3. Results and discussionsThe revie

3. Results and discussions
The review of inventoried dryer parameters (Section 2) shows a significant deficit in appropriate criteria for characterized dryers from the sub-Saharan zone. The most significant we propose for all inventoried dryer types include: the dried products used, the types of dryer users, the environmental specifications, the energy used, the drying specifications and the cost of the inventoried dryers. The thermal and economic analyses of six characteristic dryers chosen from all the inventoried dryers follow.
3.1. Inventoried dryer types
Although there are many types of dryer technologies in scientific literature [2,11,12,21], the variety of technology really used in the field is limited (Tables 4–6, Fig. 2). The inventoried dryers were grouped into 10 types, and characterized by their data in Tables 7 and 8. According to the criteria classification of Table 1, all were small-scale dryers applied to solid products in a thin layer. They ran in batch or semi continuous mode, and were controlled manually. The inventoried dryers could be classed according to six types of energy income modes and described in Tables 4–6.
3.1.1. Sun drying Sun drying was the most used, representing 70% of the total dryers surveyed. This method uses a surface such as the ground, asphalt roads, cemented areas or tables (Table 4, 7). The use of a protective sheet (fabrics, trays, sieves or mats) facilitated product handling and avoided direct contact with the ground. A typical case involved drying on fertilizer bags. In some cases, products were covered with fabric to reduce insect and bird contamination (e.g. sun drying of coconuts covered with a fine net in Burkina Faso).
3.1.2. Direct solar dryers Three types of direct solar dryers were found: tent, direct cupboard and greenhouse (Tables 5, 7 and 8). All their structures were made of wood. Covers were made of polyethylene film for tent and greenhouse dryers, and glass for direct cupboard sides. Table 7 shows that their capacities were limited from 5 to 20 kg of product which is lower than those of sun drying.
3.1.3. Indirect solar dryers Despite the numerous research publications on indirect solar dryers [2,13,22], only two types of this model were found: the shell dryer and the indirect cupboard dryer (Tables 5, 7 and 8). All the Shell dryer manufacturers received the same training which resulted in the same design, two black painted metal cones joined by a hinge [30,63] (see Shell photo in Table 5). The indirect cupboard dryer was closer to the M5 003, a much-studied chimney dryer see [64]. It had a wooden structure, insulated with kapok, covered with sheet metal painted in black. In that solar dryer, the hot air circulation is reached through a glass cover solar collector and a chimney.