Manual on solar dryers for fruits and vegetables

Several decades ago, thermal dryer, mainly used fossil fuels as an energy source to heat the air, which was carried out dehumidification. However, with the rising prices of traditional fuels back in the 1970-ies, solar energy has been regarded as a source of energy, representing great value for dehydration of products, therefore, the use of this energy for the purpose of drying began to grow.

Which means drying and why it is needed?

Drying is the removal of water that is in the soft tissues of a product, with the aim of imparting certain characteristics that facilitate handling, its preservation or use.

Drying in the case of food products is a process which helps to preserve it. Due to the fact that most of the liquid is missing, for this reason, dehydrated food product can not rot. It is possible to carry out the dehydration of various fruits, vegetables, meat, fish, etc. So that they can keep a natural appearance for many months.

There are a large number of other products that also have a biological origin, such as wood, which can be dehydrated to the most simplified treatment. When using wood in the construction industry, in furniture, or as a biofuel, in this case, a proper drying process. Improper drying of excess moisture in wood can lead to warping, cracking or exudation in already manufactured furniture or may have difficulty burning that also may lead to "unwanted" "explosion" when used as fuel.

How is the product drying?

There are several ways to remove moisture from products. In this article we will only talk about the processes that affect the use of solar heat. Actually you can use any energy source to generate the heat required for drying.

Among these sources are solar energy, which is ideal for this task because it is free, and it can operate in a temperature range suitable for drying, also the efficiency can be perfect.

Drying heat is from inside the product to dry into an environment that will promote the evaporation of the water contained in its interior. This is because products have a tendency to balance between their internal moisture content and the environment. If the environment is sufficiently warm and dry, the product tends to lose its internal moisture content to the extent that he can not fully recover, even if it is placed in a moist environment. Therefore, ideal conditions for dehydration are air masses that surround the product in the presence of elevated temperature and low relative humidity. They contribute to the emergence of air flow and significantly increase them for the updates of the environment around the product with the purpose of dehydration by replacing the already humidified air others dry and warm in the drying process.

1 - the Air in the manifold arrives fresh, having averaged relative humidity. Heat generated by the sun, leads to an increase in air temperature, and acquires the ability to hold more moisture. Because external moisture is missing, its relative humidity drops.

2 - Hot air and low relative humidity from the solar collector raising the temperature of the products and cause them to evaporate the water contained in them. Warm and dry air readily absorbs moisture that is released from the product, and in the process increases its relative humidity, thus reducing its temperature. In the end, still warm and moist air is discharged from the dryer into the atmosphere.

Details of solar dryers

The internal structure of solar dryers is particularly drying zone to the drying process of the products was most effective. There are differences in the form and arrangement of each of these zones depending on the model. In some models, multiple zones can be located in the same place, have similarity, and may not have defined boundaries. Key areas are:

Absorption area - the area that receives direct solar radiation and converts it into heat, whereby products to be dried.

The drying area is the place where the product is drying.

The area out moisture - a place where the air is saturated with moisture, is etched into the atmosphere.

Area of the fresh air intake point through which the air comes in to replace one that was corroded.

The air circulation system air circulation around the product to be dried is very important because it corrodes already extracted the moisture, maintaining a dry environment, which speeds drying.

Depending on the technique used to move air, there are two systems: Natural convection is the natural movement, leading to the rise of hot air. Air, when heated, reduces its density, and has a tendency to move up in a dense medium. This phenomenon is called convection. In solar dryers - this natural air movement is used to iterate through the area where the product is dried, then is removed from the system. The outlet creates a compression that leads to the entry of outside air into the system and heated up again by recycling process. Convection circulation is maintained the whole period, while under the heat of the sun. This method is suitable for small systems of natural drying. The advantage is that it has no costs and disadvantages, which are available in the dryer with a complex design, but the power of air movement may be insufficient to achieve the level of updates a suitable environment.

- Forced circulation - use electrical equipment, such as exhaust fan, where the fan serves to move air. This system is suitable for large and complex systems. The disadvantage is that it requires a power source, although, if the user will use photovoltaic panels, the energy of the whole system can be obtained from the sun. There are a large number of solar dryers. From the most simple place on the street, to the most complex intended for industrial drying, and medium size for small businesses or home use.

Types of solar dryers Drying in the open air - without a doubt this is the most simplest and the oldest of all existing systems. Probably this system has been used by mankind since prehistoric times for food drying. This method is still used in many parts of the world for the purpose of economy and simplicity. However, this simplicity imposes more restrictions on its use. Its use is limited to warm, Sunny and dry days. In areas with high humidity environmental using this method is ineffective, or even completely impossible. But in desert areas this system can be widely applied without any problems. An example of such a case are red tomatoes that many tribes in the Sahara desert are dried in the sun in a hot and dry atmosphere of the desert, to keep the product in the course of the year until the next harvest. Other disadvantages of this technique lie in the fact that the product undergoing drying is vulnerable to natural rainfall, atmospheric pollution and exposure to animals and insects. There are also a number of products, which easily dry up even living inside and in the shade. Good examples are some types of peppers (Chile), which dry up without difficulty. When using this drying technology, the drying zone of the product is the same surface on which are placed the products. The air freely enters and exits and circulation system is the only way girder convection flows.

A solar dryer of the chamber type - these types of dryers are compact wardrobe. The area of accumulation of solar energy is the same as the area of the drying chamber. It has a small hole at the bottom, where it comes fresh air, and another hole in the upper part is designed to warm air came out with a certain level of humidity. In this type of dryer, the air circulation is by natural convection. Because the air has a lot of obstacles when moving, the air flow is slowed down, therefore, the efficiency is not very high. These systems are capable of drying small amounts of food. They are mainly used for drying food.

Solar collectors and dryers of the chamber type.

These dryers consist of a solar collector where the air is heated and rises into the Cabinet, where items are placed for drying.

Solar dryer, complete with solar panel Area of collection of solar rays is its own air manifold, although there are some models that also have a transparent surface for capturing solar radiation in the cupboard. The possibility of opening a Cabinet to capture solar radiation depends on the substances to be dried. If these are products that are sensitive to ultraviolet light, the appearance of which may be corrupted, in this case, you should choose a closed system. Inlet air hole is located in the lower part of the manifold, while the exhaust hole is located at the top of the wardrobe.

Type of air circulation is through natural convection. The location of the collector at the bottom of the equipment has a certain slope, and, along with the exhaust air hole in the upper part facilitates the movement of air, which is faster than in the case of a dryer of the chamber type. These dryers are suitable for food, herbs, flowers, etc. in low to medium volume, depending on the size and capacity of equipment. By combining several modules of this type, it is possible to dry the volume of production at industrial levels.

A solar dryer with battery (bunker) – this system is similar to systems cell-type using the panel, the only difference in the large size. Instead of a closet you will have a hopper for drying a much larger volume. In addition, some of the collectors will be more because of the need to develop much more heat. This type of equipment has a system of forced air circulation, as a greater volume of products subject to drying, makes the movement of air by natural convection.

Solar dryer with battery (bunker)

The dryer is a greenhouse-type - this system consists of a large greenhouse, similar to that used in agriculture. In this case, the heat generated in the greenhouse, used for drying products. In itself it is the same scheme as the model chamber type, the only difference is in the proportions and materials used in greenhouse design. Some of the models proposed for drying wood, bring innovation, namely, to have wheels that allow to avoid excessive load on the structure due to the weight of the tree.

The dryer is a greenhouse-type

These constructions have the system forced circulation to reach the level of proper renewal of the air, which only one system can not reach the convection.

Dryers with closed collectors – these types of dryers have separate air solar collectors and the drying chamber. The hot air passes through the manifolds into the chamber through the respective ducts. Has a system of forced air circulation, which should have more power. Solar dryer with closed manifolds

Rasul Telibekov, Club, SAP, Republic of Kazakhstan