Wolfgang Sheffler and his colleagues have installed solar heating
equipment to provide cooking heat in many community kitchens since 1986,1,2,3,4 mostly in India. The
heating equipment consists of concentrating reflectors that
move to track the movement of the sun. In some configurations the
reflected and concentrated sunlight enters a nearby kitchen directly to strike a
cooking pot or frying surface.7 In other configurations, the
concentrated sunlight is used first to create steam4 which is
transported by pipes to a nearby kitchen.
A heliostat is a device
that
reflects sunlight in a fixed direction as the sun moves. Scheffler's
heliostat has a concentrating reflector turned by a clockwork powered
by
gravity or photovoltaic
panels. The rotating reflector produces a converging beam of
sunlight
aligned with its axis of rotation, which is parallel to the axis of the
earth. The converging beam of reflected sunlight enters the
kitchen through a hole in its wall.
When the rotating reflector (in this case the primary
reflector) is placed on the polar side of the kitchen, a fixed secondary
reflector inside the kitchen redirects the beam upward to the
bottom of a pot or frying surface. The
rotating reflector turns at a rate of
one revolution per day (15 degrees per hour) to keep the reflected beam
aligned with the
axis
of rotation as the sun moves.
The shape of the concentrating reflector approximates a
paraboloid.
The seasonal variation in the height of the sun requires changing not
only
the angle between the concentrating reflector and its axis of rotation,
but
also
the shape of the reflector. The geometry of the framework
supporting
the reflecting surface is designed so that the action of changing the
angle
between the concentrating reflector and its axis of rotation also
squeezes or stretches the concentrating reflector to flatten the
reflector or make it more dished.
The reflecting surface of the concentrating reflector consists of a number of facets, usually
flat glass mirrors. A concrete pad and a welded steel space frame
provide support for the concentrating reflector. The area of the concentrating
reflectors in the configurations shown above is usually about 10 square
meters, delivering approximately 2 kW to the cooking pot or solar receiver.
The reflector may be positioned on the equatorial side of the kitchen. From this position, reflected sunlight
travels upward along the axis of rotation, eliminating the need for a
secondary reflector.
Scheffler reflectors have been used to make steam for solar steam kitchens in India.4,6 The Tirumala temple in the south Indian city of Tirupathithat3, for example, uses many Scheffler reflectors to make steam for cooking in a nearby kitchen. In solar steam kitchens, concentrating reflectors are arranged on both the equatorial and polar sides of solar receivers attached to the steam pipes that transport the steam to the kichen..
At least one heat storage system for deferred cooking has been deployed with a Scheffler reflector.5 The heat store consisted of an iron mass in an insulated container. One end of the iron mass was exposed to the focus of the heliostat during the day, then capped with an insulated lid to save the collected heat for cooking or frying up to 24 hours later.
References
1]
Scheffler Community Parabolic Solar Cooker, http://www.gadhia-solar.com/communitycooker.html
2] Solare Bruecke, Scheffler's web site,
http://www.solare-bruecke.org/
3] India's Temples Go Green, Time magazine, July 7, 2008., http://www.time.com/time/world/article/0,8599,1820844,00.html
4] Steam cooking system, Ghadia Solar Energy Systems, http://www.gadhia-solar.com/steamcookingsystem.html
5] Installation
of a heat storage system at the Baha'i Vocational Institute for
Rural
Women -- Good photos of reflector frames, mounts, clockwork mechanism,
and heat storage system, http://www.barli.org/solar-storage.html
6] Parabolic solar concentrators for cooking, food processing and other applications, http://www.solare-bruecke.org/infoartikel/Papers_%20from_SCI_Conference_2006/127_deepak_gadhia.pdf
7] First
Solar Community Kitchen in Nepal,
another Scheffler system.
http://spot.fho-emden.de/hp/weiler/itahari.html. Photographs of
the interior of the cooking shed of a Scheffler solar kitchen.