Int. J. Renewable Energy Technology, Vol. 3, No. 2, 2012
A technical note on performance testing of a solar box cooker provided with sensible storage material on the surface of absorbing plate Abhishek Saxena* Department of Mechanical Engineering, Moradabad Institute of Technology, Ramganga vihar-II, Moradabad-244001, India Fax: +915912452207 E-mail:
[email protected] *Corresponding author
Varun Faculty of Mechanical Engineering Department, National Institute of Technology, Hamirpur-177 005 (HP), India E-mail:
[email protected]
Ghanshyam Srivastava Eshan Institute of Technology, Agra-Delhi Highway, Farah, Mathura-281001, India E-mail:
[email protected] Abstract: A box type solar cooker having a double glass cover and a plane mirror reflector has been tested for its thermal performance. In the present study, performance of solar box cooker has been compared by using two different sensible heat storage materials (sand and granular carbon). By using these materials as a mixture and spread it over absorber tray in the form of thin layer and fully packed with a float glass shows the significant improvement in the performance of box type solar cooker. Keywords: heat storage; performance; box-cooker; sand; carbon. Reference to this paper should be made as follows: Saxena, A., Varun and Srivastava, G. (2012) ‘A technical note on performance testing of a solar box cooker provided with sensible storage material on the surface of absorbing plate’, Int. J. Renewable Energy Technology, Vol. 3, No. 2, pp.165–173. Biographical notes: Abhishek Saxena is an Assistant Professor in the Department of Mechanical Engineering in Moradabad Institute of Technology, Moradabad-244001, India. He is also a member of three international societies of renewable energy. Currently, he is working on his area of interest, i.e., alternative energy fuels and solar hybrid energy systems.
Copyright © 2012 Inderscience Enterprises Ltd.
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A. Saxena et al. Varun is an Assistant Professor in the Department of Mechanical Engineering in National Institute of Technology, Hamirpur-177 005 (HP), India. He has more than 40 international publications. Beside this, he is a life time member of ‘SESI’. He has recently got the best research award (2010) from reputed national society of India. Ghanshyam Srivastava is the Director of Eshan Institute of Technology, Mathura-281001, India. He is an active member of various reputed universities of India as an expertise of thermal engineering especially in gas power turbines. His area of interest is thermal engineering and renewable energy.
1
Introduction
The continuous increase in the level of greenhouse gas (GHG) emissions and the increase in fuel prices are the main driving force to utilise various source of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. Cooking is the major necessity for people all over the world. It accounts for major share of energy consumption in developing countries (Pohekar et al., 2005). There is a critical need for the development of alternative, appropriate, affordable methods of cooking for use in developing countries (Funk and Larson, 1998). Cooking with the sun has become a potentially viable substitute for fuel wood in food preparation in most of the developing countries. Energy requirement for cooking accounts for 36% of total primary energy consumption in India (Sharma et al., 2009). India is blessed with good sunshine. Most parts of the country receive mean daily solar radiation in the range of 5–7 kWh/m2 and having more than 275 sunny days in a year (Mani and Rangarajan, 1982). Lof (1963) has described the principles of cooking. He has pointed out that energy requirement is maximum during the sensible heating period. On an average during cooking 20% of heat is spent in bringing food to boiling temperature, 35% of heat in vaporisation of water and 45% of heat is spent in convection losses. Some of the accessible energy has been consumed in cooking fluid in the cooking vessel and the rest is lost to the ambient due convective heat transfer, occurs among the food, interior box wall surfaces as well as inside surface of the cover from cooking utensils. The complete thermal analysis of the cooker is complex due to the three-dimensional transient heat transfers involved. Several solar cooker models have been developed, out of these solar cookers; hot box type solar cooker is the simplest to operate and is capable of cooking a variety of Indian foods where low/medium temperatures are required. In this paper, an experimental study has been carried out at Moradabad, (latitude-28°58’ N and Longitude-78°47’ E) Uttar Pradesh to investigate the figure of merits for two different materials sand (desert) and granular carbon which are used as a storage material and the properties are given in Table 1.
A technical note on performance testing of a solar box cooker Table 1
167
Properties of desert sand, granular carbon and float glass
S. no.
Properties
Sand –3
Granular carbon
Float glass
1
Density (kg m )
1,450
460
2,800
2
Thermal conductivity (W/m-K)
0.23
0.11
0.74
3
Specific heat (kJ/kg-K)
0.87 2 –1
0.93 6
2 –1
0.80 6
4
Thermal diffusivity
0.35 (m s /10 )
1.02 (m s /10 )
0.0034 (mm2/s)
5
Heat capacity
1.28 (J/m3k × 10–6)
-
-
Source: Warner (2004), Wang et al. (2006), Bottani and Tascon (2007) and Bejan and Kraus (2003)
Due to its property, sand gets hot rapidly in sunshine and cools as the ambient temperature falls. On the other side, carbon (granular having emmsivity-0.7) gets hot slowly in sunshine as well as cool in same manner and store heat energy for a short period of time in itself (Saxena, 2009). Both the materials have good absorbing capacity of heat energy. The formulas to estimate two figure of merits, F1 and F2 and given below (Mullick et al., 1987): •
first figure of merit:
F1 = •
T p − Ta Hs
second figure of merit: ⎡ 1 ⎛ T −T ⎞ ⎤ 1 − ⎜ w1 a ⎟ ⎥ F1 ( MC ) w ⎢⎢ F1 ⎝ H ⎠ ⎥ F2 = ln . ⎢ 1 ⎛ Tw2 − Ta ⎞ ⎥ At ⎟⎥ ⎢1 − F ⎜ H ⎠⎦ 1⎝ ⎣
2
Solar box cooker
The cooker consists of a double walled hot box, both the boxes, outer and inner made of 22 SWG aluminium sheet. The dimensions of the outer and inner box are 540 × 540 ×160 mm3 and 455 × 435 × 65 mm3. The space between the outer tray and outer box was filled of insulating material (glass wool) and separated by an aluminium-based alloy frame. The inner tray and outer box was painted black with the dull black paint, to absorb maximum heat energy through solar radiation. Box is covered from the top with a double glass window and placed towards south. The leakage from the box to the surroundings is minimised by having a rubber gasket (1.5 mm thick) in between the double glass cover (10 mm glazing) and the box. One 4 mm thick plane mirror reflector was fixed over the box to get additional heat energy. This reflector can be adjusted, with the help of hinges fixed on either side of the box, to any desired inclination so that the incident solar radiation onto it could be reflected onto the box window. Four cooking utensils of aluminium alloy of 160 mm diameter can be
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kept inside the cooker for cooking. The cooking vessels are painted black from outside. Figure 1 show the box type solar cooker used in the present study. Figure 1
3
Box type solar cooker (see online version for colours)
Experimentation
A large number of solar cookers have been developed in many countries. These cookers are broadly divided into direct or focusing type, indirect or box type and advanced solar cookers. The procedure followed to evaluate the performance of solar cookers consists of determining one of the following: 1
cooking time for different food products
2
the time required for a sensible heating of a known quantity of water up to the boiling point or the stagnation plate temperature recorded in at test without load.
The box cooker is kept horizontal in sunlight with its reflector facing south. The variation of temperature in the cooker is measured; the ambient temperature is measured using a K-type thermocouple meter with an accuracy of ±1°C. The solar insolation (mW/cm2) on the horizontal surface is directly measured by a standard device ‘SURYA-MAPI’ (CEL-201) with an accuracy of 1 mW/cm2. The experiments were conducted during similar weather conditions when more or less the solar radiation received was same nearly at solar noon, outdoors with and without load in April 2008 to evaluate F1 and F2 to study the effect of using sand and granular carbon used as heat storage material. The experiments have been conducted in the days of the month at 11:00 AM, when Ta was noted around 30°C and radiation was observed around 750 W/m2. The experiments were continued until the maximum temperature of the cooking fluid (water) was achieved. The effect due to clouds has been considered and shown in the temperature-time curves during the tests which resulted in suddenly rising and falling of radiation. A float thin glass sheet has been used over the layer of heat storage material, to completely seal the spreaded material on the absorber tray of solar cooker.
A technical note on performance testing of a solar box cooker
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Results and discussion
To apply the thermal test procedures for box type solar cookers to the present solar box cooker, experiments were performed on April 2008. The results of the temperature-time curves for different combination plane absorber surface: 1
sand is spread over the absorber surface
2
granular carbon is spread over the surface
3
a mixture of sand and granular carbon is spread over the absorber surface are plotted in Figures 2 to 5, respectively.
The readings of the cooking trials have been taken for an hourly period. The values of radiation have been taken as an average value for an hour. The results reported in the present work are (for Moradabad, India) obtained by conducting experiments/cooking trials during a year. The amount of water used in 1 kg. Water is considered to be a good working medium for cooking operations where a large amount of heat can be transferred from the heating surface to food. The cooked food has been noticed tasteful nutritious and of good quality. Some eatables are cooked in solar cooker and their results discussed in the Table 2. Figure 2 shows that the Tw reached up to a temperature 91°C without use of any extra material accept a simply absorber tray of Al. Figure 3 shows that using a thin layer of sand over the absorber tray (well packed and sealed from a float glass) helps to increase the inner temperature of box cooker and cooking fluid. An improvement was noticed in the performance of SBC comparison to use a simply absorber tray of Al. Float glass has been considered due to having high transmissivity (0.4 µm to 2.5 µm and emissivity 0.86) and melting point (250°C). Figure 2
Temperature distribution for simple black sheet
Note: Without any material spread over the surface.
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Figure 3
Temperature distribution for sand spread over the surface
Figure 4
Temperature distribution for granular carbon spread over the surface
Figure 4 shows that using granular carbon over the absorber tray for 1 mm thickness; a slow increment inside the box cooker was noticed but after one hour of starting of testing it was more than previous two tests. The food cooked in this test was noted well hot at 20.00 hrs. Figure 5 shows that results (combination of material) are better as compared to other three previous (without any material, sand and granular carbon) results obtained. A combination of material which store good amount of energy and retain it for time, by implementing this method for small duration solar cooker can work efficiently due to its stored energy.
A technical note on performance testing of a solar box cooker Figure 5
Table 2 S. no.
171
Temperature distribution for mixture of sand and granular carbon spread over the surface
Time taken in cooking of some eatables Substance
Ta (°C)
T (food) (°C)
Time taken (min.)
Result
1
Water (500 grams)
40
99
117
Good hot (boil)
2
Pulse (300 grams)
40
102
109
Ripped (tasty)
3
Rice (250 grams)
38
97
96
Ripped (tasty)
4
Mutton (boneless) (250 grams)
41
98
131
Hard ripped (appetising)
Figure 6 shows the cooked food which is cooked in the solar box cooker by using mixture as a heat storing media. In this study, an annual performance of solar cooker provided with a mixture of material spread on the absorber tray has been estimated for the different months by considering the actual values on the day of cooking trials (Table 3). Figure 6
Roasted kababs (mutton) prepared in the box type solar cooker (see online version for colours)
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5
Year around performance of box type solar cooker from April 2008 to March 2009
S. no.
Month
Ta
Tsbc
Tw
Radiation
Wind speed
1
April
42
145
99
950 w/m2
2
May
40
120
97
3
June
38
127
4
July
36
5
August
6
Time ()
F1
F2
5.38 m/s
12.00 to 14.00
0.12
0.45
900 w/m2
5.8 m/s
12.00 to 14.00
0.13
0.44
98
850 w/m2
5.6 m/s
12.00 to 14.00
0.13
0.54
105
96
830 w/m2
4.12 m/s
12.00 to 14.00
0.11
0.51
35
100
95
800 w/m2
3.97 m/s
12.00 to 14.00
0.11
0.49
September
31
98
93
820 w/m2
5.33 m/s
12.00 to 14.00
0.11
0.44
7
October
31
96
89
800 w/m2
5.33 m/s
12.00 to 14.00
0.12
0.45
8
November
24
92
84
780 w/m2
3.42 m/s
12.00 to 14.00
0.12
0.50
9
December
21.5
91
82
750 w/m2
2.67 m/s
12.00 to 14.00
0.12
0.45
10
January
21
90
80
720 w/m2
4.22 m/s
12.00 to 14.00
0.12
0.47
11
February
25
94
85
750 w/m2
4.01 m/s
12.00 to 14.00
0.13
0.51
12
March
32
115
98
850 w/m2
4.89 m/s
12.00 to 14.00
0.13
0.46
Conclusions
Experimental studies were conducted to see the effect of sand and granular carbon used as the heat absorbing material on the surface of absorber plate in solar box cooker. The result indicates that using these materials, there is a considerable improvement in the cooker performance and the best performance is obtained when a mixture of both the material is used. In the present study, a layer of 1 mm is provided on the surface of the absorber plate. By using these materials the cooking time inside the cooker is reduced. Thicker the layer obviously store excellent amount of heat energy in the presence of good sun-shine but take more time to achieve stagnation. In the present work, objective is to cook the healthy food as well as to reduce the cooking time, hence, thin layer of the material (almost equal to the thickness of absorber tray and cooking vessel) has been considered which is observed good for a quality output in less time.
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References Bejan, A. and Kraus, A.D. (2003) Heat Transfer – Handbook, Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Bottani, E.J. and Tascon, J.M.D. (2007) Adsorption by Carbons, Published by Elsevier Ltd. Funk, P.A. and Larson, D.A. (1998) ‘Parametric model of solar cooker performance’, Solar Energy, Vol. 62, pp.63–68. Lof, G.O.G. (1963) ‘Recent investigation in the use of solar energy for cooking’, Solar Energy, Vol. 7, pp.125–133. Mani, A. and Rangarajan, S. (1982) ‘Solar radiation over India’, Allied publishers, New Delhi. Mullick, S.C., Kandpal, T.C. and Saxena, A.K. (1987) ‘Thermal test procedure for box type solar cooker’, Solar Energy, Vol. 39, pp.353–360. Pohekar, S.D., Kumar, D. and Ramachandran, M. (2005) ‘Dissemination of cooking energy alternatives in India – a review’, Renewable and Sustainable Energy Reviews, Vol. 9, pp.379–393. Saxena, A. (2009) ‘To enhance the thermal efficiency (cooking performance) of a solar box cooker with analysis of heat curves’, MTech thesis (unpublished), UP Technical University, Lucknow, India. Sharma, A., Chen, C.R., Murty, V.V.S. and Anant, S. (2009) ‘Solar cooker with latent heat storage systems: a review’, Renewable and Sustainable Energy Reviews, Vol. 13, pp.1599–1605. Wang, L.W., Wang, R.Z., Lu, Z.S., Chen, C.J., Wang, K. and Wu, J.Y. (2006) ‘The performance of two adsorption ice making test units using activated carbon and a carbon composite as adsorbents’, Carbon, Vol. 44, pp.2671–2680. Warner, T.T. (2004) Desert Meteorology, Published in the United States of America by Cambridge University Press, New York.
Nomenclature F1
First figure of merit
F2
Second figure of merit
H
Solar radiation (W/m2)
T
Temperature (°C)
M
Mass of water (Kg)
C
Specific heat of water (kJ/kg K)
A
Aperture area (m2)
sbc
Solar box cooker
a
Ambient
w
Water
s
Substance
Subscript
