Drying for agricultural and marine products are one of the most attractive and cost-effective application of solar energy. Numerous types of solar dryers have been designed and developed in various ...parts of the world, yielding varying degrees of technical performance. Basically, there are four types of solar dryers; (1) direct solar dryers, (2) indirect solar dryers, (3) mixed-mode dryers and (4) hybrid solar dryers. This paper is a review of these types of solar dryers with aspect to the product being dried, technical and economical aspects. The technical directions in the development of solar-assisted drying systems for agricultural produce are compact collector design, high efficiency, integrated storage, and long-life drying system. Air-based solar collectors are not the only available systems. Water-based collectors can also be used whereby water to air heat exchanger can be used. The hot air for drying of agricultural produce can be forced to flow in the water to air heat exchanger. The hot water tank acts as heat storage of the solar drying system.
A comprehensive review of the various designs, details of construction and operational principles of the wide variety of practically-realised designs of solar-energy drying systems reported ...previously is presented. A systematic approach for the classification of solar-energy dryers has been evolved. Two generic groups of solar-energy dryers can be identified, viz passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (often referred to as hybrid solar dryers). Three sub-groups of these can also be identified, viz integral-type (direct mode), distributed-type (indirect mode) and the mixed-mode type. The appropriateness of each design type for application by rural farmers in developing countries is discussed. Some very recent developments in solar drying technology are highlighted.
•CFD and experimental analysis of a compact solar dehumidification system.•Analyzing the effect of integrating baffles on drying performance.•Exergy efficiency of the drying chamber was achieved ...between 44.91 and 55.28%.•Maximum deviation between numerical and experimental findings obtained as 6.5%.
Drying foods and medicinal plants is a process with high energy consumption because high amount of energy is needed for evaporating the water from the surface of the fresh materials. Therefore, developing cost-effective and simple-structured drying systems with low energy demand is a crucial matter. In the present study, compact type solar dryers have been designed, manufactured and tested experimentally. Within this context, thermal and flow behavior of compact dryers with and without baffles have been analyzed using CFD method. Then, fabricated compact dryers have been experimented at different conditions to determine their performance. Experimental and numerical findings of this study illustrated the effective design of the developed compact solar dryers. However, compact solar dryer with baffles exhibited better performance than the solar dryer without baffles. According to the experimental results, mean specific moisture extraction rate in compact dryers with and without baffles obtained between the ranges of 0.55–0.69 and 0.47–0.62 kg/kWh, respectively. In addition, average exergy efficiency of drying chambers was attained over the range of 44.91–55.28%. Overall findings of this study indicated successfulness of a compact solar dryer which developed using simple and lightweight materials.
•Four air channel modification has been tested.•Comparison of triple-pass and quadruple-pass solar air collectors.•Greenhouse dryer performance has been upgraded to achieve high thermal ...performance.•Comparison of integrated mode and single mode.•Maximum instantaneous collector efficiency was obtained as 90.30%.
Solar air collectors can be utilized in different applications like process air preheaters, dryers, and fresh air providing in buildings. In addition to solar air collectors, greenhouse dryers can be utilized as direct type solar dryers. In the present study, a quadruple-pass solar air collector (QPSAC) which accompanied with a pilot-scale greenhouse dryer (GD) has been analyzed experimentally and numerically. In the first section of the study, some kinds of upgrading have been done to choose the effective design of dryer and solar collector. In the second section, manufacturing of successful designs has been performed and performance of them have been analyzed experimentally. Red pepper and kiwi have been dried in the fabricated drying system as products. The performance tests have been done at 0.008 and 0.010 kg/s flow rates. The achieved mean thermal efficiency for QPSAC is in the range of 71.63–80.66%. The highest instantaneous temperature difference in QPSAC was obtained as 28.10 °C at 0.008 kg/s air flow rate. Also, SMER obtained in the range of 0.21–0.36 kg/kWh. Experimental results indicated that integrating QPSAC with GD decreased drying time considerably. Moreover, maximum deviation between CFD and experimental findings is 10%.
Over the past three decades there has been nearly exponential growth in drying R&D on a global scale. Improving of the drying operation to save energy, improve product quality as well as reduce ...environmental effect remained as the main objectives of any development of drying system. A solar assisted chemical heat pump dryer is a new solar drying system, which have contributed to better cost-effectiveness and better quality dried products as well as saving energy. A solar collector is adapted to provide thermal energy in a reactor so a chemical reaction can take place. This reduces the dependency of the drying technology on fossil energy for heating. In this paper a review on advanced of solar assisted chemical heat pump dryer is presented (the system model and the results from experimental studies on the system performance are discussed). The review of heat pump dryers and solar assisted heat pump dryer is presented. Description of chemical heat pump types and the overview of chemical heat pump dryer are discussed. The combination of chemical heat pump and solar technology gives extra efficiency in utilizing energy.
•An efficient solar fruit and vegetable dryer was developed for household usages.•The developed dryer was optimized using both experimental and numerical methods.•The proposed numerical simulation ...has provided accurate insight into the thermo-fluid behavior of the dryer.•The 16.4% thermal efficiency of the dryer proved its favorable efficiency in comparison with some other apparatuses.•The conversion procedure is believed to be inspiring for converting typical direct dryers into indirect ones.
In the present study, a low-cost solar fruit and vegetable dryer was designed, constructed, and tested for small-scale production, especially in households. An optimizing procedure was performed to convert a typical widely-used direct cabinet dryer into a new indirect dryer using experimental data and numerical simulations. Several well-designed experiments accompanied by CFD simulations using finite volume method were carried out. It was concluded that the proposed conversion procedure could be achieved utilizing a blackened absorber plate, an air circulating unit, and a honeycomb air manifold. The drying capacity of the dryer could be increased by more than 50% in comparison with the direct type by adding a new tray to the optimized dryer. The thermal efficiency of the proposed solar dryer was estimated 16.4%, indicating that the desirable energy performance of the dryer was superior to some available designs of solar dryers. Besides, it was observed that the proposed numerical approach was able to produce a reliable qualitative and quantitative insight into both the thermal and fluid mechanical behavior of the dryer. Finally, it was claimed that not only was the developed apparatus suitable and efficient for the household usage, but also the optimized version of the dryer could be inspiring for those who are currently using typical direct cabinet dryers to convert their dryers into indirect ones.
Uncertain price rise and rapid depletion of fossil fuels accelerated the development of renewable energy sources in the form of alternative power sources. Solar is an abundant, renewable and ...sustainable energy source that attracted many eminent researchers across the world to work in the field of solar energy applications. In developing countries, solar drying is capable of fulfilling the increasing demand of healthy, low cost natural foods and need for sustainable income. Solar dryers used for agricultural product drying can be proved to be most useful device from energy conservation point of view that not only save energy but also saves a lot of time, occupying less area, improves product quality and personnel life style. A solar crop drying system does not solely depends on solar energy for its functioning besides many recent advancements in drying process which combines other form of auxiliary heating source with sun energy are also in recent trends for reducing fuel consumption. In this paper, a review of various types of solar dryers namely, direct solar dryers, indirect solar dryers, hybrid solar dryers and their various drying applications are represented.
•Drying experiments were performed in natural and forced convection indirect solar dryers (NCISD and FCISD).•The final MC of guava slices was reached at 14, 18 and 24 h in FCISD, NCISD and open sun ...drying.•The increment of 34.19% on Def, 55.59% of h and 55.55% of hm were observed in FCISD.•The increment of ηcoll and ηdry were 16.63% and 26.19%, respectively, in FCISD.•FCISD performed well in all aspects compared to NCISD.
The performance of the dryer, thermal properties and the drying kinetics of guava were evaluated in natural convection indirect solar dryer (NCISD) and forced convection ISD (FCISD) and compared. The average final moisture content (MC) of guava slices (kept in all four trays) was reached from 5.5355 to 0.0244 kg/kg of db. The final MC was attained at 14, 18 and 24 h in FCISD, NCISD and open sun drying (OSD) methods, respectively. Effective diffusion coefficient (Def), surface transfer coefficients (h and hm), collector and dryer efficiencies (ηcoll and ηdry), specific energy consumption and activation energy were estimated for both setups. The Page models were the best models for indicating the drying curve of guava slices in NCISD, FCISD and OSD. The specific energy consumption (SEC) of guava slices was 2.108 and 1.675 kWh/kg in NCISD and FCISD, respectively. The average increment of 34.12% on Def, 55.59% of h, 55.55% of hm, 16.63% of ηcoll, 26.19% of ηdry and decrement of 20.54% on SEC were observed in FCISD than NCISD. The activation energy of guava slices was 136.98 and 116.49 kJ/mol in NCISD and FCISD, respectively. 16.63% and 26.19% improvements in ηcoll and ηdry, respectively, were noticed in FCISD compared to NCISD. The performance of ISD is improved in FCISD by attaching DC fans driven by PV modules.
Solar energy has long been utilised in crop conservation. The use of solar dryers as a drying method overcomes the drawbacks of the traditional approach to drying crops under the sun. Previous ...studies were coherently described and focused on the development and thermal convection of greenhouse dryers, structural analysis, crops cultivation for offering choices to reader about solar greenhouse dryers in agriculture. Based on the previous related studies, the current work proposes an evolve and detail review of solar-assisted greenhouse dryers, namely, passive dryers, active dryers and hybrid dryers, from the aspects of system performance, collector performance and the drying characteristics of various crops. Drying crops in a solar greenhouse dryer protects crops from insects, rain and dust. Thus, these dryers are preferred in natural and conventional drying. This review provides a satisfactory guide for researchers to compare, develop and improve solar greenhouse drying systems to achieve optimum crop drying rates based on thermal convection mode.
⁃Solar assisted greenhouse dryer from the aspects of system performances of various crops was described.⁃The solar greenhouse dryers are protected from insects, rain, and dust.⁃The drying rate of crops inside a greenhouse could be controlled in forced convection.⁃A hybrid dryer could provide satisfactory thermal energy storage with an auxiliary heater in a greenhouse system.⁃The hybrid dryers are suitable for use on a large scale by industries.⁃This review is suitable for researchers aiming to develop and improve solar greenhouse drying systems.
•Literature data of different indirect type solar dryers (ITSD) were discussed.•ITSD with sensible and latent heat storage system was reviewed.•ITSD with roughened solar collector was extensively ...reviewed.•Chemical pre-treatment, payback period and economic analysis were discussed.•Important findings and summaries of ITSD were listed and tabulated.
Inadequate preservation techniques and poor storage provisions lead to deterioration in the quality of agricultural products. Advanced processing techniques have been used to reduce postharvest losses of agricultural goods. Drying is a processing technique used for food product preservation. Drying by solar energy is an ancient food preservation technique. Solar dryers of various sizes, capacities and designs are available for drying applications in agricultural industries. Indirect type solar dryer (ITSD) is one of the prominent dryers used to dry food products and this type of dryer with its unique features, types, and different technique incorporated to improve its performance has not been investigated so far in any detail. The purpose of this work is to review the features and benefits of ITSD. A commonly used classification of different types of solar dryers is also presented. Heat transfer enhancement on ITSD and the influence of pre-treatment before drying are also effectively reviewed. Payback period and cost analysis of ITSD are discussed. Important findings on ITSD have been reviewed, discussed and tabulated. The most dominant parameters affecting the drying rate are air temperature and velocity, followed by solar radiation, type of product, initial moisture content and total mass of the product. Passive solar dyers were easy to fabricate compared to active dryers. The drying rate of pre-treated foods was high and the quality of the product remained intact after drying.
