**4. Conclusions**

This study investigated the feasibility of an industrial-scale application of indirect solar thermal drying of pineapples as part of a Togo-specific pineapple processing circular economy strategy, and the main factors affecting the respective drying process. Drying time and pineapple slice thickness proved to be significant, whereas the solar radiation intensity was found to affect the drying process to a lesser extent. As part of the experiments, drying kinetics were also determined for both slice thicknesses of 6–8 mm and 12–14 mm. The corresponding drying curves were characterized by two parts with different drying rates, which were delimited by the critical times of 350 min and 100 min, respectively.

Considering the best obtained residual moisture content in the dried pineapple fruit of 29.4 wt % and that of the reference product (13.7 wt %), the performance of the solar dryer alone was deemed insufficient. In order to meet the required product quality, either the solar thermal drying process had to be extended for a further 10–12 h period (which may not be feasible with respect to the local climatic conditions) or another post-solar drying procedure had to be added. Nevertheless, solar pre-drying of the pineapple fruit followed by a conventional drying process resulted in significant fossil fuel savings of around 66%. This lays the groundwork for the successful implementation of the proposed circular economy strategy.

A modification of the solar thermal collector to increase its efficiency was also briefly discussed. The improved design of the absorber, changes to the thermal insulation, and the addition of a fan control system resulted in a considerable increase in the drying air temperature at the collector outlet (83.4 ◦C instead of the original 56.8 ◦C).

Ultimately, the combination of the solar thermal drying process with post-solar drying based on the combustion of biogas produced from agricultural wastes leads to improved environmental sustainability and also supports the circular economy in the agricultural sector. The proposed strategy, therefore, could help farmers in developing countries to meet the growing demand for the sustainable processing of tropical fruit or other agricultural products.

**Author Contributions:** Conceptualization, M.R., Y.O.A., and Z.J.; methodology, M.R.; investigation, L.D. and M.W.; resources, M.R.; writing—original draft preparation, L.D., M.R., and M.W.; writing—review and editing, V.T. and Z.J.; visualization, V.T.

**Funding:** This research was funded by the Czech Ministry of Education, Youth and Sports/EU Operational Programme Research, Development and Education, Grant No. CZ.02.1.01/0.0/0.0/16\_026/0008413, "Strategic partnership for environmental technologies and energy production", and by the Augsburg University of Applied Sciences.

**Conflicts of Interest:** The authors declare no conflict of interest.
