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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 W/m 2 , 1100 W/m 2 ), 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.Energies 2019, 12, 2841 2 of 18 the most common are dried fruit and juice, usually in organic quality, which are exported primarily to European countries. The processing itself takes place in various decentralized small-and medium-sized enterprises. As described in [3], a typical such enterprise processes 1-2 t/d of fresh fruit, while the resulting pineapple waste (peels, cores, stems, and crowns) is about 40% of this amount. Other sources give even higher figures up to 75% [4]. The waste, together with discarded fruits with high moisture content and various other farm production wastes (cow dung, etc.) can be efficiently utilized for energy recovery via biogas production, and the resulting sludge can be used as fertilizer. While sophisticated biogas production technologies are employed in the developed countries, the technologies in developing countries must be tailored to the local conditions. Both the adaptation of European biogas production technologies to the specific requirements of pineapple waste processing in Togo and the corresponding experimental results were discussed by the authors of the present paper in [3]. Key Role of the Drying ProcessDrying-that is, a continuous or intermittent process associated with heat and mass transfer-significantly influences the shelf life, appearance, composition, taste, shape, structure, and other characteristics of the product. With respect to the fact that this process is probably the most energy-intensive one in the food pr...

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Plastic immersed heat exchangers are used in various applications where chemically neutral and noncorrosive equipment is required. Their potential competitors, namely, polymeric hollow-fiber bundles, were investigated. Three different fiber bundles were tested as immersed coolers of a hot reservoir. Two types of polypropylene hollow fibers with various outside diameters were employed. Fibers were twisted with different curvature to achieve better distribution and improve natural convection on their outer surface. Calculation by experimental results was compared with the equation for natural convection across the horizontal cylinder, which overestimates heat transfer coefficients and can be applied for only rough estimation. Experimentally achieved pressure drops agreed well with theoretical prediction for laminar flow.