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420 mm
Turning mirror
Cooled radiation
protection
Infrared camera and
optical equipment
Concentrated
sunlight with
a maximum
intensity of
5000 Suns
Glass cover
Focal plane and
samples
3D table
275°C
20°C
10 solar constants, that is, 10 times the solar radiation in Earth
orbit. Such high thermal loads can be particularly well reproduced
in a solar furnace. Although the spectrum is not quite the same
as at Mercury, due to the influence of Earth’s atmosphere, it simu-
lates space conditions much more realistically than the artificial
light sources previously used in the aerospace industry for tests of
this kind.
Also appreciated by the aerospace industry is the advanta-
geous size of the solar furnace for testing individual spacecraft
components. Hiring a vacuum chamber and the associated
services for testing individual components when an entire satellite
will fit into it – as is the case with the Large Space Simulator (LSS)
at the ESA ESTEC Test Centre in Noordwijk, the Netherlands –
entails considerable expense. As the largest vacuum chamber in
Europe, with a diameter of 10 metres and an installation depth of
15 metres, the LSS is simply too big to conduct long-term stability
tests for individual solar cells only a few tens of centimetres
across.
The first task in the development of the BepiColombo
mission was to carry out thermal tests on two different samples
of the solar cell structures that will be used on the spacecraft.
The cells, measuring 20 by 20 centimetres, had to be uniformly
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SPACE EXPERIMENTS
irradiated at an intensity of up to 10 solar constants (about 14
kilowatts per square metre), matching the conditions in Mercury
orbit. Both endurance and thermal shock tests were carried out.
During the course of this series of tests, systems to help repli-
cate the conditions to be encountered as realistically as possible
were developed and built by the solar furnace team. A cooling
system to simulate the extreme temperature difference between
the forward facing and rear sides was needed, so the DLR engi-
neers at the solar furnace developed a system that could be
used to simulate the actual temperatures – down to minus 150
degrees Celsius – to which the BepiColombo solar cells will be
exposed.
Long-term irradiation of solar cells simulates conditions
near the Sun
For specific experimental requirements, the test environ-
ment is frequently not suitable for directly simulating the actual
conditions required. So, during the 18 years that the solar furnace
has been operating, there has been continuous improvisation,
conversion, repair and, especially, new construction. For the
solar cell tests, the engineers developed an experimental setup
in which two cooled hollow chambers are symmetrically opened
to slowly expose the enclosed solar array to the radiation. The
Test facility for extreme
conditions
DLR researchers are using the high-flux solar furnace and the xenon
high-flux solar simulator to test new technologies. For this, they are
working with concentrated sunlight and artificial light, enabling
radiation strengths of up to five megawatts per square metre and
temperatures of up to 2000 degrees Celsius. Researchers and appli-
cation specialists from academia and industry have a wide range of
options for developing and qualifying processes in which concen-
trated solar radiation is used. This particularly involves the chemical
storage of solar energy and its application in high-temperature
chemical and metallurgical processes.
Temperature distribution on the solar cell
array for the BepiColombo spacecraft
Plate with sample carriers for the lunar rock
simulant JSC-1 – image after sample pyrolysis.
The xenon high-flux solar
simulator
A high-intensity artificial light source based on elliptical reflec-
tors and short-arc xenon lamps enhances the solar furnace during
the winter, when there is insufficient solar radiation, and for
long-term experiments. The shortwave radiation emitted by the
source, which has an output of around 25 kilowatts, is delivered
as concentrated energy at a distance of three metres, over a
target area of 100 square centimetres and with a power density
of up to 4.1 megawatts per square metre.
unit has a system of channels that can be fed with liquid
nitrogen. Sensors on the pump ensure a constant level of
nitrogen within the cooling unit.
To make the tests as comprehensive and reliable as
possible, the loads on the solar cells for BepiColombo have to
be examined at the optical boundary of the visible spectrum.
The solar furnace system also provides artificial light sources
for this if specific parts of the spectrum cannot be adequately
filtered out. The artificial light sources are also an important
prerequisite for long-term tests.
Prolonged thermal or infrared loads on the solar cells
were investigated for the BepiColombo project. To achieve this,
harsher conditions than those expected for the actual mission
were established. This is intended to ensure that this extremely
important component has sufficient ‘stability reserves’. To
achieve this, the test cells were irradiated with an infrared high-
flux light source under vacuum conditions for 1000 hours, or
42 days, non-stop. The researchers set the target temperature
at 250 degrees Celsius, which was increased by another 50
degrees over the final 10 hours. In addition, the surface temper-
ature distribution during the heating phase was observed with
an infrared camera.
En route to Mercury, the solar array will also be exposed
to ultraviolet light. There is a particular threat of damage to
the solar cells from radiation at this wavelength, which could
possibly lead to a complete failure of the power supply. It must
be ensured in advance that this assembly can survive heavy
loads for extended periods without being damaged. In the initial
test phase, the arrays were irradiated with concentrated ultravi-
olet light to determine the maximum operating temperature.
In the second test phase, six solar cell modules were irradi-
ated with 2000 esh (equivalent sun hours). ‘esh’ is the annual
energy input divided by the maximum radiant flux. This means that,
with double the irradiation than that on Earth’s surface, the expo-
sure time for the solar cells is halved for the same energy output.
Six to 10 times the radiation load can be expected for the trip to
Mercury, leading to a 2000 esh performance requirement for the
solar array, or an exposure time of six days non-stop. Hence, this
experiment was not carried out using the solar furnace, but using a
special high-pressure ultraviolet lamp delivering 460 watts in UV A
and 50 watts in UV B via a parabolic reflector. The harshness of this
radiation was clear as the reflective layer on the paraboloid began
to disintegrate towards the end of the test. But the irradiated solar
cells had to remain undamaged.
Most recent test shows radiation resistance of aluminium
and titanium
The most recent space test in the solar furnace ended in
the summer of 2012, following six months of irradiation. In the
tests under the optical space component (CSO) programme’s
‘Solar Irradiation Test for the CSO MLI Blankets and Coatings’,
run by the Centre National d’Etudes Spatiales (CNES) in Toulouse,
various materials such as aluminium, titanium and multi-layer
insulation with special coatings were tested for their resistance to
concentrated radiation. In doing so, not only were flux densities
that occur in space simulated, but also the absence of convection
cooling of the samples in the vacuum of space. The substantial
challenge here consisted of precisely maintaining the specified
test parameters such as the duration and intensity of the radia-
tion. Power densities of 17 to 230 kilowatts per square metre
were applied. The effect of very high power densities on the
samples could be seen after the experiment – their colour and
structure had changed, or they had become brittle.
The aim of the test campaign was principally to determine
precisely how much material evaporates from such coatings. To
do this, ‘witness plates’ were placed in the vacuum chamber.
These are highly polished metal discs that are cooled and so
adsorb the outgassing products coming from the irradiated
sample. This deposit can be qualitatively and quantitatively iden-
tified using spectroscopic techniques. To achieve the highest
possible accuracy, an ‘empty test’ had to be conducted first, to
determine the level of contamination due to the chamber itself.
Furthermore, the contamination of the chamber was minimised
by means of an extensive cleaning process, including a three-
day bake-out phase.
The flux density measurement technology developed
by DLR’s engineers is a worldwide leader. The especially high
requirements of customers in the aerospace industry with
regard to the precision of these measurements also led to
improvements in the solar furnace systems. These are now,
in turn, benefitting experimenters from the original core area
of renewable energy sources.
•
About the author:
Gerd Dibowski is a mechanical engineer with an education as an
electrical craftsman who has contributed to the construction of
the solar furnace and the high-flux solar simulator. Following a
number of years in experimental operation, he has been head of
the large solar furnace facility since 2006.
Christian Raeder beside the large vacuum chamber. Radiation enters through
the circular quartz glass window.
Engineer Christian Willsch, solar furnace operations, in front of a test set-up for
radiation flux measurement devices.
More information:
