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• Effectiveness of geothermal/solar drying system for onion in cloudy and rainy weather is studied. • Energy, exergy and environmental analysis of the drying system have been determined. • Solar fraction and geothermal fraction have been verified. • Average payback period of the system was found to be 0.74 years. • Low total color change of dried onion was observed. Preservation of onion surplus during rainy and cloudy days is a big challenge for farmers in the so-far area in northern Sahara of Algeria. Thus, available renewable energies such as geo-exchange and solar energy could be an effective solution. In the current study, an insulated north wall solar dryer (INWSD) integrated with geo-exchange heat exchanger (GHE) with inlet temperature of 80 °C was tested for drying onion slices under cloudy and rainy weather. Solar radiation and ambient temperature were in the range of 0–447 W m−2 and 9.65–26.53 °C, respectively. Drying process took 28 consecutive hours. Average diffusivity was found to be 2.81 × 10−10 m2 s−1 by considering the variation of relative humidity and temperature inside the drying chamber. Specific energy consumption varied between 0.66 and 1.75 kWh/kg with an average of 1.71 kWh/kg. Thermal efficiency was in the range of 1.48%–48.90%, with an average value of 9.52%. The low CO 2 emissions of 46.69 kg/year and average payback period of 0.74 year were obtained using the INWSD. Dried onion was with low total color change at 3.79 compared to fresh samples. Therefore, INWSD is a promising technique by comprehensively considering the 3E (energy, economic, economic-environmental) and color analysis. [ABSTRACT FROM AUTHOR]

BACKGROUND: The economic value of many fruit and vegetables is often linked to the ability to preserve such natural products. In the case of tomatoes, the manufacture of 1 kg of powder or concentrate requires the evaporation of about 16 kg water kg−1 dry matter. In the present work, the effectiveness and economic analysis of solar drying systems for drying of tomato paste with and without geothermal water heat exchanger (GWHE) were studied. RESULTS: The GWHE integration significantly increased the recorded drying temperature up to 53.41 °C, with an improvement of 11.5 °C compared to the conventional dryer. Moreover, the drying temperature inside the drying chamber exceeded ambient temperature after sunset and at night by 30.8 °C. GWHE ensured a continuous drying process, while drying time was 22 consecutive hours, compared to the basic dryer with 18 h in 3 days. Effective moisture diffusivity was found to be 1.617 × 10−9 and 1.202 × 10−9 m2 s−1 for hybrid and basic drying systems, respectively, using the slope. Three methods were used to perform economic analysis. The cumulative present worth of savings, at 23 382.6 USD, is much higher than the capital cost of the hybrid dryer, at 467 USD. The payback period is quite acceptable, at 2.21 years as compared to dryer lifetime of 15 years. Therefore, the hybrid dryer will dry tomato paste without additional charges for 12.79 years and is recommended for a large‐scale tomato paste dryer. CONCLUSION: The hybrid system with GWHE ensures a continuous drying process during night‐time, in addition to reduced drying time. The economic study revealed a high feasibility for the hybrid dryer, with a short payback period. © 2021 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

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 CO 2 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. • 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 FROM AUTHOR]

In order to explore the feasibility of suitable drying technology of tomato-paste, four solar drying methods were employed, namely (i) Basic Solar-drying Mode (SDM), (ii) Solar-Geothermal Drying (SGD), (iii) Convective Ventilated Solar Drying (CVD), and (iv) Combination of Ventilation and Geothermal Drying (VHD) and their effect on exergy efficiency, economic and quality attributes were investigated. Results showed that the drying time using different drying methods was 10, 9, 8, and 4.5 h for SDM, SGD, CVD, and VHD, respectively. Proposed model could better describe tomato paste drying behavior among eight tested mathematical models. Highest drying efficiency and specific energy consumption were observed for VHD method as 68.27% and 3.21 kWh/kg, respectively. Highest exergy efficiency was recorded for SGD and VHD as 54%. Payback period values were 0.28, 0.325, 0.3, and 0.169 years for SDM, SGD, CVD, and VHD, respectively. Color analysis revealed that CVD method was more suitable with the lowest total color change of 22.41 and highest brightness of the red color of 1.34. SGD method preserved 73.85% and 59.42% of phenol and flavonoid compounds, respectively, while VHD method preserve the highest flavonoid compounds as 82.24% compared to fresh sample. Geothermal energy combined with ventilation is recommended as a suitable drying method for tomato paste in terms of drying time, energy, exergy efficiency, economic, and quality attribute. • Solar drying of tomato paste was performed using four compared drying methods. • Highest exergy and drying efficiency were attained for ventilated/solar geothermal drying as 54% and 68.28%, respectively. • The drying systems are with a payback period in the range of 0.169–0.325 years. • Short drying time reduced total color change of dried paste. • Solar/geothermal drying preserve phenolic and flavonoids significantly. [ABSTRACT FROM AUTHOR]

A laboratory-scale hot air convective dryer (HACD) was used to dry date fruits surplus at 0.5 ± 0.02 kg kg−1 to a safe moisture level of 0.35 kg kg−1 under drying temperatures (T) of 50–70 °C with airflow rates (Q) of 0.18 and 0.28 m3 h−1. Kinetics, modeling, energy, economic feasibility, and color difference of the date fruits were analyzed and then optimized using response surface methodology (RSM) with two drying factors and eight responses. The drying time to reach the desired moisture level ranged from 9 to 28 h under different drying conditions. The proposed mathematical model expressed the moisture behavior over drying time with the highest R 2 = 0.9998 and 0.9999, respectively, for 0.18 and 0.28 m3 h−1. Effective moisture diffusivity (D eff) was calculated using the starting accessibility parameter and varied between 9.9 × 10−8 and 3.3 × 10−7 m2 s−1. The energy analysis revealed that the specific energy consumption (SEC) varies between 22.04 and 43.05 kWh.kg−1 and the drying efficiency (η) are 16.91, 16.14, and 25.38% for 50, 60, and 70 °C, respectively. The RSM optimization using a multi-level design with two centurium points denoted the optimum responses for the highest drying temperature. The lowest payback period (P b) of the drying system was estimated to be 0.45 years for T = 70 °C and Q = 0.28 m3 h−1. The optimum drying time, D eff , total energy consumption (E t), SEC , η , and total color change (ΔE) were estimated to be 9 h, 3.3 × 10−7 m2 s−1, 33.91 kWh, 22.04 kWh.kg−1, 27.69%, and 1.69, respectively. [ABSTRACT FROM AUTHOR]

An indirect solar-electric dryer (ISED) was constructed to valorize the date surplus under drying temperatures of T = 50–65 °C. The effective moisture diffusivity (D eff) values of dates were 4.59 × 10−7, 5.58 × 10−7, 7.29 × 10−7, and 1.04 × 10−6 m2 s−1, respectively, for 50, 55, 60, and 65 °C with convective mass transfer coefficient ranging between 9.18 × 10−7 and 2.08 × 10−6 m s−1. The drying behavior of date fruits using ISED was described by Wang and Singh's model with 99.98% alignment with experimental data. The lowest specific energy consumption (SEC) was 3.71 kWh.kg−1 resulting in T = 65 °C. The highest solar energy fraction (63.2%) and the lowest electric energy fraction (36.8%) were obtained under 60 °C drying temperature. The highest exergy efficiency (η Ex = 73%) was attained at 50 °C. The highest waste exergy ratio and the lowest sustainability index 0.39 and 2.54, respectively were obtained for T = 65 °C. The improvement potential factor increased with the decrease in η Ex and varied between 17.1 and 64.7. Low CO 2 emissions amount of 17.23 kg/year was reached, and impressive payback period values of 0.26, 0.25, 0.12, and 0.08 years were estimated, respectively, for T = 50, 55, 60, and 65 °C. The ISED under 65 °C drying temperature is recommended for date fruits. [ABSTRACT FROM AUTHOR]

• Three laboratory-scale solar drying systems were investigated for drying food surplus. • Highest specific energy consumption was observed for system with heat pump (HPD). • HPD outperformed the best in terms of energy and enviro-economic feasibility. • P b and CO 2/emissions of HPD were estimated to be 2.98 years and 10.99 kg/year. • Farmers and industrials are encouraged to invest in HPD systems. Three laboratory-scale drying systems, which are direct solar dryer (SD), solar dryer-assisted solar water collector (SCD), and solar dryer provided with a heat pump (HPD) to generate hot water to the heat exchanger, are considered in this work for drying tomato surplus. The study reported the kinetics, energy, environmental, and economic analysis as crucial investment factors. Outcomes showed that SD, SCD, and HPD took, respectively, 16 h within two drying days, 14 and 13 h, to reduce the moisture content from 14.68 ± 0.02 to 0.13 (g/g), with a significant diffusional stage for both SCD and HPD. Using a heat pump increased the specific energy consumption (SEC) of HPD ranging from 0.87 to 2.79 kJ/kg. The annual saving attained 9.5 × 104, 1.2 × 105, and 1.57 × 105 DZD with feasible payback periods at 3.67, 4.95, and 2.98 years for SD, SCD, and HPD, respectively. The systems had a CO 2/emission of 36.60, 100.5, and 43.68 kg/year and net CO 2/mitigation at 9.21, 25.18, and 10.99 tons/year, respectively, for SD, SCD, and HPD. Among the examined drying systems, HPD outperformed the best in energy and enviro-economic and can be considered to invest at a large scale by the industrials or farmers. [ABSTRACT FROM AUTHOR]