Case Study of Solar Stoves

Made and Used in the Peruvian Andes

For the Peru Children’s Trust.

  Contents

  1. Introduction
  2. Why use a solar stove?
  3. Firewood and Health issues.
  4. How does a solar stove work?
  5. Precautions for use of solar stoves.
  6. Materials required.
  7. Parts list & technical drawings.
  8. Tools required.
  9. Building the solar stove.
  10. Maintenance and caring for your solar stove.
  11. Cooking tips.

 

 

 

  1. Introduction.

The Peru Children’s Trust is a small Christian charity run by a couple called Manuel and Gay Reynaga. They work in the high Andes of Peru in the city and shantytowns of Huancayo. The main work of the charity is to support 100 of the more disadvantaged children through sponsorship, which pays towards schooling costs. They also support the families of these children with medical and pastoral care and have initiated a number of small business development schemes from carrot farming to brick manufacturing.

In 1997 Manuel and Gay came to our Youth Group (EnigmaSt Mary’s Maidenhead) to talk about the Peru Children’s Trust. Among other things they mentioned the fact that much time and money is spent on collecting firewood and buying gas to cook with. They also mentioned that the sun is very hot during the day and it should be possible to use the sun’s energy to cook with.

Being an engineer, this got me thinking, so I did some research and found a number of useful resources. The most inspiring book was ‘Cooking with the Sun: How to Build and Use Solar Cookers’ by Beth and Dan Halacy.

So during the summer of 1999 my wife and I set about making our first and second prototypes. The first was a bit wobbly and would only boil water if there were no wind. The second was much more efficient but a bit time consuming to make. But the joy and glee of boiling your first potatoes, or frying your first sausages is immense!

Our third model was more stable, simpler to make and more efficient. It is this model that we decided would be best for Peru, so this case study is mainly concerned with the manufacture and use of this solar stove.

In September 1999 a group of seven people from St Mary's went out to Peru to visit the Reynagas and to help with their work in a variety of ways. The seven included David & Gill Coe, Jim & Ali Peck, Quinton & Jody Stowell and Vicky Warren. We spent three days in Lima, did a ten day tour around Peru and spent the remainder of our four week trip with the Reynagas. Apart from visiting many of the sponsored families and passing on gifts from English sponsors, we also organised two 'Fun Days' for the 100 sponsored children. This involved sketches (The Prodigal Son), art and crafts, active games, lots of food, and for some of the older children - building stoves. It was an exhausting few days but very rewarding.

  1. Why use a Solar Stove?

The heat energy produced by the sun is immense. In equatorial regions the solar radiation can exceed 1000 Watts/m2. That is equivalent to half the power of an electric kettle whenever there is good sunlight. It only takes 10 – 15 minutes to boil water on a solar stove. And it’s free, as long as you have clear blue skies! (Not much use for us British then!) The material costs are about $10 per stove which with some sponsorship is feasible to raise, and they are fun to make.

In Peru, like many equatorial countries the main method of cooking is with wood fires or butane gas. Among the poorer families firewood is usually the main option as gas is relatively expensive. So a free source of energy for cooking is an exciting possibility.

  1. Firewood and Health issues.

The ‘Manual for solar box cookers’ published by Technology for Life, Finland quotes: -

"About 2000 million people, over one-third of the population of the world, are daily dependent on firewood as the source of their cooking and heating energy. They live in the tropics, in the most favourable areas for the use of solar energy. Every year the cutting of firewood results in the loss of 20,000 - 25,000 km2 of tropical forests (UNEP).

The use of solar cookers also brings with in important health benefits. Diseases spread through contaminated water cause 80% of the illnesses in the world (WHO). Heating water to the pasteurisation temperature of about 60 0C destroys disease organisms. This temperature is easily achieved with solar cookers. Acute respiratory infections (ARI) are the cause of death for millions of children in the world each year. The large majority of these casualties occur in the developing countries as a result of polluted indoor air due to cooking over open fires in houses without chimneys or ventilation. This problem could be greatly reduced by using solar cookers, which are, of course, completely smokeless."

  1. How does a solar stove work?

There are two basic methods of collecting enough heat from the sun to cook. These are commonly described as the ‘Solar Box Cooker’ and the ‘Solar Stove’.

Solar Box Cooker.

The basic principle is to collect the heat by letting the sun light pass through a clear glass plate into a well-insulated enclosure. The light ‘trapped’ in the box and turns into heat when it is absorbed by the black cooking pot. The secret of a good Solar Box Cooker is to have good insulation with no air gaps and a good lid reflector to get the most light into the box. Cooking times are not that quick, but temperatures of 150 0C are possible.

Technology for Life, Finland has documented the Solar Box Cooker with great accuracy. For more information: - http://www.kaapeli.fi/~tep/projektit/index.html

 

Solar Stove.

The Solar Stove uses a parabolic reflector to focus the sunlight to one point. This produces the effect of a massive magnifying glass. With an accurate reflector it is possible to get ‘times 100’ magnification. We measured 420 0C in Peru. The reflector is aligned to point at the sun, and then the black cooking pot is placed on a grill at the focal point. Being black the pot absorbs most of the sunlight and creates sufficient heat for cooking.

  1. Precautions for use of solar stoves.

Do not look at the reflection of the sun at any time. Once the stove is in alignment with the sun, and the pot is at the focal point, then there should be no light scatter. If there is ‘stray’ light then the stove may need adjustment such that all the reflected light hits the pot (this will improve your cooking as well).

When adjusting the angle of the grill use thick gloves as the grill can get very hot.

  1. Materials required.

The most important material in the stove is the reflective aluminum sheet. This I was fortunate to obtain courtesy of Nordson UV Ltd (my employer) and Anocoil (who delivered the wrong grade and never collected it!). The size of sheet was limited to 400 mm x 620mm as this fitted nicely in my suitcase. The aluminium is available from a number of suppliers, but the reflective qualities are important. We used 0.3 mm Anocoil grade 710.33 which is 86% reflective and almost mirror quality.

The main enclosure is cut from 4 mm thick ‘Triple A’ plywood. An 8ft x 4ft sheet will make 21 stoves and only costs $5 in Peru. A few other bits of wood are required to make the framework within the stove (sizes detailed below).

As the focal point gets very hot it is necessary to make the grill and grill support from metal. We used mild steel and painted it silver, but any metal will do.

Many thanks to Pat Keogh for his kind donation that covered the cost of all the materials required making 10 stoves.

  1. Parts List and Technical Drawings. General assembly drawing

To view and print all the drawings together click here…

To download an Assembly Drawing as an AutoCAD file 49KB click here…

To download an Assembly drawing as a DXF (Drawing Exchange File) 94KB click here…

To download a CorelDraw 8 Assembly drawing 50KB click here…

To download an Assembly drawing as a PostScript file 52KB click here…

To download an Assembly drawing as a Windows Metafile 34KB click here…

Description

Size

Material

Quantity

Base

710 x 570 x 4

Plywood

1

Side

710 x B x 4

Plywood

2

End

560 x A x 4

Plywood

2

Frame – side

25 x 25 x 700

Wood

2

Frame – end

25 x 25 x 510

Wood

2

Frame – corner

25 x 25 x 130

Wood

4

Rib drawings

560 x B

Cardboard

2

Rib

560 x C

Cardboard

2

Rib

560 x D

Cardboard

2

Rib

560 x E

Cardboard

2

Rib

560 x F

Cardboard

1

Cross Rib

700 x 60

Cardboard

2

Cross Rib

700 x 30

Cardboard

2

Grill

350 x 12 x 2.5

Mild steel bar

2

Grill support

16 x 430

Mild steel tube

1

Pin

5 x 105

Mild steel bar

1

Base support – long

75 x 25 x 580

Wood

1

Base support – short

75 x 25 x 100

Wood

1

Reflector

620 x 400 x 0.3

Anocoil aluminium

2

 

  1. Tools Required

The Peru Children’s Trust has run a number of courses for the children in ‘workshop training’ so we were blessed with a good variety of tools and a couple of youngsters who could weld!

Useful tools include: Drill, jigsaw, hacksaw, vice, wood saw, sharp ‘Stanley’ knife, tape measure, square, metal file and hammer.

Optional tools include: Grinder and welder.

  1. Building the solar stove.

We split the work up into three sections. Manuel Reynaga was in charge of the metalwork, David Coe organised the lads (and one girl) in cutting the cardboard ribs, and I was in charge of the woodwork. We rotated the work, so they all had a go at each discipline. I was amazed how keen they all were to get things done. Once they saw the water boiling on our prototype they all wanted one!

Woodwork.

We made up some templates (thanks Ali Peck) for the sides and base, and cut them out using a 'jigsaw' power tool. The edges were filed flat. We then cut the frame parts to length and nailed the whole box together around the frame. See drawing WOOD for dimensions. Once the enclosure was assembled we made up the wooden SUPPORT at the base. This had the main purpose of holding the 16mm bar and grill in place. The support was nailed to the underside of the wooden box. To help align the stove with the sun we made a sundial by fixing a nail through the support at the edge parallel to the 16mm bar. By adjusting the stove to eliminate any 'shadow' of the nail it is easy to find the sun's location. See drawing GA for details.

Cardboard.

Finding enough cardboard for 10 stoves was surprisingly difficult, but the 'Lord provided' just enough. The reflector profile is spherical with a radius of 0.7 m (equal to the square size of the enclosure). This gives an almost parabolic shape with a focal point at 0.35 m from the reflector. We used the 'wine box' principle of interlocking cardboard to create the required shape for the reflector to fit into. Ideally the box should be square (0.7 m x 0.7 m) but as our aluminium was not big enough we had to make it only 0.56 m wide by 0.7 m long. I designed the rib shapes on the computer, but it only takes a bit of Pythagoras to work out each radius. See drawing RIBS and RIBSX for details. Each rib is 0.07 m apart and 11 ribs are required to make the shape (including the two wooden ends). Each rib has a different radius depending on its location. I have designed a GENERIC profile that can be scaled up to any size of stove. The larger the reflector area, the more effective the stove, so I have also designed a unit 1 m x 1m called Solar1000 that will give 2.5 time the heat of our Peruvian stove.

Metalwork

The hacksaw, drill and vice came in very handy. The three main items are the BAR, GRILL and PIN. The bar pushes into the wooden base and is located in place with the pin. The grill is fixed to the bar with an M5 x 20 screw with a wingnut and spring washer. This allows the grill to be secure but also adjustable as the sun changes height. The bar and grill were painted silver to reflect more light. See drawing GA for details.

Reflector

Once all the parts have been made they can be assembled into the box. The cardboard ribs should just drop in and sit flush with the base. If there are gaps under some of the ribs then it is probably because the slots in the ribs are not deep enough. Fitting the reflector is the final thing to do. We marked the reflector on 15 degree angles and then cut through with a very sharp knife leaving an area in the middle uncut. Take care of your fingers, we only had one accident! See REFLECTOR drawing for details. We then placed the reflector on top of the box and ribs and cut the edges to length. The reflector was then taped down around the edges, which produced an accurate focal point. We used shiny aluminium foil to tape the reflector in place. We found that the best method was to tape alternate edges first. This enabled us to push the other segments firmly on top of the first segments. It is worth spending time in getting each segment just right as this affects the focusing.

  1. Maintenance and caring for your solar stove.

When not in use, keep the stove in the dry. To store the unit in winter, remove the grill and grill support and turn upside down.

A dirty reflector will slow down the cooking times. Clean the reflector with a dry cloth or 'alcohol' if available.

  1. Cooking tips.

Black pots work a lot better than silver pots. The pot needs to absorb as much light as possible and silver tends to reflect the light. Dull or 'matt' finishes absorb more light than 'shiny' surfaces.

Pots with close fitting lids keep the heat in and help the cooking process. Placing the stove in a sheltered area stops the wind from cooling the outside of the pot.

 

 

Last updated on 20th February 2005.