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Report from the Grupo Faísca in Mozambique

Largely based on our work with solar energy, but we do many other things


Our group and our work

I and a small group of young people are working on 'Technology for Young People', a series of little films and booklets and hope to get some of the films on national tv soon. We call ourselves 'The Spark Group' (O Grupo Faísca' in our Portuguese) Within this main effort we specialise in using locally-available resources to do the demonstrations and make the devices we use.

We cover a  wide range, from simple physics and chemistry to health, agriculture, solar devices and technology in general. Everything is pertinent to Mozambique especially but would probably be suitable for sub-Saharan Africa, and perhaps elsewhere, with modifications.

We are not really doing these things for a campaign directly to communities but to show young people what is possible and then encouraging them to do likewise and better. So when I explain what we are doing, you will see this bias towards education, young people, Mozambique and cheap, local resources.

We are not at all formal. In a sense we do what we fancy doing - what we happen to think. "It would be great to do that!" For the films especially we try to present stuff in an attractive (and sometimes rather comical) way. In the future we will assemble the materials in a curriculum.

More advanced work

I have another side to my interest, and that is, in a way, the higher technology of solar devices. Again I am interested in simplicity, but this time in more of what the mathematicians would call 'elegant' solutions to problems. This sometimes leads to low-cost devices. I enjoy calling up in my imagination some device, first perhaps a mere idea, a crudity. Then changing it, still in my imagination, I like to 'dream' of such devices, but my dreams are based on a long experience of practical devices of all sorts in which I have always taken an interest (including traditional technology in Asia and Africa), and on my background in Physics. In fact it is one of my great pleasures to indulge in such dreams at the end of the day over a glass of something under the stars on the veranda of one of the little outdoor bars so common in our part of Africa.

Parabolic and similar solar concentrators

Recently, I've been thinking in this way of various aspects of big solar concentrating reflectors for cooking and how to make them simply. Digging back into my high school maths, I remember that near the vertex of a parabola the curvature is approximately spherical, and the more so the nearer  to the centre of curvature and the longer the distance to the focus. So I do not need to design an exact paraboloidal device (which is difficult to make) but a spherical shape (which is easy). The focus is not so well-defined, but we do not need a fine focus to heat a pot.

Then again, I know some of the theory of non-imaging concentrators (e.g. funnels). So if I have a dispersed focus, firing it into a funnel will bring the rays together enough to go, for  example, into a ‘Scheffler through-the-wall oven’. For those who do not know this, it consists of a big parabolic reflector which sends the sunlight through a hole in the wall of a kitchen into an oven inside the kitchen. 

A challenge to make a big cooker

I have a friend to whom I demonstrated a 'Cookit' stove (a neat small portable set of mirrors which reflect sunlight and heat on to a cooking pot). He runs a little restaurant and bar a short way out into the countryside, and was impressed by the Cookit. He said, "But at weekends, people come to eat cows head. Can you make me a stove that works off the sun big enough to cook one of these?" He has a brazier (a kind of barbecue) made from an oil barrel cut in half along the axis and punched through with holes. "But it takes so much wood," he said. Firewood is costly . In fact the area where he lives is rather denuded of trees, a common problem here.

I confess I don't like roasted cows head bits, cheeks, brains, tongue, eyeballs...  especially if the cooking is so crude as to leave in a good deal of burnt hair. But it's popular here.

Anyway, that's the challenge for me to dream over. Now I know there are classic solutions (the Solar Cooking Archive sponsored by Solar Cookers International) is one of my bibles), but they are all rather expensive to make. So first I think simply, 'It's got to be a big collecting area'. There's no way around that. Considering the size of a cow's head and the need to cook it in  two or three hours, I reckon it needs a minimum of three kilowatts. So let's say a collecting area of four square metres. Sunlight comes down here at a kilowatt per square metre in our summer.

My hope is to make a shallow parabola as the reflector using a circle of hardboard (unitex, sellotex; I don't know the term for it in other parts of the world, but I mean the compressed wood dust and glue that is used for cheap panelled doors). The hardboard will have aluminium cooking foil glued on.

After a bit of πr2 and so on, I see we need an area with a diameter of about two and a half metres, which is a bit more than the eight feet square hardboard that I can buy here So a circle cut out of this would do, approximately, but be a little bit less than I need.

Now I am one of those persons who always likes to get a bit more than I need, so as to have something in hand. My wife complains that if I need a hammer, I buy two. Yes, but that's because my team keeps borrowing such tools and they sometimes don't come back. It's worth the extra one or two dollars to have a hammer on hand on a Sunday when I can't nip out and buy one and there I am frustrated with a possibly brilliant idea I can't put into practice for want of a hammer.

So there remains in my mind the idea that I'm going to be a bit short on collecting area. But my imaginary calculations are vague anyway, so perhaps it doesn't matter. Still, it niggles there in my mind, even when I’m not thinking about it. (To niggle: to agitate and annoy.) There is a story about the famous Irish mathematician Hamilton who was looking for a solution to dealing with a problem connected with the square root of minus one (concerned with 'quaternions'). After many months of vain thought with  the problem niggling in the back of his mind, the solution came to him when he was not thinking about it. In 1843, on the way to a meeting of the Royal Irish Academy, of which he was President, a flash of inspiration came to him. He jumped out of his carriage and inscribed his basic quaternion formula on Brougham Bridge, on the Royal Canal, so that he wouldn't forget it. A habitual scribbler, Hamilton is reputed to have carried out some of his calculations on his finger nails and the shell of his breakfast egg.

Non-circular concentrators

What lay niggling in my mind was the thought that reflector concentrators need not be circular and that if they were actual squares of eight feet on the side, they would give me six kilowatts. And square sheets are much easier to make and to mount in a frame. The general image of a solar concentrator is a circular object, but there is no logical reason for it to be so. And if square, why not rectangular, using the sheets of the more common size, which is eight by four feet?

Now, how could I make an eight-foot sheet of hardboard into a concave shape, a concentrating reflector, cheaply and easily? Well, one way would be to wet it with boiling water or steam and press it on to a mould. I could do this, but it would be laborious, making a concrete convex mould, etc. 

Inspiration from the gods

But is there a more 'elegant' way, more economical of effort? I sit and dream. I sometimes like to think there is a tutelary god of inventors (for the Greeks it was Daedalus) looking down on me and my efforts. He urges me, "Come on, think wide! Think laterally!" (He has read Edward de Bono.)

Making a flat sheet into a dish shape

Suppose I make a wooden frame and nail the circle of hardboard on it, then pull the middle of the circle down with a long bolt to a cross-piece at the back of the frame? Will it pull down to an approximately spheroidal or paraboloidal shape?  Quite possibly. There is no doubt a mathematical model solution but I don't know it. The Internet is wonderful, but sometimes you don't know how to ask Google for what you want. This is a case in point.

But at least I imagine that even if it does not give a good focus, the focal area may be reasonably small. (The physicists quaintly call this 'the circle of least confusion'.) It imagine it being small enough to fire into a mirror funnel to send the heat rays into the 'through-the-wall oven'. So it will be a simplified 'Scheffler' oven.

But now, more elegance. Abolish the awkward wooden frame. "Consider a suspension bridge," says god Daedalus. "Use wires". So, taking the tip, instead of pulling the middle down, I consider pulling the circumference up. Most elegantly by putting say 20 wires across the diameter. Or, since these need a considerable tension to pull the edge up, better would be a radial 'tent' of wires going from points on the circumference to a central thin rod. So, in a way, after a lot of thought, I have invented the umbrella.

So that is where I am at as I write these words. I will do the experiment when I can; that is, when I have finished writing the Teachers Guide to Technology for Grade Six which was supposed to be given in to Macmillan Publishers last month.

Non-circular again

Could I do much the same thing if the frame were square, using the common square or rectangular hardboard straight from the shop, a rectangular umbrella?  This is something I'll have to try in practice since the maths is beyond me. I've tried it practically in a half-hearted way but at present we are in our winter (June) and the sun is not strong. Another problem is that our puppet theatre happens to want a rectangle of hardboard for its scenery and has carried my piece of hardboard off, so solar technology is sacrificed to art.

A through-the-wall oven and perfect black bodies

These experiments are, as I said, to try to make a high-power through-the-wall oven. So now I 'dream' about this, and call on my old physics knowledge.

The concentrating reflector, whatever form it finally assumes, must produce a concentrated patch of radiation of (I am still imagining) about 30 cm diameter to put through a reflecting funnel into the oven.

But the hole into the oven must be as small as possible because we want as little as possible to come back out of the oven. This is common sense but it is also formalised in radiation theory, as found in any first year university text book on Heat. This defines a 'perfect black body' as a body that absorbs 100% of the heat radiation that falls on it, and reflects zero. This cannot be realised in practice. The Solar List pages suggest various surface finishes that are good, but none are very good. An approximation to the ideal 100% absorber is not a surface but a hole in a hollow sphere. The radiation goes into the sphere through the hole. The inside walls of the hollow sphere are as black as possible. The hole is as small as possible. Of what radiation goes on, only a tiny proportion goes out. The bigger the sphere and the smaller the hole, the less radiation can find its way out.

So this is the theoretical background to the 'through-the-wall' oven. As small an entrance orifice as possible, everything as black as possible inside, and as big an inside volume as possible.

However, the bigger the oven is inside, the greater the losses by conduction through the walls. And since their heat loss depends on their area, an oven of twice the linear dimensions loses heat at four times the rate. So there is a balance to be struck.

If the cooking pot inside the oven is directly behind the hole, then the incoming heat radiation beats directly on it and will reflect a lot of itself back through the hole. Thus it will be better to put the cooking pot out of the line of entry of the incoming rays.

Now remember, I am just imagining, visualising and thinking about this, so I don't know at this stage what are the real significances of these considerations. Perhaps the effects are too small to worry about. Only real physical experimentation will tell. Which takes time, space and equipment. Preferably sensitive and exact equipment which will measure heat arrival rate and temperatures exactly enough to trace improvements. Well, when I have the time and the money...

Research supervision

Or, if someone else has these facilities, I would like to supervise research on the ideas. Well, 'supervise' sounds heavy. I mean I could suggest and provide ideas and helpful observations. However, 'supervise' might be an acceptable term since I have a Masters degree and am nearly seventy years old. Does anyone who is reading this know of someone who might like such 'supervision'? By e-mail?  Here in Mozambique I haven't found anyone, but I'd like to pass my experience on. One day some of the young people who work with me will be something of this sort I hope, but they still have a long way to go. On the other hand, at my age, perhaps I don't have a long way to go, particularly if I don't give up smoking.

Complex sun-follower mechanisms

I often read with interest  (generally in the solar discussion group concerned with photo-voltaic panels) about the mechanisms to make a solar device follow the diurnal movements of the sun. They generally involve micro-processors and servos. For the Scheffler ovens we will need something to do this. Now how to do it in a rural village? Well, this is  where countries like ours have an advantage that would not be considered in richer countries. We  would use a boy or a girl. Cheap, accurate and always available. "Here's a pair of sunglasses. Keep that hot spot going into that funnel. Do it about every quarter of an hour."  And provide them with a little shelter from the sun, a chair and table to do their homework and, as salary, a supply of the books they need for school. Some who go to school in the morning could do the controlling in the afternoon, and vice versa. I imagine their friends coming to admire the oven and the controller kids explaining how it all works, and so on. And we'll put a communal guitar in the shelter so that who comes can play.

Social aspects

I am interested in the social aspects of the use of solar technology, from how to interest people in the devices, to why they prefer cooking with wood, with the pot supported on three stones. The use of three stones connects with the local mythology and the need to keep contact with people’s ancestors. This is not trivial; you find it in West Africa, in Mali for example. There is a sense in which the domestic hearth and its traditions are sacred. Changes in such cultural habits are hard to make.

The suck-down mirror basin

One of  our best demonstrations of solar heating is the highly successful mirror concentrator. We stretch and glue a piece of thin mirror plastic over a half-metre-wide plastic bowl. All the stationers' shops here sell the plastic for wrapping birthday presents. One side is mirror; the other has a pattern on it. Then with a thin tube (the covering pulled off a scrap piece of electric cable) glued through the side of the bowl, we suck air out and the plastic goes down to make a marvellous concave mirror. Very efficient. Even at this size (the 50x50cm  limit of the plastic wrapper size) and the hot spot focus directed on to the bottom of a small frying pan, it will fry an egg .

Demonstration and film

In the film we have made on this basin reflector, we show how the sunlight focuses by reflecting a beam of sunlight into our dark workshop so that it lands on the wall as a big bright spot. Then we waft a smouldering rag around, and its smoke shows the beam very clearly. Then we put our concave mirror in the path of the beam and see how the beam concentrates to a focus. It is dramatic and actually rather beautiful. It certainly demonstrates clearly how the light reflects off the mirror.

Then we take the mirror outside and focus it on to a tree stump which quickly smoulders and bits of  bark on it burst into flame.

Next we put the mirror on the ground, tilted sunward, and put a black frying pan above it with the hot spot focused on the bottom. It fries an egg in about fifteen minutes.

We were doing this in the garden of the National Institute for Educational Research where we often work, and the cooks from the kitchen came and marvelled. While our young people were eating the eggs between slices of bread, the policeman guard of the place arrived and also marvelled, just as I was going out to buy cigarettes. Our lads asked him if he would like to see how they did it. "Yes," he said but then added, "But now we'll have to wait until the white man comes back or it won't work." The lads were very amused.

To the policeman, it was just Western magic. Others ask, "Where are the batteries?" and are amazed there is nothing but air in the basin.

We make our films to help to change this ignorance and improve people's knowledge about technology, especially by using common objects and referring to commonly-known processes so that people can see and feel that it is nothing magical but all a rather every-day kind of thing.

Welding plastic

One process we have worked out is welding plastic things by focusing the sun on them with a lens. It works very well with a 10 centimetre diameter hand lens (one of those that old people used to read their bibles before spectacles were as easy to get as they are now). This was a suggestion from Barbara Kerr via Bob Culbertson. Every family here uses plastic bowls and buckets, and they are costly things for poor people to buy. Yet there are lots of them that have split and now are on scrap heaps for lack of an inexpensive method of repairing them. Well, our solar welding works well. The hot spot at the focus of the lens is our 'flame', and we usually melt a thin strip of extra plastic into the split to provide more material, as one would use a welding rod for metal.


Often, as we do these experiments, we film them, with the hope of putting them out in future on national tv. Well, not thinking perhaps to use these actual shots but as a 'draft' to guide a later film, shot more carefully. In fact at this very moment while I'm typing this, two young people out in my yard are filming the welding of plastic. It works well and will make a nice and useful film.

Welding plastic - the open water lens

The useful thing about the lens method of welding plastic is that it has an easily variable heating area and intensity, just by varying the distance and going in and out of focus.

This is potentially a very useful process if someone wants to set up a little business repairing bowls and buckets - and it could be a reasonably profitable one with a wide market. But big lenses are impossible to get except perhaps in Maputo, and even then too expensive for the kind of person that would want to set up such a business. And yet it is just the kind of thing that people want - a little income-generating activity. Now this is what our little group is usually quite good at - suggesting methods for doing something which might lead to making a living, but by inexpensive methods.

Bike wheel rim water lens

A couple of years ago, we experimented with a lens made of water. It arose from some joke about making a lens of ice. We got the rim of a bicycle wheel and stretched a sheet of transparent plastic over it and glued it on, supported it horizontally, and then poured water on to the plastic. Naturally, this depressed it into a plano-convex lens. With the sun overhead it made a good concentrator and even when the sun moved lower down the sky, it still produced a good heating effect at the focus. We just moved the frying pan around on the ground to get the hot spot on the egg, which it fairly quickly fried. We didn't get round to putting a tent of clear plastic over it to stop the wind taking heat away, but we meant to do so on some other day, but we never got round to it.

Wind rippling the top surface was a trouble but we could get rid of this by floating a layer of plastic on it, which however again we did not try.  To make it more portable, we thought of doing as is sometimes done by a lighting assistant in making films outdoors (to fill in shadows on the subject's sunlit face). He has a large circular reflector consisting of a sheet of aluminised cloth stretched on a circular frame of flexible plastic tube. After use, he grasps the circumference tubing with both hands (across a diameter). He twists one hand one way and the other hand the other way, and the circular tube does a transformation into two circles which then lie on top of each other, and the thing is half the size.  Again, we haven't tried it out. Well, as I write this, I've just been outside where I've got a similar thing (from when we were experimenting making hula-hoops for children) and confirmed that the folding idea works. It does. The idea was that it would be good to take such a foldable ring with a clear plastic membrane, on a picnic for boiling tea. It weighs nothing, folds small, and water is available from a nearby stream.

A bi-convex water lens

Then, a couple of weeks ago, we though of the same idea of a water lens for making a cheap big lens for doing our welding of plastic buckets. We glued a sheet of transparent plastic (a lot is around here as trash) over the rim of a plastic bowl whose bottom we had cut out. (Using contact glue, the kind all shoe-repairers here use). Then we poured water on the plastic to make it go down into the convex shape and put it on a stand and let the sun shine through it, focusing on to the plastic to be melted.

In fact for the first experiment the plastic sheet we used was 'Gladwrap', 'Clingfilm' or whatever - the stuff you wrap food in before putting it in the fridge. Thicker plastic needs more water on top to force it down, but we prefer common plastic because it is commoner than Clingfilm. So we now use thicker common plastic and pour hot water in to soften it, It then goes down to the curvature we need. In fact is stays in this shape when we take the water out.

The lens will only work horizontally, obviously. One problem we met was that although it melted the plastic well, the focal area was too big and melted too big an area. It melted a hole rather than produced a line weld as we wanted. That is due partly normal spherical aberration producing a diffuse focus. That is, a spherical-surfaced lens is the wrong shape except at small apertures compared with the focal length.  We could improve that by putting in less water and having a longer focal length, but then the image of the sun (which is of course what the hot spot is) would be larger, thus defeating our purpose. Also it is partly due to the possible fact that the surface is not actually even spherical. A mathematician would soon tell me because this liquid-on-a-membrane problem is a classic bit of integration. Ah, I could have done it when I was in high school but now the solution is beyond me. I should ask a mathematician here, but again... there is a lot I don't get round to doing.

Recently I reverted to thoughts about this water lens. Two of the young people who work with me have made a lens by putting water between two sheets of thin plastic sheet (Clingfilm) clamped in a metal ring of about 15cm diameter. A thin pipe goes in to fill the space between the sheets with  water, and the plastic bulges out to make a double convex lens. The metal ring is in fact the rim of a strainer (one of those hemispheres of fine net that cooks use to sieve flour through) with the wire mesh taken out. And 'clamped' gives the wrong idea - it is merely glued to the rim. All very simple and easy. It would be very difficult to get a big glass lens here in Mozambique unless one was lucky enough to come across and old enlarger. But with our lens, anyone even in a remote village can do it.

It makes a good lens and it works  as a welder of plastic almost as well as the glass hand lens. Its focus is a little more diffuse because the surface is probably not the proper shape for a lens. That is, it suffers from aberration, and this is very noticeable with such a wide lens (which we need to for its collecting area) and its small radius of curvature (which we need for getting a small spot). These factors are incompatible so we've got to get something that compromises.

Keith Warren