Community Solar Cookers Using Scheffler Reflectors

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by W. Scheffler, Group Ulog, Switzerland

INTRODUCTION

Solar energy has been used for cooking for many years, and various designs are employed. In India, for example, about 130,000 solar box cookers are used regularly and a similar number of reflector cookers are in use in China. Even a very simple system, such as folded sheets of aluminium-coated cardboard, combined with a black cooking pot in a transparent plastic bag, works and is used.

Thanks to the more recent development of the so called "Scheffler" reflector, a parabolic reflector with a flexible curvature and fixed focal area, solar cooking has also became a reality in community kitchens (such as those serving boarding schools) which cater for 100 to 400 people.

Currently, one of the world's biggest solar kitchens is nearing completion at Mount Abu, India. Twenty-four locally produced Scheffler reflectors with a total aperture area of 120 m2 will supply steam to the main kitchen of an international conference centre, catering for up to 1,500 participants.

BACKGROUND

In many developing countries energy, even for basic needs such as cooking, is in short supply while the industrialised countries use too much.

Since 1983, Wolfgang Scheffler has been working to develop a practical device for solar cooking which could be produced in developing countries. Together with Group Ulog, in Switzerland and Solare Brücke eV in Germany, he now manages a large and growing programme to develop the technology still further and to facilitate its use.

Most of the practical development work on the new device was done in Kenya and India, in close co-operation with the intended users, and using locally available materials and construction techniques.

The result is the Scheffler reflector, a flexible parabolic concentrator which adjusts its shape to the seasonal position of the sun and, because of the fixed focal area, allows convenient solar indoor cooking at high temperatures.

The technical features also make other applications feasible: a high temperature heat source for thermal storage, for cooling processes, and for process heat in general.

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Simplified diagram of the system.

THE TECHNOLOGY

Fixed Focal Area
The Scheffler reflector uses a small portion of a larger parabolic reflector to reflect sunlight sideways to a focal area. To make it easier for the reflector to track the sun, the axis of daily rotation passes through the centre of this small portion and is parallel to the earth's axis. This minimises the forces on the tracking system. Daily tracking can be operated mechanically (by clockwork) at a speed of 15 per hour.

The centre of the focal area is on the same axis, at a distance from the centre of the reflector which is determined by the chosen parabola. In this way, the concentrated light rotates on itself around this point during the day, but does not move in any direction.

Variable Parabola
As the incident angle of the sunlight in respect to earth's axis changes with the seasons ( 23.45 ), the parabola which defines the shape of the reflector also has to change. At the same time, the centre of the eccentric portion and the focal point remain fixed in space. This means that a different parabola is needed each day of the year.

To make things easier, the reflector frame is constructed for the conditions at equinox (when the sun's rays are perpendicular to the earth's axis), and then is inclined and subjected to some elastic deformation to form all the other parabolas required.

From the focal point, the light is reflected again and concentrated onto the black bottom of the cooking vessel, where it is absorbed and converted into heat. The efficiency for cooking is in the range 30% to 50%, depending on the cleanliness of the reflective surfaces and the thermal insulation of the cooking vessel. The optical efficiency at the focus area can reach up to 68%

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Inside a "traditional" solar kitchen in southern India

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PRACTICAL EXPERIENCE

The first practical cooker of this type (with a diameter of 1.2 m x 1.6 m) was built in 1986 for a mission station in the northern desert of Kenya. It is still in use today.  Since then, more and more cookers have been built each year, all manufactured locally using local materials. Currently, about 120 systems are installed, mainly at boarding schools to cook meals for the students. Cooks like using the solar reflectors since their work is reduced (they do not have to tend the fire, vessels are easier to clean) and the kitchen is smokeless.

TECHNICAL DIFFICULTIES

Problems which arise are usually technical ones and, in the beginning, these happened quite frequently. The kitchen sometimes remained unused for many months until someone happened to pass by who could solve the problem.

The major problems were malfunction of the clockwork, the relatively short lifetime of the reflective plastic film of 2 to 3 years (replacement is not difficult, but requires some extra initiative from the user), reduced reflectivity of the secondary reflector under the pot and aluminium pots with bottoms that are no longer black because they have become worn.

In the latest systems most of these difficulties have been addressed by using a sturdier clockwork mechanism and by using small glass mirrors for the reflective surface. These are easier to clean and promise a much longer lifetime.

Where there are many solar kitchens in one area, such as in Gujarat, India, the programme has organised local maintenance support to visit and service all kitchens annually, after the monsoon season.

DISSEMINATION

In the first 10-year phase of this programme the focus was on developing the technology in close contact with the intended users. Now, in the second phase, dissemination of the knowledge is the main aim together with continuing technological progress.

Interested groups which have secured finance for their project are given training over a 2-3 month period, during which one or two cooking units are built. After that the group can continue alone. Programme workers keep in touch with the users however to help if problems arise and to teach improvements.

For more information contact the CADDET Swiss National Team in Aarau.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.