*Article* **Indirect Convective Solar Drying Process of Pineapples as Part of Circular Economy Strategy**

**Yaovi Ouézou Azouma 1, Lynn Drigalski 2, Zden ˇek Jegla 3,\*, Marcus Reppich 2, Vojt ˇech Turek 3 and Maximilian Weiß 2**


Received: 28 June 2019; Accepted: 20 July 2019; Published: 24 July 2019

**Abstract:** This study investigates the industrial-scale application of a simple convective solar drying process of pineapples as part of a circular economy strategy for developing countries. A renewable energy concept is presented, which follows the circular economy aims by effectively employing a simple system for biogas production and a two-stage drying system. Both these systems meet the requirements for implementation in the specific conditions of developing countries, of which Togo, where pineapple is a major crop, is taken as an example. With respect to earlier findings available in the literature, the paper focuses on the solar drying process, which is critical to the proposed strategy. A portable solar dryer working in indirect heating mode was built and later also modified to enhance its performance. Three main factors influencing the convective drying process, namely, drying time (270 min, 480 min), solar radiation intensity (650 <sup>W</sup>/m2, 1100 <sup>W</sup>/m2), and slice thickness (6–8 mm, 12–14 mm), were considered. The statistical Design of Experiments (DOE) method was applied to reduce the number and scope of experiments. In the best case, the moisture content was reduced from 87.3 wt % in fresh samples to 29.4 wt % in dried samples, which did not meet the quality requirements for dried fruit. An additional conventional post-solar drying procedure would, therefore, still be necessary. Nonetheless, the results show that in the case of pineapple drying the consumption of fossil fuels can be decreased significantly if convective solar pre-drying is employed.

**Keywords:** drying; solar energy; sustainable processing; energy efficiency
