Parabolic solar reflectors - Solar Cooking

Parabolic solar reflectors - Solar Cooking

Single and compound curves

A lot of difficulty is encountered in the fabrication of parabolic reflectors, even with thin sheets of steel, since a paraboloid is a "compound curve", which cannot be made simply by bending a flat sheet of metal. To make a paraboloid exactly, the metal has to be stretched, which is difficult to do with simple equipment. Many fabricators have presented methods to make the construction of a parabolic shape easier by doing it approximately. Srinivasan (1979) proposes a method wherein a circular sheet was cut out into suitably shaped petals, each of which is a "single curve", which can be made by bending a flat sheet., The dimensions of the petals are carefully arrived at after detailed calculations. The petals are then joined to form the parabola, But this method too was found rather difficult, especially cutting out ‘curved petals’ from the sheet.

Fabricating Parabola from a single plane.

So, a simpler method has been proposed – (Concept I) by Ashok Kundapur– which involves cutting the circular (1-1.5 m diameter) GI sheet or aluminium into ‘petals’ with straight cuts which is easier. Large sheet of one to one-and a half m diameter is taken. Center is marked and a small circle of 15 cm in drawn from this circle. Then up to to 8 or 16 lines are drawn to the perimeter of the outer circle from the center (radius). 5 mm holes are drilled at the junction of this inner circle and the line of straight cuts. This would assist in easier overlapping (Figure 4). Then, the ‘petals’ are fixed overlapping one another, only at the edge. The width of this overlap was calculated using standard formulae (Baumeister et al. 1978; Kundapur 1995). Recently Mikes site mentioned above give more accurate dimensions of the over lap, one need enter just the number of 'petals' and length of focus in his chart and lo ! the width of overlap shows up. Once you get the values of over lap, drill small holes at the outer periphery of circle just adjacent to the radial lines based on the width of over lap so as to aid in joining of the 'petals'. Also drill a 5 mm hole in the center of the circle. Now cut the circle into petals along the radial li e perfected by pushing or pulling at the hole at the center of parabola. When finished, the parabola has to be fixed to a frame work made of Steel or Aluminium flats. The frame work has to be fixed on to a U shaped stand. (see following Figure for details). The petals are marked as 1 & 2, first


(Please await diagram)

Mechanical Mathematician

Mr Genevieve of France has described a simple way to form a parabolic dish. either from cardboard panels or with mud. He made both. He calls it the Mechanical Mathematician. http://www.sciences.univ-nantes.fr/physique/perso/gtulloue/conics/drawing/para_string.html has a super applet that shows the principle really well. In practice, a setsquare cannot be used as it would go through and destroy the parabolic shape! The mechanical mathematician makes use of the principle but does not go through the parabolic dish.So a dish can be made directly. It has several advantages. No math needs to be calculated and the material for making the mathematician can change depending on what is available. In practice, a The focal length can be changed really quickly to suit the material used. It can be set up on site pointing in the best direction, etc. find more on the web about it, mechanical mathematician and cob can be searched. Here is a simple animation to show how it works.