Iqra Ayub1

Department of Energy Systems Engineering,

University of Agriculture,

Faisalabad 38000, Pakistan

e-mail: iqra.ayub@uaf.edu.pk

Anjum MunirDepartment of Energy Systems Engineering,

University of Agriculture,

Faisalabad 38000, Pakistan

e-mail: Anjum.munir@uaf.edu.pk

Abdul GhafoorDepartment of Farm Machinery & Power,

University of Agriculture,

Faisalabad 38000, Pakistan

e-mail: Abdul.ghafoor@uaf.edu.pk

Waseem AmjadDepartment of Energy Systems Engineering,

University of Agriculture,

Faisalabad 38000, Pakistan

e-mail: waseem_amjad@uaf.edu.pk

Muhammad Salman NasirDepartment of Structures and

Environmental Engineering,

University of Agriculture,

Faisalabad 38000, Pakistan

e-mail: salmannasir@uaf.edu.pk

Solar Thermal Application forDecentralized Food Baking UsingScheffler Reflector TechnologyBaking is an energy intensive unit operation. The thermal application of solar energy isgetting attention in food processes by eliminating the facts of interrupted supply and fluc-tuated costs of nonrenewable energy sources. This study has been carried out for thedesign and development of solar bakery unit which comprises of a 10 m2 Scheffler reflec-tor focusing all the beam radiations on a secondary reflector that further concentrate thebeam radiations toward the heat receiver of solar bakery unit to heat up the air circu-lated through baking chamber employing a photovoltaic operated fan. Computationalfluid dynamic (CFD)-based three-dimensional (3D) simulation was performed to analyzethe design for uniform air distribution in the baking chamber. The system designed con-figurations gave quite good results for airflow distribution. The receiver temperaturereached between 300 and 400 �C while temperature at the inlet of baking chamber was inthe range of 200–230 �C, sufficient for most of the products to be baked. The maximumavailable solar power at receiver was calculated to be 3.46 kW having an average effi-ciency of 63%. A series of experiments were conducted for the baking of cakes and totalenergy available in baking chamber was about 3.29 kW and cake utilized 0.201 kWenergy to be baked. The average value of energy utilization ratio was found to be 45%.As a base, the study would lead to the development of an appropriate and low cost solarbaking units for the maximum retention of quality parameters and energy saving.[DOI: 10.1115/1.4040206]

Keywords: solar energy, solar baking, Scheffler reflector, CFD

1 Introduction

The food cooking especially baking process is an energy-intensive unit operation and it becomes more challenging for thedeveloping countries facing severe energy crises. It is important tomention here that a large quantity of heat energy is consumed forlow to medium scale heating applications. The firewood and fossilfuels are used for the generation of heat which causes deforesta-tion and environmental pollution. Cooking is one of the mainenergy application processes and its share of energy consumptionis more in developing countries [1]. A considerable share of heatutilization is taken by baking industry. There is need to developenvironment-friendly new cooking, baking, and heating technolo-gies to overcome the rising environmental issues.

Solar thermal energy offers an excellent opportunity for bakingindustry to decrease the consumption of primary energy sourcesand unit cost of energy availability. Various types of solar thermalcollectors, i.e., nonconcentrating collectors like vacuum tube col-lectors, flat plate collectors and concentrating type collectors, i.e.,paraboloidal concentrators, parabolic dish and heliostat are beingused worldwide to produce low to medium temperature solar ther-mal energy [2] but these collectors remain unable to deal most ofthe agricultural processes requiring high temperature. Saxenaet al. [3] developed a modified box type solar cooker, which had

reduced the cooking time for 20 min to cook the food product andenhanced the cooking power from 70.60 W (conventional vessel)to 79.80 W (new vessel). It was also found that the efficiency ofthe modified solar cooking vessel had been improved from 60%(conventional vessel) to 67.77% (modified vessel).

On the other side, Scheffler fixed focus concentrator technologyis successfully employed for moderate to high-temperature appli-cations in different parts of the world for heating, cooking, baking,and power generation [4]. These concentrators are the lateral sec-tions of paraboloid focusing all the incident beam radiation on afixed receiver throughout the year [5] and would be quite effectiveto perform the baking process. Solar thermal application for cook-ing is quite common and easy to manage but the baking processrequires a considerably higher temperature and need to be opti-mized [6]. Rinc�on and Herrera [7] conducted the research on 2Dand three-dimensional (3D) ray-tracing procedures for optimizingan innovative solar hot plate (called Tolokatzin). The temperatureof about 250 �C was obtained at a half-acceptance angle of 5 deg,which allows the concentration of sun light without any trackingfor about 40 min. These temperatures are high adequate for cook-ing. They also concluded that the Tolokatzin hot plates performan excellent job as solar cooker. Rinc�on and Herrera [8] presentedthe detail design, operational principle, and results of a new solarcooker. The use of curved optimized mirrors makes it a compactdesign and provides the possibility to work like an oven, in additionthe ability to fry. The common baking processes are involved simul-taneous heat, mass transfer, and chemical reactions. Prediction ofbaking performance is relatively difficult since the mechanisms ofthe baking process, like relationships between volume change, tem-perature, distribution of moisture content particularly the chemicalreactions are still not fully understood [9,10]. Mahavar et al. [11]

1Corresponding author.Contributed by the Solar Energy Division of ASME for publication in the

JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING

ENERGY CONSERVATION. Manuscript received September 8, 2017; final manuscriptreceived May 2, 2018; published online June 26, 2018. Assoc. Editor: JorgeGonzalez.

Journal of Solar Energy Engineering DECEMBER 2018, Vol. 140 / 061005-1Copyright VC 2018 by ASME

introduced a new parameter “required electric back-up power(Prb)” for Box type solar cookers. An analytical model was devel-oped under different meteorological and cooking time conditions.It was concluded that the electric back-up in solar box type cookeralso decreases its payback period and increases its net currentvalue. Analytical model was also in support of this box type solarcooker. Ayub et al. [12] presented a detailed thermal analysis ofsolar bakery unit. Thermal analysis of solar bakery unit presentedvariations in rate of energy utilization, energy utilization ratio,exergy losses, and exergy efficiency in range of 0.01–0.07 kW,25–75%, 0.19–1.08 kW, and 6.62–56.46%, respectively. Theoverall exergy efficiency of solar bakery unit was found to be59.26%. This research provides a comprehensive method to con-duct the thermal analysis of a solar bakery unit. Nandwani [6] hasconducted a study to compare solar ovens with conventional fire-wood and electric stoves. He concluded that roughly 16.8� 106

ton of firewood could be saved and the emission of 38.4� 106 tonof carbon dioxide per year could be prevented. Little work hasbeen reported on the solar thermal application in baking. Hassenet al. [13] developed an innovative baking system powered bysolar concentrated parabolic trough. Surface temperatures of191 �C were attained on top of the glass baking pan and Injerabaking experiments were conducted efficiently. Tesfay et al. [14]developed a new technology of indirect solar stove for baking pur-pose. In this study, three important results were achieved. The firstresult shows that this technology proves indoor solar baking ispossible. Second, Injera baking using solar energy was impossibledue to its high temperature demand. The third and important resultis the possibility of Injera baking at 135–160 �C that will bring asignificant impact in the revolution of Injera stove in Ethiopia.After experimentation, it was found that it will become more effi-cient stove if it is attached with auto trackers and heat storagemechanism. Tesfay et al. [15] discussed the solar powered heatstorage for Injera baking in Ethiopia. In this research, the develop-ment and performance evaluation of the prototype for directsteam-based baking was carried out. The experimental and COMSOL

simulation results for charging of the storage gave a similar result.The useful heat stored was found to be 374.4 kJ latent and 853 kJsensible, that is equivalent to the heat required to bake two Injerasincluding heating up power. At the end, it was concluded that thisresearch gave hope to break the bottleneck related with on-focussolar cookers. Muller et al. [16] developed a solar bakery oven ofvolume about 200 l. It gained a temperature of about 350 �C bymeans of 3 kW input power obtained from concentrated light gave40% efficiency at 300 �C. They also estimated that solar bakeryunit could save about 150 tons of firewood per annum. The draw-back of this system was nonuniform circulation of hot air througha perforated plate of the baking chamber. Rinc�on [17] presentedan innovative design of solar ovens and hot plates in Mexico andin other developing countries to avoid deforestation and environ-mental issues. This oven can be used for cooking of tortillas, friedmeat, and vegetables and for baking of hot cakes, bacon, eggs,steaks, and fries. In order to get a proper design, computer-basedprocess simulations are much important and currently beingwidely used. The computational fluid dynamic (CFD) is a state-of-the-art tool used for analyzing the airflow and heat distributionpattern based on geometric configuration. Chhanwal et al. [18]developed a 3D CFD model for an electrical baking oven. Thethree different radiation models for simulation such as discretetransfer radiation model, surface to surface and discrete ordinateswere used. Similar results were obtained from all models.

Keeping in view the above discussion, the current study hasbeen carried out for the design and development of a decentralizedsolar bakery unit. Prior to actual design, CFD-based simulationwas performed to get appropriate design configurations, whichcould provide uniform airflow in the baking chamber. The energydistribution at various positions of the solar bakery was also calcu-lated to access energy balance. This study will provide an under-standing of the significance of solar thermal application for thebaking processes.

2 Materials and Methods

2.1 Description of Solar Bakery Unit. Figure 1 shows the3D computer-based and the actually developed solar bakery unitcomprising of major parts namely: primary reflector, glass win-dow, receiver, inner and outer chambers of the baking unit, bakingtrays, solar fan, DC motor, and a solar panel. Fixed focus Schefflerconcentrator having 10 m2 surface area and geometric concentra-tion ratio of 100 was used as primary reflector. The reason ofselecting Scheffler concentrator was its ability to keep the focusstationary regardless of the position of the dish while trackingsun; moreover, it produced its focus a bit away from its framewhich made it ideal for cooking as well as baking processes [4].Major components of this Scheffler reflector are elliptical reflectorframe made up of seven crossbars, semicircular tracking channel,rotating support of steel pipe, steel reflector stand, reflecting mir-rors of 226� 152 mm, and photovoltaic tracking device for dailytracking and telescopic clamp mechanism for seasonal tracking.Figure 2(a) shows that the primary reflector was designed by con-sidering the lateral part of a specific paraboloid. The semimajoraxis and semiminor axis of elliptical reflector frame were taken as4.13 and 1.5 m respectively. Ten crossbars were equally distrib-uted along minor axis with one crossbar at the center and otherswere located at 0.51 m from the preceding on both sides as shownin Fig. 2(b). The journal bearings assembly had been welded withthe reflector frame along the line parallel to the polar axis by set-ting it in north–south direction and inclination angle (31.25 deg)with the horizontal. At the end, the axis of rotation (steel pipe) ofthe reflector assembly had been inserted into the journal bearingsto complete the reflector. The ratio of reflector and receiver dis-tances was found to be 4206 mm and 2700 mm, respectively.

Photovoltaic tracking device had been used to rotate the pri-mary reflector by chain sprocket mechanism for daily tracking.Daily tracking device rotates the reflector along an axis parallel tothe polar axis of earth with an angular velocity of one revolutionper day to counter balance the effect of earth rotation. The sea-sonal tracking devices have been made of telescopic clamp mech-anism and have been rotated manually at half the angle of solardeclination.

The edges of outer chamber of baking unit were joined pru-dently to assure that there is no direct contact with the insulationmaterial (rock wool), which would affect the quality of the bakingproduct. This chamber was made of 0.8 mm thick sheet of stain-less steel and comprised of an aligned zigzag receiver and pebbledstorage. The receiver was constructed from four pieces of ironsheet having height and length of 330 mm and 2000 mm, respec-tively, with a space of 30 mm between two sheets. The finalheight, length, and width of receiver were 330 mm, 360 mm and360 mm, respectively. The zigzag receiver was protected by a3.8 mm thick tempered glass window and it was cut into stripsto avoid the breaking of glass due to heat expansion. A DCmotor of 80 W (driven by a solar panel of 160 W) was installed inouter chamber at the back side of inner baking chamber(60� 60� 60 cm3) to operate solar fan for the circulation of air.The backside plate of the inner baking chamber was made perfo-rated (4.6%) for uniform airflow distribution through the bakingproducts. A rectangular passage (49.5 cm� 8 cm) at the bottom ofbaking chamber served as an air outlet. Two perforated (17.4%)baking trays were placed in the inner chamber to place the bakingproducts. The system was developed at Department of EnergySystems Engineering, University of Agriculture Faisalabad,Pakistan (Latitude and longitude 31.4187� N, 73.0791� E,respectively).

Concentrated solar radiations from primary reflector (Schefflerreflector) are intercepted by secondary reflector that is further con-centrated on the zigzag receiver of solar bakery unit. The air getswarm while passing through the receiver and a solar DC fan(80 W) provides draft for the hot air movement from zigzagreceiver to the baking chamber. After exiting from the bakingchamber, the air partially transfers its remaining heat to the pebble

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bed storage to store some thermal energy for 1–2 h of independentbaking. The temperature for different baking products can be setby increasing/decreasing the area of receiver expose to the pri-mary reflector.

In baking/cooking, typically there is a heating element (sun-light, fire, etc.), heat transfer medium (air, water, oil, etc.) andthe food product. The heat moves from the heating sourcethrough the medium to the food. In this baking system, naturalair is basically used as a heat transfer medium. The heat musttravel to the center of the food in order to bake/cook it

throughout. The rate of heat transfer into the food product relateto the temperature difference between two adjacent molecules inthe food. The moisture loss in the baked products increased withincreasing the final product temperature and the oven air tem-perature. High air temperature accelerates heat transfer but willproduce a greater difference between the temperature at theproduct surface and at its center due to creation of brittle sur-face. Smaller pieces will have less temperature difference thanlarger pieces. During baking, air humidity in the oven affectedthe heat transfer coefficient.

Fig. 1 Solar bakery unit (a) computer-aided design of the system (b) developed system

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2.2 Process Simulation. For the assessment of heat distribu-tion and air flow, a three-dimensional process simulation was per-formed using ANSYS FLUENT 14. Only the influencing parts (airduct from receiver and baking chamber) were modeled in DesignModeler and tetrahedral structured mesh was generated. The sim-ulation was performed in steady-state and k–e standard turbulencemodel along with second-order upwind discretization was used. Avalue of 0.1 m/s velocity was used normal to inlet boundary.Inside the baking chamber, four samples were modeled as cakesand appropriate values of coefficient of heat transfer wereassigned to these models. No-slip boundary conditions wereapplied on the walls. QUICK approach was followed to discretizethe governing equations for the system solved by using SIMPLEalgorithm. As there were only four food samples in the bakingchamber, resistance to airflow was considered negligible. The air-flow resistance at the perforated inlet was assumed uniform overthe entire of its length to examine the effect of systemsconfiguration on air flow distribution. Performing a simulationencompasses several stages including preprocessing, model start-up, calculation iteration, and postprocessing of the results. Thesimulation conditions and values are tabulated in Table 1.

2.3 Performance Evaluation of Solar Bakery Unit. Bakingexperiments were conducted to calculate the energy distribution atprimary reflector, receiver and amount of energy supplied andutilized within baking chamber to calculate the systems energyefficiency. All these experiments were conducted at the solar parkof the Department of Energy Systems Engineering, University of

Agriculture Faisalabad, Pakistan (Latitude and longitude 31.4187�

N, 73.0791� E, respectively).

2.3.1 Scheffler Reflector and Receiver. Before starting, test-ing of Scheffler reflector (primary reflector) and receiver was per-formed. For this, primary reflector was washed with water and leftfor some time to dry the reflecting surfaces. After that, a bakeryunit was placed in front of primary reflector without load in such away that receiver faced to reflector. Glass pane was removed fromthe receiver window to measure the temperature of ambient air(Tamb) and air temperature at the inlet of baking chamber (Tin) forefficiency calculations. Pyranometer with black pipe was installedat Scheffler reflector to record beam radiation (Ib) in such a waythat there would be no shadow all around the pipe. K-type thermo-couples linked to a data logger were used to measure ambient airtemperature, air temperature at baking chamber inlet and outlet,temperature inside the baking chamber after an interval.

The receiver efficiency is defined as the ratio of energy outputfor the baking process to the total energy input at receiver. Thus,receiver efficiency of the system was calculated using the belowequation [19]:

gr ¼Qd

Qr(1)

where gr is the receiver efficiency (%), Qd is the heat transferredto air as output (kW), Qr is the heat received by zigzag receiver(kW) calculated by using the below equation [20]:

QR ¼Ap:Ib

1000(2)

where Ib is the beam radiation (Wm�2), Ap is the aperture area ofScheffler reflector (m2); the aperture area was calculated by usingthe below equation [20]:

Ap ¼ Ac Cosð43:236 d=2Þ (3)

where Ac is the surface area of Scheffler reflector (m2) and d is thesolar declination.

Heat energy available at the inlet of bakery chamber was com-puted by using the below equation:

Qd ¼ m � Cp � ðTin�TambÞ (4)

where m is the mass flow rate of air (kg), Cp is the specific heat ofair at constant pressure (1.008 kJ kg�1 K�1), and DT is the temper-ature difference (K).

The mass flow rate of air can be calculated using the belowequation:

m ¼ q� V (5)

where q is the density of air (1.127 kg m�3) and V is the volumet-ric flow rate (m3 s�1).

Fig. 2 (a) Dimensions of the reflector and (b) intersectionspoints of seven crossbars (q1–q7) on an elliptical reflector frame

Table 1 Three-dimensional model properties and settings ofthe simulations

Properties Value

Number of elements 96,139Volume main body 0.317 m3

Grid type 3D, tetrahedral, structuredTurbulence model k–e standardDiscretization Second-order upwindWall friction model No slipInlet air velocity (normal to air inlet) 0.1 m s�1

Outlet pressure 0 Pa

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The volumetric flow rate was calculated using the belowequation:

V ¼ A� v (6)

where A is the cross-sectional area of receiver (m2) and v is thevelocity of air (ms�1).

So, Eq. (7) can be written as

Qd ¼ A � v � q � Cp � ðTin– TambÞ (7)

As energy travels from one system to another, the energy loss isbound to occur. So, it is necessary for one to determine how muchenergy is actually being lost during this process to optimize thesystem if possible and to assess how much energy is available tothe system under study. An experiment was conducted to calculatethe energy available at Scheffler reflector, energy available afterreflector, and energy available at solar receiver. The falling beamradiations were not perpendicular to the frame of the Schefflerfixed focus concentrator (having area Ac) due to the lateral part ofthe paraboloid and inclined downward at an angle of (43.236d/2)deg (d is solar declination), which is positive for northern hemi-sphere and negative for southern hemisphere). Therefore, theaperture of the Scheffler reflector based on solar declination and istaken as Ac Cos (43.236d/2). The pyranometer was installed onthe primary reflector at a plane to which beam radiations act per-pendicular. The orientation and installation accuracy of the pyra-nometer can also be checked for any shadow during the wholeday. The total available energy at Scheffler reflector (Ep) is thebeam radiation recorded by pyranometer time fraction of the aper-ture area Ac Cos (43.236d/2) and was calculated as

Ep ¼ Gb � Ac Cos 43:237d2

� �(8)

The energy was disseminated as absorbed radiation and reflectedradiation. The reflectivity of the material is the most importantfactor in absorption of energy. In Scheffler reflector dish mirrorswere used as reflecting material having reflectivity of more than87%. The energy available after Scheffler reflector (Epr) was cal-culated using the below equation:

Epr ¼ Rp � Ep (9)

where Rp is the reflectivity of the material used for the Schefflerreflector. This reflection of radiation is due to the inappropriatereflector profiles and insufficiency in daily and seasonal tracking.The fraction available at the focus (Ff) was taken as 0.85. Theenergy available at the receiver (Esr) of solar bakery unit wascomputed using the below equation:

Esr ¼ EprFf (10)

2.3.2 Energy Supplied and Utilized During Baking Process.After calculating the energy distribution over primary reflectorand receiver, baking process was started. Four samples of cakeswere put inside the baking chamber over two trays. K-type ther-mocouples were placed to measure the temperatures T1, T2, T3,

and T4 representing temperatures before the receiver, after thereceiver, at inlet and outlet of the baking chambers, respectively.Air temperature and velocity were recorded after 5 min of timeinterval using K-type thermocouple and anemometer, respec-tively. The total energy supplied (kW), total energy used (kW)during the baking process, and energy utilization ratio (%) werecalculated. Air velocity was also recorded using an anemometer.

The energy utilization ratio is defined as the ratio of heat energyused during baking of cakes to the total energy supplied. Thus, theenergy utilization ratio was calculated as

EUR ¼ Cp T2ð Þ�Cp T4ð ÞCp T2ð Þ�Cp T1ð Þ

(11)

where EUR is the energy utilization ratio (%), Cp is the specificheat of air at constant pressure (1.008 kJ kg�1 K�1), T1 is thetemperature before the receiver (�C), T2 is the temperature afterthe receiver (�C), T3 and T4 are the temperatures at inlet andoutlet of the baking chamber (�C), respectively, as shown inFig. 3. It was assumed that T2¼T3 because the temperature afterthe receiver and at the inlet of baking chamber were found to bealmost same.

Heat energy supplied (Qs) at the inlet of baking chamber forbaking (kW) was calculated by using the below equation:

Qs ¼ A1 � V1 � q � Cp � ðT2– T1Þ (12)

where A1 is the cross-sectional area of duct (m2), V1 is the velocityof air after receiver (ms�1), q is the density of air (1.127 kg m�3),Cp is the specific heat of air at constant pressure (1.008 kJ kg�1

K�1), and (T2�T1) is the temperature difference (K) betweentemperature before the receiver and after the receiver (at the inletof baking chamber).

Heat energy utilized during baking of cakes was computed byusing the below equation:

Qu ¼ A2 � V2 � q � Cp � ðT2– T4Þ (13)

where A2 is the cross-sectional open area of perforated plate(m2), V2 is the velocity of air (ms�1), q is the density of air(1.127 kg m�3), Cp is the specific heat of air at constant pressure(1.008 kJ kg�1 K�1), and (T2� T4) is the temperature difference(K) between baking chamber inlet and outlet temperature.

Fig. 3 Temperature distribution at different points of solar bak-ery unit

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2.4 Error Analysis. During an experiment, errors and uncer-tainties are inviteable in the measured values depending upon theaccuracy of the instrument used. The instrument selection, calibra-tion, and way to handle the devices are main reasons of errors. Itis therefore mandatory to perform a detailed error analysis for theexperimental measured and calculated values. Using the conceptof the dimensionless relative error in the individual factorsdenoted by xn, the errors in the concerned parameters were calcu-lated using the following equation [21] shown in Table 2:

U ¼ x1ð Þ2 þ x2ð Þ2 þ � � � þ xnð Þ2h i1=2

(14)

3 Results and Discussions

3.1 Simulation Results. Figure 4 shows the simulated out-comes of the baking process in the form of airflow pattern(a), pressure profile (b) and turbulence eddy dissipation (c).Figure 4(a) shows flow path lines and contours representing airvelocity through the air duct and drying chamber. It can beobserved that air flow after the receiver is quite nicely distributedalong the perforated inlet of the baking chamber, making possibil-ity for the air to enter the chamber uniformly throughout its cross-sectional area. The average air flow within the chamber was foundto be 0.5 m/s at two baking trays and increased at the outlet due tosmaller area. At the bottom of the baking chamber, low airflowwas observed due to high uplift of warm inlet air in the inlet ductcausing comparatively low airflow in the bottom of the chamber.Figure 4(b) shows contours for pressure distribution. A uniformpressure occurrence inside the baking chamber can be observedand it had maximum values before entering the chamber due tothe resistance offered by perforated plate. This effect can also besupported from the contours of eddy viscosity (Fig. 4(c)), whichshows effect of turbulence, modeled with an effective eddy vis-cosity model. More turbulence occurred at the entrance of the bak-ing chamber due to divergence of flow direction and resistance toflow of perforated inlet plate. The simulated results show that geo-metric configurations of the bakery unit demonstrated a good airflow for the baking process. These results show the significance ofcomputational approach during design process. The occurring ofuniform airflow facilitates the distribution of heat uniformlythroughout the baking chamber, which is the most importantdesign aspect to get quality oriented baked product. Temperatureis the main influencing baking parameter for quality. Shahapuziet al. [22] studied the effect of airflow on oven temperature profileand reported an increase of 5–10% in cake volume expansion ratewhen airflow condition was applied. In order to compare thenumerical and experimental airflow pattern within the bakingchamber, the entire baking area was sectioned into three zonesvertically (each of 0.234 m wide and 0.64 m high). From each sec-tion, four measurements were taken. Air velocity was recordedusing an anemometer by having provision of small openings toinsert the probe of anemometer which were closed after taken

each value. Similarly from CFD results, values were taken bydrawing planes on respective areas and then average values of air-flow were taken from the extracted data of air velocity. Figure 5shows comparison between these average velocities (predictedand experimental measured), which revealed a good correlationbased on statistical analysis. This comparison strengthened thedesign configuration of baking chamber regarding proper air flowthrough the system, which ultimately facilitated the uniform tem-perature distribution associated with working medium (air).

Table 2 Error of uncertainty for the measured and calculatedparameters

Parameter measured/calculated Unit Value

Baking chamber inlet temperature �C 60.07Baking chamber inside temperature �C 60.02Baking chamber outlet temperature �C 60.08Temperature before the receiver �C 60.08Ambient temperature �C 60.15Total uncertainty for energy at Scheffler reflector kJ/s 60.52%Total uncertainty for energy at receiver kJ/s 61.49%Total uncertainty for supplied energy kJ/s 61.92%Total uncertainty for energy utilization kJ/s 62.03%

Fig. 4 Three-dimensional CFD based results for airflowstreamlines and contours: (a) pressure distribution contours,(b) turbulence eddies, and (c) with in the modelled bakery unit

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3.2 Receiver Efficiency. During the performance evaluationof Sheffler reflector and zigzag receiver of bakery unit, differentexperiments were conducted on daily basis at no load conditionbetween 10:00 and 14:00 on Oct. 21, 2015. The surface area ofScheffler reflector (Ac) was found to be 10 m2, declination (d) on

Oct. 21, 2015 was about �10.77 �C and aperture area (Ap) wasabout 7.897 m2.

Figure 6 shows the relationship between time of day (minutes)and receiver efficiency (%). At start, receiver efficiency of solarbakery unit increased with the time and became maximum(76.3%) at 12:00 p.m. because of maximum solar radiation. Afterthat, there was observed a decline in the receiver efficiency by 8%and it remained same up to 1:15 p.m. The efficiency again reachedat maximum value at about 1:30 p.m. The fluctuation in efficiencywas due to the fluctuation in solar intensity. As the solar intensityincreases, temperature increases at receiver, which eventuallyincreases the rate of heat transfer to the incoming air leading tohigher receiver efficiency. Based on data obtained through seriesof experiments conducted, the average efficiency of receiver wasfound to be 63% as tabulated in Table 3. It was also recorded thatthe maximum temperature found inside the baking chamber wasabout 180–220 �C, which is enough to bake different products.Baking material was put inside the baking chamber when temper-ature reached to 180 �C. Therefore, the availability of the requiredbaking temperature and the obtained baking results show thepotential of solar energy for the baking processes. More tempera-ture can be obtained successfully by optimizing receiver’s designfor the enhancement of its efficiency. Second, the curvature of thesecondary reflector is also important to consider in this regard.Therefore, these results are showcase for the industrial level appli-cation of solar energy to process food items.

Fig. 5 Comparison of average predicted and average experi-mental measured air velocity in the baking chamber

Fig. 6 Relationship between time and receiver efficiency

Table 3 Parameters determined for receiver efficiency

TimeAmbient,T1 (�C)

Inside bakingchamber, T2 (�C)

T2�T1,DT (�C)

Beam radiations,Ib (Wm�2)

Air velocity,v (ms�1)

Efficiency,g (%)

11:00 a.m. 30.5 66.0 35.5 794 0.4 37.9911:15 30.0 76.5 46.5 832 0.5 59.3711:30 31.0 86.0 55.0 863 0.4 54.1611:45 31.5 93.5 62.0 870 0.4 60.56

12:00 p.m. 30.0 94.5 64.5 898 0.5 76.3012:15 29.5 97.0 67.5 893 0.4 64.2412:30 30.0 99.0 69.0 896 0.4 65.4512:45 30.0 100.5 70.5 905 0.4 66.201:00 31.5 102.0 70.5 899 0.4 66.641:15 31.5 105.5 74.0 891 0.4 70.591:30 32.0 110.5 78.5 896 0.3 55.841:45 30.0 113.0 83.0 892 0.4 79.082:00 31.0 120.0 89.0 889 0.3 63.81

Avg.¼ 63%

Fig. 7 Relationship between energy supplied and energy utili-zation ratio with baking time

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3.3 Energy Supplied and Consumed for Baking. Theamount of heat transferred to air through receiver is actually theamount of energy available for the baking process. The relation-ship between amount of energy supplied and energy utilizationratio during the baking process has been shown in Fig. 7. Theinside baking temperature was 180 �C for 30 min. It can beobserved that the rate of energy consumed decreased with bakingtime which ultimately lowered energy utilization ratio. At thestart, energy consumed in the baking was about 75%, whichdecreased to 25% at the end of the baking process (50% less con-sumption as compared to the energy consumed at the start of theprocess). Rate of energy utilization decreases as product tends tobe baked/dried due to the less removal of moisture through uppercrusty surface creating resistant to heat and mass transfer. The dif-ference between amount of energy supplied and energy consumedat the early stages of the baking process could be due to the use ofless number of samples to be baked (one cake). Putting morecakes on both trays would definitely increase the rate of energyconsumption. Second, some amount of heat losses would alsooccur through the provision kept for data measuring inside thebaking chamber. The measured data for the calculation of theseparameters are shown in Table 4.

3.4 Energy Distribution. The loss of energy on its way fromprimary reflector to baking chamber was also estimated to

optimize the system if possible and to assess how much energy isavailable for the system under study. Energy available at Schefflerreflector, energy available after reflector, and energy available atsolar receiver were calculated using the respective equations asdescribed before.

Figure 8 shows that about 39% (4.69 kW) of energy was avail-able at Scheffler reflector (primary reflector), and out of which34% (4 kW) of energy was available for secondary reflector whilethe energy available at the receiver was calculated to be 28%(3.468 kW). This energy is further transferred to the incomingair for baking process. So, receiver efficiency to transmit itsavailable heat to air is of much important, which was 63%as reported above. Second, to get uniform baked products, uni-form airflow within the baking chamber is of main factor. Sab-ovics et al. [23] reported a significant effect of bakingtemperature variation on different selective baking qualityparameters (bread moisture, water activity, acidity and pHparameters, as well as hardness of bread crumb). That tempera-ture distribution is linked with airflow pattern, which depends onsystems configuration.

It is obvious from both simulation and experimental results thatair was quite uniformly distributed inside the baking chamber.The energy difference at Scheffler reflector and the receiver aredue to energy losses. These losses include reflectivity of mirrordue to incomplete absorbance and heat losses from different partsof solar bakery unit, i.e., through conduction, convection, andradiation.

4 Conclusion

Different technologies have been developed to extract maxi-mum power from solar energy for cooking purposes but very lessattention has been paid to demonstrate the true potential of thisresource and how effectively it can substitute the conventionalresources of energy for food baking. This study reported thedevelopment of a solar based baking unit for the demonstrationof decentralized solar energy application in baking industry onscientific basis. Before the development process, CFD-based 3Dsimulation was also performed to assess the functionality of thedesign based on uniform airflow pattern. The system designedconfigurations gave quite good results for airflow distribution. Thedevelopment was made using locally available materials andinfrastructure to reduce its initial cost and making it affordable forpoor community members. Further improvements can be made inthe system by optimizing its process conditions with respect to airtemperature, humidity, and solar intensity. Energy efficiency canalso be improved by optimizing the surface area of receiver. Theobtained results revealed that by providing engineering-basedsolution, solar energy can be effectively used in the baking indus-try not only to reduce energy cost but also to perform a decentral-ized baking process. Such kind of solar thermal applications notonly help to overcome the problem of depletion of firewood andfossil fuels but also avoid the hazardous carbon emissions thatcause global warming.

Table 4 Parameter determined during baking process of cake

Time

Air temperaturebefore receiver,

T1 (�C)

Baking chamberinlet temperature,

T2¼T3 (�C)

Baking chamberoutlet temperature,

T4 (�C)

Air velocityafter receiver,

V1 (m/s)

Air velocity inbaking chamber,

V2 (m/s)

Energysupplied,Q1 (kW)

Energyutilizationratio (%)

12:00 160 180 165 0.3 0.4 0.731 7512:05 162 179 168 0.4 0.4 0.828 6412:10 167 180 173 0.4 0.5 0.634 5312:15 170 180 176 0.4 0.5 0.487 4012:20 174 180 178 0.3 0.4 0.219 3312:25 176 180 179 0.4 0.5 0.195 2512:30 176 180 179 0.4 0.5 0.195 25

Fig. 8 Energy distribution at Scheffler reflector and receiver

061005-8 / Vol. 140, DECEMBER 2018 Transactions of the ASME

Funding Data

� The Department of Energy Systems Engineering.� University of Agriculture Faisalabad.� International Centre for Development and Decent Work

(ICDD).� German Academic Exchange Service (DAAD), Germany.

Nomenclature

A ¼ cross-sectional area of receiver (m2) (ft2)Ac ¼ surface area of Scheffler reflector (m2) (ft2)Ap ¼ aperture area of Scheffler reflector (m2) (ft2)A1 ¼ cross-sectional area of duct (m2) (ft2)A2 ¼ cross-sectional open area of perforated plate (m2) (ft2)CP ¼ specific heat (kJkg�1 k�1) (BTU lb�1��F�1)Ep ¼ total available energy at Scheffler reflector (kW)

(horse-power (HP), 1 HP¼ 550 ft lbs/s,1 HP¼ 0.746 kW)

Epr ¼ energy available after Scheffler reflector (kW)(horse-power (HP), 1 HP¼ 550 ft lbs/s,1 HP¼ 0.746 kW)

Esr ¼ energy available at the receiver (kW) (horse-power (HP),1 HP¼ 550 ft lbs/s, 1 HP¼ 0.746 kW)

EUR ¼ energy utilization ratio (%)Ff ¼ fraction available at the focus (kW) (horse-power (HP),

1 HP¼ 550 ft lbs/s, 1 HP¼ 0.746 kW)Ib ¼ beam radiation (W/m2) (W/ft2)

ma ¼ air flow rate (kg s�1) (lb s�1 or slug s�1)Qd ¼ heat given by receiver to air as output (kW) (horse-power

(HP), 1 HP¼ 550 ft lbs/s, 1 HP¼ 0.746 kW)QR ¼ heat received by zigzag receiver (kW) (horse-power (HP),

1 HP¼ 550 ft lbs/s, 1 HP¼ 0.746 kW)Qs ¼ heat energy supplied at the inlet of baking chamber for

baking (kW) (horse-power (HP), 1 HP¼ 550 ft lbs/s,1 HP¼ 0.746 kW)

Qu ¼ heat energy utilized during baking of cakes (kW)(horse-power (HP), 1 HP¼ 550 ft lbs/s,1 HP¼ 0.746 kW)

Rp ¼ reflectivity of the material used for the Scheffler reflector (%)T ¼ temperature (�C) (�F)

Tamb ¼ temperature of ambient air (�C) (�F)Tin ¼ temperature at inside baking chamber (�C) (�F)T1 ¼ temperature before the receiver (�C) (�F)T2 ¼ temperature after the receiver (�C) (�F)T3 ¼ temperature at inlet of baking chamber (�C) (�F)T4 ¼ temperature at outlet of baking chamber (�C) (�F)v ¼ velocity of air (ms�1) (ft s�1)V ¼ volumetric flow rate (m3 s�1) (ft3 s�1)

V1 ¼ velocity of air after receiver (ms�1) (ft s�1)V2 ¼ velocity of air after perforated plate (ms�1) (ft s�1)d ¼ solar declinationgr ¼ receiver efficiency (%)q ¼ density of air (kg m�3) (slug ft�3)

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