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INTRODUCTION, SOLAR COOKING: How does a solar box cooker work? How to pasteurize water in a solar cooker? How to tell when and if water reached the right temperature of pasteurization? Temperature in a solar cooker, Weather and climate conditions for solar cookers, reference


Today one-sixth of the world population, which is 1,1 billion of people, lacks access to improved water supply. Improved water supply is something that most of the European population takes for granted, it means: household connection, public standpipe, borehole, protected dug well, protected spring or rainwater collection. Lack of such facilities leaves a lot of people without the basic need - safe drinking water.

This situation occurs mostly within the poorest people in developing countries. Only in Sub-Saharan Africa 42% of the population do not have access to improved water sources. And nearly two thirds of all people unserved live in Asia. Only in China live as much people without access to improved water supply as there are unserved people in all Africa alone.

Apart from dramatic situation of water supply, nearly two-fifths of the world population - which is 2,6 billion of people (half of the developing world) do not have access to improved sanitation facilities 5 which is: connection to a public sewer, connection to a septic system, four-flush latrine, simple pit latrine or ventilated improved latrine.

Many of these people live in remote rural areas and urban slums. It is estimated that rural communities have less than half the sanitation coverage of urban areas in developing countries. Only 31% of rural inhabitants in developing countries have access to any type of improved sanitation.

Poor sanitation conditions together with unprotected water sources may cause huge health hazards mainly due to water-borne diseases (especially those transmitted by the faecal-oral route). According to WHO every day more than 3900 children under the age of five die due to diarrhoea - water-borne disease. Many others including older children and adults suffer from poor health, diminished productivity and missed opportunities for education.

The common recommendation for treatment of unsafe water is boiling. This, however, is not widely practiced because number of reasons: " 1) people do not believe in germ theory of disease, 2) it takes too long, 3) boiled water tastes bad, 4) fuel is often limited or costly, 5) the heat and smoke are unpleasant."(Solar Cooking International)

The biggest problem seem to be the costs. For example, one of the school teacher in Tanzania has to spent 1/16th of her income on charcoal for cooking and boiling only 9,47 liters ( 2,5 gallons) of water per day (for herself and the family of seven). Similarly in Bangladesh, the poorest families would have to spent 11% of their income to boil water. In Peru, during the cholera outbreak Minister of Health strongly advised boiling water for 10 minutes. For the poor it would mean the cost of 29% of their income.

If a family cannot afford to buy the fuel they usually gather wood in the surroundings. But when it becomes scarce women (as they are the household fuel collectors) have to walk long distances every day to meet their families` energy needs. This time consuming activity limits women time for other household activities. It is estimated that it can take 6-8 hours or more for people from developing countries to gather enough fuel to cook the daily meal and boil the water.

Apart from this, burning wood, charcoal or dung for boiling water creates smoke that is a serious health hazard. According to the Solar Cooking International 400 to 700 million people, mainly women, suffer health problems because of the indoor air pollution.

Moreover, usage of biomass fuels for boiling of water contributes to deforestation that is one of the biggest environmental problems in developing countries. For example, in Madagascar (one of the biodiversity hotspots) over 90% of urban population in the southwest relies on wood and charcoal as their source of energy. This causes disappearing of about 10,000 hectares of forest annually. Mia MacDonald and Danielle Nierenberg in the "State of the World 2003" write: " If the urban population on this island nation continues to grow at its current rate of 5-6 percent a year, and if no alternatives become available (...) 42,500 hectares of forest will be needed annually by 2010 to meet urban demands for fuel wood and charcoal alone. Even more forest will be lost as rural dwellers also seek to meet their daily needs for fuel for heating and cooking."

Facing today's poverty and environment degradation of developing countries boiling of water (although it disinfects water saving peoples` lives) is not a sustainable solution and thus other, sustainable alternatives should be found.

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One of the alternatives to obtain safe drinking water is pasteurization in a solar cooker. This device was designed to cook using heat from sun, thus cooking environmentally friendly - without fuel usage, ash and fume production. The first solar cooker was invented by a Swiss naturalist - Horace de Saussure who was experimenting as early as 1767. Since then there are more than 60 designs and more than 100 variations of solar cookers.

There are three main types of solar cookers:

> Box-style cooker - it consists of a box with a cover, window and reflector. It is simple in use and simple to construct. Variations include turning the box toward the sunlight and the number of reflectors.

"Solar box cooker with cover, window and reflector" ,
source: Aalfs M.: "Principles of Solar Cooker Box Design", Solar Cookers International

> Panel cookers - these cookers are constructed from various flat panels that concentrate the sun's rays onto a pot. The pot is placed inside a plastic bag or under a glass bowl. It takes about an hour to build this type of a solar cooker. Moreover it is quite cheep. In the Kakuma Refugee Camp ( Kenya) solar panels are being manufactured for 2 USD each.

"Projects in Kenyan Refugee Camps",
source: Solar Cookers International: "Breakthrough in Kenyan Refugee Camps"

> Parabolic cookers - concave disks that focus the light onto the bottom of a pot. They obtain the highest temperature from all types of the solar cookers and can cook foods as fast as conventional stoves. They are not recommended however, because they are difficult to construct, they must be focused often to follow the sun, and they can cause burns and eye injury if not correctly used.

"Solar cooker use in Sarh, Chad",
source: Solar Household Energy Int.

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How does a solar box cooker work?

A solar cooker cooks, purifies water or sterilizes instruments because the interior of the solar box is heated by the energy of the sun. On sunny days even a temperature of 200oC may be obtained in specially designed solar ovens. To obtain such high temperatures (enough to cook or bake) simple principles of absorption, the greenhouse effect, reflection and concentration are used.

Solar box in itself absorb heat because its interior is painted in black or dark metal absorber plate and pots are used through which heat is conducted to cook the food.

Greenhouse effect is obtained due to window cover that tracks sunbeams as glass lets in solar heat but prevents most of it from escaping. The light energy that is absorbed by dark pots or plate is converted into longer wavelength heat energy. This energy radiates from the interior, but because it is of a longer wavelength it cannot pass back through the glass. That is why heat is trapped within the enclosed space.

"The greenhouse effect"
source Aalfs M.: "Principles of Solar Cooker Box Design" , Solar Cookers International

Additional heat gain is obtained due to the reflector constructed to reflect and concentrate sun beams into solar box. This input of solar energy results in higher cooking temperature. 87 In a solar oven there may be also specially constructed mirrors, which reflect light (and heat) from its surface, concentrating sunbeams.

"Reflectors and additional solar gain",
source: Solar Cookers International

Depending on construction type solar cookers have different heat storage capacity. If materials present in the insulated shell of a solar cooker have high density and weight, the capacity of a solar box to store heat increases. Thus, interior of a box with bricks, heavy pans, heavy foods will take longer time to heat up but will store the heat that will radiate within a box keeping it warm for much longer period of time.

"Thermal mass inside of a solar box"
source: Aalfs M.: "Principles of Solar Cooker Box Design", Solar Cookers International

>> See more on Principles of Solar Box Cooker Design (Solar Cookers International)

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How to pasteurize water in a solar cooker?

In pasteurization process water should be heated to 65�C for 6 minutes, or to a higher temperature for shorter time. This will kill most of germs, viruses and parasites. To pasteurize water one should put blackened container with water into a solar box cooker. A yield of 4- 12 litres per day may be achieved using this method. Panel type of cookers may be also used for this purpose. In this type instead of putting food one should put darkened container of water in an oven roasting bag and place it on a panel solar cooker. In this way about 2 litres of water can be pasteurized at a time.

How to tell when and if water reached the right temperature of pasteurization?

A special device was developed to check whether water has reached pasteurization temperature - WAPI (the Water Pasteurization Indicator). A prototype was constructed by Dr. Fred Barrett in 1988. The current WAPI was developed by Dale Andreatta and other graduate engineering students at the University of California.

source: "A summary of Water Pasteurization Techniques"

WAPI consists of a plastic tube with both ends heated, pinched and sealed. At the one end of the tube there is special a type of soybean fat that melts at 69�C . The tube is weighted with a washer to make it sink to the bottom of the container (where water is the coolest). There is also a nylon string attached to the tube that makes taking the indicator out of water easier and that does not lead to recontamination of the water. WAPI is placed in the container with the fat at the top of the tube. If the fat is found in the low end of the tube at any time after (even if water is already cooled down) it means that soybean fat has melted thus water reached the proper temperature and is pasteurized.

WAPI is reusable. When the fat cools and becomes solid at the bottom this side acts as a top in the next usage. This device may be bought in Solar Cooker International for about 3 US dollars.

WAPI may be used during water pasteurization regardless of the heat source. For example scarce fuel (like firewood) may be saved as heating water to pasteurization temperatures uses about 50% less energy than while boiling to 100oC. Apart from soybean fat one can also use Beeswax & Carnauba Wax (a product of Brazil) to indicate temperature. Beewax melts at 62�C which is not enough to reach the pasteurization temperature of 65�C. However, mixing a small amount of carnauba with the beeswax (1:5 ratio) raises the melting temperature of the beeswax to 70�C - 75�C.

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Temperature in a solar cooker

For water pasteurization one have to reach temperature of 65�C and such temperature is easily obtained in solar cookers as a common single-reflector box cooker can reach the temperature of 150� C on sunny days.

For cooking higher temperatures are not needed as it is enough to get about 90�C or so to obtain safe cooked food (free of pathogens). One should also remember that no food can go above 100oC anyway, until all of its water content has evaporated. It is estimated that food in a single-reflector box cooker will take about twice as long as in a conventional oven but since food cannot be burned in a solar cooker, one does not have to watch the cooker or stir any food while it is cooking thus one saves the time for other household job.

Weather and climate conditions for solar cookers

Functioning of a solar cookers varies depending on many factors such as type and size of a device, sun angles, sun conditions, location (latitude and altitude), and weather.

Areas with the biggest quantities of solar radiation are best for solar cooking. Thus latitudes of 20 and less are very good locations for solar cookers usage for all year round. At about 30 latitude, there is much less sun in the winter but still dishes that are easy to cook can be prepared on sunny day. At about 40 latitude, cooking is also possible but only during the warmest 7 - 8 months of the year. However, it was found that in Seattle, which location is 47,5 N latitude, solar cookers work properly even on January.

Apart from geographical location weather play a vital role either. For best results it should be cloudless and windless but with partial cloud cover and wind it is still possible to use a solar cooker.

One should also remember that during the early morning and late afternoon when sun angles are lower it would take much more time to cook food or pasteurize water than during the mid-day. That is why some solar cookers have movable reflectors to follow the sun beams improving in this way the device's effectiveness. Another solution is to replace whole solar cooker from time to time to follow the best sunlight. Glass orientation may play a role either. It can be seen at the illustration below that although the glass is the same size on two boxes, more sun shines through the glass at the second box. It is because the glass at the second box faces the sun more directly.

"Glass orientation at the solar box cooker
source: Aalfs M.: "Principles of Solar Box Cooker Design" , Solar Cookers International

>> See how to construct the simplest version of a solar box cooker (Solar Cookers International)

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1. Solar Cookers International:
>> "Recent Advances in Solar Water Pasteurizationl"
>> "Solar Water Pasteurizers Make safe Drinking Water in Tanzania"
>> "To Drink Without Risk: The Use of Ultraviolet Light to Disinfect Water in Developing Countries"
>> Aalfs M.: "Principles of Solar Box Cooker Design"
>> "Solar Cooking Frequently-Asked Questions"
>> "A Summary of Water Pasteurization Techniques"
>> "Health Safety"
>> Barbara Prosser Kerr, 1991,: " The Expanding World of Solar Box Cookers"
2. WHO and UNICEF, 2004: "Meeting The MDG Drinking Water And Sanitation Target - A Mid-Term Assessment of Progress"
3. WHO and UNICEF: "Global Water Supply and Sanitation assessment 2000 Report"
4. Peace Corps Environment and Forestry
5. Safe Water Systems :"Solar Water Pasteurization"
6. Kundapur A.: "Review of Solar Cookers Designs"
7. Halacy B. & D.: "Cooking with the Sun"

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