- Email: [email protected]
Performance studies of a large size nontracking solar cooker
Rem'~*able Enerqy Vol. 2, No. 4,5 pp. 4 2 1 4 3 0 Printed in Great Britain.
1992
0960-1481/92 $5.00+.00 Pergamon Press Ltd
PERFORMANCE STUDIES OF A LARGE SIZE NONTRACKING SOLAR COOKER N. M. NAHAR Central Arid Zone Research Institute, Jodhpur 342 003, India (Received 25 September 1991 ; accepted 28 November 1991) Abstraet--A large size nontracking solar cooker has been designed, fabricated and tested. The cooker is based on the hot box principle. The cooker has been tested extensively and its performance has been compared with a solar oven, a hot box solar cooker and a solar cooker (tilted absorber). The stagnation temperatures are in increasing order for the hot box solar cooker, the solar cooker (tilted absorber), the large size nontracking solar cooker and the solar oven. The performance of this solar cooker is comparable with that of a solar oven. The former is not tracked towards sun while the latter is tracked every 30 min. The efficiency of a large size nontracking solar cooker is 24.9%. The energy saved by this new solar cooker has also been calculated and its payback period has been computed by considering interest, maintenance and inflation in fuel prices and maintenance cost. The payback periods are 1.1(~3.63 years depending on which fuel it replaces. Relatively short payback periods show that the use of the cooker is economical, and it is easy to operate since no tracking is required.
1. INTRODUCTION Cooking accounts for the major share of energy consumption in developing countries. Fifty per cent of the total energy consumed in India is for cooking [1]. Most of the cooking requirement is met by noncommercial fuels such as firewood, agricultural waste and cowdung cake. The total consumption of firewood, cowdung cake and agricultural waste are 150, 75 and 50 million tonnes per year in India [2]. However, forest reserves are rapidly being depleted. By the end of this century it has been predicted that there will be a great shortage of firewood if a suitable alternative is not provided to the rural people. Fortunately, India is blessed with a b u n d a n t solar radiation [3]. The arid parts of India receive maximum radiation, i.e. 7600-8000 MJ m 2 per annum, followed by semi arid parts, 7200-7600 MJ m 2, per a n n u m and least on hilly areas where solar radiation is still appreciable, i.e. 6000 MJ m 2 per annum. Therefore, solar cookers seem to be a good substitute for cooking with firewood. There are two broad categories of solar cookers developed so far, i.e. reflector [4-9] and hot box [1015] types. The reflector type solar cooker was developed in the early 1950s [4] and it was manufactured on a large scale in India [16] but it did not become popular due to its inherent defects. It requires 10 rain tracking. Cooking can be done only in the middle of the day, in the direct sun. Its performance is affected by dust and wind which are serious limita42I
tions in arid conditions, for instance, at Jodhpur, India, there is a danger of burning. One has to stand near to the cooker for cooking, and its design is complicated. The reflecting surface deteriorates with time and hence cooking efficiency is reduced. Therefore, the hot box type solar cooker has been suggested by Garg et al. [17]. Although the solar oven [11-14] performs well, it also requires 30 min tracking, is too bulky and its cost is very high. Therefore, the simple hot box type solar cooker [18] with a single reflector is being promoted by the Indian government. The performance of the hot box solar cooker is very good during summer but it is very poor during winter in northern parts of India. It is difficult to cook two meals per day during winter because its glass window and absorbing surface are horizontal and so receive much less radiation compared to optimally inclined surfaces [19]. Solar radiation received at Jodhpur on horizontal surfaces and optimally inclined surface is shown in Table 1. From Table 1, it is clear that solar radiation received on an inclined surface is 69.37% more than a horizontal surface during the month of December and an inclined surface receives 43.8% and 22.76% more radiation than a horizontal surface during the winter season (October-March) and round the year, respectively. Considering this, a new solar cooker with tilted absorber (TA) [20] has been designed, developed and tested. Although the performance of the solar cooker (TA) is better than the hot box solar cooker and comparable with the solar even, it can only be used
422
N. M. NAHAR
Table 1. Mean daily solar radiation (MJ m- 2) on horizontal and optimally inclined surfaces Solar radiation (MJ m 2) Month
Horizontal surface
Inclined surface
Increase over horizontal surface (%)
January February March April May June July August September October November December
16.6 19.6 23.0 25.5 26.6 24.9 21.1 19.5 21.5 20.5 17.3 15.6
26.10 27.65 26.16 26.02 26.60 24.90 21.10 19.67 23.09 26.05 26.70 26.42
57.23 41.07 13.74 2.04 0.00 0.00 0.00 0.87 7.39 27.07 54.33 69.36
Mean
21.0
25.04
22.76
Sources : Refs 3 and 19. for cooking for a maximum of five people, while in rural India family size is large and people live in clusters. The solar cooker's (TA) mirror has a width to length ratio of unity, but to avoid edge effects [21-23] during off peak hours, the ratio should be greater than three. We have considered this and increased the capacity of the cooker so that it can be used for cooking 10 kg of food per day rather than the 2 kg by the solar cooker described above. This large size cooker can meet the cooking requirements of 25 people. In this paper the design and performance of this large size solar cooker is described and compared with the solar oven, hot box solar cooker and the solar cooker (TA). 2. DESIGN
2.1. Large size solar cooker The solar cooker (Fig. 1) is based on the hot box principle with dimensions of 2070 mm x 570 mm × 230 mm. The inner tray is made of aluminium sheet and the outer is of mild steel. The space between them is filled with fibreglass insulation. Two clear glass covers are provided and the inner tray is painted with blackboard paint. Four doors have been provided on the rear side for loading and unloading of the cooker. One adjustable reflector, with a width to length ratio of four to avoid edge effects, is fixed over it to boost solar radiation on the glass window. The cooker is fixed on an angle iron stand. It is designed so that its inclination should be changed once a fortnight. It is kept facing the equator. Eight specially designed
cooking utensils (Fig. 2) can be kept inside for boiling various dishes. Photographs of the device are shown in Fig. 3. 2.2. Solar oven The solar oven (Fig. 4) has a trapezoidal reflecting assembly, a cylindrical cooking chamber and an angle iron stand which has both azimuthal as well as elevation tracking. The concentration ratio of the cooker is 3.5. The detailed design of this cooker is described elsewhere [14]. 2.3. Hot box solar cooker It is a simple hot box (Fig. 5) with one reflector and hot box chamber for cooking. It has only azimuthal tracking. Its detailed design has been reported elsewhere [14]. 2.4. Solar cooker (TA) This solar cooker (Fig. 6) is also a hot box solar cooker but it has two adjustable reflectors, one at the top and another at the bottom so that the angle of the booster with the glass window can be changed as required and more radiation can be obtained on the glass window and cooking efficiency can be improved. The detailed design and performance has been described elsewhere [20]. 3. PERFORMANCE AND TESTING
The solar cooker was tested by measuring the stagnation temperature of air inside the cooking chamber in the large size solar cooker and comparing this with the solar oven, the hot box solar cooker and the solar cooker (TA) with different combinations of tracking for the solar oven, hot box solar cooker and solar cooker (TA). The large size nontracking solar cooker was always kept fixed, but its tilt was changed once a fortnight. Average diurnal variations in stagnation temperatures of different solar cookers are shown in Table 2. The solar oven, hot box solar cooker and solar cooker (TA) were tracked while the large size nontracking solar cooker was kept fixed. The stagnation temperatures are in increasing order; hot box, solar cooker (TA), large size nontracking solar cooker and solar oven. When the solar oven and the hot box were tracked but the solar cooker (TA) and the large size nontracking solar cooker were kept fixed the stagnation temperatures were measured as shown in Table 3. From Table 3 it is clear that stagnation temperatures are in increasing order; hot box, solar cooker (TA), large size nontracking solar cooker and solar oven. Similarly all cookers were kept fixed except the solar oven was tracked. The average recorded
423
Large size nontracking cooker
WOODEN FRAME
OPENABLE D O O R / ~
~"
L L,L--
~ ~...4~2 ~
2070
El
¢1
__
U
PLAIN GLASS pLAN -~ FIBRE GLASS INSULATIO~N o
CROSS SECTION AT A-A DOOR
ISOMETRIC VIEW DESIGN OF LARGE SIZE
ALL DIMENSIONS ARE IN m.m. NONTRACKING SOLAR COOKER
Fig. 1. Design of large size nontracking solar cooker.
stagnation temperatures for this case are shown in Table 4. From Table 4 it is seen that stagnation temperatures are increasing order for hot box, solar cooker (TA), large size nontracking cooker and solar oven. Similarly, stagnation temperature inside cooking chamber were also measured with a load. One kilogramme of water in four utensils was kept in the solar oven, hot box solar cooker and 4 kg of water in eight utensils were kept in the large size nontracking solar cooker. The diurnal variations in stagnation temperatures without load and with load are shown in Figs 7 and 8, respectively. Figures 7(a) and 8(a) depict stagnation temperatures for the solar oven and the hot box solar cooker, tracked, while in Fig. 7(b) and Fig. 8(b), only the solar oven was tracked, with the hot box and large size nontracking solar cooker kept fixed. From Tables 2-4 and Figs 7 and 8, it is clear that the performance of the large size nontracking solar
cooker is better than the hot box solar cooker and solar cooker (TA), and comparable with the solar oven. Its performance is better than the hot box solar cooker because it has a tilted absorber while the hot box has horizontal absorber so that more radiation can be obtained. Its performance is better than the solar cooker (TA) because the width to length ratio of the booster is one in the solar cooker (TA) while it is four in the large size nontracking solar cooker so the edge effect has been minimized. The efficiency of the nontracking large size solar cooker has also been obtained by measuring the rise in temperature of a known quantity of water in a specified time by the following relation :
muC2(te~2-tvO+mw(tw2-twO Ac
(1)
HdO
The efficiency of the large size nontracking solar
N. M. NAHAR
424
LID
J
J
190 ALUMINIUM SHEET BOX
ALL DIMENSIONS IN m.m.
COOKING UTENSIL FOR SOLAR COOKER(TA) Fig. 2. Design of cooking utensils for large size nontracking solar cooker.
cooker is found to be 24.9%. Cooking trials in this cooker have also been conducted and are shown in Table 5. All types of boiling, baking and roasting operations can be performed and the time taken for various dishes is between 75 and 210 min. Cooking time is generally a complex function of the amount and types of dishes, season and time of the day during which cooking is carried out. 4. ENERGY SAVING AND PAYBACK PERIOD
Based on duration of cooking time, it has been assumed that the large size nontracking solar cooker will cook two meals per day if the duration of bright sunshine hours is more than 7 h per day and only one meal if the duration of bright sunshine hours is more than 5 h but less than 7 h per day. By analysing the duration of bright sunshine hours at Jodhpur during 1980 to 1987 it has been found that this solar cooker will cook two meals for about 310 days and one meal only for about 22 days. The energy required for cooking per person per day is about 900 kJ of fuel equivalent per meal. The cooker is sufficient for cooking for about 25 people and it saves about 50% of cooking fuel when it is used since
deep frying, baking of Chapati (local bread) and breakfast are not possible, and hence other fuels must be used. Therefore, the cooker will save 11.25 MJ of fuel equivalent per meal. Accordingly it will save 7222.5 MJ of fuel equivalent per year. The payback periods of the large size nontracking solar cooker have been computed by considering equivalent savings in alternative fuels, i.e. firewood, coal, kerosene, LPG and electricity. Payback periods have been calculated by considering interest, maintenance and inflation in fuel and maintenance cost. The cost, calorific value and efficiencies [24, 25] of different fuels are shown in Table 6. From Table 6 it is clear that firewood is the costliest fuel which is used by rural people. Therefore, to conserve firewood and combat desertification solar cooking should be encouraged. The payback periods [26, 27] have been obtained from the following :
(E--M) n =
E-M
(l+a)
(2)
log (1 + b) The cost of the solar cooker is about Rs. 3000.00
Large size nontracking cooker
425
Fig. 3. Field installation of large size nontracking solar cooker.
(1 US$ = 25.9 Rs.), i.e. about US $116.00. The payback period has been calculated with respect to the different fuels with the following annual costs : interest rate maintenance
a = 10% M = 5% of total cost of solar cooker
inflation in fuel prices and maintenance cost
The payback period of the solar cooker has been calculated from eqn (2) with respect to different fuels. The payback periods are 1.10, 1.36, 2.20, 2.82 and 3.63 years with respect to firewood, coal, electricity,
b = 5%.
f / , - ' ~ .JJ...-'TJ
M.8. S H E E T
~,~.
'
•
FRAME
MIRROR
~-
o.~ ;S
,,
WOODEN
~..,
CASTOR
Fig. 4. Schematic of solar oven.
WHEEL
~t~ ARE t..~m.
Fig. 5. Schematic of hot box solar cooker.
N. M. NAHAR
426
~ . REFLECTOI~
j
/S/~
L P G and kerosene, respectively. The payback period is shortest with firewood because its efficiency is very poor while the payback period are longer for kerosene because kerosene is highly subsidized by the Government of India. 5. CONCLUSION
0
The performance of the large size nontracking solar cooker is always better than the hot box solar cooker and the solar cooker (TA) whether the hot box and solar cooker (TA) are tracked or kept fixed. Simultaneously its performance is almost comparable with the solar oven. The cooking time depends upon season, type of dish and time of loading. The efficiency
0 ~LL DIMF'NSIONS ,ARE IN rn.m.
Fig. 6. Schematic of solar cooker with tilted absorber.
Table 2. Average stagnation air temperature inside cooking chamber of different models of solar cookers (July-December 1987) (all cookers tracked but large size nontracking kept fixed) Stagnation temperature
(of)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1I00 1200 1300 1400 1500 1600
74.6 112.8 133.6 144.0 147.3 141.4 131.8 115.1
54.0 70.6 92.6 110.5 119.4 111.2 99.4 80.3
59.2 88.6 115.7 124.3 132.5 123.8 117.1 89.7
Solar radiation (MJ m 2)
Large size nontracking solar cooker
Ambient air (°C)
Horizontal surface
Inclined surface
76.6 102.5 130.3 140.0 145.8 137.2 121.4 96.7
25.2 26.8 28.9 30.6 30.9 30.9 32.3 31.7
0.850 1.512 2.021 2.323 2.415 2.293 1.731 1.431
1.275 2.293 3.042 3.474 3.578 3.281 2.727 1.891
Table 3. Average stagnation air temperature inside cooking chamber of different models of solar cookers (solar oven and hot box tracked, solar cooker (TA) and large size nontracking kept fixed) Stagnation temperature
(of)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1100 1200 1300 1400 1500 1600
75.4 111.0 138.8 146.5 151.6 146.1 143.6 122.5
52.4 83.0 108.3 117.0 124.2 117.5 112.0 97.8
57.1 83.9 118.0 123.4 133.6 127.7 110.8 89.9
Solar radiation (MJ m-2)
Large size nontracking solar cooker
Ambient air (°C)
Horizontal surface
Inclined surface
75.8 103.5 133.8 141.1 145.1 138.8 125.3 105.1
27.2 29.6 31.5 33.2 34.2 35.2 35.7 34.6
1.220 1.687 2.328 2.650 2.708 2.580 2.314 1.876
1.361 2.336 3.149 3.586 3.687 3.520 3.000 2.437
427
Large size nontracking cooker Table 4. Average stagnation air temperature inside cooking chamber of different models of solar cookers (September-December 1987) (solar oven tracked and other cookers kept fixed) Stagnation temperature
(°C)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1100 1200 1300 1400 1500 1600
78.2 111.8 137.8 142.8 147.6 142.8 138.0 118.0
68.2 88.3 105.6 115.1 120.6 118.6 111.7 95.4
67.2 98.1 125.0 136.0 139.8 129.7 125.7 112.0
Solar radiation (MJ m 2)
Large size nontracking solar cooker
Ambient air CC)
Horizontal surface
Inclined surface
74.7 102.3 129.1 142.0 147.3 136.4 122.3 101.1
24.5 26.8 28.0 29.9 31.8 32.9 33.0 32.6
1.295 1.826 2.304 2.654 2.711 2.560 2.126 1.769
1.341 2.344 3.104 3.576 3.718 3.384 2.956 2.402
Table 5. Cooking trials in large size nontracking solar cooker Type and quantity of material (food + water) (g)
Time of loading Date
(h)
19 October 1987
1000
19 October 1987
1400
20 October 1987
1200
21 October 1987
1300
27 October 1987
1000
28 October 1987
1400
2 November 1987
1100
18 November 1987
1000
19 November 1987 20 November 1987 23 November 1987
1030 1000 1000
Rice (800 + 1600) Lentil (800 + 2000) Rice (800 + 1600) Lentil (800 + 1000) Rice (800 + 1600) Lentil (800 + 1600) Rice (800 + 1600) Lentil (800+ 1600) Rice (800 + 1600) Kidney bean (400 + 400) White gran (400 + 400) Pigenopea split (400 + 600) Potato (1000 + 200) Guar bean (1000+600) Rice (800 + 1600) Lentil (400 + 1200) Potato (1000+250) Rice (800+ 1600) Lentil (400 + 1100) Sweet potato (1000) Sweet potato boiling (1000 + 150) Pearl millet Pigenopea split Mixed (bajara Kheech) (400 + 1000) Bajara Kheech (400 + 1600) Pearl millet split (400 + 1600) Pearl millet flour batir (500)
Solar radiation (MJ m 2)
Duration of cooking (min)
Optimum tilt
Horizontal surface
120 180 120 165 90 120 75 120 98 200 200 210 90 90 90 150 90 90 90 90 90 150
6.95 10.73 5.08 5.99 5.65 7.43 4.32 6.55 4.69 8.94 8.94 9.14 4.69 4.69 4.63 5.93 4.63 5.45 5.45 5.45 5.45 8.68
4.80 7.47 3.74 4.41 4.00 5.25 3.06 4.64 3.19 5.84 5.84 5.98 3.19 3.19 2.71 3.95 2.71 3.65 3.65 3.65 3.65 5.4 t
160 180 90
8.45 10.00 5.00
5.14 6.36 2.60
Cooking material soaked in water over night. of the large size n o n t r a c k i n g solar cooker is 24.9%. The p a y b a c k period varies between 1.10 a n d 3.63 years depending u p o n the fuel it replaces. The payback periods are in increasing order with respect to
the fuels, firewood, coal, electricity L P G and kerosene. The use of the large size n o n t r a c k i n g solar cooker will conserve conventional fuels which will help c o m b a t desertification.
428
N.M.
NAHAR
(o) --
(b)
-" S O L A R
•--*
AMBIENT
~ ' - - - - ~ HOT
a-----t
LARGE SIZE
o~o
SOLAR RADIATION INCLINED SURFACE
o---<~
S O L A R RADIATION H O R I Z O N T A L SURFACE
OVEN AIR
BOX
IL
'
E ~ x
~100 :i
-,
X
4.O~ z o IQ
z z
2.O I: A<
/
@
I I0
l
I 12
TIME
I
I 14
I
I IG
I I0
(Hrs)
I
I 12
TIME
I
l 141.
i
I 16
(Hrl)
Fig. 7. Diurnal variations in stagnation temperature (without load) of different solar cookers. (a) Solar oven and hot box tracked, large size solar cooker kept fixed. (b) Solar oven tracked but hot box and large size solar cooker kept fixed.
(o)
(b)
e--e
SOLAR
----
AMBIENT
z
x HOT
OVEN AIR
BOX
I--A
LARGE
o--o
S O L A R RADIATION INCLINED SURFACE S O L A R RADIATIONNORIZONTAL SURFACE
o----o
150
SIZE
ag N
4-O~
~:1oo E
zo b-
hi )..
_[ < z
¢[ .OJF' a*
5G l /
2
IZ
.J 0 U)
m .I
,o
I.
,~
m
,
,,
TIME IHr8 I
t
,6
,;
,'~ TIME
'
,;,
,'~
(Hr8)
Fig. 8. Diurnal variations in stagnation temperatures (with load) of different solar cookers (a) same as 7, (b) same as 7.
Large size nontracking cooker
429
Table 6. Calorific value, efficiencies and cost of different fuels Cost of fuel per MJ of energy
S. No.
Type of fuel
Calorific value (MJ kg i)
Efficiency (%)
Cost
(Rs.)
1 2 3 4 5
Firewood Coal Kerosene Electricity L.P.G.
19.89 27.21 45.55 3.6 kW h ~ 45.59
17.3 28.0 48.0 76 60.0
1.50 kg ' 2.75 kg t 2.70 kg 0.64 kWh 5.20 kg i
0.23 0.23 0.15 0.20 0.16
L
1.0 US$ = 25.9 Rs. Sources : (Refs 26 and 27).
Acknowledgements--The author is grateful to Dr J. Venkateswarlu, Director, Central Arid Zone Research Institute, Jodhpur and Dr P. C. Pande, Head, Division of Wind Power and Solar Energy Utilisation, for providing the necessary facilities and constant encouragement for the present study. NOMENCLATURE a compound interest, per cent yr Ac aperture area of solar cooker, m 2 h inflation in fuel prices and maintenance cost, per cent yr C cost of solar cooker, Rs Cp specific heat of material of cooking utensil E energy saving, Rs yr i H solar radiation on glass plane, MJ m 2 h m, mass of cooking utensils, kg mw mass of water in cooking utensils, kg M maintenance cost, Rs yr n payback period, yr Q period of test t,,~ initial temperature of cooking utensils, °C tu2 final temperature of cooking utensils, 'C tw~ initial temperature of water in cooking utensils, 'C
tw2 final temperature of water in cooking utensils, 'C q efficiency of solar cooker REFERENCES 1. M. Fritz, Future Energy Consumption. Pergamon Press, New York (1981). 2. A. C. Pandya, Energy Jbr agricultural in India. Central Institute of Agricultural Engineering, Bhopal, India Tech. Bull. No. CIAE/81/20 (1981). 3. IMD, Solar Radiation Atlas of India. India Meteorological Department, New Delhi, India (1985). 4. M. L. Ghai, Design of reflector typedirect solar cookers. J. of Sci. & Indu. Res. 12A, 165-175 (1953). 5. J. A. Duffle, Reflector solar cooker design. Trans. Conj~ on Use q[' Solar Energy. University of Arizona, Tucson 3(II), 79-86 (1955). 6. G. O. G. L6fand D. Fester, Design and performance of a folding umbrella type solar cooker. Proc. UN Conf. on New Sources of Energy Rome, Paper 5/100, 5, 347 352 (1961).
7. H. Tabor, A solar cooker for developing countries. Solar Energy 10, 153 157 (1966). 8. U. Ya. Umarov, A. K. Alimav, D. N. Aavutdinev, A. Sh. Khodzhaev and A. H. Gaturav, Multifacet solar power kitchen. Applied Solar Energy (Geliotekhnika) 12, 61 -62 (1976). 9. M. Von Oppen, The sun basket. Appropriate Technology 4, 8 10 (1977). 10. M. K. Ghosh, Utilisation of solar energy. Science Culture 22, 304-312 (1956). 11. M. Telkes, Solar cooking ovens. Solar Energy 3, 1 II (1959). 12. H. P. Garg, A solar oven for cooking. Indian Farming 27, 7 9 (1976). 13. K. S. Malhotra, N. M. Nahar and B. V. Ramana Rao, Optimization factor of solar ovens. Solar Energy 31, 235 237 (1983). 14. N. M. Nahar, Performance studies on different models of solar cookers in arid zone conditions of India. Proc. 7th Miami International Conf. on Alternative Energy Sources. Hemisphere, Washington, D.C. 1431 1439 (1986). 15. I. A. Olwi and A. M. Khalifa, Computer simulation of solar pressure cooker, Solar Energy 40, 259 268 (1988). 16. M. L. Ghai, B. S. Pandher and Harikishan Dass, Manufacture of reflector type direct solar cooker. J. Sci. lndi. Sci. Res. 12A, 212 216 (1953). 17. H. P. Garg, H. S. Mann and K. P. Thanvi, Performances evaluation of five solar cookers. Proc. ISES Congress, New Delhi (Edited by F. de Winter and M. Cox) SUN 2. pp. 1491 1496. Pergamon Press. New York (1978). 18. M. Parikh and R. Parikh, Design of a flat-plate solar cooker for rural applications. Proc. National Solar Energy Convention of India, Bhavnagar, Central Salts and Marine Chemical Research Institute, Bhavnagar, India, 257 261 (1978). 19. IMD, Solar radiation on inclined surfaces. India Meteorological Department. Pune, India (1980). 20. N. M. Nahar, Performance and testing of an improved hot box solar cooker. Energy Convers. Manag. 30, 9 16 (1990). 21. D. K. McDaniels, D. H. Lavndes, H. Mathew, J. Reynolds and R. Gray, Enhanced solar energy collection using reflector solar thermal collector combination. Solar Energy 17, 277 283 (1975). 22. S. C. Seital, Collector performance enhancement with fiat reflectors. Solar Enerqy 17, 291-295 (1975).
430
N. M. NAHAR
23. S. L. Grassie and N. R. Sheridan, The use of planar reflectors for increasing the energy yield of flat plate collectors. Solar Energy 19, 663~68 (1977). 24. NCAER, Domestic Fuels in India. National Council of Applied Economic Research, New Delhi. Asia Publishing House, Bombay (1959). 25. DNES, Solar Hot Water Systems. Department of Non-
conventional Energy Sources, Ministry of Energy, New Delhi (1985). 26. K. W. B6er, Payback of solar systems. Solar Energy 20, 225-232 (1978). 27. J. A. Duffle and W. A. Beckman, Solar Engineering of Thermal Processes. John Wiley, New York (1980).
1992
0960-1481/92 $5.00+.00 Pergamon Press Ltd
PERFORMANCE STUDIES OF A LARGE SIZE NONTRACKING SOLAR COOKER N. M. NAHAR Central Arid Zone Research Institute, Jodhpur 342 003, India (Received 25 September 1991 ; accepted 28 November 1991) Abstraet--A large size nontracking solar cooker has been designed, fabricated and tested. The cooker is based on the hot box principle. The cooker has been tested extensively and its performance has been compared with a solar oven, a hot box solar cooker and a solar cooker (tilted absorber). The stagnation temperatures are in increasing order for the hot box solar cooker, the solar cooker (tilted absorber), the large size nontracking solar cooker and the solar oven. The performance of this solar cooker is comparable with that of a solar oven. The former is not tracked towards sun while the latter is tracked every 30 min. The efficiency of a large size nontracking solar cooker is 24.9%. The energy saved by this new solar cooker has also been calculated and its payback period has been computed by considering interest, maintenance and inflation in fuel prices and maintenance cost. The payback periods are 1.1(~3.63 years depending on which fuel it replaces. Relatively short payback periods show that the use of the cooker is economical, and it is easy to operate since no tracking is required.
1. INTRODUCTION Cooking accounts for the major share of energy consumption in developing countries. Fifty per cent of the total energy consumed in India is for cooking [1]. Most of the cooking requirement is met by noncommercial fuels such as firewood, agricultural waste and cowdung cake. The total consumption of firewood, cowdung cake and agricultural waste are 150, 75 and 50 million tonnes per year in India [2]. However, forest reserves are rapidly being depleted. By the end of this century it has been predicted that there will be a great shortage of firewood if a suitable alternative is not provided to the rural people. Fortunately, India is blessed with a b u n d a n t solar radiation [3]. The arid parts of India receive maximum radiation, i.e. 7600-8000 MJ m 2 per annum, followed by semi arid parts, 7200-7600 MJ m 2, per a n n u m and least on hilly areas where solar radiation is still appreciable, i.e. 6000 MJ m 2 per annum. Therefore, solar cookers seem to be a good substitute for cooking with firewood. There are two broad categories of solar cookers developed so far, i.e. reflector [4-9] and hot box [1015] types. The reflector type solar cooker was developed in the early 1950s [4] and it was manufactured on a large scale in India [16] but it did not become popular due to its inherent defects. It requires 10 rain tracking. Cooking can be done only in the middle of the day, in the direct sun. Its performance is affected by dust and wind which are serious limita42I
tions in arid conditions, for instance, at Jodhpur, India, there is a danger of burning. One has to stand near to the cooker for cooking, and its design is complicated. The reflecting surface deteriorates with time and hence cooking efficiency is reduced. Therefore, the hot box type solar cooker has been suggested by Garg et al. [17]. Although the solar oven [11-14] performs well, it also requires 30 min tracking, is too bulky and its cost is very high. Therefore, the simple hot box type solar cooker [18] with a single reflector is being promoted by the Indian government. The performance of the hot box solar cooker is very good during summer but it is very poor during winter in northern parts of India. It is difficult to cook two meals per day during winter because its glass window and absorbing surface are horizontal and so receive much less radiation compared to optimally inclined surfaces [19]. Solar radiation received at Jodhpur on horizontal surfaces and optimally inclined surface is shown in Table 1. From Table 1, it is clear that solar radiation received on an inclined surface is 69.37% more than a horizontal surface during the month of December and an inclined surface receives 43.8% and 22.76% more radiation than a horizontal surface during the winter season (October-March) and round the year, respectively. Considering this, a new solar cooker with tilted absorber (TA) [20] has been designed, developed and tested. Although the performance of the solar cooker (TA) is better than the hot box solar cooker and comparable with the solar even, it can only be used
422
N. M. NAHAR
Table 1. Mean daily solar radiation (MJ m- 2) on horizontal and optimally inclined surfaces Solar radiation (MJ m 2) Month
Horizontal surface
Inclined surface
Increase over horizontal surface (%)
January February March April May June July August September October November December
16.6 19.6 23.0 25.5 26.6 24.9 21.1 19.5 21.5 20.5 17.3 15.6
26.10 27.65 26.16 26.02 26.60 24.90 21.10 19.67 23.09 26.05 26.70 26.42
57.23 41.07 13.74 2.04 0.00 0.00 0.00 0.87 7.39 27.07 54.33 69.36
Mean
21.0
25.04
22.76
Sources : Refs 3 and 19. for cooking for a maximum of five people, while in rural India family size is large and people live in clusters. The solar cooker's (TA) mirror has a width to length ratio of unity, but to avoid edge effects [21-23] during off peak hours, the ratio should be greater than three. We have considered this and increased the capacity of the cooker so that it can be used for cooking 10 kg of food per day rather than the 2 kg by the solar cooker described above. This large size cooker can meet the cooking requirements of 25 people. In this paper the design and performance of this large size solar cooker is described and compared with the solar oven, hot box solar cooker and the solar cooker (TA). 2. DESIGN
2.1. Large size solar cooker The solar cooker (Fig. 1) is based on the hot box principle with dimensions of 2070 mm x 570 mm × 230 mm. The inner tray is made of aluminium sheet and the outer is of mild steel. The space between them is filled with fibreglass insulation. Two clear glass covers are provided and the inner tray is painted with blackboard paint. Four doors have been provided on the rear side for loading and unloading of the cooker. One adjustable reflector, with a width to length ratio of four to avoid edge effects, is fixed over it to boost solar radiation on the glass window. The cooker is fixed on an angle iron stand. It is designed so that its inclination should be changed once a fortnight. It is kept facing the equator. Eight specially designed
cooking utensils (Fig. 2) can be kept inside for boiling various dishes. Photographs of the device are shown in Fig. 3. 2.2. Solar oven The solar oven (Fig. 4) has a trapezoidal reflecting assembly, a cylindrical cooking chamber and an angle iron stand which has both azimuthal as well as elevation tracking. The concentration ratio of the cooker is 3.5. The detailed design of this cooker is described elsewhere [14]. 2.3. Hot box solar cooker It is a simple hot box (Fig. 5) with one reflector and hot box chamber for cooking. It has only azimuthal tracking. Its detailed design has been reported elsewhere [14]. 2.4. Solar cooker (TA) This solar cooker (Fig. 6) is also a hot box solar cooker but it has two adjustable reflectors, one at the top and another at the bottom so that the angle of the booster with the glass window can be changed as required and more radiation can be obtained on the glass window and cooking efficiency can be improved. The detailed design and performance has been described elsewhere [20]. 3. PERFORMANCE AND TESTING
The solar cooker was tested by measuring the stagnation temperature of air inside the cooking chamber in the large size solar cooker and comparing this with the solar oven, the hot box solar cooker and the solar cooker (TA) with different combinations of tracking for the solar oven, hot box solar cooker and solar cooker (TA). The large size nontracking solar cooker was always kept fixed, but its tilt was changed once a fortnight. Average diurnal variations in stagnation temperatures of different solar cookers are shown in Table 2. The solar oven, hot box solar cooker and solar cooker (TA) were tracked while the large size nontracking solar cooker was kept fixed. The stagnation temperatures are in increasing order; hot box, solar cooker (TA), large size nontracking solar cooker and solar oven. When the solar oven and the hot box were tracked but the solar cooker (TA) and the large size nontracking solar cooker were kept fixed the stagnation temperatures were measured as shown in Table 3. From Table 3 it is clear that stagnation temperatures are in increasing order; hot box, solar cooker (TA), large size nontracking solar cooker and solar oven. Similarly all cookers were kept fixed except the solar oven was tracked. The average recorded
423
Large size nontracking cooker
WOODEN FRAME
OPENABLE D O O R / ~
~"
L L,L--
~ ~...4~2 ~
2070
El
¢1
__
U
PLAIN GLASS pLAN -~ FIBRE GLASS INSULATIO~N o
CROSS SECTION AT A-A DOOR
ISOMETRIC VIEW DESIGN OF LARGE SIZE
ALL DIMENSIONS ARE IN m.m. NONTRACKING SOLAR COOKER
Fig. 1. Design of large size nontracking solar cooker.
stagnation temperatures for this case are shown in Table 4. From Table 4 it is seen that stagnation temperatures are increasing order for hot box, solar cooker (TA), large size nontracking cooker and solar oven. Similarly, stagnation temperature inside cooking chamber were also measured with a load. One kilogramme of water in four utensils was kept in the solar oven, hot box solar cooker and 4 kg of water in eight utensils were kept in the large size nontracking solar cooker. The diurnal variations in stagnation temperatures without load and with load are shown in Figs 7 and 8, respectively. Figures 7(a) and 8(a) depict stagnation temperatures for the solar oven and the hot box solar cooker, tracked, while in Fig. 7(b) and Fig. 8(b), only the solar oven was tracked, with the hot box and large size nontracking solar cooker kept fixed. From Tables 2-4 and Figs 7 and 8, it is clear that the performance of the large size nontracking solar
cooker is better than the hot box solar cooker and solar cooker (TA), and comparable with the solar oven. Its performance is better than the hot box solar cooker because it has a tilted absorber while the hot box has horizontal absorber so that more radiation can be obtained. Its performance is better than the solar cooker (TA) because the width to length ratio of the booster is one in the solar cooker (TA) while it is four in the large size nontracking solar cooker so the edge effect has been minimized. The efficiency of the nontracking large size solar cooker has also been obtained by measuring the rise in temperature of a known quantity of water in a specified time by the following relation :
muC2(te~2-tvO+mw(tw2-twO Ac
(1)
HdO
The efficiency of the large size nontracking solar
N. M. NAHAR
424
LID
J
J
190 ALUMINIUM SHEET BOX
ALL DIMENSIONS IN m.m.
COOKING UTENSIL FOR SOLAR COOKER(TA) Fig. 2. Design of cooking utensils for large size nontracking solar cooker.
cooker is found to be 24.9%. Cooking trials in this cooker have also been conducted and are shown in Table 5. All types of boiling, baking and roasting operations can be performed and the time taken for various dishes is between 75 and 210 min. Cooking time is generally a complex function of the amount and types of dishes, season and time of the day during which cooking is carried out. 4. ENERGY SAVING AND PAYBACK PERIOD
Based on duration of cooking time, it has been assumed that the large size nontracking solar cooker will cook two meals per day if the duration of bright sunshine hours is more than 7 h per day and only one meal if the duration of bright sunshine hours is more than 5 h but less than 7 h per day. By analysing the duration of bright sunshine hours at Jodhpur during 1980 to 1987 it has been found that this solar cooker will cook two meals for about 310 days and one meal only for about 22 days. The energy required for cooking per person per day is about 900 kJ of fuel equivalent per meal. The cooker is sufficient for cooking for about 25 people and it saves about 50% of cooking fuel when it is used since
deep frying, baking of Chapati (local bread) and breakfast are not possible, and hence other fuels must be used. Therefore, the cooker will save 11.25 MJ of fuel equivalent per meal. Accordingly it will save 7222.5 MJ of fuel equivalent per year. The payback periods of the large size nontracking solar cooker have been computed by considering equivalent savings in alternative fuels, i.e. firewood, coal, kerosene, LPG and electricity. Payback periods have been calculated by considering interest, maintenance and inflation in fuel and maintenance cost. The cost, calorific value and efficiencies [24, 25] of different fuels are shown in Table 6. From Table 6 it is clear that firewood is the costliest fuel which is used by rural people. Therefore, to conserve firewood and combat desertification solar cooking should be encouraged. The payback periods [26, 27] have been obtained from the following :
(E--M) n =
E-M
(l+a)
(2)
log (1 + b) The cost of the solar cooker is about Rs. 3000.00
Large size nontracking cooker
425
Fig. 3. Field installation of large size nontracking solar cooker.
(1 US$ = 25.9 Rs.), i.e. about US $116.00. The payback period has been calculated with respect to the different fuels with the following annual costs : interest rate maintenance
a = 10% M = 5% of total cost of solar cooker
inflation in fuel prices and maintenance cost
The payback period of the solar cooker has been calculated from eqn (2) with respect to different fuels. The payback periods are 1.10, 1.36, 2.20, 2.82 and 3.63 years with respect to firewood, coal, electricity,
b = 5%.
f / , - ' ~ .JJ...-'TJ
M.8. S H E E T
~,~.
'
•
FRAME
MIRROR
~-
o.~ ;S
,,
WOODEN
~..,
CASTOR
Fig. 4. Schematic of solar oven.
WHEEL
~t~ ARE t..~m.
Fig. 5. Schematic of hot box solar cooker.
N. M. NAHAR
426
~ . REFLECTOI~
j
/S/~
L P G and kerosene, respectively. The payback period is shortest with firewood because its efficiency is very poor while the payback period are longer for kerosene because kerosene is highly subsidized by the Government of India. 5. CONCLUSION
0
The performance of the large size nontracking solar cooker is always better than the hot box solar cooker and the solar cooker (TA) whether the hot box and solar cooker (TA) are tracked or kept fixed. Simultaneously its performance is almost comparable with the solar oven. The cooking time depends upon season, type of dish and time of loading. The efficiency
0 ~LL DIMF'NSIONS ,ARE IN rn.m.
Fig. 6. Schematic of solar cooker with tilted absorber.
Table 2. Average stagnation air temperature inside cooking chamber of different models of solar cookers (July-December 1987) (all cookers tracked but large size nontracking kept fixed) Stagnation temperature
(of)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1I00 1200 1300 1400 1500 1600
74.6 112.8 133.6 144.0 147.3 141.4 131.8 115.1
54.0 70.6 92.6 110.5 119.4 111.2 99.4 80.3
59.2 88.6 115.7 124.3 132.5 123.8 117.1 89.7
Solar radiation (MJ m 2)
Large size nontracking solar cooker
Ambient air (°C)
Horizontal surface
Inclined surface
76.6 102.5 130.3 140.0 145.8 137.2 121.4 96.7
25.2 26.8 28.9 30.6 30.9 30.9 32.3 31.7
0.850 1.512 2.021 2.323 2.415 2.293 1.731 1.431
1.275 2.293 3.042 3.474 3.578 3.281 2.727 1.891
Table 3. Average stagnation air temperature inside cooking chamber of different models of solar cookers (solar oven and hot box tracked, solar cooker (TA) and large size nontracking kept fixed) Stagnation temperature
(of)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1100 1200 1300 1400 1500 1600
75.4 111.0 138.8 146.5 151.6 146.1 143.6 122.5
52.4 83.0 108.3 117.0 124.2 117.5 112.0 97.8
57.1 83.9 118.0 123.4 133.6 127.7 110.8 89.9
Solar radiation (MJ m-2)
Large size nontracking solar cooker
Ambient air (°C)
Horizontal surface
Inclined surface
75.8 103.5 133.8 141.1 145.1 138.8 125.3 105.1
27.2 29.6 31.5 33.2 34.2 35.2 35.7 34.6
1.220 1.687 2.328 2.650 2.708 2.580 2.314 1.876
1.361 2.336 3.149 3.586 3.687 3.520 3.000 2.437
427
Large size nontracking cooker Table 4. Average stagnation air temperature inside cooking chamber of different models of solar cookers (September-December 1987) (solar oven tracked and other cookers kept fixed) Stagnation temperature
(°C)
Time (h)
Solar oven
Hot box
Solar cooker (TA)
900 1000 1100 1200 1300 1400 1500 1600
78.2 111.8 137.8 142.8 147.6 142.8 138.0 118.0
68.2 88.3 105.6 115.1 120.6 118.6 111.7 95.4
67.2 98.1 125.0 136.0 139.8 129.7 125.7 112.0
Solar radiation (MJ m 2)
Large size nontracking solar cooker
Ambient air CC)
Horizontal surface
Inclined surface
74.7 102.3 129.1 142.0 147.3 136.4 122.3 101.1
24.5 26.8 28.0 29.9 31.8 32.9 33.0 32.6
1.295 1.826 2.304 2.654 2.711 2.560 2.126 1.769
1.341 2.344 3.104 3.576 3.718 3.384 2.956 2.402
Table 5. Cooking trials in large size nontracking solar cooker Type and quantity of material (food + water) (g)
Time of loading Date
(h)
19 October 1987
1000
19 October 1987
1400
20 October 1987
1200
21 October 1987
1300
27 October 1987
1000
28 October 1987
1400
2 November 1987
1100
18 November 1987
1000
19 November 1987 20 November 1987 23 November 1987
1030 1000 1000
Rice (800 + 1600) Lentil (800 + 2000) Rice (800 + 1600) Lentil (800 + 1000) Rice (800 + 1600) Lentil (800 + 1600) Rice (800 + 1600) Lentil (800+ 1600) Rice (800 + 1600) Kidney bean (400 + 400) White gran (400 + 400) Pigenopea split (400 + 600) Potato (1000 + 200) Guar bean (1000+600) Rice (800 + 1600) Lentil (400 + 1200) Potato (1000+250) Rice (800+ 1600) Lentil (400 + 1100) Sweet potato (1000) Sweet potato boiling (1000 + 150) Pearl millet Pigenopea split Mixed (bajara Kheech) (400 + 1000) Bajara Kheech (400 + 1600) Pearl millet split (400 + 1600) Pearl millet flour batir (500)
Solar radiation (MJ m 2)
Duration of cooking (min)
Optimum tilt
Horizontal surface
120 180 120 165 90 120 75 120 98 200 200 210 90 90 90 150 90 90 90 90 90 150
6.95 10.73 5.08 5.99 5.65 7.43 4.32 6.55 4.69 8.94 8.94 9.14 4.69 4.69 4.63 5.93 4.63 5.45 5.45 5.45 5.45 8.68
4.80 7.47 3.74 4.41 4.00 5.25 3.06 4.64 3.19 5.84 5.84 5.98 3.19 3.19 2.71 3.95 2.71 3.65 3.65 3.65 3.65 5.4 t
160 180 90
8.45 10.00 5.00
5.14 6.36 2.60
Cooking material soaked in water over night. of the large size n o n t r a c k i n g solar cooker is 24.9%. The p a y b a c k period varies between 1.10 a n d 3.63 years depending u p o n the fuel it replaces. The payback periods are in increasing order with respect to
the fuels, firewood, coal, electricity L P G and kerosene. The use of the large size n o n t r a c k i n g solar cooker will conserve conventional fuels which will help c o m b a t desertification.
428
N.M.
NAHAR
(o) --
(b)
-" S O L A R
•--*
AMBIENT
~ ' - - - - ~ HOT
a-----t
LARGE SIZE
o~o
SOLAR RADIATION INCLINED SURFACE
o---<~
S O L A R RADIATION H O R I Z O N T A L SURFACE
OVEN AIR
BOX
IL
'
E ~ x
~100 :i
-,
X
4.O~ z o IQ
z z
2.O I: A<
/
@
I I0
l
I 12
TIME
I
I 14
I
I IG
I I0
(Hrs)
I
I 12
TIME
I
l 141.
i
I 16
(Hrl)
Fig. 7. Diurnal variations in stagnation temperature (without load) of different solar cookers. (a) Solar oven and hot box tracked, large size solar cooker kept fixed. (b) Solar oven tracked but hot box and large size solar cooker kept fixed.
(o)
(b)
e--e
SOLAR
----
AMBIENT
z
x HOT
OVEN AIR
BOX
I--A
LARGE
o--o
S O L A R RADIATION INCLINED SURFACE S O L A R RADIATIONNORIZONTAL SURFACE
o----o
150
SIZE
ag N
4-O~
~:1oo E
zo b-
hi )..
_[ < z
¢[ .OJF' a*
5G l /
2
IZ
.J 0 U)
m .I
,o
I.
,~
m
,
,,
TIME IHr8 I
t
,6
,;
,'~ TIME
'
,;,
,'~
(Hr8)
Fig. 8. Diurnal variations in stagnation temperatures (with load) of different solar cookers (a) same as 7, (b) same as 7.
Large size nontracking cooker
429
Table 6. Calorific value, efficiencies and cost of different fuels Cost of fuel per MJ of energy
S. No.
Type of fuel
Calorific value (MJ kg i)
Efficiency (%)
Cost
(Rs.)
1 2 3 4 5
Firewood Coal Kerosene Electricity L.P.G.
19.89 27.21 45.55 3.6 kW h ~ 45.59
17.3 28.0 48.0 76 60.0
1.50 kg ' 2.75 kg t 2.70 kg 0.64 kWh 5.20 kg i
0.23 0.23 0.15 0.20 0.16
L
1.0 US$ = 25.9 Rs. Sources : (Refs 26 and 27).
Acknowledgements--The author is grateful to Dr J. Venkateswarlu, Director, Central Arid Zone Research Institute, Jodhpur and Dr P. C. Pande, Head, Division of Wind Power and Solar Energy Utilisation, for providing the necessary facilities and constant encouragement for the present study. NOMENCLATURE a compound interest, per cent yr Ac aperture area of solar cooker, m 2 h inflation in fuel prices and maintenance cost, per cent yr C cost of solar cooker, Rs Cp specific heat of material of cooking utensil E energy saving, Rs yr i H solar radiation on glass plane, MJ m 2 h m, mass of cooking utensils, kg mw mass of water in cooking utensils, kg M maintenance cost, Rs yr n payback period, yr Q period of test t,,~ initial temperature of cooking utensils, °C tu2 final temperature of cooking utensils, 'C tw~ initial temperature of water in cooking utensils, 'C
tw2 final temperature of water in cooking utensils, 'C q efficiency of solar cooker REFERENCES 1. M. Fritz, Future Energy Consumption. Pergamon Press, New York (1981). 2. A. C. Pandya, Energy Jbr agricultural in India. Central Institute of Agricultural Engineering, Bhopal, India Tech. Bull. No. CIAE/81/20 (1981). 3. IMD, Solar Radiation Atlas of India. India Meteorological Department, New Delhi, India (1985). 4. M. L. Ghai, Design of reflector typedirect solar cookers. J. of Sci. & Indu. Res. 12A, 165-175 (1953). 5. J. A. Duffle, Reflector solar cooker design. Trans. Conj~ on Use q[' Solar Energy. University of Arizona, Tucson 3(II), 79-86 (1955). 6. G. O. G. L6fand D. Fester, Design and performance of a folding umbrella type solar cooker. Proc. UN Conf. on New Sources of Energy Rome, Paper 5/100, 5, 347 352 (1961).
7. H. Tabor, A solar cooker for developing countries. Solar Energy 10, 153 157 (1966). 8. U. Ya. Umarov, A. K. Alimav, D. N. Aavutdinev, A. Sh. Khodzhaev and A. H. Gaturav, Multifacet solar power kitchen. Applied Solar Energy (Geliotekhnika) 12, 61 -62 (1976). 9. M. Von Oppen, The sun basket. Appropriate Technology 4, 8 10 (1977). 10. M. K. Ghosh, Utilisation of solar energy. Science Culture 22, 304-312 (1956). 11. M. Telkes, Solar cooking ovens. Solar Energy 3, 1 II (1959). 12. H. P. Garg, A solar oven for cooking. Indian Farming 27, 7 9 (1976). 13. K. S. Malhotra, N. M. Nahar and B. V. Ramana Rao, Optimization factor of solar ovens. Solar Energy 31, 235 237 (1983). 14. N. M. Nahar, Performance studies on different models of solar cookers in arid zone conditions of India. Proc. 7th Miami International Conf. on Alternative Energy Sources. Hemisphere, Washington, D.C. 1431 1439 (1986). 15. I. A. Olwi and A. M. Khalifa, Computer simulation of solar pressure cooker, Solar Energy 40, 259 268 (1988). 16. M. L. Ghai, B. S. Pandher and Harikishan Dass, Manufacture of reflector type direct solar cooker. J. Sci. lndi. Sci. Res. 12A, 212 216 (1953). 17. H. P. Garg, H. S. Mann and K. P. Thanvi, Performances evaluation of five solar cookers. Proc. ISES Congress, New Delhi (Edited by F. de Winter and M. Cox) SUN 2. pp. 1491 1496. Pergamon Press. New York (1978). 18. M. Parikh and R. Parikh, Design of a flat-plate solar cooker for rural applications. Proc. National Solar Energy Convention of India, Bhavnagar, Central Salts and Marine Chemical Research Institute, Bhavnagar, India, 257 261 (1978). 19. IMD, Solar radiation on inclined surfaces. India Meteorological Department. Pune, India (1980). 20. N. M. Nahar, Performance and testing of an improved hot box solar cooker. Energy Convers. Manag. 30, 9 16 (1990). 21. D. K. McDaniels, D. H. Lavndes, H. Mathew, J. Reynolds and R. Gray, Enhanced solar energy collection using reflector solar thermal collector combination. Solar Energy 17, 277 283 (1975). 22. S. C. Seital, Collector performance enhancement with fiat reflectors. Solar Enerqy 17, 291-295 (1975).
430
N. M. NAHAR
23. S. L. Grassie and N. R. Sheridan, The use of planar reflectors for increasing the energy yield of flat plate collectors. Solar Energy 19, 663~68 (1977). 24. NCAER, Domestic Fuels in India. National Council of Applied Economic Research, New Delhi. Asia Publishing House, Bombay (1959). 25. DNES, Solar Hot Water Systems. Department of Non-
conventional Energy Sources, Ministry of Energy, New Delhi (1985). 26. K. W. B6er, Payback of solar systems. Solar Energy 20, 225-232 (1978). 27. J. A. Duffle and W. A. Beckman, Solar Engineering of Thermal Processes. John Wiley, New York (1980).