Solar Dryer | Solar Drying for Fruits and Vegetables, Fish & Seafood | VINASAY

The heat from the sun coupled with the wind has been used to dry food crops for preservation for several thousand years. This sun drying using direct sunlight and open air harnessing power of the sun as solar heat has been used for food preservation of agricultural commodities and products since early stages of mankind in best ways Other crops such as timber need to be dried before they can be used effectively, in building for instance.

The sun is the central energy producer of our solar system. It has the form of a ball and nuclear fusion take place continuously in its centre. A small fraction of the energy produced in the sun hits the earth and makes life possible on our planet. Solar radiation drives all natural cycles and processes such as rain, wind, photosynthesis, ocean currents and several other which are important for life. The whole world energy need has been based from the very beginning on solar energy. All fossil fuels (oil, gas, coal) are converted solar energy.

The solar radiation intensity outside the atmosphere is in average 1,360 W/m² (solar constant). When the solar radiation penetrates through the atmosphere some of the radiation is lost so that on a clear sky sunny day in summer between 800 to 1000 W/m² (global radiation) can be obtained on the ground.

This sun-drying has often developed into solar-drying, where the drying area is in an enclosed ventilated area – often with polythene, acrylic or glass covering as a more efficient harnessing of the elements of the drying operation. There are innumerable designs in use and each has its advantages and disadvantages. However, there are three basic designs upon which others are based: solar cabinet dryer, Solar Tent, tent-dryer, Solar Dehydrator and solar tunnel dryer.

These are discussed below after a brief description of the principles of drying/dehydrator.

Basic principles of drying and design consideration of drying depends upon:

• Temperature, humidity (moisture content) and quantity of air used

• Size of the pieces being dried

• Physical structure and composition

• Air flow patterns within the drying system

Heat is not the only factor which is necessary for drying. The condition, quality and amount of air being passed over and through the pieces to be dried determine the rate of drying. The amount of moisture contained in the air to be used for drying is important and is referred to as absolute humidity. The term relative humidity (RH) is more common and is the absolute humidity divided by the maximum amount of moisture that the air could hold when it is saturated. RH is expressed as a percentage and fully-saturated air would have an RH of 100%. This means that it cannot pick up any more moisture. Air containing a certain quantity of water.

at a low temperature will, when heated, have a greater capacity to hold more water. The table below gives an example of air at 29oC with an RH of 90%. Such air, when heated to 50oC will then have an RH of only 15%. This means that instead of only being able to hold only an extra 0.6 grams of water per kilogram (at 29oC), it is able to hold 24 grams per kilogram. Its capacity to pick up moisture has been increased because it has been heated.

When placed in a current of heated air, food initially loses moisture from the surface. This is the constant rate period. As drying proceeds, moisture is then removed from inside the food material, starting near the outside. Moisture removal becomes more and more difficult as the moisture has to move further from deep inside the food to the surface. This is the falling-rate period. Eventually no more moisture can be removed and the food is in equilibrium with the drying air.

During the falling-rate period, the rate of drying is largely controlled by the chemical composition and structure of the food. Design of a dryer depends upon the drying rate curve of the material to be dried but these curves are indicative only and depend upon the factors mentioned above.

The heat required to evaporate water is 2.26kJ/kg. Hence, approximately 250MJ (70kWh) of energy are required to vaporise 100kg water. If the ambient air is dry enough, no heat input is essential. The greatest potential for drying crops in a short time is when the ambient air is arid and warm. If the air is warm then less air is needed. This temperature will itself depend mainly on the air temperature but also on the amount of solar radiation received directly by the food being dried.

Solar drying Technique and Operation

Variety of Solar dryers need ventilation to be able to dry crops effectively. Air movement can be by natural convection or can be assisted using fans. Solar food drying can be used in most areas but how quickly the food dries is affected by the variables indicated above, especially the amount of sunlight and relative humidity. Typical drying times in solar dryers are from 1 to 3 days depending on sun, air movement, humidity and the type of food to be dried. Most dryers are black inside, either painted or with black polythene inserts to absorb as much solar radiation as possible.

Solar driers compared to fuel-fired dryers

The choice between using solar radiation or fuel-fired dryers using, for instance, wood, charcoal, diesel, gas or electricity depends upon the equipment capital cost, cost of raw material to be dried, operating costs of running the dryer and the likely price obtained for the final dried product. Fuel heating allows much better control of the drying operation than solar heating and does not depend on the sun to be shining. However, it is possible to combine solar drying with a fuel-source to reduce fuel costs. Such systems include pre-heating of air by solar energy.

Choice of solar dryer

The choice between alternative types of solar drier will depend on local requirements including scale of operation as well as the budget available. If intended for smallholder farmers drying crops for their own needs then capital cost may well be the main constraint and so low-cost plastic-covered tent or box driers may be the most suitable choice. However, commercial farmers with an assured market for their product may consider banks of fan-assisted, glass-covered solar dryers more appropriate for their needs./ Solar dehydrator.

 Type of Solar Dryers

All drying systems can be classified primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classified broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers) and solar-energy dryers. Strictly, all practically-realised designs of high temperature dryers are fossil fuel powered, while the low temperature dryers are either fossil fuel or solar-energy based systems.

To classify the various types of solar dryers, it is necessary to simplify the complex constructions and various modes of operation to the basic principles. Solar dryers can be classified based on the following criteria:

·      Mode of air movement

·      Exposure to insulation

·      Direction of air flow

·      Arrangement of the dryer

·      Status of solar contribution

Solar dryers can be classified primarily according to their heating modes and the manner in which the solar heat is utilised. In broad terms, they can be classified into two major groups, namely:

1.      active solar-energy drying systems (most types of which are often termed hybrid solar dryers); and

2.      passive solar-energy drying systems (conventionally termed natural-circulation solar drying systems).

Three distinct sub-classes of either the active or passive solar drying systems can be identified (which vary mainly in the design arrangement of system components and the mode of utilisation of the solar heat, namely :

·      integral-type solar dryers;

·      distributed-type solar dryers; and

·      mixed-mode solar dryers.

Natural convection is used on the diminution of the specific weight of the air due to heating and vapour uptake. The difference in specific weight between the drying air and the ambient air promotes a vertical air flow. Natural convection dryers therefore can be used independent from electricity supply. However, the airflow in this type of dryer is not sufficient to penetrate higher crop bulks. Furthermore the air flow comes to a standstill during night and adverse weather conditions. The risk of product deterioration due to mould attack and enzymatic reactions is high.

Furthermore the mode of drying can be differentiated into direct and indirect, depending whether the product is directly exposed to solar radiation or dried in the shade. In direct mode, the product itself serves as absorber, i.e. the heat transfer is affected not only by convection but also by radiation according to the albedo of the product surface. Therefore, the surface area of the product being dried has to be maximized by spreading the crop in thin layers. To obtain uniform final moisture content, the crop has to be turned frequently.

Using integral (direct) mode of drying, is should be noted, that sunlight may affect certain essential components in the product e.g. chlorophyll is quickly decomposed. Due to the limitation of the bulk depth, such dryers need large ground surface areas. If grounds are scarce, indirect mode type of dryers are preferred for drying larger quantities.

Processes during solar drying

The objective of a dryer is to supply the product with more high temperature than is available under ambient conditions, thereby increasing sufficiently the vapour pressure of the moisture held within the crop and decreasing significantly the relative humidity of the drying air and thereby increasing its moisture carrying capacity and ensuring sufficiently low equilibrium moisture content.[6]

One type of solar dryer It was designed for the particular requirements of rice but the principles hold for other products and design types, since the basic need to remove water is the same.

Air is drawn through the dryer by natural convection. It is heated as it passes through the collector and then partially cooled as it picks up moisture from the rice. The rice is heated both by the air and directly by the sun.

Processes during sun drying

Exposing agricultural products to wind and sun is the preservation method practiced over centuries throughout the world. Cereals, legumes and green forages are dried in the field immediately after harvesting. Fruits, vegetables, spices and marine products as well as threshed grains are spread out in thin layers on the ground or trays, respectively. Other methods include hanging the crop underneath a shelter, on trees or on racks in the field.

For better understanding of the processes during solar drying, the process of sun drying is described in first.

During sun drying heat is transferred by convection from the surrounding air and by absorption of direct and diffuse radiation on the surface of the crop. The converted heat is partly conducted to the interior increasing the temperature of the crop and partly used for effecting migration of water and vapour from the interior to the surface. The remaining amount of energy is used for evaporation of the water at the surface or lost to ambient via convection and radiation. The evaporated water has to be removed from the surrounding of the crop by natural convection supported by wind forces.

Under ambient conditions, these processes continue until the vapour pressure of the moisture held in the product equals that held in the atmosphere. Thus, the rate of moisture desorption from the product to the environment and absorption from the environment are in equilibrium, and the crop moisture content at this condition is known as the equilibrium moisture content. Under ambient conditions, the drying process is slow, and in environments of high relative humidity, the equilibrium moisture content is insufficiently low for safe storage.

Due to the hygroscopic properties of all agricultural products, during sun drying the crop can either be dried or rewetted. Especially during night time when ambient temperature in general is decreasing, causing a simultaneous increase of the humidity, remoistening effects can occur either by condensation of dew or by vapour diffusion caused by osmotic or capillary forces.

As described above this method of drying has a lot of disadvantages. A technical alternative to the traditional method of sun drying is solar drying.

Sun drying versus solar drying

Solar drying is a possible replacement for sun drying or for standard dehydration processes. In terms of sun drying, solar drying is competing with an approach that is deeply entrenched in the way of life for most potential users. Sun drying is by no means a perfect process with problems arising due to potential contamination of the produce, variability in drying times, rain damage and so on. However some of the reasons proposed for the lack of success in adoption of solar drying are as follows:

−     Solar food dryers have often been too expensive or initial investment capital or loan facilities were unavailable.

−     Solar Tunnel dryers have often been too complicated or poor training of local entrepreneurs and technicians was provided.

−     Solar dryers have often required too big changes from traditional methods.

−     Solar drying chamber have not been built for long term use.

−     There is a lack of incentive to improve the quality of the product. People are willing to pay nearly the same amount for discoloured or damaged foods and there is therefore no incentive for producers to risk higher amounts of money in a dryer when there is not a great return.

When comparing solar drying to the conventional dehydration processes a new range of issues arises. These include:

·      Solar dryers must provide the equivalent performance to that of the conventional processes in terms of capacity, labour input, the quality of the final product, the total drying costs and reliability.

·      A backup heating system should be installed to ensure drying during the critical periods when the weather is bad.

Advantages of solar drying can be summarized as follows:

+  The higher temperature, movement of the air and lower humidity, increases the rate of drying.

+  Food is enclosed in the dryer and therefore protected from dust, insects, birds and animals.

+  The higher temperature deters insects and the faster drying rate reduces the risk of

spoilage by micro organisms.

+  The higher drying rate also gives a higher throughput of food and hence a smaller drying area (roughly 1/3).

+  The dryers are water proof and the food does not therefore need to be moved when it rains.

+  Dryers can be constructed from locally available materials and are relatively low cost.

+  More complete drying allows longer storage

Active solar cabinet dryers

Active solar dryers are also called forced convection or hybrid solar dryers. Optimum air flow can be provided in the dryer throughout the drying process to control temperature and moisture in wide ranges independent of the weather conditions. Furthermore the bulk depth is less restricted and the air flow rate can be controlled. Hence, the capacity and the reliability of the dryers are increased considerably compared to natural convection dryers.

It is generally agreed that well designed forced-convection distributed solar dryers are more effective and more controllable than the natural-circulation types.

The use of forced convection can reduce drying time by three times and decrease the required collector area by 50 %. Consequently, dryer using fans may achieve the same throughput as a natural convection dryer with a collector six times as large. Fans may be powered with utility electricity if it is available, or with a solar photovoltaic panel. Almost all types of natural convection dryers can be operated by forced convection as well.

Active ventilated cabinet solar dryers

If utility electricity is available it is cheaper to connect the fans to the grid, compared to a connection to a PV installation. Besides the fans also an electronic controller may be connected to the grid, which is able to adjust the appropriate temperature by variable speed of the fan.

In a PV-powered solar panel system, the fan is directly coupled to the solar module, working without an accumulator and load controller. Increasing solar radiation increases the module’s output, thus speeding up the fan. This has the advantage of permitting a simple temperature control merely by appropriately designing the components of the PV system, thus obviating any additional control devices as long as the system is suitably dimensioned.

Cabinet solar dryers

Here, the crop is located in trays or shelves inside a drying chamber. If the chamber is transparent, the dryer is termed an integral-type or direct solar dryer. If the chamber is opaque, the dryer is termed distributed-type or indirect solar dryer. Mixed-mode dryers combine the features of the integral (direct) type and the distributed (indirect) type solar dryers. Here the combined action of solar radiation incident directly on the product to be dried and pre-heated in a solar air heater furnishes the necessary heat required for the drying process.

In most cases the air is warmed during its flow through a low pressure drop thermosyphonic solar collector and passes through air ducts into the drying chamber and over drying trays containing the crops. The moist air is then discharged through air vents or a chimney at the top of the chamber.

The cabinet is a large wooden or metal box. It should be insulated properly to minimise heat losses and made durable (within economically justifiable limits). Construction from metal sheets or water resistant cladding, e.g. paint or resin, is recommended.

Inside the box internal runners are fitted to support the trays of food being processed. A general rule of thumb is that one m² of tray area is needed to lay out 10 kg of fresh produce. Access to the inside of the dryer is via hinged doors at the rear of the cabinet. The drying trays slide on rails on the inside of the cabinet so that they can be removed from the dryer for loading, unloading and cleaning.

Heated air flows through the stack of trays until the entire product is dry. Clearly, as the hot air enters below the bottom tray, this tray will dry first. The last tray to dry is the one at the top of the chamber. The advantages and disadvantages of this system are:

+  simple chamber

+  low labour costs – simply load and then unload

+  the food need not be exposed to the direct rays of the sun which reduces the loss of colour and vitamins.

+  heat storage systems can be applied

−       a tendency to over-dry the lower trays

−       low efficiency, in terms of fuel consumption, in the later stages of drying when most of the trays are dry.

Further major drawbacks for natural convection solar dryers are the poor moist air removal which reduces drying rate and the very high internal temperatures with the likelihood of over

heating the product. Drying air temperatures as high as 70 °C – 100 °C may be reached with these dryers. These temperatures are excessive for most products. The most severe constraints are on beans (35°C), rice (45°C), and all grains if they are to be used for seed (45°C).

In a natural convection system, the flow of air is caused by the fact that the warm air inside the dryer is lighter than the cooler air outside. This difference in density creates a small pressure difference across the bed of grain, which forces the air through it. 

It can be seen that if the incoming air is heated by only 10-30°C then the presence of a chimney on top of the dryer would make little or no difference, unless it acted efficiently as a solar collector and raised the temperature of the air significantly. So a solar chimney increases the buoyancy force imposed on the air stream and provides a higher air flow velocity and, thus, a more rapid rate of moisture removal.

It should be noted that even if the difference in densities is as much as 0.5 kg/m³, then the resulting pressure difference is only 0.5 Pa (5 millionths of atmospheric pressure) per metre of chimney. For comparison, forced convection systems commonly operate with pressure differences of 100-500 Pa.

One of the earliest designs to enhance ventilation in cabinet solar dryers is the solar and wind-ventilated dryer. The design uses a ventilator which depends entirely on the wind effect. Air is drawn through the dryer by wind-powered rotary vanes located on top of the dryer chimney. Temperature and air flow rates are controlled by a damper.

Need Of Solar Dryers in Africa:

In the majority of African countries, agriculture represents the biggest part of the economy. 80-90% of the working population is employed in agriculture. Despite these large numbers, national food production still does not meet the needs of the population. The lack of appropriate preservation and storage systems caused considerable losses, thus reducing the food supply significantly.

The dent in food production caused by crop-failures as well as significant seasonal fluctuations in availability can be ironed out by food conservation, e.g., by drying. Sun drying of crops is a traditional method of food preservation in a lot of African countries due solar irradiance being very high for the most of the year. There are some drawbacks relating to the traditional method of drying, i.e., spreading the crop in thin layers on mats, trays or paved grounds and exposing the product to the sun and wind. These include poorer quality of food caused by contamination by dust, insect attack, enzymatic reactions and infection by micro-organisms. Also this system is labour- and time intensive, as crops have to be covered at night and during bad weather, and the crops continually have to be protected from attack by domestic animals. Non-uniform and insufficient drying also leads to deterioration of the crop during storage. Serious drying problems occur especially in humid tropical regions where some crops have to be dried during the rainy season.

In order to ensure continuous food supply to the growing population and to enable the farmers to produce high quality marketable products, efficient and at the same time affordable drying methods are necessary. Studies have shown that even small and most simple oil-fired batch dryers are not applicable for the most farmers, due to lack of capital and insufficient supply of energy for the operation of the dryers. The high temperature dryers used in industrialised countries are found to be economically viable in developing countries only on large plantations or big commercial establishments. Therefore the introduction of low cost and locally manufactured solar dryers offers a promising alternative to reduce the tremendous post harvest losses. The opportunity to produce high quality marketable products seems to be a chance to improve the economic situation of the farmers. However, taking into account the low income of the rural population in developing countries, the relatively high initial investment for solar dryers still remains a barrier to a wide application.