Part four - focus control and testing
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| Drilling a small hole in
the dish allowed me to thread an air fitting into the dish. This is
connected through some clear Tygon tubing to a small vacuum pump.
The idea is to let the air pressure push the Mylar into a curved surface. This will allow the light to be focused into a small area. The same principle of a curved mirror is used in telescopes and reflectors, but in those cases the curve is "fixed" because it is made of solid glass or metal. The variable curvature of this reflector is its unique property. But this curve is not suitable for a telescope because it is a catenary (a mathematical shape) with a broad focus rather than a parabola (which would produce a sharp focus). |
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| A cheap plastic project
box contains the small vacuum pump. A typical 12 volt gel cell
battery powers the pump.
Once the mirror is aimed at a target, the pump is operated until the light is focused into the smallest area. If you overfocus, you can open a small bleeder valve on the air line to allow the mirror to "uncurve" slightly. For power generation, the mirror would be aimed at a boiler and the heat developed would operate a small steam engine and generator. With a larger dish, a small steam turbine could be operated. |
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| Here is the completed
unit. The dish mounts easily on the pivot and the air line is small
and stays out of the way.
Even though this solar collector is small, it is excellent for demonstrating the operation of such a system. In practical applications, larger units can easily be made. Any flat surface with a circular wall that is airtight can be used as a replacement for the dish. A pair of plywood sheets could be used to create a 2.6 meter (or 8 foot) diameter unit. If a number of units were constructed, they could be moved by servomotors under computer control to keep the illumination on a generator constant. The nice thing about this design is that a small vacuum sensor can be used to measure to focal distance, and it could be automatically adjusted by a digital controller. |
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| With the focus adjusted,
this plastic pot was located about 5 meters from the dish. In a few
seconds, it began to melt and expand.
A scrap of plywood placed at the focus began to smoke and then ignited after thirty seconds. This is because the focus is not tight- the illuminated area is a 10 centimeter wide circle. This design is far better than the solar furnace design for power generation or cooking applications where other power is not available. |
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Using very little other than small items and surplus, a "farm" of sunlight collectors could be assembled very cheaply. Much larger area collectors can capture far more sunlight. This dish collects about 1.5 kilowatts, but a 2.6 meter one would collect about 5.3 kilowatts. This ignored losses- for instance, the Mylar is not a perfect reflector and a portion of the light leaks through it. Still, this is one of the cheapest methods of sunlight collection.