http://solcooker.tripod.com/solar4.htm#Spherical%20Mirrors
Being a pioneering
design, this type of cooker has seen maximum variations, Besides, the design
offers several advantages. One of the most important is that the mode of
cooking is very much identical to day to day cooking. There are two major types
of concentrator solar cookers: (I) cookers which concentrate the light from
below, and (ii) cookers which concentrate the light from above.
Heat coming from
below
is most convenient for routine cooking, hence, many designers have concentrated
on this type of cookers. There are a lot of designs in this category, and they
are classified on the basis of the type of reflectors used (I) Spherical reflector
(S), (ii) Parabolic reflectors (P), (iii) Fresnel types (F), (iv) Cylindro
parabolic (CP), and (v) Mirrors plane (MP).
Spherical mirrors
are
the simplest type of reflectors, very easy to build and use. Focusssing sun
rays are also easy, and if one opts for a moving vessel to meet the focus,
cooking can also be done very easily. Such a design was suggested for the first
time in the year 1961 by Stam (1961) (Type S1, Figure 1). He suggested a large
reflector of 4.0 m diameter made of local material which could even include
mud, and the reflector surface suitably smoothed with fine mud/cement and
coated with aluminized polyester. An appropriate technology handbook describes
a simple method of construction of the spherical mirror in the ground (a tall
tripod with a long string to which a stone is attached at the tip, will act as
a guide for excavating a hollow in the ground) and after finishing and
stabilizing the interior, the reflector material could be stuck to make it into
a spherical mirror. Such a mirror, of about 2.0 m in diameter, would do useful
work for at least five to six hrs a day.
The cooking vessel
could
be hung from the tripod or a suitable stand and positioned to cooking vessel
could be hung from the tripod or a suitable stand and positioned to meet the
focus (Type S 1a). Dr. Halacy 91974) suggests a similar design (Type S 1b,
Figure 2). He uses two full and several half cardboard ribs to fabricate the
base with a mylar film as a reflector. This device was meant mainly for
campers. Bamboo and/or other locally available materials could be used to
fabricate such hemispherical baskets (Type S1).
Recently
Prof.Quintone
of UK has takenup this design and is trying to popularise it in places like
Peru. In his beautifully designed site he presents detailed instructions on
design and use.
Source:- TIDE., March 1998, 8-1, pp 1-37,
For
Comments, suggestions, contributions contact
Unfortunately, this
simple design has not attracted much attention, but on a very big scale, like
in power generation (as in Marseilles France), such a hemispherical mirror is
used (Jet Propulsion Laboratory 1981). Scientists of Australia (Anon. 1979) have
presented a similar design.
Medved et al.,
propose
an interesting design ( 1996 ) called as 'SOLAR BALL'. Designated here as Type
S 2 . It is an inflatable plastic ball with lower part of reflective material.
Cooking vessel is kept at the base. It is an interesting variation but there
appears to be some serious limitations with reference to size of the Ball as
well as size and handling of the cooking vessel.
Recently the sperical
geometry seems to have made a come
back and we see that in India. At Auroville,such a mirror is being built to
cook meals for 1000 persons. It is 15 meters in diameter and 7 meters above
ground level. The sun's rays, trapped by a huge hemispherical mirror, focus on a
cylindrical boiler which follows the sun's position by means of a computerised
tracking device. On a clear day, sufficient steam at a temperature of 150°C can
be generated in this boiler to cook two meals a day for 1,000 people.For
further details a visit to their site is a must.
(http://www.auroville.org/research/ren_energy/solar_bowl.htm) The figures
presented here are from their site.
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 make a parabola from a flat sheet from which a parabola
of desired focus and diameter are to be constructed.
Mike Scirocco is not
new
to visitors of this site, and also those who are trying to fabricate a
parabolic reflector. A vist to his site is a mustMr. Mike Scirocco's
site Mike has
updated his already excellent program
on the subject, it is a freeware, it is easy to use, makes the task of
fabricating Parabola very easy.
But how to start
with
making one ? Here I present the proposal in three easy steps.
The mud block has to be
made from roughly sieved
mud, this will aid in finishing process. This can even be hand sieving by
removing large stones and fiber particles. During drying process cracks appear,
but yo need not bother.
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.
Mikes program makes
it
easier to excavate a parabolic shape in the mud. You need to know the diameter
of the reflector, and it is generally 120 to 140 cms. Larger the
better.
The next parameter
to
select is the Focal length. Once these parameters are given the program gives
you exact measurements in X and Y axis of parabola and thus it is easy to
excavate the parabola.
TWO TYPES of
parabolic
reflectors are recognized in this review: (i) rigid parabolas, and (ii) collapsible
or folding 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).
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,
For
Comments, suggestions,contributions contact
Please
add, Solar or Solar Cooker, in the subject to avoid your
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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).
Fig
3a. Parabolic Mirror array form Mt. Abu ( Source SCI site )
Recently Sintex
Plastics
of India seems to have entered the Solar Market in a big way. They had been
manufacturing Solar Water Heaters of Plastic, but now they are trying their
hand at Solar Cookers, the Parabolic as well as Box type Cookers. I am yet to
receive the samples for evaluation. The photo presented here has been copied
from their site, though it does not display any innovations, the price is very
competitive at just Rs. 2500. Of course, much depends on the reflective coating
too, and again I am yet to receive details regarding the same. With a long
lasting Reflective material, and at the price, this Parabolic Cooker should
steel the market.
Another recent
addition
to the family of Parabolic cooker comes from the prolific inventor Mr Deris of
California. The Parabola it self is simple, he seems to have discarded the
interesting design of square Parabola invented by himself. The parabola he uses
is held vertically, and it directs the focused rays on to a Cooking vessel kept
on a reflector kept horizontally on the ground.
Interestingly way
back
in 1980, I had proposed that a reflector/focusing device kept outside or in a
suitable position would point focus the image in the house which could then be
used for cooking. This design you would see under Type ---but such a concept
has already taken shape, independently at Bahai institute in India and with the
help of a parabolic reflector kept out side the hut cooking even Chapathis are being
cooked.
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Fig 3a1 & 2 Photos
from Bahai Institute, India ( Ref. their site on the web )
The following diagram
shows the principle of Scheffler's reflective cooker design in detail.
The photos bellow
shows
Scheffler's reflective cooker, a 2.7 m dia cooker, just out side an Indian
Home, The photos are from ecosolar.com. The second photo shows a lady cooking
in a small kitchen.
The parabolic
cookers
are being uses in Africa as well and the photo below confirms it
further.
Tinytech's Community
Cooker
Recently Zhu and Kim have evolved a parabolic Cooker which they claim as non tracking. May be the geometry is such that it may not require constant tracking. In wich case it would be more of a deep parabola as detailed later. The Photo is from Solar cooking archives.
Source:- TIDE., March 1998, 8-1, pp 1-37,
Attempt are on
worldwide
to form parabolas from other materials. In India it is being made from mud. The
photo included, taken form Solarcooking.org site, confirms this.
Many interesting
variations have been reported, and some of the following photos, taken from
Solar cooking.org, are self explanatory.
Note the light
but
stable stand arrangement, looks like it could be easily tucked in a corner and
easily transported as well. ( Ref. solracooking.org site )
This is indeed a
novel
design by Deris of USA, because metal sheet comes in the shape of rectangle or
square, and thus a sq. parabola is easy to make,no wastage! but details were
not available ( Ref. solracooking.org site ).
Prof Ajay Chandak of
India has come out with another variation of Square parabola. Here he has used
strips of reflector material rather than converting the entire sheet as done by
Derris. Ajay report that the results are better than that of regular parabola.
Recently Mr
VELJIBHAI K
DESAI of TINYTECH PLANTS,Tagore Road, RAJKOT - 360002, INDIA, (
+919227606570 , +91
281 245 5730 , 91
281 246 7552 (FAX), tinytech@tinytechindia.com,
http://www.tinytechindia.com) is manufacturing larger design of this type of
square parabola capable of cooking 5 kg (+14 kg of water so a total of 19 kg)of
rice per batch,within 60 min. The unit is foldable and can be easily stored. He
uses German made Aluminium Reflector strips. Tinytech is manufacturing many
more alternate energy devices, a visit to their site is a must.
'Old is
gold' it is claimed and this has been proved once again by Prof Ajay
Chandak of India. In what could be considered as worlds biggest project Prof. Ajay has
installed 363 Parabolic community cookers. Each reflector has a diameter of 2.3 meters.
Fixed on to a rigid 'A' type stand, the cooker can take care of 22 lit pressure cooker and
can also be used for wide variety of cooking. These cookers are installed in tribal schools
located in remote areas of Maharashtra(India). Each cooker cost around Rs 26,000/- (US
$575)and is supplied with large Pressure cooker, sun glasses, gloves etc. The picture bellow
shows cooker in use at a Hotel. Note the small platform for the cook to stand on and cook.
The Cook can use an umbrella tied to a stand. Prof Ajay selected Parabolic cooker, because
Scheffler's cookers involved more cumbersome technique to fabricate and costly installing
and tracking mechanism, making it rather costly. Author congratulates Prof Ajay on the
excellent work done by him along with his team.
The Interesting
variation was designed by active workers from Nepal Sri Ram Ashis Sharma and
Hannu Virtanen. Other details were not available at the time of uploading this
details. ( Ref. solracooking.org site )
A new and interesting
design of Parabolic Cooker has been presented by a German Group. It is called 'Papillon',
for it looks like a Butter fly with wings open. Though a Parabolic Cooker I would classify
the same under Duel Parabola which has been in vogue in China and Nepal. But Papillon could
be considered as an improved version over that of Nepal Design, if the claims made by the
designer are certified by an independent group. I find it very attractive and have a feeling
that it would be as efficient as it is claimed. But am not sure about wind stability. I feel
it can be improved by widening the base. Each wing like reflector measurers One meter
square, so together it make it a Two Sq. meter reflector delivering 1.2 Kw . The Designer
group, BSW Alternative Energie, has listed a lot of features and data on the cooker and a
visit to their site(www.bsw-energie.de ) is a must. It would be better if the design can be
made in such a way that it can be assembled only with nut-bolts ( without brazing,) and
other activities like forming a Parabola at the site. Without any doubt the efficiency could
be improved by 'enclosing' the cooking vessels. I am expecting further details from BSW as
well as any test reports from Solarcooking International or Dr Paul Funk.
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Recently Zhu and Kim have evolved a parabolic Cooker which they claim as non tracking. May be the geometry is such that it may not require constant tracking. In wich case it would be more of a deep parabola as detailed later. The Photo is from Solar cooking archives.
Mr Bernhard Muller of Germany
(http://www.mueller-solartechnik.com)has comeout with a low cost design of Parabolic cooker which is
dubbed as "Kundu Kar". This design is
being successfully used in Africa.
Prof. Von Oppen
(1977),
while working in India, proposed a do-it-yourself deep parabola (PRD 1). The
focus of the reflector was inside the rim of the mirror, almost at the base.
This design was unique in several respects; it made use of locally available
materials like bamboo, paper pulp, Aluminium foil for the reflector, and a
unique and simple method for tracking the sun. The method was simple, a master
mould was made to form the shape of the deep parabola, paper mache was smeared
and dried, fine paper pulp was used to make the interior smooth, and this shell
was reinforced with bamboo from outside. The interior of the shell was then
coated with Aluminium foil. The cooking vessel was hung from a string (Figure
7). Focusing was essential only once in 30 minutes. Provision was made for
automatic tracking.
A pipe of about 15cm
diameter and 1 m height was buried near the basket and filled with water. A
bottle, half filled with water, was hung on into this pipe with the help of a
string, and the other end of the string was tied to the basket. A small hole at
the bottom of this pipe let out water and as the water level went down, the
half-filled bottle sank, turning the basket suitably. However, effective
cooking power was estimated to be only 250W. Initially, the cooker created a
sensation. There were many difficulties in using the cooker, bright reflected
light hit the eyes, it was difficult to handle the hot cooking vessel, and the
basket was not stable in strong wind (GATE 1979).
In 1979, the German
Appropriate Technology Group (GATE ) proposed another less deeper parabola
(PRD2, Figure 8). The reflector was 1.4 sq. m and was supported on a firm
stand. The cooking vessel was to be hung from a separate stand (GATE 1979).
Kapur (1982) described a similar design with further improvements in the stand
like adding castor wheels (Figure 9).
Source:-
TIDE., March 1998, 8-1, pp 1-37,
For
Comments, suggestions, contributions contact
Please
add, Solar or Solar Cooker, in the subject to avoid your
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Another interesting
design from Nepal uses two parabolic mirrors, made of various materials
including clay and coated with probably with Aluminised polyester. ( For details
see SCI site )
Mr Hannu Verten and
his
team is doing excellent work at Nepal and his site is worth visiting for
excellent photos, plans and comments.( listed under Links of Solar 16 )
The Classical Chinese design presented here bellow is one such
example.
Asymmetrical
parabolas
An asymmetrical
parabolic reflector enables a cook to be as close as possible to the cooking
vessel. In fact, in some of the earlier designs, like in case of Ghai’s design
(Type PRS 2 and also in PRD 2), part of the reflector was cut out to facilitate
this. But Tabor (1966) proposed a unique design (PRA 1). He used several
smaller parabolic mirrors and arranged them in an asymmetrical parabolic
configuration (Figure 10). The unit has a ‘U’ frame rotating on a pivot fixed
to a strong base.
Tabor's design seems
to
have made a comeback. Recently the Solar Cooker group from Mexico have devised
a bigger version of this type of cooker. The device has Orientation/Tracking
unit also
The frame has a
stand at
the top for the cooking vessel. Focal length of the reflector was 83 cm and the
area was 0.8 sq. m. A temperature of 300o C could be attained though the
effective cooking
power was estimated to be 185 W.
Source:-
TIDE., March 1998, 8-1, pp 1-37,
For
Comments, suggestions, contributions contact
Please
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Concept II, PRA 2a
envisages the use of strips of reflector. This would make the fabrication easy
and may perhaps increase the efficiency and maintenance of the reflector.
Further, an insulating cover around the cooking vessel would increase the
efficiency and would add aesthetic value.
For
those who wish to
cook from inside the house, or at least near the place where such shapes are
used to trap radio waves,) Murthy (1982) presented an interesting variation
designated here as PRA 4 (Figure 13). The device is a large asymmetrical
parabola of Auminium sheet hooked on to a strong stand which can rest on a
window frame (a provision had been made to adjust for the declination angle of
the sun window.). The author calls it an offset feed parabola, a by-product of
space technology (Figure 13). The author claims that the design delivered 994
W, which appears rather high ( the reflector used was only an Aluminium sheet).
Murthy suggests a south – facing window for better results. An insulated cover
around the cooking vessel would increase efficiency. The design should
incorporate easy folding facility such that it could be shifted easily from one
window to another.
Prof
Ajay, is a
prolific designer based at Dhule, Maharastra, India. His design called as
Balcony Cooker is presented here. I classify it as PRA4a as it is very close to
Murthy’s concept. The Balcony Cooker must have been evolved independently by
Ajay. Ajay’s another design called Hybrid cooker is presented on the next page
under plane mirror section, almost at the end of the page.
Source:- TIDE., March 1998, 8-1, pp 1-37,
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Comments, suggestions, contributions contact
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