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BIPV INSTALLATIONSWORLDWIDE INASP ECHNOLOGYH. MauNs, M. Schmid, B.Blersch, P. Lechner, H. SchadeRWE SC HOTT Solar GmbH, PhototronicsHermann-Oherth-Strasse 1 I , 8 5 64 0 P u t z b ~ n n , e r ma ny

ABSTRACTThe thin-film technology based on amorphous silicon (a-Si) offers a range of attractive features that are ideallysuited for building-integrated photovoltaic installations(BIPV). Solar modules may be assembled to custom-specific BIPV elements for roofs and facades, and thusmay combine various functions, namely electricitygeneration, thermal insulation, shading, and evcn satisfyaspects of architectural design. Some of these functionsrely on the fabrication of semitransparent modules thatexhibit a color-neutral see-through effect (AS1 THRU").Compared to other PV technologies, a-Si modules showonly a minor reduction in power output at elevatedtemperatures and at lower light levels, and thus offersuperior energy yields per peak power. Recent BIPVinstallations worldwide demonstrate various designpossihilitics and build on the track record of pastinstallations since 1992.

1. INTRODUCTIONAmong the thin-film technologies to date only thosebased on thin-film silicon have reached a significantcontribution to the PV market worldwide. Althoughhistorically the amorphous silicon (a-Si) technology was

mainly used for consumer applications, it has morerecently been increasingly applied to large-area bu ilding-integrated electricity generation. This trend is in line withthe aim of providing peak power to electricity grids, andthus to contribute to the goals o f attaining about 10 G Wof PV power installed worldwide by 2010. The suitabilityof a thin-film technology based on silicon for building-integrated photovoltaics (BIPV) is suppo rted by a numb erof attractive features that apply to the technology ingeneral, and to BIPV in particular. Equally important, therealization of these features has already beendemonstrated with impressive examples, and ultimatelymay also econom ically prove to be attractive.

2. FEATURES OF THEa-Si BASED TECHNOLOGYInspite of lower stabilized efficiencies obtained afterlight-induced degradation and photostabilization(Staehler-Wronski-effect) [I], there are several attractivefeatures that distinguish thin-film silicon technologiesfrom other PV technologies, namely:they are silicon-based technologies, this implies an

abundant materials supply and non-toxicconstituents,they involve low process temperatures, thisfacilitates the use of low-cost substrate materials,such as float-glass, and leads to moderate energy

consumption, as well as short energy payback times121,they allow for large-area deposition processes hyplasma-enhanced chemical vapour deposition(PECVD),they are often based on two-terminal stacked cellstructures, allowing thereby for highervoltagellower current devices, and for theincorporation of different band gaps for extendedspectral response,they possess low temperatwe coefficients of thephotovoltaic data [3],they permit a monolithic series connection of cellsto modules, and, hence, a variability of outputvoltages,they have a very substantial cost reduction potential

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~41 .All these features lend themselvcs very advantageously toapplications in BIPV, as will be detailed below, in termsof both achievable building-relevant p roperties andenergy yields, and may verf well outweigh the onlydrawback of a-Si PV, namely the lower efficiencies, ascompared to crystalline solar modules.

3. A-Si MODULE INCORPORATION FORBIPVA-Si modules applied in BIPV combine as a singlecomponent

electricity generation,D shading and glare protection,

architectural design.thermal insulation,

Depending on the particular application, thesefunctions may be differently weighted. Below theirindividual relevances are described .3.1 Electricity generationThis function is of course central to any BLPVapplication. It is governed by the photovoltaic propertiesof the solar cell structure, specifically by their spectraland temperature dependencies. Compared to thecrystalline silicon (c-Si), a-Si modules are morefavourably adapted to the operationa l conditions.The spect ral response, due to the high energy gapof a-Si, provides high blue response and low red response.The spectral factor [ 5 ] , defined by a ratio of short-circuitcurrents measured under the actual illumination andstandard AM 1.5, respectively, serves to quantifydifferent spectral responses. At a given site, for a-Si thespectral factor reaches its highest value both during

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midday and also during the seasons from spring to fall,while the opposite applies to c-Si. These periods, bothdaily and seasonally, correspond to lower AM values, andcoincide with higher levels of insolation [ 61 . AS aconsequence, the energy yield (generated energy per peakpower) of a-Si modules may he higher than that of c-Simodules (see below), provided the spectral factor SF>I ,which for the same insolation period implies SF

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Obviously, the efficiency of semitransparent modulesis reduced by the percentage of their optical transmission.However, besideselectricity generation theyserve additional functionsin terms of light andtemperature management inbuildingsFig.4: Cross section of an ASIT HRU " doubleglazing unitDouble glazing units with integrated SemitransparentAS1 THRUmmodules, as shown in cross section in Fig. 4,combine the following properties essen tial for low powerbuildings:- The total energy transmittance (g-value) reachesvalues down to 10%. which usually cannot bereached by conventional shading systems, such asvenetian blinds. When designing large-area glassfacades or sky lights, a low g-value is important tolower the cnergy requirements for air conditioning.The heat transfer coefficients (U-valucs) are similarto those of conventional double glazing units,typically I .2 Wim'K.

3.3 Archi tectural designThe app licability of a-Si modules in B IPV is manifoldand extends to all elements of a building shell, namelyroofs, facades, windows, doors, and awnings. Althoughthe substrate sire for the semiconductor deposition isfixed by the particular deposition equipmcnt, the finalmodule sizes can be adapted to the specific size of thedesired building element by cutting and formation oflaminates. Thus a high degree of flexibility in design isachieved by incorporaling the modules, opaque orsemitransparent, through more or less conventionalmethods used in the building industry, such as laminatingtechniques, or mounting and fixturing procedures.Thereby static requirements and many building coderegulations can automa tically be m et, or obtained in closecooperation with these industries. As an example,glassiglass construction with PVB sheet lamination hasrecently been qualified for overhead glaring applications[ 131. The feature of semitransparcncy described aboveprovides a wide range of additional design possibilitiesthat may s e n e energy management in buildings, l ivingcomfort, and aesthetics.3.4 Cast considerat ionsThe usual valuation of PV products is made in termsof priceiWp, However, for BIPV the valuation is moresuitably based on pricelm'. This type of pricing resultsfrom a number of overriding aspects that must beconsidered. namelyenergy yield in order to determine the return obtainedfrom fee ding electricity into the grid, as w ell as fromsavings in conventional utility costs, temperaturecontrol installations and s ervice;the cost comparison with conventional roof or faqadebuilding elements;the particular design, and the combination ofelectrical and thermal fun ctions;the aestethetic appearanc e.

With the examples shown below various combinations of 'functions, design, and appearance of BIPV realized in a -Si technology worldwide are demonstrated.

4. BIPV EXAMPLES4.1 B avarian Minist ry of Environmental Protect ion,MunichThe faqade of theBavarian Ministry ofEnvironmental Protectionin Munich, Germany, isone of lhe oldest PVfaqades based on a-Simodules. The poweroutput of the 6.5 kWp PVfaqade has been monitoredover a period of about 9years [12], compare Fig. 2.The faqade is built byusing an aluminium profilesystem, and was completedin Aug. 1993.

Fig. 5: ASI' Faqade of the Bavarian Ministry ofEnvironmental Protection4.2 Suprem e Building Authori ty of the State ofBavaria, M unich

Fig. 6: Semitranspare nt light roof installation of lheSupreme Building AuthorityThe light roof of the Supreme Building Authority inMunich, Bavaria, has been realized with ASITHRU'semitransparent modules. Thc system was installed inM ay 1993 and was one of the first installations with suchmodules. The 2 .8 kWp system is installed to provideshading of a cafeteria area.

4.3 Universidade Federal de Santa Catarina,FlorianopolisIn September 1997, the Universidade Federal deSanta Catarina, Florianopolis, installed the first thin-filmBIPV system in Brazil. The performance data areavailable elsewhere [141.

4.4 University of Hong KongIn October 2000, an a-Si test facade has beeninstalled by the University of Hong Kong. The system hasbeen monitored since installation, performance data areavailable elsewhere [IS].

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4.5 Paul L oebe Haus, Berl inThe Paul Loebe Haus in Berlin is used by themembers of the German Parliament for ofices andconference rooms. The building roof is designed with6048 semitransparent solar blinds. The one-axis trackingallows higher electrical yield and allows sun control forthe interior. The total power of the installation reaches123kWp. At the date of installation in April 2001, thesystem was the largest building integrated PV installationusing semitransparent thin film modules.

Fig. 6 : SmartSolarFab'The south facing facade of RWE SCHOTT Solar'sproduction facility in Alzenau has been build withASITHRU" double glazing units. Beside elecmcityprod uction , the 540111 faca de provide ther mal insula tionand shading to the building. The system has beeninstalled in 2002.4.7 Phase 11 Facilit ies of the Disaster Reduct ion andHu ma n Renovat ion Inst i tut ion (DIU), Kobe-City

Fig. 7:The DRI Facilities were created as a response to the

great Hanshin-Awaji earthquake in Kobe-City. For themain building windows between the 3d and the 71h floor,customized AS1 THRU" laminates provide shade to thebuilding interior and glare protection. The grid-connectedsystem has a total power of 20.6 kWp.4.8 St il lwell Avenue Te rminal , New Y orkAn almost 6000m2 project for New York City'sTransit will be the world's largest BlPV thin filminstallation when completed in 2005. It uses custom-designed modules constructed for overhead-glazing. Thepartially transparent glass laminates are a combination ofclear glass strips and thin-film a-Si sola r modules.

Building of the DRI, Kobe-City

Fig. 8: Stillwell Avenue Terminal (artist'simpression - Copyright NYCT)AcknowledgementPortions of this work are derived from research anddevelopment partly funded by the GermanBundesministerium fur Wirtschaft und Technologie.

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M. Bottger, Proc. lo th European PVSEC, Lisbon1991,pp. 1184-1187[I31 Deutsches lnstitut fir Bautechnik, Berlin[I41 R. Riither, M. M. Dacoregio, A. A. M ontenegro,Proc. 17th European P VSEC , Munich (2001), p. 2697[I51 K. H. Lam, J. Close, E. W. C. Lo , Proc. 17thEuropean PVSEC, Munich (2001), p. 820

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