Available online at www.sciencedirect.com
ScienceDirect Solar Energy 122 (2015) 148–155 www.elsevier.com/locate/solener
Design, development and testing of a small scale hybrid solar cooker S.B. Joshi b,⇑, A.R. Jani a a
Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India b G.H. Patel College of Engineering and Technology, Gujarat, India Received 20 June 2014; received in revised form 21 July 2015; accepted 18 August 2015
Communicated by: Associate Editor S.C. Bhattacharya
Abstract A Small Scale Box type solar cooker (SSB) weighing 4.8 kg is modified into a novel photovoltaic and thermal hybrid solar cooker named as Small Scale Box type Hybrid solar cooker (SSBH) weighing 6.5 kg. Five solar panels each of 15 W are attached with this cooker. Cooking time is reduced due to the photovoltaic power generated by solar panels along with the solar thermal power. Any time cooking facility makes it a user friendly cooker. The design features of this hybrid solar cooker include proper sizing of solar photovoltaic panels, battery and dc heaters. The performance and costing of the cooker is compared with a baseline Solar Box type Cooker (SBC) weighing 12.3 kg. Efficiency of Improved Small Scale Box type Hybrid solar cooker (ISSBH) is 38%. Estimated cost ($120) of the developed solar cooker can be further reduced upon commercialization to make it more affordable and popular. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Small Scale Box type Hybrid solar cooker; Solar thermal energy; Solar photovoltaic energy
1. Introduction Major amount of energy consumption in the rural areas accounts for cooking in developing countries (Ramchandra and Shruthi, 2007). Solar cookers seem to be a good substitute for cooking with conventional natural sources like wood and animal dung. Most commonly used solar cookers are solar panel type, box type and parabolic type (Cuce and Cuce, 2013). Achievable temperature in box type solar cooker is lower than that of the parabolic reflector type solar cooker as it utilizes only thermal energy (Nahar et al., 1994). Reflector type cookers require the tracking of the sun after every 15 min which restricts its popularity. The cooker direction has to be changed for better efficiency according to movement of the sun. Use of this type of cooker becomes tedious job for a user to follow the track ⇑
Corresponding author. Mobile: +91 9723309295. E-mail address:
[email protected] (S.B. Joshi).
http://dx.doi.org/10.1016/j.solener.2015.08.025 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
of the sun (Ahmad, 2000). As this cooker has open ends, blowing of winds reduces the efficiency of cooking drastically. User’s eyes and skin can also be damaged due to direct reflection from the reflector of the cooker (Kimambo, 2007). The advantages and disadvantages of some popular solar cookers are summarized in Table 1. Different types of solar cookers and ovens have been designed, developed and tested by many scientists (Suhail, 2013; Panwar et al., 2013). Solar cookers with booster mirrors which are relatively faster than the conventional box type solar cookers have also been developed (Mirdha and Dhariwal, 2008). Community type solar cookers for indoor cooking which are having very attractive features have also been developed (Kaushik and Gupta, 2008). Number of scientists have put in efforts in development and testing of solar cookers of box type (Kumar, 2005), concentrator type (Hermim et al., 2010; Purohit and Purohit, 2009) and oven type (Suharta et al., 1998). Solar ovens can achieve very high temperature range
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Nomenclature F1 TP Ta Is M Cp A
first figure of merit stagnation plate temperature ambient temperature solar insolation on horizontal surface at stagnation temperature mass specific heat of water glazing area
of 200–500 °C and are used for frying purpose but they are very bulky and costly which make them less convenient for use (Telkes, 1959). In the earlier work of the authors, solar cooker was tested for certain analysis with dual axis solar tracker (Joshi and Jani, 2013a, 2013b, 2013c). A very small scale and light weight (1.6 kg) casserole type solar cooker was developed and tested for its maximum utilization. Different recipes were cooked in this casserole type cooker for its actual cooking performance (Joshi and Jani, 2013a, 2013b, 2013c). A photo voltaic and thermal hybrid solar cooker was tested using a fixed power supply of 30 W (Joshi and Jani, 2013a, 2013b, 2013c). Photovoltaic and thermal hybrid solar cooker makes it possibly a 24 h usable cooker as it is connected with the battery having capacity of 45 A h. This fully charged battery can be used for about 3 h without recharging. This duration can further be increased by employing other suitable batteries. The casserole type (1.6 kg) solar cooker with some modifications can also be utilized for drying applications. Bitter gourd chips and potato chips were dried in this solar dryer (Joshi et al., 2014). Conventional Solar Box type Cooker (SBC) meets with the figure of merit F1 value as required by BIS (Bureau of Indian Standards) but still not popular as it can be used only during certain limited hours in day time. This fact has become the rationale of present work.
t1 t2 SBC
initial time final time Solar Box type Cooker (BIS standard dimension) SSB Small Scale Box type solar cooker SSBH Small Scale Box type Hybrid solar cooker ISSBH Improved Small Scale Box type Hybrid solar cooker
This paper reports the design, development and testing of a prototype Small Scale Box type Hybrid (SSBH) solar cooker with five foldable solar panels attached to it. The SSBH solar cooker incorporates photovoltaic and thermal effects simultaneously. It is connected with a 45 A h battery which is charged by the solar panels. This cooker can work with fully charged battery for about 3 h. During the idle conditions, the solar panels charge the battery which permits the cooking at night as well as for lighting purposes. The SSBH also facilitates the use of solar cooker by the people living in the high – rise buildings by placing it in side projection of the apartment with limited sunshine. A modification to SSBH into Improved Small Scale Box type (ISSBH) solar cooker is also reported in this paper. It offers following novel and user friendly features: (1) Cooking at any time convenient to the user (2) Fast cooking (3) Four to five meals preparation in a day (4) Affordable cost (5) Small size and light weight (6) Unattended cooking. 2. Design Fig. 1(a) shows the photograph of Small Scale Box type (SSB) solar cooker (4.8 kg) with single pot. Fig. 1(b) shows the photograph of the SSBH solar cooker (6.5 kg) with folded solar panels and Fig. 1(c) shows the SSBH solar cooker with opened out solar panels. The Small Scale
Table 1 Advantages and disadvantages of some typical solar cookers. Sr. no.
System
Advantages
Disadvantages
1
3 4
Panel cooker Collector cooker
5
Concentrating (reflector) cooker (SK-14)
6 7
Parabolic dish type cooker Modified solar cooker with booster mirrors Community size solar cooker for indoor cooking (Scheffler cooker)
Operates under direct as well as diffuse radiation Requires little intervention by users, easy to construct Low cost and small size Operates under direct as well as diffuse radiation Quite efficient, cooker can achieve extremely high temperatures High energy efficiency Higher temperature throughout the day High energy efficiency
Slow cooking and limited cooking hours, bulky
2
Solar Box type Cooker (SBC) (base line solar cooker) Solar oven
8
Requires large concentration area, costly Very sensitive to wind Bulky and costly Requires intervention by users, strong reliance on direct beam, more sensitive to wind, relatively high cost, burns or eye damage (Kimambo, 2007) Very sensitive to wind (Panwar et al., 2013) Large size (Mirdha and Dhariwal, 2008) Large land area is required, very expensive (Kaushik and Gupta, 2008)
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Fig. 1. (a) SSB solar cooker, (b) SSBH solar cooker with folded solar panels, and (c) SSBH solar cooker with open solar panels.
Box type (SSB) solar cooker of Fig. 1(a) is converted into a SSBH solar cooker by attaching five solar panels as shown in Fig. 1(b) and (c). The SSBH solar cooker is further improved by changing the type of the heater and named as Improved Small Scale Box type Hybrid (ISSBH) solar cooker. ISSBH is designed for better efficiency. The dimensions of SSB solar cooker Fig. 1(a) are shown in column 3 of Table 2. Dimensions of the outer box are 0.30 0.30 0.15 m3 and of the inner box are 0.23 0.23 0.07 m3. The outer and inner boxes of the cooker are made up of aluminum. The space between them is filled with glass wool insulation and separated by a wooden frame. The inner box is painted black with blackboard paint. Two clear window glass panes of 0.04 m thickness have been fixed over it with a wooden frame which can be opened and 0.04 m thick plane mirror reflector is fixed over it. The tilt of the reflector can be varied from 0° (closed lid) to 120° from the horizontal plane depending upon the season. The absorber area is 0.16 m2. One aluminum pot of 0.2 m diameter was kept inside it for cooking. The line diagram of the SSB solar cooker is given in Fig. 2. Few modifications were done in the SSB solar cooker to convert it into a SSBH solar cooker. The line diagram of
SSBH solar cooker is as shown in Fig. 3. The outer box along with the reflecting mirror was replaced by a solar module consisting of a set of five solar panels. The dimension of each solar panel was kept 0.30 0.30 0.005 m3. The weight of each solar panel is 0.518 kg as the cells were mounted on FRP (Fiber-Reinforced Plastic) sheet which is unbreakable and lighter than glass. The mono-crystalline Si cell efficiency was 19.3% and the solar panel efficiency was 15.78%. Five PV Panels each of 15 W were connected with each other with hinges. The set of five PV panels was connected with the cooker. These solar PV panels are detachable. One dish type dc heater was kept on the collector plate inside the box of the SSBH solar cooker. In ISSBH solar cooker the dish type dc heater has been replaced by three rod type heaters each of 25 W capacity. These rod type heaters were fixed between the outer box and inner box of the ISSBH solar cooker. 3. Experimental arrangement Details of measuring instruments used for solar cookers are stated in Table 3. The least count which is the smallest
Table 2 Dimensions of some box type solar cookers. Parameter
Standard solar cooker with four pots
Solar cooker with single pot
Novel hybrid solar cooker with single pot
Weight (kg) Shape External dimensions (m3)
Pot dimensions (m2) diameter height Absorber paint Insulation (m) No. of glass covers Reflecting mirror Solar PV module (m3)
12.3 Box type lbh 0.52 0.52 0.20 0.45 0.45 0.054 lbh 0.43 0.43 0.078 0.19 0. 065
4.8 Box type lbh 0.30 0.30 0.15 0.23 0.23 0.014 lbh 0.23 0.23 0.07 0.19 0.065
6.5 Box type lbh 0.30 0.30 0.15 0.23 0.23 0.014 lbh 0.23 0.23 0.07 0.19 0.065
Black board paint Glass wool Two One –
Black board paint Glass wool Two One –
Heaters
–
–
Black board paint Glass wool Two – Five solar PV panels each of 0.30 0.30 0.005 One dish type heater (SSBH) Three rod type heaters (ISSBH)
Aperture area (m2) Volume (m3) Internal dimension (m3)
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Fig. 2. Line diagram of the SSB solar cooker.
Fig. 3. Line diagram of the SSBH solar cooker.
Table 3 Details of measuring instruments used for testing solar cookers. Parameter measured
Instrument/sensor used
Unit
Least count
Range
Accuracy
Total solar radiation Temperature
Pyranometer/061712/(RTC/PYR/03) (1) Pt-100 sensor (2) Rishabh 135 multimeter (3) KEW temperature indicator RTD Reptech RB-6005 Precision balance Data acquisition adaptor Rish 232 DT830D Digital multimeter DT830D Digital multimeter Meter scale
W/m2 °C
1.00 0.01 0.1 0.01 0.001 50 106 0.1 0.1 0.001
0–4000 10 to 500 200 to 850 0 to 500 0 to 6 50 106 to 60 0–1000 0–10 0–1
±2% ±2% ±0.25% ±2% 5% 50 106 ±5% ±5% 0.001
Mass of water and cooking pots Cooking time Voltage Current Dimensions of solar cooker
number that can be displayed on the instrument is also mentioned in the table.
4. Results and discussions SSB solar cooker (4.8 kg) was selected and its performance was checked by conducting figure of merit test also known as F1 test given by Bureau of Indian Standards
kg s Volt Ampere M
(BIS). Tests on SSBH and ISSBH solar cooker were also conducted as under: a. Outdoor testing of SSB solar cooker (Using only thermal effect) b. Indoor testing of SSBH solar cooker (Using fixed and variable power supply) c. Indoor testing of ISSBH solar cooker (Using only photovoltaic effect)
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d. Outdoor testing of ISSBH solar cooker (Using both thermal effect and photovoltaic effect simultaneously) a. Outdoor testing of SSB solar cooker SSB solar cooker was selected and its performance was checked by conducting figure of merit test also known as F1 test given by Bureau of Indian Standards (BIS). F1 is defined as the ratio of optical efficiency to the heat loss factor by bottom absorbing plate and is a measure of the differential temperature gained by the absorbing plate at a particular level of solar insolation. The figure of merit F1 is calculated from the standard Eq. (1) (Garg and Prakash, 2000). F1 ¼
Tp Ta Is
ð1Þ
Fig. 4 shows the temperature vs. time plot for SSB solar cooker with error bar. The collector temperature for both these days at 10:00 am was 18.2 °C and 18 °C respectively which increased with time and reached to 97.7 °C and 108.9 °C. The initial hot air temperature was 19 °C and 17.8 °C and finally they reached to 111 °C and 118 °C respectively. These days were windy days with wind velocity approximately 16–18 km/h. Table 4 shows the values of F1 calculated during 18th to 21st January 2013. Dimensions (Table 2, column 3) of the SSB solar cooker are therefore suitably so chosen as to make it more useful to cater to the needs of small families. BIS has published the specification for SBC with standard dimensions as shown in Table 2, second column. According to BIS, the F1 of solar cooker must be F1 P 0.12 m2 K/W. The figure of merit F1 of SSB solar cooker is experimentally determined which was slightly lower than the BIS requirement so the SSB cooker was converted into SSBH solar cooker by incorporating photovoltaic heating along with the thermal one. SSBH and ISSBH solar cooker are merely prototype designed for test case study. It may be noted that no BIS specifications have been published for the novel hybrid photovoltaic and thermal category. The design features of the hybrid solar cookers incorporate attachments of five solar panels making it working on photovoltaic and
thermal effect simultaneously thereby reducing the cooking time and thus eliminating the limitation of lower figure of merit. After it gets popular BIS may come out with relevant specifications to provide F1 for hybrid solar cookers. It is hoped that, the presently determined value of F1 will match with the expected standards of the BIS for such small scale solar cookers. b. Indoor testing of SSBH solar cooker Temperature vs. time plot for SSBH solar cooker with error bars has been shown in Fig. 5. This is an empty cooker test. The cooker was connected with power supply and 50 W power was supplied to it for 90 min. It was observed that the initial air temperature was 24.7 °C which reached to 137.1 °C within 90 min. At 12:10 pm, the power supply was switched off and after that both heater and hot air temperatures started to fall with nearly same rate. At 13:40 pm, the temperatures of heater and hot air were 45.3 °C and 39.1 °C respectively. Efficiency of the hybrid cooker was calculated by taking the ratio of output power Po and input power Pi and the results are given in Table 5. The temperature measurements of water were recorded at an interval of 10 min (600 s). Initially temperature of water increases very rapidly then it remains nearly constant. Eqs. (2) and (3) give the formulae to calculate the input power Pi and output power Po respectively of a solar cooker. The efficiency of the SSBH solar cooker can be calculated by Eq. (4). Pi ¼ V I
ð2Þ
P o ¼ mC p DT =Dt
ð3Þ
Efficiency g ¼
Po Pi
ð4Þ
Initially, the hybrid cooker was tested inside the laboratory by connecting it with a fixed power supply of 30 W. The cooking done with 30 W was taking a long time. To reduce the cooking time, it was required to increase the power. For this purpose, a fixed power supply was replaced by a variable power supply. Indoor and outdoor performance tests of the hybrid solar cooker were conducted. c. Indoor testing of Improved Small Scale Box type Hybrid (ISSBH) solar cooker Table 6 shows the indoor testing of ISSBH solar cooker where 0.385 g water was taken in the pot and the pot was kept in the solar cooker for testing. The readings for temperature of water were recorded at an interval of 10 min. Fig. 6 shows the indoor testing of ISSHB solar cooker. Boiling temperature was reached by nearly 70 min. The average efficiency of the cooker with three heaters was 36.4%.
Fig. 4. Temperature vs. time plot for SSB solar cooker with error bar.
d. Outdoor testing of Improved Small Scale Hybrid Box (ISSBH) type solar cooker
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Table 4 F1 for SSB solar cooker. Date
Insolation (W/m2)
Ta (°C)
Plate temp. Tp (°C)
Tp–Ta (°C)
F1 (°C m2/W)
Mean F1 (C m2/W)
18th January 19th January 20th January 21st January
788 827 930 948
27.0 27.0 27.2 27.0
97.7 97.0 108.9 106.0
70.7 70.0 81.7 79.0
0.090 0.085 0.088 0.083
0.087
Standard deviation r = 0.0027. Standard error of mean S = 0.0014. 65% confidence interval for F1 0.082 6 0.087 6 0.091.
Fig. 5. Temperature vs. time plot for SSBH solar cooker with error bar. Fig. 6. Indoor testing of ISSBH solar cooker.
The cooker was tested in open space on the ground to incorporate the thermal heating along with the photovoltaic heating. Table 7 shows the outdoor testing of ISSHB solar cooker. The boiling temperature was achieved within 40 min. The heating time was considerably reduced in the case of outdoor heating. Fig. 7 shows the outdoor testing of the ISSHB solar cooker. The input thermal power and total input power were calculated by Eqs. (5) and (6) respectively.
Thermal Power P iðThÞ ¼ Is=Cooker aperture area of ISSBH solar cooker
ð5Þ
ð0:23 0:23 m2 Þ
Here P i ¼ Photovoltaic power P iðPVÞ þ Thermal Power P iðThÞ
ð6Þ
Table 8 shows the cost comparison for some typical solar cookers. The estimated cost of ISSHB solar cooker is $120 which has an interesting advantage. It consists of
Table 5 Efficiency of SSBH solar cooker. Time (s)
TW (°C)
DT (°C)
Po (W)
Pi = V I (W)
Efficiency g (%)
600 1200 1800 2400 3000
27.5 41.0 66.0 83.9 97.8
13.5 25.0 17.9 13.9
11.3 20.9 14.9 11.6
48.8 48.8 48.9 48.8
23.2 42.9 30.6 23.8
Average efficiency (%)
30.1
Table 6 Indoor testing of ISSHB solar cooker. Time (h)
Time (s)
Twater (°C)
DT (°C)
Po (W)
Pi = V I (W)
g (%)
10:20 10:30 10:40 10:50 11:00 11:10 11:20 11:30
0 600 1200 1800 2400 3000 3600 4200
33 45 56 67 78 87 95 103
12 11 11 11 9 8 8
32.2 29.5 29.5 29.5 24.2 21.5 21.5
76.9 73.9 74.5 73.1 74.2 74.0 73.9 72.4
43.6 39.6 40.4 39.8 32.7 29.1 29.7
Average efficiency (%)
36.4
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Table 7 Outdoor testing of ISSHB solar cooker. Time (h)
Time (s)
Twater (°C)
DT (°C)
Po (W)
Pi(PV) = V I (W)
Is (W/m2)
Pi(th) (W)
Pi (W)
g (%)
12:00 12:10 12:20 12:30 12:40
0 600 1200 1800 2400
34 48 68 88 100
14 20 20 12
37.6 53.7 53.7 32.2
77.2 74.5 74.2 70.4
803.1 793.7 791.8 795.1
42.5 42 41.9 42.1
119.7 116.5 116.1 112.5
31.4 46.1 46.3 28.6
Average efficiency (%) 38.1
The residents of rural and hilly area may be benefitted by the development of the hybrid solar cooker. The battery charged by solar PV panels can also be used to supply power for other requirements like emergency lighting. It is a portable cooker with an affordable cost of nearly $120. Commercialization of this novel improved small scale hybrid solar cooker may further reduce the cost thereby making it more popular. There is a scope of improvement in the present design to make it useful on larger scale. Acknowledgements Fig. 7. Outdoor testing of ISSHB solar cooker.
Table 8 Cost comparison of some typical solar cookers. Sr. no.
Type of solar cooker/oven
Approximate cost in $
1 2 3 4 5 6 7 8
SBC SSBC SSHB ISSHB Hybrid solar cooking oven Global sun oven Sport solar oven Hot pot simple solar cooker
40 15 125 120 299 275 150 99
solar PV module and a battery both of which may form a part of the already existing emergency light system in many cases. In such cases, the approximate cost of the cooker reduces to $15 only (excluding the cost of solar PV module and a battery). 5. Conclusions The novel prototype Improved Small Scale Box type Hybrid solar cooker (ISSBH) specially designed for small family reveals 38% efficiency. The solar panels designed for 75 W output, consisting of five solar panels 15 W each is an integral part of the SSBH and ISSBH solar cookers. The cooking time is reduced considerably by incorporating the photovoltaic effect with the thermal one. ISSBH offers novel and user friendly features like cooking at any time convenient to the user, fast cooking, four to five meals preparation in a day, affordable cost, small size and light weight, unattended cooking. The cost of $120 will reduce to $15 for buyers having solar emergency lighting system and vice versa.
The authors thank Director SPRERI (Sardar Patel Renewable Energy Research Institute), Vallabh Vidyanagar, Gujarat, India for availing the radiation data. The authors also thank Dr. Siva Reddy, Head, Solar division, SPRERI for literature support. Instrumental support provided by Mr. Shreelal Jha, Department of Physics is appreciated here. References Ahmad, B., 2000. Users and disusers of box solar cookers in urban India implications for solar cooking projects. Solar Energy, 209–215. Cuce, E., Cuce, P.M., 2013. A comprehensive review on solar cookers. Appl. Energy 102 (C), 1399–1421. Garg, H.P., Prakash, J., 2000. Solar Energy Fundamentals and Applications. Tata McGraw-Hill Publishing Company Limited, New Delhi. Hermim, A., Belhamel, M., Boukar, M., Amar, M., 2010. Experimental investigation of box-type solar cooker with a finned absorber plate. Energy 35 (9), 3799–3802. Joshi, S.B., Jani, A.R., 2013a. Certain Analysis of a Solar Cooker with Dual Axis Sun Tracker. IEEE Explore, Ahmadabad, India, p. 5. Joshi, S.B., Jani, A.R., 2013b. A New Design of Solar Cooker for Maximum Utilization. Center for Ionics University of Malaya (CIUM), Malaysia, Kuala Lumpur, pp. 287–292. Joshi, S.B., Jani, A.R., 2013c. Photovoltaic and thermal hybridized solar cooker. ISRN Renew. Energy 2013, 5. http://dx.doi.org/10.1155/2013/ 746189 746189. Joshi, S.B., Thakkar, H.R., Jani, A.R., 2014. A novel design approach of small scale conical solar dryer. Int. J. Latest Technol. Eng., Manage. Appl. Sci. (4), 258–261 Kaushik, S.C., Gupta, M.K., 2008. Energy and exergy efficiency comparison of community size and domestic size paraboloidal solar cooker performance. Energy Sustain. Dev. 9 (3), 60–74. Kimambo, C., 2007. Development and performance testing of solar cookers. J. Energy South Afr. 18, 41–51. Kumar, S., 2005. Estimation of design parameters for the thermal performance evaluation for box-type solar cookers. Renew. Energy, 1117–1126. Mirdha, U.S., Dhariwal, S.R., 2008. Design optimization of solar cooker. Renew. Energy 33, 530–544.
S.B. Joshi, A.R. Jani / Solar Energy 122 (2015) 148–155 Nahar, N.M., Marshall, R.H., Brinkworth, B.J., 1994. Studies on a hot box solar cooker with transparent insulating materials. Energy Convers. Manage. 35, 784–791. Panwar, N.L., Kothari, S., Kaushik, S.C., 2013. Energetic and exergetic study of three different type of solar cookers. J. Renew. Sustain. Energy 5, 1–12. Purohit, I., Purohit, P., 2009. Instrumentation error analysis of a paraboloid concentrator type solar cooker. Energy Sustain. Dev. 13 (4), 255–264.
155
Ramchandra, T.V., Shruthi, B.V., 2007. Spatial mapping of renewable energy potential. Renew. Sustain. Energy Rev. 11 (7), 1460–1480. Suhail, Z.F., 2013. A gravity based tracking system for box type solar cookers. Solar Energy 92, 62–68. Suharta, H., Abdallah, K., Sayaigh, A., 1998. The solar oven: development and field testing of users made designs in Indonesia. Solar Energy 64, 121–132. Telkes, M., 1959. Solar cooking ovens. Solar Energy 3, 1–11.
