Better sunbathing for satellites

Future telecommunications and broadcasting satellites can have more
power and add more channels without need for a major redesign. More
efficient solar cells offer 50 per cent more electrical energy than
their predecessors, from panels of the same size as before. This is
one of many areas of technology where the European Space Agency
helps Europe’s industries to keep abreast of the latest developments.

Solar panels extend from spacecraft like the wings of birds. These
have become a familiar feature of the Space Age, although spinning
satellites may wear their solar cells like a coat. Certain other
spacecraft rely on batteries, fuel cells or radioactive generators.
But converting the free and ever-renewable energy of sunlight into
electricity, in photovoltaic cells, is by far the most popular
source of power in space.

A junction in a semiconductor, containing different impurities on
either side, responds to light by creating an electric voltage.
Until recently, silicon was the favoured semiconductor for solar
cells. Now gallium arsenide has begun to replace it. The first
gallium-arsenide cells were only slightly more efficient, converting
19 per cent of the solar energy compared with around 16 per cent in
good silicvaluells. In the latest gallium-arsenide based cells, the
efficiency jumps to 27 per cent.

How’s the trick done? Each semiconductor junction needs a minimum
energy in the particles of sunlight, the photons, if it is to
generate a voltage. Engineers can exploit the fact that gallium
arsenide is transparent, and stack three junctions on top of each
other. The bottom layer absorbs red light and the middle layer
green light. The most energetic blue light activates the outermost
junction. The result is that more of the available photons are
captured, than in a single-junction device.

"Although these triple-junction cells aren’t cheap to make, there’s
a big payoff," says Lothar Gerlach, who is in charge of
solar-generator engineering in the power division at ESTEC, ESA’s
science and technology centre in the Netherlands. "To change and
re-qualify an existing satellite design with a new and larger solar
array is far more expensive than improving what you have already,
just by using more efficient solar cells. Or if you don’t need
more power, you can have a smaller solar array. Then less fuel is
needed to control the satellite’s attitude in space, and more
payload can go on board."

An ESA contractor, RWE Solar GmbH (formerly called ASE), is
Europe’s largest producer of solar cells for space. It is now
preparing for large-scale production of triple-junction
gallium-arsenide based solar cells for space applications, at its
plant in Heilbronn, Germany. ESA will use them in spacecraft such
as PROBA-2 for technology development, GOCE for charting the
Earth’s gravity, and Herschel and Planck for astronomy.

A distinguished history

An artificial sun, intense light flashes, and high-temperature
ovens are among the test facilities available for solar-cell
engineering at ESTEC. Above the Earth’s atmosphere the cells face
a solar intensity 36% stronger than on the ground. They are
subject to drastic changes of temperature whenever a satellite
passes in or out of the Earth’s shadow, causing thermal fatigue.
ESTEC’s test facilities are used for research, development,
evaluation and trouble-shooting, from single solar cells to
complete arrays.

Before any solar cells are accepted by ESA for use in space, they
must pass qualification tests. The official centre for this
purpose is Spasolab, at Spain’s Instituto Nacional de Técnica
Aeroespacial (INTA) in Torrejón de Ardoz. Spasolab also helps
with testing during the development of new solar cell components.

ESA and its industrial contractors in Europe have long experience
in solar-cell technology. When the Hubble Space Telescope was
first launched in 1990, it was powered by solar arrays provided
by ESA. In 1993, the first set of solar arrays was replaced by a
second ESA set. The astronauts servicing Hubble retrieved one of
the older wings and returned it to Earth.

Here was a special chance to investigate the experience of the
solar cells in space. Experts examined them for impacts by
micrometeorites, manmade debris and high-energy atomic particles.
They also evaluated effects of atomic oxygen and thermal fatigue.
Now Hubble’s second set of arrays, both retrieved in March 2002,
will undergo a similar post-flight investigation.

Another special effort has met ESA’s requirement for solar cells
operating far from the Sun, at low light intensities and low
temperatures. These will power the Rosetta spacecraft to be
launched in January 2003, which will rendezvous with Comet
Wirtanen far beyond the orbit of the planet Mars. Hitherto,
spacecraft venturing so far afield have had to use radioactive
power sources. Europe’s engineering and industrial teams have
achieved an amazing 25 per cent efficiency in specially designed
silicon solar cells operating at low temperatures.

At the other extreme, ESA needs gallium-arsenide based solar
cells suitable for very bright light and high temperatures.
The BepiColombo mission to the innermost planet, Mercury, will
encounter such severe conditions. The Solar Orbiter spacecraft
will swoop even closer to the Sun, where the intensity of
sunlight will be 25 times higher than in the Earth’s vicinity.
But the solar cells for these missions must also be efficient at
the distance of the Earth, because they will propel the spacecraft
on their journeys, by electric rockets. In ESA’s Cosmic Visions
2020 science programme, these spacecraft are due for launches in
2011 or 2012.

The next step: thin-film solar cells

Weight is seldom a problem in solar-power systems on the Earth’s
surface, but space engineers have to worry about the total mass
of a satellite at launch. They are therefore interested to know,
not just how many watts of power they’ll get from a square metre
of solar cells, but how many watts per kilogram. Typical figures
at present are 50 for silicon and 110 for gallium arsenide. That
can be pushed to 400 watts per kilogram for thin-film solar cells
now under development.

In thin-film devices, the semiconductor is laid as a coating on a
thin, flexible but strong support made of metal or plastic. The
cell thickness, including the support structure, can be less than
three hundredths of a millimetre. The conversion is less efficient
than in rigid cells, but that does not matter if the spacecraft
can unfurl and control large, lightweight wings of thin-film
solar panels.

"For planetary missions, you can even imagine gigantic solar sails
coated with thin film solar cells," says Lothar Gerlach. "Then you
could use the Sun’s energy in two ways at once. The pressure of
sunlight on the sail gives direct propulsion, while sunlight
converted into electric energy drives an electric rocket and
powers the spacecraft with its scientific payload.

* Nuna wins the World Solar Challenge!
http://www.esa.int/export/esaCP/ESAXXA8VTTC_FeatureWeek_0.html

Better sunbathing for satellites

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