The Scheffler-Reflector

The Idea

The basic idea that lead to the development of the Scheffler- Reflectors was to make solar cooking as comfortable as possible. At the same time the device should be build in a way that allows it to be constructed in any rural welding workshop in southern countries after a certain period of training. The locally available materials must be sufficient.
 


The Technology

To make cooking simple and comfortable the cooking-place should not have to be moved, even better: it should be inside the house and the concentrating reflector outside in the sun.

The best solution was a eccentric, flexible parabolic reflector which rotates around an axis parallel to earth-axis, synchronous with the sun. Additionally the reflector is adjusted to the seasons by flexing it in a simple way.



How does this work?

The reflector is a small lateral section of a much larger paraboloid. The inclined cut produces the typical elliptical shape of the Scheffler-Reflector. The sunlight that falls onto this section of the paraboloid is reflected sideways to the focus located at some distance of the reflector.

The axis of daily rotation is located exactly in north-south-direction, parallel to earth axis and runs through the centre of gravity of the reflector.

That way the reflector always maintains it's gravitational equilibrium and the mechanical tracking device (clockwork) doesn't need to be driven by much force to rotate it synchronous with the sun. The focus is located on the axis of rotation to prevent it from moving when the reflector rotates. The distance between focus and centre of the reflector depends on the selected parabola. During the day the concentrated light will only rotate around its own centre but not move sideways in any direction. That way the focus stays fixed, which is very useful, as it means the cooking-pot doesn't have to be moved either.

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In the course of the seasons the incident angle of the solar radiation varies + / - 23,5° in relation with the perpendicular to earth-axis. The paraboloid has to perform the same change of inclination in order to stay directed at the sun. Otherwise it's not possible to obtain a sharp focal point. But the centre of the reflector and the position of the focus are not allowed to move.

This is only possible by shaping the reflector after an other parabola for each seasonal inclination-angle of the sun, i.e. for each day of the year. This means the reflector has to change its shape.

The reflector-frame is build for equinox. By inclining and elastically deforming the reflector-frame all other parabolas can be achieved with sufficient accuracy.



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Changing the inclination and deforming the reflector are mechanically combined: the two pivots A, at each side of the reflector-frame, and pivot B, in the centre of the reflector, do not form a line, but B is located below. That way inclining the reflector leads to a change in its depth, the centre of the reflector is lifted up (big radius of crossbars) or pressed down (small radius of crossbars) relative to the reflector-frame. It's enough to adjust the upper and lower end of the reflector (C and D) to their correct position to obtain a sufficiently exact reflector-shape. The setting is done by a telescopic bar at each end of the reflector.

Adjusting the reflector-shape has to be done manually every 2-3 days. When all concentrated light enters the opening of the cooking-place installed at the focal point the correct reflector-shape is achieved.

After passing the opening the light is redirected by a small reflector (secondary reflector) to the black bottom of the cooking pot. There it is absorbed and transformed into heat. The efficiency for cooking, i.e. heating water from 25°C to 100°C, can reach up to 57% and depends on the cleanliness of the reflector-surface and the state of insulation of the cooking-pot. At the focal-point itself we have measured optical efficiency of up to 75% (with 2mm ordinary glass mirrors). Depending on the season an elliptical reflector of 2,8m x 3.8m (standard size of 8m² Scheffler-Reflector) collects the sunlight of a 4,3m² to 6,4m² area, measured perpendicular to the direction of the incident light (aperture). That way the cooking power varies with the season. As an average a 8m² Reflector can bring 22 litres of cold water to boiling temperature within one hour (with 700W/m² direct solar radiation).

There are many options for the design of the cooking-place. Mostly it is integrated into a kitchen building and provides the possibility to use firewood for cooking when the sun doesn't shine. Depending on the type of food which is cooked there is no need for a secondary reflector. This increases the efficiency and simplifies maintenance. Instead of a cooking-place a backing-oven, steam-generator or heat-storage can be installed at the focal-point.


Dissemination

The first well functioning Scheffler-Reflector (size: 1,1m x 1,5m) was built by Wolfgang Scheffler in 1986 at a mission-station in North-Kenya and is still in use .

Since then the technology has been continuously improved and passed on to many motivated people. That way the number of installed reflectors increases from year to year .

For a number of years mainly 8m² size reflectors were constructed for canteen kitchens. After the year 2000 mostly 10m² Scheffler-Reflectors are installed.

It´s difficult to tell how many Scheffler Reflectors exist, as there is no central registration and many workshops work independently. 2004 there were about 750 reflectors in 21 countries, that coresponds to about 200 solar kitchens, including 12 solar steam kitchens with 10 to106 reflectors per installation. The biggest solar kitchen of the world in Abu Road, Rajastan (India) is catering for up to 18 000 visiters of a Yoga center.
Now, 2006, there might be around 950 Scheffler Reflectors worldwide.




Local Production

During the whole time of development of the Scheffler-Reflectors - up to now 23 years- care was taken to use simple materials and construction techniques common for rural settings. For that reason Wolfgang Scheffler did a great deal of his developments in Kenya and India where he learned local construction techniques. Together with local craftsmen he built a number of first solar kitchens and installed them at interested schools.

The main construction of the reflectors (apart from the mirror surface) consists of steel. We use profiles that are common in construction of furniture, water-installation and housing. This material is relatively cheaply available everywhere.

For the reflector surface various materials can be used. Silvered glass mirrors - as they are used for bathroom-mirrors - are most common.

The combination of affordable materials, common tools and un-complicated techniques of fabrication to create a product with high-tech qualities enables interested groups to make something with their own hands which will benefit them in a sustainable way.

A good example is the construction of the worlds largest solar-kitchen in Abu Road, Rajastan, by the Brahma Kumaris. Because they did most of the work involved themselves, the whole installation ( 800m² of Reflector surface + steam system + back-up boiler) could be built for only 100 000 €. As they cook for a maximum of 18 000 people this equals 125,-€ per m² or 5,5 € per person.

That way 400 liter diesel per day can be saved.

At the moment the Brahma Kumaris run six such solar kitchens (the other five are smaller).

Sunshine is locally available everywhere and shall add value locally and therefore increase the local quality of living. Access to energy is the base of development and economic action.


Solar-Economy

An example for a starting solar-economy in Burkina Faso is heating shea-nuts in locally produced solar ovens (solar box-cooker) as one of the steps necessary to make shea-nut butter. Usually the nuts are heated on a iron plate over a wood-fire. That way some of the nutshells get charred. In the solar oven the nuts are heated slowly and uniformly, without getting burnt. The effect is double: firewood is saved and the quality of the product is raised. Through this savings and higher gains the solar oven can be financed.

Developments

The 2m² Scheffler-Reflectors are now well designed for the use in Europe (especially for southern Europe). We have developed a construction from aluminium, the reflector-surface is made from clear-glass and the daily tracking is run automatically by a photovoltaic-control.

In co-operation with Indian engineer Ronny Sabawalla Wolfgang Scheffler developed and constructed a 50m² Scheffler-Reflector which is now being tested to deliver energy for crematoriums.

Besides that Scheffler-Reflectors are build in varieties of 2,7m², 10m² and 12m² (the last one with a longer focal-length in order to reduce problems of shadow from an existing building).

For larger energy-consumption and when kitchen and reflectors have a separate location the reflectors can produce steam, which is lead to the kitchen through steam-pipes. The biggest installation of this kind is used in India at a yoga-centre to cater for a maximum of 18 000 persons. Here the steam is also used as a medium of storage (2 hours full power without sunshine).
Smaller systems use a well insulated solid iron-block which is heated to about 400°C at the focal-point. For 8m² Reflectors a 300kg iron-block is appropriate, for 2m² Reflectors a 50kg iron-block. The energy can be stored over night and in case of the 300kg storage even over several days.
Recently some work was done on providing steam for autoclaves in hospitals. Bigger systems can use the same way of making steam as the large steam kitchens do. For small rural hospitals an insulated iron block gets heated by a 10m² reflector and provides steam at the instance.
Since 2006 16m² reflectors are manufactured in India, they are mostly used for process heat in industries.

Experiences

People happily like to use solar-cookers when the cooker is practical, delivers enough energy for their needs and when they don’t have to change their cooking-habits too much.

Through the integration into the house a Scheffler-Reflector provides similar comfort like a gas-stove (comfortable height of work, no smoke, plus: time- or money-saving for acquiring cooking-fuel).
The weak point up to now has been the daily tracking of the reflector with a mechanical clockwork. The advantage is that all the parts for such clockworks are available everywhere inexpensively. But when the mechanism fails the cook is often neither able to repair it nor willing to do the tracking manually. Therefore he/she will go back to their old wood-fire.
A good solution is a maintenance-service, but because of large distances and other reasons it doesn’t yet exist everywhere. Very reliable and with little need for maintenance is the PV-tracking device as we use it for the 2m² reflectors in Europe. But (apart from India) the components are normally not available in the country.

Long-term Operation

Since 1987 about 200 "small" solar-kitchens with 1 to 3 Scheffler-Reflectors of 8m² each were built - mainly to cater for boarding schools in rural areas. How well these kitchens are used depends on various local conditions and the involved individuals and institutions. In some places they are used enthusiastically on every sunshiny day - even in the morning before the sun comes up (where there is a storage-system integrated). In other places the kitchens are used rarely or even not at all. Best results show in India, where - among the kitchens with 1 to 3 reflectors- about half are used on control-visits.

An entirely different picture shows for the big installations of steam-powered kitchens. The systems are always in use (apart from monsoon-time) and well maintained. All technical problems are solved - sometimes after consultation- by the operators themselves. Solar Energy is used to its maximum and the owners are proud of the installation.

1998 the first big solar steam kitchen for 1000 people was inaugurated. Until now many more have been set up, even Indias biggest temple, the Tirupati Temple in Andra Pradesh is equipped with 105 reflectors.

 

Some Data on the 8m² and 10m² Scheffler-Reflector
Maximum temperature reached at focal-point

1020°C
Maximum optical efficiency (reflector-surface from clear-glass/ordinary glass)

84%

75%
Average cooking-power at 700W/m² insolation, with normal glass-mirrors (8m² Refllector)

2,2kW

(1,7kW in summer and 2,5kW in winter)
Maximum number of pots per reflector

3
Number of reflectors of the largest kitchen

106
Largest number of people catered for by one kitchen

18000
Cost of materials for one reflector (in India)

approximatly 550,-Euro
Overall number of world-wide installed reflectors (2004)

over 750
Used materials

Steel-profiles, glass-mirror


Internet-addresses about Scheffler-Reflectors


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