A Comprehensive Review on Solar Cookers

Applied Energy 102 (2013) 1399–1421

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

A comprehensive review on solar cookers Erdem Cuce ⇑, Pinar Mert Cuce School of the Built Environment, University of Nottingham, University Park, NG7 2RD Nottingham, UK

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Article history: Received 24 May 2012 Received in revised form 7 August 2012 Accepted 2 September 2012 Available online 1 October 2012 Keywords: Solar cooker Efficiency Cooking power PCM Exergy

a b s t r a c t In this paper, a thorough review of the available literature on solar cookers is presented. The review is performed in a thematic way in order to allow an easier comparison, discussion and evaluation of the findings obtained by researchers, especially on parameters affecting the performance of solar cookers. The review covers a historic overview of solar cooking technology, detailed description of various types of solar cookers, geometry parameters affecting performance of solar cookers such as booster mirrors, glazing, absorber plate, cooking pots, heat storage materials and insulation. Moreover, thermodynamic assessment of solar cooking systems and qualitative evaluation of thermal output offered by solar cookers are analyzed in detail. Complex designs of solar cookers/ovens with and without heat storage material are illustrated and furthermore possible methods to be able to enhance the power outputs of solar cooking systems are presented. Feasibility analysis, environmental impacts and future potential of solar cookers are also considered in the study. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Energy is a thermodynamic quantity that is often understood as the capacity of a physical system to do work. Besides its physical meaning, energy is vital for our relations with the environment [1]. Research to resolve problems related to energy is quite significant since life is directly affected by energy and its consumption [2]. Fossil fuel-based energy resources still predominate with the highest share in global energy consumption. However, clean energy generation becomes more and more crucial day by day due to the growing significance of environmental issues. Especially after the oil crisis of 1973 with soaring fuel prices, a strong stimulation of research into renewable energy technologies is observed. Currently, renewable energy resources supply about 14% of total world energy demand and their future potential is remarkable [3,4]. Among the clean energy technologies, solar energy is recognized as one of the most promising choice since it is free and provides clean and environmentally friendly energy [1,5–10]. The Earth receives 3.85 million EJ of solar energy each year [11]. Solar energy offers a wide variety of applications in order to harness this available energy resource. Among the thermal applications of solar energy, solar cooking is considered as one of the simplest, the most viable and attractive options in terms of the utilization of solar energy [12]. Wood is still the primary energy source in much of the developing world since it is seen the cheapest way to obtain the energy required. However, this situation causes some serious ecological ⇑ Corresponding author. E-mail address: [email protected] (E. Cuce). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.09.002

problems such as deforestation [13]. Especially in rural areas of Africa, a major amount of total available energy resource is utilized for cooking. The energy required for cooking is supplied by noncommercial fuels like firewood, agricultural waste, cow dung and kerosene [14]. Similarly, in India, energy demand for cooking accounts for 36% of total primary energy consumption. As reported by Pohekar et al. [15], 90% of rural households in India are still dependent on biomass fuels. People in rural areas are left no choice but to walk several kilometers every day to collect firewood. On the other hand, people in urban areas spend too much money on firewood which can be considered a major expenditure especially for poor families. Besides the environmental and economic burden of firewood use, there are some serious health problems such as burns, eye disorders and lung diseases originate from the utilization of firewood [13]. It is also emphasized by the World Health Organization (WHO) that 1.6 million deaths per year are caused by indoor air pollution [16]. Therefore, there is a rising attention concerning the renewable energy options to meet the cooking requirements of people in developing countries. It is well-known that most of the thickly populated countries from the developing part of the world are blessed with abundant solar radiation with mean daily illumination intensity in the range of 5–7 kW h/m2 and have more than 275 sunny days in a year [17,18]. From this point of view, it can be easily said that solar cookers have a big potential in these countries in order to meet the energy demand especially in the domestic sector. In addition, utilization of solar cookers provides many advantageous like no recurring costs, high nutritional value of food, potential to reduce drudgery and high durability [17]. Hence, in this paper, a comprehensive review of solar cooking technology is presented. Appropriate recommenda-

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tions are made in order to enhance current performance of solar cookers and future potential of this technology is evaluated. 2. Historic overview of solar cooking The history of solar cookers goes back to the eighteenth century. Halacy and Halacy [19] reports that the first experiments on solar cookers were carried out by a German Physicist named Tschirnhausen (1651–1708). In 1767, French–Swiss Physicist Horace de Saussure attempted to cook food via solar energy. He constructed a miniature greenhouse with 5 layers of glass boxes turned upside down on a black table and reported cooking fruit [20]. English astronomer Sir John Herschel attempted to cook food in a similar insulated box on an expedition to South Africa in 1830. A French Mathematician Augustin Mouchot integrated the heat trap idea with that of the burning mirror in 1860 and built an efficient solar oven. He also succeeded to create a solar steam engine but it was too large to be practical. In 1876, W. Adams developed an octagonal oven equipped with 8 mirrors and he reported that the oven cooked rations for 7 soldiers in 2 h [21]. One year later, Mouchot designed solar cookers for French soldiers in Algeria, including a shiny metal cone, made from a 105.5° section of a circle [20]. He also wrote the first book on Solar Energy and its Industrial Applications. In 1891, Clarence Kemp, an American plumbing and heating manufacturer, invented the first commercial solar water heater for bathing and dishwashing. In 1894, Xiao’s Duck Shop in Sichuan, China, roasted ducks via the principle of solar cooking. In 1930s, France sent many solar cookers to its colonies in Africa. On the other hand, India began to investigate solar energy as an option for avoiding deforestation. In 1940s, Dr. Maria Telkes in the USA analyzed various types of solar cookers including some heat storage materials also published a book named Solar Ovens in 1968 [19,20]. The first commercial box-type solar cooker was produced by an Indian pioneer named Sri M.K. Ghosh in 1945 [22]. In 1950s, Indian researchers devised and constructed commercial solar ovens and solar reflectors, but they were not readily accepted due to the lower-cost alternatives. Also, United Nations Food and Agriculture Association (FAO) investigated water-heating capacities of a parabolic cooker and an oven type cooker. In 1961, a United Nations Conference on New Sources of Energy including many authorities on solar cooking technology was held. In 1970s, as a result of the increasing fuel prices due to the oil crisis, an intensive interest on renewable energy technologies was observed worldwide especially in China and India [23]. Barbara Kerr in the USA constructed several types of concentrating and box-type solar cookers using recycling materials and aluminium foil. In 1979, water pasteurization was performed using box-type solar cookers by Dr. Metcalf and his student Marshall Longvin. In 1980s, especially the Governments of India and China expanded national promotion of box-type solar cookers. Heather Gurley Larson wrote

first US solar cookbook, Solar Cooking Naturally, in 1983 [20]. Mullick et al. [24] presented a method to analyze the thermal performance of solar cookers in 1987. In 2000, Funk [25] proposed an international standard for testing solar cookers. It was observed that the resulting solar cooker power curve is a useful device for evaluating the capacity and heat storage ability of a solar cooker. Especially in recent years, intensive efforts have been made to be able to enhance the cooking power capacity of solar cookers. Numerous analytical, numerical and experimental studies on novel designs of solar cookers have been carried out by many researchers. Today, solar cooking technology is very promising with its potential in order to narrow the gap between renewable and conventional power sources.

3. Solar cookers A solar cooker or solar oven is a device which utilizes solar energy to cook food. Solar cookers also enable some significant processes such as pasteurization and sterilization. It is a clear fact that there are countless styles of solar cookers in the world and they are continually improved by researchers and manufacturers. Therefore, classification of solar cookers is a hard work. However, it may be asserted that most of the solar cookers today fall within three main categories called solar panel cookers, solar box cookers and solar parabolic cookers as shown in Fig. 1.

3.1. Solar panel cookers Solar panel cookers may be considered the most common type available due to their ease of construction and low-cost material. In solar panel cookers, sunlight is concentrated from above [26]. This method of solar cooking is not very desirable since it provides a limited cooking power. On the other hand, this type of solar cookers is highly appreciated by people living or travelling alone. Solar panel cookers utilize reflective equipment in order to direct sunlight to a cooking vessel which is enclosed in a clear plastic bag. Solar panel cooker of Dr. Roger Bernard (CooKit) is one of the most popular designs in this category [17]. Only cardboard and foil shaped was utilized to manufacture the CooKit. It was an affordable, convenient and effective solar cooker which enabled to preserve nutrients without burning or drying out. Bernard also investigated how the solar cooking technology is taken up by populations [27]. Performance of solar panel cookers highly depend on reflected radiation thus, they do not seem effective under cloudy conditions [28]. In recent years, some efforts have been made in order to expand the utilization areas of panel cookers. Kerr and Scott [29] designed and built a solar powered apparatus for sterilization. They also indicated that the prescribed system can be used for cooking and food preserving purposes.

Fig. 1. Types of solar cookers: (a) solar panel cooker; (b) solar parabolic cooker; and (c) solar box cooker.

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Fig. 2. Components of a solar box cooker [20].

3.2. Solar box cookers History of solar cooking technology started with the invention of box-type solar cookers. The first solar box cooker was invented by a French–Swiss naturalist named Horace de Saussure in 1767. Especially in the twentieth century, this solar cooker type demonstrated a considerable development in terms of design and performance parameters. A solar box cooker basically consists of an insulated box with a transparent glass cover and reflective surfaces to direct sunlight into the box [20]. The inner part of the box is painted black in order to maximize the sunlight absorption. Maximum 4 cooking vessels are placed inside the box [30,31]. A detailed description of solar box cookers is illustrated in Fig. 2. Each component of the box cooker has a significant influence on cooking power. Therefore, optimization of these parameters is vital for obtaining maximum efficiency. It is observed from the cooker designs of 1950s that the food is directly exposed to sunlight [32–34]. Telkes [35] focused on box type cookers and noted that they are slow to heat up, but work well even where there is diffuse radiation, convective heat loss caused by wind, intermittent cloud cover and low ambient temperatures [36]. At the beginning of 1960s, Schaeffer [37] presented a report on the current situation of solar box cookers. In the following years, outdoor testing of box-type solar cookers was carried out by several researchers [38–42]. Garg et al. [43] compared performances of five available solar cookers [44]. After the 1980s, researchers especially focused on optimization of geometry parameters of solar box cookers since they have a dominant effect on performance. In this context, some researchers analyzed the booster mirror effect on efficiency of box-type solar cookers. Dang [45] investigated the concentrators for flat plate collectors and explained that booster mirrors can be utilized in order to increase the efficiency of solar collectors since it provides extra solar radiation. The results indicated that the effectiveness of concentrators highly depends on the angle of mirrors. Garg and Hrishikesan [46] presented a comprehensive analysis of a system consisting of a flat plate collector integrated with two reflectors. They proposed a model which was numerically simulated for conditions prevailing in three different Indian stations for three different months. They found that the enhancement is maximum for the month of December in all the three stations for both horizontal and tilted surfaces. Narasimha et al. [47–50] comprehensively analyzed the solar cookers augmented with booster mirrors. They provided a single adjustable booster mirror to a solar box cooker and calculated the total energy falling on the cooking aperture for the latitude of 18°N (Warangal City, India) and for five different declinations of the sun. The results showed that the total energy was

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enhanced at all hours of the day by intermittent adjustment, continuous adjustment and fixed orientation of the supporting mirror [47]. They also analyzed elongation effect (ratio of length/width of booster mirror) on total energy collection. Rectangular apertures were found more efficient than the equal are of square aperture in terms of total energy absorbed. On the other hand, the efficiency was approximately the same for a value of elongation [48]. Energy contribution by the booster mirror became increasingly significant with an increase in latitude of the location [49]. El-Sebaii et al. [51] constructed and tested a box-type solar cooker with multi-step inner reflectors. A transient mathematical model was proposed for the cooker. The transient performance of the cooker was determined by computer simulation for typical summer and winter days in Tanta, Egypt. They observed that the cooker is able to boil 1 kg of water in 24 min when its aperture area equals 1 m2. Habeebullah et al. [52] introduced an oven type concept to minimize the amount of heat losses and maximize the concentrated solar energy. They expressed that if the solar box cooker is augmented with four booster mirrors, heat losses due to wind will reduce since wind will not be in direct contact with the glazed surface. Results of the mathematical model indicated that oven type receiving pot has both a higher fluid temperature and overall receiver efficiency compared to the bare receiver type, working under similar conditions. El-Sebaii and Aboul-Enein [53] presented a transient mathematical model for a box-type solar cooker with a one-step outer reflector hinged at the top of the cooker. The model was based on analytical solution of the energy balance equations using Cramer’s rule for different elements of the cooker. The boiling and characteristic boiling times of the cooker were decreased by 50% and 30%, respectively, on using the cooker around midday. Buddhi et al. [54] designed and analyzed a solar cooker augmented with three reflectors and a phase change material storage unit. The experimental results showed that late evening cooking is possible in the solar cooker proposed. Algifri and Al-Towaie [55] carried out a research in order to find out effect of the cooker orientation on its performance. The analysis was applied to a cooker placed at Aden, Yemen. They found that the reflector tilt angle and the elevation angle are related by the relationship 3R  2a ¼ 180 and the cooker which satisfies this condition gives the best performance. Mirdha and Dhariwal [56] theoretically investigated several designs of solar cookers in order to optimize their performance. Various combinations of booster mirrors were analyzed as shown in Fig. 3 to be able to arrive at a final design, aimed at providing a cooker, which can be fixed on a south facing window. The results indicated that the proposed new cooker can provide higher temperature throughout the day and round the year. They also noted that the cooker can be used for preparation of two meals in a day and to keep the food warm in late evening. Some researchers focused on glazing factor in solar box cookers [57–62]. It is well known from the literature that there are various glazing materials such as glass, fibreglass, and acrylics which are commonly used in box-type solar cookers. Single pain glass and double pain glass are the most common structures which enable to receive a higher solar transmission. Optimization of the gap between panes is a significant problem since a large air gap may encourage convective heat transfer and cause a heat loss. In literature, recommended air gap depth varies from 1 to 2 cm [20,57–59]. Absorption of long wave radiation emitted by collector plates increases the glass temperature and this increment causes heat loss from the cooker to the surrounding atmosphere. Therefore, transparent insulating materials are suggested in order to improve the efficiency of solar box cookers [63,64]. Absorber tray is one of most significant component of a solar box cooker. Solar radiation passes through the glazing part and absorbed by a surface painted black called absorber tray. An absorber tray first of all should have a remarkably high absorptivity in order

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Fig. 3. (a) Conventional box-type solar cooker with south facing mirror; (b) solar box cooker with south tilted collecting surface and south facing mirror; (c) cooker with south tilted collecting surface and north facing mirror; and (d) cooker with south tilted collecting surface, north facing mirror and a fixed south facing vertical mirror [56].

to transfer maximum radiant energy to food in the cooking pot [65,66]. Harmim et al. [67] experimentally investigated a box-type solar cooker with a finned absorber plate as shown in Fig. 4. Tests were carried out on the experimental platform of the Renewable Energies Research Unit in Saharan Environment of Algeria at Adrar. The results indicated that solar box cooker equipped with fins was about 7% more efficient than the conventional box-type solar cooker. The time required for heating water up to boiling temperature was reduced about 12% when a finned absorber plate was used.

Comparative results are illustrated in Fig. 5. Pande and Thanvi [68] designed, developed and tested an efficient solar cooker. The significant part of the proposed cooker was its stationary mode and maximum capture of energy through improved design and optimum tilt of the system. They found that the cooker could save about 40% of the cooking fuel via the proposed absorber. Shrestha [69] concluded that if the external surface of the absorbing top plate is treated with selective coating, it demonstrates a better performance compared to the simple black coated absorber tray.

Fig. 4. (a) Schematic of the finned absorber plate; (b) conventional (A) and improved (B) solar box cooker [67].

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Fig. 5. (a) Finned and ordinary absorber plate temperatures and (b) comparison between internal air temperatures of cooker ‘‘A’’ and internal air temperatures of cooker ‘‘B’’ [67].

Thulasi Das et al. [70,71] carried out some simulation analysis on performance parameters of solar box cookers like the thickness and size of the absorber plate, emissivity of the vessels and insulation thickness. Anderson et al. [72] investigated performance of coloured solar collectors. They showed that coloured solar collector absorbers can make remarkable contributions to heating loads. Although their thermal efficiency is lower than highly developed selective coating absorbers, they offer the advantage of sensitive integration with buildings. Tripanagnostopoulos et al. [73,74] also analyzed coloured absorbers. They obtained that unglazed collectors with coloured absorbers are in general of low efficiency and might be used in low temperature solar applications. Amer [75] presented a novel design of solar cooker in which the absorber is exposed to solar radiation from the top and the bottom sides. A set of plane diffuse reflectors was used to direct the radiation onto the lower side of the absorber plate. Results under the same operating conditions showed that the absorbers of the solar box cooker and the double exposure cooker attain 140 and 165 °C, respectively. Kumar [76] carried out a thermal analysis in order to evaluate natural convective heat transfer coefficient in a trapezoidal enclosure of box-type solar cooker. It was underlined that the major advantage of using a trapezoidal shaped absorber tray is the absorption of a higher fraction of incident solar radiation falling on the aperture at larger incident angles, due to a more exposed surface area. Ogunwole [77] designed, constructed and test a solar cooker which absorber was a square base pot, blackened with smoke and was made of stainless steel. In the design, aluminium foil was used as reflectors. An average temperature of 100 °C was obtained from the cooker for an ambient temperature of 34 °C. Any type of cooking vessel can be used in solar box cookers but generally cylindrical shaped cooking vessels made of aluminium or copper are recommended. As reported by Saxena et al. [20], number of cooking vessels in a solar box cooker may vary depending on the quantity and the nature of the food. Khalifa et al. [78] conducted some experiments on an Arafa cooker, basically a point focus concentrator featured with Pyrex pots. The tracking was performed manually for every 15–20 min. It was observed that cooking food by directly reflected solar radiation decreases the cooking time. Gaur et al. [79] revealed that performance of a solar cooker may be improved if a utensil with a concave shape lid is used instead of a plain lid. Narasimha Rao and Subramanyam [80,81] investigated effects of some modifications on cooking vessels and analyzed performance enhancement of solar box cookers. They observed that raising the cooking vessel by providing a few lugs would make the bottom of the vessel a heat transfer surface. This change would improve the performance of the system by improving the heat transfer rates in both heating and cooling modes [80].

Fig. 6. Solar box cooker with a conventional cylindrical cooking vessel on the floor of the cooker and another vessel with central annular cavity kept on three lugs spaced at 120° [82].

They also found that cooking vessel with central annular cavity on lugs performs much better than the conventional vessel kept on the floor of the cooker [81]. Reddy and Narasimha Rao [82] compared performances of conventional solar box cooker and improved cooker having cooking vessel with central annular cavity as it is illustrated in Fig. 6. The experiments were conducted for several days using water and thermic fluid as working medium. The results indicated that when the vessel with central annular cavity is placed on lugs in the cooker interior, the hot air circulation through the gap between the bottom of the cooking vessel and the floor of the cooker and through the central annular cavity improves the heat transfer to the water in the vessel and results in the reduction of cooking time. Harmim et al. [83] experimentally investigated a box-type solar cooker with two different cooking vessels: the first one conventional and the second one identical to the first in shape and volume but its external lateral surface augmented with fins. They found that cooking time considerably reduces with the finned design. The average difference in power was calculated 7.49 W. Srinivasan Rao [84] analyzed the effects of fins attached inside the central cavity on cooker performance. A maximum temperature gain of 17 °C was observed with new design of cooking vessel in comparison of conventional type. Some researchers performed intensive efforts on solar box cookers in order to allow late evening cooking. In this context, a great deal of solid–liquid phase change materials (PCMs) were investigated for heating and cooling applications [85–92]. At the end of 1980s, Ramadan et al. in Tanta University [93] augmented

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Fig. 7. Schematic of the solar cooker based on evacuated tube solar collector with PCM storage unit [96].

a simple flat plate solar cooker with a jacket of sand as heat storage material. They observed a considerable longer cooking period with heat storage medium. Six hour per day of cooking time was reported. Haraksingh et al. [94] used coconut oil as the heat transfer fluid in a double-glazed flat plate collector solar cooker. Temperatures of approximately 150 °C were achieved between 10:00 and 14:00. Nandwani et al. [95] constructed a solar hot box with two similar compartments. They compared the behaviour of a metallic slab filled with a phase change material for short term storage with that of a conventional absorbing sheet. Advantage of the heat storage material could not be confirmed due to some reasons like high transition temperature and low quantity of PCM as well as losses while opening the door. Sharma et al. [96] investigated thermal performance of a prototype solar cooker based on an evacuated tube solar collector with PCM storage unit. The design had separate parts for energy collection and cooking coupled by a PCM storage unit as shown in Fig. 7. It was observed noon cooking did not affect the evening cooking and evening cooking using PCM heat storage was found to be faster than noon cooking. They also noted that the system is expensive but shows good potential for community applications. Hussein et al. [97] experimentally investigated a novel indirect solar cooker with outdoor elliptical cross section integrated indoor PCM thermal storage and cooking unit. Magnesium nitrate hexahydrate (Tm = 89 °C, latent heat of fusion 134 kJ/kg) was used as the PCM inside the indoor cooking unit of the cooker. They found that the cooker proposed can be used for heating or keeping the meals hot at night and early morning for breakfast of the next day. Chen et al. [98] numerically studied PCMs used as the heat storage media for solar box cookers. Magnesium nitrate hexahydrate, stearic acid, acetamide,

acetanilide and erythritol were selected as PCMs. For a two-dimensional simulation model based on the enthalpy approach, calculations were made for the melt fraction with conduction only. Stearic acid and acetamide were found to be good compatibility with latent heat storage system. It was also noted that the initial temperature of PCM does not have very important effects on the melting time. El-Sebaii et al. [99] utilized acetanilide and magnesium chloride hexahydrate as PCM in solar box cooker and obtained 134 °C of stagnation temperature. They also presented transient mathematical models of single slope-single basin solar still with and without PCM under the basin liner of the still [220]. Oturanc et al. [100] constructed and tested a solar box cooker which uses engine oil as heat storage material. It was observed that the cooker was successful to cook only light meal like rice, eggs macaroni, etc. under the climatic conditions of Turkey. Mawire et al. [101,102] carried out some simulation studies on an oil-pebble bed thermal energy storage system for a solar cooker. It is well known from the literature that insulation is one of most crucial key points for a solar cooker to provide an efficient cooking [103,104]. Insulation in a solar box cooker should not be limited to the walls of the frame box and absorber tray since a remarkable amount of heat loss occurs through the glazing [20]. In this context, Nahar et al. [105,106] carried out some studies on utilization of transparent insulation material (TIM) in solar box cookers. Under an indoor solar simulator, they tested a hot box solar cooker with glazing surface consisting 40 and 100 mm thick TIM. The stagnation temperature with the 40 mm TIM was found to be 158 °C, compared with 117 °C without the TIM [105]. A double reflector hot box solar cooker with TIM was designed, constructed, tested and its performance was compared with a

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Fig. 8. (a) Double reflector solar box cooker with TIM and (b) conventional hot box solar cooker [106].

single reflector hot box solar cooker without TIM. Fig. 8 depicts the field installation of the proposed cookers. 40 mm thick honeycomb made of polycarbonate capillaries was placed between two glazing surfaces in order to minimize the heat loss due to convection. The efficiencies were determined to be 30.5% and 24.5% for the solar box cooker with and without TIM, respectively. Energy saving by using a solar cooker with TIM was estimated to be 1485 MJ of fuel equivalent per year [106]. Mishra and Prakash [107] evaluated the thermal performance of solar cookers with four different insulation materials readily available in rural areas. Their performance was compared with that of the glass wool. It was aimed at minimizing the cost of the cooker with a view to enhance its widespread application in the rural Indian environment. Bjork and Enochsson [108] experimentally investigated three different insulation materials (glass wool, melamine foam and corrugated sheets of cellulose plastics) in terms of condense formation, drainage and moisture dependent heat transmittance. It was noted that the all materials provide best insulation in dry form. Nyahoro et al. [109] carried out a simulation study on an indoor, institutional solar cooker. The cooker storage unit consisted of a cylindrical solid block and it was insulated by a material with thermal conductivity of 0.1 W/mK and specific heat capacity of 1000 J/kgK. 3.3. Solar parabolic cookers The first solar parabolic cooker was developed by Ghai [110] in the early 1950s at the National Physical Laboratory, in India. Then, Lof and Fester [111] investigated various geometries and mounting configurations of parabolic cookers. These type of cookers attracted people immediately all over the world due to their outstanding performance. Solar parabolic cookers can reach extremely high temperatures in a very short time and unlike the panel cookers or box cookers, they do not need a special cooking vessel. However, a parabolic cooker includes risk of burning the food if left unattended for any length of time because of the concentrated power. A solar parabolic cooker simply consists of a parabolic reflector with a cooking pot which is located on the focus point of the cooker and a stand to support the cooking system. Ozturk [112–115] conducted several experimental researches on solar parabolic cookers and analyzed the performance parameters in terms of thermodynamic laws. Ozturk experimentally examined energy and exergy efficiencies of a simple design and the low cost parabolic cooker under the climatic conditions of Adana which is located in Southern Turkey (at 37°N, 35°E). The

energy output of the parabolic cooker was determined to be 20.9–78.1 W, whereas its exergy output was in the range of 2.9– 6.6 W. The results showed that the energy and exergy efficiencies of the parabolic cooker were calculated between 2.8–15.7% and 0.4–1.25%, respectively [114]. He also compared energy and exergy efficiencies of box-type and parabolic-type solar cookers. Experimental study indicated that the power output of the box-type cooker ranged from 8.2 to 60.2 W, whereas it varied between 20.9 and 73.5 W for the parabolic cooker. On the other hand, the exergy output of the solar box cooker ranged from 1.4 to 6.1 W, whereas it was in the range of 2.9 to 6.6 W for the parabolic cooker. It was also observed that the energy and exergy efficiencies of the box-type and the parabolic-type cookers were in the range of 3.05–35.2%, 0.58–3.52% and 2.79–15.65%, 0.4–1.25%, respectively [115]. Arenas [116] described a portable solar kitchen with parabolic solar reflector that folded up into a small volume. The experimental study indicated that the solar cooker reached an average power output of 175 W, with an energy efficiency of 26.6%. Al-Soud et al. [117] designed, operated and tested a parabolic cooker with automatic two axes sun tracking system. The test results showed that the water temperature inside the cooker’s tube reached 90 °C when the maximum registered ambient temperature was 36 °C. A parabolic cooker was investigated from the exergy viewpoint by Petela [118]. According to the results, the exergy efficiency of parabolic cooker was relatively very low approximately 1% while the energy efficiency ranged from 6% to 19%. Shukla [119] presented the energy and exergy efficiencies of two types of parabolic solar cookers which were tested in summer and winter in the climatic conditions of India. The results showed that the energy output of the community and domestic solar cookers varied from 2.73 to 43.3 W and 7.77 to 33.4 W, respectively whereas the exergy output of the cookers ranged from 1.92–2.58 W to 0.65–1.45 W, respectively. On the other hand, the energy efficiencies of the community and domestic solar cookers were in the range of 8.3–10.5% to 7.1–14.0%, respectively. Pohekar and Ramachandran [120] conducted a survey about present dissemination of nine cooking energy alternatives in India to compare their technical, economic, environmental/social, behavioural and commercial issues. Liquefied Petroleum Gas (LPG) stove was found the most preferred device, followed by kerosene stove, solar box cooker and parabolic solar cooker in that order while electric oven had the lowest ranking. They also determined utility assessment of parabolic cooker as a domestic cooking device in India. The study indicated that if the parabolic cookers have to become a reality

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Fig. 9. Schematic diagram of (a) cylindrical and (b) rectangular box-type of solar cookers [145].

the utility has to be increased. They stressed that the advantages of parabolic cookers in terms of technical, behavioural and commercial should be improved [121].

4. Different designs of solar cooking systems In recent years, researchers highly focused on producing novel designs of solar cookers to provide the most appropriate operating conditions and hence obtain efficient cooking. Nahar et al. [122– 129] presented numerous studies to enhance the performance of solar cookers with low cost modifications. Khalifa et al. [130,131] also conducted some studies on new design concentrating type solar cookers. Tiwari and Yadav [132] devised a new box-type solar cooker integrated with a single reflector at the hood. In their design, the base of the oven acted as the lid unlike the conventional solar box cooker and hence the problem of preheating was solved as faced in conventional box-type solar cooker. The results showed that the newly designed cooker was more efficient compared to the conventional cooker. Nandwani [133] experimentally and theoretically investigated a solar oven in the climatic conditions of Costa Rica. The cooker was augmented with a reflector to increase the illumination intensity on absorbing plate. Maximum plate temperature measured was between 130 and 150 °C. Thermal efficiency of the cooking system varied from 30% to 40%. At University of Jordan in the early 1990s, Al-Saad and Jubran [134] developed a low cost clay solar cooker. The most outstanding features of the cooker were that it was made cheap, locally available materials. In addition, no skilled labour was in need in order to operate the cooker. In their design, absorber plate of the cooker was replaced with locally available black stones. Using black stones instead of absorber plate allowed storing solar energy, hence making late cooking possible. Grupp et al. [135] presented a novel box-type solar cooker consisted of a fixed cooking vessel in good thermal contact with a conductive absorber plate. The novel cooker provided easier access to the cooking pots and less maintenance due to the protection of all absorbing and reflecting surfaces. Outdoor tests also indicated that 5 L of water per m2 of opening surface could be brought to full boiling in less than 1 h. Nandwani and Gomez [136] experimentally investigated two folding and light solar ovens constructed by Solar Box Cookers International (SBCI) in the climatic conditions of Costa Rica. Performances of the cookers were compared with a conventional oven during 30 days. The tests were conducted at load and no load condition, and with or without a reflector. Cardboard ovens were found to be 15–25% less efficient than the conventional oven.

Wareham [137] developed a solar cooker stove called SUNSTOVE which is an affordable, easy to use, suitable for family, rugged and stackable for shipping. By using the SUNSTOVE, the reduce fuel consumption decreased the cost of living and helped to improve the health of the people. The unit of SUNSTOVE held four pots with 2 L. The cooker pasteurized water in 15 min at 71 °C and it did not burn foods. The cooker’s sides had wings to increase the solar collecting area to provide for the elimination of reflectors and to reduce internal volume to be heated [137]. Beaumont et al. [138] designed a family sized ultra-low cost solar cooker in Tanzania. The hot box style cooker was developed to be built on site by the users with minimal tools, skills or special materials. The cooker consisted of a shallow 1 m2 square hole in the ground, insulated with straw and lined with adobe, a glass or plastic roof and a 1 m2 aluminized plastic reflector with guy ropes for adjustment. It provided cooked for 10–12 people on clear days with midday and dusk. A 4 L load of water brought up to cooking temperature in 60–70 min. Suharta et al. [139] designed three different solar cookers called HS 7534, HS 7033 and the newest design HS 5521. They carried out various experiments for comparison of these cookers’ cooking performance and the other parameters. It was calculated oven temperature of 202 °C between 12:00 and 12:45 p.m. on October in 1997 for type of HS 7033. It was found that these solar cookers have a good heat storage capability; therefore they can be used for consecutive cooking. Volume of HS 5521 was 35% of HS 7033’s and it was cheaper than HS 7033. Although it was seen that HS 5521 had the same heat collection rate with the others, it was able to cook as fast as HS 7033. Sonune and Philip [140] developed a Fresnel type domestic SPRERI concentrating cooker. The cooker was found capable of cooking food for a family which consisted of 4 or 5 people. The highest plate bottom temperature was calculated 255 °C in approximately 40 min while ambient temperature was 30 °C and direct solar radiation was 859 W/m2. Negi and Purohit [141] compared the performances of a conventional box type cooker and a concentrator cooker. The experimental results obtained showed that the concentrator solar cooker provided stagnation temperature 15–22 °C higher than the conventional box type cooker using a booster mirror. It was also observed that the boiling point of water with concentrator cooker is reached faster, by 50–55 min, than the conventional box type cooker. It was seen that the solar cooker utilizing non-tracking reflectors provided increased heat collection and faster cooking compare to the conventional box type cooker. El-Sebaii and Ibrahim [142] experimentally tested a solar box cooker for two different configurations under the weather

E. Cuce, P.M. Cuce / Applied Energy 102 (2013) 1399–1421

Fig. 10. Schematic diagram of truncated pyramid-type solar cooker [147,148].

conditions of Tanta, Egypt. The experiments were conducted during July 2002 with and without load. The cooking power (P) was correlated with the temperature difference (DT) between the cooking fluid and the ambient air. Linear correlations between P and DT had correlation coefficients higher than 0.90 satisfying the standard. It was also underlined that the improved cooker was able to cook many kinds of food with an overall efficiency of 26.7%. In Cornell University, Rachel Martin et al. [143] devised novel solar ovens for the developing world. Different types of solar ovens like fix cooker, bowl cooker, cone cooker, box type cooker and parabolic type cooker were constructed and tested in Nicaragua in fall of 2005 and the spring of 2006. Nandwani [144] designed, constructed and tested a hybrid multifunctional solar cooking system in Costa Rica. The device proposed enabled cooking, drying and heating/pasteurizing purposes in a single system. Kurt et al. [145] experimentally investigated the effect of box geometry on performance of solar cookers. Two different model solar box cookers, which are in rectangular and cylindrical geometries as shown in Fig. 9 were constructed using the same material and tested under the same operating conditions. Performance parameters of each cooker were determined for 0.5, 1 and 1.5 kg of fresh water. The thermal efficiency increased from 12.7% to 36.98% for cylindrical and 9.85% to 28.25% for rectangular model, when the amount of water was increased from 0.5 to 1.5 kg. The cylindrical model provided higher thermal efficiency and lower characteristic boiling time than the rectangular model. Schwarzer and Silva [146] described four types of solar cookers (flat plate collector with direct use, flat plate collector with indirect use, parabolic reflector with direct use, parabolic reflector with indirect use) in terms of their basic characteristics and test procedures. They also presented a simplified analytical model to design simple cooking systems. At Sardar Patel Renewable Energy Research Institute, Kumar et al. [147,148] designed, fabricated and tested a novel solar box cooker: truncated pyramid-type solar cooker. The truncated pyramid geometry illustrated in Fig. 10 allowed concentrated the illumination intensity towards the bottom and the glazing surface on the top facilitated the trapping of energy inside the cooker. One of the salient features of the novel cooker was to totally eradicate the need of a solar tracking system. Maximum absorber plate stagnation temperature was determined to be 140 °C and water temperature inside the cooker reached 98.6 °C in 70 min. In addition two figures of merit, F1 and F2 were found to meet the standards prescribed by the Bureau of Indian Standards for solar box-type cookers. They also observed the financial viability of the device

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via a simple economic analysis [148]. Bello et al. [149] investigated performance analysis of a simple solar box cooker in the climatic conditions of Nigeria. The average efficiency of the cooker was estimated to be 47.56%. It was recommended that the device proposed might be used as a pre-cooking and alternative to domestic cooking stove. Grupp et al. [150] developed a metering device for the determination of solar cooker use rate. The device allowed recording food temperature, ambient temperature and illumination intensity level. Moreover, the assessment of fuel savings and greenhouse-gas emission reduction compared to other cooking options was available with the proposed system. Zhou and Zhang compared the performances of two different solar cooking systems by simulation method: solar energy storage vessel between vacuum tube collector and plate collector. The temperature distribution, energy releasing rate and liquid fractions during the energy releasing process were compared for summer and winter conditions. The plate collector storage vessel was found more reliable and suitable for the climatic conditions of Nanjing [151]. Kurt et al. [152] estimated performance parameters of solar box cookers with and without reflector using artificial neural network. The experimental data set consisted of 126 values. 96 values were used for training/learning of the network and the rest of the data for testing/validation of the network performance. The results indicated that the thermal performance parameters of a solar cooker can be determined with a high degree of accuracy via artificial neural network. Hernandez-Luna and Huelsz [153] developed a solar oven for the intertropical zones and evaluated its performance. Temperature measurements of the oven were performed using 36 thermocouples type T and the data was recorded by a data acquisition system. Cooking tests showed that the oven is suitable to cook three basic Mexican meals: beans, nixtamal and corn cobs. A conservative estimation of the wood savings per solar oven is 850 kg per year which accounts for the 30% firewood used to cook by a typical Mexican rural family. Prasanna and Umanand [154,155] proposed a hybrid solar cooking system where the solar energy was transported to the kitchen. The thermal energy source was used to supplement the Liquefied Petroleum Gas (LPG) which was in common use in kitchens. In the prescribed system, solar energy was transferred to the kitchen by means of a circulating fluid. Energy gain from the sun was maximized by changing the flow rate dynamically. It was concluded from the results that as using the novel cooking system proposed, cooking can be carried out at any time of the day with time taken being comparable to conventional systems. Saitoh and El-Ghetany [156] devised a solar watersterilization system with thermally controlled flow. They carried out a heat transfer analysis in order to determine the effects of environmental conditions on the behaviour of the system. Thermal and biological tests of the water samples during the sterilization process were obtained. It was found that the proposed system can be used in clear-sky areas with a high illumination intensity potential to produce a large amount of sterilized water. Chaudhuri [157] estimated the electrical backup for an Indian solar cooker to be able to use the cooker throughout the year. It was found that approximately 160 W heater would be sufficient for cooking. Abu-Malouh et al. [158] designed, constructed and tested a spherical type solar cooker augmented with automatic sun tracking system. The system components are illustrated in Fig. 11. The experimental results indicated that the temperature inside the pan reached more than 93 °C in a day where the maximum ambient temperature was 32 °C. It was underlined that this temperature is suitable for cooking purposes and was obtained by means of a two axes solar tracking device. All measured parameters in the study are depicted in Fig. 12. As it is easily seen from the results for three different days, temperature inside pan and temperature outside pan have almost the same behaviour as a function of time.

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Fig. 11. Spherical type solar cooker: (a) the whole system; (b) the pan and the dish; and (c) the control devices [158].

On the other hand, ambient temperature increases from morning till noon and then it gradually decreases till sunset. In University of Nigeria, Ekechukwu and Ugwuoke [159] designed and constructed a solar box cooker and analyzed its performance for with and without plane reflector. The experiments were carried out with four cooking vessels each capable of holding 1 kg of water. Absorber plate temperatures with and without reflector were found to be 138 and 119 °C, respectively. Boiling times for 1 kg of water were determined to be 3600 s and 4200 s for with and without reflector, respectively. Jaramillo et al. [160] developed a novel solar cooker for intertropical zones called optogeometrical design. In their design, the oven box had seven faces instead of the six faces of most common designs reported in the literature. The most outstanding feature of this oven was that the oven needed only four simple movements to be able to obtain sufficient solar concentration throughout the year. The results showed that, at noon, the solar cooker achieves a concentration level greater than 1.95 during the whole year. Mohamad et al. [161] constructed and tested a simple wooden, hot box solar cooker with one reflector under the climatic conditions of African Sahel Region. It was observed that the cooker reached 160 °C under field conditions of Giza, Egypt. Different types of foods were successfully cooked such as rice, meat, fish, and beans. The cooking time varied from 1 to 2.5 h. Hussain et al. [162] investigated performance analysis of a box-type solar cooker with auxiliary heating. The reason of using an auxiliary heater was the cloudy days in Bangladesh which make solar cooking impossible. Six heating elements were connected in series to generate 150 W heat from 220 V AC source and were placed below the absorber plate. It was found that the use of auxiliary heating equipment allows cooking on most cloudy days. Schwarzer et al. [163] developed indoor and outdoor solar cookers with or without storage as shown in Fig. 13 for families and institutions in different countries of the world. Thermal storage was provided with a tank which was filled with pebbles. Vegetable oil was used as the working fluid which flows in cooper pipes. Approximately 250 systems were constructed in various sizes and installed in different countries for different purposes. It was stressed in the study that large-scale use of solar cookers in developing countries can only be possible through the development with financial aid.

5. Performance analysis of solar cookers Thermal performance of solar cookers can be determined by an elaborate analysis of the optical and thermal characteristics of the cooker materials and the cooker design or by experimental testing under operating conditions [20]. However, as stated by Lahkar and Samdarshi, it is very difficult to compare the cookers’ performance reported by previous researchers and establish the criteria required for selection of a cooker which can provide a successful and satisfactory cooking [12]. There are some performance parameters such as energy and exergy efficiency, cooking power, figures of merit, and parameter index which are commonly used for performance investigation of solar cooking systems. These parameters have been analyzed theoretically and experimentally by many researchers in order to provide the most appropriate operating conditions for solar cookers. 5.1. Theory of solar cookers In the mid of 1980s, overall utilizable efficiency for a solar box cooker was developed by Khalifa et al. [164] and presented by the following formula:

gu ¼

QF Q in

ð1Þ

where QF is the useful heat stored in the food for a temperature rise of DT. Qin is the solar input and for a constant illumination intensity level GNR, collector area Ac and cooking time Dt, it is determined as follows:

Q in ¼ GNR Ac Dt

ð2Þ

For the mass of water M, the specific boiling time ts and the characteristic boiling time tc are calculated by the Eqs. (3) and (4), respectively.

DT Ac M ts G tc ¼ GNR

ts ¼

ð3Þ ð4Þ

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Fig. 13. (a) Outdoor cooker with thermal storage installed in an elementary school in northern Chile, South America; (b) outdoor cooker without thermal storage installed in Mali, Africa; and (c) indoor solar cooker with three circular pots (80, 40 and 20 L) and one rectangular flat pot (60 L) installed in a school in Nicaragua, Central America [163]. Fig. 12. Variation of (a) ambient temperature; (b) illumination intensity level; (c) temperature inside pan; and (d) temperature outside pan with time [158].

F 2 ¼ F 0 g0 C R ¼ GNR is a reference radiation level and commonly taken to be 900 W/m2. G is the average illumination intensity level. There are two figures of merit F1 and F2 which are largely used for evaluating thermal characteristics of any solar cooker type. The first figure of merit F1 is determined by conducting the no load stagnation temperature test and given as follows [12]:

F1 ¼

T ps  T a G

ð5Þ T a

In Eq. (5), Tps and are maximum absorber plate temperature and average ambient temperature, respectively. The second figure of merit F2 is obtained by the full load water heating test as follows [12]:

  F 1 ðMCÞw 1  ð1=F 1 ÞððT w1  T a Þ=G Þ ln 1  ð1=F 1 ÞððT w2  T a Þ=G Þ As

ð6Þ

where F0 is heat exchange efficiency factor, g0 is optical efficiency, CR is heat capacity ratio, (MC)w is product of the mass of water and its specific heat capacity, A is absorber area, s is time interval, Tw1 is initial temperature of water and Tw2 is final temperature of water. It can be concluded from Eq. (6) that the second figure of merit is more or less independent of climatic variable. Eq. (6) can be rearranged in terms of time constant s0 as follows [12]:

s0 ¼

  F 1 ðMCÞw 1  ð1=F 1 ÞððT w1  T a Þ=G Þ ln 1  ð1=F 1 ÞððT w2  T a Þ=G Þ AF 2

ð7Þ

The measurements required to estimate the F1 and F2 are illumination intensity falling on the surface of solar cooker, ambient

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E. Cuce, P.M. Cuce / Applied Energy 102 (2013) 1399–1421 Table 1 Thermal performance parameters, their expressions developed by several researchers and range of values [12]. Author 1. Mullick et al. [24]

Parameters

Expression

F1

T ps T a G F0 o CR

F2

g

ln

i

1ð1=F 1 ÞððT w1 T a Þ=G Þ 1ð1=F 2 ÞððT w2 T a Þ=G Þ

0.254–0.490

700MC w DT 600G

348.83 W at DT = 50 °C

3. Khalifa et al. [164]

gu

Qf/Qin

ts

DTAc M t s G GNR ðMC w þM1 C u ÞðT w2 T w1 Þ

7.4–29.6% 25.843–85.757 min m2/kg

g

CA

temperature, wind speed, initial water temperature and final water temperature. Mullick et al. [165] carried out some tests in order to determine the F2 through the experimental data. They observed that the F2 increases with increase in number of cooking vessels if load is kept constant and equally distributed. This is attributed to an improvement in the heat exchange efficiency factor (F0 ) with number of cooking vessels [12]. Funk [25] developed a cooking power expression for solar cookers as follows:

MC w dT w P¼ dt

ð8Þ

where P is the cooking power, M is the mass of water, Cw is specific heat of water, dTw is temperature difference of water and dt is the time interval. Funk [25] also presented a term called standard cooking power which is given as follows:

700MC w DT 600G

ð9Þ

where Ps is the standard cooking power and DT is the temperature difference. It is clear from the Eq. (9) that in order to calculate the standard cooking power, the reference illumination intensity level should be 700 W/m2. Patil et al. [166] developed an expression for the cooking time using the standard cooking power:

MC w Ps ðT w1 Þ ln Ps ðT w2 Þ C3 N

ð10Þ

where N is number of pots and C3 is coefficient which characterizes the solar cooker. Nahar [18,106] developed an expression in order to determine the efficiency of solar cookers:

g¼

¼

h

Ps

4. Nahar [18]

s¼

0.12–0.16 m2 K/W F 1 ðMCÞw As

2. Funk [25]

tc

Ps ¼

Range of values

ðMC w þ M1 C u ÞðT w2  T w1 Þ Rs CA 0 Gdt

ð11Þ

where g is the efficiency of the cooker, M1 mass of cooking utensil, Cu is specific heat of cooking utensil, C is concentration ratio and G is the illumination intensity. A brief of the reported expressions by several researchers on performance parameters of solar cookers is given in Table 1. 5.2. Analytical models of solar cookers In Indian Institute of Technology, Yadav and Tiwari [167] carried out a simple transient analysis to get the overall picture of the performance of solar box cookers. They found that the time required to obtain the stagnation temperature is largely dependent on the heat capacity of water or the ingredient to be cooked in the cooking vessel. If the heat capacity of the contents of the cooking vessel has greater value, then the cooking period becomes long. Medved et al. [168] presented a new solar heater named SOLARBALL which was shaped as an inflatable hemisphere. A mathematical and numerical model was developed to analyse solar radiation and heat transfer in such a solar heater. The numerical model was verified by a series of experiments. It was found that typical optical

RT 0

20.1–66.7 min m2/kg 27.5%

Gdt

Table 2 Values of variables used in calculations [219]. Variable

Value

Variable

Value

Ac (BC) Ac (CC) At Ag

0.492 m2 1.545 m2 0.174 m2 0.235 m2

C (CC) M1 M2

8.88 1.477 kg 4.751 kg 30 °C

Cw C (BC)

4186 J/kgK 2.09

GT

Ta Tw2 Ta

95 °C 30 °C

2

906 W/m

efficiency and overall heat transfer coefficient of the hemispherical solar heater are between 0.45–0.50 and 0.6–1.6 W/m2K, respectively. The time required for the preparation of hot drinks and heating of food was found entirely acceptable. Kablan [169] evaluated energy saving potential of solar water heating systems in Jordan between the years of 2001–2005. He calculated that the total savings over the entire period are estimated to be 46.28 million US$ if solar water heaters are used instead of commonly used LPG powered cookers. Diallo et al. [170] theoretically investigated the performance analysis of a solar cooker with tilted walls. The northern side wall was tilted at an angle of 38° and other walls were tilted at an angle of 9° relative to the vertical. All these walls were covered with a thin reflective aluminium film. Theoretical results were in agreement with the experimental results with an inaccuracy less than 2%. Fared et al. [171] presented a mathematical model based on an electric resistances analogy which describes and simulates the thermal behaviour of a solar stove. The mathematical model included three different heat transfer mechanisms between different surfaces of the solar stove and the environment. The proposed model allowed predicting the solar stove entropy generation and its efficiency. Saitoh and El-Ghetany [156] constructed a solar water-sterilization system with thermally controlled flow and analyzed it theoretically and experimentally. Thermal and biological tests of the water samples during the sterilization process were obtained. Overall efficiency of the hot box solar cooker was found to be 35%. Effect of the plate thickness on the performance of the cooker was theoretically investigated. The results indicated that crucial parameters for the solar water-sterilization system are the level of contamination of water, type of bacteria, type and size of the transparent water container, the intensity of solar radiation, the water temperature inside the transparent container, the quantity of water being exposed, environmental conditions, exposure duration and water flow rate. Recently, Lahkar et al. [219] have developed a novel performance parameter called cooker opto-thermal ratio (COR) based on Hottel–Whillier–Bliss (HWB) equation. A single step test procedure has been used to obtain COR and to establish its utility in inner-cooker comparison, box type (BC) and concentrating type (CC) solar cookers have been tested initially. COR has been defined as follows:

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Fig. 14. Rise in water temperature with time for BC and CC [219].

COR ¼

g0 C UL

ð12Þ

where g0 is the optical efficiency, C is the concentration ratio and UL is the heat loss factor. The experimental data which is illustrated in Table 2 has been fitted in HWB equation to determine the relevant parameters. In Table 2 M1, M2, T a , Tw2, Ac, At, Ag, Cw, C and GT refer to mass of water for BC, mass of water for CC, average ambient temperature, final temperature of water, aperture area, pot surface area for CC, glazed surface area for BC, specific heat capacity of water, concentration ratio for CC and average total solar radiation on the plane of aperture, respectively. Rise in water temperature (Tw) with time (t) is given in Fig. 14. Calculated parameters in the study are listed in Table 3. In Table 3 F, UL and Tfx refer to heat exchange efficiency factor, heat loss factor and maximum achievable fluid temperature, respectively. The results indicated that COR is a robust performance parameter derived from HWB equation analytically. A cooker with a higher value of COR may be graded higher than the one having a lower value of COR. Al-Soud et al. [117] constructed, operated and analyzed a parabolic cooker with automatic two axes sun tracking system as illustrated in Fig. 15. The experiments were performed for three days from 8:30 h to 16:30 h in the year 2008. The test results indicated that the water temperature inside the cooker’s tube reached 90 °C when the maximum registered ambient temperature was 36 °C. It was also noticed that the water temperature increases when the ambient temperature gets higher or when the solar intensity is abundant. This is in favour of utilizing the proposed cooker in many developing countries, which are characterized by high solar insulations and high temperatures. Besides cooking, the aforementioned cooker could be utilized for warming food, drinks as well as to pasteurize water or milk.

Table 3 Mean values of parameter set with COR, experimental variables and maximum achievable fluid temperature [219]. Parameters

BC

2

FUL/C (W/m K) Fg0 COR Tfx (°C)

CC

Mean

Std. deviation

Mean

Std. deviation

1.576 0.213 0.136 147.75

0.138 0.008 0.011 4.950

2.260 0.348 0.155 161.82

0.011 0.013 0.007 16.688

respectively. Olwi and Khalifa [174] presented an elaborate analysis on a solar cooker used for meat grilling. Several experiments were performed in order determine the effects of thermal parameters on cooking performance. In addition, a mathematical model was developed. Heat balance equations were solved via 4th order Runge–Kutta method. It was observed that an air-tight solar cooker with double glazing and maximum meat charge provide the best performance and highest efficiency for the solar grill. Similarly to Olwi and Khalifa [174], Bidotnark and Turkmen [175] used 4th order Runge–Kutta method to investigate thermal performance of a hot box solar cooker named ITU-2 which was manufactured in Istanbul Technical University, Turkey. Jubran and Alsaad [176] presented the theoretical analysis and performance investigation of a single, as well as double, glazed box-type solar cooker with or without reflectors. The mathematical model was based on heat balance equations arranged for various components of the cooker. In the study, the properties of the cooking materials and the overall heat loss coefficient were allowed to vary as a function of the absorber plate and food temperature. Effects of thermal parameters on cooking performance were investigated.

5.3. Numerical models of solar cookers

5.4. Modeling and simulation

El-Sebaii [172] numerically analyzed a box-type solar cooker with outer-inner reflectors. Numerical calculations were carried out for different tilt angles of the outer reflector on a typical winter day (20 January) in Tanta, Egypt. The optimum tilt angle of the outer reflector was 60°. For this specific value, it was observed that the specific and characteristic boiling times were decreased by 50% and 35%, respectively, compared to the case without the outer reflector. The overall utilization efficiency of the cooker was determined to be 31%. Terres et al. [173] numerically investigated the heating of bee honey, olive oil, milk and water in a solar box cooker integrated with internal reflectors. In the study, climatic values of Mexico City for February 26, 2006 were used. It was observed that the maximum simulation temperatures were 91.8, 91.6, 86.2 and 85.3 °C that correspond to bee honey, olive oil, milk and water,

In North West University, Mawire et al. [177] carried out discharging simulations for an oil/pebble-bed thermal energy storage system (TES). Accuracy of the model was verified by the experimental results. Discharging results of the TES system were presented using two different methods. The first method discharged the TES system at a constant flow rate while the second method changed the flow rate in order to provide a desired power at a constant load inlet temperature. It was observed from the results that the TES system at a constant flow rate demonstrate a higher rate heat utilization. However, this is not beneficial to the cooking process since the maximum cooking temperature is not maintained for the duration of the discharging period. On the other hand, the controlled load power discharging method has a slower initial rate of heat utilization but the maximum cooking temperature is

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Fig. 15. Schematic of the two axes sun tracking system [117].

maintained for most of the discharging process and this is what is expected for the cooking process. Mawire and McPherson [178] simulated the temperature distribution of an oil-pebble bed TES system under a variable heat source during charging. The charging outlet temperature was controlled by a combined feedforward and PID feedback control structure to maintain thermal stratification during the experiment and the simulations. In the study, Schumann model and modified Schumann model were simulated in order to analyse thermodynamic behaviour of the TES system. It was found that the discharging results were in good agreement with the experimental results. Thulasi Das et al. [70,71] presented thermal models for the solar box cookers augmented with different number of cooking vessels. The effect of parameters such as the thickness and size of the absorber plate, emissivity of the vessel, insulation thickness, and cooking time were studied. Different cooker sizes were simulated in order to assess their adequacy in cooking. It was found that the black paint on the vessels could be avoided if weathered stainless steel or aluminium vessels are used. In addition, the cooker with inner dimensions of 0.6  0.6  0.1 m3 was found to be adequate to cook lunch and dinner on a clear day even in the winter months. Besides these specific studies, some researchers focused on solar energy models in recent years [179– 182]. Jebaraj and Iniyan [183] presented a review on energy models including renewable energy models. 5.5. Experimental work Purohit [184] carried out a large number of experiments on a box-type solar cooker in the climatic conditions of New Delhi, India. He determined absorber tray temperature (Tps), ambient temperature (Tas) and illumination intensity (Hs) in order to determine first figure of merit (F1). Similarly, he measured initial water temperature (Tw1), final water temperature (Tw2), average ambient temperature ðT a Þ, average illumination intensity (H) and time difference in which water temperature rises from Tw1 to Tw2 to be able to calculate second figure of merit (F2). The measured and calculated parameters are listed in Tables 4 and 5. In Indian Institute of Technology, Kumar [185] presented a simple test

procedure for determination of design parameters to predict the thermal performance of a solar box cooker. In order to determine two figures of merit (F1 and F2), a series of outdoor experiments were conducted on double glazed solar cooker with aperture area of 0.245 m2. Experimental setup for determination of F1 and F2 is illustrated in Fig. 16. The parameters required, optical efficiency and heat capacity of the cooker were calculated using the linear regression analysis of experimental F2 data for different load of water. The results indicated that optical efficiency and heat capacity of the cooker are crucial design parameters to be able to predict the thermal performance of solar cookers. Kumar et al. [186,187] experimentally investigated the heat loss from a parabolic concentrator solar cooker with and without wind condition. Values of the heat loss factor for the tilted reflector were compared with those obtained with the reflector in a horizontal position. It was found that a parabolic reflector is not required for heat loss determination. It was also noted that thermal performance of a parabolic concentrator solar cooker depends greatly on the wind speed. In Taiwan, Yeh et al. [188] experimentally and analytically investigated a novel design for inserting an absorbing plate to divide the air duct into two channels (the upper and the lower) for double-flow operation in solar air heaters with fins attached over and under the absorbing plate. Both the theoretical predictions and experimental results indicated that the optimal fraction of airflow rate in upper and lower subchannels is around the value of 0.5. They also examined the effect of the flow-rate ratio of the two air streams of flowing over and under the absorbing plate on the enhancement of collector efficiency. It was underlined that providing fins attached on the collector, will improve the collector efficiency. Moreover, constructing the collector with fins attached may scarcely increase the fan power. Rathore and Shukla [189] experimentally analyzed two different solar cookers: flat plate box type solar cooker (SBC) and parabolic solar cooker (SPC). The experiments were carried out at the roof top of Renewable Energy Lab, Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University (BHU), Varanasi, India during month of October and November 2008. The cookers were operated under the same climatic conditions. It was found that the daily average

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Table 4 First figure of merit (F1) of a typical Indian solar box cooker obtained from outdoor testing [184]. Tps (°C)

Tas (°C)

Hs (W/m2)

F1

106.84 111.05 105.66 118.44 117.52 104.71 106.52 118.78 118.67 108.39 112.61 112.21 107.97 111.88 102.54 105.59 111.42 111.56 111.29 125.35 104.04 101.52 122.01 105.23 100.58

26.33 27.05 26.33 28.81 28.81 26.33 24.64 29.71 29.71 27.05 28.65 26.76 27.05 26.76 23.14 24.64 26.76 28.65 28.65 31.84 24.64 23.14 29.16 27.05 23.14

603 630 603 687 687 603 631 687 687 630 651 663 630 663 619 631 663 651 651 737 631 619 742 630 619 Average value of F1 Standard deviation Standard error of mean

0.1335 0.1333 0.1316 0.1316 0.1302 0.1299 0.1298 0.1296 0.1294 0.1291 0.1289 0.1288 0.1284 0.1284 0.1283 0.1282 0.1276 0.1274 0.1269 0.1268 0.1258 0.1258 0.1251 0.1248 0.1243 0.1285 0.0024 0.0005

Table 5 Second figure of merit (F2) of a typical Indian solar box cooker obtained from outdoor testing [184]. Tw1 (°C)

Tw2 (°C)

T a (°C)

H (W/m2)

T (s)

F2

60.00 60.10 60.00 60.00 60.34 60.59 60.00 60.00 60.00 60.00 60.10 60.00 60.00 60.00 60.00 60.02 61.37 60.83 60.59 60.00 60.00 60.39 60.39 60.59 60.10

90.00 90.27 90.00 90.00 90.03 90.03 90.00 90.00 90.00 90.00 90.03 90.00 90.00 90.00 90.00 90.29 91.54 90.03 90.03 90.00 90.00 90.76 90.79 90.27 90.03

38.05 32.00 38.03 38.30 28.09 22.62 36.90 38.00 38.18 36.74 25.23 35.55 34.59 36.90 35.58 37.27 35.95 30.17 30.95 35.45 37.70 37.31 35.59 32.16 28.19

712 798 692 696 803 885 764 712 655 631 865 631 731 764 728 819 767 738 842 631 676 767 890 809 800

5100 5520 5640 5340 6120 5580 4740 5220 6120 6900 5640 7200 5400 4740 5340 4500 5100 7320 5280 7500 6060 5100 4200 5760 6600 Average value of F2 Standard deviation Standard error of mean

0.4997 0.4985 0.4864 0.4852 0.4838 0.4829 0.4792 0.4789 0.4787 0.4787 0.4780 0.4752 0.4749 0.4747 0.4732 0.4721 0.4667 0.4665 0.4652 0.4649 0.4647 0.4618 0.4577 0.4572 0.4543 0.4744 0.0117 0.0023

temperature of water in the SPC was 333 K and for SBC was 326 K and the daily average difference between the temperature of water in the cooking vessel and the ambient air temperature was 31.6 K for SPC and 26.4 K for SBC. The energy output of the SPC varied from 0.65 to 39.3 W and 7.44 to 33.49 W for SBC, whereas its exergy output was in the range of 0.92 to 2.58 W for SPC and for SBC it varied from 0.65 to 1.45 W. The energy efficiency of the SPC varied from 0.42% to 5.27% and for the SBC it varies from 4.7% to 29.81%.

Fig. 16. Experimental setup for determination of (a) F1 and (b) F2 [185].

Prasanna and Umanand [154] developed a hybrid solar cooking system as shown in Fig. 17 where the solar energy was transported to the kitchen. The thermal energy source was used to supplement the Liquefied Petroleum Gas (LPG) which was in common use in kitchens. In the prescribed system, cooking could be carried out at any time of the day with time taken being comparable to conventional systems. Design and sizing of different components of the system were described with equations. 5.6. Effective parameters on performance of solar cookers It is well known in literature that thermal performance parameters of solar cookers are highly dependent on the main components of the cookers. If a solar box cooker is considered, these components will be the booster mirror, glazing, absorber tray, cooking vessel, heat storage material and insulation as expected. On the other hand, characteristic features of the reflective surfaces will play the main role if a solar panel cooker or a parabolic cooker is evaluated. 5.6.1. Booster mirror A booster mirror is quite significant for a solar cooker since it allows higher illumination intensity falling on the transmitting surface of the cooker hence higher working temperatures which enhance the efficiency. Ibrahim and Elreidy [190] investigated the performance of a solar cooker integrated with a plane booster mirror reflector under the climatic conditions of Egypt. The experiments lasted 2 years for various operating conditions. Cooker position and the tilt angle of the booster mirror were adjusted in order to maximize the sunlight concentration. It was observed that a good meal for a family of four was cooked in 3–4 h. It was also found that better heat transfer occurred when the cooking pot

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Fig. 17. Block diagram of the hybrid solar cooking system [154].

was covered with an airtight plastic transparent cover rather than using an ordinary metallic cover. Gayapershad et al. [191] evaluated the performances of two solar cooking units: a low-cost, low-technology Sunstove unit and the more expensive Ishisa box unit. The cookers were tested with and without tracking system under summer radiometric conditions at the Solar Thermal Applications Research Laboratory (STARlab) between December 2005 and April 2006. The Ishisa box unit also augmented with external mirror panels. The Sunstove unit could not succeed to boil water. The maximum water temperature reached in the Sunstove unit was found to be 88 °C for tracked conditions. On the other hand, the Ishisa box unit enabled boiling water for both tracked and non-tracked conditions. The tracked unit reached the boiling temperature 20 min earlier than the untracked unit. It was noted that the Ishisa box unit benefitted from tracking efficiently via its external booster mirrors. In Indian Institute of Technology, Shukla and Gupta [192] presented an energy and exergy analysis of a concentrating solar cooker. The cooker was devised for community cooking and integrated with a linear parabolic concentrator which concentration ratio is 20. The experiments were carried out in both summer and winter conditions. Through the experimental results, the average efficiency of the solar cooker was determined to be 14%. Heat losses caused low efficiency were classified as optical losses (16%), geometrical losses (30%) and thermal losses (35%). The rest of the losses were due to edge losses, etc. The maximum temperature that the water in the cooker reached was 98 °C during the tests. 5.6.2. Glazing Barker [193] interestingly showed that, if it is not needed to exceed 100 °C, an efficient solar cooker can be made for less than 5$ with materials that are available almost everywhere. He underlined that multiple glazings and highly insulated boxes are not necessary in the proposed design. A double glazed transparent polyethylene plastic film was used as a glazing material in the cooker. It was concluded that most of the foods can be cooked in this very low cost cooker with a 0.25 m2 collector area. Bell mentioned about the glazing selection for various heat transfer applications. One or more sheets of glass or other diathermanous (radiationtransmitting) material was utilized in order to transfer the solar energy to the collector/absorber plate. The transparent cover was used to minimize convection losses from the absorber plate through the restraint of the stagnant air layer between the absorber plate and the glass. It also enabled reducing radiation losses from the collector as the glass is transparent to the short wave radiation received by the sun but it is nearly opaque to long-wave thermal radiation emitted by the absorber plate [20,194,195]. Hussain and Khan [196] experimentally investigated a low cost

box-type solar cooker made of two paper carton boxes with crumpled newspaper balls as insulation. The cooker was supported by a reflector covered with aluminium foil. Experimental results obtained from the novel cooker were compared with a standard costlier solar box cooker. It was observed that the water temperature rapidly increase in novel cooker compared to the standard cooker. Two figures of merit of the new cooker also found satisfactory.

5.6.3. Absorber plate Absorber tray of a solar cooker is a crucial component since it absorbs the useful energy from sun to be able to succeed cooking process. Geometric structure of an absorber plate is quite significant as well as its thermophysical properties. In order to maximize the illumination intensity falling on the absorber tray and enhance the heat transfer from the absorber tray to the food in cooking vessels, absorber tray is a key item which allows various modifications. Harmim et al. [67] devised and constructed a box-type solar cooker with a finned absorber plate to maximize the solar energy absorption. The results showed that solar box cooker integrated with fins was approximately 7% more efficient than the conventional solar box cooker. The time required for water to boil was reduced approximately 12% when a finned absorber plate was utilized. In Turkey, Ozkaymak [197] experimentally investigated the performance of a hot box solar cooker. The cooker has a cylindrical geometry as shown in Fig. 18, with a 38 cm inner diameter, 40 cm outer diameter and 25 cm height. The outer wall of the cooker was made of 1 mm thick metal sheet tray. The absorber plate was made of thin copper sheet, which was painted black for absorbing solar radiation better. Glass wool insulation was used on the bottom and sides of the cooker to minimize thermal losses through conduction. A clear window glass of 4 mm thickness was fixed over the inner tray. Three 4 mm thick plane mirror reflectors were placed around the cooker. The three reflectors were kept fixed. The constant tilt of the reflector is 678 from the horizontal plane. The cooking pot was a black painted aluminium pot with 10 cm diameter and 16.5 cm height. The experiments were carried out during July and August 2004 at Karabuk, Turkey. The solar cooker with three reflectors was exposed to solar radiation between 10.00 a.m. and 4 p.m. It was observed from the experimental results that absorber plate temperature was over 100 °C during a period of 5 h which is a sufficient time to cook most of the foods. Mawire et al. [198] developed a thermal energy storage system using a packed pebble bed. An electrical hot plate heated up oil circulating in a copper absorber plate which charges the storage system. A Visual Basic program was developed to acquire data for monitoring the storage system and to maintain a nearly constant outlet temperature from the charging point. It was concluded

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Fig. 18. Hot box solar cooker with cylindrical cooking vessel and experimental setup [197].

that the results obtained can be used to characterize the cooking system. 5.6.4. Cooking equipment Cooking pots are the items which are in conduction with the absorber tray in order to receive the absorbed energy and transfer it to the food. Any type of cooking vessel can be used in solar cookers but generally rectangular and cylindrical shaped cooking vessels made of aluminium or copper are recommended. Saxena et al. [20] emphasized in their comprehensive review that number of cooking pots in a solar box cooker may vary depending on the quantity and the type of the food. Gaur et al. [79] found that performance of a solar cooker can be enhanced if a cooking vessel with a concave shape lid is used instead of a plain lid. Joshi et al. [199,200] presented experimental and numerical studies on solar cookers in the early 2012 in order to provide an efficient design including cooking equipment. They aimed at increasing the solar cooking efficiency from 10–25% to 60% or more. In their novel design, the cooking pots gained energy from condensing steam on the outside surface. The cooking charge (water + rice or lentils and/or vegetables) received heat by the mode of natural convection. The results of CFD indicated that optimum heat flux is in the range of 16,200–25,000 kcal/h m2 where m2 is the bottom surface area of the cooking system. Cooking pots with perforations were recommended for higher efficiency. Franco et al. [201] introduced a multiple use communal solar cooker. The parabolic concentrator and the cooking pot are shown in Fig. 19. The cooking pot with 10 L capacity was painted black and placed on the focus of the concentrator. Stew as food was tested in the cooking system. A stew is generally made with potatoes, noodles or rice, meat, vegetables like peppers and carrots, and spices. The cooking is done in water, adding the ingredients according to the time span each one needs to be cooked. It was expressed in the study that about 18 kg of food can be cooked using only one concentrator. They noted that about 18 kg of stew can be cooked on each solar cooker within 3 or 4 h. 5.6.5. Heat storage material It is a clear fact from the literature that solar cookers are very promising devices in the upcoming future. However, there are some handicaps concerning the solar cooking technology. Perhaps, the most challenging point of solar cookers is that they are not able to serve when the sun goes down. Some researchers performed intensive efforts on solar box cookers in order to allow late evening cooking. PCMs were considered as a solution in most cases. Bushnell [202] designed, constructed and evaluated a solar energy

Fig. 19. (a) Communal solar cooker and (b) cooking pot [201].

storing heat exchanger as a step toward a solar cooking concept. The solid–solid transition of pentaerythritol was the principal mechanism for energy storage. The methods for describing the system performance were explained and applied to a test system containing a controllable replacement for the solar input power. This first stage of this research work followed by a heat exchanger, which was connected to a concentrating array of CPC cylindrical troughs. Author also described the size of the solar collector area and mass of PCM mass needed in order to provide adequate energy for several family-size meals with sufficient storage to cook at night and 1 or 2 days later. The performance was described from efficiency measurements and the determination of a figure of merit. Bushnell and Sohi [203] also designed a modular phase change heat exchanger with pentaerythritol used as a PCM for thermal storage (solid–solid phase change at 182 °C) was tested in an oven by circulating heat transfer oil which was heated electrically in a manner to simulate a concentrating solar collector. Thermal energy retention times and cooking extraction times were determined, and along with the efficiencies, were compared with the results previously reported for a non-modular heat exchanger. Buddhi and Sahoo [204] designed a box-type solar cooker as shown in Fig. 20 with latent heat storage for the composite climatic conditions of India. The experimental results demonstrated the feasibility of using a phase change material as the storage medium in solar cookers. It also provided a nearly constant plate temperature in the

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and exergy output and efficiency because of changes in cooker configuration. It was also seen that the exergy analysis was more convenient than the energy analysis for predicting solar cooker efficiency.

  mw cpw ðT wf  T wi Þ =t energy output Eo ¼ ¼ g¼ energy input Ei It Asc Fig. 20. Solar box cooker with thermal energy storage material (G: double glass lid, A: absorber tray, B: PCM tray, C: pot container, P: PCM and I: glasswool insulation) [204].

late evening. The experimental results were also compared with those of a conventional solar cooker. The test of the cooker was performed without a cooking load. The results indicated that solar cooker with PCM provides an environment in which the cooking is possible even the sun goes away. The absorber plate temperature of the solar cooker remained constant at about 70 °C for a long period of time. 5.6.6. Insulation It is well-documented in literature that insulation is one of most crucial key points for a box type solar cooker to be able to provide an efficient cooking [103,104]. All materials with low thermal conductivity may be used as an insulation material in solar cookers. However, the main purpose for material selection should be minimizing heat loss from the solar cooker to the environment with minimal cost. Vandana [205] devised and constructed a very low cost for Indian women who are burdened with household work, agriculture work and care of animals in addition to all time financial crisis. The proposed fireless cooker was insulated with strawboard and tested in terms of cooking efficiency. The results indicated that the fireless cooker of strawboard could both cook as well as keep the food hot with in safe temperatures well above 6 h. Nyahoro et al. [206] presented an explicit finite-difference method to simulate the thermal performance of short-term thermal storage for a focusing, indoor, institutional, solar cooker. The cooker storage unit consisted of a cylindrical solid block. The block was enclosed in a uniform layer of insulation except where there were cavities on the top and bottom surfaces to allow heating of a pot from storage and heating of the storage by solar radiation. A paraboloidal concentrator focused solar radiation through a secondary reflector onto a central circular zone of the storage block through the cavity in the insulation. The storage was charged for a set period of time and heat was subsequently discharged to a pot of water. In these simulations a pot of cold water was placed on the hot storage block and the time then estimated until the water either boiled or the temperature of the water reached a maximum value. Simulations were made for a given pot capacity with the storage block made from either cast iron or granite (rock). The effects on cooker performance were compared for a variety of height to diameter ratios of the storage block and size of the area of solar input zone. Bollin [207] proposed a detailed study about the transparent insulation in various solar applications including solar cookers with thermal energy storage.

where g is energy efficiency, mw is water mass, cpw is specific heat of water, Twf is final temperature of water, Twi is initial temperature of water, t is time, It is total instantaneous solar radiation and Asc is intercept area of solar cooker.

h i T wf exergy output Exo mw cpw ðT wf  T wi Þ  T o ln T wi =t h i ¼ w¼ ¼ exergy input Exi a Asc It 1  4T 3T s

ð14Þ

where w is exergy efficiency, To is outside temperature, Ta is ambient temperature and Ts is sun temperature. Kumar et al. [208] investigated a truncated pyramid type solar box cooker (TPSBC) in terms of exergy and energy efficiencies. Two cooking vessels which filled 2 L of water were used for conducting full load test. During the test period, the booster mirror was covered with black cloth. The water temperature inside the vessels reached 90.6 °C from 60 °C in 70 min whereas the initial water and ambient temperatures were 43.18 °C and 33.43 °C, respectively. The maximum and minimum values of insulation were observed as 929 W/m2 and 376 W/m2, respectively. The maximum and minimum energy gained from water inside the solar cooker was calculated 20.8 kJ and 7.5 kJ, respectively. An interesting result in the article was the shift in the output exergy peak from that of the output energy peak on the time scale, which is a direct consequence of the decrease in the exergy lost after the water temperature became >60 °C. In addition exergy analysis of solar box cookers was a practical, comprehensive and realistic tool for solar cookers’ performance evaluation. The schematic view of TPSBC is illustrated in Fig. 21. It is necessary to determine the exergy of incoming solar radiation for conducting second law analysis of solar cookers. In this context, Petela [118] defined an expression for the utilizable part of the solar energy as follows:

w¼1þ

 4 1 T0 4 T0  3 T 3 T

ð15Þ

where w is maximum efficiency ratio, T0 is ambient temperature and T is absolute temperature. It is understood from the Eq. (15) that for T0 = 300 K and T = 6000 K, approximately 0.93 G is the utilizable part of the incoming energy where G is the illumination intensity.

6. Thermodynamic assessment of solar cookers Energy and exergy analysis provide an alternative means of evaluating and comparing solar cookers. Ozturk [115] defined energy and exergy efficiency for the solar cookers as given in Eqs. (13) and (14), respectively. Several studies were carried out about this topic. However, the first study on energy and exergy efficiencies of solar cookers was conducted by Ozturk [115]. It was stressed in his article that there was large difference in energy

ð13Þ

Fig. 21. Schematic model of TPSBC with cooking vessels [208].

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7. Methods to enhance solar cooking performance There are many opportunities in order to improve the performance of solar cookers. First of all, amount of absorbed solar energy may be increased via a concentrating system. Fresnel lens is a good choice to achieve this purpose. Especially in recent years, many applications of Fresnel lens have been recorded in not only solar cookers but also other solar energy technologies [209–214]. However, if a photovoltaic cell is considered, when the PV cell is supported with a Fresnel lens it definitely should be cooled by an efficient cooling system for a desired increment in power output. Otherwise, as reported by Wu et al. [215] efficiency of the cell dramatically decreases depending on the huge temperature increase of the cell. Amount of solar energy falling on the surface of a solar cooker can also be enhanced with reflecting mirrors or surfaces. Secondly, thermophysical properties of the absorber tray play an important role on the performance parameters of solar box cookers. Absorber trays should be selected from materials with high thermal conductivity and painted black. It is also possible to develop new materials with higher absorptivity coefficients. As recommended by Harmim et al. [67], absorber plate can be constructed with extended surfaces in order to enhance the heat transfer from absorber tray to food in the cooking vessels. Saxena et al. [20] reported a cooking vessel modified to reduce the cooking time for a solar box cooker. The cooking vessel had a trapezoidal shape which absorbs a good amount of solar radiation due to its exposed surface area and made of aluminium with a 150 mm bottom end diameter and 180 mm top end diameter. A series of lugs in a curvature form at the bottom of vessel was provided as shown in Fig. 22 to enhance the heat transfer. The lid became hot and generated a current of hot air, which circulated inside the box cooker. The heat carrying by this hot air circulation, reached to the food via the most sides of the vessel. A heat transfer between food and the lid took place by means of convection in the air layer between the food and the lid. The air convection was effective in transferring heat from the food to the lid and vice versa. The total depth of the cooking vessel was 600 mm + 40 mm. The radius of curvature of a lug was 2.5 mm. To measure the temperature of cooking fluid stored in the modified cooking vessel during the testing a lid holder openable knob (screw threaded) was provided on the top of cooking vessel. There was also a locking system of lid to the cooking vessel for proper closing. The testing was performed

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to determine the cooking power. Thirdly, an efficient and low cost insulation should be provided in order to avoid heat loss from the walls of the cooker to the ambient. Transparent insulation materials (TIMs) are highly recommended by many researchers for the insulation of glazing [105,106]. Finally, solar cookers should be used with thermal energy storage materials (water, rock, pebble, PCMs, etc.) to enable late evening cooking. 8. Environmental impacts of solar cookers Nandwani [216] carried out a study on the ecological benefits of solar cookers. The study aimed at estimating the energy used for cooking in Costa Rica and comparing advantages and limitations of solar ovens with conventional firewood and electric stoves. The payback period of a common hot box type solar oven, even if used 6–8 months a year, was found to be around 12–14 months. Even if only 5% of persons facing fuel shortages in the year 2005 use solar ovens, roughly 16.8 million tons of firewood will be saved and the emission of 38.4 million tons of carbon dioxide per year will be prevented according to the results. Escobar [217] proposed a low cost solar cooker which was designed and developed at the School of Physics seeking to reduce the consumption of wood as an energy source. According to National statistics, this source of energy represents 53% of the primary energy consumed in the country. The solar cookers were made with cardboard, glass, aluminium foil iron sheet, and vegetable residues as thermal insulator, other insulators were polyurethane residue which testing has determined its thermal resistance. The economics savings by using the prescribed cooker in terms of wood burning and electricity were properly highlighted. Wentzel and Pouris [14] investigated the development impact of solar cookers in South Africa. Their observations were based on field tests in South Africa that started in 1996 to investigate the social acceptability of solar cookers and to facilitate local production and commercialization of the technology. It was concluded that only 17% of solar cooker owners do not use their stoves after purchase. Active solar cooker users utilise their stoves on average for 31% of their cooking incidences. Solar cooking technology may be a very good opportunity especially in rural areas of developing world in order to avoid deforestation. Solar cookers are quite attractive to deal with the health problems in developing countries caused by firewood use and minimize the CO2 emission all over the world.

Fig. 22. A modified cooking pot for solar box cookers [20].

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9. Future potential of solar cookers As reported by Panwar et al. [4], renewable energy resources will play an important role in the world’s future. According to the global renewable energy scenario, proportion of the solar thermal applications will be about 480 million tons oil equivalent by 2040 [218]. Average cost of solar cookers decreases day by day on the contrary their power output and efficiency considerably increases. In the upcoming future, widespread use of this technology is expected hopefully not only in developing countries but also throughout the world. Nowadays, solar cookers are also available to use in the areas with limited solar radiation depending on the developments in solar power concentrating systems and material technology. In addition, the most challenging point of solar cookers, unavailable to use when sun goes away, is overcome with thermal energy storage techniques. Briefly, it is anticipated that solar cooking technology will be demanded by a huge group of people in the near future because of its outstanding features. 10. Conclusion In this study, a comprehensive review of the available literature on solar cookers is presented. The review covers a historic overview of solar cooking technology, detailed description of various types of solar cookers, performance analysis and thermodynamic assessment of solar cookers, novel designs on solar cooking technology, key items to enhance solar cooking efficiency and also ecological aspects of solar cookers. Specific findings obtained in the review are given as follows:  Fresnel lenses or at least two booster mirrors should be used in solar cookers in order to maximize the incoming solar radiation.  Glazing should be double for a satisfactory insulation.  Absorber plate/tray should be painted black and augmented with extended surfaces for better heat transfer.  Cylindrical shaped cooking vessels made of aluminium or cooper and painted black should be preferred for a higher cooking efficiency.  TIMs should be utilized between glazings in order to avoid notable heat loss from the top of the cooker.  Glasswool, rockwool, strawboard or sawdust can be used for the insulation of side walls and bottom.  To enable late evening cooking water, rock, pebble, PCMs, etc. should be utilized as thermal energy storage material beneath the absorber tray.  Maximum payback period of solar cookers is about 2 years and this time may be shorter depending on the design, frequency of use and location.  Solar cooking technology is a key item in order to deal with deforestation and environmental pollution.

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