## 4 Discussion
### Drying Performance
Specifically, in Replication 4, which had the higher efficiency of the first 24 h of drying, the average daily wind speed was recorded at its lowest values (0.21-1.97 km/h). During Replications 1 to 3, the average wind speed was between 4.76 and 5.15 km/h (first 24 h). The ambient average temperature was almost 3 \({}^{\circ}\)C higher in Replication 2 (Table 4), but drying efficiency was not higher in comparison with Replication 4 for both potatoes and bananas. The lower drying efficiency (first 24 h) of the first three replications was probably due to the high relative humidity of the environment during the drying process. After drying, both potatoes and bananas changed drastically in color from light yellow to dark yellow and dark brown. In addition, in the potato, we had a slight shrinkage of the shape of the cube, with the middle of its surfaces receding toward their interior, forming slight curves. The proposed system can be compared with similar methods tested in solar dryers. For banana drying, there are similar cases in the literature that the drying is performed in a direct solar system (greenhouse-type drier) with forced ventilation in Thailand (cases study 1: C.S.1) [22] and in an indirect system (solar collector) also with forced ventilation in Egypt (case study 2: C.S.2) [17]. In C.S.1, the moisture content reduced from 70% to 40% of the total weight in the first 24 h. In C.S.2, this percentage reduction depends on the inclination of the solar drier. In the most favourable position, the moisture content reduced from 85% to almost 20-25% in the first 24 h. Based on Figure 7, in the examined case, the weight loss reduction is about 40-50% in the first 24 h for each tube type, while when the conditions are favourable, this percentage can vary from 50% to 60%. This percentage is mostly moisture, so it seems that the proposed drying method efficiency is comparable to other solar drying methods. However, the disadvantage was that the efficiency was dependent on climatic conditions (temperature and prevailing winds), so the drying rate was not constant. Moreover, it was observed that even if the concentrated air flow inside the tubes leads to a high drying rate, in C.S.1 and C.S.2 systems, larger amounts of banana slices could be dried as the available surface was larger. On the other hand, this drying rate variation and less available space in the examined system is encountered with zero energy consumption. In any case, there are many factors that also need to be considered, such as the weather conditions, initial product characteristics, etc. Beyond the drying process, the initial weight and thickness of the samples should be taken into consideration. In C.S.2 samples, the thickness was between 0.5 and 1 cm, while in the examined system, it was 1 cm of thickness in all cases. The size of the samples is almost the same, so any difference occurs due to the different operations of the compared systems. Similar findings were also observed for potato drying. The system will be compared with the case of an indirect solar dryer in the region of Algeria (Case study 3: C.S.3), where the drying was performed in a cabinet that air heats up in a collector that was forced to move by the assistance of an artificial ventilation system [21]. A similar system studied in the region of India comprised a specially designed triple-pass solar dryer utilizing the techniques of thermal storage and collector insulation to enhance its efficiency (Case study 4: C.S.4 [23]). In C.S.3, the potato slices reached the desired moisture content (13% wet basis) in almost 3 h, while in C.S.4, the same result was reached in 4.5 h. In the examined system, the potato drying took almost 40-48 h in the most favourable case. However, it should be noticed that the potato samples in this study had a thickness of 1 cm, while in C.S.3 and C.S.4 was 2 mm and 3 mm, respectively. Furthermore, in these case studies, the potato samples were initially blanched, which strongly enhanced the drying process.