| Boiling Point No. 36 - November 1995 |
Over 70 per cent of the wood consumed in underdeveloped countries is used for cooking' This not only creates an ecological disaster but inhaling the smoke is a major health hazard for the families. Solar energy is the only free and abundant fuel in most developing countries. Where the sun shines and the wood has been burned, the world will (must) learn to use this energy to cook the basic foods.
Field acceptance of solar cooking constitutes the major barrier to successful projects. Daily use by people of all races and economic positions is essential for complete acceptance. Even if it is only used for camping or special events, being seen in use by the better off will raise its market image. Existing religious, social, and government organizations must be the leaders and promoters of solar cooking.
The following observations, comments and conclusions are a guide for a solar cooker programme:
All peoples in developed or undeveloped countries want a high-quality solar cooker. Home-made units are good for those who build them. Solar Cookers International, Sacramento, CA, has done an excellent job promoting solar cooking at the grass roots level. In most countries, a rugged user-friendly, aesthetic cooker is required. Home-made quality will not be accepted in the market.
Hundreds of thousands of units are required in every developing country. In many areas the trees
are gone and electricity will not be available for years. These quantities cannot be built in your back yard. In India large quantities of solar box cookers were built by companies subsidized by the government, and in Pakistan by a charity. China subsidized large quantities of parabolic cookers. These projects lacked training programs and there are not enough funds in the world to subsidize the millions of solar cookers needed. Solar cooker programs must be self-supporting.
The market is the poorest of the poor. The cooker must be a low-cost, affordable unit. Financial help such as arrangements for installment payments are needed. Material manufactures have assisted non-profit groups manufacturing solar cookers by extending their lowest price regardless of quantity. However, material must be purchased in large quantities if these low prices are to be maintained. The use of scrap or used materials (de, litho off-set printing plates) and re-ground plastics for moulding will also reduce and stabilize the price. A label on each unit could advertise the sponsors.
Seed money to purchase the first units is required. Local or foreign conservation groups, trust, governments, foundations, etc should be contacted for support. Units should be distributed by established local organizations who appoint a commissioned salesman in each community. (Do not give exclusive territories.) The price of a unit should include a commission for the local distributor and salesman. Giveaway programs establish the value of the unit as zero and defeat themselves, e.g. India's initial prograrnme. The proceeds (less commissions) are used to purchase additional units thus establishing a self-supporting programme. NGOs, religious groups, service organizations and commercial businesses are all needed and welcomed in this marketing effort.
Parts and units should be produced in large quantities for local and foreign assemblers. This amortizes the moulds and tooling over a greater number of units and guarantees low cost and high quality. Manufacturers must be located in those countries where equipment, facilities, low-cost materials and scrap are available to produce an affordable unit. The major cost is materials, tooling and machinery. Each developing country should create sales jobs by developing a market and marketing programme before making the capital investment for a mould and equipment to build units.
Units must be stackable for shipping and packaging. The post offices in some countries will ship one or two at a low cost. Ten stackable units will fit into the back seat of an automobile (a small van or combi can hold 50). Larger quantities can be carried by local truckers returning empty to rural areas. In some cases empty containers being relocated to other countries are available at low rates. One 20-foot sea container can hold approximately 800 units, or 500 units plus 100 black enamelled two and four litre pots.
Taxes, duties and tariffs on solar cooker materials, parts and units must be low or zero. Government taxes (including VAT) can destroy a programme. These problems must be addressed and solved at the start of the project. Government co-operation and/or support is essential. The country gains the ecological benefits and the reduction of air pollution. Trees, shrubs and forests are preserved. The reduced fuel consumption reduces the cost of living and improves the health and life of the people, especially the women and children.
A solar cooker that meets the above criteria is the Sunstove. It is
• Affordable (Less than $25.00)
• Lightweight (4kg)
• Weather resistant (aluminium and plastic)
• Easy to use (No external reflectors to adjust. The internal sides are 'winged' out to increase the area of the solar collection window).
• Rugged (Moulded housing). It will pasteurize water ( 15 minutes at 71 °C) and slow-cooks rice, corn meal, beans, potatoes, vegetables (without water), stews, fish, meat, etc for a family of six
(food cooks at 82°C - some beans need to be boiled for 15 minutes to eliminate poisons).
The best time for cooking is from 9.00 to 15.00. Cooking time (two to five hours) will vary with the time of day, latitude, type and quantity of food. The Sunstove will not bum food. The water used in cooking rice, corn meal, millet, beans, etc acts as a heat sink limiting the cooking temperature to 100°C.
Point the cooker at the sun. Enjoy your food. When the sun doesn't shine, use some of the wood you have saved!
For details of the Sunstove see the following article.
Sunstove Organisation PO Box 21960, Crystal Park 1515, South Africa. Tel and fax +2711969 2818.
by Richard C Wareharn of the Sunstove Organization
In the past two years approximately 1,500 Sunstoves have gone out into South Africa and surrounding countries. Figure 1 shows three types of cooker; the one on the right was our first plastic design and was distributed mainly via the Girl Guides Association. This model worked well, and about 900 were sold, but the plastic outer case, only being stapled together, did not stand up well to transport handling.
Therefore we have changed to the centre model in the picture which has a blow-moulded outer shell and is altogether more rugged. To date, more than 600 have been sold. We are trying to keep it affordable for the people who need it the most, and are a non-profit organization. Nevertheless, it may still be out of reach of many people.
Because of this, and hopefully to attract funding for the project, we have designed the model on the left of the photograph which we call our Job-creation unit. This can be made in small communities without expensive equipment. Whether it can be made more cheaply remains to be seen.
Since the 1940s hundreds of different solar cooking units have been designed and tested. The goal was to reduce the traditional use of wood by cooking with solar energy. Most designs failed because - of high cost, short life, scarce materials, assembly problems, difficulty of use, lack of training, high freight costs and duties, etc.
The challenge was to design and build a simple, affordable unit that developed the temperature needed to cook the local foods. Education and training was also needed to gain social acceptance.
The most expensive part of a solar cooker has been the external flat or parabolic reflectors, which become ineffective due to gradual loss of reflectivity, destruction by the wind, and lack of adjustment or focusing. This has made solar cookers without external reflectors the most desirable.
It was interesting that the cost of US$65.00 for material and labour for solar cookers made in Pakistan is the same as the wooden box units being built by several groups in Kenya. This is typical for 'home-made' units. The Sunstove cost in South Africa is US$22.00, including materials, labour, administration, overheads and amortization of moulds and tooling (excluding VAT). We will start manufacturing Sunstoves in the USA for the Caribbean islands and surrounding countries this year. The Minneapolis Foundation will use plastic re-grind for the case and scrap aluminium for the interior which reduces the cost.
Sunstove design, specifications and capabilities.
The Sunstove is a practical solar cooker:
• easy to use
• no reflectors
• weather resistant.
Its sides were 'winged' out to increase the solar collecting area and to reduce the internal volume to be heated. The sloped internal walls of this 'hot box' reflect, radiate and conduct the sun's energy to the black cooking pots. This creates the effect of an 'enclosed parabolic' cooker that reduces the losses caused by oxidation or stain on the reflectors. Aluminium coloured interior walls are recommended but black metal walls only increase the cooking time by a maximum of 10 per cent. A transparent plastic cover retains the heat absorbed by the black pots.
The Sunstove has an area sufficient to collect the solar energy needed to cook six to eight litres of food. The internal dimensions will accommodate two cooking pots, one six litre and one two litre. The plastic cover is sloped to collect and direct the sun's energy to the black pots thus increasing the temperature and decreasing the cooking time.
The Sunstove rapidly reaches cooking temperatures as high as 125°C and will easily pasteurize water or milk. Cooking time will vary according to local conditions, i.e. hours of sunshine, quantity and type of food, time of day, latitude, etc.
Overloading the cookers with food can increase the heat and time required for cooking past the capabilities of the unit. The cooker needs to be protected from wind and animals.
'Point it toward the sun and enjoy your food!'
NOTE: The Sunstove was designed for mass production.
Tooling was purchased in 1992 in Africa and production of the Sun- stove started in 1993. Labour, materials and overheads cost under US$25.00 per unit.
• Black pots with a cover and no plastic handles.
• Small pots for small quantities.
• Start cooking when you can see your shadow.
• Best cooking between 10.00 am and 3.00 pm.
• Stir only when absolutely necessary.
• Stirring cools the food and slows the cooking.
• The Sunstove will cook when the sun shines for more than 30 minutes each hour.
• When the sun does not shine, use the wood you saved when the sun did shine.
The Sunstove is designed to cook for a family of six. That is one pot holding 20 cups (five litres) of your basic food plus a small pot with one to two litres (four to eight cups) of vegetables, stew or dip. Each litre of food will take from half to one hour to cook.
Point it towards the sun. For maximum heat and energy adjust to the sun every two hours. Place the Sunstove in an area protected from the wind and animals.
Typical cooking times
Vegetables, potatoes, mealie meal, rice, millet, chicken pieces, fish take up to three hours.
Meats, dried peas and beans, large portions of food take up to four hours.
Sunstove Organization, PO Box 21960, Crystal Park, 1515, RSA
by Dr. T.B. Scheffler, Department of Physics, University of Pretoria
At the Transvaal Provincial Administration's Witvinger Nature Reserve, six 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 Sunstove for user acceptance. At the Department of Physics of the University of Pretoria we are privileged to be able to further improve the Sunstove - which is already the world's best. Aspects being investigated are aimed at:
• increasing the thermal performance without increasing the cost.
• making the unit even more user friendly.
• making it suited to further applications.
• developing suitable black pots and other accessories.
The Sunstove initiative came from Mr Richard C Wareham, Milwaukee, Wl - an American with long-time strong links to South Africa. Since 1992 he has designed and sponsored several models which have been manufactured and marketed The manufacture has been in Springs, RSA. The most thermally efficient solar cooker is probably of the vacuum tube type, which retails at several hundred dollars.
Stephen Gitonga, IT Kenya
In Kenya, interest in solar cooking developed mainly after the United Nations New and Renewable Energy Conference held in Nairobi in 1981. By 1987 about 80 cookers had been made and sold to various missions and development organizations in northern and central Kenya.
In other parts of Kenya (mostly dry areas), both concentrating-type solar cookers and box-type were being tested for use in the institutional and household sectors. After preliminary work, however, it was concluded that the concentrating-type solar cookers had no place in cooking activities in Kenya. The field of solar cooking in Kenya today is dominated by solar box cookers (SBC), and concentrating-type cookers are no longer promoted.
There are three basic designs of SBC being promoted in Kenya today, cookers with:
• a double glass panel with reflector
• a double glass panel without reflector
• a single glass panel without reflector
Solar cooker projects in Kenya
Currently there are five major organizations promoting solar box cookers in nine projects in Kenya. There are many other small initiatives dominated by school teachers, members of the Peace Corps, women's groups, and individual enthusiasts. These projects are supported by a strong team of both local and international solar box cooker promoters and researchers. They include Solar Cookers International (SCI) and its affiliates, Kenyatta University Appropriate Technology Centre, and the Kenya Girl Guides Association. SCI plays a leading role in solar cooker promotion, while funding comes from individual donors mainly from Switzerland, the Netherlands, Belgium and Germany.
The general objectives for the ongoing solar box cooker projects in Kenya are variously stated as:
• to reduce deforestation,
• to reduce the work burden for women,
• to improve health standards,
• to provide rural poor communities with an alternative energy source and in some cases to create commercial enterprises by producing and selling SBCs.
It is not evident that any of the stated objectives has yet been achieved.
A review of solar box cooker activities in Kenya by ITDG revealed that the level of activity of existing projects is minimal, concentrating mainly on awareness creation and training. The projects studied operate in specific niches, reaching only a very small section of the community. The evidence collected indicates a relatively low level of penetration by current SBC projects. Currently, the success of the projects is measured by the number of cookers disseminated - despite the fact that only 17 per cent of all cookers are used once or more per week when the weather is favourable. It was estimated by GEF/UNDP that a total of 2200 cookers had been disseminated by October 1994.
Factors affecting adoption
One major factor in the adoption of solar cookers in Kenya is the degree to which the technology can be used to undertake existing traditional cooking activities. Of the people interviewed in the review survey 90 per cent found the cooker to be too slow. Fifty-four per cent complained that it could not cook their preferred dishes, and in many cases the cooker could not cook enough for all family members. Sixty-seven per cent had misgivings about leaving their cooker or food unattended and so only used them when they were present to watch over them.
The fear of other people interfering with food or even stealing the cooker has also affected adoption. In some areas where the technology is promoted there is real scarcity of food, especially in the dry parts of the country, and people will not experiment with the little food that they have. The cooker is seen as a very expensive item by over 53 per cent of the respondents especially since it can only cook during the day. In seven out of the nine project areas visited, firewood is freely available and there is little incentive for people to buy or use the cooker. In two cases the cooker is promoted in areas with low insolation (sunshine) levels, making it difficult to use effectively. Strong winds and dust disrupt solar cooking in two project
areas studied, although this problem could be solved by making the cooker more robust.
Socio-economic factors appear to influence adoption more than the technical features of the cooker. The survey showed that the choice of dissemination method or approach has affected adoption. This is true especially where the wrong choice of target groups and regions is made, for example, in Nuu Division of Mwingi District in Kenya where there is sufficient sunshine but there is also plenty of firewood. The purchasing power of the community is very low and there is no basic infrastructure to sustain any locally made solar cooker activity (local artisans, training and production materials). So although the area is sunny, adoption rates are very poor. Experience in India seems to suggest that promotion of solar box cookers is likely to be most successful in better off, urban areas. They should also be promoted for use as heat storage cookers when the sun is not strong.
The technology clearly has limitations such as:
• it can never completely replace other cooking systems
• it cannot cook certain preferred and common foods
• it can only be used at times of sufficient insolation
• it requires radical changes in the way cooking is perceived, especially with regard to common perceptions of the kitchen and the cooking hearth, and cannot cook quickly.
• the stoves currently promoted cannot cook for the number of people for whom food is frequently needed.
• it is at present too expensive to offer to poor communities.
For SBCs to be successfully promoted in Kenya in the absence of credit facilities they need to be affordable for the identified markets and socio-economic groups. This requirement is at present difficult or perhaps impossible to meet if rural populations are to be targeted. The cookers need to be strong enough to withstand hot sunshine, rain, humid conditions and strong winds. SBC designs need to be appropriate for the materials available in Kenya, and incorporate good insulation properties to conserve heat during cloudy periods.
The survey did not find any evidence of any relationship between SBC promoters and other energy programmes. The SBC sector has developed in isolation from broader energy or conservation initiatives, and no evidence of interest in linkages was found. Where there has been integration it has been with initiatives in other areas (health, evangelism etc) undertaken by organizations concerned. This has denied the organizations concerned an opportunity to learn from experiences of other biomass energy technologies and their promoter organizations.
Lessons from stoves and household energy programmes
Stoves and household energy programmes in Kenya have passed through a process of technology development (research and development), technology testing, production development, pricing, dissemination (including marketing), monitoring (including modification of the technology and dissemination approach), evaluation and project reviews. Issues of sustainability have come very much to the fore and are now paramount considerations in all household energy activities. Non-stove inputs - technical and sociological - are also now recognized as important contributions in the biomass energy sector, including a range of conservation techniques and practices. These processes are still continuing, and the supporters of stoves programmes have learned that flexibility and innovation are important ingredients of any successful adoption of stove/cooker technology. In the process, they have made many mistakes that SBC promoters can now avoid. They include choosing the wrong target groups or regions, giving subsidies to a level that threatened sustainability of the projects after funds were withdrawn, producing pieces of cooking equipment that could not be fabricated easily and cheaply, duplication of efforts and repeatedly 're-inventing the wheel'.
The SBC projects visited in Kenya had unclear and varied objectives, inappropriate target groups and unsustainable methods of promotion. The target communities should be much more closely involved in all stages of product development. Promoters should exchange ideas and experiences more regularly, and learn more from the lessons of other household energy activities.
IT Kenya, Household Energy
Regional Project, PO Box 39493,
Nairobi. Tel: +254 2
446243/442108. Fax +254 2
A letter from Elisabeth Castiglioni of The Institute of Cultural Affairs, Nairobi.
In 1990 we started integrating solar cooking into most of our regular programmes, the first pilot project being in Kakamega, Western Kenya, with 20 women's groups Since that time, approximately 1000 cookers have been distributed to women who have been thoroughly instructed, over a period of one to two years, in the use and function of the cookers, and who enthusiastically praise this relatively new cooking method.
I do not want to hide the shortcomings of this cooking method; solar cookers only work when the sun shines. They cannot yet function early in the morning, at night or on cloudy days. But here, on the equator, sunshine is plentiful for most months of the year! Especially with meals which take a long time to cook (maize, beans, tough meat etc.), it is a great relief to get one's cooking done without supervision, without constantly having to add water, firewood, charcoal etc., without the risk of burning and overcooking, not to mention the cost of the fuel required. Which mother in rural areas, without a solar cooker, can bake bread and cakes?
We never hide the fact that you need another cooking stove besides the solar cooker, and we actively support Intermediate Technology's Stoves and Household Energy (SHE) programme. We do not consider solar cooking to be in competition with IT's work.
We also do not contest the fact that many women in rural areas cannot yet afford a well-built, solid, solar box cooker. But is this a reason to ban solar cooking? To categorically declare that the cookers are unacceptable and inappropriate to the needs of the poor? That they cannot be recommended? There are a large number of families who can afford this expense. Many family members, with an average-paid job, buy cookers for their mothers, sisters and relatives in rural areas. International donors often subsidize promotion programmes, and our organization encourages companies and other organizations to distribute cookers on a payment-by-instalment basis.
The cost of solid solar cookers is often grossly exaggerated. Kenya is still experiencing high inflation, yet we today build quality cookers in our workshop, here in Nairobi, at a selling price of KSh 3,500 (approximately US $65). This price includes the cost of locally available quality materials, salaries, some overheads and profit; and it allows us to work sustainably. During one-week training courses, we teach carpenters from remote rural areas how to build their own cooker, and we encourage them to include solar cooker production and promotion in their income-generation projects.
The food to be cooked is prepared in the house as usual, and only then is the pan carried outside and placed in the solar cooker. No food (potatoes, cassava, yams, corn and beans etc.) needs to be boiled before being placed in the cooker. Since solar cooking is 'slow cooking' and requires about double the time used with a regular cooker, we recommend that dry beans and corn be soaked overnight. We do not want to list all the advantages of solar cookers but here are some: no fire, no smoke, no accidents, no fuel or maintenance costs.
There is, however, the social aspect to consider. Cooking practices need to change. Lunch should be in the cooker at about 10 am, if it is to be ready when the children come from school. The housewife has to plan her meals in advance. She will also miss the social interactions with daughters, in-laws and friends who come to chat in the smoky kitchen during the long mornings. Constant supervision, adding water and firewood is no longer necessary, but she may miss tasting the food.
Women will give up some of the time-consuming chores in the kitchen and have to reorganize their accustomed daily routine, which may cause problems. It is hard to change traditions, but women, in time, will eventually handle this issue. They will realize and appreciate the new freedom they acquire with more hours of free time!
We are impressed by the aims and work of Intermediate Technology and urge you to support the cause of improving and promoting solar cookers.
The Institute of Cultural Affairs, Rose Avenue, Ngong Road, PO Box 21679, Nairobi, Kenya. Tel: +254 2 7243/4/729375 Fax: +254 2 729375
by James Muriithi, Kenya Ministry of Energy/G7Z-SEP
Much of the earth's heat energy comes from the sun. In only one minute, more heat and light reach the earth from the sun than man can produce in a year. In spite of this energy being free, mankind has not made much use of it for his own purposes and most of the energy consumed in the Third World countries still comes from wood.
In the rural areas of Kenya 93 per cent of the energy requirements are met by wood, mainly in its primary form - firewood. Up to 83 per cent is used for cooking, 11 per cent for heating and 6 per cent for lighting. Although firewood is free in monetary terms in most rural areas, in other respects it is certainly not, as its continued use has grave consequences for several aspects of the environment.
In view of the increasing shortage of firewood for domestic cooking, the Ministry of Energy/GTZSEP programme and some local NGOs such as Trans-World Radio Nairobi and CITC-Kapsabet are exploring the possibilities of using solar energy for cooking. They have concentrated on the design and production of solar box cookers. Many such cookers have been designed and tried out in many countries over the last 30 or more years and seem to provide the best prospect of being accepted by users. A solar box (see Figure 1) is basically a box comprising of:
• an outer wooden box (wood is strong, easy to work and a poor conductor of heat)
• an inner box of sheet metal, without a lid and with the inside painted black/gray for maximum absorption of solar radiation
• insulating material such as cotton wool, straw, saw-dust etc, between the two boxes
• double glass lid (to reduce loss of heat by conduction)
• a reflecting sheet attached to the lid to direct the sun's rays into the box and on to the cooking pots
The sun's rays, direct and from the reflector, pass through the glass into the cooker and are converted into heat. The dark interior of the box absorbs the sun's energy while the glass prevents the heat rays from escaping (the greenhouse effect) and the insulation minimizes heat lost by conduction. The air inside the box may reach temperatures of up to 150-200°C. The food to be cooked is placed in darkened pots with dark lids.The solar box is directed at the sun and the reflecting lid is adjusted to direct the sun's rays into the cooker as the sun moves.
A pot of maize and beans, soaked overnight and cooked when there is maximum sunlight, might take about five hours to cook. Water placed in the cooker can reach average temperatures of 60 to 70°C and sometimes 100°C. Depending on the size and materials used, the costs of the solar cookers range (in 1995) from KSh 2,000 to 3,000 (US$33 to 50).
The advantages of the solar box cooker include:
• uses free energy
• smoke free
• very little or no attendance to food when cooking as it will not burn or stick to the pan and there is no need to stir
• food will remain hot when cooking is finished
The disadvantages include:
• cannot cook during the night
• cannot cook when the weather is not favourable i.e. when it is raining, cloudy etc
• takes a long time to cook a meal double the time of other cookers
• cooking has to be done outside and the solar cooker has to be moved now and then to follow the sun
• one person cannot handle the big size cookers
• cannot be used for roasting maize cobs
• cannot be used for space heating in the house
Up to the present, solar cookers have not been widely adopted in Kenya for the following reasons:
• the price of KSh 2,000 to 3,000 is too high when compared with the KCJ at about KSh 100
• the cookers are not strongly promoted
• people are disillusioned because they take too long to prepare a meal
• people are used to waiting by the stove whilst the meal is being prepared. They need to recognize that if they want to have the meal at 1.00 pm they should put the food into the solar box at about 8.00 am and then attend to their other activities.
• very few organizations or individuals are producing solar box cookers
Further development work and testing is being done on our solar box cookers to make them more suitable and affordable to our rural households.
We look forward to receiving further information about the success of the project.
Kenya Ministry of Energy/GTZ-SEP,
PO Box 30582, Nairobi, Kenya
by Kedar N. Nag and A.N. Mathur; University of Udaipur; India
In India, cooking and water heating consume 43 per cent of the total domestic energy. A recent survey of a rural area of Karnataka State has shown that the domestic sector alone consumes 87 per cent of energy which is comprised mostly of fuel-wood, cow dung, farm waste and oil. In the urban areas of Delhi, Bombay and Calcutta, commercially available energy sources meet 66 per cent of their needs for domestic use and in other rural and urban areas 12-19 per cent.
In order to reduce the use of commercial fuel in urban areas, the technology to harness solar energy through the use of solar cookers and water heaters has been developed. The popularization of solar-based domestic equipment will depend on sociological, technological, political and economical factors. Normally it takes about a decade to shift from one source of energy to another.
A box-type solar cooker, known as an 'Udaipur Cooker', with a metallized polyester reflector on both sides of the box has been fabricated and costs less than a box type solar cooker with flat collector sides. The collector plates of both cookers are made of 25s gauge aluminium sheets painted black. The temperature of the Udaipur cooker was 15°C to 17°C higher than that of the flat-side box cooker with and without food. The maximum temperature during the month of August was 104°C in the Udaipur Cooker in comparison to 87.5°C in the box-type cooker.
The metallized polyester film (90 per cent reflectivity) pasted on parabolic section hardboard has been provided on two side walls to reflect solar radiation on the container. The focus of the parabolic reflector is at the centre of the 'collector'. The double glass cover reduces the heat-convection losses due to wind. The outer box, made of wood, is painted with enamel paint. Rubber strip seals are provided around the lid of the cooker.
The above article was published in 1983 in the Indian journal Agricultural Mechanization in Asia, Africa and Latin America.
The reported design and performance of the 'Udaipur Cooker' seem to be better than that of the many solar box cookers 'invented' and promoted in the subsequent 13 years but it has still not been widely taken up.
by Dale Andreatta of S.E.A. Inc.
The lack of clean drinking water is a major health problem in the developing world. To reduce this health risk, ways of producing clean water at an affordable cost are needed and people need to be educated about germs and sanitation, lest they accidentally recontaminate their clean drinking water. Recently, several of us at the University of California at Berkeley have attacked the first of these requirements. Information on pasteurization is available from Solar Cookers International, 1724 11th Street, Sacramento, California, USA 95814. In this article we describe a new low-cost device that pasteurizes water.
In pasteurization, water is heated to 149°F (65°C) for about six minutes, killing all the germs, viruses, and parasites that cause disease in humans, including cholera and hepatitis A and B. This is similar to what is done with milk and other beverages. It is not necessary to boil the water as many people believe. Pasteurization is not the only way to decontaminate drinking water, but it is particularly easy to scale down so that the initial cost is low.
The new device is called a solar puddle, and it is essentially a 'puddle in a greenhouse'. One form of the solar puddle is shown in the figure below, though many variations are possible.
Begin by digging a shallow pit. The test puddle was a 'family-size' unit, about three and a half feet square, but it could be larger or smaller. The pit is filled with two to four inches of solid insulation. We used wadded paper, but straw, grass, leaves, or twigs could be used. This layer of insulation should be made flat, except for a low spot in one corner of the puddle for the drain out syphon.
The level of the bottom of the puddle must be above the level of the surrounding ground to permit syphoning.
Put a layer of clear plastic and then a layer of black plastic over the insulation, with the edges of the plastic extending up and out of the pit. Two layers are used in case one develops a small leak. We used inexpensive polyethylene sheet from a hardware store, though special UV stabilized plastic would last longer.
Put in some water and flatten out the insulation so that the water depth is even to within about half an inch throughout the puddle, except in the trough which should be about one inch deeper than the rest. Put in more water so that the average depth is one to three inches depending on how much sunshine is expected. A pasteurization indicator (available from Solar Cookers International, Tel: +916 444 6616, should go in the trough since this is where the coolest water will collect.
Put a layer of clear plastic over the water, again with the edges extending beyond the edges of the pit. Form an insulating air gap by putting one or more spacers on top of the third layer of plastic (large wads of paper will do) and putting down a fourth layer of plastic, which must also be clear. The thickness of the air gap should be two inches or more.
Pile soil or stones on the edges of the plastic sheets to hold them down. The puddle is drained by siphoning the water out, placing the siphon in the trough and holding it down by a rock or weight. If the bottom of the puddle is flat, well over 90 per cent of the water can be siphoned out.
Once the puddle is built it would be used by adding water each day, either by folding back the top two layers of plastic in one corner and adding water by bucket, or by using a fill siphon. The fill siphon should not be the same siphon that is used to drain the puddle, as the fill siphon is recontaminated each day, while the drain siphon must remain clean. Once in place the drain siphon should be left in place for the life of the puddle.
The only expensive materials used to make the puddle are a pasteurization indicator (about $2), a siphon tube (about $1), and four sheets of plastic (about $2 for the size tested). All of these items are easily transportable, so solar puddles might be an excellent option for refugee camps if the expertise were available for setting them up. On days with good sunshine the required temperature was achieved even with 17 gallons of water (two and a half inch depth).
The device seems to work even under conditions that are not ideal. Condensation in the top layer of plastic doesn't seem to be a problem, though if there is a lot of condensation the top layer should be pulled back to let it evaporate. Small holes in the top layers do not make much difference. The device works in wind, or if the bottom insulation is damp. The water temperature is uniform throughout the puddle to within 2°F ( 1 °C).
After some months the top plastic layers weaken under the combined effects of sun and heat and have to be replaced, but this can be minimized by avoiding hot spots such as places that are exposed to the sun but not cooled by the water. Another option would be to use an acrylic grade of plastic that is stronger and more resistant to sunlight but is more expensive. The two bottom layers of plastic tend to get tiny tears unless one is very careful in handling them, (that is why there are two layers on the bottom). A tiny hole may let a little water through and dampen the solid insulation, but this is not a big problem.
There are many variations of the solar puddle. It is also possible to build a puddle with wooden sides, on the ground, on a table top, or on the roof of a building. We have been able to put the top layer of plastic into a tent-like arrangement that sheds rain. This would be good in a place that gets frequent brief showers. Adding a second insulating layer of air makes the device work even better, though this adds the cost of an extra layer of plastic.
One could make a water heater by roughly tripling the amount of water so that the maximum temperature was only 120°F (50°C) but this water would stay warm well into the evening. This water would not be pasteurized.
One could help solve the problem of dirty vessels by putting drinking cups into the solar puddle and pasteurizing them along with the water. The solar puddle could possibly cook foods like rice in emergencies, for example in a refugee camp.
S.E.A. Inc., 7349 Worthington-
Galena Road, Columbus, Ohio
43085. Tel: +1 614 888 4160.
Fax: +1 614 885 8014.
by Dennis Anderson and Kulsum Ahmed, World Bank
For developing countries in particular, solar energy is an abundant and environmentally attractive resource, with enormous economic promise.
Each year, the earth receives an energy input from the sun equal to 15,000 times the world's commercial energy consumption and 100 times the world's proven coal, gas and oil reserves. Modern solar-electric schemes are capable of converting 10 to 20 per cent of the incident energy into a form useful for consumption, and in theory they would need less than 1 per cent of the world's land area to meet all its energy needs - which is less than the land areas now occupied by hydro reservoirs, and not much more than is planted for potatoes. Yet, despite the abundance and attraction of the solar energy resource, only a tiny fraction is used.
The situation, however is changing. The last two decades have seen major technological developments for harnessing solar energy through the use of photovoltaic cells.
Until recently, the argument for alternatives to fossil fuels - the justification for the nuclear power programs of the 1950s, 1960s, and 1970s - was that a backstop technology was needed in case we ran out of them.
But the world's proven reserves of fossil fuels are very large, over 800 billion tons of oil equivalent energy (t.o.e.), of which 70 per cent is coal and 30 per cent oil and gas - enough to last a century at today's levels of consumption and for 50 years allowing for the growth of demands in developing countries.
Furthermore, these are only the commercially proven reserves, which have expanded appreciably for many decades. Industry estimates of ultimately recoverable reserves are about 4,600 billion tons, including 1,400 billion tons in oil shales and tar sands - sufficient to last us for the next 150 years or so, assuming continued growth in world demand over the next century.
Thus the old backstop argument is no longer valid. Rather, the case is being made on other grounds. The economic case is that the technologies will eventually compete with fossil and nuclear fuels - and also with hydro-electricity.
The solar schemes have no net emissions of carbon dioxide, particulate matter, sulphur dioxide, or nitrous oxide. In the case of carbon emissions, solar energy is the only alternative currently available for development for domestic cooking in the Third World and to stabilize carbon accumulations in the atmosphere should the need arise.
What can be done to encourage the development and wider use of renewables in a way consistent with the aims of good policy making?
First, the industrial countries in particular need to diversify their R&D portfolios. Not only does solar energy receive funding that is minuscule compared with fossil and nuclear technologies (about 5 per cent of public R&D in energy) but its share of a declining budget has also been shrinking for the past 13 years. International collaboration on R&D also needs to be promoted, as it is in other areas such as agriculture.
Reprinted from Finance and Development 1993