Traditional methods of food drying is to spread the foodstuffs 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 meat 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.
The solar dryer is a relatively simple concept. 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: a) natural convection dryers, which are solar dryers that use the natural vertical convection that occurs when air is heated and b) forced convection dryers, in which the convection is forced over the food through the use of a fan and c) tunnel dryers.
While several different designs of the solar dryers exist, the basic components of a solar dryer are illustrated in Figure 1. In the case of a forced convection dryer, an additional component would be the fan.
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
The design of a tunnel dryer is illustrated in Figure 2. In addition, the technology teaser image at the top of this description is an image of the inside of a tunnel dryer.
Natural Convection Dryer Large scale design
Generally, natural convection dryers are sized appropriately for on-farm use. One design that has undergone considerable development by the Asian Institute of Technology in Bangkok, Thailand is shown in Figure 3. This natural covenction dryer is a large scale structure: the collector is 4.5 meters long and 7 meters wide and the drying bin is 1 meter long and 7 meters wide. The structure consists of three main components: a solar collector, a drying bin and a solar chimney. The drying bin in this design is made of bamboo matting. In addition to the collector, air inside the solar chimney is heated which also increases the thermal draught through the dryer. The solar chimney is covered with black plastic sheet in order to increase the thermal absorption. A disadvantage of the dryer is its high structural profile which poses stability problems in windy conditions, and the need to replace the plastic sheet every 1-2 years.
Figure 4 shows a smaller design for a natural convection dryer. The capacity of this dryer is ten times smaller than the capacity for food drying in the larger design. However, the design is simple to build and is less susceptible to stability problems.
Natural Convection dryer small scale design
These solar food dryers 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 (see Figure 5). 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.
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.
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.
Therefore, it is essential to not only monitor the moisture content of the foodstuffs, but to also monitor temperature, mould growth, appearance and smell of food, air flow, etc. Whether a natural convection dryer, a forced convection dryer or a tunnel dryer is appropriate depends on the amount of food, the climate and the demands placed on the end-product (how long does it need to be stored, in what quantities, etc.). A typical pattern of several of these factors is shown in Figure 6.
In addition, 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.
According to the FAO (no date), the most common drying method of grain in tropical developing countries is sun drying. The process of sun drying starts when the crop is standing in the field prior to harvest; maize may be left on the standing plant for several weeks after attaining maturity. However, this may render the grain subject to insect infestation and mould growth. In addition, it prevents the land being prepared for the next crop and is vulnerable to theft and damage from animals.
A more controlled practice is to bring the foodstuffs into a structure which is specifically designed for food drying. This removes the issue of bacterial contamination, theft and insect infestation. Modern variations are to dry food in special enclosed drying racks or cabinets and expose the food to a flow of dry air heated by electricity, propane or solar radiation.
Although it is difficult to establish the current status of the technology in terms of market penetration as data on this technology is insufficient, some general remarks can be made about the market potential.
There seem to be no major design barriers to a solar dryer: the design is easy to build with a minimum of materials required. This is especially true for the natural convection dryer, which doesn't require any machinery or energy source (next to the solar energy source). In contrast, the forced convection flow, the electricity heated design and the propane fuelled dryers all require some form of machinery and require an external heat source (in the form of electricity or propane). This complicates their designs and makes their operational costs more expensive. However, these designs possibly do have lower food loss rates due to more constant air flow.
Related to the previous remark, the easy design cuts costs. The design can be made primarily from materials found in the local surroundings. For instance the frame of the structure can be constructed from wood, bamboo or any other natural product that is strong enough. This characteristic enhances the market potential of this product.
The technology provides several socio-economic benefits. As the FAO (2010) notes, 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.
While there is insufficient data at the moment to elaborate fully on the financial requirements and costs of this technology, certain general remarks can be made.
For natural convection dryers, the financial requirements are low. The structure is made from components that are mostly easily available (wood, bamboo, other strong construction materials). However, the major cost components are likely to be the glass sheets required to trap the heat, and the plastic sheets need to be available. Operational costs of the natural convection technology are limited to labour costs. Forced convection dryers have higer initial costs and higher operational costs, as the fan needs to be purchased and operated.
As mentioned, dried food products might yield a higher price on the market as it can be sold out-of-season (the fresh food version might no longer be on the market in a particular season, which might increase the price of the dried version of the food.
FAO, 2010. “Climate-Smart” Agriculture - Policies, Practices and Financing for Food Security, Adaptation and Mitigation. Food and Agriculture Organization of the United Nations 2010. Document can be found at: http://www.fao.org 
FAO, no date. Information retrieved from the following websites: http://www.fao.org/docrep/t0395e/T0395E04.htm , http://www.fao.org/docrep/x0209e/x0209e06.htm  and http://www.fao.org/docrep/t1838e/T1838E0v.htm