Optimizing Solar Drying: A Critical Review of Shapes, Orientation, and Future Prospects for Hybrid Solar Dryers

This comprehensive review focuses on the performance of solar dryers, with a specific emphasis on their structural shape and orientation. Researchers have extensively examined these design parameters, often employing Computational Fluid Dynamics (CFD) to assess thermal attributes and predict temperature distribution, airflow patterns, and temperature profiles within the structures. Geographical location significantly influences solar dryer shape preferences, with the parabolic shape finding favor in tropical regions for its superior solar radiation capture and storm resistance, while even-span and Quonset shapes are popular elsewhere. Solar dryer orientation is another crucial factor, with east-west alignment consistently proving optimal due to its ability to maximize year-round solar radiation absorption and, consequently, enhance drying efficiency. Economic considerations, however, fall beyond the scope of this review, which predominantly focuses on thermal aspects. This investigation reveals diverse global preferences for solar dryer shapes and orientation, highlighting the necessity of considering geographical factors in design choices. While CFD and shape/orientation dynamics have provided valuable insights, there remains room for future research to expand into transient state simulations under various conditions, contributing to a more comprehensive understanding of solar dryer performance. Such insights promise to promote sustainable and efficient drying processes, benefitting agricultural and drying applications across the globe.

Industrial and Small-Scale Biomass Dryers:An Overview

The quality of the drying process depends mainly on the efficient use of thermal energy.Sustainable systems based on solar energy takes a leading role in the drying of agro-products because of low operating cost.However,they are limited in use during off–sun periods.Biomass dryer is one of the simplest ways of drying because of its potential to dry products regardless of time and climate conditions.The other benefit is that crop residues could be used as fuel in these systems.However,the major limitation of the dryer is unequal drying because of poor airflow distribution in the drying medium,which can be improved by integrating some design changes in the dryer.This review analyses the two types of biomass dryers:industrial biomass dryers and small biomass dryers for food product,along with their efficiency.Further,studies on technical,sustainability and economic aspects are expected to provide a greater understanding of biomass drying.

Analysis and survey of the application of solar dryers in eastern Nigeria

A survey on the use of solar dryers for drying purposes was carried out in some communities in eastern Nigeria.The objective of the study was to ascertain the extent uses of solar dryers by local farmers and the rural populace.The survey method included oral interview and questionnaire administration.The results of the study revealed that the practical use of solar dryers is absent domestically and industrially.However,great enthusiasm was shown by the farmers in the use of solar crop dryers if their performance is satisfactory with respect to quality and quantity of products dried at a given time interval.Most farmers rejected the idea of establishing communally maintained and operated systems but preferred commercialized systems where payments are made per unit quantity of products dried.The study therefore recommends the development of affordable solar crop dryers with auxiliary heat sources to mitigate the effects of daily and seasonal fluctuation in solar radiation in order to cope with the demands of a profit oriented commercial drying system.Such a system holds a lot of promises for entrepreneurs.

Experimental Study and Thermal Modelling of Cocoa Shell Convective Drying in an Indirect Solar Dryer

The concern of the present work is the convective drying of empty cocoa shells in an indirect solar dryer. Some drying experiments, using one sample, were carried out. During the experiments, the sample is introduced in the drying chamber. Then at steady time intervals, the sample is withdrawn from the drying chamber, for a rapid weighing. After each weighing, the sample is reintroduced in the dryer. At each time interval, the ambient temperature of the drying chamber and its relative humidity γ are measured by a thermo-hygrometer. From the experimental data, a theoretical determination of the moisture evaporated from the product was performed and a good agreement was found between the theoretical and experimental values, confirmed by the value of the RMSE. Those calculations used the constants in the Nusselt number found in literature. Then those constants were evaluated again, to get new values more suitable with the experimental data. The dimensionless numbers of Nusselt, Grashof and Prandtl were calculated. That allowed the calculation of the average value of the Nusselt number. The average convective heat transfer coefficient was determined.

Some construction parameters of unglazed perforated solar dryers for medicinal plants

Erecting solar dryers for medicinal plants require the best design option to suit such plants.Investigations were carried out on some design parameters of the unglazed perforated solar dryers under the prevailing weather conditions of Ismailia,Egypt.Effects of different parameters on the heat exchange effectiveness were investigated.Parameters considered in this study including hole(perforation)diameters,dimensions apart,material being used and its thickness all were investigated to reveal the effects of suitable suction velocity,prevailing wind speed and its direction under the simulated incident solar radiation of the investigation site.Based on the obtained results for heat exchange effectiveness under controllable conditions,solar drier was built and investigated outdoors under the prevailing weather conditions of the investigation site.Holes diameter of 3 mm for all the investigated plates and under the different conditions resulted the highest obtained heat exchange effectiveness.Diamond shape for Holes distribution with dimensions of 29 mm×29 mm apart gave higher heat exchange effectiveness above the commonly used distribution(square shape).Temperature increase above ambient per unit of incident radiation was found as(0.0162±0.0007)℃m^(2)/W,when the best designed performance was investigated in the open environment.Using that perforated surface as an absorber in constructed dryer to dry Rosemary(Rosmarinus officinalis L.),reduced the moisture content from 65%(wet basis)to 38.1%on eight hours on the first drying day of the 18th April,2010 while the reduction in moisture content of the sample dried in the shade area was from 65%(wet basis)to 42.2%under the same prevailing weather conditions at the same time.

Heat and Mass Transfer for a Nanofluid Flow in Fluidized Bed Dryer in Presence of Induced Magnetic Field

This research entails the study of heat and mass transfer of nanofluid flow in a fluidized bed dryer used in tea drying processes in presence of induced magnetic field. A mathematical model describing the fluid flow in a Fluidized bed dryer was developed using the nonlinear partial differential equations. Due to their non-linearity, the equations were solved numerically by use of the finite difference method. The effects of physical flow parameters on velocity, temperature, concentration and magnetic induction profiles were studied and results were presented graphically. From the mathematical analysis, it was deduced that addition of silver nanoparticles into the fluid flow enhanced velocity and temperature profiles. This led to improved heat transfer in the fluidized bed dryer, hence amplifying the tea drying process. Furthermore, it was noted that induced magnetic field tends to decrease the fluid velocity, which results in uniform distribution of heat leading to efficient heat transfer between the tea particles and the fluid, thus improving the drying process. The research findings provide information to industries on ways to optimize thermal performance of fluidized bed dryers.

Drying rates of some fruits and vegetables with passive solar dryers

Two passive solar dryers were designed and constructed with available local materials.The passive solar dryers which were direct and indirect types were tested with pepper(Capsicum annum L.),okro(Abelmoschus esculentus L.)and vegetables(Amaranthus hybridus L.)in order to evaluate the drying rate of these produces.The moisture content of 78.9%(w.b.)for 180 g freshly harvested peppers was reduced to 24.0%(w.b.).The drying rate in the direct passive solar dryer was found to be higher than the indirect passive solar dryer.The initial moisture content of 92%(w.b.)for 1000 g okro was reduced to 20%(w.b.).The drying rate in the direct passive solar dryer was also found to be higher than in the indirect passive solar dryer.The initial moisture content of 90%(w.b.)for 400 g vegetable was reduced to 20%(w.b.).The drying rate with the direct passive solar dryer was found to be higher than that with indirect passive solar dryer.During the course of drying,after each crop was kept inside the drying system,the temperature of the drying was monitored at an-hour interval;the moisture content was also monitored at a three-hour interval until there was no more change in the weight of the crop.The crops dried faster with the direct passive solar dryer than with the indirect passive solar dryer.As a result of the highest temperature attained in the direct passive solar dryer,the rate of moisture removal was the highest in this dryer.

Waste Heat Recovery from a Drier Receiver of an A/C Unit Using Thermoelectric Generators

Thermoelectric generators(TEGs)are considered promising devices for waste heat recovery from various systems.The Seebeck effect can be utilized to generate power using the residual heat emitted by the filter dryer receiver(FDR)of an air conditioning(A/C)system,which would otherwise go to waste.The study aims to build a set of thermoelectric generators(TEG)to collect the waste heat of the FDR and generate low-power electricity.A novel electrical circuit with two transformers is designed and fabricated to produce a more stable voltage for operation and charging.The thermoelectric generator(TEGs)was installed on the FDR of the A/C unit.The test showed that climate conditions have a significant impact on the output power generated from the system.The results showed that the peak voltage recorded in the current study is 5.2 V per day(wet,cold,and wind weather)with an output power of 0.2 W.These values are acceptable for powering the load and charging a single battery with 3.5 V as the voltage increases battery 0.1 V/20 min charge.A case study of operating the emergency signs in a building was considered.The current heat recovery system is deemed to be easily installed and can be connected to a network of TEGs to produce more power.

Experimental Study of a Modified Icaro Solar Dryer: The Case of Coffee Drying

Food losses in the developing country are thought to be 50% of the fruits and vegetables grown and 25% of harvested food grain. Food preservation can reduce wastage of a harvest surplus, allow storage for food shortages, and in some cases facilitate export to high-value markets. Drying is one of the oldest methods of food preservation. Drying makes produce lighter, smaller, and less likely to spoil and helps to minimize the moisture content in coffee beans as high moisture content during storage is certain to ruin the taste and appearance of coffee. This work presents the results of an experimental study of forced convection drying of coffee cherries in a modified Icaro solar dryer. The study aims to validate the numerical models developed for further research. The experimental tests envisaged also aim to determine the mass loss curves of the product by fixing or calculating its initial mass (1 kg), its initial water content (70%), the ambient temperature, the drying airflow (0.02 m3.s-1 to 0.09 m3.s-1) and the exchange coefficients. The influence of these aerothermal parameters on the drying time of a most commercialized coffee variety (Robusta) was studied. Finally, the results revealed an increase in the efficiency of the heat transfer air and a reduction in the water content of the coffee cherry from 70% to 9.87%, after 30.2 hours.

Experimentation of a Forced Convection Solar Dryer for Drying Sweet Potatoes at the Higher Institute of Technology of Mamou-Guinea

This study was carried out at the Mamou Higher Institute of Technology during the period from March 10 to April 15, 2022, with the aim of designing and testing a solar dryer with forced convection by drying potatoes. The dryer was designed using local materials. Its main geometric parameters are: 1) height of the drying chamber (90 cm), 2) length of the drying chamber (50 cm), 3) width of the drying chamber (43 cm), 4) surface of the racks (0.1806 m2), 5) surface of the heat accumulator (0.2537 m2). The experiment focused on the vacuum test of the dryer for two days and that of the drying of the sweet potato for three days from 8:30 a.m. to 5:30 p.m. The average vacuum test temperature values of the three environments are respectively accumulator (43°C), dryer chamber (41°C) and ambient environment (34°C). Four kilograms (4 kg) of boiled sweet potato were dried. The average temperatures in the accumulator and in the drying chamber during the three days of drying are respectively 33°C and 39°C. The final mass of the dried product is 1.2 kg, with a quantity of water extracted of 2 liters or 63% of the initial mass of the product. The average drying rate is 0.074 kg/h. The drying kinetics showed a decreasing rate in the absence of the heating period and the constant rate period.

Experimentation of a Forced Convection Solar Dryer for Drying Sweet Potatoes at the Higher Institute of Technology of Mamou-Guinea

This study was carried out at the Mamou Higher Institute of Technology during the period from March 10 to April 15, 2022, with the aim of designing and testing a solar dryer with forced convection by drying potatoes. The dryer was designed using local materials. Its main geometric parameters are: 1) height of the drying chamber (90 cm), 2) length of the drying chamber (50 cm), 3) width of the drying chamber (43 cm), 4) surface of the racks (0.1806 m2), 5) surface of the heat accumulator (0.2537 m2). The experiment focused on the vacuum test of the dryer for two days and that of the drying of the sweet potato for three days from 8:30 a.m. to 5:30 p.m. The average vacuum test temperature values of the three environments are respectively accumulator (43°C), dryer chamber (41°C) and ambient environment (34°C). Four kilograms (4 kg) of boiled sweet potato were dried. The average temperatures in the accumulator and in the drying chamber during the three days of drying are respectively 33°C and 39°C. The final mass of the dried product is 1.2 kg, with a quantity of water extracted of 2 liters or 63% of the initial mass of the product. The average drying rate is 0.074 kg/h. The drying kinetics showed a decreasing rate in the absence of the heating period and the constant rate period.

Iodine Determination by Microwave Plasma Torch Atomic Emission Spectrometer Coupled with Online Preconcentration Vapor Generation Technique

This article focuses on iodine determination by microwave plasma torch atomic emission spectrometry （MPT-AES） coupled with online preconcentration vapor generation method. A new desolvation device, multistrand Nafion dryer, was used as the substitute for condenser desolvation system. Some experimental conditions, such as preconcentration time, acidity of sample solution, rinsing solution acidity and dynamic linear range were investigated and optimized. The new desolvation system eliminates the problem of decreasing emission intensity of I（I） 206.238 nm line with the increase of working time on a conventional condenser desolvation system, thus greatly improving the reproducibility.

Quantifying the effect of persistent dryer climates on forest productivity and implications for forest planning:a case study in northern Germany

Background: Forest management decisions are based on expectations of future developments. For sound decisions it is essential to accurately predict the expected values in future developments and to account for their inherent uncertainty,for example the impact of climate change on forests. Changing climatic conditions affect forest productivity and alter the risk profile of forests and forest enterprises. Intensifying drought stress is seen as one major risk factor threatening forest management in the north German lowlands. Drought stress reduces tree growth and vitality and might even trigger mortality. But so far, it is not possible to quantify effects of a persistent dryer climate on forest productivity at a level suitable for forest management.Methods: We apply a well-established single-tree forest growth simulator to quantify the effect of persistent dryer climates on future forest productivity. We analyse the growth of Scots pine(Pinus sylvestris L.), European beech(Fagus sylvatico L.) and oak(Quercus robur L. and Quercus petraea(Matt.) Liebl.) in two forest regions in the north German lowlands for a time interval of 60 years until 2070. The growth response under three different climate projections is compared to a baseline scenario.Results: The results show clear differences in volume increment to persistent dryer climates between tree species. The findings exhibit regional differences and temporal trends. While mean annual increment at biological rotation age of Scots pine and oak predominantly benefits from the projected climate conditions until 2070, beech might suffer losses of up to 3 m^3·ha^(-1)yr^(-1) depending on climate scenario and region. However, in the projection period2051 to 2070 the uncertainty ranges comprise positive as well as negative climatic effects for all species.Conclusions: The projected changes in forest growth serve as quantitative contributions to provide decision support in the evaluation of, for example, species future site suitability and timber supply assessments. The analysis of productivity changes under persistent dryer climate complements the drought vulnerability assessment which is applied in practical forestry in northwestern Germany today. The projected species' productivity has strong implications for forest management and the inherent uncertainty needs to be accounted for.

Heat Transfer Optimization in Air Flat Plate Solar Collectors Integrated with Baffles

This paper presents an experimental analysis for comparisons of conventional flat plate solar collectors and collectors integrated with different numbers of baffles. Heat transfer between absorber plate and drying fluid (air) has been one of the major challenges in the design and operations of the indirect solar dryer systems. In this experiment, efficiency of air flat plate solar collector integrated with 2, 3, 4 and 8 baffles was studied and compared with the ordinary collector. The results showed that integrating solar collector with baffles significantly increased the efficiency of the system. It was noted that collector with 2, 3, 4 and 8 baffles had a mean efficiency of 29.2%, 31.3%, 33.1% and 33.7% respectively while with no baffles was 28.9%. The analysis showed that when there were less than four baffles in the collector, heat transfer was dominant over pressure drop and hence high efficiency. However, when the number of baffles exceeded four, the effect associated with an increase in pressure drop highly observed compared to heat transfer coefficient, thus resulted to insignificant increase in efficiency. Therefore, the optimum number of four baffles was commended for the designed model for optimum efficiency.

Development and Quality Analysis of a Direct Solar Dryer for Fish

The study investigates drying characteristics of common species of fish in the tropics, catfish (Clarias gariepinus) and tilapia fish (Oreochromis niloticus) using a direct solar dryer. The drying process was carried out during the dry and wet season under natural (0 m/s) and forced convective drying (1.5 m/s, 2.5 m/s, 3.5 m/s fan speed). Results obtained showed that the drying air attained by the dryer was satisfactory and the maximum temperature difference between the dryer and the ambient temperatures was 35°C. The moisture content of the dried samples was 13.97% for catfish and 13.35% for tilapia fish during dry season and during the wet season it was 15.68% for catfish and 14.9% for tilapia fish while for the open sun dried samples it was 21.7% for catfish and 17.0% for tilapia fish. Maximum drying efficiency of 74.3% was recorded for the dryer during dry season and the dried samples at 3.5 m/s fan speed were better in drying rates. The proximate compositions of the fish before and after drying were determined. There was significant difference in proximate composition before and after drying (P < 0.05) and no significant difference in proximate composition at fan speeds considered for both fish species (P < 0.05).

Performance Analysis and Drying Kinetics of Maize in an AflaSTOP Dryer

The performance of an AflaSTOP dryer which utilises biomass energy for drying maize was investigated. The drying behaviour of maize grains in the dryer was also investigated using ten (10) thin-layer mathematical models. The models were compared based on coefficient of determination (R2) and Root Mean Square Error (RMSE) values between experimental and predicted moisture ratios. At an average drying air temperature of 50°C and drying air velocity of 2.5 m/s, maize at average moisture content (MC) of 17.5% (wb) was dried to an average MC of 11.5% (wb) in three (3) hours. The drying and thermal efficiency were calculated as 81.1% and 29.6% respectively. Overall, drying took place in the falling rate period. The Logistics model was best to describe the thin-layer drying kinetics of maize in the dryer with R2 value of 0.9902 and RMSE value of 0.04908.

Effect of Drying Air Velocity on Drying Kinetics of Tomato Slices in a Forced-Convective Solar Tunnel Dryer

The objective of this work is to analyse the extent to which a change in the drying air velocity may affect the drying kinetics of tomato in a forced-convective solar tunnel dryer. 2 m?s?1 (V1) and 3 m?s?1 (V2) air speeds were applied in similar drying air temperature and humidity conditions. Main drying constants calculated included the drying rate, the drying time and the effective water diffusivity based on the derivative form of the Fick’s second law of diffusion. Henderson and Pabis Model and Page Model were used to describe the drying kinetics of tomato. We found that solar drying of tomato occurred in both constant and falling-rate phases. The Page Model appeared to give a better description of tomato drying in a forced-convective solar tunnel dryer. At t = 800 min, the drying rate was approximately 0.0023 kg of water/kg dry matter when drying air velocity was at 2 m/s. At the same moment, the drying rate was higher than 0.0032 kg of water/kg dry matter when the drying air velocity was 3 m/s. As per the effective water diffusivity, its values changed from 2.918E?09 m2?s?1 to 3.921E?09 m2?s?1 when drying air velocity was at 2 and 3 m?s?1 respectively, which is equivalent to a 25% increase. The experimentations were conducted in Niamey, on the 1st and 5th of January 2019 for V2 and V1 respectively. For both two experiments, the starting time was 9:30 local time.

A Study on contact drying with flexible screen

Three types of material as platen in contact drying process were tested in Composition Board Laboratory of Northeast Forestry University in 1998. Poplar veneers were dried under’0.3 MPa at 160℃ or 180℃. Through comparison a non-metal flexible material was chosen as screen for designing contact dryer and replacing curved metal plate. The new type contact dryer solved the problem of long auxiliary time by loading wet veneers into press and unloading dried veneers out of press at the same time. This increased productivity. Veneer’s moisture content(MC) varied with drying time in index law. The process of contact drying with non-metal flexible screen was analyzed by stages of preheating, high-speed drying, retarded drying and extremely slow drying.

Hydrodynamics of Continuous Spiral Dryer with Rotatory Conical Sleeves: Experiments versus CFD Simulations

In this study, a continuous and airtight twinspiral dryer was developed in accordance with the characteristics and challenges in the process of disposing polysilicon slurry. Computational fluid dynamics(CFD)simulations were used to investigate the flow field in the rotating twin-spiral continuous dryer and an original discrete phase model was also elaborated to compare with the cold-modeling experimental results. The corresponding flow field was obtained using the available inlet velocity of 0.05-0.3 m/s and the rotational speed of the inner cone of 12-44 r/min, the residence time distribution, and tracked particles trajectory. Results showed that the residence time of the tracer particles in the cone cylinder was about 15.8-25.4% of the time spent out of it, and the particle's residence time was much shorter in contrast to the rotational speed and inlet velocity. The external ribbon had a larger influence on the fluid, thereby leading to a larger velocity in the region outside the cone compared to that in the region inside the cone. In addition, the appearance of the vortex and boundary layer separation at the back of the ribbon and the spoke bar had secondary diversion effects on the fluid. Furthermore, the inlet velocity had little influence on the flow field while the rotational speed of the cone greatly affected the flow field. Hence, the CFD simulations showed good agreement with the experimental results.

ON A NEW DRYER FOR SLAG OR FLY-ASH

Granulated blastfurnace slag or fly-ash , as admixture of slag cement or pozzolana cement, has to be dried before it is ground with clinker, for it contains water. Here, a new dryer for drying slag or fly-ash is recommended, which is called suspension dryer.