The main objective of the project is to end up with a European supplier for thin film solar cell technology for large platforms.
The objectives of the current phase are twofold:
Firstly, the general layout and infrastructure for the demonstration module based on thin film solar cells will be established. This demonstration module will be implemented on the @-Sat, which will be equipped with a classical planar solar array.
Secondly, a breadboard model will be designed, manufactured and tested to verify the initial design solution of the thin film solar array. This breadboard model will be equipped with a number of the latest developed thin film cells in Europe.
The key issue for this project is the survival of the thin film solar cells connected as a module in an acoustic noise environment.
The potential of using thin film solar cell in solar arrays for large satellites is to decrease the costs to a level of roughly 50% and to decrease the mass required per Watt significantly.
The demonstration module must be exposed to representative conditions and the chosen method is to stow the blanket type thin film solar cell structure between two standard rigid panels and to deploy it sideways.
The latter principle is shown in the figure on the right.
In phase 1 the general concept has to be worked out to select the general layout in close co-operation with the developers of the @-Sat. An initial specification of the thin film panel will be made and a number of possible design layouts shall be worked out in more detail fulfilling the mass requirement of 600 g/m2 or less for a panel including thin film solar cells.
One of the main problem areas is the behaviour of the blanket in the stowed condition. The gap between the rigid panels allows a certain movement of the solar cell blanket and this shall not lead to damage of the solar cell surface. The local blanket vibration motion in response to an acoustic noise environment is a point of concern as this may lead to high local loads on the solar cells resulting in flaking off the solar cell coatings from the titanium surface.
In phase 2 the selected concept will be further analysed and optimised. The viability of the design will be demonstrated by making a number of breadboard models. Engineering tests will be performed on samples with prototype solar cells and these samples will be exposed to thermal cycling and acoustic noise. Modules samples will be made consisting of 2 types of solar cells
Phase 1 and 2 have been completed. Different concepts for the demonstrator have been worked out. The interfaces of the demonstrator with the @-SAT have been discussed with the system engineers of the @-Sat.
The future thin film solar array will be equipped with solar blankets. The flight demonstration panel has to fulfil certain structural requirements and shall have similar interfaces as a normal rigid panel. The chosen panel layout to comply with these requirements is a rigid panel with rectangular holes and in these holes representative pieces of the thin film solar blanket are mounted. The chosen layout of the panel is shown in figure 2.
Figure 2: Demonstration panel with thin film solar cell blanket patches.
Three representative test samples of the solar cell blanket have been made. One of the test samples is shown in figure 3. Each test sample was equipped with 10 prototype thin film solar cells having a dimension of 40 x 80 mm. The CIGS solar cells are manufactured on 25 µm thick titanium foil and 18 cells were made by the HMI (the Hahn Meitner Institute, Berlin) and 12 cells by ZSW (Zentrum für Solarenergie und Wasserstoff Forschung, Stuttgart). The solar cells are mechanically and electrically connected to each other using an innovative design ("The Matrix") developed by Dutch Space. The solar cell blanket weight is only 240 g/m2 and provides a very high packing density of solar cells. An experimental high emissivity coating was applied on 18 solar cells (front and rear side).
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|Figure 3: Solar blanket mounted in a frame, the blanket is equipped with 10 prototype solar cells and 38 dummy cells|
The three test samples have been exposed to a thermal and acoustic noise environment. The overall results of the average solar cell efficiency are given in table 1.
All solar cells and their interconnections survived the qualification acoustic noise levels without a trace of degradation. However, the power degradation due to integration and thermal cycling is significant and this needs further investigations and improvements.
The test results of the CIGS thin film solar cells on titanium foil (25 µm) and the Matrix connection method provided valuable data and this work was a significant step forward in the development of the thin film solar array.
The follow-on programme is currently running to investigate the cause of the solar cell degradation and to implement improvements.
Table 1: Overall Test Programme and Performance Results
|Performance measurements during hte test programme||Solar cell efficiency (AMO, average value)|
|Single solar cells||9.3 %|
|Integrated solar cell pairs||7.6 %|
|After first thermal cycling programme 20 cycles + 100 ºC / -180 ºC||7.2 %|
|After acoustic noise test programme ARIANE 5 , OASPL 147 dB||7.2 %|
|After second thermal cycling programme 180 cycles + 100 ºC / -180 ºC||6.1 %|