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available energy resource. Among the thermal applications of solarenergy, solar cooking is considered as one of the simplest, the mostviable and attractive options in terms of the utilization of solarenergy [12].

Wood is still the primary energy source in much of the develop-ing world since it is seen the cheapest way to obtain the energyrequired. However, this situation causes some serious ecological

part of the world are blessed with abundant solar radiation withmean daily illumination intensity in the range of 57 kW h/m2

and have more than 275 sunny days in a year [17,18]. From thispoint of view, it can be easily said that solar cookers have a big po-tential in these countries in order to meet the energy demandespecially in the domestic sector. In addition, utilization of solarcookers provides many advantageous like no recurring costs, highnutritional value of food, potential to reduce drudgery and highdurability [17]. Hence, in this paper, a comprehensive review of so-lar cooking technology is presented. Appropriate recommenda-

Corresponding author.

Applied Energy 102 (2013) 13991421

Contents lists available at

lseE-mail address: laxec5@nottingham.ac.uk (E. Cuce).after the oil crisis of 1973 with soaring fuel prices, a strong stimu-lation of research into renewable energy technologies is observed.Currently, renewable energy resources supply about 14% of totalworld energy demand and their future potential is remarkable[3,4]. Among the clean energy technologies, solar energy is recog-nized as one of the most promising choice since it is free andprovides clean and environmentally friendly energy [1,510]. TheEarth receives 3.85 million EJ of solar energy each year [11]. Solarenergy offers a wide variety of applications in order to harness this

for poor families. Besides the environmental and economic burdenof rewood use, there are some serious health problems such asburns, eye disorders and lung diseases originate from the utiliza-tion of rewood [13]. It is also emphasized by the World HealthOrganization (WHO) that 1.6 million deaths per year are causedby indoor air pollution [16]. Therefore, there is a rising attentionconcerning the renewable energy options to meet the cookingrequirements of people in developing countries. It is well-knownthat most of the thickly populated countries from the developing1. Introduction

Energy is a thermodynamic quantthe capacity of a physical system tomeaning, energy is vital for our rel[1]. Research to resolve problems reicant since life is directly affected b[2]. Fossil fuel-based energy resourchighest share in global energy cenergy generation becomes more adue to the growing signicance of en0306-2619/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.09.002t is often understood asrk. Besides its physicalwith the environmentenergy is quite signif-y and its consumptionpredominate with the

ption. However, cleanre crucial day by dayental issues. Especially

problems such as deforestation [13]. Especially in rural areas ofAfrica, a major amount of total available energy resource is utilizedfor cooking. The energy required for cooking is supplied by non-commercial fuels like rewood, agricultural waste, cow dung andkerosene [14]. Similarly, in India, energy demand for cooking ac-counts for 36% of total primary energy consumption. As reportedby Pohekar et al. [15], 90% of rural households in India are stilldependent on biomass fuels. People in rural areas are left no choicebut to walk several kilometers every day to collect rewood. Onthe other hand, people in urban areas spend too much money onrewood which can be considered a major expenditure especiallyExergy cookers are also considered in the study. 2012 Elsevier Ltd. All rights reserved.A comprehensive review on solar cooker

Erdem Cuce , Pinar Mert CuceSchool of the Built Environment, University of Nottingham, University Park, NG7 2RD N

a r t i c l e i n f o

Article history:Received 24 May 2012Received in revised form 7 August 2012Accepted 2 September 2012Available online 1 October 2012

Keywords:Solar cookerEfciencyCooking powerPCM

a b s t r a c t

In this paper, a thorough rperformed in a thematic wndings obtained by reseaThe review covers a historof solar cookers, geometryglazing, absorber plate, coassessment of solar cookiners are analyzed in detail. Care illustrated and furthecooking systems are prese

Applied

journal homepage: www.ell rights reserved.gham, UK

w of the available literature on solar cookers is presented. The review isin order to allow an easier comparison, discussion and evaluation of theers, especially on parameters affecting the performance of solar cookers.verview of solar cooking technology, detailed description of various typesrameters affecting performance of solar cookers such as booster mirrors,g pots, heat storage materials and insulation. Moreover, thermodynamicstems and qualitative evaluation of thermal output offered by solar cook-plex designs of solar cookers/ovens with and without heat storage materialre possible methods to be able to enhance the power outputs of solar. Feasibility analysis, environmental impacts and future potential of solar

SciVerse ScienceDirect

Energy

vier .com/locate /apenergy

tions are made in order to enhance current performance of solarcookers 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 rst experiments on solarcookers were carried out by a German Physicist named Tschirn-hausen (16511708). In 1767, FrenchSwiss Physicist Horace deSaussure attempted to cook food via solar energy. He constructeda miniature greenhouse with 5 layers of glass boxes turned upsidedown on a black table and reported cooking fruit [20]. English

rst US solar cookbook, Solar Cooking Naturally, in 1983 [20].Mullick et al. [24] presented a method to analyze the thermal per-formance of solar cookers in 1987. In 2000, Funk [25] proposed aninternational standard for testing solar cookers. It was observedthat the resulting solar cooker power curve is a useful device forevaluating the capacity and heat storage ability of a solar cooker.Especially in recent years, intensive efforts have been made to beable to enhance the cooking power capacity of solar cookers.Numerous analytical, numerical and experimental studies on noveldesigns of solar cookers have been carried out by many research-ers. Today, solar cooking technology is very promising with itspotential in order to narrow the gap between renewable and con-

panel cookers utilize reective equipment in order to direct sun-

They also indicated that the prescribed system can be used for

1400 E. Cuce, P.M. Cuce / Applied Energy 102 (2013) 13991421astronomer Sir John Herschel attempted to cook food in a similarinsulated box on an expedition to South Africa in 1830. A FrenchMathematician Augustin Mouchot integrated the heat trap ideawith that of the burning mirror in 1860 and built an efcient solaroven. He also succeeded to create a solar steam engine but it wastoo large to be practical. In 1876, W. Adams developed an octago-nal oven equipped with 8 mirrors and he reported that the ovencooked rations for 7 soldiers in 2 h [21]. One year later, Mouchotdesigned solar cookers for French soldiers in Algeria, including ashiny metal cone, made from a 105.5 section of a circle [20]. Healso wrote the rst book on Solar Energy and its Industrial Applica-tions. In 1891, Clarence Kemp, an American plumbing and heatingmanufacturer, invented the rst commercial solar water heater forbathing and dishwashing. In 1894, Xiaos Duck Shop in Sichuan,China, roasted ducks via the principle of solar cooking.

In 1930s, France sent many solar cookers to its colonies in Afri-ca. On the other hand, India began to investigate solar energy as anoption for avoiding deforestation. In 1940s, Dr. Maria Telkes in theUSA analyzed various types of solar cookers including some heatstorage materials also published a book named Solar Ovens in1968 [19,20]. The rst commercial box-type solar cooker wasproduced by an Indian pioneer named Sri M.K. Ghosh in 1945[22]. In 1950s, Indian researchers devised and constructed com-mercial solar ovens and solar reectors, but they were not readilyaccepted due to the lower-cost alternatives. Also, United NationsFood and Agriculture Association (FAO) investigated water-heatingcapacities of a parabolic cooker and an oven type cooker. In 1961, aUnited Nations Conference on New Sources of Energy includingmany authorities on solar cooking technology was held. In 1970s,as a result of the increasing fuel prices due to the oil crisis, anintensive interest on renewable energy technologies was observedworldwide especially in China and India [23]. Barbara Kerr in theUSA constructed several types of concentrating and box-type solarcookers using recycling materials and aluminium foil. In 1979,water pasteurization was performed using box-type solar cookersby Dr. Metcalf and his student Marshall Longvin. In 1980s,especially the Governments of India and China expanded nationalpromotion of box-type solar cookers. Heather Gurley Larson wroteFig. 1. Types of solar cookers: (a) solar panel cooker; (bcooking and food preserving purposes.light to a cooking vessel which is enclosed in a clear plastic bag.Solar panel cooker of Dr. Roger Bernard (CooKit) is one of the mostpopular designs in this category [17]. Only cardboard and foilshaped was utilized to manufacture the CooKit. It was an afford-able, convenient and effective solar cooker which enabled topreserve nutrients without burning or drying out. Bernard alsoinvestigated how the solar cooking technology is taken up by pop-ulations [27]. Performance of solar panel cookers highly depend onreected radiation thus, they do not seem effective under cloudyconditions [28]. In recent years, some efforts have been made in or-der to expand the utilization areas of panel cookers. Kerr and Scott[29] designed and built a solar powered apparatus for sterilization.ventional power sources.

3. Solar cookers

A solar cooker or solar oven is a device which utilizes solarenergy to cook food. Solar cookers also enable some signicantprocesses such as pasteurization and sterilization. It is a clear factthat there are countless styles of solar cookers in the world andthey are continually improved by researchers and manufacturers.Therefore, classication of solar cookers is a hard work. However,it may be asserted that most of the solar cookers today fall withinthree main categories called solar panel cookers, solar box cookersand solar parabolic cookers as shown in Fig. 1.

3.1. Solar panel cookers

Solar panel cookers may be considered the most common typeavailable due to their ease of construction and low-cost material. Insolar panel cookers, sunlight is concentrated from above [26]. Thismethod of solar cooking is not very desirable since it provides alimited cooking power. On the other hand, this type of solar cook-ers is highly appreciated by people living or travelling alone. Solar) solar parabolic cooker; and (c) solar box cooker.

Ener3.2. Solar box cookers

History of solar cooking technology started with the inventionof box-type solar cookers. The rst solar box cooker was inventedby a FrenchSwiss naturalist named Horace de Saussure in 1767.Especially in the twentieth century, this solar cooker type demon-strated a considerable development in terms of design and perfor-mance parameters. A solar box cooker basically consists of aninsulated box with a transparent glass cover and reective surfacesto direct sunlight into the box [20]. The inner part of the box ispainted black in order to maximize the sunlight absorption. Maxi-mum 4 cooking vessels are placed inside the box [30,31]. A detaileddescription of solar box cookers is illustrated in Fig. 2. Each compo-nent of the box cooker has a signicant inuence on cookingpower. Therefore, optimization of these parameters is vital forobtaining maximum efciency.

It is observed from the cooker designs of 1950s that the food isdirectly exposed to sunlight [3234]. Telkes [35] focused on boxtype cookers and noted that they are slow to heat up, but workwell even where there is diffuse radiation, convective heat losscaused by wind, intermittent cloud cover and low ambient temper-atures [36]. At the beginning of 1960s, Schaeffer [37] presented areport on the current situation of solar box cookers. In the follow-ing years, outdoor testing of box-type solar cookers was carried outby several researchers [3842]. Garg et al. [43] compared

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

E. Cuce, P.M. Cuce / Appliedperformances of ve available solar cookers [44].After the 1980s, researchers especially focused on optimization

of geometry parameters of solar box cookers since they have adominant effect on performance. In this context, some researchersanalyzed the booster mirror effect on efciency of box-type solarcookers. Dang [45] investigated the concentrators for at platecollectors and explained that booster mirrors can be utilized in or-der to increase the efciency of solar collectors since it provides ex-tra solar radiation. The results indicated that the effectiveness ofconcentrators highly depends on the angle of mirrors. Garg andHrishikesan [46] presented a comprehensive analysis of a systemconsisting of a at plate collector integrated with two reectors.They proposed a model which was numerically simulated for con-ditions prevailing in three different Indian stations for three differ-ent months. They found that the enhancement is maximum for themonth of December in all the three stations for both horizontal andtilted surfaces. Narasimha et al. [4750] comprehensively analyzedthe solar cookers augmented with booster mirrors. They provided asingle adjustable booster mirror to a solar box cooker andcalculated the total energy falling on the cooking aperture for thelatitude of 18N (Warangal City, India) and for ve different decli-nations of the sun. The results showed that the total energy wasenhanced at all hours of the day by intermittent adjustment, con-tinuous adjustment and xed orientation of the supporting mirror[47]. They also analyzed elongation effect (ratio of length/width ofbooster mirror) on total energy collection. Rectangular apertureswere found more efcient than the equal are of square aperturein terms of total energy absorbed. On the other hand, the efciencywas approximately the same for a value of elongation [48]. Energycontribution by the booster mirror became increasingly signicantwith an increase in latitude of the location [49]. El-Sebaii et al. [51]constructed and tested a box-type solar cooker with multi-stepinner reectors. A transient mathematical model was proposedfor the cooker. The transient performance of the cooker was deter-mined by computer simulation for typical summer and winter daysin Tanta, Egypt. They observed that the cooker is able to boil 1 kg ofwater in 24 min when its aperture area equals 1 m2. Habeebullahet al. [52] introduced an oven type concept to minimize theamount of heat losses andmaximize the concentrated solar energy.They expressed that if the solar box cooker is augmented with fourbooster mirrors, heat losses due to wind will reduce since windwill not be in direct contact with the glazed surface. Results ofthe mathematical model indicated that oven type receiving pothas both a higher uid temperature and overall receiver efciencycompared to the bare receiver type, working under similar condi-tions. El-Sebaii and Aboul-Enein [53] presented a transient mathe-matical model for a box-type solar cooker with a one-step outerreector hinged at the top of the cooker. The model was basedon analytical solution of the energy balance equations usingCramers rule for different elements of the cooker. The boilingand characteristic boiling times of the cooker were decreased by50% and 30%, respectively, on using the cooker around midday.Buddhi et al. [54] designed and analyzed a solar cooker augmentedwith three reectors and a phase change material storage unit. Theexperimental results showed that late evening cooking is possiblein the solar cooker proposed. Algifri and Al-Towaie [55] carried outa research in order to nd out effect of the cooker orientation on itsperformance. The analysis was applied to a cooker placed at Aden,Yemen. They found that the reector tilt angle and the elevationangle are related by the relationship 3R 2a 180 and the cookerwhich satises this condition gives the best performance. Mirdhaand Dhariwal [56] theoretically investigated several designs ofsolar cookers in order to optimize their performance. Various com-binations of booster mirrors were analyzed as shown in Fig. 3 to beable to arrive at a nal design, aimed at providing a cooker, whichcan be xed on a south facing window. The results indicated thatthe proposed new cooker can provide higher temperature through-out the day and round the year. They also noted that the cooker canbe used for preparation of two meals in a day and to keep the foodwarm in late evening.

Some researchers focused on glazing factor in solar box cookers[5762]. It is well known from the literature that there are variousglazing materials such as glass, breglass, and acrylics which arecommonly used in box-type solar cookers. Single pain glass anddouble pain glass are the most common structures which enableto receive a higher solar transmission. Optimization of the gapbetween panes is a signicant problem since a large air gap mayencourage convective heat transfer and cause a heat loss. In litera-ture, recommended air gap depth varies from 1 to 2 cm [20,5759].Absorption of long wave radiation emitted by collector plates in-creases the glass temperature and this increment causes heat lossfrom the cooker to the surrounding atmosphere. Therefore, trans-parent insulating materials are suggested in order to improve theefciency of solar box cookers [63,64].

Absorber tray is one of most signicant component of a solar

gy 102 (2013) 13991421 1401box cooker. Solar radiation passes through the glazing part and ab-sorbed by a surface painted black called absorber tray. An absorbertray rst of all should have a remarkably high absorptivity in order

1402 E. Cuce, P.M. Cuce / Applied Energy 102 (2013) 13991421to transfer maximum radiant energy to food in the cooking pot[65,66]. Harmim et al. [67] experimentally investigated a box-typesolar cooker with a nned absorber plate as shown in Fig. 4. Testswere carried out on the experimental platform of the RenewableEnergies Research Unit in Saharan Environment of Algeria at Adrar.The results indicated that solar box cooker equipped with ns wasabout 7% more efcient than the conventional box-type solar coo-ker. The time required for heating water up to boiling temperaturewas reduced about 12% when a nned absorber plate was used.

Fig. 3. (a) Conventional box-type solar cooker with south facing mirror; (b) solar box cooktilted collecting surface and north facing mirror; and (d) cooker with south tilted collec

Fig. 4. (a) Schematic of the nned absorber plate; (b) conComparative results are illustrated in Fig. 5. Pande and Thanvi[68] designed, developed and tested an efcient solar cooker. Thesignicant part of the proposed cooker was its stationary modeand maximum capture of energy through improved design andoptimum tilt of the system. They found that the cooker could saveabout 40% of the cooking fuel via the proposed absorber. Shrestha[69] concluded that if the external surface of the absorbing topplate is treated with selective coating, it demonstrates a betterperformance compared to the simple black coated absorber tray.

er with south tilted collecting surface and south facing mirror; (c) cooker with southting surface, north facing mirror and a xed south facing vertical mirror [56].

ventional (A) and improved (B) solar box cooker [67].

They also found that cooking vessel with central annular cavity onlugs performs much better than the conventional vessel kept onthe oor of the cooker [81]. Reddy and Narasimha Rao [82] com-pared performances of conventional solar box cooker and im-proved cooker having cooking vessel with central annular cavity

een

Fig. 6. Solar box cooker with a conventional cylindrical cooking vessel on the oorof the cooker and another vessel with central annular cavity kept on three lugsspaced at 120 [82].

EnerThulasi Das et al. [70,71] carried out some simulation analysis onperformance parameters of solar box cookers like the thicknessand size of the absorber plate, emissivity of the vessels and insula-tion thickness. Anderson et al. [72] investigated performance ofcoloured solar collectors. They showed that coloured solar collectorabsorbers can make remarkable contributions to heating loads.Although their thermal efciency is lower than highly developedselective coating absorbers, they offer the advantage of sensitiveintegration with buildings. Tripanagnostopoulos et al. [73,74] alsoanalyzed coloured absorbers. They obtained that unglazed collec-tors with coloured absorbers are in general of low efciency andmight be used in low temperature solar applications. Amer [75]presented a novel design of solar cooker in which the absorber isexposed to solar radiation from the top and the bottom sides. Aset of plane diffuse reectors was used to direct the radiation ontothe lower side of the absorber plate. Results under the same oper-ating conditions showed that the absorbers of the solar box cookerand the double exposure cooker attain 140 and 165 C, respec-tively. Kumar [76] carried out a thermal analysis in order to eval-uate natural convective heat transfer coefcient in a trapezoidalenclosure of box-type solar cooker. It was underlined that themajor advantage of using a trapezoidal shaped absorber tray isthe absorption of a higher fraction of incident solar radiation fallingon the aperture at larger incident angles, due to a more exposedsurface area. Ogunwole [77] designed, constructed and test a solarcooker which absorber was a square base pot, blackened with

Fig. 5. (a) Finned and ordinary absorber plate temperatures and (b) comparison betw[67].

E. Cuce, P.M. Cuce / Appliedsmoke and was made of stainless steel. In the design, aluminiumfoil was used as reectors. An average temperature of 100 C wasobtained from the cooker for an ambient temperature of 34 C.

Any type of cooking vessel can be used in solar box cookers butgenerally cylindrical shaped cooking vessels made of aluminium orcopper are recommended. As reported by Saxena et al. [20], num-ber of cooking vessels in a solar box cooker may vary depending onthe quantity and the nature of the food. Khalifa et al. [78]conducted some experiments on an Arafa cooker, basically a pointfocus concentrator featured with Pyrex pots. The tracking was per-formed manually for every 1520 min. It was observed that cook-ing food by directly reected solar radiation decreases the cookingtime. Gaur et al. [79] revealed that performance of a solar cookermay be improved if a utensil with a concave shape lid is used in-stead of a plain lid. Narasimha Rao and Subramanyam [80,81]investigated effects of some modications on cooking vessels andanalyzed performance enhancement of solar box cookers. They ob-served that raising the cooking vessel by providing a few lugswould make the bottom of the vessel a heat transfer surface. Thischange would improve the performance of the system by improv-ing the heat transfer rates in both heating and cooling modes [80].internal air temperatures of cooker A and internal air temperatures of cooker B

gy 102 (2013) 13991421 1403as it is illustrated in Fig. 6. The experiments were conducted forseveral days using water and thermic uid as working medium.The results indicated that when the vessel with central annularcavity is placed on lugs in the cooker interior, the hot air circulationthrough the gap between the bottom of the cooking vessel and theoor of the cooker and through the central annular cavity improvesthe heat transfer to the water in the vessel and results in the reduc-tion of cooking time. Harmim et al. [83] experimentally investi-gated a box-type solar cooker with two different cooking vessels:the rst one conventional and the second one identical to the rstin shape and volume but its external lateral surface augmentedwith ns. They found that cooking time considerably reduces withthe nned design. The average difference in power was calculated7.49W. Srinivasan Rao [84] analyzed the effects of ns attachedinside the central cavity on cooker performance. A maximum tem-perature gain of 17 C was observed with new design of cookingvessel in comparison of conventional type.

Some researchers performed intensive efforts on solar boxcookers in order to allow late evening cooking. In this context, agreat deal of solidliquid phase change materials (PCMs) wereinvestigated for heating and cooling applications [8592]. At theend of 1980s, Ramadan et al. in Tanta University [93] augmented

Ener1404 E. Cuce, P.M. Cuce / Applieda simple at plate solar cooker with a jacket of sand as heat storagematerial. They observed a considerable longer cooking period withheat storage medium. Six hour per day of cooking time wasreported. Haraksingh et al. [94] used coconut oil as the heat trans-fer uid in a double-glazed at plate collector solar cooker. Tem-peratures of approximately 150 C were achieved between 10:00and 14:00. Nandwani et al. [95] constructed a solar hot box withtwo similar compartments. They compared the behaviour of ametallic slab lled with a phase change material for short termstorage with that of a conventional absorbing sheet. Advantage ofthe heat storage material could not be conrmed due to some rea-sons like high transition temperature and low quantity of PCM aswell as losses while opening the door. Sharma et al. [96] investi-gated thermal performance of a prototype solar cooker based onan evacuated tube solar collector with PCM storage unit. The de-sign had separate parts for energy collection and cooking coupledby a PCM storage unit as shown in Fig. 7. It was observed nooncooking did not affect the evening cooking and evening cookingusing PCM heat storage was found to be faster than noon cooking.They also noted that the system is expensive but shows good po-tential for community applications. Hussein et al. [97] experimen-tally investigated a novel indirect solar cooker with outdoorelliptical cross section integrated indoor PCM thermal storageand cooking unit. Magnesium nitrate hexahydrate (Tm = 89 C,latent heat of fusion 134 kJ/kg) was used as the PCM inside theindoor cooking unit of the cooker. They found that the cooker pro-posed can be used for heating or keeping the meals hot at night andearly morning for breakfast of the next day. Chen et al. [98] numer-ically studied PCMs used as the heat storage media for solar boxcookers. Magnesium nitrate hexahydrate, stearic acid, acetamide,

Fig. 7. Schematic of the solar cooker based on evacuategy 102 (2013) 13991421acetanilide and erythritol were selected as PCMs. For a two-dimen-sional simulation model based on the enthalpy approach, calcula-tions were made for the melt fraction with conduction only.Stearic acid and acetamide were found to be good compatibilitywith latent heat storage system. It was also noted that the initialtemperature of PCM does not have very important effects on themelting time. El-Sebaii et al. [99] utilized acetanilide and magne-sium chloride hexahydrate as PCM in solar box cooker and ob-tained 134 C of stagnation temperature. They also presentedtransient mathematical models of single slope-single basin solarstill with and without PCM under the basin liner of the still[220]. Oturanc et al. [100] constructed and tested a solar box coo-ker which uses engine oil as heat storage material. It was observedthat 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 onan oil-pebble bed thermal energy storage system for a solar cooker.

It is well known from the literature that insulation is one ofmost crucial key points for a solar cooker to provide an efcientcooking [103,104]. Insulation in a solar box cooker should not belimited to the walls of the frame box and absorber tray since aremarkable amount of heat loss occurs through the glazing [20].In this context, Nahar et al. [105,106] carried out some studieson utilization of transparent insulation material (TIM) in solarbox cookers. Under an indoor solar simulator, they tested a hotbox solar cooker with glazing surface consisting 40 and 100 mmthick TIM. The stagnation temperature with the 40 mm TIM wasfound to be 158 C, compared with 117 C without the TIM [105].A double reector hot box solar cooker with TIM was designed,constructed, tested and its performance was compared with a

d tube solar collector with PCM storage unit [96].

TIM

Enersingle reector hot box solar cooker without TIM. Fig. 8 depicts theeld installation of the proposed cookers. 40 mm thick honeycombmade of polycarbonate capillaries was placed between two glazingsurfaces in order to minimize the heat loss due to convection. Theefciencies were determined to be 30.5% and 24.5% for the solarbox cooker with and without TIM, respectively. Energy saving byusing a solar cooker with TIM was estimated to be 1485 MJ of fuelequivalent per year [106]. Mishra and Prakash [107] evaluated thethermal performance of solar cookers with four different insulationmaterials readily available in rural areas. Their performance wascompared with that of the glass wool. It was aimed at minimizingthe cost of the cooker with a view to enhance its widespread appli-cation in the rural Indian environment. Bjork and Enochsson [108]experimentally investigated three different insulation materials(glass wool, melamine foam and corrugated sheets of celluloseplastics) in terms of condense formation, drainage and moisturedependent heat transmittance. It was noted that the all materialsprovide best insulation in dry form. Nyahoro et al. [109] carriedout a simulation study on an indoor, institutional solar cooker.

Fig. 8. (a) Double reector solar box cooker with

E. Cuce, P.M. Cuce / AppliedThe cooker storage unit consisted of a cylindrical solid block andit was insulated by a material with thermal conductivity of0.1 W/mK and specic heat capacity of 1000 J/kgK.

3.3. Solar parabolic cookers

The rst solar parabolic cooker was developed by Ghai [110] inthe early 1950s at the National Physical Laboratory, in India. Then,Lof and Fester [111] investigated various geometries and mountingcongurations of parabolic cookers. These type of cookers attractedpeople immediately all over the world due to their outstandingperformance. Solar parabolic cookers can reach extremely hightemperatures in a very short time and unlike the panel cookersor box cookers, they do not need a special cooking vessel. However,a parabolic cooker includes risk of burning the food if left unat-tended for any length of time because of the concentrated power.A solar parabolic cooker simply consists of a parabolic reectorwith a cooking pot which is located on the focus point of the cookerand a stand to support the cooking system.

Ozturk [112115] conducted several experimental researcheson solar parabolic cookers and analyzed the performance parame-ters in terms of thermodynamic laws. Ozturk experimentallyexamined energy and exergy efciencies of a simple design andthe low cost parabolic cooker under the climatic conditions ofAdana which is located in Southern Turkey (at 37N, 35E). Theenergy output of the parabolic cooker was determined to be20.978.1 W, whereas its exergy output was in the range of 2.96.6 W. The results showed that the energy and exergy efcienciesof the parabolic cooker were calculated between 2.815.7% and0.41.25%, respectively [114]. He also compared energy and exergyefciencies of box-type and parabolic-type solar cookers. Experi-mental study indicated that the power output of the box-type coo-ker ranged from 8.2 to 60.2 W, whereas it varied between 20.9 and73.5 W for the parabolic cooker. On the other hand, the exergy out-put of the solar box cooker ranged from 1.4 to 6.1 W, whereas itwas in the range of 2.9 to 6.6 W for the parabolic cooker. It was alsoobserved that the energy and exergy efciencies of the box-typeand the parabolic-type cookers were in the range of 3.0535.2%,0.583.52% and 2.7915.65%, 0.41.25%, respectively [115]. Arenas[116] described a portable solar kitchen with parabolic solar reec-tor that folded up into a small volume. The experimental studyindicated that the solar cooker reached an average power outputof 175W, with an energy efciency of 26.6%.

Al-Soud et al. [117] designed, operated and tested a parabolic

and (b) conventional hot box solar cooker [106].

gy 102 (2013) 13991421 1405cooker with automatic two axes sun tracking system. The testresults showed that the water temperature inside the cookers tubereached 90 C when the maximum registered ambient temperaturewas 36 C. A parabolic cooker was investigated from the exergyviewpoint by Petela [118]. According to the results, the exergyefciency of parabolic cooker was relatively very low approxi-mately 1% while the energy efciency ranged from 6% to 19%. Shu-kla [119] presented the energy and exergy efciencies of two typesof parabolic solar cookers which were tested in summer and winterin the climatic conditions of India. The results showed that the en-ergy output of the community and domestic solar cookers variedfrom 2.73 to 43.3 W and 7.77 to 33.4 W, respectively whereasthe exergy output of the cookers ranged from 1.922.58W to0.651.45 W, respectively. On the other hand, the energy efcien-cies of the community and domestic solar cookers were in therange of 8.310.5% to 7.114.0%, respectively. Pohekar and Rama-chandran [120] conducted a survey about present disseminationof nine cooking energy alternatives in India to compare theirtechnical, economic, environmental/social, behavioural and com-mercial issues. Liqueed Petroleum Gas (LPG) stove was foundthe most preferred device, followed by kerosene stove, solar boxcooker and parabolic solar cooker in that order while electric ovenhad the lowest ranking. They also determined utility assessment ofparabolic cooker as a domestic cooking device in India. The studyindicated that if the parabolic cookers have to become a reality

solar box cooker and hence the problem of preheating was solvedas faced in conventional box-type solar cooker. The results showed

d (b

Enerthat the newly designed cooker was more efcient compared to theconventional cooker. Nandwani [133] experimentally and theoret-ically investigated a solar oven in the climatic conditions of Costathe utility has to be increased. They stressed that the advantages ofparabolic cookers in terms of technical, behavioural and commer-cial should be improved [121].

4. Different designs of solar cooking systems

In recent years, researchers highly focused on producing noveldesigns of solar cookers to provide the most appropriate operatingconditions and hence obtain efcient cooking. Nahar et al. [122129] presented numerous studies to enhance the performance ofsolar cookers with low cost modications. Khalifa et al. [130,131]also conducted some studies on new design concentrating type so-lar cookers. Tiwari and Yadav [132] devised a new box-type solarcooker integrated with a single reector at the hood. In their de-sign, the base of the oven acted as the lid unlike the conventional

Fig. 9. Schematic diagram of (a) cylindrical an

1406 E. Cuce, P.M. Cuce / AppliedRica. The cooker was augmented with a reector to increase theillumination intensity on absorbing plate. Maximum plate temper-ature measured was between 130 and 150 C. Thermal efciency ofthe cooking system varied from 30% to 40%. At University of Jordanin the early 1990s, Al-Saad and Jubran [134] developed a low costclay solar cooker. The most outstanding features of the cooker werethat it was made cheap, locally available materials. In addition, noskilled labour was in need in order to operate the cooker. In theirdesign, absorber plate of the cooker was replaced with locallyavailable black stones. Using black stones instead of absorber plateallowed storing solar energy, hence making late cooking possible.Grupp et al. [135] presented a novel box-type solar cooker con-sisted of a xed cooking vessel in good thermal contact with a con-ductive absorber plate. The novel cooker provided easier access tothe cooking pots and less maintenance due to the protection of allabsorbing and reecting surfaces. Outdoor tests also indicated that5 L of water per m2 of opening surface could be brought to full boil-ing in less than 1 h. Nandwani and Gomez [136] experimentallyinvestigated two folding and light solar ovens constructed by SolarBox Cookers International (SBCI) in the climatic conditions of CostaRica. Performances of the cookers were compared with a conven-tional oven during 30 days. The tests were conducted at load andno load condition, and with or without a reector. Cardboard ovenswere found to be 1525% less efcient than the conventional oven.Wareham [137] developed a solar cooker stove calledSUNSTOVE which is an affordable, easy to use, suitable for family,rugged and stackable for shipping. By using the SUNSTOVE, the re-duce fuel consumption decreased the cost of living and helped toimprove the health of the people. The unit of SUNSTOVE held fourpots with 2 L. The cooker pasteurized water in 15 min at 71 C andit did not burn foods. The cookers sides had wings to increase thesolar collecting area to provide for the elimination of reectors andto reduce internal volume to be heated [137]. Beaumont et al. [138]designed a family sized ultra-low cost solar cooker in Tanzania. Thehot box style cooker was developed to be built on site by the userswith minimal tools, skills or special materials. The cooker consistedof a shallow 1 m2 square hole in the ground, insulated with strawand lined with adobe, a glass or plastic roof and a 1 m2 aluminizedplastic reector with guy ropes for adjustment. It provided cookedfor 1012 people on clear days with midday and dusk. A 4 L load ofwater brought up to cooking temperature in 6070 min. Suhartaet al. [139] designed three different solar cookers called HS 7534,HS 7033 and the newest design HS 5521. They carried out variousexperiments for comparison of these cookers cooking performanceand the other parameters. It was calculated oven temperature of202 C between 12:00 and 12:45 p.m. on October in 1997 for type

) rectangular box-type of solar cookers [145].

gy 102 (2013) 13991421of HS 7033. It was found that these solar cookers have a good heatstorage capability; therefore they can be used for consecutivecooking. Volume of HS 5521 was 35% of HS 7033s and it wascheaper than HS 7033. Although it was seen that HS 5521 hadthe same heat collection rate with the others, it was able to cookas fast as HS 7033. Sonune and Philip [140] developed a Fresneltype domestic SPRERI concentrating cooker. The cooker was foundcapable of cooking food for a family which consisted of 4 or 5 peo-ple. The highest plate bottom temperature was calculated 255 C inapproximately 40 min while ambient temperature was 30 C anddirect solar radiation was 859W/m2. Negi and Purohit [141] com-pared the performances of a conventional box type cooker and aconcentrator cooker. The experimental results obtained showedthat the concentrator solar cooker provided stagnation tempera-ture 1522 C higher than the conventional box type cooker usinga booster mirror. It was also observed that the boiling point ofwater with concentrator cooker is reached faster, by 5055 min,than the conventional box type cooker. It was seen that the solarcooker utilizing non-tracking reectors provided increased heatcollection and faster cooking compare to the conventional box typecooker.

El-Sebaii and Ibrahim [142] experimentally tested a solar boxcooker for two different congurations under the weather

Enerconditions of Tanta, Egypt. The experiments were conductedduring July 2002 with and without load. The cooking power (P)was correlated with the temperature difference (DT) between thecooking uid and the ambient air. Linear correlations between Pand DT had correlation coefcients higher than 0.90 satisfyingthe standard. It was also underlined that the improved cookerwas able to cook many kinds of food with an overall efciency of26.7%. In Cornell University, Rachel Martin et al. [143] devised no-vel solar ovens for the developing world. Different types of solarovens like x cooker, bowl cooker, cone cooker, box type cookerand parabolic type cooker were constructed and tested in Nicara-gua in fall of 2005 and the spring of 2006. Nandwani [144] de-signed, constructed and tested a hybrid multifunctional solarcooking system in Costa Rica. The device proposed enabled cook-ing, drying and heating/pasteurizing purposes in a single system.Kurt et al. [145] experimentally investigated the effect of boxgeometry on performance of solar cookers. Two different modelsolar box cookers, which are in rectangular and cylindrical geome-tries as shown in Fig. 9 were constructed using the same materialand tested under the same operating conditions. Performanceparameters of each cooker were determined for 0.5, 1 and 1.5 kgof fresh water. The thermal efciency increased from 12.7% to

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

E. Cuce, P.M. Cuce / Applied36.98% 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. Thecylindrical model provided higher thermal efciency and lowercharacteristic boiling time than the rectangular model. Schwarzerand Silva [146] described four types of solar cookers (at platecollector with direct use, at plate collector with indirect use, par-abolic reector with direct use, parabolic reector with indirectuse) in terms of their basic characteristics and test procedures.They also presented a simplied analytical model to design simplecooking systems.

At Sardar Patel Renewable Energy Research Institute, Kumaret al. [147,148] designed, fabricated and tested a novel solar boxcooker: truncated pyramid-type solar cooker. The truncatedpyramid geometry illustrated in Fig. 10 allowed concentrated theillumination intensity towards the bottom and the glazing surfaceon the top facilitated the trapping of energy inside the cooker. Oneof the salient features of the novel cooker was to totally eradicatethe need of a solar tracking system. Maximum absorber plate stag-nation temperature was determined to be 140 C and water tem-perature inside the cooker reached 98.6 C in 70 min. In additiontwo gures of merit, F1 and F2 were found to meet the standardsprescribed by the Bureau of Indian Standards for solar box-typecookers. They also observed the nancial viability of the devicevia a simple economic analysis [148]. Bello et al. [149] investigatedperformance analysis of a simple solar box cooker in the climaticconditions of Nigeria. The average efciency of the cooker was esti-mated to be 47.56%. It was recommended that the device proposedmight be used as a pre-cooking and alternative to domesticcooking stove.

Grupp et al. [150] developed a metering device for the determi-nation of solar cooker use rate. The device allowed recording foodtemperature, ambient temperature and illumination intensity le-vel. Moreover, the assessment of fuel savings and greenhouse-gasemission reduction compared to other cooking options was avail-able with the proposed system. Zhou and Zhang compared theperformances of two different solar cooking systems by simulationmethod: solar energy storage vessel between vacuum tubecollector and plate collector. The temperature distribution, energyreleasing rate and liquid fractions during the energy releasing pro-cess were compared for summer and winter conditions. The platecollector storage vessel was found more reliable and suitable forthe climatic conditions of Nanjing [151]. Kurt et al. [152] estimatedperformance parameters of solar box cookers with and withoutreector using articial neural network. The experimental dataset consisted of 126 values. 96 values were used for training/learn-ing of the network and the rest of the data for testing/validation ofthe network performance. The results indicated that the thermalperformance parameters of a solar cooker can be determined witha high degree of accuracy via articial neural network.

Hernandez-Luna and Huelsz [153] developed a solar oven forthe intertropical zones and evaluated its performance. Tempera-ture measurements of the oven were performed using 36 thermo-couples type T and the data was recorded by a data acquisitionsystem. Cooking tests showed that the oven is suitable to cookthree basic Mexican meals: beans, nixtamal and corn cobs. A con-servative estimation of the wood savings per solar oven is 850 kgper year which accounts for the 30% rewood used to cook by atypical Mexican rural family. Prasanna and Umanand [154,155]proposed a hybrid solar cooking system where the solar energywas transported to the kitchen. The thermal energy source wasused to supplement the Liqueed Petroleum Gas (LPG) whichwas in common use in kitchens. In the prescribed system, solar en-ergy was transferred to the kitchen by means of a circulating uid.Energy gain from the sun was maximized by changing the ow ratedynamically. It was concluded from the results that as using thenovel cooking system proposed, cooking can be carried out atany time of the day with time taken being comparable to conven-tional systems. Saitoh and El-Ghetany [156] devised a solar water-sterilization system with thermally controlled ow. They carriedout a heat transfer analysis in order to determine the effects ofenvironmental conditions on the behaviour of the system. Thermaland biological tests of the water samples during the sterilizationprocess were obtained. It was found that the proposed systemcan be used in clear-sky areas with a high illumination intensitypotential to produce a large amount of sterilized water. Chaudhuri[157] estimated the electrical backup for an Indian solar cooker tobe able to use the cooker throughout the year. It was found thatapproximately 160 W heater would be sufcient for cooking.Abu-Malouh et al. [158] designed, constructed and tested a spher-ical type solar cooker augmented with automatic sun tracking sys-tem. The system components are illustrated in Fig. 11. Theexperimental results indicated that the temperature inside thepan reached more than 93 C in a day where the maximum ambi-ent temperature was 32 C. It was underlined that this temperatureis suitable for cooking purposes and was obtained by means of atwo axes solar tracking device. All measured parameters in the

gy 102 (2013) 13991421 1407study are depicted in Fig. 12. As it is easily seen from the resultsfor three different days, temperature inside pan and temperatureoutside pan have almost the same behaviour as a function of time.

; (b

EnerOn the other hand, ambient temperature increases from morningtill noon and then it gradually decreases till sunset. In Universityof Nigeria, Ekechukwu and Ugwuoke [159] designed and con-structed a solar box cooker and analyzed its performance for withand without plane reector. The experiments were carried outwith four cooking vessels each capable of holding 1 kg of water.Absorber plate temperatures with and without reector werefound to be 138 and 119 C, respectively. Boiling times for 1 kg ofwater were determined to be 3600 s and 4200 s for with and with-out reector, respectively. Jaramillo et al. [160] developed a novelsolar cooker for intertropical zones called optogeometrical design.In their design, the oven box had seven faces instead of the six

Fig. 11. Spherical type solar cooker: (a) the whole system

1408 E. Cuce, P.M. Cuce / Appliedfaces of most common designs reported in the literature. The mostoutstanding feature of this oven was that the oven needed onlyfour simple movements to be able to obtain sufcient solar con-centration throughout the year. The results showed that, at noon,the solar cooker achieves a concentration level greater than 1.95during the whole year. Mohamad et al. [161] constructed andtested a simple wooden, hot box solar cooker with one reectorunder the climatic conditions of African Sahel Region. It was ob-served that the cooker reached 160 C under eld conditions ofGiza, Egypt. Different types of foods were successfully cooked suchas rice, meat, sh, and beans. The cooking time varied from 1 to2.5 h. Hussain et al. [162] investigated performance analysis of abox-type solar cooker with auxiliary heating. The reason of usingan auxiliary heater was the cloudy days in Bangladesh which makesolar cooking impossible. Six heating elements were connected inseries to generate 150 W heat from 220 V AC source and wereplaced below the absorber plate. It was found that the use ofauxiliary heating equipment allows cooking on most cloudy days.Schwarzer et al. [163] developed indoor and outdoor solar cookerswith or without storage as shown in Fig. 13 for families andinstitutions in different countries of the world. Thermal storagewas provided with a tank which was lled with pebbles. Vegetableoil was used as the working uid which ows in cooper pipes.Approximately 250 systems were constructed in various sizesand installed in different countries for different purposes. It wasstressed in the study that large-scale use of solar cookers in devel-oping countries can only be possible through the developmentwith nancial aid.5. Performance analysis of solar cookers

Thermal performance of solar cookers can be determined by anelaborate analysis of the optical and thermal characteristics of thecooker materials and the cooker design or by experimental testingunder operating conditions [20]. However, as stated by Lahkar andSamdarshi, it is very difcult to compare the cookers performancereported by previous researchers and establish the criteria re-quired for selection of a cooker which can provide a successfuland satisfactory cooking [12]. There are some performance param-eters such as energy and exergy efciency, cooking power, guresof merit, and parameter index which are commonly used for

) the pan and the dish; and (c) the control devices [158].

gy 102 (2013) 13991421performance investigation of solar cooking systems. These param-eters have been analyzed theoretically and experimentally bymany researchers in order to provide the most appropriate operat-ing conditions for solar cookers.

5.1. Theory of solar cookers

In the mid of 1980s, overall utilizable efciency for a solar boxcooker was developed by Khalifa et al. [164] and presented by thefollowing formula:

gu QFQin

1

where QF is the useful heat stored in the food for a temperature riseof DT. Qin is the solar input and for a constant illumination intensitylevel GNR, collector area Ac and cooking time Dt, it is determined asfollows:

Qin GNRAcDt 2

For the mass of water M, the specic boiling time ts and the charac-teristic boiling time tc are calculated by the Eqs. (3) and (4),respectively.

ts DT AcM 3

tc tsG

GNR4

EnerE. Cuce, P.M. Cuce / AppliedGNR is a reference radiation level and commonly taken to be900W/m2. G is the average illumination intensity level. Thereare two gures of merit F1 and F2 which are largely used for eval-uating thermal characteristics of any solar cooker type. The rstgure of merit F1 is determined by conducting the no load stagna-tion temperature test and given as follows [12]:

F1 Tps Ta

G5

In Eq. (5), Tps and Ta are maximum absorber plate temperature

and average ambient temperature, respectively. The second gureof merit F2 is obtained by the full load water heating test as follows[12]:

Fig. 12. Variation of (a) ambient temperature; (b) illumination intensity level; (c)temperature inside pan; and (d) temperature outside pan with time [158].gy 102 (2013) 13991421 1409F2 F 0g0CR F1MCw

Asln

1 1=F1Tw1 Ta=G1 1=F1Tw2 Ta=G

6

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

s0 F1MCwAF2 ln1 1=F1Tw1 Ta=G1 1=F1Tw2 Ta=G

7

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

Fig. 13. (a) Outdoor cooker with thermal storage installed in an elementary schoolin northern Chile, South America; (b) outdoor cooker without thermal storageinstalled in Mali, Africa; and (c) indoor solar cooker with three circular pots (80, 40and 20 L) and one rectangular at pot (60 L) installed in a school in Nicaragua,Central America [163].

were in agreement with the experimental results with an inaccu-racy less than 2%. Fared et al. [171] presented a mathematical mod-el based on an electric resistances analogy which describes andsimulates the thermal behaviour of a solar stove. The mathematicalmodel included three different heat transfer mechanisms betweendifferent surfaces of the solar stove and the environment. The pro-posed model allowed predicting the solar stove entropy generationand its efciency.

Saitoh and El-Ghetany [156] constructed a solar water-steriliza-tion system with thermally controlled ow and analyzed it theo-retically and experimentally. Thermal and biological tests of thewater samples during the sterilization process were obtained.

seve

on

F1MA

uTwT

0Gdt

Enertemperature, wind speed, initial water temperature and nal watertemperature. Mullick et al. [165] carried out some tests in order todetermine the F2 through the experimental data. They observed thatthe F2 increases with increase in number of cooking vessels if load iskept constant and equally distributed. This is attributed to animprovement in the heat exchange efciency factor (F0) withnumber of cooking vessels [12]. Funk [25] developed a cookingpower expression for solar cookers as follows:

P MCwdTwdt

8

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

Ps 700MCwDT600G 9

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

s MCwC3N

lnPsTw1PsTw2 10

where N is number of pots and C3 is coefcient which characterizesthe solar cooker. Nahar [18,106] developed an expression in orderto determine the efciency of solar cookers:

g MCw M1CuTw2 Tw1CA

R s0 Gdt

11

where g is the efciency of the cooker, M1 mass of cooking utensil,Cu is specic heat of cooking utensil, C is concentration ratio and G isthe illumination intensity. A brief of the reported expressions byseveral researchers on performance parameters of solar cookers is

Table 1Thermal performance parameters, their expressions developed by

Author Parameters Expressi

1. Mullick et al. [24] F1 TpsTa

G

F2 F 0goCR 2. Funk [25] Ps 700MCwDT600G3. Khalifa et al. [164] gu Qf/Qin

ts DTAcMtc tsG

GNR

4. Nahar [18] g MCwM1CCAR

1410 E. Cuce, P.M. Cuce / Appliedgiven in Table 1.

5.2. Analytical models of solar cookers

In Indian Institute of Technology, Yadav and Tiwari [167] car-ried out a simple transient analysis to get the overall picture ofthe performance of solar box cookers. They found that the timerequired to obtain the stagnation temperature is largely dependenton the heat capacity of water or the ingredient to be cooked in thecooking vessel. If the heat capacity of the contents of the cookingvessel has greater value, then the cooking period becomes long.Medved et al. [168] presented a new solar heater named SOLAR-BALL which was shaped as an inatable hemisphere. A mathemat-ical and numerical model was developed to analyse solar radiationand heat transfer in such a solar heater. The numerical model wasveried by a series of experiments. It was found that typical opticalefciency and overall heat transfer coefcient of the hemisphericalsolar heater are between 0.450.50 and 0.61.6 W/m2K, respec-tively. The time required for the preparation of hot drinks andheating of food was found entirely acceptable. Kablan [169] evalu-ated energy saving potential of solar water heating systems inJordan between the years of 20012005. He calculated that the to-tal savings over the entire period are estimated to be 46.28 millionUS$ if solar water heaters are used instead of commonly used LPGpowered cookers. Diallo et al. [170] theoretically investigated theperformance analysis of a solar cooker with tilted walls. The north-ern side wall was tilted at an angle of 38 and other walls weretilted at an angle of 9 relative to the vertical. All these walls werecovered with a thin reective aluminium lm. Theoretical results

ral researchers and range of values [12].

Range of values

0.120.16 m2 K/W

Cws ln

11=F1Tw1Ta=G 11=F2Tw2Ta=G h i

0.2540.490

348.83 W at DT = 50 C

7.429.6%25.84385.757 min m2/kg

20.166.7 min m2/kg

2Tw1 27.5%

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

Variable Value Variable Value

Ac (BC) 0.492 m2 C (CC) 8.88Ac (CC) 1.545 m2 M1 1.477 kgAt 0.174 m2 M2 4.751 kgAg 0.235 m2 Ta 30 CCw 4186 J/kgK Tw2 95 CC (BC) 2.09 Ta 30 C

GT 906 W/m2

gy 102 (2013) 13991421Overall efciency of the hot box solar cooker was found to be35%. Effect of the plate thickness on the performance of the cookerwas theoretically investigated. The results indicated that crucialparameters for the solar water-sterilization system are the levelof contamination of water, type of bacteria, type and size of thetransparent water container, the intensity of solar radiation, thewater temperature inside the transparent container, the quantityof water being exposed, environmental conditions, exposure dura-tion and water ow rate.

Recently, Lahkar et al. [219] have developed a novel perfor-mance parameter called cooker opto-thermal ratio (COR) basedon HottelWhillierBliss (HWB) equation. A single step test proce-dure has been used to obtain COR and to establish its utility ininner-cooker comparison, box type (BC) and concentrating type(CC) solar cookers have been tested initially. COR has been denedas follows:

performance and highest efciency for the solar grill. Similarly toOlwi and Khalifa [174], Bidotnark and Turkmen [175] used 4th or-der RungeKutta method to investigate thermal performance of ahot box solar cooker named ITU-2 which was manufactured inIstanbul Technical University, Turkey. Jubran and Alsaad [176] pre-sented the theoretical analysis and performance investigation of asingle, as well as double, glazed box-type solar cooker with orwithout reectors. The mathematical model was based on heatbalance equations arranged for various components of the cooker.In the study, the properties of the cooking materials and the overallheat loss coefcient were allowed to vary as a function of the

ure

EnerCOR g0CUL

12

where g0 is the optical efciency, C is the concentration ratio and ULis the heat loss factor. The experimental data which is illustrated inTable 2 has been tted in HWB equation to determine the relevantparameters. In Table 2M1,M2, Ta, Tw2, Ac, At, Ag, Cw, C and GT refer tomass of water for BC, mass of water for CC, average ambient tem-perature, nal temperature of water, aperture area, pot surface areafor CC, glazed surface area for BC, specic heat capacity of water,concentration ratio for CC and average total solar radiation on theplane of aperture, respectively. Rise in water temperature (Tw) withtime (t) is given in Fig. 14. Calculated parameters in the study arelisted in Table 3. In Table 3 F, UL and Tfx refer to heat exchangeefciency factor, heat loss factor and maximum achievable uidtemperature, respectively. The results indicated that COR is a robustperformance parameter derived from HWB equation analytically. Acooker with a higher value of COR may be graded higher than theone having a lower value of COR.

Al-Soud et al. [117] constructed, operated and analyzed a para-bolic cooker with automatic two axes sun tracking system as illus-trated in Fig. 15. The experiments were performed for three daysfrom 8:30 h to 16:30 h in the year 2008. The test results indicatedthat the water temperature inside the cookers tube reached 90 Cwhen the maximum registered ambient temperature was 36 C. Itwas also noticed that the water temperature increases when theambient temperature gets higher or when the solar intensity isabundant. This is in favour of utilizing the proposed cooker inmany developing countries, which are characterized by high solarinsulations and high temperatures. Besides cooking, the aforemen-

Fig. 14. Rise in water temperatE. Cuce, P.M. Cuce / Appliedtioned cooker could be utilized for warming food, drinks as well asto pasteurize water or milk.

5.3. Numerical models of solar cookers

El-Sebaii [172] numerically analyzed a box-type solar cookerwith outer-inner reectors. Numerical calculations were carriedout for different tilt angles of the outer reector on a typical winterday (20 January) in Tanta, Egypt. The optimum tilt angle of theouter reector was 60. For this specic value, it was observed thatthe specic and characteristic boiling times were decreased by 50%and 35%, respectively, compared to the case without the outerreector. The overall utilization efciency of the cooker was deter-mined to be 31%. Terres et al. [173] numerically investigated theheating of bee honey, olive oil, milk and water in a solar box cookerintegrated with internal reectors. In the study, climatic values ofMexico City for February 26, 2006 were used. It was observed thatthe maximum simulation temperatures were 91.8, 91.6, 86.2 and85.3 C that correspond to bee honey, olive oil, milk and water,respectively. Olwi and Khalifa [174] presented an elaborate analy-sis on a solar cooker used for meat grilling. Several experimentswere performed in order determine the effects of thermal param-eters on cooking performance. In addition, a mathematical modelwas developed. Heat balance equations were solved via 4th orderRungeKutta method. It was observed that an air-tight solar cookerwith double glazing and maximum meat charge provide the best

with time for BC and CC [219].

Table 3Mean values of parameter set with COR, experimental variables and maximumachievable uid temperature [219].

Parameters BC CC

Mean Std. deviation Mean Std. deviation

FUL/C (W/m2 K) 1.576 0.138 2.260 0.011Fg0 0.213 0.008 0.348 0.013COR 0.136 0.011 0.155 0.007Tfx (C) 147.75 4.950 161.82 16.688gy 102 (2013) 13991421 1411absorber plate and food temperature. Effects of thermal parame-ters on cooking performance were investigated.

5.4. Modeling and simulation

In North West University, Mawire et al. [177] carried out dis-charging simulations for an oil/pebble-bed thermal energy storagesystem (TES). Accuracy of the model was veried by the experi-mental results. Discharging results of the TES system were pre-sented using two different methods. The rst method dischargedthe TES system at a constant ow rate while the second methodchanged the ow rate in order to provide a desired power at a con-stant load inlet temperature. It was observed from the results thatthe TES system at a constant ow rate demonstrate a higher rateheat utilization. However, this is not benecial to the cooking pro-cess since the maximum cooking temperature is not maintainedfor the duration of the discharging period. On the other hand, thecontrolled load power discharging method has a slower initial rateof heat utilization but the maximum cooking temperature is

axe

Enermaintained for most of the discharging process and this is what isexpected for the cooking process. Mawire and McPherson [178]simulated the temperature distribution of an oil-pebble bed TESsystem under a variable heat source during charging. The chargingoutlet temperature was controlled by a combined feedforward andPID feedback control structure to maintain thermal straticationduring the experiment and the simulations. In the study, Schu-

Fig. 15. Schematic of the two

1412 E. Cuce, P.M. Cuce / Appliedmann model and modied Schumann model were simulated inorder to analyse thermodynamic behaviour of the TES system. Itwas found that the discharging results were in good agreementwith the experimental results. Thulasi Das et al. [70,71] presentedthermal models for the solar box cookers augmented with differentnumber of cooking vessels. The effect of parameters such as thethickness and size of the absorber plate, emissivity of the vessel,insulation thickness, and cooking time were studied. Differentcooker sizes were simulated in order to assess their adequacy incooking. It was found that the black paint on the vessels could beavoided if weathered stainless steel or aluminium vessels are used.In addition, the cooker with inner dimensions of 0.6 0.6 0.1 m3was found to be adequate to cook lunch and dinner on a clear dayeven in the winter months. Besides these specic studies, someresearchers focused on solar energy models in recent years [179182]. Jebaraj and Iniyan [183] presented a review on energy mod-els including renewable energy models.

5.5. Experimental work

Purohit [184] carried out a large number of experiments on abox-type solar cooker in the climatic conditions of New Delhi, In-dia. He determined absorber tray temperature (Tps), ambient tem-perature (Tas) and illumination intensity (Hs) in order to determinerst gure of merit (F1). Similarly, he measured initial water tem-perature (Tw1), nal water temperature (Tw2), average ambienttemperature Ta, average illumination intensity (H) and timedifference in which water temperature rises from Tw1 to Tw2 tobe able to calculate second gure of merit (F2). The measuredand calculated parameters are listed in Tables 4 and 5. In IndianInstitute of Technology, Kumar [185] presented a simple testprocedure for determination of design parameters to predict thethermal performance of a solar box cooker. In order to determinetwo gures of merit (F1 and F2), a series of outdoor experimentswere conducted on double glazed solar cooker with aperture areaof 0.245 m2. Experimental setup for determination of F1 and F2 isillustrated in Fig. 16. The parameters required, optical efciencyand heat capacity of the cooker were calculated using the linear

s sun tracking system [117].

gy 102 (2013) 13991421regression analysis of experimental F2 data for different load ofwater. The results indicated that optical efciency and heat capac-ity of the cooker are crucial design parameters to be able to predictthe thermal performance of solar cookers. Kumar et al. [186,187]experimentally investigated the heat loss from a parabolic concen-trator solar cooker with and without wind condition. Values of theheat loss factor for the tilted reector were compared with thoseobtained with the reector in a horizontal position. It was foundthat a parabolic reector is not required for heat loss determina-tion. It was also noted that thermal performance of a parabolicconcentrator solar cooker depends greatly on the wind speed. InTaiwan, Yeh et al. [188] experimentally and analytically investi-gated a novel design for inserting an absorbing plate to dividethe air duct into two channels (the upper and the lower) for dou-ble-ow operation in solar air heaters with ns attached overand under the absorbing plate. Both the theoretical predictionsand experimental results indicated that the optimal fraction of air-ow rate in upper and lower subchannels is around the value of0.5. They also examined the effect of the ow-rate ratio of thetwo air streams of owing over and under the absorbing plate onthe enhancement of collector efciency. It was underlined thatproviding ns attached on the collector, will improve the collectorefciency. Moreover, constructing the collector with ns attachedmay scarcely increase the fan power. Rathore and Shukla [189]experimentally analyzed two different solar cookers: at platebox type solar cooker (SBC) and parabolic solar cooker (SPC). Theexperiments were carried out at the roof top of Renewable EnergyLab, Department of Mechanical Engineering, Institute of Technol-ogy, Banaras Hindu University (BHU), Varanasi, India during monthof October and November 2008. The cookers were operated underthe same climatic conditions. It was found that the daily average

EnerTable 4First gure of merit (F1) of a typical Indian solar box cooker obtained from outdoortesting [184].

Tps (C) Tas (C) Hs (W/m2) F1

106.84 26.33 603 0.1335

E. Cuce, P.M. Cuce / Appliedtemperature of water in the SPC was 333 K and for SBC was 326 Kand the daily average difference between the temperature of waterin the cooking vessel and the ambient air temperature was 31.6 Kfor SPC and 26.4 K for SBC. The energy output of the SPC variedfrom 0.65 to 39.3 W and 7.44 to 33.49 W for SBC, whereas its exer-gy output was in the range of 0.92 to 2.58 W for SPC and for SBC itvaried from 0.65 to 1.45 W. The energy efciency of the SPC variedfrom 0.42% to 5.27% and for the SBC it varies from 4.7% to 29.81%.

111.05 27.05 630 0.1333105.66 26.33 603 0.1316118.44 28.81 687 0.1316117.52 28.81 687 0.1302104.71 26.33 603 0.1299106.52 24.64 631 0.1298118.78 29.71 687 0.1296118.67 29.71 687 0.1294108.39 27.05 630 0.1291112.61 28.65 651 0.1289112.21 26.76 663 0.1288107.97 27.05 630 0.1284111.88 26.76 663 0.1284102.54 23.14 619 0.1283105.59 24.64 631 0.1282111.42 26.76 663 0.1276111.56 28.65 651 0.1274111.29 28.65 651 0.1269125.35 31.84 737 0.1268104.04 24.64 631 0.1258101.52 23.14 619 0.1258122.01 29.16 742 0.1251105.23 27.05 630 0.1248100.58 23.14 619 0.1243

Average value of F1 0.1285Standard deviation 0.0024Standard error of mean 0.0005

Table 5Second gure of merit (F2) of a typical Indian solar box cooker obtained from outdoortesting [184].

Tw1 (C) Tw2 (C) Ta (C) H (W/m2) T (s) F2

60.00 90.00 38.05 712 5100 0.499760.10 90.27 32.00 798 5520 0.498560.00 90.00 38.03 692 5640 0.486460.00 90.00 38.30 696 5340 0.485260.34 90.03 28.09 803 6120 0.483860.59 90.03 22.62 885 5580 0.482960.00 90.00 36.90 764 4740 0.479260.00 90.00 38.00 712 5220 0.478960.00 90.00 38.18 655 6120 0.478760.00 90.00 36.74 631 6900 0.478760.10 90.03 25.23 865 5640 0.478060.00 90.00 35.55 631 7200 0.475260.00 90.00 34.59 731 5400 0.474960.00 90.00 36.90 764 4740 0.474760.00 90.00 35.58 728 5340 0.473260.02 90.29 37.27 819 4500 0.472161.37 91.54 35.95 767 5100 0.466760.83 90.03 30.17 738 7320 0.466560.59 90.03 30.95 842 5280 0.465260.00 90.00 35.45 631 7500 0.464960.00 90.00 37.70 676 6060 0.464760.39 90.76 37.31 767 5100 0.461860.39 90.79 35.59 890 4200 0.457760.59 90.27 32.16 809 5760 0.457260.10 90.03 28.19 800 6600 0.4543

Average value of F2 0.4744Standard deviation 0.0117Standard error of mean 0.0023gy 102 (2013) 13991421 1413Prasanna and Umanand [154] developed a hybrid solar cookingsystem as shown in Fig. 17 where the solar energy was transportedto the kitchen. The thermal energy source was used to supplementthe Liqueed Petroleum Gas (LPG) which was in common use inkitchens. In the prescribed system, cooking could be carried outat any time of the day with time taken being comparable to con-ventional systems. Design and sizing of different components ofthe system were described with equations.

5.6. Effective parameters on performance of solar cookers

It is well known in literature that thermal performance param-eters of solar cookers are highly dependent on the main compo-nents of the cookers. If a solar box cooker is considered, thesecomponents 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 reective surfaceswill play the main role if a solar panel cooker or a parabolic cookeris evaluated.

5.6.1. Booster mirrorA booster mirror is quite signicant for a solar cooker since it

allows higher illumination intensity falling on the transmittingsurface of the cooker hence higher working temperatures whichenhance the efciency. Ibrahim and Elreidy [190] investigatedthe performance of a solar cooker integrated with a plane boostermirror reector under the climatic conditions of Egypt. The exper-iments lasted 2 years for various operating conditions. Cookerposition and the tilt angle of the booster mirror were adjusted inorder to maximize the sunlight concentration. It was observed thata good meal for a family of four was cooked in 34 h. It was alsofound that better heat transfer occurred when the cooking pot

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

cooker with a nned absorber plate to maximize the solar energyabsorption. The results showed that solar box cooker integrated

ybr

Enerwas covered with an airtight plastic transparent cover rather thanusing an ordinary metallic cover. Gayapershad et al. [191] evalu-ated the performances of two solar cooking units: a low-cost,low-technology Sunstove unit and the more expensive Ishisa boxunit. The cookers were tested with and without tracking systemunder summer radiometric conditions at the Solar Thermal Appli-cations Research Laboratory (STARlab) between December 2005and April 2006. The Ishisa box unit also augmented with externalmirror panels. The Sunstove unit could not succeed to boil water.The maximum water temperature reached in the Sunstove unitwas found to be 88 C for tracked conditions. On the other hand,the Ishisa box unit enabled boiling water for both tracked andnon-tracked conditions. The tracked unit reached the boiling tem-perature 20 min earlier than the untracked unit. It was noted thatthe Ishisa box unit benetted from tracking efciently via its exter-nal booster mirrors. In Indian Institute of Technology, Shukla andGupta [192] presented an energy and exergy analysis of a concen-trating solar cooker. The cooker was devised for community cook-ing and integrated with a linear parabolic concentrator whichconcentration ratio is 20. The experiments were carried out in bothsummer and winter conditions. Through the experimental results,the average efciency of the solar cooker was determined to be14%. Heat losses caused low efciency were classied as opticallosses (16%), geometrical losses (30%) and thermal losses (35%).The rest of the losses were due to edge losses, etc. The maximumtemperature that the water in the cooker reached was 98 C duringthe tests.

Fig. 17. Block diagram of the h

1414 E. Cuce, P.M. Cuce / Applied5.6.2. GlazingBarker [193] interestingly showed that, if it is not needed to ex-

ceed 100 C, an efcient solar cooker can be made for less than 5$with materials that are available almost everywhere. He under-lined that multiple glazings and highly insulated boxes are notnecessary in the proposed design. A double glazed transparentpolyethylene plastic lm was used as a glazing material in the coo-ker. It was concluded that most of the foods can be cooked in thisvery low cost cooker with a 0.25 m2 collector area. Bell mentionedabout the glazing selection for various heat transfer applications.One or more sheets of glass or other diathermanous (radiation-transmitting) material was utilized in order to transfer the solarenergy to the collector/absorber plate. The transparent cover wasused to minimize convection losses from the absorber platethrough the restraint of the stagnant air layer between the absor-ber plate and the glass. It also enabled reducing radiation lossesfrom the collector as the glass is transparent to the short waveradiation received by the sun but it is nearly opaque to long-wavethermal radiation emitted by the absorber plate [20,194,195].Hussain and Khan [196] experimentally investigated a low costwith ns was approximately 7% more efcient than the conven-tional solar box cooker. The time required for water to boil wasreduced approximately 12% when a nned absorber plate was uti-lized. In Turkey, Ozkaymak [197] experimentally investigated theperformance of a hot box solar cooker. The cooker has a cylindricalgeometry as shown in Fig. 18, with a 38 cm inner diameter, 40 cmouter diameter and 25 cm height. The outer wall of the cooker wasbox-type solar cooker made of two paper carton boxes with crum-pled newspaper balls as insulation. The cooker was supported by areector covered with aluminium foil. Experimental results ob-tained from the novel cooker were compared with a standard cost-lier solar box cooker. It was observed that the water temperaturerapidly increase in novel cooker compared to the standard cooker.Two gures of merit of the new cooker also found satisfactory.

5.6.3. Absorber plateAbsorber tray of a solar cooker is a crucial component since it

absorbs the useful energy from sun to be able to succeed cookingprocess. Geometric structure of an absorber plate is quite signi-cant as well as its thermophysical properties. In order to maximizethe illumination intensity falling on the absorber tray and enhancethe heat transfer from the absorber tray to the food in cookingvessels, absorber tray is a key item which allows various modica-tions. Harmim et al. [67] devised and constructed a box-type solar

id solar cooking system [154].

gy 102 (2013) 13991421made of 1 mm thick metal sheet tray. The absorber plate was madeof thin copper sheet, which was painted black for absorbing solarradiation better. Glass wool insulation was used on the bottomand sides of the cooker to minimize thermal losses through con-duction. A clear window glass of 4 mm thickness was xed overthe inner tray. Three 4 mm thick plane mirror reectors wereplaced around the cooker. The three reectors were kept xed.The constant tilt of the reector is 678 from the horizontal plane.The cooking pot was a black painted aluminium pot with 10 cmdiameter and 16.5 cm height. The experiments were carried outduring July and August 2004 at Karabuk, Turkey. The solar cookerwith three reectors was exposed to solar radiation between10.00 a.m. and 4 p.m. It was observed from the experimentalresults that absorber plate temperature was over 100 C during aperiod of 5 h which is a sufcient time to cook most of the foods.Mawire et al. [198] developed a thermal energy storage systemusing a packed pebble bed. An electrical hot plate heated up oilcirculating in a copper absorber plate which charges the storagesystem. A Visual Basic program was developed to acquire datafor monitoring the storage system and to maintain a nearly con-stant outlet temperature from the charging point. It was concluded

storing heat exchanger as a step toward a solar cooking concept.The solidsolid transition of pentaerythritol was the principal

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

Enerthat the results obtained can be used to characterize the cookingsystem.

5.6.4. Cooking equipmentCooking pots are the items which are in conduction with the ab-

sorber tray in order to receive the absorbed energy and transfer itto the food. Any type of cooking vessel can be used in solar cookersbut generally rectangular and cylindrical shaped cooking vesselsmade of aluminium or copper are recommended. Saxena et al.[20] emphasized in their comprehensive review that number ofcooking pots in a solar box cooker may vary depending on thequantity and the type of the food. Gaur et al. [79] found that per-formance of a solar cooker can be enhanced if a cooking vessel witha concave shape lid is used instead of a plain lid. Joshi et al.[199,200] presented experimental and numerical studies on solarcookers in the early 2012 in order to provide an efcient designincluding cooking equipment. They aimed at increasing the solarcooking efciency from 1025% to 60% or more. In their novel de-sign, the cooking pots gained energy from condensing steam on theoutside surface. The cooking charge (water + rice or lentils and/orvegetables) received heat by the mode of natural convection. Theresults of CFD indicated that optimum heat ux is in the range of

2 2

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

E. Cuce, P.M. Cuce / Applied16,20025,000 kcal/h m where m is the bottom surface area ofthe cooking system. Cooking pots with perforations were recom-mended for higher efciency. Franco et al. [201] introduced a mul-tiple use communal solar cooker. The parabolic concentrator andthe cooking pot are shown in Fig. 19. The cooking pot with 10 Lcapacity was painted black and placed on the focus of the concen-trator. Stew as food was tested in the cooking system. A stew isgenerally made with potatoes, noodles or rice, meat, vegetableslike peppers and carrots, and spices. The cooking is done in water,adding the ingredients according to the time span each one needsto be cooked. It was expressed in the study that about 18 kg of foodcan be cooked using only one concentrator. They noted that about18 kg of stew can be cooked on each solar cooker within 3 or 4 h.

5.6.5. Heat storage materialIt is a clear fact from the literature that solar cookers are very

promising devices in the upcoming future. However, there aresome handicaps concerning the solar cooking technology. Perhaps,the most challenging point of solar cookers is that they are not ableto serve when the sun goes down. Some researchers performedintensive efforts on solar box cookers in order to allow late eveningcooking. PCMs were considered as a solution in most cases.Bushnell [202] designed, constructed and evaluated a solar energygy 102 (2013) 13991421 1415mechanism for energy storage. The methods for describing thesystem performance were explained and applied to a test systemcontaining a controllable replacement for the solar input power.This rst stage of this research work followed by a heat exchanger,which was connected to a concentrating array of CPC cylindricaltroughs. Author also described the size of the solar collector areaand mass of PCMmass needed in order to provide adequate energyfor several family-size meals with sufcient storage to cook atnight and 1 or 2 days later. The performance was described fromefciency measurements and the determination of a gure of mer-it. Bushnell and Sohi [203] also designed a modular phase changeheat exchanger with pentaerythritol used as a PCM for thermalstorage (solidsolid phase change at 182 C) was tested in an ovenby circulating heat transfer oil which was heated electrically in amanner to simulate a concentrating solar collector. Thermal energyretention times and cooking extraction times were determined,and along with the efciencies, were compared with the resultspreviously reported for a non-modular heat exchanger. Buddhiand Sahoo [204] designed a box-type solar cooker as shown inFig. 20 with latent heat storage for the composite climatic condi-tions of India. The experimental results demonstrated the feasibil-ity of using a phase change material as the storage medium in solarcookers. It also provided a nearly constant plate temperature in the

and exergy output and efciency because of changes in cooker con-guration. It was also seen that the exergy analysis was moreconvenient than the energy analysis for predicting solar cookerefciency.

g energy outputenergy input

EoEi

mwcpwTwf Twi

=tItAsc

13

where g is energy efciency,mw is water mass, cpw is specic heat ofwater, Twf is nal temperature of water, Twi is initial temperature ofwater, t is time, It is total instantaneous solar radiation and Asc isintercept area of solar cooker.

w exergy outputexergy input

ExoExi

mwcpw Twf Twi To ln TwfTwi

h i=t

It 1 4Ta3Tsh i

Asc14

Energy 102 (2013) 13991421late evening. The experimental results were also compared withthose of a conventional solar cooker. The test of the cooker wasperformed without a cooking load. The results indicated that solarcooker with PCM provides an environment in which the cooking ispossible even the sun goes away. The absorber plate temperatureof the solar cooker remained constant at about 70 C for a longperiod of time.

5.6.6. InsulationIt is well-documented in literature that insulation is one of most

crucial key points for a box type solar cooker to be able to providean efcient cooking [103,104]. All materials with low thermal con-ductivity may be used as an insulation material in solar cookers.However, the main purpose for material selection should be mini-mizing heat loss from the solar cooker to the environment withminimal cost. Vandana [205] devised and constructed a very lowcost for Indian women who are burdened with household work,agriculture work and care of animals in addition to all time nan-cial crisis. The proposed reless cooker was insulated with straw-board and tested in terms of cooking efciency. The resultsindicated that the reless cooker of strawboard could both cookas well as keep the food hot with in safe temperatures well above6 h. Nyahoro et al. [206] presented an explicit nite-differencemethod to simulate the thermal performance of short-term ther-mal storage for a focusing, indoor, institutional, solar cooker. Thecooker storage unit consisted of a cylindrical solid block. The blockwas enclosed in a uniform layer of insulation except where therewere cavities on the top and bottom surfaces to allow heating ofa pot from storage and heating of the storage by solar radiation.A paraboloidal concentrator focused solar radiation through a sec-ondary reector onto a central circular zone of the storage blockthrough the cavity in the insulation. The storage was charged fora set period of time and heat was subsequently discharged to apot of water. In these simulations a pot of cold water was placedon the hot storage block and the time then estimated until thewater either boiled or the temperature of the water reached a max-imum value. Simulations were made for a given pot capacity withthe storage block made from either cast iron or granite (rock). The

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].

1416 E. Cuce, P.M. Cuce / Appliedeffects on cooker performance were compared for a variety ofheight to diameter ratios of the storage block and size of the areaof solar input zone. Bollin [207] proposed a detailed study aboutthe transparent insulation in various solar applications includingsolar cookers with thermal energy storage.

6. Thermodynamic assessment of solar cookers

Energy and exergy analysis provide an alternative means ofevaluating and comparing solar cookers. Ozturk [115] denedenergy and exergy efciency for the solar cookers as given inEqs. (13) and (14), respectively. Several studies were carried outabout this topic. However, the rst study on energy and exergyefciencies of solar cookers was conducted by Ozturk [115]. Itwas stressed in his article that there was large difference in energywhere w is exergy efciency, To is outside temperature, Ta is ambi-ent temperature and Ts is sun temperature. Kumar et al. [208] inves-tigated a truncated pyramid type solar box cooker (TPSBC) in termsof exergy and energy efciencies. Two cooking vessels which lled2 L of water were used for conducting full load test. During the testperiod, the booster mirror was covered with black cloth. The watertemperature inside the vessels reached 90.6 C from 60 C in 70 minwhereas the initial water and ambient temperatures were 43.18 Cand 33.43 C, respectively. The maximum and minimum values ofinsulation were observed as 929 W/m2 and 376W/m2, respectively.The maximum and minimum energy gained from water inside thesolar cooker was calculated 20.8 kJ and 7.5 kJ, respectively. An inter-esting result