All about the solar dryer

In this article, we review all aspects of natural food dehydration with a solar dryer: the different drying methods - how solar dryers work - the advantages of solar dehydration. Enjoy your reading!

Traditional air drying of food

The traditional method of drying food is to place the food in the sun, in the open air. This method, called natural drying or air drying, is effective for small amounts of food. The amount of surface area needed to dry in the sun increases with the amount of food, and because the food is placed in the open air, it is easily contaminated. Therefore, one of the main reasons why sun-drying is not easy to achieve with larger quantities of food is that control and monitoring becomes increasingly difficult with increasing quantities of food.

How does the solar dryer work?

Unlike sun drying, where food is exposed to direct sunlight, the solar drying method uses indirect solar radiation. The principle of the solar drying technique is to collect solar energy by heating the air volume in solar collectors and to lead the warm air from the collector to the drying chamber. The products to be dried are placed in the drying chamber.

In this closed system, consisting of a solar collector and a drying chamber, without direct exposure of the meat to the environment, the drying of the meat is more hygienic, as there is no secondary contamination of the products by rain, dust, insects, rodents or birds. The products are dried only with hot air. There is no direct impact of solar radiation on the product. Solar energy produces hot air in the solar collectors. The increase in temperature in a given volume of air decreases the relative humidity of the air and increases the water absorption capacity of the air. A steady flow of hot air in the drying chamber, flowing through and over the pieces of meat, allows for continuous and efficient dehydration.

What is the capacity of a solar dryer?

The solar dryer or solar dehydrator is a relatively simple concept. An important feature of the drying devices is the size of the solar collectors. Depending on the amount of product to be dried, the collectors must have the capacity to supply sufficient quantities of hot air to the drying chamber. Collectors that are too small for the amount of food to be dried will result in failed attempts and food spoilage.

What is the drying process?

The food drying process consists of 3 steps:

1. Converting light into heat: the presence of black inside a solar dryer improves the efficiency of the transformation of light into heat.

2. Trapping heat: insulating the air inside the dryer from the outside air makes a significant difference. Using a transparent solid, such as a plastic bag or glass lid, allows light to enter, but once the light is absorbed and converted to heat, the plastic bag or glass lid traps the heat inside to dehydrate the food. This makes it possible to achieve similar temperatures in cold, windy weather as in hot weather.

3. Move heat to the food: Both the natural convection dryer and the forced convection dryer use heated air convection to move heat to the food. When the heat is redirected into the hot chamber, the humidity in the air decreases drastically allowing dehumidification and drying of fruits for example.

The 3 types of solar dryer

There is a wide variety of solar dryer models. Basically, solar dryers can be classified into three groups:

1. Solar dryer with natural convection

Solar dryers use natural vertical convection that occurs when air is heated. In general, natural convection dryers are appropriately sized for farm use.

The structure consists of three main components: a solar collector, a drying tank and a solar chimney. Natural convection dryers, which are smaller in size, are essentially wooden boxes with vents at the top and bottom. Food is placed on wire frames that slide into the boxes. An appropriately sized solar air heater with south-facing plastic glazing and a black metal absorber is connected to the bottom of the boxes. Air enters from the bottom of the solar air heater and is heated by the black metal absorber. The hot air rises along the food and exits through the vents at the top.

In operation, these dryers produce temperatures of 54 to 82°C, which is desirable for most food drying and pasteurization. With these dryers, it is possible to dry food in one day, even when the weather is partly cloudy, foggy and very humid. Inside, there are thirteen racks that can hold 35 to 40 medium-sized apples or peaches cut into thin slices.

2. Solar dryer with forced convection

Convection is forced onto the food through the use of a fan. In the case of forced convection dryers, the structure can be relatively similar. However, the forced convection dryer requires a power source for the fans to provide the airflow. The forced convection dryer does not require an incline for the airflow, but the collector can be placed horizontally with the fan at one end and the drying pan at the other end.

In addition, the forced convection dryer is less dependent on solar energy because it provides the airflow itself; this allows the design to operate in weather conditions in which the natural convection dryer does not operate. Since inadequate ventilation is a major cause of food loss in solar dryers, and is exacerbated by intermittent heating, it is essential to achieve adequate ventilation. The addition of forced convection flow, for example provided by a PV solar cell connected to a fan, will prevent food loss.

3. Solar tunnel dryer

The structure of a tunnel dryer is relatively simple with a polyethylene wall that is particularly resistant to high temperatures. Unlike other types of dryers, the structure is large enough for one person to enter.

How to successfully dry food?

Drying is an important step in the food production process. The main argument for drying food is to preserve it longer. However, it is important to note that the process is not limited to removing moisture from the food.

Other quality factors are influenced by the choice of drying conditions and equipment:

Moisture content: It is essential that the food after drying has a moisture content suitable for storage. The desired moisture content will depend on the type of food, the length of storage and the available storage conditions. The drying operation is also essential to minimize the range of moisture levels in the batch of food, as insufficiently dried portions of food can lead to spoilage of the entire batch.

Nutritional value: Food constituents can be negatively affected when excessive temperatures are reached.

Mould growth: The rate of growth of micro-organisms depends on the water content of the food, the temperature and the degree of physical damage to the food.

What are the advantages of a solar dryer?

This technology has several socio-economic advantages. One of the main problems facing developing countries today is food security. The solar dryer, using this renewable energy, can improve food security, allowing for better preservation of food after drying compared to food that has not been dried. The solar dryer reduces the drying time and facilitates the drying of food compared to the sun drying method.

The solar dryer saves fuel and electricity when replacing dryer variants that require an external energy source in the form of electricity or fossil fuel. In addition, solar dryers reduce drying times compared to solar drying. While dryers powered by fossil fuels or electricity may have some advantages (smoother airflow and higher temperatures), the financial barriers associated with these technologies may be too high for marginal farmers. For example, electricity may not be available or may be too expensive, and drying with fossil fuels may involve high initial and operating costs.

Fruit, vegetables and meat dried in a solar dryer are of better quality and more hygienic than sun-dried fruit, vegetables and meat. As mentioned, due to the closed system design, contamination of food is avoided or minimised. In addition, the food is not vulnerable to rain and dust, unlike the open sun-drying system.

In rural areas where farmers grow fruit and vegetables without adequate drying facilities, they have to sell the food on the market soon after harvest. When food production is high, farmers need to sell the food at a low price to prevent it from losing value through decomposition. Therefore, the solar food dryer may be able to avoid the financial losses that farmers face in these situations. Dried food can be stored for longer and retain its quality for longer. In addition, dried fruits and vegetables can be sold as differentiated products, which can increase their market value. For example, dried meat can be processed into a variety of different products.

The food will be drier and less bulky. Therefore, in addition to a longer storage time, the food is also easier to transport after drying with the help of the integrated racks in the solar dryer.

How to make a solar dryer?

The manufacture of a solar dryer requires only simple materials which are accessible in all large surfaces and reflective mirror.
Solar Brother offers you the PDF plans to download for free by clicking here.

Discover OMY, the modular solar dryer designed by Solar Brother :