https://orcid.org/0000-0001-8867-1113
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ABSTRACT
In this work, analytical modeling and theoretical analysis of a recently developed unique design of a solar cooker cum dryer (SCCD) have been conducted to predict its thermal characteristics under a given set of input parameters. The energy balance equations of the main components of the cooking compartment are iteratively solved to determine the unknown temperatures of the SCCD unit when it operates as a pure cooker and when the same acts as an air heater for the food drying compartment. The influence of design parameters such as air velocity, glazing thickness, spacing between glazing, back reflector, and surface coatings of the absorber plate has been studied and quantified. The analytical model has been tested by simulating and validating the modeling results with the experimental data for different days of the year Moreover, further experiments are conducted on the SCCD in drying mode to quantify its thermal performance in natural versus forced convection of the airflow through the SCCD and to demonstrate its effectiveness in comparison to open sun food drying. The obtained results show that the SCCD unit attain cooking and drying temperatures in the suitable range of 80–135℃ and 50–70℃, respectively. The experimental results of the SCCD unit as a food dryer revealed that the Page model with higher adequacy is found to be the best fit for all three modes of food drying in the SCCD unit. It is found that in comparison to drying of tomatoes in the open sun which took ⁓13 hrs, forced convection of airflow in the SCCD unit resulted in the least time of ⁓7.5 hrs to achieve the desired level of moisture content. The energy and exergy efficiency of the SCCD in cooking mode was estimated to be in the range 2.1–15.5% and 1.1–5.6%, while it was 7.5–16.5% and 26.6–50.7% in drying mode, respectively.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Nomenclature
| A | = | Area (m2) |
| Hc | = | Convective heat transfer coefficient (W/m2. K) |
| hcond | = | Conductive heat transfer coefficient (W/m2. K) |
| hr | = | Radiative heat transfer coefficient (W/m2. K) |
| I | = | Hourly incident radiations (W/m2) |
| Ib | = | Beam component of incident solar radiations (W/m2) |
| Id | = | diffuse component of incident solar radiations (W/m2) |
| k | = | Thermal conductivity of air (W/m. k) |
| K | = | Drying rate (gm hr−1) |
| mw | = | Mass of water (kg) |
| hfg | = | Latent heat of vaporization of water (kJ/kg) |
| Twi | = | Initial temperature of water (o C) |
| Twf | = | Final temperature of water (o C) |
| t | = | Time (sec) |
| Mi | = | Initial moisture content (-) |
| Mf | = | Final moisture content (-) |
| Mwb | = | Moisture content on wet basis (-) |
| Mdb | = | Moisture content on dry basis (-) |
| Me | = | Equilibrium moisture content (-) |
| M.R | = | Moisture ratio (-) |
| Nu | = | Nusselt number (-) |
| Pr | = | Prandtl number (-) |
| Q | = | Heat transfer rate (W) |
| Rb | = | Beam radiation tilt factor (-) |
| Ra | = | Rayleigh number (-) |
| R2 | = | Coefficient of determination |
| RMSE | = | Root mean square error |
| S | = | Absorbed solar radiations (W/m2) |
| Sref | = | Solar radiations absorbed from reflector (W/m2) |
| SSE | = | Sum of square error |
| T | = | Temperature (o C) |
| UL | = | Overall heat loss coefficient (W/m2.k) |
| Vair | = | Velocity of air (m/sec) |
| FC | = | Forced convection |
| NC | = | Natural convection |
| OD | = | Open sun drying |
| F | = | View factor |
| X | = | Length of absorber plate (m) |
| Y | = | Width of absorber plate (m) |
| = | X/L | |
| = | Y/L | |
| ղ | = | Energy efficiency |
| Ex | = | Exergy efficiency |
| β | = | Slope/surface tilt (deg) |
| δ | = | Declination angle (deg) |
| γ | = | Surface azimuth angle (deg) |
| ε | = | Emittance (-) |
| θ | = | Incidence angle (deg) |
| ρg | = | Ground reflectance (-) |
| ρr | = | Reflectance of reflector (-) |
| ω | = | Hour angle (deg) |
| σ | = | Stefan Boltzmann constant (W/m2. k4) |
| ϕ | = | Latitude angle (deg) |
| ( | = | Transmittance - absorptance product (-) |
| ( | = | Transmittance - absorptance product for beam radiations (-) |
| ( | = | Transmittance - absorptance product for diffuse radiations (-) |
| ( | = | Transmittance - absorptance product for beam radiations from reflector (-) Superscripts |
| a | = | Ambient |
| s | = | sky |
| c | = | SCCD in cooking mode |
| d | = | SCCD in drying mode |
| ins | = | Insulation |
| p | = | Absorber plate |
| g1 | = | Inclined inner glass cover |
| g2 | = | Inclined outer glass cover |
| p-g1 | = | Absorber plate to inclined inner glass cover |
| p-air | = | Absorber plate to chamber air |
| air-g1 | = | Chamber air to inclined inner glass cover |
| g2-sky | = | Outer glass cover to sky |
| g1-g2 | = | Inner to outer inclined glass cover |
| g2-a | = | Inclined outer glass cover to ambient |
| ins-a | = | Insulation to ambient |
| n | = | Number of observations |
| m | = | Number of constants |
Additional information
Notes on contributors
Muhammad Yasin Khan
Mr. Muhammad Yasin Khan runs a private business and is not officially affiliated with any academic institute. He has been working as an independent researcher in the field of renewable energy. He has already published a number of research papers without having a particular institutional affiliation.
Khalid Mahmood
Khalid Mahmood received his PhD in Mechanical Engineering from the University of Manchester, UK. He is Associate Professor at the Department of Mechanical Engineering, HITEC University. His research interests include Laser Material Processing, Additive Manufacturing, and Heat Transfer.
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