2. Spectrum of potential users
The spectrum of potential users for solar cookers is huge. A family in a refugee camp will be interested in the cheapest cooker (for only a few dollars), a community kitchen of a school, for example, will look for something more powerful and a middle class family will prioritise convenience and design. Therefore, it is not possible to talk about a single appropriate solar cooker; different solar cooker types have to be taken into account.

3. Types of solar cookers
First experiments with solar cooking were carried out more than half of a century ago; since then, a tremendous variety of cookers have been proposed by inventors, researchers and do-it-yourself enthusiasts. Today, four types of solar cookers are propagated seriously:

Solar panel cookers as an ultra cheap solution A metallic cardboard reflector heats the cooking pot, which is insulated by a transparent plastic bag. - Very cheap, lightweight - Metallic cardboard is often imported - Limited power and lifespan - Needs to be adjusted to the path of the sun every 15-20 min - Successfully used in refugee camps
Solar box cookers for family use The greenhouse principle is used for cooking and baking (two boxes, one within the other, separated by insulation; single or double glazing; additional reflector). - Easily transportable, depending on the size; medium price range - Can be built with locally available materials - Medium range power and lifespan - Cooking needs minimal attention - Successfully propagated in Switzerland, Germany, India, Pakistan
Small parabolic reflectors with manual solar tracking for large families (> 10 persons) Aluminium facets in the form of a parabolic mirror reflect the bundled sunlight onto the cooking pot. - Transportable, medium price range - Aluminium facets often need to be imported - Relatively high power, medium life span of aluminium facets - Successfully propagated in Germany, China, India
Large parabolic reflector system with automatic tracking for community kitchens (50+ people) A sophisticated, large parabolic mirror reflects the sunlight directly under the cooking pot inside the kitchen. The reflector automatically follows the sun (mechanical clockwork or solar cell). - Generally fixed installation, upper price range - All parts available locally - High power, medium to long lifespan - Successfully propagated in India

4. Cost and global potential of solar cooking
For easier comparison, we have estimated the cost of the four cooker types described above on the basis of their power (kW) and of the energy they produce (kWh) (see Tab. 1, the sales prices are taken from India [ 4 ]). It should be noted that solar cookers are built locally in many countries; the costs involved therefore vary from place to place. The prices are of solar cookers produced in small series; mass production would result in cheaper prices.

The first comparison reflects the costs involved in obtaining a certain efficiency for solar cooking i.e. the cost of one solar cooker kW. The cost is estimated using power at food level, i.e. after losses of reflection, etc., and for a regular use during 10 years. One solar cooker kW costs between 240 and 540 US $. This is at least 8 times less than the cost of photovoltaic electricity (not taking the cost of an electrical range into account). Larger community kitchens are on the more expensive side; this is justified by their more regular use and the extended comfort.

The second comparison focuses on the energy of the cookers. For calculating the cost of each kWh at food level, a typical climate has to be defined. Household cookers are estimated to be used 3 or 4 hours per day on 150 sunny days per year, community cookers are defined as being used for 7 hours per sunny day.

According to our calculations, the cost of a solar cooker kWh is between 5 to 6 US cents if the cooker is used as defined above. How does a solar cooker kWh compare to the other cooking fuels? One solar cooker kWh at food level is about the same as 5 kWh of wood (20% efficiency of a wood-fire), so replacing the use of one kWh of biomass (wood) by solar cooking costs 1 US cent. As a gas stove is more efficient than a wood fire, we can compare one kWh of solar heat at food level with 3 kWh of gas (33% efficiency). This means: replacing one kWh of gaz by solar cooking costs about 2 US cents! (Note that the cost values for solar cooking include the cost of the stove.)

The worldwide consumption of fire wood is close to exceeding 2 billion tons per year, most of the wood used for cooking [ 1 ]. In areas with a favourable climate, it is possible to reduce the energy required for cooking by 1/3 to ½. Just how much solar cooking could contribute to worldwide reduction of fire wood consumption has still to be investigated in more detail.
However, more important than the total amount of biomass saved is the fact that solar cooking can help where help is most needed: areas with severe fire wood scarcity are usually dry and sunny.

Estimated cost of cooker without pot; production in small series; climate: 150 sunny days per year * Replacement of aluminium facets twice in 10 years ** Installation cost not included

Table 1. Solar cookers according to cost per kW and per kWh at food level on the basis of regular use over 10 years. One solar cooker kW(h) at food level replaces about 5 kW(h) of wood or 3 kW(h) of gas. (Note that the cost values for solar cooking include the cost of the stove.)

5. Promotional work for solar cooking
For more than 15 years, the ULOG group has been promoting solar box cookers and 'Scheffler-type' parabolic cookers in developing countries and within Europe [2]. The ULOG group is a 'loosely organised' NGO concentrating mainly on small-scale (and low-cost) projects. An example of one such project is the financial and long-term moral support of a solar cooker family enterprise in Burkina Faso.

Looking back, we have to admit that the 'good news' of solar cooking is not spreading as fast as the idealistic initiators envisaged. Ever present is the question why some projects succeed and others fail. Asked blatantly, despite decades of propagation, why is solar cooking not yet used on a large scale if it is so simple, cheap and from the European point of view so 'logical'? The answer does not lie in the technology alone. The experiences made in many ULOG projects, as well as the author's hands-on experience as a solar cook, are presented in another paper [3].

6. Outlook
Even if solar cooking is a 'slow starter', there is hope: If projects are implemented seriously, if the dissemination of information is intensified, if cookers are optimised to the cooking habits of the target group and utilisation is actively supported, if solar cooking is not limited to a solution for the «poor», if we practise what we preach and if we find appropriate financing schemes, there will be a bright and sunny future for solar cooking.

7. References
[ 1 ] 'Moving Ahead with Solar Cookers', Brochure of the German Technical Cooperation GTZ, 1999
[ 2 ] U. Oehler, W. Scheffler, 'The use of indigenous materials for solar conversion', Sol. En. Mat. and Solar Cells 33 (1994), p.379-387
[ 3 ] http://www.solarcooking.org/papevar2.htm
[ 4 ] Price list of Gadhia Solar Energy Systems, 86 Old GIDC Gundlav Valsad, 396035 Gujarat India