Team GreenAirEnergy of the renewable energy challenge 2012 / 13

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Hi there, I am Felix and this is Lukas from the
reech team GreenAirEnergy. Today we would like to present to you our new plant – a rather big solar
low-temperature Stirling engine. You want to know how this engine works? It’s as simple as this: The
sun shines onto this surface, turning it very hot. Inside this hollow box here
there’s lots of air. The air pushed back and forth by a displacer piston.
The displacer piston is driven by the mechanics you can see here, and it’s one of the innovative
features about this system that there is just a single interface between this mechanism and the outside
world – right here, below this point. That’s the
only place that needs to be sealed. It’s the only mechanical interface. Now,
as the air is pushed back and forth, it heats up when it’s near the top, and
cools down when it’s near the bottom, expanding and contracting in the process. We make
use of this cyclical expansion and contraction by means of this large bellows here. This is our
driving piston. It extends as the air expands, and deflates as the air contracts, which
provides for the rotation of the engine. This has to happen in exactly the right phase, which
is why the phase difference between these two connecting rods is very important, and so we’ve got
lots of adjusting screws everywhere. As you may have noticed, the plant for the most part consists
of quite simple materials, such as wood … the friction coating on this wheel here
is made from an old bicycle tire … and, this part here, for instance, has been taken from a truck,
it’s a pneumatic air suspension for trucks. This is also part of our approach – which was to build this
system in the simplest possible way. This is why we deliberately dispensed with complex focusing
and reflective optics and opted for this low-temperature Sterling principle instead,
which absorbs sunlight directly through an absorber plate and converts it into heat.
The idea of simplicity is a central theme underlying our concept. Ultimately,
this here is supposed to be a power plant, which means that we want to generate
electricity. So we’ve got a generator, a second-hand hub dynamo, which is ideal for this
application. It supplies alternating current. We’ve assembled the electronics ourselves
to turn it into direct current, and there’s an USB port. With a low-temperature Stirling engine
of this kind with such a large piston the average internal pressure is extremely important. The average
internal pressure is related to the internal volume. This volume can be adjusted through this control
valve here. I can use it to release air. Fact is, as the engine heats up, the average
temperature of the air rises and it expands in volume, so that the internal pressure –
as you can see quite well here – causes the piston to stop. The flywheel is no longer able to
contract the piston beyond this point, since the pressure has become too great.
So I extend the piston, release some air – the whistling you can hear
now is the additional volume that has been built up due to the rise in temperature – and,
when some air has been released, with some luck
the plant will be running again!

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