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Selection of the best design

 

It is interesting to note that attempts to select the best design have been made since the 1960s but the best design is still elusive. Another fact is that almost all the past reviewers have observed that the existing designs were not up to the mark and found it FIT to propose new designs. In fact, this review is also not an exception, and the author has taken the liberty of presenting almost 12 new designs or variations. So the list could go on and render decision-making difficult for the interested public.

In 1961, VITA brought out a very detailed review. This was indeed a landmark review, and formed the basis of this research work at Udupi. There were also several other reviews like those of Gupta (1975) , Garg (1978), GATE Group (GATE 1979), Bowman, Blatt (1978), and many more (Habeebullah, Khalifa, Olwi 1995; Khalifa, Taha, Akurt 1985) on the subject. Some of the reviewers concentrated on the marketability, like why the solar cookers are not so popular, etc., and among such reviews that of GEDA (1979) and the more recent review of Wentzel (1966) are of importance. Recently the team led by Dr. Paul A Funk has formulated an International Standard for testing of Solart Cooker. I have included the details as per his directions, the details could be found if you click here

Evaluation of cookers

To start with, one has to concentrate on the performance evaluation and cost of the cookers. Many parameters have been considered and, at best, one can only summarize these findings.

VITA (1961) considered eight parameters to judge a cooker and they are : (I) cooking performance, (ii) durability, (iii) cost, (iv) weight, (v) portability, (vi) ease of operation, (vii) ease of manufacture, and lastly, (viii) adaptability to local skills and materials. After detailed reasoning they found none of the designs that existed came up to mark and were obliged to present three new designs. Walton et al. (1977) conducted another survey of the state-of-the-art; they built and tested seven types of cookers and concluded that none were up to the mark. Of the lot, they recommended three types, but the best design according to them was the reflector type.

Bowman (1978) presented another detailed review. He evaluated the cookers on the basis of 15 points (Table 1). They were:

Figures in the parentheses represent the weightage given to each parameter. Prof. Bowman admits that the parameters selected have a Haitian bias, and hence, are a bit arbitrary, so he feels that solar scientists should evolve a standard set of parameters for testing a cooker. ( Author feel that, care should be taken that an ordinary woodstove or a liquefied petroleum gas stove should not ‘fail’ when subjected to such a test, in other words, such conventional stoves should form the basis of comparison for solar stoves as well). Prof. Bowman tested about eight designs. Out of a total of 140 marks (Table 1) he allocates 91 for the Wisconcin parabolic cooker, 85 to Telkes Halacy, and 66 to the Brace Research Wisconcin parabolic cooker, 96 to Telkes, and 56 to the Brace Research Cooker. He evidently did not find a good design and was compelled to present the FIT concept like Type MP 3. For people who prefer the Telkes type of box cooker he modified it too and made it more efficient (Type BFF 1d) The new cooker with foam glass insulation could heat oil up to 200 C and boil 1 lit of water in 45 minutes. The author of the current review has made some more alterations to this design (Type BFF 1 e) but the tests did not show any improvement.

Lof (1963) did some analysis of solar cookers using computers. He has incorporated data of tests conducted by the Food and Agricultural Organization. In his compilation, he presents tests and specifications of about 10 solar cookers of which two were proposed but evidently not fabricated designs. The charts present the data but it is difficult to compare and select. But the scale tilts in favor of focusing the parabolic type cooker.

Next on the list is the very methodical survey conducted by GATE (1979). They have compiled data on 16 cookers. In future, all the cookers are to be tested as per the pattern laid down by them. Three broad categories under which they consider the cookers are: construction (under which 10 points are tested), efficiency (15 points), and handling and costs (5 points). Together 30 points are considered. Quantitative as well as qualitative values are allocated (Table 2)

Table 1. Evaluation of seven cookers

Name of cookers Wiscon-

Cin*

Solar

Chef

Skewer

Type

Collapsible

parabola

Telkes

Oven*

Telkes

Halacy’s

Brace

cooker

Code in this Review PRS1a MP 1a CP 2 PC 1 c BFF 1 BFM 1 IDT 1 a
Characteristics---------- Points
Time to boil water

Maximum temperature

Energy storage

Capacity

Versatility

Effectiveness

Ease of use

Ease of maintenance

Durability

Wind stability

Portability

Material cost

Cost of imported Items

Ease of manufacture

Transportability

Total score

10

10

10

10

10

5

10

10

5

10

10

10

10

10

10

140

10

10

0

5

5

5

7

2

2

7

10

4

8

7

9

91

5

3

1

2

5

1

5

5

4

8

10

8

9

5

4

75

0

0

0

0

0

0

10

5

3

0

10

10

10

9

10

67

0

0

0

1

0

0

2

0

0

0

10

9

10

4

10

56

4

9

10

10

6

5

10

7

5

7

10

6

9

5

3

106

4

8

3

10

6

5

7

4

1

1

10

7

9

5

5

85

0

2

0

3

0

0

10

3

2

10

10

8

7

5

6

66

Source. Bowman, Blatt (1978) (Style of the table has been altered slightly)

• Based on previously published results.

Table 2 is very illustrative and useful. The design adjudged as the best, or which got maximum marks was the Stam type which scores 67 marks, next was Von Oppen design with 66 marks. Sobaco got 48, Chinese asymmetrical scored 60, and so on. But some of the other promising designs, like that of FIT concept OF Bowman, do not find a place here, mainly because they were new. Interestingly, the Sobaco design gets low marks. The group also evaluates the cookers on the basis of regional characteristics, such as solar insulation, local requirements, and preferences (Table 3).

Their analysis recommends Von Oppen type cooker (Type PRD 1 in this review) for India (261 marks) whereas the box – type cooker (type BWM 1a) gets only 204 marks. For Kenya the best suited cooker would be the steam cooker (Type IDT 1d) with 291 marks. For China the best cooker would be Chinese type (Type IDT 1d) with 291 marks. For China the best cooker would be Chinese type (Type PRA 2). Not satisfied with any cooker, they fabricated a new type of parabolic cooker designated here as Type PRD 2.

In recommending this design the group lists the following advantages:

Selection of best design continued --

Table 2. Evaluation of solar cookers according to characteristics ( I Part )

Name of the cookers

Von oppen

Box type

Garg type

Four mirror

Code

PRD1

BWM1

BSM 1d

BFF1

Construction----------------------------------

Principle

Radiation

Dimension

Aperture (m3)

Focal length

Intensification factor

Reflector material

Weight (kg)

Stability

Portaility

*****

Efficiency

Effective power (w)

Degree of efficiency

Max. temperature (oC)

Cooking cap. (Kg)

Number of pots

Cooking hrs. (Hrs)

Keeping warm (hrs)

Roasting?

Baking?

Access to food ?

Stirring?

Repositioning?

How often?

Stray light

General handling

*****

Production

Complicated?

Duration (years)

Local materials

Costs (US $)

Power costs (w/US%)

Total

-

Reflector

Direct

0.9x0.9x0.6

0.7

0.1

-

Alu. Foil

4

Bad

Excellent

*

~

250

50

170

2.5

1

9-16

0

Yes

Fair

Complicated

Complicated

Simple

20

Incon.

Bad

*

~

No

5

Exclusively

6

40

-

-

-

4

-

-

-

-

4

1

4

*

~

2

4

3

2

1

3

0

4

3

2

2

3

2

1

1

*

~

4

2

4

4

4

66

-

Absorber

Global

0.6x0.6x0.2

0.1

2

-

Mirror

8

Excellent

Excellent

*

~

100

40

90

4

4

11-15

1

No

O

None

No

-

None

-

Bad

*

~

A little

5

Exclusively

44

2.3

-

-

-

4

-

-

-

-

3

4

4

*

~

0

3

0

3

4

1

2

0

0

0

0

2

2

4

1

*

~

3

2

4

2

0

54

-

Abs.+refl.

Direct

0.6x0.6x0.1

0.4

-

-

Mirror

20

Excellent

Good

*

~

160

50

140

4

4

10-16

1-5

No

No

None

No

Simple

30

None

Bad

*

~

A little

5

Exclusively

25

5.6

-

-

-

4

-

-

-

-

3

4

3

*

~

1

4

2

3

4

2

3

0

0

0

0

3

3

4

1

*

~

3

2

4

3

3

62

-

Abs. + refl.

Direct

0.8x0.8x1.2

0.5

-

8

Mirrors

31

Good

Good

*

~

170

45

250

3

2

10-16

2

Difficult

Difficult

None

No

Simple

30

None

Bad

*

~

Rather

5

Exclusively

44

3.9

-

-

-

4

-

-

-

-

2

3

3

*

~

1

3

4

2

2

2

4

-

0

0

0

3

3

4

1

*

~

1

2

4

2

1

61

Source. GATE (1979)

Note. The cookers delivering 370, 470, and 570 W of output are getting the same marks, while only the four mirror model is shown to develop 250oC. This may be due to the fact that in Sobaco and Chinese types the cooking vessels were exposed to wind; better results could be obtained it they are enclosed as suggested.

 

TABLE 2. ( II Part ) Evaluation of solar cookers according to characteristics

Name of the cookers Steam cooker Indoor steam Sobaco Chinese
Code used in this review IDT1 IDT4 PC 3 PRA 2
Construction----------------

Principle

Radiation

Dimension

Aperture (m3)

Focal length

Intensification factor

Reflector material

Weight (kg)

Stability

Portaility

*****

Efficiency

Effective power (w)

Degree of efficiency

Max. temperature (oC)

Cooking cap. (Kg)

Number of pots

Cooking hrs. (Hrs)

Keeping warm (hrs)

Roasting?

Baking?

Access to food ?

Stirring?

Repositioning?

How often?

Stray light

General handling

*****

Production

Complicated?

Duration (years)

Local materials

Costs (US $)

Power costs (w/US%)

Total

-

Reflector

Global

1.5x1x1.4

0.8

-

-

Cr-Ni-steel

60

Good

Very bad

*

~

150

21

100

4

2

9-17

1

No

No

Complicated

Complicated

Very simple

120

None

Good

*

~

Rather

7

Exclusively

62

2.4

-

-

-

2

-

-

-

-

0

3

0

*

~

1

1

1

3

2

4

2

-

0

1

1

4

4

4

3

*

~

1

2

4

1

0

48

-

Reflector

Global

3x1.5x1.5

3.6

-

-

Pol. Alu

-

Fixed

Excellent

*

~

470

14

100

6

2

10-16

1

No

No

Yes

Yes

-

-

None

Excellent

*

~

Rather

10

Exclusively

150

3.1

-

-

-

2

-

-

-

-

2

4

0

*

~

4

0

1

4

2

2

2

-

0

4

4

2

4

4

4

*

~

1

4

4

0

1

57

-

Reflector

Direct

2.5x1.8x2

1.7

-

-

-

70

Good

Very bad

*

~

370

28

180

5

2, special

8-16

0.5

Yes

Yes

Complicated

Difficult

Simple

10

Slight

Bad

*

~

Yes

10

Mostly

300

1.2

-

-

-

0

2

-

-

-

0

3

0

*

~

4

1

4

4

1

4

1

4

4

1

0

3

0

3

1

*

~

0

4

3

0

0

48

-

Reflector

Direct

1.8x1.6x1

-

-

-

-

100

Excellent

Very bad

*

~

560

36

180

5

2

9-17

0

Yes

No

Yes

Yes

Complicated

15

Incon.

Good

*

~

Rather

10

Exclusively

70

8

-

-

-

3

-

-

-

-

0

4

0

*

~

4

2

4

4

2

4

0

4

0

4

4

1

1

1

3

*

~

1

4

4

0

3

60

Source. GATE (1979)

Note. The cookers delivering 370, 470, and 570 W of output are getting the same marks, while only the four mirror model is shown to develop 250oC. This may be due to the fact that in Sobaco and Chinese types the cooking vessels were exposed to wind; better results could be obtained it they are enclosed as suggested.

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

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Selection of best design Continued -- ( Last Part )

Table 3. Evaluation of solar cookers according to regional requirements

Name Code Used here in this review INDIA China Kenya Sahel Average Sum From Table 2
Von oppen

Box type B

Garg type

Telkes type

Stam cooker

Steam cooker

Sobaco

Chinese

Vita Fresnel

FRD 1

BWM 1a

BSM 1d

BFF 1

IDT 3

IDT 4

PC 3

PRA 2

F1

261

204

231

239

179

226

200

237

191

234

175

207

223

179

236

239

268

165

250

204

250

209

212

291

195

264

165

258

149

195

201

174

260

235

315

172

251

183

121

218

186

253

217

271

176

66

54

62

61

48

57

48

60

--

Source. GATE (1979)

There are many more reviews and surveys such as that of the UN, Olwi et al. (1994), Habeebullah, Khalifa, Olwi (1995), Khalifa, Taha, Akyurt (1985), and Wentzel (1966). The list is rather extensive and an attempt is made to cover the most important, if not, all of them, in this review. Recently from Brigham Young University, USA, Prof. Stevens has conducted test along with his colleague McMillen on specially Funnel type of Cookers. The details are available on SCI. They have tested Solar Funnel Cookers and have suggested some improvements such as using Wire Baskets etc.

Dr Phillips Fairers et. al of Florida Solar Energy Center, (FSEC) have excellent testing facilities, and they have tested mostly COOKIT type of Panel Cookers. They state that High Back Funnel Cooker perform well

Apart from taking several parameters into consideration it is essential to get the opinion of the housewives if the solar cookers are to become popular.In this regard Mr Ravindra Pardesi of India, an inventor, is working with his wife Parvati, and have designed at least two new types of Cookers. A 12 sided double reflector type and Double funnel type. Both of these are referred to under solar 7 of this site.

Recently Paul A Funk of USA along with other scientists has laid out a detailed procedure for testing Solar Cookers. This was corollary to the 3rd International conference on Solar Cookers held at Avinashlingam University of Home Science at Coimbatur of India. The paper has been reproduced in SCIs Solar Cooker Review. But I wish that Professor and his team had tested a couple of cookers and reported the results.

To see Dr Paul A Funk et.al., formulations click here

Some recommendations

It was the desire of the author to build and test all the important models, especially the newer ones so as to select at least three or four good designs. When the choice is available, the selection process becomes easy. As regards the box-type cookers, the general contention is that if they are manufactured on a large scale they would be cheaper, but then that would mean manufacturing several other related components such as steel sheets, etc. and although it could generate several jobs, cost of cookers may not be reduced, and subsidies would not support them for long. The author strongly recommends do-it-yourself designs for villagers.

Before that, there is a need to standardize the materials. One can not suggest ordinary Aluminium foil or aluminized polyester sheets for reflectors. They lose their shine very soon. It is felt that the most likely candidate for reflectors is polished stainless steel, but user data for this is not available. For example, how long will the shine last? When the shape is lost, then can the sheet be polished locally ? If yes, then can it be done by the housewife or at least by a traditional knife sharpener with his slightly modified leg operated grinder ?

It is of common knowledge that even good glass mirrors do not last long in open sunshine (Buckwalter, McVay 1980; Howe 1981). In fact, there exists a long list of materials which need testing under the sun, that is sun worthiness. Which glass mirror is to be recommended ?

As regards glazing, the author had recommended ‘UV stablized’ transparent polyester materials. But they did not last for more than three months (Kundapur 1980). One cannot insist on tempered glass for the box-type cookers as it is very costly, low iron glass is not easily available, and 4 mm glass would be costly too. Which is the best alternative ? What if one uses ordinary 3 mm glass? What will be the difference in performance between ordinary and iron-free glass ? Data is not available easily. A small difference will not matter, and the author has used only 3 mm glass, and got satisfactory results.

Similarly, glass wool or mineral wool insulation cannot be recommended for the villagers, the material is hazardous. One has to standardize and suggest composites made from agricultural wastes like rice husk or straw or any other locally available material. Insulating materials like foam glass is a good alternative (with a k of 0.036 and a weight of 137 kg/m3, it is as good alternative (with a k of 0.036 and a weight of 137 kg/m3, it is as good as mineral wool) and for urban cookers too one can definitely recommend the same. This could make the cookers cost less. Further, the problem of broken glass could also be solved to a certain extent as broken glass could be used in the manufacture of foam glass.

Absorptive coating calls for special attention. Is there anything better, safer, and cheaper than common blackboard paint ? What about the special absorptive stickers, especially for the cooking vessels, or certain special treatments to darken the surface ? This is the area which requires serious attention and research. Only when the scientists have definite answers for aspects like insulation, absorptive coating, etc. can they think of a massive popularization, program. This, by itself, will boost the use of solar cookers.

Conclusions

Solar cookers are here to stay, or to put it in the other way, if we have to preserve our planet then each and every one of us has to use one or the other type of solar gadget, be it a cooker a solar water heater or solar photovoltaic panel.

Dr Paul A Funk et.al., have formulated another set of parameters for testing solar cookers the details could be found if you click here


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S4 - Parabolic Concentrators     S5-Parabola & 0thers   S6 - light from above
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