The Azimuth Project
The Double-Cylindrical PointFocus (Rev #7)

The idea that nobody has wanted - so far

The parabola has the well-known property of reflecting axis-parallel rays to a point, as can be seen in this video clip. If we rotate the parabola around its axis, we create a parabolic disc. It has the well-known property of reflecting parallel rays (= planar wave fronts) to a point - if they are incident along the axis direction of the disc - which can be seen in this video clip.

Now, it is a beautiful (and little known) geometrical fact of great practical consequence that we can avoid the astronomical costs associated with creating a large parabolic disc, and harness the work-power of the sun in a more efficient way by bending two flat mirror sheets into the shape of two parabolic cylinders. If these parabolic cylinders are properly configured, the incoming parallel light rays (= planar wave fronts) will create an exact point focus after two successive reflections. This is due to

The Double Cylindrical Point Focus Principle:

If the focal line of the first cylinder is identical to the generating line of the parabola that is the intersection of the second cylinder with a plane perpendicular to its axis, then the incoming rays will be reflected to a perfect point.

The basic ray trace of the Double-Cylindrical Point Focus

The DCPF principle was discovered on 16 November 1976 by Ambjörn Naeve while working with Lloyd Cross, a well-known physicist and holographer who co-founded the School of Holography and the Multiplex Company in San Francisco, California. The original insight was recorded on this piece of paper, and a geometric proof of the DCPF principle can be found here.

Between 9-12 December 1976, Lloyd Cross and Ambjörn Naeve built the first point-focusing mirrors based on the DCPF principle. A short description of this intense event, including some pictures, can be found here.

Lloyd and Ambjörn with the first DCPF mirror
A sketched ray-trace of the first DCPF mirror

A computer-based animation of the double-cylindrical point-focusing mirrors in action, created by Ambjörn Naeve using the Graphing Calculator, can be seen in this video clip, and a live “burning demo” with a DCPF mirror-pair, built by Tomas Elofsson in 1989, can be seen in this video clip.

On 1 July 1989, in Gusum, Sweden, the DCPF mirror-pair shown in the latter video clip (1.5 m^2 primary mirror) was used in an experiment by Ambjörn Naeve and Tomas Elofsson that succeeded in melting limestone (2560 ˚C) in free air. In the pictures below, Tomas Elofsson is shown tuning the mirrors and melting the limestone sample:

Tuning the primary mirror
Tuning the secondary mirror
Melting the limestone sample in a open carbon crucible

A collection of video clips on the double-cylindrical point focus

The DC pointfocus videos by MathRehab on YouTube

Four major advantages of the double-cylindrical point focus

in comparison with the classical parabolic disc point focus

1) It is easier to build in large sizes, since cylindrical surfaces are curved in only one direction and therefore can be BENT into shape from a flat sheet, hence avoiding the “astronomical costs” that occur when a doubly-curved mirror has to be CAST into a fixed shape.

2) The focal point can be positioned outside of the solar influx area between the mirrors, where it is freely available to perform work, as in the DCPF wheelbarrel design.

3) The focal point can be moved by rotating the second cylinder around the focal line of the first cylinder. This fact can be used to create various forms of solid-state heat engines, for example by successively heating and cooling a series of bi-metal coils.

4) The mirror configuration can be approximated with planar strips, the number of which grows LINEARLY with the overall size of the construction – as opposed to the parabolic disc, where the number of planar approximators grows QUADRATICALLY with construction size.

Possible applications of the double-cylindrical point focus

As has been demonstrated above, the DCPF provides a cheap and efficient way to create “a huge burning glass”, which can achieve very high temperatures. Therefore, it enables local (= rural) development of a multitude of technologies that presently require advanced and expensive high-tech labs, such as:

Solar-powered steel plants that recycle scrap metal into a valuable resource for the local community. A discussion and some designs of what a solar steel plant could look like can be found here.

• Cheap and efficient super-heated external combustion engines (steam, sterling, …), whose work-power (like that of all heat engines) is directly proportional to the difference (in ˚K) between input and output temperatures.

• Highly efficient solar electricity generators, which concentrate the energy on high-performance photo-voltaic cells that must be water-cooled to avoid melting.

The idea that nobody wanted (1976-2001)

The story (some parts of which are in Swedish) of trying to get people interested in the double-cylindrical point focus - from its discovery in 1976 and up until 2001 - can be found here. One of the chapters in this story took place in July 1995 in Gunnarskog in Värmland/Sweden, where Ambjörn Naeve and Tomas Elofsson recorded a video with their daughters acting as solar energy consultants, helping a farmer to melt horse shoes and load batteries by concentrating light on a solar panel.

These activities created a big stir in the local community, with the paper “Värmlandsbygden” publishing a major story on the DCPF with a frontpage headline stating: “They have the solar power that melts mountains” (“De har solkraften som smälter berg”). Some local politicians got involved, interested in creating jobs for the local community, and it looked like things would start moving when an application for funding was sent to the Swedish Innovation and Industry Development Agency (NUTEK), which at that time was responsible for strengthening regional growth by furthering industrial applications of innovations. However, the application was dismissed on grounds that included the following reasons (translated from Swedish):

“A problem with the suggested design is that the temperature becomes very high. This means that the heat losses are increased, which makes the [DCPF] technique less interesting in energy applications. Moreover, concentration of light by mirrors is part of the research that is already carried out within the solar energy domain - although with different configurations than the DCPF, which lead to lower temperature levels and lower heat losses.”

The DCPF goes virtual

Optical properties of Concis and Quadrics

Caustics involving conic reflectors
Point-focusing properties of Conics
Point-focusing properties of Quadrics
Practical burning demos of the DCPF mirrors

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