# Technical Performance of an Inflatable Solar Dryer for Drying Amaranth Leaves in Kenya
## 3 Results and Discussion
### Drying Performance of the ISD Compared to Conventional Sun Drying  
Solar radiation and PV voltage, along with temperature and relative humidity inside the ISD, were compared to ambient conditions for a four-day drying experiment on 8-11 December 2016 and are shown in Figure 6. During the measurement period, bright cloud fields passed through and the sun was frequently shining through openings, causing frequent fluctuations in measured solar radiation and resulting in high peaks up to 1070 W/m\({}^{2}\). Same fluctuations were reflected in PV voltage, where values of 10.22 to 18.75 V were obtained for solar radiation of 510 to 950 W/m\({}^{2}\). Maximum PV voltage was 18.91 V and was achieved on December 8 (day 1) at 14:00. During the night, the ambient temperature and relative humidity could reach up to 18.0 \({}^{\circ}\)C and 92.7%. The highest recorded temperature inside the ISD was 63.4 \({}^{\circ}\)C on December 9 at 14:09, while the ambient temperature was 45.4 \({}^{\circ}\)C. Drying conditions at temperatures of 40 \({}^{\circ}\)C or above were typically achieved in the ISD from 12:00 to 16:00. During the night, the ISD could show a lower temperature and a higher relative humidity than the ambient condition. Fluctuations of the temperature could be attributed to the opening of the zipper while mixing the leaves, which resulted in heat loss. In total, mixing the leaves on all 21 tra  
Figure 6: Solar radiation and voltage generated by the PV modules (**top**) and temperature and relative humidity in the ISD and ambient conditions (**bottom**) during the drying experiment on 8–11 December 2016. 4.5 min. In order to minimize the opening time of the zipper, the mixing process should be done as efficiently as possible. The drying curve of blanched amaranth leaves inside the ISD and under sun drying is presented in Figure 7. The total drying time was 68.08 h, whereas the final moisture content of the leaves dried in the ISD and sun drying was 11.6% and 24.7% w.b., respectively. It can be seen that the ISD showed a better drying performance than sun drying, particularly on day 3. In the night between days 3 and 4, a visible reduction of the moisture content in the leaves under sun drying was detected. This might be due to lower relative humidity of the ambient air and occurrence of wind, which was observed during that night. A lower relative humidity of the air causes a higher vapor pressure deficit of the air, which accelerates the drying rate [26]. A different pattern was reported in drying paddy rice using a previous type of the ISD, which resulted in a lower drying performance than sun drying during the dry season [13]. This might not be the case for amaranth leaves. Unlike paddy rice, amaranth leaves have no hulls or shells that form a resistance to moisture diffusion and thus reduce the drying rate [27]. By visual inspection, it was observed that the leaves were not dried uniformly. On the last day, leaves in the corners of the trays were not completely dried. This might have been caused by shading of the leaves by the black bottom layer, which was raised at both sides when the tunnel was inflated. To improve the design, the width of the black bottom layer in the ISD should be reduced. Another aspect that lowered the drying performance of amaranth leaves was the presence of stalks. It was observed that the proportion of stalks in blanched leaves was around 41%. At the end of drying, the stalks were less dry than the leaves, as shown in Figure 8. The stalks have an outer layer, which prevents water evaporation and therefore prolongs the overall drying time. In common practice, solar drying of leafy vegetables without the stalks takes only about 12 h [28]. The optimum time and method to dry the stalks should be further studied. The stalks are also a valuable commodity for amaranth leaf processors. In order to reduce the load in the dryer and accelerate the drying process, it is recommended to first unload the leaves soon after they are completely dried and then proceed with the stalks.