The first action for the morning cooks is to turn the big parabolic mirror towards sunrise. Then they place a pot of water into the mirror's focus for morning coffee. The rest happens on its own. The cooks are satisfied: "The Scheffler mirror is very easy to use. The food cooks quickly when there is direct sun." The Scheffler mirror was built in Tamera in cooperation with the Swiss travelling advocate for sustainable development, Alec Gagneux. "The advantage of this technology is," he explains, "that it runs without expensive photovoltaics and can be built and repaired relatively easily without specialist education. What distinguishes the Scheffler mirror from other parabolic solar mirrors is its convenience of use. It is possible to cook inside the house -- a decisive advantage in hot countries where no one wants to stand outside in the midday sun to cook."
The Technology of the Scheffler Mirror
A Scheffler mirror is a fixed-focus solar reflector. The original idea came from Jürgen and Hans Kleinwächter. The inventor Wolfgang Scheffler developed it further for the needs of economically poor, sun-rich countries. The invention is not patented: its construction plans are available to all, following the open-source philosophy (http://www.solare-bruecke.org/). Sunlight is focused on a fixed point, and remains focused there by means of a simple automatic adjustment system even as the relative position of the sun changes during the day. Wolfgang Scheffler developed an intelligent clockwork adjustment mechanism: the mirror moves automatically to follow the course of the sun with the help of a mechanism built from bicycle parts. The tracking mechanism of the Scheffler mirror means that a solar oven, for instance, can run throughout a sunny day without needing any manual adjustment, thus enabling the construction of solar kitchens. The mirror continuously supplies concentrated light energy. The distance between the hot focus (for instance the baking oven in a house) and the centre of the reflector (the mirror in front of the house) is defined by the curve chosen for the mirror.
Where Are Scheffler Mirrors Used?
For the past 20 years the mirrors have been used in many southern countries. The Barefoot College in India holds regular courses on the Scheffler mirror, enabling women to create their own independent and environmentally friendly energy source in rural areas. Different modules can be placed in the mirror's focus, such as a steam generator or heat-store in place of the cooking pot or oven.
How Effective Is this Technology?
The output of a reflector with a surface of 10 square metres varies depending on the season of the year between 2.2 kW during summer and 3.3 kW during winter, assuming solar radiation of 700 watts per square metre. At our latitudes, the power (energy per unit time) of the Scheffler mirror is higher in winter than in summer as the area of the mirror is used more effectively when the sun is lower in the sky. The total energy received during a day is however still greater during summer as there are more hours of sunshine.