Single layer solar drying behaviour of Citrus aurantium leaves under forced convection

Convective solar drying experiments in thin layers of Citrus aurantium leaves grown in Marrakech, morocco, were conducted. An indirect forced convection solar dryer consisting of a solar air collector, an auxiliary heater, a circulation fan and a drying cabinet is used for the experiments. The air temperature was varied from 50 to 60 °C; the relative humidity from 41% to 53%; and the drying air flow rate from 0.0277 to 0.0833 m³/s. Thirteen statistical models, which are semi-theoretical and/or empirical, were tested for fitting the experimental data. A nonlinear regression analysis using a statistical computer program was used to evaluate the constants of the models. The Midilli–Kucuk drying model was found to be the most suitable for describing the solar drying curves of Citrus aurantium leaves with a correlation coefficient (r) of 0.99998, chi-square (χ²) of 4.664 × 10⁻⁶ and MBE of 4.8381 × 10⁻⁴.     

Performances and improvement potential of solar drying system for palm oil fronds

In this study, an indirect forced convection solar drying system was tested for drying of palm oil fronds. The drying of 100 kg of palm oil fronds via solar drying system reduced the moisture content from 60% (w.b) to 10% (w.b) in 22 h (3 d of drying). During the drying process, the daily mean values of the drying chamber inlet temperature, drying chamber outlet temperature, drying chamber air temperature, and solar radiation ranged from 26 °C to 75 °C, 25 °C–65 °C, 26 °C–67 °C, and 96 W/m² to 1042 W/m² respectively, with corresponding average values of 53 °C, 46 °C, 48 °C, and 580 W/m². At average solar radiation of about 600 W/m² and air flow rate 0.13 kg/s, the collector, drying system and pick-up efficiencies were found about 31%, 19% and 67% respectively. The specific moisture extraction rate (SMER) was 0.29 kg/kWh. The exergy efficiency varied between 10% and 73%, with an average of 47%. In addition, the improvement potential of solar drying system for palm oil fronds ranged from 8 W to 455 W, with an average of 172 W.     

Investigation of a new solar greenhouse drying system for peppers

Solar drying is the oldest preservation technique of agricultural products using several types of solar crop dryers based mostly on solar energy, which is abundant, renewable and sustainable. This study aimed to modeling a new solar greenhouse drying system (SGDS) for the drying of red peppers. The proposed mixed-mode (SGDS) consists of two main parts, namely a flat plate solar air collector and an experimental greenhouse. A mathematical model is developed using the TRNSYS simulation program to predict the change in the drying kinetics during the drying process under our proposed (SGDS). The experimental part consisted in testing the solar air collector to investigate its performance. The test showed that this solar air collector has a good performance; its efficiency varies between 0, 5 and 0, 65. The model was validated with the observed data and showed good agreement with experimental values. The influence of the area of the product to be dried, airflow rate and collector area, on moisture content changes, air temperature and humidity inside the greenhouse was studied. For the case study of this SGDS, the results obtained from simulation showed that the optimum values of area of the product to be dried, the exhaust airflow rate and the collector area were found to be 40 m², 250 kg/h and 2 m², respectively.     

Thermophysical characterization by solar convective drying of thyme conserved by an innovative thermal-biochemical process

The solar drying for preservation of food products is the most attractive and promising application. It leads to reduce the water activity in the product and thereby prolongs its shelf life by decreasing the microbial contamination. Thymus satureioides, a medicinal and aromatic plant, was conserved using a nondestructive innovative preservation process integrating sustainable energy. The process is based on a combination of pretreatment by aqueous solutions of natural organic acid followed by a partial solar convective drying. Therefore, it is overridden to understand the mechanisms influencing the treated thyme drying process compared with the drying kinetics of witness thyme. The effect of drying conditions (season, temperature ranging from 50 to 80 °C and drying volume flow rates of 150 and 300 m³/h) and the pretreatment conditions (concentration and volume of citric acid) on the process efficiency were studied. Furthermore, the effects of temperature, season, drying volume flow rates and citric acid on drying rate and total phenolic content were also investigated. Moreover, nine mathematical models have been used to describe the drying curves. The best mathematical models which represented treated and witness thyme were Wang-Singh and Midilli-Kucuk models. Hence, the characteristic drying curve of the pretreated product was determined.     

Thin layer solar drying and mathematical modeling of mulberry

In this study, drying parameters of mulberry grown in Elaz?? were investigated as experimental and theoretical using solar dryer system. The drying experiments were conducted at seven different drying mass flow rates varied between 0.0015 and 0.036 kg s^?1. As results of the drying experiments were conducted at different drying mass flow rates, it was shown that the drying time was decreased with the drying mass flow rate. This paper also presents a new mathematical modeling of thin layer solar drying of mulberry samples. In order to estimate the suitable form of solar drying curves, 10 different mathematical models to be in the literature and new model were applied to the experimental data and compared according to their correlation coefficients (R) and chi‐squared (χ²), which were predicted by non‐linear regression analysis using the Statistica Computer Program. It was concluded that the Midilli model and the newly developed model represent drying characteristics better than the other models. The effective moisture diffusivity values were in the range 3.47×10^?12–1.46×10^?9 m² s^?1. Copyright © 2009 John Wiley & Sons, Ltd.     

Crop drying by indirect active hybrid solar – Electrical dryer in the eastern Algerian Septentrional Sahara

In the present work, a new specific prototype of an indirect active hybrid solar–electrical dryer for agricultural products was constructed and investigated at LENREZA Laboratory, University of Ouargla (Algerian Sahara). In the new configuration of air drying passage; the study was done in a somewhat high range of mass flow rate between 0.04 and 0.08 kg/m² s a range not properly investigated by most researchers. Experimental tests with and without load were performed in winter season in order to study the thermal behavior of the dryer and the effect of high air masse flow on the collector and system drying efficiency. The fraction of electrical and solar energy contribution versus air mass flow rate was investigated. Slice tomato was studied with different temperatures and velocities of drying air in order to study the influence of these parameters on the removal moisture content from the product and on the kinetics drying and also to determine their suitable values. Many different thin layer mathematical drying models were compared according to their coefficient of determination (R²) and reduced chi square (χ²) to estimate experimental drying curves. The Middli model in this condition proved to be the best for predicting drying behavior of tomato slice with (R² = 0.9995, χ² = 0.0001). Finally an economic evaluation was calculated using the criterion of payback period which is found very small 1.27 years compared to the life of the dryer 15 years.     

Anthocyanins and betalains as light-harvesting pigments for dye-sensitized solar cells

We present the photoelectrochemical properties of dye-sensitized solar cells using natural pigments containing betalains and anthocyanins as sensitizers. The dyes extracted from grape, mulberry, blackberry, red Sicilian orange, Sicilian prickly pear, eggplant and radicchio have shown a monochromatic incident photon to current efficiency (IPCE) ranging from 40% to 69%. Short circuit photocurrent densities (J_sc) up to 8.8 mA/cm², and open circuit voltage (V_oc) ranging from 316 to 419 mV, were obtained from these natural dyes under 100 mW/cm² (AM 1.5) simulated sunlight. The best solar conversion efficiency of 2.06% was achieved with Sicilian prickly pear fruits extract. The influence of pH and co-absorbers on natural sensitizers, were investigated and discussed.     

Potential for biofuels from the biomass of prickly pear cladodes: Challenges for bioethanol and biogas production in dry areas

Prickly pear is a term used to refer to several species of cactus belonging primarily to the genus Opuntia. In general, these species present an exceptional ability to produce biomass in soil and climate conditions unfavorable for most plant species, in part due to their high water use efficiency. Given the current increase demand for renewable energy and the future prospect of more limited water resources, the potential use of prickly pear cladodes for biofuel production deserves to be investigated. The objectives of this study were to gather information on the chemical composition of prickly pear biomass from the most cultivated varieties in NE Brazil, discuss the potential of processing biomass for ethanol and biogas production and to point out gaps in know-how and priorities for research on this topic. We quantified in the tree varieties studied significant amounts of uronic acids (10.7%) and oxalic acid (10.3%), confirming the reports of high amounts of pectin and calcium oxalate in cladodes of prickly pear. The estimated potential of ethanol production for prickly pear (1490–1875 L ha⁻¹ yr⁻¹) was low when compared to traditional biomass sources (sugarcane and sugar beet, for example). However, it appears that prickly pear stands out as a biomass with potential for high production rates of methane (3717 m³ ha⁻¹ yr⁻¹), being comparable to traditional energy crops. Further studies are needed to assess more consistently both the sustainability of biomass production as the potential for ethanol, and biogas production, specially for newly released varieties of prickly pear.     

Mathematical modelling of a thin layer drying of apricots in a solar energized rotary dryer

In this paper we present a mathematical modelling of a thin layer forced solar drying of apricots. An indirect forced convection solar dryer consisting of a solar heater and a rotary column cylindrical drying (RCCD) cabinet was used in the experiments. Air heated by the solar air heater was forced through the apricots by an electrical fan. Moreover, the natural sun drying experiments were conducted for the comparison at the same time. Fourteen different thin layer mathematical drying models were compared according to their coefficients of determination (r,χ², RMSE) to estimate solar drying curves. The effects of the drying air temperature, velocity and the rotation speed of column on the drying model constants and coefficients were predicted by multiple regressions using a linear type model. Copyright © 2004 John Wiley & Sons, Ltd.     

Mathematical modeling of thin-layer drying of fermented and non-fermented sugarcane bagasse

This work reports hot-air convective drying of thin-layer fermented and non-fermented sugarcane bagasse. For this purpose, experiments were carried out in a laboratory-scale dryer assessing the effects of solid-state fermentation (SSF) on the drying kinetics of the processing material. The fermented sugarcane bagasse in SSF was obtained with the use of Kluyveromyces marxianus NRRL Y-7571. Drying experiments were carried out at 30, 35, 40 and 45 °C, at volumetric air flow rates of 2 and 3 m³ h^?1. The ability of ten different thin-layer mathematical models was evaluated towards representing the experimental drying profiles obtained. Results showed that the fermented sugarcane bagasse presents a distinct, faster drying, behavior from that verified for the non-fermented material at the same conditions of temperature and volumetric air flow rate. It is shown that the fermented sugarcane bagasse presented effective diffusion coefficient values of about 1.3 times higher than the non-fermented material. A satisfactory agreement between experimental data and model results of the thin-layer drying of fermented and non-fermented sugarcane bagasse was achieved at the evaluated experimental conditions.     

Improvement in greenhouse solar drying using inclined north wall reflection

A conventional greenhouse solar dryer of 6 m² × 4 m² floor area (east-west orientation) was improved for faster drying using inclined north wall reflection (INWR) under natural as well as forced convection mode. To increase the solar radiation availability onto the product (to be dried) during extreme summer months, a temporary inclined wall covered with aluminized reflector sheet (of 50 μm thickness and reflectance 0.93) was raised inside the greenhouse just in front of the vertical transparent north wall. By doing so, product fully received the reflected beam radiation (which otherwise leaves through the north wall) in addition to the direct total solar radiation available on the horizontal surface during different hours of drying. The increment in total solar radiation input enhanced the drying rate of the product by increasing the inside air and crop temperature of the dryer. Inclination angle of the reflective north wall with vertical (β) was optimized for various selective widths of the tray W (1.5, 2, 2.5 and 3 m) and for different realistic heights of existing vertical north wall (h) at 25°N, 30°N and 35°N latitudes (hot climatic zones). Experimental performance of the improved dryer was tested during the month of May 2008 at Ludhiana (30.56°N) climatic conditions, India by drying bitter gourd (Momordica charantia Linn) slices. Results showed that by using INWR under natural convection mode of drying, greenhouse air and crop temperature increased by 1-6.7 °C and 1-4 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 13.13% of the total drying time. By using INWR under forced convection mode of drying, greenhouse air and crop temperature increased by 1-4.5 °C and 1-3 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 16.67% of the total drying time.     

Induced flow for ventilation and cooling by a solar chimney

An experimental and numerical model of a solar chimney was proposed in order to predict its performance under varying geometrical features in Iraqi environmental conditions. Steady, two dimensional, turbulent flow was developed by natural convection inside an inclined solar chimney. This flow was investigated numerically at inclination angles 15° to 60°, solar heat flux 150–750 W/m² and chimney thickness (50, 100 and 150) mm. The experimental study was conducted using a single solar chimney installed on the roof of a single room with a volume of 12 m³. The chimney was 2 m long; 2 m wide has three gap thicknesses namely: 50, 100 and 150 mm. The performance of the solar chimney was evaluated by measuring the temperature of its glass cover, the absorbing wall and the temperature and velocity of induced air. The results of numerical model showed that; the optimum chimney inclination angle was 60° to obtain the maximum rate of ventilation. At this inclination angle, the rate of ventilation was about 20% higher than 45°. Highest rate of ventilation induced with the help of solar energy was found to be 30 air changes per hour in a room of 12 m³ volumes, at a solar radiation of 750 W/m², inclined surface angle of 60°, aspect ratio of 13.3 and chimney length of 2 m. The maximum air velocity was 0.8 m/s for a radiation intensity of 750 W/m² at an air gap of 50 mm thickness. No reverse air flow circulation was observed even at the largest gap of 150 mm. The induced air stream by solar chimney can be used for ventilation and cooling in a natural way (passive), without any mechanical assistance.     

Performance analysis of solar chimney using mathematical and experimental approaches

A mathematical model based on one‐dimensional energy and mass balance across the solar chimney has been developed. The air flow characteristics such as exit velocity and temperature are evaluated with respect to the collector inclination angle, hourly solar radiation, ambient temperature, and wind speed. The model is validated by comparing the performance parameters obtained, with the experimental results and also with the experimental data of different geometrical range and environmental conditions from the literature. An average deviation of 8% for exit air velocity and 1.35% for exit air temperature is obtained for the solar chimney with absorber inclination angle 30°, collector area 0.41 m², and chimney height 0.24 m. The experimental daily average and maximum exit air velocity during the month of April are 0.5 and 0.88 m/s, respectively. The predicted optimum operating conditions are 75° inclination angle, 0.63 m² absorber area, and 0.48‐m chimney height. The maximum average exit air velocity and temperature numerically obtained are 0.64 m/s and 331 K, respectively, when operating with optimum conditions. It is observed that the exit air velocity increases 33% by increasing the absorber area from 0.5 to 3 m² for a solar chimney with 0.5 m height. An increase in exit air velocity of 52% was obtained by increasing the chimney height from 0.5 to 3 m for a solar chimney with 0.64 m² absorber area. A reduction in exit air velocity of 4% was observed for the increment in wind flow over the glass cover from 1.5 to 3 m/s. These results confirm that the solar chimney could be designed based on the predicted monthly performance by the present model.     

Drying kinetics of pumpkinseeds

The drying of pumpkinseeds was investigated in the present study. Pumpkinseed has a thin liquid film on its surface and has rough‐surfaced peel. The most important parameter affecting its drying kinetics and increasing the drying rate is the temperature of the drying air. Drying was carried out with the surrounding air first at ambient temperature, and secondly an experimental rig was built to increase air temperature by using solar energy. A single layer of the pumpkinseed was dried in two different ways by free and forced convection. In these methods, pumpkinseeds were either placed in the tray exposed to the natural environment or placed in sieves on the experimental rig. The drying curves and variation of drying rate curves obtained were dependent on product moisture content. The drying air velocity can take moisture until saturation has a secondary effect on the moisture transfer. In the analysis, pumpkinseed was considered like a measurable rectangular prism. Fick's analytical solution, including effective diffusion coefficient, was applied to the present model. The results obtained from the present analytical model were compared with the experimental data and a good agreement was found. Copyright © 2000 John Wiley & Sons, Ltd.     

Simulation of a solar timber drier

Numerical simulation and experimental measurement are used to examine the performance of a glasshouse-type solar timber dryer. The solar kiln dries about 5 m³ of timber from green to equilibrium in about 3 weeks. A fairly simple model (2 differential equations) gives a reasonable fit to the drying curves. It is found that fastest drying is obtained from a design with low heat capacity but good (forced) circulation of air through the timber stack.     

Thermal analysis of natural convective air drying of shrinking bodies

In the present paper, a method for determination of external mass transfer coefficient h_m, during drying of shrinking bodies is described under simulated natural convective air drying conditions. The effects of sample shrinkage and air temperature on h_m during drying of cylindrical potato samples of diameter 0.01 m and length 0.05 m were experimentally investigated at air temperatures 40, 50 and 60°C. The mass transfer coefficient considering shrinkage was found to be independent of sample moisture content during drying process with mean values varying from 1.06 × 10^?7 to 2.60 × 10^?7 m s^?1 for temperature range 40–60°C. However, calculated values of h_m, with no shrinkage effect taken into account, were found to be overestimated. The experimental error in terms of percent uncertainty in mass transfer coefficient measurements was computed and found to be in the range of 0.4–2.0%. It was demonstrated that higher drying air temperature caused increased values of h_m and the variation followed Kelvin's law type relation. A mathematical model to predict the drying process of cylindrical bodies with convective mass transfer boundary condition at air–solid interface is proposed. The low range of various errors between the results of moisture content ratio predicted by the model and those obtained experimentally indicates that the present methodology is capable of simulation of drying kinetics of potato cylinders. Copyright © 2006 John Wiley & Sons, Ltd.     

Heat transfer coefficient and thermal losses of solar collector and Nusselt number correlation for rectangular solar air heater duct with longitudinal fins hold under the absorber plate

An experimental investigation has been carried out for a series of system and operating parameters in order to analyze the effect of mass flow rate on heat transfer and Nusselt number characteristics in solar air heater. Experiments are performed at different air mass flow rates; varying from 0.012 to 0.016 kg/s, about hot summer days of Mai 2012. Hourly values of global solar radiation and some meteorological data (temperature, wind speed, relative humidities, etc.) for measuring days are obtained from the Biskra city of Algeria. The experiments encompassed the flow Reynolds number in the range 965.48–1301.4. Longitudinal fins were used inferior the absorber plate for an increase the heat exchange and render the flow fluid in the channel uniform. The effects of mass flow rate of air on the outlet temperature, Nusselt Number, Reynolds Number, Prandtl Number, the heat transfer in the thickness and length of the solar air collector were studied. For this effect was have created a new correlation correspondent of solar air collector with using fins it was written Nu = κ₀Re^1.36Pr^?0.68exp(0.342m)h ^[?0.018Pr].     

Experimental characterisation and modelling of thin layer direct solar drying of Amelie and Brooks mangoes

Direct solar drying characteristics of Amelie and Brooks mangoes were experimentally determined using a solar dryer made up of four trays and used under weather conditions of fruit harvest period. Direct solar drying curves were established, fitted using 10 mathematical models and simulated with a direct solar drying model. Effective diffusivity, drying rates and drying efficiency were estimated for each drying day and each variety. Results showed that at least four days were necessary to reach the range of preservation water contents. Drying curves depended on variety and were suitably fitted by “two-term” and “Approximation of diffusion” models (with R² ≥ 0.9888, RMSE ≤ 0.0283, E ≤ 9.1283% and χ² ≤ 1.3314 × 10⁻⁴). Drying rates and drying efficiency significantly decreased with the number of drying days (respectively between 0 and 0.15 g kg⁻¹ s⁻¹ and between 0 and 34%) and were very close for the two varieties. Diffusivity weakly varied with variety and strongly decreased with the number of drying days between 2.7906 × 10⁻¹¹ and 1.8489 × 10⁻¹⁰ m²/s. Drying kinetics were suitably simulated by the direct solar drying model (with: Amelie: R² = 0.989 and E = 7.623%, Brooks: R² = 0.9924 and E = 4.961%). The final water content was about 24.83% for Amelie and 66.32% for Brooks and Amelie was the most suitable variety for direct solar drying.     

Mathematical modeling of thin layer solar drying of sultana grapes

Thin layer solar drying experiments were conducted for Sultana grapes (cv. Thompson seedless) grown in Antalya, Turkey. An indirect forced convection solar dryer consisting of a solar air heater and a drying cabinet was used in the experiments. Air heated by the solar air heater was forced through the product by an electric fan. In order to examine the effect of drying air temperature and velocity on a thin layer drying of Sultana grapes, twenty-two experiments were performed. Eight different thin layer mathematical drying models were compared according to their coefficient of determination to estimate solar drying curves. The effects of drying air temperature and velocity on the model constants and coefficients were predicted by the regression models. According to the results, a two-term drying model could satisfactorily describe the solar drying curve of Sultana grapes with a correlation coefficient (r) of 0.979. The constants and coefficients of this model could be explained by the effect of drying air temperature and velocity.     

ANFIS modelling of a natural convection greenhouse drying system for jaggery: an experimental validation

The aim of the current research work is to generate an adaptive-network-based fuzzy inference system (ANFIS) model so as to forecast the jaggery temperature, the greenhouse air temperature and the moisture evaporation for drying of jaggery inside the greenhouse for natural convection mode. The experiment was conducted separately for 0.75 and 2.0 kg of jaggery pieces having dimensions of 0.03×0.03×0.01 m³ for complete drying. The jaggery was dried in a roof-type even span greenhouse with a floor area of 1.20×0.78 m². An ANFIS model was developed in MATLAB software so as to calculate the jaggery temperature, the greenhouse air temperature and the moisture evaporated and was also used to forecast the thermal performance of the greenhouse on the basis of solar intensity and ambient temperature. This model was experimentally validated. It was shown that the analytical and experimental results for jaggery drying are in good agreement.