Traditional methods of food drying is to place the foodstuffs in the sun in the open air. This method, called sun drying, is effective for small amounts of food. The area needed for sun drying expands with food quantity and since the food is placed in the open air, it is easily contaminated. Therefore, one major reason why sun drying is not easily performed with larger quantities of food is that the monitoring and overview becomes increasingly more difficult with increasing food quantities. In contrast to sun drying, where the food is exposed directly to the sun, the solar drying method uses indirect solar radiation. The principle of the solar drying technique is to collect solar energy by heating-up the air volume in solar collectors and conduct the hot air from the collector to an attached enclosure, the meat drying chamber. Here the products to be dried are laid out.
In this closed system, consisting of a solar collector and a meat drying chamber, without direct exposure of the meat to the environment, meat drying is more hygienic as there is no secondary contamination of the products through rain, dust, insects, rodents or birds. The products are dried by hot air only. There is no direct impact of solar radiation (sunshine) on the product. The solar energy produces hot air in the solar collectors. Increasing the temperature in a given volume of air decreases the relative air humidity and increases the water absorption capacity of the air. A steady stream of hot air into the drying chamber circulating through and over the meat pieces results in continuous and efficient dehydration.
Responds to the following needs
- Food security and preservation
- Clean energy
- Small-scale farmers and food producers
- Subsistence agriculture
Relevant CTCN Technical Assistance
- Mali: Design and financing for crop drying and storage technologies to strengthen food security in the face of climate change
Relevant Technology Needs Assessment
- Promotion of solar dryers technology and capacity building for technicians and farmers in Kenya
- Kenya TNA Technology fact sheet
The solar dryer is a relatively simple concept. An important feature of solar drying devices is the size of the solar collectors. Depending on the quantity of goods to be dried, collectors must have the capacity to provide sufficient quantities of hot air to the drying chamber. Collectors which are too small in proportion to the amount of food to be dried will result in failed attempts and spoiled food.
The basic principles employed in a solar dryer are:
- Converting light to heat: Any black on the inside of a solar dryer will improve the effectiveness of turning light into heat.
- Trapping heat: Isolating the air inside the dryer from the air outside the dryer makes an important difference. Using a clear solid, like a plastic bag or a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, a plastic bag or glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.
- Moving the heat to the food. Both the natural convection dryer and the forced convection dryer use the convection of the heated air to move the heat to the food.
There are a variety of solar dryer designs. Principally, solar dryers can be categorized into three groups:
Natural convection dryers: Solar dryers that use the natural vertical convection that occurs when air is heated. Generally, natural convection dryers are sized appropriately for on-farm use. The structure consists of three main components: a solar collector, a drying bin and a solar chimney. Natural convention dryers that are smaller in scale are basically wooden boxes with vents at the top and bottom. Food is placed on screened frames which slide into the boxes. A properly sized solar air heater with south-facing plastic glazing and a black metal absorber is connected to the bottom of the boxes. Air enters the bottom of the solar air heater and is heated by the black metal absorber. The warm air rises up past the food and out through the vents at the top. While operating, these dryers produce temperatures of 130–180° F (54–82° C), which is a desirable range for most food drying and for pasteurization. With these dryers, it’s possible to dry food in one day, even when it is partly cloudy, hazy, and very humid. Inside, there are thirteen shelves that will hold 35 to 40 medium sized apples or peaches cut into thin slices.
Forced convection dryers: The convection is forced over 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 air flow. The forced convection dryer doesn't require an incline for the air flow however, the collector can be placed horizontallly with the fan at one end and the drying bin at the other end. In addition, the forced convection dryer is less dependent on solar energy as it provides the air flow itself; this allows the design to work in weather conditions in which the natural convection dryer doesn't work. As inadequate ventilation is a primary cause of loss of food in solar food dryers, and is made worse by intermittent heating, it is essential to realize proper ventilation. Adding a forced convection flow, for instance provided through a PV- solar cell connected to a fan, will prevent the loss of food.
Tunnel dryers: The structure of a tunnel dryer is relatively simple. The basic design components of a tunnel dryer are the following:
- A semi circular shaped solar tunnel in the form of a poly house framed structure with UV stabilized polythene sheet
- The structure is, in contrast to the other dryer designs, large enough for a person to enter
Important quality factors: Drying is an important step in the food production process. The main argument for food drying is to preserve the food for longer periods of time. However, it is important to note that the process is not just concerned with the removal of moisture content from the food. Additional quality factors are influenced by the selection of drying conditions and equipment:
- Moisture Content. It is essential that the foodstuff after drying is at a moisture content suitable for storage. The desired moisture content will depend on the type of food, duration of storage and the storage conditions available. The drying operation is also essential in minimizing the range of moisture levels in the batch of food as portions of under-dried food can lead to deterioration of the entire batch.
- Nutritive value. Food constituents can be adversely affected when excessive temperatures are reached.
- Mould growth. The rate of development of micro-organisms is dependent on the food moisture content, temperature and the degree of physical damage to the food.
- Appearance and smell of the food. For example, the colour of milled rice can be adversely affected if the paddy is dried with direct heated dryers with poorly maintained or operated burners or furnaces.
Co-benefits of this technology
- The technology provides several socio-economic benefits. One of the main issues facing developing countries today is the issue of food security. The solar food dryer can improve food security through allowing the longer storage of food after drying compared to food that hasn't been dried.
- The solar dryer can save fuel and electricity when it replaces dryer variations that require an external energy source in the form of electricity or fossil fuel. In addition solar food dryers cut drying times in comparison to sun drying. While fossil fuel or electrically powered dryers might provide certain benefits (more consistent air flow and higher temperatures), the financial barriers that these technologies provide might be too high for marginal farmers. For instance, electricity might be not available or too expensive and fossil fuel powered drying might pose large initial and running costs.
- Fruits, vegetables and meat dried in a solar dryer are better in quality and hygiene compared to fruits, vegetables and meat dried in sun drying conditions. As mentioned, due to the closed system design, contamination of food is prevented or minimized. In addition, the food is not vulnerable to rain and dust, compared to the open system design of sun drying.
- In rural areas where farmers grow fruits and vegetables without proper food drying facilities, the farmers need to sell the food in the market shortly after harvesting. When food production is high, the farmers have to sell the food at low price to prevent the food from losing value through decomposition. Therefore, the solar food dryer might be able to prevent the financial losses farmers in these situations face. Dried food can be stored longer and retain quality longer. Moreover, dried fruits and vegetables might be sold as differentiated products which possibly enhances their market value. For example, dried meat can be processed into a variety of different products.
- Drying food reduces its volume. Therefore, in combination to longer storage times, the food is also more easily transported after drying which potentially opens up additional markets to the producer of the food.
- Piloting climate change adaptation in food production and food security on low-lying atolls of Solomon Islands
- Solar dryers for income generation in India