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A solar Scheffler reflector unique in our country
A Scheffler reflector is a solar device for the concentration of solar light that allows, amongst other applications, to cook with solar energy. The aim that led to the development of the Scheffler reflector was to make solar cooking more comfortable. With this idea, the German physician Wolfgang Scheffler designed a solar cooker that allowed the cooking focus to be still while the sun kept its apparent path in the sky, and that had a structure that would allow cooking from inside of the building.
In 2003, in an exhibit of the work of the artist Andreas Wegner, an eight square meters surface Scheffler reflector was set up in Madrid. The author of the work and the inventor of these reflectors, Wolfgang Scheffler, gave this device up to Terra Foundation to be used as a tool for solar demonstration in our country.
At that moment, Terra Foundation established a relationship with the Museo Nacional de la Ciencia y la Técnica de Catalunya (National Museum of the Science and Technics of Catalonia), located in the city of Terrassa, with the aim to install this reflector on the roof of the modernist building of the Museum. The work was not completely installed until mid 2006. The Scheffer reflector installed in the mNACTEC has a reflective surface of 8 m2 and makes a 3.8 x 2.8 meters' ellipse. The installation also includes a protective metallic door and a switch to turn off the mechanism of the daily solar tracking. The structure of the Scheffler reflector has 5 main parts: the support structure (1), the reflector (2), the mechanism of daily rotation (3), the mechanism of seasonal adjustment (4) and the cooking focus (5).
A feasible technology
Another of the characteristics of the Scheffler reflectors is that they can be built in any rural welding workshop and with materials that are affordable and locally available. For this reason, the Scheffler reflector is made up mainly of steel pieces and mirror glass. For the models destined to Europe, a small photovoltaic device for the tracking is also used, which feds a small electric engine and it has a frame of aluminium. For the secondary focus or cooking focus, high reflecting aluminium sheets are used. In other countries, these materials are substituted by others that have a similar function but which are available locally. This first Scheffler reflector set up in Spain is one of the few of its characteristics available in Europe. It was hand-built in 2003 with pieces of iron, aluminium and standard mirrors. The autumn of 2003 the reflector was displayed in Madrid to prepare food.
Due to its current location in the National Museum of the Science and Technics, this Scheffler reflector is the only one in Europe to be used as a device for demonstrations about the possibilities of the solar energy in the treatment of food (cooking, drying of food, making up of cooking recipes, etc...).
A surprising technology
The shape of the reflector is the result of a small lateral section of a circular paraboloid. The light reflected through this section is concentrated on the focus of the circular paraboloid. The distance between the focus and the reflector depends on the initial parabola selected. The reflecting structure is orientated to the south and it is situated in front of the house, so that the solar rays are concentrated on the focus, which is inside the house. The shape of the Scheffler reflector is a parabolic mirror that turns in a synchronised way with the sun around a rotational axis parallel to the Earth polar axis. This axis is located in a north-south direction and goes through the centre of gravity of the own reflector, keeping it in equilibrium, so that the daily mechanism to follow up needs to make a minimum effort to follow the daily rotation. In the same way, in order to not misplacing the focal point, this is placed in the projection of the rotation axis, so that during the day the concentrated light goes round itself and it never moves laterally.
By means of the tracking mechanism the reflector rotates daily around its axis to keep the focus still and thanks to the distortion of the frame of the reflector; this adjustment of its shape during the different seasons of the year it is done manually. During the seasons, the incidence angle of the solar radiation may vary up to ± 23,5º in relation to the inclination of the terrestrial axis. For this reason, it is necessary that the reflector can bend with an identical angle to align with the sun and to concentrate the solar radiation at a maximum, without varying neither the position of the reflector nor the cooking focus. This is only possible if the shape of the reflector varies with each new solar angle through the seasons of the year, so that each day we have a different shape of the parabola. To achieve this variation, the frame of the reflector distorts by itself to adjust to the angle variation.
Once the reflector concentrates the solar radiation in the focal point, a fraction of the light impacts straight over the wall of the cooking pot and another fraction is reflected through a second smaller reflector - called secondary reflector - to the base of the pot where it is absorbed and converted into heat. Depending on the season, the 8m2 reflector collects the solar light in a perpendicular surface - called opening area. This fact makes the cooking power vary slightly depending on the season of the year.