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

defect. In this task, the influence of process pressure on strenght of ultralight (< 1 kg/m 2 ) sandwich
structures was also investigated and modeled, and the results depicted in Figure 15 show that the
optimal pressure for such ultralight structure is not 1 bar as usually considered for classic vacuum bag
processing, but it close to 0.5 bar. This important information was transfered to company Decision SA
in charge of the manufacture of several components for Solar Impulse.
Stress in 0° layer [MPa]
400
350
300
250
200
150
100
50
Smooth skin without defects
Smooth skin with 2mm defects
Smooth skin with 5mm defects
0 10 20 30 40 50
Adhesive quantity [g/m 2 ]
Figure 13: Compressive stress in the skin at
failure in 4-points bending tests as a function
of adhesive weight and with and without bonding
defects.
Comp. load per unit width [N/mm]
40
38
36
34
32
30
28
26
24
22
20
Wrinkling load
Core compression
Core shear
Local skin strain
Experimental data
2
0 20 40 60 80 100 120
Adhesive weight in menisci [g/m 2 Critical half-wavelength
]
Figure 14: Compressive load in the skin at failure in
4-points bending test as a function of adhesive
weight. The loads predicted for local instability coupled
with local core or skin failure are displayed.
Figure 15: influence of process pressure on strenght of ultralight sandwich structures
Task 7. Development of asymmetric ultralight sandwich structures
The strength of asymmetric sandwich structures with incorporated solar cells has been further studied.
Sandwich samples with one carbon skin and one skin comprising one solar cell were produced with
-0.3 and -0.9 bar relative process pressure (prototype shown in Figure 17). The sandwich beams were
tested in 4 point bending with the cell in tension. The failure was due to the tensile failure of the cell.
The stress at failure was slightly lower than during pure tensile tests (Figure 1). No significant differences
were found between the two process pressures. The bonding of solar cells on honeycomb has
been evaluated. The contact angle of adhesive on the silver backside of the cell was measured and
was similar to that measured on cured carbon prepreg. The measurement of the debonding energy
using cantilever beam method was particularly difficult due to the brittleness of the cells. However, it
was observed during solar skin debonding that honeycomb core tore with only 5 g/m 2 of adhesive,
which showed thus a very high adhesion of the adhesive on the silver-coated backside of the solar
cells.
Task 8. Production of prototype modules
Several novel demonstrators were produced, some of which are shown in Figures 16-19. LPI made a
complete encapsulated flexible module (Figure 16) consisting of 11 cells. A curved asymmetric c-Sihoneycomb-carbon
fiber composite ultralight sandwich structure (800 g/m 2 ) was also at LTC (Figure
17). This structure with 1 skin made of c-Si cells showed balanced mechanical performance (stiffness
and strength) and is unique. A new structural element has also been developed by CCLab (Figure 18).
This is a multifunctional sandwich panel made of glass polyester skins and polyurethane foam core.
Solar cells have been encapsulated on one skin. It is expected that this new structural element can be
implemented in building applications to serve the needs for structural integrity, energy production, and
thermal insulation of the building. The new structural element would also be modeled by CCLab to
simulate its behavior.
Ultralight Photovoltaic Structures, Y. Leterrier, EPFL
196/290
6
5
4
3
Critical half-wavelength [mm]
6/8