 2.2) can routinely be deposited [24,31,32]. Intermediate refractive indices would be accessible by the synthesis of mixed oxides. Nanocomposite mixed oxides can be modeled in the framework of effective medium theories, such as e. g. the Bruggeman or the Ping Sheng theory [33,34],

both providing analytical expressions for the resulting optical properties. With all coating processes, care has to be taken for a superior film homogeneity, which is essential for interference filters. Vacuum processes yield in general high quality films, but a considerable investment into the vacuum coating machines is necessary already in the start-up phase. The scale-up of a vacuum process, which has been developed in the laboratory, is possible, but non-trivial [35]. This is much easier for SolGel dip-coating. Once the right solutions and withdrawal speeds are found, the size of the glass pane does not alter the basic process parameters. Here, one main problem is to avoid dust, which creates defects and harms the coating quality. Costs rise with the repeated baking of multilayered coatings on large glass panes. One solution to the problem can be special precursors, which enable a film hardening by ultraviolet light [36].

Conclusions

Multilayered interference filters of dielectric thin films have been designed for the application as energy-efficient coloration of collector cover glasses. The optical behavior of the designed multilayers is analysed by computer simulations yielding the CIE color coordinates, the relative luminosity, the degree of solar transmission, and a figure of merit which is a measure for the energy effectiveness of the coloration. For several types of multilayer design the computer simulations yield promising results. Hereby, constraints such as a realistic choice of refractive indices, a limited number of layers in the stack, and a not excessive total stack thickness have been respected. The way for the experimental realisation of energy-efficient colored glazed solar collectors has thus been opened up.

Acknowledgements

Financial support of this work has been provided by the Swiss Federal Office of Energy SFOE. Authors are grateful to Dr. I. Hagemann, and Dr. P. W. Oliveira for inspiring discussions.

Whether a crack in a brittle material initiates from a damage or not, depends on both, the size of the damage and the extent of the effective load. According to fundamental fracture mechanics relations a crack of the length a initiates, if the

Stress Intensity Factor K| = f о (n a)1/2

exceeds a critical value Kc, which is specific for the given material. о is the applied tensile stress and f is a factor which takes into account the particular geometric conditions. At low stresses о, therefore, large damage sizes a can be tolerated, and at minor damages large stresses can be borne.

Endeavours towards a reduction of loss due to fracturing in a process line, therefore, must aim at both, a reduction of the size of pre-damages in the wafers and a reduction of the effective loads. The manufacturing processes must be optimised under these aspects.

A process which was found to introduce often many pre-damages into a wafer is the wire-sawing from the ingot. An SEM micro-photograph of a
sawing trace is shown in Figure 10.

The influence of such sawing traces on the strength of the wafers is demonstrated in Figure 11. The strength values (arbitrary units) measured in concentric ring tests are plotted for a group of wafers with minor sawing traces (black bars on the left). For comparison the strength values for a group of wafers with distinctly visible traces are given (grey bars on the right). The sawing traces in this example reduce the strength by about 30 percent.

Obviously an analysis and improve­ment of the wire sawing process has a great potential for loss reduction.

One type of manufacturing processes which can implicate mechanical loads to the wafers are high temperature procedures. Due to rapid heating or cooling rates thermally induced stresses can develop. An example is shown in Figure 12. The wafers are etched in a hot bath, and subsequently the hot wafers are immersed into a cool bath (in order to stop the etching process). The quenching process was simulated numerically by means of finite element analysis and the stress field arising in the wafer was calculated. The areas of tensile stresses are represented in Figure 12 by dark zones. The maximum tensile stress occurs at the edges of the wafer on the position of the »waterline« (blackspots marked by arrows). By a numerical variation of the process parameters like temperatures, immersion speed, timing etc. the process can be optimised with respect to the mechanical loading.

Other high temperature processes in cell manu­facturing which can generate thermally induced stresses are tempering procedures like diffusion or fast firing. In Figure 13 the temperature of a wafer is plotted which was measured during its passage through the diffusion furnace (grey curve). The black curve represents the maximum thermal tensile stress induced in the wafer. The stresses were calculated numerically applying finite element analysis. The peak value of the tensile stress in this example is in the order of 40 MPa. Pre-damaged wafers can be destroyed by stresses of this magnitude.

Relatively large stresses can also arise during the tampon printing process for the purpose of metallization, especially with not perfectly planar wafers. Therefore, the deformation of the complete system tampon — wafer — underlay was analysed by means of numerical modelling and simulation and additional verification experi­ments. The resulting stresses in the wafer were calculated. An example is given in Figure 14. A tampon (not drawn in the Figure) is pressed against a non-planar wafer resting on an underlay. The deformation of wafer and underlay can be seen. The stress field in the wafer is represented by the grey colouring (black symbolises high tensile stress). The picture on the top of Figure 14 is valid for a stiff
underlay. The maximum stress (set to 100 percent) occurs inside of the curved segment of the wafer. When the underlay underneath the curvature is made from a soft material, the area of high stresses is spread over a larger region of the wafer (picture in the middle), and the stresses in the elbow become significantly smaller (83 percent). A continuous transition of the stiffness in the underlay leads to a further decay of tensile stresses in the sensitive elbow region (45 percent, picture below).

Applying such numerical simulations it could be shown by variation of the influencing process parameters that the tensile stresses caused by the tampon printing procedure can be reduced by a factor of 3.7

During the last few years operational experience with solar systems was gained in Germany from the "1000 Dacher Programm”, a government programme economically supporting the installation of solar equipment on the roofs of buildings. A number of reasons for system unavailability were discovered [1], as shown in Table 1, the main reason being component failures.

Table 1: Percentage of failure causes

Not only equipment failures were monitored, but also repair times and skill requirements for repair were recorded in [1] (cf. Figure 2). All of these aspects are addressed in the simulation procedure.

Not every failure directly leads to the unavailability of the energy supply system. Depending on the system configuration and timeliness of repair production may be maintained, as can be shown by simulation.

Using building integrated photovoltaics BIPV can contribute to lowering the need for purchased electric energy and increasing security of supply for the project. It can also make the good environmental profile of the project more visually present.

Section east — west

Installing PV on the roof is one of the most obvious solutions in the building, as the large, unfragmented surface receives abundant sunlight. An area of more than 2000 m2 is well suited for PV installation, and flat roof elements is preferred to maintain the clear outline of the building. Elevating the northern roof corner of the closed volume to the west, would make the entrance situation more expressive and dramatic and the roof surface very feasible for PV. Using excess heat from the PV for domestic hot water will increase the efficiency of the system.

The skylight area is about 350 m2, but is unfortunately tilted to the north, which make it unsuited for PV integration. Tilting it to the south would not deteriorate light conditions noticeably, as the light reaches it destination as secondary light anyway. This location is a good place to express the environmental profile of the building through crystalline PV arrays with well dimensioned and generous spacing between the cells. This will give interesting light qualities in the central atrium, which is dominating the interior. Installation in large sections on the windows is not feasible with the shade conditions on the fagade.

Only a ribbon on the lower part of the windows is suitable in terms of output, but a fixed installation here would affect the view from the work stations in a negative way.

The walls have large proportions of window area and are curved. Overhanging canopies and galleries make them poor areas for installing PV. Some narrow belts can be added at the edge of the external galleries to the south, but as this can look aesthetically detached from the fagade, it is not recommended. The canopy and external galleries to the south create complicated shading conditions. This makes wall integration of PV somewhat difficult.

Using PV as solar shading is a logical response to regulating the solar conditions to the south. Presently, local shading is handled with interior blinds. This solution is cheap, but can contribute to high interior temperatures. Using exterior elements to increase the efficiency of the PV elements and at the same time reducing internal heat load is the most feasible approach. The large window surfaces and overhangs create complex conditions. The elements need to be placed on the exterior of the galleries to avoid shade. The distance between the wall and the elements is favourable in terms of ventilating the elements. Mobile elements can be connected to the web system of workplace control. Sun tracking PV elements can give good electricity outcomes of the system and give the fagade a dynamic expression.

The slightly tilted canopy that runs along most of the edge of the roof is well suited for installing PV in a section. It is important to use canopy elements that have both a PV and a regular version with the same expression.

The building is placed on the south west corner of the site, bordering to a green bridge structure that covers the highway. The triangular site has an area to the north and to the east, towards the river. The northern end of the site lies somewhat undefined behind the car access. Free standing sculptural PV elements can be installed on this part to create an interesting and more elaborated entrance situation. PV can be fitted over some of the guest parking, but this would make the entrance situation quite crowded. It is therefore not recommended.

The east side of the site has an open area over the subterranean cafeteria. Using transparent PV elements could supply this important facility with abundant natural light and give it less of a dungeon feel. Several smaller light shafts can make the underlying space evenly lit and less dominated by the installation, and would fit better in the terrain than one large shape.

Sketches illustrating some concepts for PV application and facade consequences, including integration on two roof surfaces, the skylight, in light shafts, in the terrain and as long lamellas on the south fagade for shade.

Sketches illustrating some concepts for PV application and facade consequences, including integration on two roof surfaces, the skylight, in light shafts, in the terrain and as a large sliding sheet on the south facade.

The project is finished with most of the initial investigations of the building and the site, and will continue with elaborating different concepts for PV integration. Common approaches of applying PV on roof surfaces give the least implications for the aesthetical image, but traditional methods for applying PV on the facades have proved difficult concerning both solar access and aesthetics. The project continues to investigate further possibilities for PV solutions for facades and for installation on the site that can contribute to a dynamical expression and adding features to the building that can improve the working environment.

References:

"Tomorrows office”, Santa Raymond, Roger Cunliffe "The creative office”, Jeremy Myerson, Philip Ross The article “Drontene”, Sissel Morseth Gromholt

The report “Passsive climate measures for Pynten”, Tor H Dokka, Marit Tyholt

Michael Niggemann, Markus Glatthaar, Andreas Gombert, Andreas Hinsch, Volker Wittwer, Birger Zimmermann, Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr.2, 79110 Freiburg, Germany

The common architecture for organic solar cells is planar with the organic light-absorbing layer sandwiched between a semitransparent ITO electrode and an aluminum counter electrode. The presented work focuses on the development of alternative solar cell archi­tectures wherein substitution of cost-intensive ITO is envisioned with respect to an optimi­sation of the device performance. The pursued approach is to built up organic solar cells on micro structured substrates. Two types of structures — micro prisms and buried nano electrodes — are under investigation. The micro prism structure with a period of 100^m can be regarded as a folded planar cell. The benefits are an increased absorbance due to a twofold reflection at the photoactive layer and the substitution of the ITO-electrode by a metal-grid supported polymer electrode. Besides experimental results, optical simula­tions and the calculation of optimum dimensions are presented. The substrate for buried nano electrodes is made by replication of holograpical originated structures. At least one planar electrode — preferably the ITO-electrode is substituted by a comb-like array of verti­cal electrodes embedded in the active polymer. The substitution of both planar electrodes by buried nano-electrodes results in a interdigital electrode set-up. The period of the mi­cro structure is in the range of the light-wavelength (720nm) of the incident light. First de­vices have been built and the results are presented.

Introduction

The bulk heterojunction concept for organic solar cells (OSC) based on a donor — type conjugated polymer blended with methanofullerenes forming an interpenetrat­ing donor/acceptor network is very promising [1]. A power conversion efficiency of 5.0% under AM 1.5 illumination has been reported for these cells [2]. Poly — 3(hexylhiophene)(P3HT) and 3,7-dimethyloctyloxy methyloxy polyparaphenylene vinylene (MDMO-PPV) are examples for donor-type conjugated polymers in combination with the electron acceptor [6,6]-Phenyl C61 — butyric acid methyl ester (PCBM, a C60-derivative). The most commonly investigated planar OSC is built up as follows: An ITO (indium tin ox­ide) coated glass or plastic substrate is coated with a p-conducting conjugated polymer poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) named PEDOT/PSS forming the p-contact. Subsequently the photoactive layer is spin-coated. The n-contact is repre­sented by an evaporated aluminum electrode. The efficiency is limited in this case by the small absorbance of the two photoactive components in the range of the solar spectrum combined with the necessity of a small film thickness of the absorbing layer. The thick­ness of the absorbing layer (100-200nm) is restricted by the limited transport properties of at least on type of charge carriers. Increasing the film thickness significantly will lead to a reduction of the fill factor and the overall device efficiency. New cell architectures should contribute to a cost reduction by substituting ITO with the challenge of improving the device efficiency. Microstructured plastic substrates which can be made in a replica­tion process offer a wide range of design possibilities. The two presented concepts, micro prisms and buried nano-electrodes, differ in their dimensions of the structure by two or­ders of magnitude (figure 1a/b). The micro prism structure can be regarded as a folded planar cell (figure 1a). The dimensions of the micro prisms (lattice distance=100(um) are three orders of magnitude larger than the thickness of the applied organic films. Light from normal incidence on the substrate is reflected twice and contributes to a gain in ab-

sorbance. The ITO-electrode is substituted by a highly doped p-conducting polymer layer (PEDOT CPP105d) with a supporting metal grid which is located in the grooves of the prism structure. Optical simulations were performed with supporting experiments. The thin film system is treated with wave optics and the prism structure can be well described with geometrical optics. A significant increase of the overall light absorbance in contrast to a planar device was calculated. It has to be emphazised that the prismatic structure has to be regarded as an example for a linear microstructure. Other types of structures with different shapes like paraboloids are under investigation. In contrast to the concept of the micro prism solar cell, the dimensions of the buried nano electrodes are compara­ble to the thickness of the organic films. The period of the structure is in the range of the light- wavelength (720nm) of the incident light. Two concepts — asymmetric and interdig­ital buried electrodes — are presented (figure 1b). In the case of buried nano-electrodes at least one planar electrode — preferably the ITO-electrode is substituted by a comb-like array of vertical electrodes embedded in the active polymer. The influence of asymmet­ric electrodes on the charge extraction is an evident question and has not been answered yet. In this case, the description with wave optics becomes a necessity.

Certain problems of PDISPL implementation can be solved in pulsed mode, including :

— Photo-electric transformation of PDISPL radiation;

— Specification of the products generated at PDISPL operation;

To solve these problems a laboratory bench was developed and fabricated featuring the following characteristics:

— duration of the pumping pulse 1.510-3 sec;

— spectral distribution of the energy as in black body spectrum with luminescent temperature 3500 — 5000 K;

— intensity corresponding to ~3105 solar constants (~4-104W/cm2).

A crystal of GaSb made in the Institute of Physics and Technology by Ioffe (FTI) was used as the conversing element. Duration of laser pulse was 650 ^sec and the energy, after release, was 1.1 mJ or 2.2 mJ. These values were chosen jointly with FTI for obtaining the optimal conversion ratios of laser energy into the electrical one.

Selection of the operating composition for iodine laser pumped by solar light is one of the fundamental problems. All the experience of experimental studies of iodine photo-dissociation lasers offers the conclusion — the only class of compositions suitable for implementation as the operational substances are the saturated fluorine organic compositions with one iodine atom, the so-called perfluoroalkyliodides (PFAI), having the common formulation RFI, where RF are fluorine substituted radicals of different construction (linear, branched, cyclic, with ether groups, etc.) The literature [4-6] cites the data on the following compositions: linear: CF3I — C6 F13I

isomeres: iso-C3F7I, 2- C4 F5I, t-C4F9I, 2 — C6 F13I ethers: C3F7OI, h30-C3F7OI

The available spectral-kinetic data on these compositions are presented in tables 3,4.

The following conclusions can be done on selection of the operational compositions for PDISPL based on the data presented in the tables.

Presently the best choice for PDISPL is t-C4F9I. Firstly, this composition possesses relatively wider absorption band with the maximum shifted to the "red” region as compared to the majority of optional compositions. Secondly, t-C4F9I possesses the unique kinetic characteristics. The constant of recombination velocity into the original state for this composition is twice as higher as for KR+R, withdrawal of radicals in reaction 2R-o — R2. This fact can be possibly explained by the necessity of overcoming

the conformational energy barrier at formation of (CF3)3 — (CF3)3 as opposed to reaction I+(CF3)3 -o — (CF3)3I, following the normal radical, that is without activation energy mechanism. The drawbacks of t-C4F9I include its aggregate state: within the range of normal temperatures and pressures this composition does not have the liquid state.

Conclusions

The resulting PDISPL model will be implemented in follow-up applied research activities. One of those implies further research aimed at optimization of the effectiveness of laser energy conversion into the electrical one at 1.315 pm wavelength. Above all, the technologies and methods developed will be implemented for creation of more powerful simulators of solar radiation.

The scientific results obtained on different spheres of interest cannot be named final. The authors have highlighted the ways for upgrading the effectiveness of PDISPL model together with updating the methods pre-developed.