3E analysis and mathematical modelling of garlic drying process in a hybrid solar-electric dryer

Highlights

• The hybrid solar-electrical drying of garlic cloves has been carried out.

• Mathematical modelling, and characteristic drying curve have been determined.

• The environmental exergy issues of the dryer have been verified.

• Payback period of the dryer proposed in Algeria was estimated at 0.08 years.

• Total phenol compound and DPPH were determined.

Abstract

The current study aims to explore the hybrid solar-electric drying (HSED) of garlic cloves, tests were carried out at three different temperatures 50, 60 and 70 °C under natural convection mode and forced convection mode. Drying air velocity of 4.1 and 6.9 m s⁻¹ have been studied experimentally in forced convection mode. Midilli-Kucuk model can be used to predict the drying behaviour of garlic cloves. The energy payback time of the forced convection dryer are 0.62 and 0.32 years and the net CO₂ mitigation was 72.61 and 140.81 ton for air velocity of 4.1 and 6.9 m s⁻¹, respectively. The highest exergy efficiency of 89.86% was achieved at 50 °C drying temperature in natural convection, with a range of 69.61–89.86%. The highest environmental impact factor of 40.35 was observed for the lowest exergy efficiency. The proposed HSED in Algeria has a significant payback period of only 0.08 years compared to Morocco with 0.7 years. The effect of air velocity on the total phenol compound and 2-diphenyl-1-picryl hydrazyl (DPPH) was found to be significant, and the highest values were recorded for HSED under natural convection mode. The proposed HSED in Algeria was found to be sustainable both environmentally and economically.

Introduction

Garlic (Allium sativum L., Alliaceae) has played one of the most important nutritional and medicinal roles in human beings for centuries [1]. It was produced since ancient times and used in many cultures as a flavouring powder due its benefits in both curative and preventive medicine. Unfortunately, as a result of the lack of transportation, processing and storage, 30% of global garlic production is eliminated each year [2].

According to the Algerian Ministry of Agriculture [3], Algeria achieved in 2019 a large surplus in garlic production rising to 8 thousand tons with a total cultivated area of 11700 ha. The price per kilogram reached as low as 0.078 USD in the harvest season and most producers subsequently suffered from large losses of surplus production which was eventually burned. On the other hand, its price raised sharply costing 15.5 USD per kilogram in the rest of the seasons due to the lack of an effective way of preservation.

In many regions of the world, where the garlic is grown, the drying process is the most common way to preserve the surplus. Nowadays, it takes a long time to dry crops with open sun and also harms the environment by emitting odours [4]. Therefore, numerous scientific works have been done to provide an innovative highly effective drying process. Solar energy is always used in many agriculture operations. Solar drying has been utilized since ancient times to dry plants, seeds, natural products, meat, angle, wood and other rural and forest items [5]. The solar dryer results in instant heat and mass transfer as the product heated, dehumidified and transported by the preheated air. It improves the quality of the final product, reduces the cost of production without causing GHGs, which are harmful to the global environment [6]. Their reliability and intermittent weather are one of the most common issues faced by solar dryers i.e., unstable quality of sun-oriented radiation during the stormy period or cloudy days and its inaccessibility at night-time. By hybridization, the drying process can continue throughout the night hours using the back-up energy from the stored heat and/or secondary source. Thus the product is spared from conceivable disintegration by microbial invasion [7].

Hybrid solar dryers are systems in which solar energy is an additional energy source used to heat the drying air. Heating frames and supplementary electric heater, fans, biomass or fossil fuel all are used to ensure heated air circulation. They are typically used in forced convection mode [8]. The hybrid systems allow control of aerothermal conditions inside the drying chamber such as temperature, air velocity, humidity, etc. The ability of hybrid systems results in a good control of drying kinetics and extraction of drying characteristics model. Also, since the temperature can be controlled, the effective diffusion coefficient and activation energy of the product can be deduced. These characteristics are specific parameters to extend the wide spread use of hybrid solar dryers and to ensure and control the dry product’s value.

Chandrasekar et al. [9] investigated experimentally the effect of split (A/C) condenser unit integrated into an indirect type solar dryer on the drying kinetics of grapes, and exergy of the drying system. The hybrid dryer reduced drying time by 16.7% compared with open sun drying. The integration of the condenser increased the inflow exergy. Yahya et al. [10] studied the energy and exergy analysis of a fluidized bed solar dryer with biomass furnace for drying of paddy. The specific energy consumption and thermal efficiencies of the drying system were in the range of 4–4.46 kWh and 13.45%–16.28%, respectively. The exergy efficiencies were found to be low due to the increased exergy losses and it was 47.6% and 49.5% for average drying temperature of 61 and 78 °C, respectively. Atalay [11] analysed the energy and exergy data of indirect solar dryer with packed bed thermal energy storage for drying of orange slices. The dryer was tested with both with and without energy storage. The average specific energy consumption was determined as 1.8897 and 1.9804 J/kg, and the highest exergy efficiency for the drying system was found to be 66.58% and 68.37%, for the two cases, respectively. In this study, improvement potential was found to be in the range of 0.016–0.07 kW. Suherman et al. [12] studied the hybrid solar dryer with a burner unit for drying of sugar-palm vermicelli. Increasing drying temperature from 40 to 100 °C increased the dryer efficiency from 13.02 to 17.02%, energy utilization ratio from 0.18 to 0.32 and exergy efficiency from 67.4 to 83.6%, respectively. Nwakuba et al. [13] developed a hybrid electric-solar dryer to dry red pepper at drying temperatures of 50, 60 and 70 °C, and air velocity of 1.14, 2.29 and 3.43 ms⁻¹. The highest exergy efficiency was recorded for the lowest drying temperature of 50 °C and lowest air velocity of 1.14 ms⁻¹, while the low efficiency was detected as 35.8% at 70 °C and 3.43 ms⁻¹ due to the increase of exergy losses and outflow exergy. The developed dryer was able to achieve a 30.3% higher CO₂ reduction in comparison with that obtained from a hybrid solar-biomass dryer with 398.86–3872.7 tons. Ndukwu et al. [14] designed a hybrid-solar biomass dryer for drying plantain slices. The exergy efficiency of the dryer ranges from 10.6 to 95.13% with biomass furnace (SD₁) and 5.6–93% without biomass furnace (SD₂). The amount of CO₂ entering to the atmosphere reduced from 307.4 up to 3074 tons and from 44 up to 440 tons per year for SD₁ and SD₂, respectively. The hybrid biomass-solar dryer can save between 20.5 and 205.17 USD/year and 2.94 and 29.443 USD/year for SD₁ and SD₂, respectively. Zachariah et al. [15] tested a photovoltaic-assisted mixed-mode solar dryer with and without thermal energy storage for drying of bitter gourd. The dryer assisted with thermal storage (PCM) can keep the drying temperature suitable for drying process in the range of 40°–50 °C for a longer duration. The dryer’s overall thermal efficiency was found to be 18.6% and 10.8%, respectively, with and without PCM storage. The dryer with thermal storage was found to be both environmentally and economically sustainable with an energy payback time and a discounted payback period of 1.91 and 0.8 years, respectively. A fluidized-bed solar-assisted drying system for paddy grains drying was studied by Mehran et al. [16]. The heating system generally include a solar water heater and an infrared lamp powered by photovoltaic panels as the primary and main heating sources, along with a gas water heater as the secondary heating source, which was used together to provide the required thermal energy. The experiments were conducted based on variables air velocity of 7, 8 and 9 m s⁻¹, drying temperature of 35, 45, and 55 °C and return air gate status. The specific energy consumption varied between 17.07 and 9.76 and 15.02 kWh/kg for drying temperatures of 35, 45, and 55 °C, respectively, while the lowest value was recorded at 45 °C. Furthermore, it was found to be 11.95, 14.20 and 15.69 kWh/kg for drying system in closed, semi-open and open status of the return air gate, respectively. Singh et al. [17] investigated the energy and exergy analysis using a hybrid solar drying system with four drying methods namely (i) heat pump drying (HPD), (ii) infrared-assisted heat pump drying (IHPD), (iii) solar assisted heat pump drying (SAHPD) and (iv) solar-infrared-assisted heat pump drying (SIAHPD). The highest values of both energy and specific energy consumptions were 2.98 kWh and 1.033 kWh/kg, respectively, which were obtained from IAHPD system. Drying efficiency, energy efficiency and total exergy losses of 61.92%, 58.5% and 0.598 kW were recorded for SIAHPD, SAHPD, and SIAHPD systems, respectively. Fresh chamomile flower was dried using a developed hybrid solar dryer [18]. The dryer consists mainly of a main drying chamber and additional small chamber, heat exchanger, and thermal storage unit. The dryer reduced moisture content of chamomile from 72 to 6% (w.b) in 30 and 33 h for main drying chamber and additional drying chamber, respectively. The volatile oil content in dried chamomile flowers by the hybrid solar dryer was high then the samples dried directly under sun, this may due to that the solar dried sample was properly dried with uniform temperature using the hybrid dryer.

All papers reviewed above confirm that the hybrid solar dryers contribute to improving drying kinetics, energy and exergy efficiency with the limitation of high exergy losses. Also, hybrid solar dryers are proving to be sustainable in environmental and economic terms.

In Algeria, many experimental investigations and mathematical modelling studies have been accomplished to improve the hybrid solar drying of several agro-products such as tomato [19,20], potatoes [21], dates [22,23], pumpkin slice [24], henna leaves [25], camel meat [26]. The increase in garlic production in Algeria caused great losses for farmers due to the inability to preserve the surplus. Additionally, there is a real research gap in the treatment of garlic by solar drying means. However, from the literature review, there is no information about the indirect type solar drying of garlic cloves assisted electric heater. As a result, the present study was conducted using a hybrid solar-electric drying system to valorise the surplus of garlic in Algeria. The HSED operated with three drying temperatures 50, 60 and 70 °C under natural convection (NC) and air velocity of 4.1 and 6.9 m s⁻¹. Therefore, the primary objectives of the present study are:

• : Mathematical modelling and characteristic drying curve of dried garlic.

• : Environmental analysis and energy payback time of the HSED.

• : Exergy analysis of the drying process, waste exergy ratio, sustainability exergy index and improvement potential as well as the environmental impact factor were determined.

• : Economic analysis of the HSED in Algeria and Morocco cases for drying of garlic.

• : The effect of drying temperatures and air-drying velocity on total phenolic compound and DPPH.