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Wooden membrane with integrated flexible photovoltaic foil
Walter Klasz*, Alexander Grasser ², Michael Flach³, Pavel Sevela 4
*Institute of Design, University of Innsbruck,
Innsbruck, Austria, Walter.Klasz@uibk.ac.at
(2) Institute of Experimental Architecture, University of Innsbruck
(3) Unit for Timber Engineering, University of Innsbruck
(4) Unit for Energy Efficient Buildings, University of Innsbruck
Abstract
This paper presents a method to design lightweight architecture out of bent timber with integrated
photovoltaic foil as a structural and waterproof building skin.
The first chapter explains the physical form-finding process of bent only configurations where material fatigue
plays no relevant role in the long time load capacity of the architecture. The final configurations consist of
pressure arches and tensile surfaces self-interlocking each other. The geometrical freedom of possible forms
is primarily determined by the limits of bending plywood merged with photovoltaic foil into double curved
forms.
The second chapter presents the benefits in the assembling process. Elastic bending behaviour of wood is
used to build membrane-like structures without the necessity of any scaffold nor using heavy foundations.
Parametric digital design – respectively the software rhino-grasshopper – supports the optimization of the
forms providing digitally related construction files for an effective workflow simultaneously.
The last chapter analyses exemplarily one architecturally relevant configuration – a hybrid between a
membrane and a shell – and presents the calculation of its electrical efficiency related to the form.
Figure 1: Collage: physical and parametric design to apply flexible PV-foil on structural wooden membranes
Keywords:
Integrated Photovoltaic, Sustainable Lightweight-Design, Assembling process, Active Bending Wood,
Constructional Membranes, Wooden self-formed shell
1. Form-finding process
1.1 The form finding concept of bent wooden membranes
1.1.1 Initial situation and context
The basic idea to facilitate the building process of curved architecture by using the elastic bending behaviour
of materials is an ancient concept and was recently rediscovered in contemporary architecture as
summarized in the paper Active Bending from the year 2013 [1]. In 2015 Klasz has presented the built case
study A cloud for fresh snow [2] explaining the concept to use membranes in a hybrid configuration with
active bending wood. The benefit is that material fatigue of wood – the reducing bending force – plays no
relevant role for the long time load capacity of the structures [3]. Based on this knowledge this paper
presents the new concept to replace the textile membranes with wooden membranes including integrated
photovoltaic foils. All components are planar wooden elements: The primary structure consists of longitudinal
wooden boards. The secondary structure consists of thin planar plywood merged with photovoltaic foil.
Figure 2: Collage Spherical tetrahedron: left: a cloud for fresh snow (wood + textile membrane); middle: geometry
drawn with rhino-membrane; right: geometry built in a model out of straight wooden members
In contrast to grid shells like the Multihalle Mannheim by Frei Otto in 1975 or the Polydôme at the Campus
Lausanne [4] by Julius Natterer (see Fig. 3) this paper presents statically self-interlocking configurations
replacing heavy foundations and simplifying the assembling process. The main difference concerning the
assembling is that the final form is realized from the beginning of the installation process by bent timber-
boards in a self-organizing process. The second difference deals with the building skin: Instead of putting a
waterproof layer on top after the construction is finished – like done in Mannheim and Lausanne – the new
concept consist in using waterproof PV-wood-panels as a structural building skin. The basic typologies (see
chapter 1.2.1 and 1.2.2.) present solutions, where the structural skin works in the final configuration statically
as a membrane and not as a grid shell. Hybrid typologies (see chapter 1.2.3.) include both: membrane-like
surfaces and grid-shells merging into each other smoothly.
Figure 3: Polydôme at the Campus EPFL Lausanne by J. Natterer; left: Photo: W. Klasz 2015; middle: axonometry of
the structure; right: photo of the assembling process of the straight wooden members up to 19m; images middle and right
see Schweizer Baublatt 1992 [4].
1.1.2 Bent primary structure
Active Bending simplifies setting up the curved primary structures. In the final configuration, these elements
are hold in position by the membrane-like secondary structure and therefore these compressed bows
(pressure arches) have geometrically no moment-forces – although the remaining elasticity supports in the
redundant structure the stiffness according to different load cases. Relevant load cases to dimension these
elements are those during the assembling process. The self-weight of the elements limits the upscaling of
this concept (see perspectives). Selected boards of the secondary structure are installed to get the primary
structure bent in its wanted position (see Fig. 4 – image in the middle). In other words, parts of the secondary
structure bend the primary boards in their position, while finding their own position simultaneously.
Figure 4: Collage onion-like structure: left: three primary wooden members joined at the ends to each other; middle:
three members of the secondary structure bend the primary members; right: the longitudinal secondary members bend
the vertical ones easily in the membrane-like configuration.
1.1.3 Bent secondary water-prove membrane-like structure
The secondary structure is continuously completed causing the more and more precise form-finding of the
primary structure and the whole configuration. After having installed a tight grid of boards the thin plywood
sheets with integrated photovoltaic-foil are fixed to the structure. One benefit is, that all sheets are cut out of
plane plywood without any pre-curving in the third dimension. The other advantage is the easy installation of
these parts to the grid and the individually possible maintenance. In the final configuration, they work
statically as membranes with mainly tensile forces, whereas the outer primary bows have pressure forces
only. The more the bending force of the wood releases (increasing material fatigue) the more precisely the
materials find the wanted self-interlocking position. This process can be improved by pre-stressing the
tensile surfaces, which can effectively done by a prolongation of the outer primary beams (the detailing will
be done in ongoing applied research on the planed full-scale case study – see perspectives.)
Figure 5: Statically self-interlocking models with partly fixed membrane-like elements out of thin plywood
left: spherical tetrahedron; right: onion-like configuration.
1.1.4 Limits and freedom of bending the PV - wood panel
The Austrian company Sunplugged is developing in ongoing research machines to produce flexible
photovoltaic foil of a width of 40cm and a length of maximum 200cm. The foils can be produced in any planar
form and fixed so precisely to each other that it cannot be seen visually from a distance of more than one
meter. As the investigation of the behaviour and function of the foil is more precise with one single piece of
foil (less complexity – no connection points) the dimension for the test-panel is chosen with 40cm to 80cm –
a possible grid for several full-scale projects.
The photovoltaic-foil – thermally covered with a waterproof translucent material – is clued to a 4mm sheet of
birch-plywood. In order to simulate different curvatures the wooden grid of boards of a cross section of
60mm to 12mm is bent by steel cables fixed with screws at each corner of the test-panel (see Fig. 6).
Figure 6: Prototype 01 of a bent PV-wood-panel with a low-tech water-prove assembling system
1.2 Self-interlocking configurations and potential applications
1.2.1 Spherical tetrahedron
The spherical tetrahedron (see Fig. 7) is a space-symmetrical form out of six pressure arches and four
membranes statically self-interlocking each other. The commissioned research lab A cloud for fresh snow
uses this basic form in a more complex configuration adding 3 high points at each façade-surface. For a
habitable use of this structures the roof can be realized in a wooden PV-membrane with an offset wooden
layer (as can be seen in a different configuration in Fig. 12) providing a stronger resistance against snow-
loads. During the assembling, the first layer works statically like a membrane and with the wooden offset-
layer the roof becomes a self-formed shell-structure. Turning the form upside down and scaled in the vertical
dimension (see Fig. 7 - below), it can be developed for example as a hard tent – lifted off the ground.
Figure 7: Spherical tetrahedron and applied variations
above: commissioned research lab “A cloud for fresh snow” 2015 [2 und 3].
below: project “Hard tent” 2015 by W. Klasz + M. Stefanova, V. Menegon, J. Marx, C. Harm
1.2.2 Onion-like structure
The onion-like structure is an axial symmetrical form consisting of three pressure arches and three
membranes statically self-interlocking each other. The proportions and curvatures can be adapted to the
different applications of longitudinal spatial solutions. Figure 8 shows exemplarily two applications in the field
of product design: A self-formed sledge and a self-formed wooden kayak – both easily built up purely using
the skin as a structure without any ribs.
Figure 8: Onion-like structure and applied variations
right above: Compact sledge 2015 by W. Klasz + N. Meyer, T. Wopfner
right below: Concept model by W. Klasz for a bent only self-interlocking kayak
1.2.3 Redundant hybrid forms between a shell and a membrane
This third typology bases on the concept to merge bent pressure arches planar with membrane-like surfaces
in a configuration of two self-interlocking four point sails. The bending only assembling method determines
the primarily form-finding parameters and leads away from the four point sails. The resulting forms work in
some areas statically like a shell and in others like a membrane – depending on the load cases. This has a
relevance in terms of wanted redundant constructions providing the necessary security for users.
Figure 9 shows on the left hand side a parametrically designed basic form. On the right hand side, the more
complex form of the self-formed PV-wood-lounge represents one possible application [5].
Figure 9: A self-formed lounge [5] – developed by W. Klasz as an exemplarily architectural application for the wood –
PV panels in a self-forming and statically self-interlocking configuration (note: the basic parametric model was done after
the experimental physical studies).
2. Simplified assembling process
2.1 Using elastic bending to largely self-form the wooden membrane-like structures
Figure 10: Collage of bent only structures comparing: left: Assembly with boundary bows; right Assembly by bending
the PV-Wood panels without boundary bows (primarily bows only – based on the parametric model)
The concept to simplify the assembling of double curved wooden structures bases on the idea to install the
architecture without a scaffold and without ribs by providing a self-formed primary framing, which merges
with the finale structure visually and statically fully integrated. Instead of putting photovoltaic at the end onto
the structure, the PV-wood-panels – respectively the longitudinal boards (see Fig. 10 right hand side) are
used themselves to let the structure emerge in the wanted form.
2.2 Using parametric digital design to optimize efficiency
The artistically and statically optimized form-finding is done experimentally with the help of scaled physical
models following the elastic bending behavior of wood (see Fig. 10 left side). Using the software rhino-
grasshopper this forms are defined and controlled digitally in a second step. Efficiency is optimized by the
designer on three levels: statically, aesthetically and energetically.
A holistic approach for sustainable design bases on the concept that aesthetics plays an important role in the
socio-cultural acceptance and as such in the lifecycle of a building [6]. The designer David Trubridge [7]
stresses the fact that beauty matters (title of many of his public speeches). This expert on bending wood
uses physical studies and digital design-tools always simultaneously in the field of furniture and lighting
design. The beauty of the assembling process has a close relation to the aesthetics of the built structures.
Aesthetics and self-formed assembling have a close relation. Parametric digital design enables the changing
of proportions due to the specific user, the different bending behavior of local materials and due to different
site-conditions.
Figure 11: Laser-cut PV-wood-panels optimized in form and dimension by digital parametric design
Statically the optimization bases on the concept of a redundant structure. Some areas of the structure (Fig.
11, 12) work statically like a wooden membrane, others temporarily or locally like a grid-shell and in the final
configuration as a composite-shell consisting of two wooden double bent surfaces – one inside and one
outside merged with the flexible PV-foil providing the waterproof skin. Depending on the positioning of the
foundation points and the load-cases the surface works statically as a membrane or a shell (a deeper
research on this topic will be done in ongoing investigations – see perspectives).
Figure 12: Fixing the second wooden-surface onto the structure providing a self-formed composite shell
3. Electrical Energy Concept
3.1 Electrical self-supply and waterproof roofing versus optimized orientation
Based on the explained concept of fully integrated photovoltaic foil in the structural building skin, the focus of
the electrical concept is to achieve a holistic solution achieving energy gaining surfaces during the whole day
and during different seasons. The fact that in this case some areas are only during specific times electrically
efficient, is compensated with the general benefit of providing a waterproof structural building skin.
This skin may be the structural roof as shown in the concept of a summer lounge (Fig. 13 left) or it may be a
double curved structurally working parapet as shown in the concept of a winter-chalet (Fig.13 right). The
basic digital parametrical model (Fig. 9 top left) can be used to develop customized applications in the
interdisciplinary team, where the client and the site play an important role, too. The holistic solution is found
in a self-forming design process [8].
3.2 Case Study: A self-formed lounge
The self-formed lounge presents exemplarily the application of structurally working PV-wood-panels.
Architecture is about putting material into the best relation to each other providing attractive space for
humans. The concept follows the idea of a natural leaf: A leaf combines structure, skin and photosynthesis in
a functional and aesthetical appealing way and it grows without any scaffold. The case study a self-formed
lounge tries to apply this approach for energetically self-sufficient self-formed bent only architecture.
Figure 13: A self-formed Lounge – Analyzing the coherence between structural PV-wood-panels and the architectural
concept: Conceptual sketches for possible summer and winter applications by W. Klasz; Analyses of Efficiency of
surfaces based on the location Innsbruck by P. Sevela (left: summer, right: winter)
4. Summary
This paper proves following integrated solution in a conceptual stage: flexible photovoltaic foil - glued to flat
plywood - provides a PV-wood-panel, which serves as a structural building skin. Three types of geometrical
configurations – all of them able to be assembled out of flat bent wood without any scaffold, foundation or
ribs necessary – present a new fully integrated light weight wooden design language. Due to different
applications, membrane-like surfaces become a shell by fixing an offset wooden layer. Parametric digital
design control allows an effective adaption to the specific use and site and it provides production files
simultaneously making the double curved statically self-interlocking and self-formed structures affordable
and competitive.
5. Perspectives
The interdisciplinary group (conceptual designer, parametrical expert, timber engineer, developer of the
flexible PV-foil and engineer for energy-efficient buildings) works in ongoing research on the presented topic
focusing on the optimization for a full-scale prototype. The research focuses following issue: How and how
far can materiality and energy be reduced in the production and assembling process of the presented self-
interlocking bent only configurations? Secondly, the limits in scaling up this structures – related to their
holistic efficiency – will be investigated.
6. Acknowledgement
This work was partly supported by the Austrian Klima-und Energiefonds within the framework of the research
project SynerCIS- Synergizing Austrian Breakthrough Innovations for CI(G)S solar cells.
The flexible PV-foil for the bending-test was given by Andreas Zimmermann, CEO of www.sunplugged.at.
Special thanks also to the Workshop team at Unitec, NZ, to the Workshop-Team at the unit Structure &
Design, University of Innsbruck, to David Trubdrige and to Dr. Switbert Greiner - Founder of art-
engineering.net.
7. References
[1] Lienhard J., Alpermann H., Gengnagel C. and Knippers J.: Active Bending, A Review on Structures
where Bending is used as a Self-Formation Process; Institute of Building Structures and Structural
Design, University of Stuttgart, Germany; 2013
[2] Klasz W., Filz G.: A Cloud for Fresh Snow
‐
Research Lab – a hybrid solution of minimal surface
prestressed by bending active boundary conditions forming a spherical tetrahedron, Paper at the
conference IASS Amsterdam 2015
[3] Klasz W., Greiner S.: Active Bending to facilitate the installation process of membranes in statically
self-locking spatial structures – comparison of two case studies, Paper at the conference Structural
Membranes, Barcelona 2015
[4] Holzbau in Schweizer Baublatt Nr. 2 1992, ed.: Schück Söhne AG, Druck- und Verlagsunternehmen
CH – 8803 Rüschlikon, p.3,4
[5] Klasz W.: A self-formed lounge – Finalist in the international conceptual design award www.s-arch.net,
June 2016 (Developed as Researcher in Residence at Unitec, Auckland, April 2016
[6] Klasz W.: Solar driven Form Finding as a Source for Aesthetics – A Case Study; Church and municipal
Center Rif by klaszkleeberger, Paper at the 10th Conference on Advanced Building Skins, Bern 2015
[7] David Truebrigde – Interviewed by W. Klasz on the 1st of march 2016 in his studio in Hasting, NZ
(see: https://www.davidtrubridge.com and see the booklet to the exhibition of Klasz in Auckland, April
2016: Inbetween – page 7)
[8] Klasz W., Bacher M., Woodruffe P.: Self-formation and Innovation, Peer reviewed Paper in Esempi di
Architettura 2015,Vol.2,N.2
[9] Flach M.: CFA Chaumont in Séquences Bois (Réalisations) / Nr 16
[10] Flach M.: Piscine à vagues de Saint-Quentin-en-Yvelines in Les cahiers techniques du batiment, Nr.
186
[11] Sevela P.: The final report of project SynerCIS – Synergizing Austrian Breakthrough Innovations for
CI(G)S solar cells
