WARNING

Concentrating sunlight is potentially hazardous.

The dangers of fire, burns and eye damage are real and appropriate precautions must be taken whenever the device is exposed to sunlight.

WARNING

The Collector:

This concentrating solar collector is in the form of a high aspect ratio right conical frustum (HARF).
It is easy to construct and very effective so please take the warning notice seriously and use with caution.

What is a High Aspect Ratio Frustum:

A frustum is what is left of a cone when a smaller cone is cut from the top of a larger cone. The aspect ratio refers to the relative thickness of the frustum compared to its diameter, and a high aspect ratio implies a large difference. In this case a much larger diameter than thickness.

A conical collector works when the reflective inner face concentrates rays along the axis of the cone.
This is most effective when a right angle cone is used.

History:

Conical collectors are not common but are not new. They are mentioned generically on p335 of Solar Engineering of Thermal Processes (Duffie and Beckman, 2^nd Edition, Wiley) and have been used in practice as described in Revisiting Solar Power's Past By Charles Smith.

This document is published widely on the internet and can be found at http://www.cs.ntu.edu.au/homepages/jmitroy/sid101/solar1/Smith.html amongst other locations.
The images below are of late 19^th century French and early 20^th century American designs described in Smith's article.

Why a High Aspect Ratio Frustum:

In a high aspect ratio frustum collector most of the cone is cut away to improve the concentration at the focus. This works because a circle’s area increases by a square of its radius.

This means, for example, that the outer 50% of a circle's area occurs in the last 29.3% of the radius and the outer 25% of area in the last 13.4% of the radius. By contrast the inner 25% of the radius only accounts for 6.25% of the collector area, or 1/7 of the outer 25%.

Using a right angle cone, similar to the French design above, in which the focal interval necessarily equals the radius of the base, each 25% of the focal interval will collect, from point to base, incident radiation in the proportions 6.25%, 18.75%, 31.25% and 43.75% respectively.

As can be seen in the diagram above each collector interval is the same length but from top to bottom gathers a decreasing proportion of the received total.

The principal of the HARF is to exploit this geometrical feature to maximise the concentration ratio, keeping the form and structure simple and allowing higher receiver temperature or optimal use of concentrator photo voltaics.

A HARF provides higher concentration than a parabolic trough and is simpler to construct than a parabolic dish. Like a parabolic trough a right conical frustum’s reflecting surface is only curved in one plane and can be simply generated by bending. A HARF, like a parabolic dish, requires two axis tracking but has other advantages:

• A HARF generally provides higher concentration than a parabolic trough (more correctly, a HARF can provide an eqivalent concentration to any parabolic trough with much lower levels of construction and reflective surface accuracy).
• The HARF receiver is axially symetrical and will likely be either simpler to construct or more effective in operation.
• A HARF is simpler to construct than a parabolic dish.

The large empty central space initially prompts a design urge to fill it but it has some potentially useful characteristics:

• Empty space is cheap and the cost of filling it will build another, more efficient collector.
• The aerodynamics may be advantageous and the shape is self draining and the surface easy to clean.
• Shading of the proximate ground area will be less dramatic if an open design is used.

How to generate a HARF:

A cone can be fashioned from a sector, ie. that part of a circle bounded by two radii and the included arc.
A right cone is fashioned from a sector with an included angle of (approx) 254.56°.

Taking an annulus of this sector will produce a frustum:

This is a simple form to generate, lay out and produce.

As explained previously, using a thinner annulus produces a higher relative concentration ratio due to disproportionate shortening of the receiver. Annuli of the order of 20% to 12.5% produce effective concentration ratios that should produce superheated steam with much less difficulty than parabolic troughs and much less expense than parabolic dishes.

Concentration ratios comparable to paraboloic dishes can be achieved using a curved secondary reflector or a cone and fresnel lens combination.

Good results can be obtained for an annulus as large as 25% of the major radius.

That is the ratio used in the experimental solar oven below which achieved sustained temperatures of 190° Celcius in full sun, despite the relative unsophistication of both the collector and receiver.

Prototype Solar Oven:

WARNING

Concentrating sunlight is potentially hazardous. The dangers of fire, burns and eye damage are real and appropriate precautions must be taken whenever the device is exposed to sunlight.

WARNING

The collector area is 0.25 square metres delivering approximately 250 watts in full sun.
The receiver is a cylinder of 7 cm diameter and 10 cm length having an area of 220 square cm.
This gives a concentration ratio of 11.36 to 1.

The original collector was started a few years ago as proof of the HARF concept, using whatever was at hand (avmat engineering).
It has undergone several transformations which accounts for its slightly byzantine appearance.
It was initially fitted with an unshrouded copper tube receiver circulating water to a home made calorimeter.
The results were just as the textbook predicted they should be so work commenced on a one square meter apeture which is still in train.

This collector/reflector is aluminised mylar (space blanket) on an adhesive plastic substrate (contact) glued to a frustum of laminated cellulose fibre (3 ply).
It is all pretty basic and rough but that may be of interest in itself. How much can be done with how little.

The frustum is attached to a stable base (more 3 ply) and the whole affair is mounted on horizontal trunnions. The base is stabilised with intersecting ribs that leave an opening in the centre of the base for mounting receivers and running tubes and/or  wires.

The use of plywood for the frustum was an unfortunate choice as its 'grain' became obvious as soon as it was curved. Extensive ribbing was required to correct the distortions.

A better choice of material would have been sheet metal or plastic.

The receiver is a a thinwall tube of copper shim with an untreated surface.
The tube end is sealed with an insulating lid and the whole is insulated with a generous still air gap.

The receiver assembly is shown below with the less-than-perfect reflector behind it.

In hindsight a tin can of the right dimensions would have worked as well given an appropriate coating of copper plating or matte black.

The trunion arms are mounted on a tricycle undercarriage where the 'front' wheel is set at right angles to the axle of the 'rear' wheels. The result is that the base pivots around a vertical axis providing azimuth to the trunions elevation.

The elevation is motorised, as will the be the 'front' wheel of the base (it is a work in progress) once the MkII collector is complete.

The elevation was motorised first as a proof-of-concept of sun tracking circuitry and because it was a huge nuisance trying to hold it 'on sun' in elevation.

The tracker uses green LEDs as detectors and a few transistors to switch a pair of double pole double throw relays.

Between them the relays provide the three output states (forward/off/reverse) for a small motor/gearbox.
The drive has a small drum on the output shaft and simply winches the collector up and down.

The inspiration for the tracker was taken from the archives of Duane C Johnson on his remarkable website www.redrok.com.
Duane has published several circuits using LED sensors and a couple of these were hybridised to get the result required.

Performance exceeded expectations, being stable and sensitive to between one and two degrees of sun movement.
The results are achieved in part by 'shading' the sensors and using reasonably long gnomon (about 120 mm).

The motor needs 1.5 volts and motor scale current is delivered by a single 'D' cell.
The control circuit requires about 6 volts at modest current levels so to remove the need for a dual voltage power supply the circuit is powered by a small array of photo voltaic cells set around the top edge of the collector.
In truth it was just as easy to do that as cobble together a battery pack and it has a gee-whiz factor.

Testing the Contraption:

Nothing fancy was attempted. Once the tracker was plugged in it moved straight on sun and azimuth was adjusted via gentle big-toe-nudges.

Some condensation appeared inside of the glass and was wiped out without producing blisters (it was reasonably warm by this time).

The digital camera worked overtime as the temperature reading climbed to ever dizzying heights until it became apparent that, as the sun dropped lower, the best result had been and gone.

Temperatures between 180°C and 190°C were maintained for about an hour and a maximum temperature of 190.4°C was achieved on this occaision.

copyleft © 2006 I Sykes.