https://orcid.org/0000-0003-4356-8184
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ABSTRACT
The technical performance and economic analysis of a developed hybrid solar-electric dryer (HSED) were investigated in this research work with respect to energy, exergy, and environmental sustainability. Experimental investigations were performed with and without load during the rainy season, in which the sunshine periods were affected by intertropical discontinuity, prompted by cold-prevalent wind interference from the Atlantic Ocean and regional water catchment. Thermal characteristics and drying efficiency of the hybrid dryer, as well as the effect of different drying temperatures (50, 60, and 70°C), air velocities (0.5, 1.0, and 1.5 ms−1), and sample thicknesses (10, 15, and 20 mm) on the overall and specific energy usage for 1,500 g batch size of fresh sliced tomato samples, were investigated. The percent energy contributions of the solar and electric heat units at varying air velocities were also determined. Results obtained indicate that the mean solar collector efficiencies during sunshine hours ranged between 24.6 and 70.3%. The total and specific energy consumption of tomato slices (Lycopersicon esculentum) varied between 5.61–120.31 kJh and 5.18–167.59 kJhg−1, respectively. Analysis of variance results show that drying air temperature, sample thickness, and velocity of air were significant at P > 0.05. The percent energy contribution by solar and electric heat units varied between 44.57–56.24% and 43.76–55.43%, respectively. Drying time and drying efficiency ranged between 130 ± 7 and 330 ± 5 min and 4.33–36.38%, respectively. The average energy efficiency of the hybrid system increased from 15.67 to 38.17%, whereas the mean exergy efficiency varied between 32.2 and 87.9%. The sustainability indicators such as ratio of waste exergy, sustainability index, and improvement potential of the HSED ranged from 0.083 to 0.158, 1.93–6.77, and 0.099–0.289 kJs−1, respectively. Optimum drying conditions for improved final quality were given. The economic analysis established that the HSED could save up to $1,490.33 per annum with a low payback period (0.72 years), thus making the dryer cost effective and economically viable. Prospects for improvement and future works were highlighted.
Nomenclature
| A | = | area (m2) |
| Acc | = | annual capital cost |
| Afc | = | annualized running fuel cost |
| Amc | = | annualized maintenance cost |
| As | = | annualized salvage value |
| Cc | = | annual capital cost |
| Cd | = | cost of fresh tomato per kg batch of dried samples ($/kg) |
| Ce | = | unit cost of electricity ($/kWh) |
| Ceq | = | cost per equivalent dried kg batch of tomato product in the open market ($) |
| Cft | = | cost of fresh tomato ($) |
| Ch | = | cost of drying 1kg of sliced tomato sample ($) |
| Cpa | = | specific heat capacity of air (J/kgK) |
| Ec | = | electricity cost ($) |
| Ee | = | experimental energy consumption (kJh/g) |
| Ex | = | exergy (kJ/s) |
| F | = | efficiency factor |
| Fr | = | heat removal factor |
| h | = | enthalpy of drying air (kJkg−1) |
| Hp | = | heater power (1500 W) |
| i | = | interest rate (10%) |
| Ig | = | incident solar intensity W/m2 |
| Ip | = | improvement potential |
| Kc | = | capital recovery factor |
| Kh | = | heater geometric factor |
| Ks | = | sinking fund factor |
| = | molar ratio of moisture (0.0004) | |
| L | = | latent heat of vaporization of moisture (kJkg−1). |
| l | = | lifespan of dryer (10 years) |
| M | = | mass flow rate of air (kg/s) |
| Ma | = | molar mass of air (29 gmol−1) |
| md | = | mass of dried tomato per batch (kg) |
| mf | = | mass of fresh tomato per batch (kg) |
| Mm | = | mass of moisture expelled (kg). |
| Mv | = | molar mass of vapor (18.153 gmol−1), |
| Nw | = | number of working days per year |
| P | = | pressure of product moisture (Pa) |
| Pb | = | payback period (years) |
| Pe | = | predicted energy consumption (kJh/g) |
| Ph | = | heater power (kW) |
| Q | = | total heat energy inflow to the drying chamber (kJs−1) |
| Qr | = | energy utilization ratio |
| r | = | mean inflation rate (11.32% as at year 2019). |
| Rm | = | universal gas constant for moisture (0.446 kJkg−1) |
| S | = | sample slice thickness (mm) |
| T | = | temperature (oC) |
| t | = | drying time (h) |
| tb | = | drying time per batch (5½ h). |
| = | temperature of the Sun (4,350 K) | |
| Si | = | sustainability index |
| Ul | = | overall heat loss coefficient |
| V | = | air velocity (ms−1). |
| = | waste exergy ratio | |
| = | rate of mechanical work output (kJs−1) |
Subscripts
| a | = | air or ambient |
| c or col | = | collector |
| e | = | electric |
| i or 1 | = | inlet or initial |
| l | = | loss |
| o or 2 | = | outlet or final |
| t | = | time or total |
Greek Symbols
| = | effective transmittance of glass (0.06) | |
| = | absorbance of glass (0.84) | |
| = | solar collector efficiency (%) | |
| = | energy efficiency of hybrid dryer (%) | |
| = | drying efficiency (%) |
Abbreviations
| EU | = | energy utilization (J/s) |
| HSED | = | hybrid solar-electric dryer |
| $a | = | annual savings |
| $d | = | daily savings |
| $kg | = | money saved in drying sliced tomato |
| $1 | = | savings in the first year |
Acknowledgments
The authors appreciate the assistance of the technical staff of the Agricultural and Bioresources Engineering Department of the Federal University of Technology, Owerri, Nigeria.
Declaration of interest
The authors have declared no competing interest among them.