Solar Cooking on the Roof

December 26, 1999

Copyright (c) 1999 David M. Delaney

Abstract:   Bashir Ahmad reports that many people in Gujarat have only an insecure rooftop as a solar cooking place.  This article suggests how to simplify carrying a solar cooker and pot to and from such a cooking place.

Contents

Introduction
The Goal
Suitable Cookers
A Suitable Pot
A One Handed Carry
Prototyping a Greenhouse Pot
References
Author's Address

Introduction

An article by Bashir Ahmad [Ahmad1] describes why people in Gujarat do not use their solar cookers.

Over a period of 20 years, an Indian national program has arranged to put 45,000 solar cookers in the hands of people in Gujarat, a state of India with a population of 40 million.  Ahmad says that 75% of the participants in this program do not use their cookers, and half the remainder use their cookers only sporadically.  In other words, only one eighth of the participants obtain the promised benefits of solar cooking.

In Gujarat, conditions seem ideal for solar cooking.  The sun shines intensely for nine months a year. Fuels for cooking are expensive.  Rice and dal, the staple foods of the region, are easily cooked in solar cookers.  In spite of these favorable circumstances,  difficulties surrounding solar cooking prevent many users from continuing with it.

One difficulty has to do with the cooking place. Many people must resort to the roof of a building or some other location away from their living space.  Because these locations are not secure, cookers cannot be left at the cooking place, but must be carried there before each cooking session, together with food and pots.  After the session, all must be returned.

The box cookers and the pots used in Gujarat are not suitable for this frequent two-way transport, being too large, heavy, or awkward. To give solar cooking its best chance of acceptance in these circumstances, cookers and pots should be as easy to carry as possible  This article makes some suggestions in this direction.

I do not claim that solving this problem removes the other issues that Ahmad identifies.  His article should be required reading for anyone who claims to offer "appropriate technology".

But dissatisfaction is sometimes a threshold phenomenon.  When a task offers one problem too many, we may become intolerant of all its problems.  Sometimes, reducing the total burden just a little will soften our intolerance of the remaining problems.

The Goal

A small person should be able to carry pot, food, and cooker with one arm.   One hand should remain free for steadying contact with walls and stair rails, for opening doors, and for clearing space at the destination to put down the pot and cooker.

This goal can be met with folding panel cookers [Bernard1]and pots of a specific design.
 

Suitable Cookers

A solar panel cooker consists of a number of flat reflecting surfaces, one serving as the floor of the cooker, and the others as walls.  The cooker has no roof, and admits sunlight freely from above.  When the cooker is working, a black steel or aluminum food container rests on a small stand on the floor.  The stand elevates the food container so that  light reflected from the floor may strike the bottom surface of the container.  A transparent greenhouse encloses the food container and the stand. Rays of sunlight bounce, sometimes more than once, from the walls and the floor of the cooker to strike the surfaces of the food container, heating it. The greenhouse retains the heat. The food cooks.

For this discussion, the most important thing about panel cookers is that they can be folded flat.  Here's a panel cooker consisting of two pieces, each of which has a single fold:

Folding panel cookers are sometimes designed to be carried in backpacks or vehicles for days at a time.  These cookers trade cooking capacity for space, providing meals for only one or two people.  For rooftop cooking, we can make a folding cooker as large (and as powerful) as we please, provided that its user can carry it easily for short distances.  When folded, even a large solar panel cooker fits comfortably under one arm if fitted with a shoulder sling.

Roger Bernard, in his book La  Cuisson Solaire Facile, [Bernard3] describes several folding solar panel cookers, including the one pictured above, which he calls the "Mini". See his book for details of how the two pieces of the Mini are held together while cooking. (The geometry of the reflective surfaces of the Mini is identical to that of another of Bernard's cookers, the very effective,  but non-folding, ROB. [Bernard2] ) With shoulder slings, all of the cookers in Bernard's book are suitable for rooftop cooking.  The smaller designs can be made larger, if necessary, without becoming difficult to carry on stairs.

A Suitable Pot

Although the cooker pictured above (the Mini) is suitable for rooftop cooking, the pot, greenhouse, and stand shown with it are not suitable. As three items,  they number more than the one available hand. The best way to carry food, whether hot or cold, is in the pot.  The best way to carry a greenhouse enclosure is to integrate it with the pot -- when you carry the greenhouse enclosure you carry the pot.  The best way to carry the stand is to extend the bottom of the greenhouse so that it doubles as the stand.

A greenhouse pot  [Delaney1]  is a nested assembly consisting of a rigid transparent greenhouse (which also serves as a stand), a food container, and a lid for the food container.  The greenhouse is of clear glass or plastic.  The food container is of steel or aluminum, black on the outside.  The lid is of clear glass, ideally, or perhaps plastic.  Steel or aluminum lids, black on top, are also effective. [Delaney2]

The food container nests in the greenhouse while food is prepared, cooked, and served.  It leaves the greenhouse only for cleaning.  Handles on the food container would serve no purpose -- any need for handles being met by the handles of the greenhouse.
 

A One Handed Carry

I remember a picture of a woman carrying two milk buckets hanging by ropes from the ends of a wooden yoke resting on her shoulders. The ropes are long, so the buckets swing near the ground. The milk won't spill, because the long ropes reduce the forces on the buckets due to  movements of the yoke.  This remembered picture suggests how a single bare hand can safely carry a full, hot, greenhouse pot.

Consider a sling made from a continuous loop of soft rope or cloth tube.

The sling should be long enough to place the center of mass of the pot about half a meter below the hand that grasps the sling.  When the arm is extended straight down, the bottom of the greenhouse should hang just high enough to clear stairs.

The sling is not needed while preparing, cooking, and serving food. To avoid getting in the way, it attaches quickly to the greenhouse pot before transport, and detaches quickly from it after transport.

Dual purpose handles on the greenhouse provide points of attachment. Each handle forms an inverted necked hook that catches a loop of the sling.  The sling remains attached only while it is under tension.

Protoyping the Greenhouse Pot

No one has yet manufactured a greenhouse pot.  Prototypes are needed to test usability and user satisfaction before investing in tooling.    A prototype should have handling characteristics similar to those of the design intended for mass production.  A reasonable prototype can be assembled from available bowls, with the addition of a few specially fabricated components.

While the fabrication of prototype pots will have high labor costs,  tooling costs for fabrication would be much lower than for a mass production design.  A few hundred or a thousand units of a prototype could be manufactured before their total cost would equal the cost of the same number of units of a mass production unit.  Beyond some small number of thousands, a unit designed for mass production would have a much lower unit cost.

Glass and steel bowls are the starting point for the prototype.  For the greenhouse, clear glass salad or mixing bowls are available in a small variety of sizes.   For the food container, stainless steel mixing bowls are available in a wider variety of sizes.  The challenge is to find a pairing of glass and steel bowls in which the steel bowl will hang in the glass bowl suspended by its lip, without projecting too far above the rim of the glass bowl.  The steel bowl should have no handle of any kind, and should have a prominently flared lip. There should be a space  3 to 8 centimeters high between the bottom of the steel bowl and the bottom of the glass bowl.

Once a suitable pair of bowls are chosen, the lid and handles probably have to be fabricated.  If the chosen steel bowl has a suitable lid, or the glass bowl suitable handles, so much the better.  This is unlikely.

A disk cut from window glass makes an adequate lid.  The diameter of the disk should be larger than the diameter of the top of the steel food container,  to provide a good seal, and to eliminate the need for precise centering.  Window glass will break if it drops even a short distance onto a hard surface. As the pot heats, a simple glass disk will tend to slide sideways, opening a crack to the atmosphere.  The disk slides when it becomes lubricated by condensed water vapor and begins to lift slightly as steam pressure increases in the food container.  Sliding would be particularly unfortunate when the pot is swinging on a sling.  These problems have been solved by Ernst Goetz, [Goetz1] who has developed a rubber profile, or bumper, for glass disk lids.

Goetz's profile is a 12 mm diameter grooved rubber rope which glues to the edge of the disk.  The groove which, runs the length of the rope, accepts the edge of the glass.  The groove is 4 mm wide and 6 mm deep to fit 4 mm window glass.  The profile comes in long pieces which can be cut to size for different glass disks.  When fitted to the disk and glued in place, the profile not only protects the disk from knocks, but also provides a 4 mm projection normal to the disk on both surfaces. This projection, which forms a circle around the outer edge of both surfaces of the disk, centers the disk on the food container and prevents lateral movement.

With a Goetz profile, the glass disk must have a diameter large enough that the profile clears the lip of the glass bowl.  If the profile rests on the glass bowl, the water seal between the glass lid and the steel food container will be spoiled.

Without the Goetz profile, a bead of silicone sealer around the outer edge of the lower surface of the lid will provide centering and prevent lateral movement, but will not provide protection against knocks.

In the drawing below, no separate handle is shown for the lid.  The rubber profile will serve as a handle, but will require two hands.  If a single handed lid is desired, a drawer pull can be glued to the center of the top surface of the lid.

Handles for the greenhouse may be shaped from wood.  The handles may be fastened to the greenhouse with epoxy or cyanoacrylate glue.   The base of the handle (the surface glued to the glass bowl) may be shaped approximately to the surface of the bowl to reduce the size of the gaps that must be bridged with glue. The use of a good crack filling glue might make this shaping step unnecessary.

The length of the handles, i.e. their extension from the side of the greenhouse, and their upward tilt, are determined by the need to position the groove for the sling so that the sling does not touch the rubber profile of the lid, and to raise the point of attachment of the sling well above the center of mass of the pot.  The drawing below suggests approximate dimensions for the handles.

The hypothetical food container shown has a volume of three liters.  It cooks only a little more than 2 liters of food, however, because some unused space at the top of the container is reserved to permit water to slosh around without spilling.

An alternative to these rectangular wooden handles, a handle yoke, exploits a projecting lip on the greenhouse bowl.  The cost of materials suitable for a handle yoke  may be higher in some places.  Transparent polycarbonate 6 mm thick is suitable.

If permanently attached, which is certainly convenient, a handle yoke of an opaque material like plywood may throw too much shade on the pot.   The user may be content to attach a handle yoke only for transport, however.   This choice may mildly improve security against theft, a hot pot being harder to handle without handles.

The greenhouse bowl fits into the hole of the yoke.  The diameter of the hole is equal to the outer diameter of the greenhouse just below its lip.  The whole greenhouse except its lip will pass through the hole. The handle yoke should be glued to the greenhouse so that its  upper surface bears against the lower aspect of the lip of the greenhouse.  With the handle yoke in this position, the glue is not stressed by the weight of the pot.
 

References

[Ahmad1] Ahmad, Bashir. Solar Cooking Technology - How Far are Technology Promoters and Users From Each Other?, http://solarcooking.org/gujarat.htm, November 1999, Department of Buildings and Energy, Technical University of Denmark, email: ba@ibe.dtu.dk.

[Bernard1] Bernard, Roger. The Solar Panel Cooker,  Journal of Solar Box Cooking #16,  http://www.solarcooking.org/spc.htm

[Bernard2]  Bernard, Roger. Improving the Solar Panel Cooker,
Journal of Solar Box Cooking No. 18, http://www.solarcooking.org/newpanel.htm.

[Bernard3] Bernard, Roger.  "La Cuisson Solaire Facile", Editions Jouvence, 1999, ISBN 2-88353-172-2.  91 pages.

[Delaney1] Delaney, David M.  Pot with Integrated Greenhouse for Solar Panel Cooker,  March 28, 1999,  http://www.geocities.com/~dmdelaney/intgrnhouse/conv-pot.htm

[Delaney2] Delaney, David M. Comparison of Lids for Greenhouse Pots for Solar Panel
Cookers, November 15, 1999,  http://www.geocities.com/~dmdelaney/lidcompare/lidcompare.htm

[Goetz1] Goetz, Ernst. Impact resistant glass lids for pots for solar cookers,  pamphlet obtained directly from the author, September 1999, Ernst Goetz,  Herrenmattstrasse 11, 4132 Muttenz, Switzerland, solarco@bluewin.ch
 

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Author's Address:

David M. Delaney
142 Waverley Street, Apt. 2A
Ottawa, Canada
K2P 0V4