## 2 Materials and Methods  
### Drying Experiment  
The experiments were conducted in Bama, Burkina Faso (1.374437\({}^{\circ}\) N, 4.389404\({}^{\circ}\) E). The fresh paddy rice was obtained from the nearby plantation areas, and was provided by a farmer cooperative. After harvest, the paddy rice was kept in a storage house for approximately 24 h before drying. The drying experiments were conducted in three batches on 20, 22 and 23 November, 2017. For each batch, the experiments started at around 10:00 and were terminated at around 16:00.  
In this study, an ISD (SBD 50, GrainPro Inc., Zambales, Philippines) was used for the field tests, as shown in Figure 1. The overall length and width of the ISD are 25 and 2 m, respectively. The ISD has a flexible body, which consists of two waterproof plastic films, namely, lower and upper cover, which are joined by a heavy-duty zipper. The lower cover, or also known as the drying floor where paddy rice is spread, is made of a black reinforced polyvinyl chloride (PVC), while the upper is made of a transparent UV-resistant low-density polyethylene (LDPE). An area with a length of 1 m from the ISD inlets was used as a solar air heater, later called a collector area, by not spreading the paddy rice on this part of the black plastic film.  
The ISD was equipped with two DC axial fans (RDF2589B12N18S, Runda Electronics CO., LTD, Shenzhen, China), which were attached at the inlets. The fans were driven by a 12 V storage battery with a capacity of 100 Ah (DIN100 SMF, Guangzhou Tongli Storage Battery Co., Ltd, Guangzhou, China). The battery was charged by two solar panels (P100, Resun Solar Energy Co., Ltd, Jiangsu, China). The system was controlled by a solar charge controller (SR-SL 10 A, Shenzhen Shuori New Energy Technology Co., Ltd, Shenzhen, China). The amount of paddy rice dried in the ISD in batch 1, 2 and 3 was 587, 975 and 975 kg, respectively.  
When the fans were switched on, the ISD body was inflated by air pressure, which built up inside. As solar radiation passed through the transparent upper cover, the inflated ISD body trapped the solar radiation by the greenhouse effect. Consequently, the air and product temperature in the ISD increased. As shown in Figure 2, the fans at the ISD inlets could easily suck unwanted material, like dust, into the ISD, due to sandy soils and a dusty environment. As a control measure, in batch 2 and 3, the ISD inletswere covered with two cardboard sheets at the left and right side, while the solar panel was placed right in front of the inlets.  
The bulk height of the paddy rice in the ISD was 22 mm in batch 1, and 40 mm in batch 2 and 3. In all batches, the bulk width was around 1.83 m. As a control, for conventional sun drying, around 80 kg of paddy rice was spread on a \(2.5\times 2.2\) m tarpaulin. The bulk width and height were the same as the paddy rice in the ISD, while the length was adjusted proportionally. When short rains occurred in batch 1, paddy rice dried under the sun was covered with a tarpaulin, while the ISD was kept running. The rainfall duration was generally less than seven minutes.  
The paddy rice drying in the ISD and under the sun was mixed every hour. For conventional sun drying, a rake was used, while for the ISD, a roller bar was used to mix the paddy rice without disturbing the drying process. The roller bar was delivered together with the ISD. Each end of the roller bar was attached to a castor wheel. In order to tow the roller bar underneath the drying floor along the ISD, two operators pulled a rope tied to each wheel. The purpose was to transmit an oscillating movement to the paddy rice during mixing [22].  
Figure 1: **Top**: Conventional sun drying and inflatable solar dryer (ISD). **Bottom**: A rake for mixing while sun drying (**left**) and a roller bar for mixing the paddy rice in the ISD (**right**).  
Figure 2: Unwanted dust suction by the fans into the inflatable solar dryer (ISD) (**left**), and a control measure to prevent dust by covering the ISD inlets (**right**).