Heliotrope General

3733 Kenora Drive, Spring Valley, CA 91977 1-800-552-8838

Construction Notes for the

Berkeley

Thermonuclear

Paraboloid

©1993 David Baty

Build a low tech, high performance parabolic type solar cooker for about $20! This solar cooker is based on the shape of another parabolic cooker, the SK 12, developed by the State Technical College in Altotting, Germany (HP#31, p. 65). Our design uses a fundamentally different type of construction and permits using a variety of construction materials. The use of a structural composite shell in lieu of the SK 12's skeletal frame dramatically reduces material costs and simplifies construction.

The Design

A flexible design that allows a variety of materials lets you use stuff that is easy to find, and lets you build to your budget. You can build a functional cooker for next to nothing in material costs. Spending more money initially (up to $20) can result in a cooker which is more durable, easier to build, and more easily repaired.

These are construction notes rather than a rigid set of step by step instructions. I want to encourage you to think about the building process, and, if necessary, to adapt construction techniques to the available tools and materials. Different possibilities are suggested along the way.

The building process is fairly labor intensive for building only one cooker. However, one virtue of this building process is that not only is the finished composite shell

Above: A Thermonuclear Paraboloid making espresso in Berkeley, California.

(or "dish" as I call it below) the body of a cooker, but it will also serve as a form on which to make another dish. Thus one dish can be the form for another dish. This design is perfect for a cottage industry.

The Pattern

The essence of this design is a simple wood or metal pattern which can create and re-create the parabolic cone. The parabolic cone is the inside face of the finished parabolic cooker.

The desired shape of the pattern is shown on the Construction Detail for Pattern & Form. This shape can be transferred to any reasonably stiff material — wood (probably easiest), metal, or plastic. The resulting pattern is attached to a 30 inch or longer piece of pipe, dowel, or other round stock to use as a vertical bar. I used an unthreaded piece of 34 inch pipe. This bar sits on a bearing of some hard material — almost anything: a scrap of metal, glass, tile, a rock

The pattern is clamped onto the bar in such a way that its location on the bar can be moved up or down. My pattern was a piece of wood, attached to my pipe with two pieces of plumbing strap and two machine bolts. Any village handyman should be able to cobble together a functional equivalent from whatever wood, metal, wire, and/or vines are locally available.

The Form

The first step in building the form is to create a pivot around which the bar and pattern can rotate. This pivot is no more than a pile of mortar (sand and cement mixed approximately 4:1) piled up around the bar a foot or so high. The bar should be sitting on the bearing, pretty close to vertical. Rotate the bar slightly once or twice before the mortar hardens and the cement sticks to the bar. Let the cement set at least overnight.

Axis of Parabola

Construction Detail for Pattern & Form:

Construction Detail for Pattern & Form:

first cut lengths

first cut lengths

Aluminum Cuts: (Assuming 8" Roll

7 1 i/2"l scraP

then diagonals finally trim ends

center hole

After making this mortar bearing I piled sand (almost anything could be used as a filler here) over the pile of mortar. There was 1)2 to 2 inches of space between the sand and where the pattern could rotate.

I made the finished surface of the form using more mortar (sand and cement 4:1) and rotating the pattern to trim excess mortar from the pile. This procedure may feel like a labor intensive mess until you begin using the pattern. It is easy to generate a precise shape using the pattern to trim and smooth the layer of mortar that is the finished surface of the form. The surface of the form doesn't need to be exact, but since this form can be used to create more parabolic dishes, it's worth taking the time to get it as nice as you can. Let the finished form harden at least overnight.

The Dish

I was familiar working with mortar and impatient to see how well this technique would work for creating a thin composite shell. So I decided to make the first dish of ferro-cement with two layers of chicken wire and mortar (sand and cement 3:1).

I removed the pattern from the bar and covered the form with a very thin (0.27 mil) piece of plastic sheet to prevent the dish from sticking to the form. Regular plastic wrap, wax paper, etc. would work as well. Above the plastic wrap I laid approximately two layers of light weight chicken wire. The wire will lay quite close to the form if you make a few radial cuts in the wire towards the edge of the dish and tighten the chicken wire at the center by twisting loops. Extra wire towards the edge of the dish is probably a good idea for strength.

When the wire was pretty close to the surface of the form, I reattached the pattern to the bar. The pattern was approximately 34 inch above the form. By rotating the pattern above the form I could see where the wire needed to be worked closer to the form. I didn't mind if the pattern touched the wire but I wanted the pattern to move easily around the form.

Above: After the form is done, Cody When the pattern would swing fairly easily around the form without hanging up on the wire, I mixed a somewhat cement rich mortar — about 3 parts sand to 1 part cement. I then used the pattern to shape a 58 inch to 34 inch thick shell the same way I'd used it to create the form. When making the dish I had a little trouble with wayward chicken wire and I wished I'd spent a little more time with the wire before applying the mortar.

After the dish is formed, a number of small holes are poked in the still soft mortar about an inch and a half from the rim. These holes are evenly spaced and correspond to the number of aluminum reflector pieces used. In this case I needed 28 holes a little less than 6 inches apart. The holes allow a piece of wire to lace the ends of the reflector pieces to the rim of the dish. These holes can be made with a nail or small stick. Finally, I covered the dish with more plastic wrap, shaded it, and let the cement cure for about 4 days.

To remove the dish from the form I made several small wooden wedges and gently tapped them between the edge of the dish and the form. The dish came loose easily. Two people were able to lift it off and turn it

Brewer constructs a parabolic cooker.

upright. The finished dish weighs about 100 pounds, easily moved by two people. Adjusting its orientation to the sun takes one person. The dish feels stable and shows no sign of being troubled by animals or children. Since it is made of cement it will not rot or burn.

This dish requires nothing fancy in the way of a mount or support. A shallow hole in the ground works nicely. We used an old car tire at the contest. Point the cooker toward the sun by pulling up on one edge of the dish.

The Reflector

The reflector material is sheet aluminum 0.010-0.020 inches thick. This material is commonly available, at least in the U.S., as aluminum roof flashing in rolls of various widths by 50 feet long. One roll weighs about three pounds and provides more than enough material for one cooker. The cost of the reflector material is about two thirds of the cost of the whole cooker.

One could build this cooker using aluminum foil, reflectorized mylar, or flattened aluminum cans for less money. However, this initial savings will likely involve a sacrifice of performance, durability, repairability, and ease of construction. Given the moderate cost, durability, ease of working, ability to conform to an efficient parabolic curve, and ease of repair it would be hard to beat this aluminum flashing. Initially cheap is often not ultimately cheap.

The aluminum is easily cut with a sharp knife and straight edge. Make two or three firm passes with the knife and the scored aluminum readily bends and breaks along the scored line.

The number of reflector strips must correspond to the number of holes previously cast in the dish. The reflector could be made of various width strips depending on the availability of aluminum. The wider the strip the less cutting of aluminum, but the more the shape of the reflector will depart from the desired parabolic shape. The design of this cooker is based upon the use of an eight inch wide aluminum roll. Twenty-eight reflector pieces are required to easily cover the interior of the dish. The aluminum is cut (see p. 35) to minimize waste.

After the reflector pieces are cut, punch or drill a hole near the narrow end. Stack all of the pieces up, drill a small hole near the narrow end and drop a small machine bolt through the hole. This assembly of reflector pieces can be secured to the inside face of the dish by placing the bolt in the center hole of the dish. Secure the bolt in place with a nut and washer(s) on the outside. Leave the nut loose enough to allow the reflector pieces to be fanned out evenly around the dish so that the entire inside surface of the dish is covered. This same arrangement can be accomplished with punch (nail, sharp stick) and short length of wire or string, as locally available.

The outer edges of the reflector are simply laced to edge of the dish. Overlap two adjacent reflector pieces over one of the holes cast in the rim of the dish. The hole in the dish is used as a guide to punch or drill the hole in the reflector pieces. A piece of wire is threaded through these holes and in and out the other holes around the dish. Separate wires could be used at each overlap of the reflector pieces, but using one wire laced around the entire dish gives a nicer appearance.

The Pot Support

The pot support keeps the cooking pot at the focus of the parabolic cone regardless of the angle of the cooker, and provides an easy way to aim the cooker at the sun. This support is simply a couple of slender pieces of wood or metal about 60 inches long. These are sprung across the rim of the cooker at right angles to one another, wired or tied together where they cross.

If the cooker is tilted, one of these cross pieces should be oriented so that it runs from the high point to the low

Above: David Baty and Cody Brewer at the 1993 Home Power Solar Cooker Contest.

point of the dish. This cross piece will provide a support from which to hang a cooking pot at the focus of the parabola. The other cross piece stabilizes the first piece against movement from side to side. The shadow of the intersection of these pieces on the center of the parabola indicates when the cooker is aimed directly at the sun.

The theoretical focal length is 11 inches, a point safely inside the parabolic curve. The focus of this parabola is soft; it concentrates the sun's energy to a hot spot, but not a dangerously hot point. This spot is easily located with one's hand when the cooker is aimed at the sun. It is also useful to learn how the focal point changes when the cooker is no longer aimed directly at the sun. I imagine that learning to cook on a parabolic type cooker would be analogous to cooking over a fire. The cooker must be tended from time to time to maintain heat by reorienting to the sun, just as a fire must be fed more fuel to maintain heat. Within both the cooker and a fire there are naturally hotter and cooler areas, and both heat sources if left untended will gradually cool off.

The cooker develops temperatures high enough for frying food. Popcorn and espresso are now culinary events that are quite easily done in a solar cooker. One can cook more than a single dish of beans and rice during the course of a day. We have used it to boil a quart of water in about 12 minutes. One parabolic cooker has the potential of doing the same amount of cooking as several box type cookers. Higher temperatures allow more types of cooking techniques. A parabolic cooker can be used to purify drinking water, heat bath water, etc. During less than ideal weather a parabolic cooker is more likely than a box cooker to have the solar gain required to cook a meal.

The Future

Cody Brewer and I have been experimenting with this cooker for just a little over two months and have experimented with different materials for the dish and construction details. We are planning to do a "production run" of 20-50 dishes to refine construction techniques for building these cookers in non-industrialized areas. We encourage anyone interested in the cooker, particularly its production, to contact us.

Access

Author: David Baty, 2929 M.L.King Way, Berkeley, CA 94703 • 510848-5951.

A version of these construction notes and full sized blueprint of the pattern are available for $5 postage paid at the above address.

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