Cooking with the Sun

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Introduction

A solar cooker uses the energy of direct sunlight to heat, cook, bake or pasteurize food or drink. It only works when the sun shines, so solar cooking can never be a stand-alone technology. Solar energy can be an ideal component of the energy mix of a household to complement other combustion-based stoves that can produce heat on demand based on other fuels. To which degree solar energy can replace other fuels depends on local dishes and cooking habits. It is also very much dependent on the local climatic conditions, which can vary tremendously over the course of a day or a year. All in all, over the year in a whole, fuel savings amount to roughly 30%-40%.

In any case a detailed assessment of the baseline cooking habits and important factors such as convenience, availability, affordability and reliability are essential to elaborate on the potentials of supplemental solar technologies to an existing household energy mix:

• The sunlight might be strong all year round, but in the tropics it is restricted to approximately 12 hours a day, with variable intensity.
• If cooking at dawn or sunset is a common feature in the daily routine of the target community, a solar cooker may not be suitable. One option is that food is precooked on the solar cooker and reheated with other cooking possibilities. Food could also be placed in a fireless cooker – an isolated container – to keep it warm, for later consumption.
• If overcast at noon is a frequent occurence, food preparation for lunch might be unreliable on a solar cooker.
• If sitting on the ground while cooking is a very important feature, some solar cookers aren´t acceptable because of their design.
• If constant attention by the cook is required for certain meals, solar cookers might not be acceptable, as the cook is exposed to the sun the whole time.
• If heavy stirring of food in a big pot is required to prepare the staple food, women or girls may find it inconvenient to cook on some of the bigger solar cooker models.
• If theft at night is a big problem, poorer households may not be able to store a huge solar cooker in their house, because of the limited space.
• If the area around a house is covered with trees for shade, a solar cooker might have to be moved too far away from the house to be acceptable.
• Solar cookers are ideal if cooking is not bound to happen at a certain time, e.g. for pasteurization of drinking water. In refugee camp situations or in locations, where other fuel options aren´t available solar cookers might be a good alternative.

Types of Solar Cookers

Most solar cookers that have been developed to date fall into five categories:

• Panel solar cookers;
• Parabolic solar cookers;
• Solar box cookers;
• Scheffler cookers;
• Schwarzer cookers (the latter two have been named after their inventors).
• Lytefire solar oven

Panel Solar Cookers

Panel-type solar cookers have a set of reflective panels focused on a black pot. These devices have been widely used in refugee camps as well as households. They can be very low in cost, with reflectors made of cardboard, plastic foam or other low-cost materials. Most panel solar cookers include a transparent cover or plastic bag in which a black cooking pot is enclosed. This increases the efficiency by reducing convective heat loss and exploiting the 'greenhouse effect'.

One example of a successful panel cooker is the CooKit, a simple panel cooker, which can be fold to the size of a large book for easy transport. It was widely used in the Darfur project by Jewish World Watch, in which over 260,000 were distributed. The CooKit used a cardboard reflector that was not very durable. Another example of a very durable, high-efficiency panel cooker, the HotPot, used a glass outer liner and lid enclosing a black bowl. It has a reflector made of sheet aluminium. About 20,000 of these panel cookers were distributed in Mexico. There are many more designs that are catalogued on the solar cooking wiki, www.solarcooking.org.

Parabolic Solar Cookers

Parabolic Cooker, Seidel

Parabolic cookers can be made from aluminium sheets, iron, or even concrete coated with aluminium foil. Through their parabolic shape, they focus radiation from the sun onto the bottom of the pot. They generally have a higher energy output than box cookers (see below) and can reach temperatures of up to 250 °C. These high temperatures enable users to do cooking, stir-frying and baking. Aluminium parabolic cookers are lightweight and can easily be transported. However, their production requires a high degree of precision and they are usually imported as sets, which are locally assembled. As a result, in most developing countries, aluminium cookers are too expensive for the majority of the population (US$ 100 and above). Furthermore, such cookers are prone to wind damages to the aluminium sheets. In areas of frequent dust storms, the sheets tend to become scratched over time, reducing the heat output by as much as 30%. To achieve maximum performance, parabolic cookers must be reoriented to the sun about every 20-30 minutes, which can be cumbersome and inconvenient for the cook, when large amounts of food are cooked. Moreover the cook can be "dazzled" (blinding of eyes) when the cooker is not handled properly. One solution that has been promoted is the use of sunglasses; another approach is to turn the cooker out of the direction of the sun before the food is stirred.

A variation of the parabolic cooker is the so-called Papillon, which has been promoted in Burkina Faso. It has a higher output than the well-known SK-14 parabolic model, is easier to handle and dazzling is less of a problem. However, the price of about 450€ is too high for households and public organizations in most developing countries.

Nepal Solarcooker, Dr. Agnes Klingshirn

Another parabolic cooker is the so-called butterfly cooker. It was developed in Tibet and is one of two models used in the region. Made from iron by local artisans, the design has successfully been used for many years. Its bowl is coated with reflective aluminium foil or with small mirrors, which can be replaced when damaged. Iron cookers are more expensive than cement cookers and can only be afforded by better-off households. The aluminium foil has to be replaced every two or three years. They are much heavier than cookers made of aluminium and are pushed around on two wheels fitted to the central support of the cooker. The butterfly cooker is one of the most successful solar cooker models with about 70 000 cookers built in Tibet as stated in a publication of 2004 , and their use significantly satisifies much of the overall energy demand of households. According to the Energy Research and Demonstration Center of Tibet Autonomous Region, more than 400,000 solar cookers have been in use in Tibet in 2014.

Solarcooker Circularbowl, Dr. Agnes Klingshirn

Similar to the butterfly cooker is a model that has a circular bowl made from cement. In Tibet, it is easier to construct, cheaper, and can be afforded by poorer families. This cooker has also been introduced in Afghanistan. The price of the cooker may vary considerably according to the cost of the surface mirrors and the cement. Disadvantages include a bowl that is more prone to breakage and the weight of the cooker (about 60 kg), making transport and handling difficult. Click here for a step by step introduction on how to build the cooker.

Solar Box Cookers

Solarbox

Solar box cookers are much easier to construct than parabolic cookers and are frequently made from local materials by local artisans after they have been trained (Download: Manual for solar box cookers). Box cookers are less powerful than parabolic cookers and do not reach temperatures above 200 °C. Stir-frying is not possible, which makes them unsuitable for countries where this type of cooking is common. However, they are easy to handle, orientation to the sun is not as important as for parabolic cookers, and if the sun is temporarily covered by clouds, the temperature does not drop as fast as in parabolic cookers. During cloudy weather, a box cooker can serve as a retained heat cooker. Box cookers have been successfully disseminated in Bolivia. More information on different box cooker designs is available at http://solarcooking.wikia.com/wiki/Category:Solar_box_cooker_designs

Scheffler Cookers

Scheffler cooker

A third type of solar cooker is the so-called ‘Scheffler cooker’ where sunlight is focused by a large reflector to a secondary reflector that heats the pot. In contrast to other types of solar cookers, tracking the sun is automated in Scheffler cookers. The tracking is driven either by a photovoltaic or gravity device. This makes the cooker very convenient to use. However, since the tracking device requires regular maintenance, it should only be used where this can be assured. In general, the Scheffler cooker is technically the most advanced type, but it is costly, and requires skills to make it work well. To date, a few hundred cookers have been installed worldwide in institutions, mainly in India and Latin America, e.g. in schools or ashrams. One advantage is that the secondary reflector can be located inside a building, with an opening left for the sun’s rays to pass into the building, whilst the cook can work in the shade. Scheffler cookers have a much higher output (several kilowatts) than the other cooker types. Several Scheffler cookers can be combined to form sets with an output of several hundred kilowatts. The high output makes Scheffler cookers especially suitable for institutions, but for individual households the cooker is too expensive.

Schwarzer Cookers

Schwarzer Solarcooker

A fourth type of solar cooker is a flat-plate collector cooker, named the Schwarzer cooker after its developer. This design uses collectors to heat a medium, such as steam or oil, to transfer heat to where it is needed for cooking. These cookers can be made as large as necessary, allowing their output to be matched to the needs of institutions or individual households. They are not easily transported, but once installed, the cookers do not need to be adjusted to track the sun, which makes their use more convenient. As the cooking point is separate from the collector, it is possible to cook in the shade or indoors. The inclusion of a thermal storage unit enables such cookers to be used after sunset, too. To date (2007), around 250 to 300 Schwarzer cookers have been built in India and Africa, most often by businesses.

Lytefire solar oven

A fifth type of solar cooker is a Fresnel mirrors type collector, developed by Solar Fire Concentration Ltd, a Finnish impact company. This design named “Lytefire” is freely inspired by Linear Reflection Fresnel Concentrator. Multiple small mirrors are oriented differently to approximate a parabola. The advantage of this technique is that it uses flat mirrors which are cheaper than curved mirrors. The simple design of the concentrator allows people to build and maintain it locally everywhere in the world. Solar Fire team has developed different applications allowing people to use the concentrator as a powerful source of energy to power their activities around food transformation (baking, roasting, dehydrating). Due to a great efficiency and a wide surface of mirrors (around 5 square meters for the first models), the oven gives few kilowatts of output. Thanks to this power, a professional activity can rely on it (e.g. a baking or roasting activity). To date, few Lytefire have been developed in East and West Africa, Haiti, Philippines, Brazil, Canada and in Europe (France, Switzerland). In Tanzania, Kenya, Burkina Faso and Normandy (France) professional bakers are using it to power their businesses. Another application is the Lytefire sauna.

Dissemination Strategies for Solar Cookers

Several approaches have been applied individually or in combination to disseminate solar cookers:

• State-controlled dissemination programmes at sometimes heavily subsidised prices, or free of charge (China)
• Commercial dissemination of subsidised models (China, India)
• Market launch of cookers with no or indirect subsidy (South Africa, Bolivia) or with accompanying offers of loans for buyers (Burkina Faso)
• Renting and sale at usual market prices, supported by project activities relating to advertising and awareness-raising (Burkina Faso).

So far, state-driven and commercial dissemination approaches have been successful in Tibet and Bolivia, see examples below.

All other strategies have tended to fail for various reasons. In India, household solar cooking was not in line with cooking traditions. In South Africa, a loan system for solar systems could not be established due to lack of interest on the side of the banks and cookers were still too expensive for poorer families. In Burkina Faso parabolic cookers were disseminated using a loan system, but the project suffered from low payback rates. A study has indicated that instalments were unaffordable to poorer families. This demonstrates the dilemma that poorer families, who could make the best use of the solar cooker, are usually not able to afford them.

Examples of Solar Cookers in Use

In Tibet, during the early stages of dissemination, solar cookers were heavily subsidised or even provided at no cost to poorer families. In Tibet, during the early stages of dissemination, solar cookers were heavily subsidised or even provided at no cost to poorer families. The state bought large numbers of cookers from local entrepreneurs, and this provided the start-up catalyst for the solar cooker industry, which gradually became independent. Cookers are now produced in a local factory as well as in family or craftsmen’s workshops. Initially subsidised, the cookers were sold for between €15 and €45. Subsidies are now limited to low-income families. Solar cookers are very popular in Tibet, not only in the towns and cities but also in rural areas, where alternative fuels are scarce. The reason for this is simple: by reducing the amount of dung used for domestic cooking, more dung is available for other purposes such as space heating, fertilizer or as commodity for trade.

Complementing devices like heat retainers help to delink the food preparation from the time of sunlight availability: foods can be precooked on a solar cooker and transferred into a retained heat cooker where cooking is finished. Heat retainers also keep the food or water warm until consumption. An impressive example is the use of thermos flasks in conjunction with solar cookers in the autonomous region of Tibet. Alongside solar cookers, families own up to five thermos flasks and store the water heated by the solar cooker during the day. The hot water is used for preparing both morning and evening meals (mainly soup and porridge) enabling the solar cooker to fulfil almost all the household energy needs. However, this represents a rather specific case, which may not be transferable to many other countries. (see also article on Heat Retainers).

In Bolivia, solar cookers were introduced with support from the French NGO ‘Bolivia Inti-Sud Soleil’ using a different approach. Solar cookers, mainly box cookers, were presented in villages during cooking demonstrations. Following this, training courses were offered to those artisans who expressed their interest in cooker production. More than 7000 solar cookers have been built and sold since 2000. In 2011, the project was registered under the Gold Standard in the voluntary carbon market. The success of solar cookers in Bolivia is partly due to biomass scarcity, causing many families to buy gas. An evaluation has shown that using the cooker saves money and a direct relationship exists between the frequency of solar cooking and the amount of money saved by the household. Solar cookers are offered together with fuel-efficient stoves as energy saving technologies.

In India, large scale solar cooking in temples, hotels, educational institutions, etc. is becoming more and more relevant and is subsidized by the government. A typical solar steam system (excluding cooking vessels & conventionalboiler) comprising of 96 sq.m of Scheffler dish area (6 dishes each of 16 sq. m) can save around 4,500 liters of diesel in a year. It could pay back the cost in 4 years with support available from the Ministry. One of the largest Scheffler cookers is used in Shirdi Saibaba temple in Shirdi, India. Built in 2010, it consists of seventy-three rooftop-mounted Scheffler reflectors of sixteen square meters each. The system is capable of cooking 40,000-50,000 meals per day. Though the solar steam cooking system cost nearly $300,000, government subsidies reduced the temple’s portion to about $170,000. Liquid petroleum gas use has been cut by roughly 100,000 kilograms each year, for an annual savings of approximately $45,000. The temple should recoup its investment in three to four years. 

Another example is the solar park at the base of Mount Abu, a Hindu pilgrimage site, where 35,000 people can be served with food. See also this video by Deutsche Welle. 

Basic Rules on Dissemination and Use of Solar Cookers

From the experience of numerous projects that have attempted to introduce solar cookers, the following rules emerge:

• Suitable for local dishes: It must be possible to prepare the most important local dishes on the cooker, and solar cooking must not get in the way of local cooking traditions. For instance, one of the reasons solar cooker did not succeed in India was that cooking is considered a private activity, which is not done outside. Furthermore, local dishes frequently require frying or steaming, and such dishes are difficult to prepare in a box cooker.
  
• Easy handling: Handling (especially tracking the sun) must be easy for the user, and the cooker must be stable.
• Sufficient sunlight: The climate should be favourable for using solar cookers. There should be a time of year when the sun shines every day, making solar cooking a reliable option. A climate with frequent showers at any time of the year makes the use of certain types of solar cookers more difficult.
• Right exposure to sunlight: There must be places within the living area positioned favourably for capturing sunlight, enabling solar cooking to take place. Whilst this is not generally a problem in rural areas, it may limit the use in large cities.
• Biomass fuel costly (time, money): Solar cooking is likely to succeed only where biomass is scarce and difficult to gather, at least during part of the year. This condition holds true for both Tibet and the Bolivian Altiplano, where solar cookers have been successful.
• No subsidised modern fuel: The target group does not have easy access to other, reduced-price energy sources. For instance, subsidies for electricity were considered to be one reason for the failure of commercial distribution of solar cookers in South Africa.
• Solar cookers never “stand alone” solution: Cookers should never be offered as an individual solution, but ideally as a package with other energy-saving household technologies. This strategy partly contributed to the success of solar cookers in Bolivia where they have been offered in combination with energy-efficient stoves and other low energy technologies.
• Good availability, affordability and maintenance: Cookers with a good price-performance ratio must be locally available, and after-sales service and maintenance must be assured.
• After sales services and follow-up: Thorough follow-up support for users must be provided, and structures to provide this have to be put in place during the project period. Comprehensive and thorough customer aftercare is essential for solar cookers. Especially at the beginning, people often operate them incorrectly, and falsely attribute their failure to the quality of the device. This quickly leads to the technology being rejected. To solve this problem, it is important for a contact person to be locally available. In Tibet, this was a problem during the beginning of solar cooker dissemination.
• Safe storage: In some areas the cooker must be protected against theft and should therefore be easy to move to a safe place. This can be a costly barrier for poor households.
• Promotion of additional functions: Dissemination of solar cookers should stress their important additional functions, depending on the situation – for example sterilising drinking water, preserving jams or fruits, heating clothes irons, etc.

Further Information

• Improved Cooking Portal on energypedia
• Solar Portal on energypedia
• BLEENS - Biogas, Liquefied Petroleum Gas, Electricity, Ethanol, Natural Gas, and Solar
• The Solar Cooking Wiki - The most comprehensive knowledge base of solar cookers and their use in 133 countries throughout the world. Sponsored by Solar Cookers International
• The State of the Art of Solar Cooking by Barbara Knudson (2004).This very comprehensive paper gives an excellent review of solar cooking in all parts of the world and presents the projects involved in dissemination.
• Something New Under the Sun; Manual for Solar Box Cookers – How to Build and to Use Them. Technology For Life, Finland, 1995 (New version in 1998).This manual published by a Finnish NGO treats all aspects of production and use of solar box cookers.
• Here comes the Sun. Options for using solar cookers in developing countriesby GIZ HERA (2007).The brochure gives an overview on the role and limitations of solar cooking in different countries. Further, it describes the different strategies used for dissemination of solar cookers and analyses the causes of their successes and failures.
• GTZ Manual for construction solar cooker cement by Chai Xiaoqian, Ulrich Frings (2006).The manual provides step-by-step instruction for the construction of a Chinese–type butterfly solar cooker using local material. Every step is describes and shown in pictures. The illustrations make the manual very useful.
• Construction of solar cookers and driers by Christelle Souriau & David Amelin (2014).This publication is not only a construction manual for box cookers, solar panel cookers and solar driers, it also provides useful information and nice illustrations on use of solar cookers, cooking times of different food, and cost of solar cookers.
• Lasting Impacts of a Solar Cooker Project in Bolivia by CEDESOL / Szulczewski, M. (2006).
• Lasting Impacts of Solar Cooker Projects by Szulczewski, M. (2006).A review of evaluations of solar cooking projects using box and panel solar ovens in Bolivia, Ethiopia, and Kenya provides conclusive evidence of successful technology transfer and cultural acceptance of solar cooking.
• Solar Cooking Compendium by GTZ (2004).
  • Solar Cooking Compendium Main Report: Challenges and achievements of the Solar Cooker Field Test in South Africa
  • Solar Cooking Compendium Vol 1: Scarcity of household energy and the rationale of solar cooking
  • Solar Cooking Compendium Vol 2: Social acceptance of solar stoves in South AfricaSolar Cooking Compendium Vol 3: Making the case for commercializing solar cookers in South Africa
  • Solar Cooking Compendium Vol 4: The solar cooking toolkit. Conclusions from the South African Field Test forfuture solar cooking projects

• The Solar Cooking Compendium is based on the experience gained in implementing the Solar Cooker Field Test (SCFT) in South Africa from 1996 to 2003. It consisted of Phase 1 – Global market situation of solar stoves and social acceptance test (1996 - 1998) and Phase 2 – Estimate the market potential in South Africa, manufacture of solar stoves, and test marketing (1999 - 2003). The SCFT, a pilot program, was performed under a bilateral Technical Cooperation Agreement between the Governments of the Federal Republic of Germany and the Republic of South Africa (RSA). Executing agencies were the Department of Minerals and Energy (DME) and the Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ).
• Cooking Fuels
• Photovoltaic Solar Cooking Without Batteries Using PTC Ceramic Heaters

References

This article was originally published by GIZ HERA. It is basically based on experiences, lessons learned and information gathered by GIZ cook stove projects. You can find more information about the authors and experts of the original “Cooking Energy Compendium” in the Imprint.

• If not stated all pictures are supplied by A. Seidel.

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