Parabolic mirrors

Parabolic mirrors

Parabolic geometry is well known, and it was probably the very first type of solar cooker. In this category, there are maximum variations. The reason for its popularity was the focus which was much better and sharper than that of other types of reflectors, but at the same time it was very sensitive to even a slight change in the position of the sun and hence the use of such reflectors meant constant tracking.

Many visitors to this site wanted more information on construction of Parabola. But it is very essential to learn about the geometry of the parabola. There are many a reviews, but one of the prolific programmer mathematician Mr Mike Scirocco, from California, recommends the paper presented in Solar Energy, of 1978 by Dr. M Srinivasan et.al. Mike has taken lot of trouble to convert the text into a web page. This is not all Mike has a small program which can give you a lot of dimensions on Parabola. You just have to enter the diameter and focus of parabola and you get a whole lot of data. Mike seems to have taken my suggestion seriously and is planning to add more data display boxes which would tell a interested person as to the diameter of flat circle of sheet from which a parabola of desired focus and diameter are to be constructed.

You have to visit Mr. Mike Scirocco's site Mike's excellent program is presented here, it is a freeware, you can use it

Two types of parabolic reflectors are recognized in this review: (i) rigid parabolas, and (ii) collapsible or folding parabolas.
Rigid parabolas

Under this category, three major types have been identified: (I) Shallow parabolas, where the focus is outside the rim of the reflector, (ii) Asymmetrical parabolas, which are partial parabolas, and (iii) Deep parabolas, where the focus is within the rim of the reflector (Chart I).

Shallow parabolic cookers

There are just two variations in this design. PRS 1 is probably the oldest design, it has a firm U-shaped stand which holds the cooking post as well as the reflector. It is on record that Mouchet built such a cooker for the French troops of Napoleon III. The popular model, however, is known as the Wisconsin design of 1959 (Type PRS 1a, Figure 3). In this design, the reflector has a plastic base on which mylar was stuck. The design was not stable in the wind and there was also some problem of spilling of cooking material while the reflector was adjusted, and, hence, several designers have tried to improve upon it. Types PRS 1b to 1f are basically similar but for some modifications in the stand, orientation mechanism or materials used for the reflector, etc. The WHO design, PRS If, was most the sophisticated in terms of design and materials used. The Chinese type has a reflector, which has a slightly different rectangular shape and is made by sticking small pieces of mirrors on to a parabolic base. The vessel is kept on a separate stand to avoid spilling. Test data was not available. A Wisconsin design with 1.0 m reflector could boil 1 liter of water in 10-20 minutes, provided the assembly was aimed at the sun every 15 minutes. Recently, Kumar (1994) has conducted technical analysis of the parabolic mirrors. A lot of difficulty was encountered in the fabrication of parabolic reflectors, even with thin sheets of steel, so, many have presented easier methods. Srinivasan (1979) proposes a method wherein a circular sheet was cut out into suitably shaped petals, dimensions of which 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. So, a simpler method has been proposed – (Concept I) – which involves cutting the circular (1-1½ m) GI sheet into ‘petals’ with straight cuts up to the border of a 15 cm inner circle. Before cutting, 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). However, the best parabolas could be fabricated only with the help of large lathes. VITA (1961) recommends several simpler techniques such as: (i) soil-cement depressions in the ground, (ii) molded vermiculites, (iii) wire reinforced concrete shells, and (iv) paper-mache shells. The Boeing Company made hi-tech parabolas. Composite honeycomb structure was reinforced with fiberglass epoxy sandwich, to which a reflector material, polished aluminum sheet, was stuck. The reflector surface was further protected by vacuum deposited silicon oxide coating. It was reported that such a hi-tech reflector delivered about 437 W of power compared to 300 W of spun aluminum reflector of the same size. The size of the reflector is directly proportional to the performance of the cooker, and hence, it is a very important parameter. However, in most of the cases the size of the reflector was 1 sq. m only.

Source:- TIDE., March 1998, 8-1, pp 1-37,

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The Chinese type PRS 1g had an area of about 1 ½ sq. m and the design proposed by Kulkarni (1978). PRS 1h, had an area of about 2 sq. m, this design also incorporates a cradle, enclosed suitably with a windshield, to hold the cooking vessel (Figure 5). Kulkarni reports that even chapatis could be fried in this cooker. This design was being marketed by M/s Jyothi Industries of Baroda. Another design with a 2 sq. m reflector was that of Walton et al. (1977). It is also known as Volta type (PRS 1e).

Prof. Ghai’s (Anon. 1970) parabolic concentrator was of a different type (PRS2, Figure 6). Evolved in the 1950s, the reflector was made from spun Aluminium sheet. It had a hole at the centre through which projected an arm of the stand to hold the cooking vessel. Part of the reflector was cut out to enable easy access to the vessel. The entire unit had to be turned to face the sun.

These types of cookers did not become very popular, but The German News, 38 (June-July): 5-6, 1997, informs that 180 sq. m, parabolic concentrators have been installed at an institute at Mount Abu, to generate 600 kg. of steam at 16 bar pressure and cook food for 1200 people (Herms 1977).