Mar 29, 1982 - The space between the two shells is filled with fibre glass insulation. One double walled door is also provided for keeping and taking otit food.
Solar Energy, Vol. 31, No. 2, pp. 235-237, 1983 Printed in Great Britain.
0038-092X1831080235~)3503.0010 Pergamon Press Ltd.
TECHNICAL NOTE Optimisation factor of solar ovens KULBiR SINGH MALHOTRA, N. M. NAHAR and B. V. RAMANA RAO Central Arid Zone Research Institute, Jodhpur-342003, India
(Received 29 March 1982; revision accepted 15 July 1982) INTRODUCTION The performance of a hot box solar oven depends upon the area of energy collecting window, concentration ratio of reflecting assembly, insulation between outer shell and hot box (cooking chamber) and volume of the hot box. The volume of the solar oven cooking chamber is decided by area which is generally square in shape[I--4]. Malhotra and Nahar[5] reduced the cooking chamber volume of the solar oven developed earlier by Garg[6] and got considerable improvement in its performance. As such four solar ovens with almost identical design but with cooking chambers of different volumes were fabricated and their performance studied.
the cooking chamber and of the cooking chamber volume of different ovens. It is observed that there is a steep rise in temperature with reduction in volume. Oven No. 3 is taken as the one having optimised volume because in oven No. 4 though the temperature is slightly higher, yet the space in the chamber is insufficient to keep a cooking utensil. PERFORMANCE OF OPTIMISED OVEN This optimised solar oven was taken to different villages of Jodhpur and Bikaner districts in the month of October/November where cooking trials were conducted in the presence of villagers. Some of these cooking trials are shown in Table 2. It is seen that cooking is faster during day time but is slow in morning and evening hours. The maximum time taken for cooking 400 g of rice was observed to be 65 rain when cooking started at 8 a.m. and 4 p.m., while only 40 minutes when cooking was started at 12 noon.
DESIGN DETAILS The detailed specifications of the four solar ovens fabricated for comparative study are given in Table 1. These are double walled semi cylindrical boxes made from galvanised iron sheets. The space between the two shells is filled with fibre glass insulation. One double walled door is also provided for keeping and taking otit food. The flat portion of the semicylinder acts like a window which is exposed to sunlight for receiving direct radiation. Two transparent glass sheets of size 40 x 40 x 0.3 cm, separated by 2.5 cm, are fitted over the window. Eight silvered glass reflectors, four each in triangular and square shape, are attached to the four sides of the window in such a way that the reflection factor is increased to 3.6. A mild steel angle iron stand with 4 castor wheels is provided to facilitate azimuthal tracking. A facility for elevation tracking is also provided. Figure 1 shows basic difference between the four oven designs. Figure 2 shows the relationship between the highest temperature of the ak inside
EFFICIENCY The efficiency of all four solar ovens was estimated by heating a known quantity of water in a vessel and is reported in Table I. There was an increase in the efficiency from 35.5 to 41.2 per cent in case of solar oven No. 3 when a rubber gasket was provided at the boundary of the door to stop leakage of hot air. CONCLUSION The optimisation factor (Fo) of the optimised cooker was calculated as follows: Volume of chamber
Fo = Area of window = 13.75 cm.
Table 1. Detailed specifications of four semi-cylindrical type solar ovens
8o!a~ Oven number 2 ~
I
4
I n s u l a t i o n (f~bre ~ a o 8 ) between oute~ and xnnel. shell (oN)
7.5
7.5
"/.5
7.5
Oonoent ration ratio
~. 6
3.6
3 •6
3.6
Area of engr&7 oolleotlng window (orez)
40x40
40z40
40x40
40x40
~blule of e h ~ b e r ( ~ )
~ ,150
231~00
21,980
20,750
~ f f 2 ~ i e n o y witho'~l~ r u b b e ~ gasket at ~he dooT (%)
~9.0
37.5
~5.5
~2.0
Bfflolen~ ~d.th r~tbbe~ gasket st t h e door (~)
-
-
41.2
160.0
168.5
178.0
MaxLmum 8ta~aant temperature o f t h e a i r in oook~ng e m ~ l ~ r (*0)
235
179.0
236
Technical Note
N
o
._=
e4
I 0 ill)
I ~ I~
I
I 0 I~
i
Oo H I
1
I
I
!
i
0 0 ~
3~INIV~I3dlN3£
I
T
o
0 N I
1
1 E ~3
c~
237
Technical Note Table 2. Cooking trials on optimised solar oven conducted in the month of October/November i
lWood
Quan~ty
Rio.
Time (in ~ n u t e s ) taken for oookin~ different foods when oooking was star~ed at 8 Jl i0 A~I 12 noon 2 PM 4 PM
400 ~ i n 700 65 al of water (0.15)
55 (0.54)
40 (0.73)
45 (0.65)
65 (0.~5)
RAoo b a t h w i t h vegetables
750 ~
~
7o
45
60
B r o k e n g~een gr~ in water
250 g
90
70
65
65
-
Po taro m a s t i n g
i ~
65
60
55
55
65
S w N t pot a~o ~,~ast~.ng
1 k~
60
60
40
45
45
Te~
5 oup.
50
50
25
~0
50
l~ures
~n b~aoke~e i n d i o a t e
mean tmurl¥ ~lobal s o l a r
Thus for the best performance of the hot box solar oven, the optimisation factor (or Manu Faclor) of solar oven should be nearly 14 cm provided the concentration ratio of the reflecting assembly is 3.6.
Acknowledgements--The authors are thankful to Dr. H. S. Mann, Director Central Arid Zone Research Institute, Jodhpur for his valuable encouragement in conducting the above study.
radiation
-
i n kWh/m2
REFERENCES 1. M. Telkes, Solar Stove. Trans. Con[. on the use o~ Solar Energy, Tucson, Arizona, 3(2), 87 (1955). 2. M. Telkes, Solar cooking ovens. Solar Energy 3, 1 (1959). 3. M. M. Baker, Solar oven and cooker. U.S. Patent 681,095 (1901). 4. H. A. Cherrier, Solar cooker, U.S. Patent 1, 158, 175 (1915). 5. K. S. Malhotra and N. M.Nahar, Appropriate technology for cooking in arid areas. Urja 9, 232 (1981). 6. H. P. Garg, A solar oven for cooking. Indian Farming 27, 7 (1976).
