https://orcid.org/0000-0001-8906-7153
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ABSTRACT
An active indirect solar dryer (AISD) has been used to conduct drying experiments of beetroot slices without a thermal storage unit (TSU) (mode-I) and with TSU (mode-II) using paraffin wax as a phase change material. The performance analysis and drying curves of beetroot slices were assessed in both modes and compared. The average drying efficiency and drying rate were enhanced by 48.63% and 41.72% in mode-II compared to mode-I. Two-term exponential and modified Page models were the best models for beetroot slices in mode-I and mode-II, respectively. The specific energy consumption was estimated to be 1.706 and 0.96 kWh/kg, the average moisture diffusivity was 7.11 × 10−9 and 7.22 × 10−9 m2/s, and the mean mass transfer coefficients were 3.56 × 10−3 and 3.64 × 10−3 m/s, respectively. The specific moisture extraction rate was 0.586 and 1.041 kg/kWh and the mean heat transfer coefficient was 4.103 and 4.2 W/m2K, in mode-I and mode-II, respectively. The collector outlet temperature is increased by 2.5 to 10.7°C when the heat is discharged from TSU. In comparison to mode-I, the performance of AISD is superior in mode-II, exhibiting better drying performance due to TSU in the drying section in mode-II.
Nomenclature
| Abbreviations | = | |
| AISD | = | active indirect solar dryer |
| db | = | dry basis |
| HSD | = | hybrid solar dryer |
| MC | = | moisture content |
| MR | = | moisture ratio of the slice |
| MSD | = | mixed solar dryer |
| OSD | = | open sun drying |
| PCM | = | phase change material |
| PISD | = | passive indirect solar dryer |
| RMSE | = | root mean square error |
| SAC | = | solar air collector |
| STD | = | solar tunnel dryer |
| TSU | = | thermal storage unit |
Symbols
| A | = | area exposed (m2) |
| cpa | = | specific mass heat of air (J/kgK) |
| De,aw | = | diffusivity value of air in water (m2/s) |
| De | = | effective moisture diffusivity (m2/s) |
| DR | = | drying rate of slice (kg/h) |
| Dpf | = | constant for pre-exponential factor |
| Eac | = | activation energy (kJ/mol) |
| Ei,dr | = | input energy to ISD (kWh) |
| F | = | thickness of the slice (m) |
| hmc | = | mass transfer coefficient (m/s) |
| hhc | = | heat transfer coefficient (W/m2K) |
| Isn | = | solar radiation at a time (W/m2) |
| k | = | thermal conductivity (W/mK) |
| L | = | latent heat of vaporization (J/kg) |
| Le | = | Lewis number |
| = | mass flow rate of drying air (kg/s) | |
| mev | = | amount of water evaporated (kg) |
| N | = | number of experimental observations |
| Q | = | heat transfer rate (W) |
| R2 | = | coefficient of determination |
| Rug | = | universal gas constant |
| SMR | = | specific moisture extraction rate (kg/kWh) |
| SEC | = | specific energy consumption (kWh/kg) |
| T | = | temperature at a section (°C) |
| t | = | drying duration (h) |
| U | = | uncertainty of dependent parameter |
| u1, u2 … un | = | uncertainty values for independent parameters |
| Vf | = | volume of the food material (m3) |
| x1, x2…xn | = | independent parameters |
| z | = | number of constant variables |
Greek symbols
| η | = | Efficiency (%) |
| χ2 | = | reduced chi-square |
| α | = | thermal diffusivity for drying air (m2/s) |
Subscripts
| amp | = | environment |
| col | = | collector |
| d1, d2, d3, d4 | = | Trays of 1 to 4 |
| da | = | dried air |
| dr | = | drying |
| ds | = | drying section |
| i | = | entry |
| o | = | exit |
| sf | = | surface area for the food material |
| u | = | useful |
Acknowledgements
The authors thank the Department of Mechanical Engineering, NIT Warangal (Grant No. NITW/MED/Head/2015/408 dated 3rd Dec. 2015) for the financial support.
Disclosure statement
No potential conflict of interest was reported by the authors.