Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

Objectives
The objective of this three years project (2006-2008) is to develop lightweight, efficient and enduring
prototype photovoltaic (PV) devices based on novel composite sandwich structures. The first application
of these PV devices is the Solar Impulse airplane. Two technologies are considered: monocrystalline
Si-based cells for the upper airplane skins, and dye-sensitized photovoltaic cell (DSC) for the bottom
skins. The proposed research focuses on the key materials and reliability issues needed to adapt
this lightweight photovoltaic device for the project. The key innovation lies in the functional integration
of the solar generator within a lightweight structure. Such functional structure does not exist, and the
project aims at determining its potential.
The following five EPFL laboratories collaborate to this end:
� Laboratoire de technologie des composites et polymères (LTC)
� Laboratoire de photonique et interfaces (LPI)
� Laboratoire de la construction métallique (ICOM) until March 31, 2008
� Laboratoire de construction en composites (CCLab)
� Chaire de modélisation et calcul scientifique (CMCS) until Dec. 31, 2007
The research was carried out in collaboration with the company Solvay-Solexis, for mechanical simulations
and for the development of fluoropolymer films for the encapsulation of the PV devices. The
company VHF Technologies joined the project on Oct. 1 st , 2007, in order to implement the project results
at industrial scale in the field of building integrated PV. The goal of this activity is to achieve a
profiled lamination process for flexible a-Si PV films.
The main results obtained are summarized in the report, according to the work package structure of
the project:
� WP1. Encapsulation of PV devices (resp. LTC + LPI)
� WP2. Process integration into ultralight sandwich composite structure (resp. LTC)
� WP3. Endurance analysis of the multifunctional structure (resp. ICOM/CCLAB)
A WP4 dedicated to fast predictions and robust optimization of the multifunctional structure (resp.
CMCS) was stopped in Dec. 2007. The main results were detailed in CTI project reports and in a joint
CCLab/CMCS publication submitted to Probabilistic Engineering Mechanics.
WP1. Encapsulation of PV devices (resp. LTC + LPI)
Task 1. Strength analysis of surface-modified monocrystalline Si
c-Si cells with inverted pyramidal texture (S32, RWE Schott Solar) were used. Their mechanical
strength was analyzed by LTC and the results are shown in Figure 1. The texture is not responsible for
the low strength of the cells. Rather, it is more sever defects that cause the failure. A Finite Element
simulation was carried out in collaboration with Solvay. It showed that the thermo-mechanical stresses
in the texturation caused by the CTE mismatch between silicon and encapsulant material were small
and could be disregarded. Also shown in Figure 1 are the data under tensile load, for both bare and
integrated cells (see WP2), which clearly shows that the processing of the sandwich did not damage
the cells.
Figure 1: Failure probability of smooth and textured solar cells in bending, with active Si side in tension
or in compression, and of textured cells in tension, bare and integrated into asymmetric sandwich.
Ultralight Photovoltaic Structures, Y. Leterrier, EPFL
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