The Sunstove
Solar cookers are regarded by many as a potential solution to the fuelwood crisis in rural areas - especially in Africa. Deforestation damages the environment - sometimes irreversibly. It also implies that nearly every month more time must be spent than the previous to collect sufficient fuelwood, and places an onerous burden especially on women in rural areas5.
The Sunstove, which since 1992 is being manufactured and developed in South Africa1, is today the best solar cooker worldwide in several key respects. This despite the fact that it is by no means the most thermally efficient solar cooker yet made1. Thermal efficiency in itself has proved insufficient2 for a solar cooker to be acceptable and useful to the most important target group of users - the rural and peri-urban poor. The Sunstove excels in the following key respects:
Price
At less than R200
for a durable cooker capable of taking 3 pots at a time, it compares very
well with even the materials cost ($80 - 100) of cookers manufactured
in an extensive program3 in Central America. These materials
had to be subsidized 100-300% to be affordable to the target market.
Robustness
The unit is the toughest
that money can buy. On more than one occasion units were lost from vehicles
travelling at 120km/hour on a freeway. Some damage was visible, but all
units were still functional after such accidents. Falling from about a
metre's height onto a hard surface has little or no effect on the units.
As solar cookers will also be used in remote locations, this ruggedness
is important.
User
friendliness
In contrast to other solar
cookers, where the orientation must be adjusted around 2 axes4,
the Sunstove is merely turned on its smooth flat base (around a
vertical axis) towards the sun. One need not even bend down to do this
- a foot will do! Orienting towards the sun is much less frequent
- every 2-3 hours, as against every 20-30 minutes for most other solar
cookers4.
It is, furthermore, nearly
impossible to burn food with the Sunstove, or to let it
boil over. This is because the thermal flux is much lower, and comes mainly
from above (also from the sides). Food is stirred only once, if
at all. To cook a large pot of mieliepap (maize porridge) - a staple5
of rural Africa - requires 10-20 times less woman-hours than on an
electric or gas stove. Instead of stirring for 30-60 minutes6,
a ¼ minute now suffices. Because the cooking process is slower, the end
result is better6.
Water
use
Because of the lower heat
flux and minimal stirring, evaporization is minimal, and less water
is used (for mieliepap about one third as much, and none for fresh
vegetables). In arid rural areas, where water is carried over long distances5,
this can be important. In rural settings water-borne diseases such as
cholera can take on epidemic proportions. The Sunstove can readily
pasteurise water and milk. User friendly pateurization temperature
indicators are being developed.
Wonder
(Hay) Box
The 40mm fibre-glass thermal
insulation of the Sunstove makes it suited to being used as a “Wonder
Box” - for example, to keep a large amount of food warm or hot
till an hour or two after sunset. For this a twice folded blanket (4 layers)
must be placed on the glazing of the Sunstove. Food heated to cooking
temperature will continue to cook food after sunset or on rainy days in
a Wonder Box.
Eye
friendly
In contrast to some other
solar cookers, the Sunstove is safe and indeed comfortable for
the eyes of the user. It is a pleasure to use, and the light Sunstove
(4 kg) is already being used by potjiekos enthusiasts, caravaners
and campers. With potjiekos one must remember that the heat now
comes mainly from above and the sides - not from below. The arrangement
of the food layers must therefore be modified accordingly.
Many families today cook their
main meal, with a variety of dishes, in the Sunstove.
Acceptability
to the market
For any product this is the
crucial test. Of the latest 2 models, about 8 000 have been sold. Current
sales (at above production cost) average about 200 per month.
Sunstoves have been
sold to customers from 12 countries in Africa. At the Transvaal Provincial
Administration's Witvinger Nature Reserve, 6 different types of solar
cookers from various parts of the world were tested under practical conditions,
and several types were built. It was found that not a single model could
compete with the Sunstove for user acceptance.
Compact
Transport / Storage
The Sunstove can be
supplied with the translucent lid & hinge separate. (It is fixed by
5 screws). In this way 5-10 or more units can be stacked spoon-wise into
one another for compact transport and storage. Each unit weighs only 4kg.
History
Richard Wareham from Milwaukee,
USA designed the Sunstove - in South Africa. The excellent Solar Cooker
Manual from Brace Research Institute (McGill Univ, Canada)
guided an ongoing quest for a more user friendly, rugged & affordable solar
cooker. Many models were designed & built using different materials and
production processes. Of some, a few hundred were made and marketed, plus
1 000 of a truncated conical model in 1993. The experience so gained has
each time been utilized to further improve the design.
Mr Wareham also sponsored these prototypes, as well as the tooling cost
of the current South African model. These inverted truncated pyra mids (see
diagram) were first produced (end of 1993) by Sinclair Perry - who contributed
his knowledge of production technology to the highly successful current
design. Today final assembly & dispatch is by Danie Jacobs in a suburb of
Springs.
Acknowledgement
Technology
Development Notes
2. See for example
A A Eberhard, Technological Change and Rural Development: A Case Study
in Lesotho. Ph D Thesis, University of Edinburgh, 1982. And: Dissemination
of Solar Ovens in Lesotho: Problems and Lessons. p2754-2758, Proc 8th
Solar World Congress, Perth 1993. Pergamon. 3. D M Kammen
& W F Lankford, Cooking in the Sunshine. Nature, 348
(385-6) 1990. 4. With the
parabolic dish type, the entire unit must follow the sun around 2 axes
- about every 20-30 minutes. With the box-type cooker with external mirrors
the mirror tilt angle needs frequent adjustment according to the height
of the sun above the horizon. And the entire unit must be turned
on its base towards the sun every 30-60 minutes. 5. M Sc thesis,
Cecile Thom (University of Cape Town, 1994): The application
of biogas technology in South Africa for small-scale energy production.
6. JA de Villiers
(author & publisher), Cook and Enjoy It.
The companies Acrylic Products,
Blomo, Owens Corning and CTP Webprinting supply materials and components
at low cost, thus contributing towards making the Sunstove easily
the most cost effective solar cooker available today. Roelf van Weele cuts
the thick aluminium base plates for the slightly faster model. The Sunstove
is now entering production in Calcutta, India - funded by Rotary International.
At the Department of Physics
of the University of Pretoria we have been privileged to further improve
the Sunstove - which is already the world's best. Aspects investigated
were aimed at:
1. The most thermally efficient
solar cooker is of the vacuum tube type - which retails at several thousand
Deutschmark apiece. These units (& less expensive ones using
silica aerogel transparent insulation and/or selective coatings) store
heat and permit relatively high temperature cooking even after
sunset. They cost 25 - 110 times as much as the Sunstove. These
and some other solar cookers can reach high er temperatures, and boil
a given amount of water in a shorter time.
Enquiries:
Dr.
T.B. Scheffler
Tel:(18:00
- 20:00)
+27 12 47-4185
Fax: +27 12 47-2559
E-mail:
schefflr@scientia.up.ac.za
©
Sunstove Organization
Registered as a section 21
(non-profit) company represented by Mr Wareham, Mrss Margaret Bennet and
Liz Perry of Gauteng, Mathilda Roos of Bloemfontein, and others.
P O Box 21 960,
1515 Crystal Park.
