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Team Solenik of the renewable energy challenge 2012 / 13

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I am Malte, this is Lennart, we are from
the team Solenik, and this is our plant. Well, let’s tell you a few things
about our concept now, shall we? We are working with three Fresnel lenses here.
You can see them up there. Right now they are covered up, otherwise it would bea bit
too warm just now to explain how things work. The sun is still strong, you know…
The upper lens is focused directly onto the central focal point. From the
two lenses further down the sunlight is directed via these two mirrors here –
aluminum mirrors with a fairly high efficiency
rate of 94 % – from both sides toward the focal point,
i.e. toward the heater head. Basically, the Sterling engine with the heater head
on top is the central core of our system, and within this heater head thermal energy
is converted into electrical energy. It works like this: The Stirling engine has
a warm side and a cold side. The hot end is up there, near the heater head. You can
see lots of little tubes here through which the working fluid, which is in this case dry
air, is flowing. This dry air is heated up, expands accordingly, and pushes the displacer
piston downward. Now the air is allowed to pass through this tube here into the
driving piston, which again drives the generator. The cold end of the engine is down there,
it’s being cooled by means of a water circuit. As the heater head heats up, making the
Stirling engine run, via this chain here a generator is driven which produces electrical
power for us and charges these two 12 V batteries. But the whole thing works only
when the light strikes the Fresnel lens at a directly perpendicular angle, so constant
tracking is required to make sure that the light hits the heater head in the right way. This
is done by using two axes: first, horizontally, via this axis, and, second, vertically,
via this axis here. Basically, the whole plant is adjusted automatically through
the self-made light sensor up there. It consists of four photocells and a central element that
simply casts a shadow onto the photocells, so that the position of the plant can be adjusted
in such a way that all photocells are always fully exposed to the light. So we can make sure
that the position is exactly perpendicular to the sun. In this way, with a single Fresnel lens,
we have reached temperatures as high as 1,080 °Celsius, which is truly immense. Of course, when
the Sterling engine is running, such high temperatures won’t be reached any more, because the
engine is being cooled down up there, but temperatures between 600 and 800 °C
will certainly still be possible.

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