Scheffler Solar Concentrators are pretty darn cool
As solar concentrators go, the Scheffler is downright brilliant! Its inventor Wolfgang Scheffler of Austria solved a set of key design problems that persist in all of the other families of sun gathering creations. This unique design is continuing to find uses in solar cooking, water heating, and small scale industrial applications around the world.
Scheffler's unique flattish-oval reflector is able to track the sun through any solar day of the year using only a simple motor (or even a weight-driven clockwork). That is to say, no servos, no on-the-fly calculations, no computers involved. The reflector also focuses its light down onto a stationary receiver - the point of focus does not move, even though the dish does. Finally, the Scheffler reflector is is remarkably light weight and comparatively easy and inexpensive to construct.
Some Background about Solar Concentrators
A solar concentrator is any device that takes in some area of the sun's light and then squashes it down to a smaller area, consequently making that area much brighter and potentially much hotter. A magnifying glass focusing the sun's rays to a burning spot is a classic example. Solar concentrators are generally used for converting the sun's light energy into heat energy. Large solar power plants use this solar heat to generate steam by boiling water, which drives turbine generators. Smaller concentrators are often used directly for cooking, boiling water, or for industrial process heating. They can also generate power with small stirling cycle heat engines where steam engines are not an option.
A curved mirror can produce similar results to that of a lens, though the focus is up away from the ground, not down on it. The shape of solar focusing mirrors is generally described as parabolic.
There are also concentrators shaped like long mirrored half-pipes. These focus the sun to a long line, usually onto a black pipe. This is a great idea, especially if you are trying to heat some kind of liquid, as liquids are famously good at through pipes. ;->
In general, solar concentrators are the most economical way to produce higher-order industrial temperatures (hundreds to thousands of degrees C) with solar energy. They can produce much higher temperatures than flat panel solar collectors, which are generally used for making hot water (sub boiling temperatures) or for heating air.
Solar Concentrators must follow the sun
One feature common to nearly all solar concentrators is that they must be motorized so as to follow the sun throughout the course of a given day. (In reality, when you follow the sun's movement through the day, you are actually un-following the earth's movement; hold onto that concept for later...)
Most solar concentrators are large curved mirrors, or sets of flat mirrors that move as a unit. If the concentrator is anything like a classic satellite dish, this puts the point of focus up above the mirror in the air. It also means that, as the dish tips to follow the sun, the point of focus also moves. If you are trying to boil water, it now means that you need to have a boiler hanging up there in the air, with pipes going to and from it, and a way to support the whole ordeal. Oh, and the pipes probably have to flex somewhere near the bottom to connect to the rest of your system's plumbing etc. It makes for a complex and potentially dangerous situation.
Moving a satellite dish with a boiler or heat engine or whatever up there is no easy thing either. The whole reflector-receiver setup must move as a single unit, and make a grand arc-shaped sweep across the sky. From an engineering standpoint, it becomes easier to pull off this sweeping motion by breaking it into two sorts of motion: a vertical tilting motion and a horizontal panning motion. At this point, you now need a pair of motors to drive these two distinctly separate axis. Oh, and the rates of the motors must be carefully changed throughout the day so as to create this arc shaped motion path. (How's your calculus?) That pretty much means you end up using a computer. Computers require software and motor position sensors and a digital clock. Oh, and electricity.
By this line of reasoning, the simple idea of focusing the sun's light has come to require a somewhat stilted receiver apparatus up there in the focus, some rather competitive structural engineering to keep everything together as it moves, and a complex automation system to follow the sun. Simple? Not at all!
Scheffler is sheer simplicity
After that bit of background, I'll restate the virtues of the Scheffler concentrator:
- One motor with simple continuous clockwork motion
- Stationary receiver
- Lightweight low cost reflector structure
Without going overboard on detail, I'd love to just touch on how such a device is possible - how does a floppy looking oval mirror pull it off?!
1. One motor: Instead of trying to follow the sun by honing-in on it vertically and horizontally, the reflector essentially un-follows the earth's motion by rotating on a pole that runs parallel with the earth's axis. In astronomy, this is called a polar mount. If you set up a reflector like this at 30 degrees latitude, then you make sure the Scheffler's pole is pointing due north and is tilted at 30 degrees. As the earth turns (counterclockwise as viewed above the north pole) the Scheffler reflector turns clockwise. The rate of the reflector's rotation just has to be equal to that of the earth's rotation, or 1 turn per 24 hours.
2. Stationary receiver: It is great to have a receiver doesn't have to move! If it's a boiler, it means it can be heavy and just sit there without risk of being tipped or dropped or having its pipes bent off. If it's a bbq, it means your burgers stay where you put them so you can flip them when it's time - you don't need a ladder or anything! The Scheffler reflector pulls off this marvelous feat by directing the incoming light directly down and along that polar axis of rotation. Since the axis stays the same no matter which way the reflector has swiveled, the light always comes right down to the same place. It cant' be stressed enough how amazing and convenient this is! I've built a number of medium and large solar concentrators, and they all suffered from out-of-reach-and-moving-receiver-syndrome. The Scheffler reflector is manually tipped up or down slightly each day to adjust for the increasing or decreasing elevation of the sun through the seasons.
3. Lightweight low cost: Since the reflector has only need to move itself - not also a receiver apparatus - it has only to support its own weight. The best versions of this unique reflector have a frame of semi-flexible metal tubes, and are covered with a skin of silvered plastic or tiled glass mirrors. The semi-flexible nature of the reflector also enables some fine adjustment to the overall concave curve throughout the year. Adjusting the curve lets the reflector's focal point of light stay sharp and round upon the receiver for any given tilt of the mirror.
If you are considering installing some sort of solar concentrator for heating, cooking, or an industrial purpose, I seriously recommend looking into the Scheffler before delving into robotic motors and computerized systems with steam pressure flex lines. Remember, not everything great needs to be complicated!
Here are some links to pages featuring Wolfgang Scheffler's work. I only hope I can find a good excuse to put one on my house some day!