Natural Convection Solar Dryer Research Articles

Diffuse Solar System Design and Utilization in Agriculture and Domestic Applications

Diffuse solar radiation is a component of total solar radiation that is good for low temperature grade heating. Since the portion of the scattered radiation from the sun, which consists of short and long waves, that reaches the earth is diffused, its utilization in Agriculture as this paper suggested, has multiple phase change material, which as well could serve as heat exchange for domestic (residential) applications. The Diffuse Solar System designed, consists of three functional units: the solar collector, thermal storage (heat sink) and a dryer cabinet. The thermal storage ranges from 140 – 1500F (60 – 500C) for heating system and 60 – 700F (15 – 200C) for residential cooling. Likewise, a natural convection solar dryer with collector efficiency of about 42% and drying efficiency of about 53% would dry tomatoes with initial moisture contents of 94% wet basis, to safe moisture content of about 4% wet basis and 35.2 – 86.8% ascorbic acid (vitamin C) retention with acceptable colour and texture. Keywords: Solar radiation, Heat sink, Ascorbic acid, Collectors, Reflectors, Yeast/mould count and Efficiency.

COMPARATIVE PERFORMANCE OF SOLAR TUNNEL GREENHOUSE AND OPEN SUN DRIING SYSTEMS DURING SEEDLESS GRAPE DRYING

Drying is simply the process of moisture removal from an agricultural product, and can be performed by different methods. The main purpose of this research work was to study the thermal performance of two different solar driers (active and passive dryers) based on the energy balance equations for the drying air and the drying product during dehydration process of seedless grapes in August 2009 under the climatic conditions of El-Mansoura city (31.05oN and 31.37 oE), Egypt. Three identical solar tunnel greenhouse driers (STGD) were functioned as an active solar drier, and an open sun drier (OSD) under direct solar radiation without cover (as a passive solar drier) to dry seedless grapes. The three active solar driers were operated under three different mass flow rates of 0.122, 0.183, and 0.259 kg/s. Three different shapes of grapes (clusters, parts, and grains) with total quantity of 33 kg, were spread on wire mesh tray with floor surface area of 2.0 m2. The results of greenhouse solar driers were compared individually with open sun drying. During the drying process, it was observed that, the hourly average air drying temperatures inside the three active solar dryers were increased to 49.9, 45.5, and 40.1oC, respectively, whereas the hourly average ambient air temperature was 31.6oC. The seedless grapes could be dried from initial moisture content of 79.1%wb to 17.1, 18.5, and 20.8%wb within drying time of 61, 61, and 71 hrs, using the three active solar dryers, respectively, while it was taken 96 hrs under open sun drying method to reach the final moisture content of 21.2%wb. The daily average thermal efficiencies for the four different solar drying modes, respectively, were 22.37%, 21.57%, 19.12%, 1and 10.81%. The quality of raisin (dried grapes) in terms of physicochemical parameters such as colour retention and water activity were evaluated. The overall study concluded that good quality dehydrated raisin could be produced by using active solar dryers as compared with passive sun drying method. The produced raisin of grapes under different drying methods had various colours. They, respectively, were light brown and dark brown when the grapes dried under active solar dryers and passive sun drying method. The experimental work showed that, the solar tunnel greenhouse drying (active solar dryer) was satisfactory and competitive to a natural convection solar drying process (passive dryer).

Energetics study of a natural convection solar crop dryer

SYNOPSIS The energetics of a low cost, locally fabricated, natural convection type solar crop dryer in India has been studied. Other types of dryers are available commercially in the market, but they are costly and their total energy input is high. This analysis may be helpful to manufacturers who wish to know the energy requirement of the different components of the systems, and to reduce GHG emissions. Locally grown chilli has been dried to evaluate the energetics of the dryer. Energy payback period has been evaluated when displacing different conventional fuels and found to be 1.1 0, 1.55, 1.32 and 1.77 years in the case of fuelwood, light diesel oil (LDO), coal and natural gas respectively. The energetics of such a simple and affordable dryer should be further analysed to reduce national and global energy demand.

Energy and exergy analyses of OMW solar drying process

The energy and exergy analyses of the drying process of olive mill wastewater (OMW) using an indirect type natural convection solar dryer are presented. Olive mill wastewater gets sufficiently dried at temperatures between 34 °C and 52 °C. During the experimental process, air relative humidity did not exceed 58%, and solar radiation ranged from 227 W/m 2 to 825 W/m 2. Drying air mass flow was maintained within the interval 0.036–0.042 kg/s. Under these experimental conditions, 2 days were needed to reduce the moisture content to approximately one-third of the original value, in particular from 3.153 g water/g dry matter down to 1.000 g water/g dry matter. Using the first law of thermodynamics, energy analysis was carried out to estimate the amounts of energy gained from solar air heater and the ratio of energy utilization of the drying chamber. Also, applying the second law, exergy analysis was developed to determine the type and magnitude of exergy losses during the solar drying process. It was found that exergy losses took place mainly during the second day, when the available energy was less used. The exergy losses varied from 0 kJ/kg to 0.125 kJ/kg for the first day, and between 0 kJ/kg and 0.168 kJ/kg for the second. The exergetic efficiencies of the drying chamber decreased as inlet temperature was increased, provided that exergy losses became more significant. In particular, they ranged from 53.24% to 100% during the first day, and from 34.40% to 100% during the second.

Design of mixed-mode natural convection solar crop dryers: Application of principles and rules of thumb

A mixed-mode natural convection solar crop dryer (MNCSCD) designed and used for drying cassava and other crops in an enclosed structure is presented. A prototype of the dryer was constructed to specification and used in experimental drying tests. This paper outlines the systematic combination of the application of basic design concepts, and rules of thumb resulting from numerous and several years of experimental studies used and presents the results of calculations of the design parameters. A batch of cassava 160 kg by mass, having an initial moisture content of 67% wet basis from which 100 kg of water is required to be removed to have it dried to a desired moisture content of 17% wet basis, is used as the drying load in designing the dryer. A drying time of 30–36 h is assumed for the anticipated test location (Kumasi; 6.7°N,1.6°W) with an expected average solar irradiance of 400 W/m 2 and ambient conditions of 25 °C and 77.8% relative humidity. A minimum of 42.4 m 2 of solar collection area, according to the design, is required for an expected drying efficiency of 12.5%. Under average ambient conditions of 28.2 °C and 72.1% relative humidity with solar irradiance of 340.4 W/m 2, a drying time of 35.5 h was realised and the drying efficiency was evaluated as 12.3% when tested under full designed load signifying that the design procedure proposed is sufficiently reliable.

Experimental investigation of a solar dryer with natural convective heat flow

A direct type natural convection solar dryer is designed. It is constructed in local materials (wood, blades of glass, metals) then tested experimentally in foodstuffs drying (cassava, bananas, mango). It is about an experimental approach which consists in analyzing the behavior of the dryer. The study relates mainly kinetics and establishment of drying heat balances. The influence of significant parameters governing heat and mass transfers, such as solar incident radiation, drying air mass flow and effectiveness, is analyzed in order to evaluate its thermal performances. Experimental data can be represented by empirical correlations of the form M( t)= M i exp( −kt) for representation of drying process. The resolution of these drying equations makes-possible to predict total drying time of each product. Moreover, this drying process allows to reduce the moisture content of cassava and sweet banana approximately to 80% in 19 and 22 h, respectively to reach the safety threshold value of 13%. This value permits the conservation of these products about one year without deterioration. The determination of parameters, like ambient temperature, drying chamber temperature, drying air mass flow and incident heat fluxes, allow to predict the drying effectiveness for modeling and refining the dimensioning of the elaborate prototype.

Solar dryer with thermal storage and biomass-backup heater

An indirect type natural convection solar dryer with integrated collector-storage solar and biomass-backup heaters has been designed, constructed and evaluated. The major components of the dryer are biomass burner (with a rectangular duct and flue gas chimney), collector-storage thermal mass and drying chamber (with a conventional solar chimney). The thermal mass was placed in the top part of the biomass burner enclosure. The dryer was fabricated using simple materials, tools and skills, and it was tested in three modes of operation (solar, biomass and solar–biomass) by drying twelve batches of fresh pineapple ( Ananas comosus), with each batch weighing about 20 kg. Meteorological conditions were monitored during the dehydration process. Moisture and vitamin C contents were determined in both fresh and dried samples. Results show that the thermal mass was capable of storing part of the absorbed solar energy and heat from the burner. It was possible to dry a batch of pineapples using solar energy only on clear days. Drying proceeded successfully even under unfavorable weather conditions in the solar–biomass mode of operation. In this operational mode, the dryer reduced the moisture content of pineapple slices from about 669 to 11% (db) and yielded a nutritious dried product. The average values of the final-day moisture-pickup efficiency were 15%, 11% and 13% in the solar, biomass and solar–biomass modes of operation respectively. It appears that the solar dryer is suitable for preservation of pineapples and other fresh foods. Further improvements to the system design are suggested.

Modelling and experimental studies on a mixed-mode natural convection solar crop-dryer

A mathematical model for drying agricultural products in a mixed-mode natural convection solar crop dryer (MNCSCD) using a single-pass double-duct solar air-heater (SPDDSAH) is presented. The model was developed in parallel with experimental work. The model comprises the air-heating process model, the drying model and the technical performance criteria model. The governing equations of the drying air temperature and humidity ratio; the material temperature and its moisture content; and performance criteria indicators are derived. The model requires the solution of a number of interrelated non-linear equations and a set of simultaneous differential equations. Results from experimental studies used for generating the required experimental data for validating the model are presented. Results of simulation runs using the model are presented and compared with the experimental data. It is shown that the model can predict the performance of the MNCSCD fairly accurately and therefore can be used as a design tool for prototype development.

Modeling the performance of the reversed absorber with packed bed thermal storage natural convection solar crop dryer

A transient analytical model has been presented to study the new concept of a solar crop dryer having reversed absorber plate type collector and thermal storage with natural airflow. The performance of 1 × 1 m 2 area of crop dryer with packed bed and airflow channel was evaluated for drying of onions. The model was solved to compute the air temperatures and various functional components of the drying systems for a day of the month of October for the climatic condition of Delhi (India). The parametric study involved the effect of width of airflow channel and height of packed bed on the crop temperature. The effect of thermal storage is observed on the natural mass flow rate in the drying system. A 30° inclined absorber plate with in-built thermal storage and 0.12 m width of airflow channel induced the mass flow rate in the range of 0.032–0.046 kg s −1 during the drying process. The thin layer drying equation was used to study the drying rate and hourly reduction in moisture content in the crop trays. It has been observed that the crop moisture content and drying rate decreases with the drying time of the day. A reversed absorber plate of 1 m length and 1 m breadth with 0.15 m packed bed could dry 95 kg of onion from a moisture content of 6.14–0.27 kg water/kg of dry matter in a 24 h drying period.

Study of the solar drying of grapes by three different processes

The objective of this work is to analyse the drying of the Sultanine grape variety by three different solar processes. Three drying kinetics have been established respectively in a natural convection solar drier, under a tunnel greenhouse and in open sun. These tests show that the solar tunnel greenhouse drying is satisfactory and competitive to a natural convection solar drying process.

A Comparative Study of Performance of Natural Convection Solar Dryers

This publication is based on experimental study that was made on the performance of three types of natural convection solar dryers, classified as direct dryer, indirect dryer and mixed mode dryer.

Optimization of mixed-mode and indirect-mode natural convection solar dryers : Simate, I. N. Renewable Energy, 2003, 28, (3), 435–453

Optimization of mixed-mode and indirect-mode natural convection solar dryers : Simate, I. N. Renewable Energy, 2003, 28, (3), 435–453

Experimental investigation of an indirect type natural convection solar dryer

An indirect type natural convection solar dryer is designed, constructed and investigated experimentally under Tanta prevailing weather conditions. The system consists of a flat plate solar air heater connected to a cabinet acting as a drying chamber. The air heater is designed to be able to insert various storage materials under the absorber plate in order to improve the drying process. Sand is used as the storage material. Drying experiments have been conducted with and without storage materials for different spherical fruits, such as seedless grapes, figs and apples, as well as vegetables, such as green peas, tomatoes and onions. The solar irradiance, temperature distribution in different parts of the system, ambient temperature and relative humidity of the inlet and outlet drying air have been recorded. The equilibrium moisture content M e for seedless grapes is reached after 60 and 72 h when the system is used with and without storage material, respectively. Therefore, the storage material reduces the drying process by 12 h. In order to accelerate the drying process, the drying products are divided into pieces and then chemically treated by dipping the samples into boiling water containing 0.4% olive oil and 0.3% NaOH for 60 s. However, the required time to achieve M e for the chemically treated seedless grapes, when the system is used with sand as a storage material, is drastically reduced to 8 h. Moreover, we found that the storage and chemical pretreatment have caused significant decreases of the drying time for all the investigated crops. The present system is capable of drying 10 kg of chemically treated grapes or green peas during 20 h of sunshine.

Optimization of mixed-mode and indirect-mode natural convection solar dryers

This paper presents a comparison of optimized mixed-mode and indirect-mode natural convection solar dryers for maize. The mixed-mode and indirect-mode solar drying simulation models were validated against results from a laboratory solar dryer with experiments carried out under a solar simulator at the University of Newcastle upon Tyne, UK. The models are now run under variable solar conditions in order to optimize the dryers and compare their performance. The inputs to the simulation models were Zambian weather conditions and materials. The solar drying simulations are combined with the cost of the dryer materials and a search technique that finds the dryer dimensions at the minimum drying cost. Optimization gave a shorter collector length for the mixed-mode solar dryer (1.8 m) than for the indirect-mode dryer (3.34 m) of the same grain capacity (90 kg). The drying cost, annual cost and initial cost of the mixed-mode dryer are lower than those of the indirect-mode although the quantity of dry grain obtained from the mixed-mode for the whole year is less than for the indirect-mode; the drying costs are 12.76 and 16.05 US$/ton for mixed-mode and indirect-mode dryers, respectively.

Design, development and performance testing of a new natural convection solar dryer

Mechanical drying of agricultural products is an energy consuming operation in the post-harvesting technology. Greater emphasis is given to using solar energy sources in this process due to the high prices and shortages of fossil fuels. For these purposes, a new natural convection solar dryer consisting of a solar air heater and a drying chamber was developed. This system can be used for drying various agricultural products like fruits and vegetables. In this study, grapes were successfully dried in the developed solar dryer. The qualitative analysis showed that the traditional drying, i.e. shade drying and open sun drying, dried the grapes in 15 and 7 days respectively, while the solar dryer took only 4 days and produced better quality raisins.

Natural convection solar dryer with biomass back-up heater

A direct-type natural convection solar dryer and a simple biomass burner have been combined to demonstrate a drying technology suitable for small-scale processors of dried fruits and vegetables in non-electrified areas of developing countries. From a series of evaluation trials of the system, the capacity of the dryer was found to be 20–22 kg of fresh pineapple arranged in a single layer of 0.01-m-thick slices. The overall drying efficiency of the unit was calculated to be ∼9%. During the same trial, the drying efficiency of the solar component alone was found to be 22%. Other trials estimated the efficiency of the burner in producing useful heat for drying to be 27%. Key features of the biomass burner were found to be the addition of thermal mass on the upper surface, an internal baffle plate to lengthen the exhaust gas exit path and a variable air inlet valve. Further modifications to further improve the performance of both the solar and biomass components of the dryer are suggested.

SIMULATION OF THE MIXED-MODE NATURAL-CONVECTION SOLAR DRYING OF MAIZE

A mathematical model for mixed mode natural convection solar drying of maize grain is presented. The drying is described by a deep bed procedure that includes conduction within the grain bed. The conduction is due to radiative energy falling on the upper surface of the bed. The results show that temperatures at the top and bottom of the bed are higher than that in the middle resulting in two drying fronts one at the top and the other at the bottom of the bed and moving in opposite directions. This results in more uniform moisture content distribution than in an indirect dryer. The results are verified against experimental data from a prototype mixed mode natural convection maize solar dryer. The laboratory solar dryer was constructed at Newcastle University, U.K. and the experiments carried out under a solar simulator. The agreement between theory and experiment is very good.

Thin-layer solar drying characteristics of rough rice under natural convection

Thin-layer solar drying experiments were conducted at Matsuyama, Japan, with medium grain rough rice. The range of average drying air temperature was 22.3–34.9°C, and the relative humidities were between 34.5% and 57.9%. The initial moisture contents were in the range of 37.07–37.69% dry-basis. A mixed-mode natural convection solar grain dryer was used for this experiment. The data of sample weight and dry- and wet-bulb temperatures of the drying air were recorded continuously from morning to evening for each test. The drying data were then fitted to the Page model, based on the ratios of the differences between the initial and final moisture contents and the equilibrium moisture content (EMC). The model gave a good fit for the moisture content with an average standard error of 0.00387 dry-basis. Both the drying parameters N and K are linear functions of temperature and relative humidity. This single-layer drying equation can be used for the simulation of deep bed drying of rough rice in mixed-mode type natural convection solar dryers.

Natural convection solar crop-dryers of commercial scale in Ghana: design, construction and performance

SYNOPSIS Traditional open sun-drying, the predominant method of food preservation in Ghana, has serious drawbacks leading to significant post-harvest losses (between 20 and 30%). Mechanical dryers can provide high quality products with minimum losses but they require sources of electrical or chemical energy, which, in the case of Ghana, are costly or scarce. This makes the development of an effective natural convection solar dryer an attractive alternative. A mixed-mode natural convection solar crop-dryer (a dryer in which the crop is dried by a combination of the direct absorption of solar radiation and by natural convection where air, heated by solar energy, is passed over the crop) has been designed. Two commercial dryers, based on this design and capable of handling between 1,000 and 1,500 kg of various agricultural products (such as cassava, pepper, maize, okra, etc.) have been constructed and partially tested at Agona-Asafo in the central region of Ghana. The results of these preliminary tests indicate that it takes 3–5 days to dry 1,000 kg of pepper from moisture content level of 78% to a safe storage level of 8%. The design, construction details and the results of the preliminary tests are presented in this paper.

Simulation of the indirect natural convection solar drying of rough rice

A mathematical model to simulate the indirect natural convection solar drying of rough rice is presented. The temperatures along the collector are described by a numerical solution. The drying of the grain is described by the introduction of a deep-bed solution procedure. The variation of the air temperature in the collector and across the air bed is incorporated in the prediction of the thermal buoyancy effect. The results show that, at the low air flow rates produced by natural convection, the drying wave moves slowly and the layer of grain cannot be considered uniform, with serious overdrying taking place in the bottom layer. The deep bed model is therefore essential in order to obtain a realistic simulation. Also, due to the temperature drop across the bed, the chimney height makes little contribution to buoyancy. Sensitivity analysis showed the system to be most sensitive to grain bed depth and collector length and least to chimney height and cover to absorber spacing. The simulation results have a number of practical implications. The grain bed should be turned regularly to avoid over-drying and give a tempering effect. The collector and plenum chamber upstream of the grain bed should be well sealed to contain the buoyancy pressure. The effect on performance of reducing or removing the chimney should be examined.