Combining Solar Thermal and Membrane Technologies. Sustainable fruit preservation in developing countries – a case study in laboratory

Abstract: This work investigates how the heat produced by a solar dryer can improve the drying process of newly developed breathable membrane bags for drying and thus preserving fruits in developing countries and the increased efficiency of the process. For that purpose, a solar dryer was built and tested in an indoor solar simulator with similar irradiation as the sun. Due to a limited time span, only one dryer was built, that could be adapted to both direct and indirect drying, i.e. with and without direct solar radiation applied on the membrane bags. The solar irradiation is transformed into thermal energy inside the dryer, increasing the temperature and therefore lowering the relative humidity, contributing to a higher drying process. Also, direct radiation on the bag increases the drying rate. The drying rate is defined as the mass of water evaporated per area and time and is given in kg/m2/h. The results show that a significant improvement of the drying rate of about 56 % was measured with direct drying in the solar collector, compared to conventional open air sun drying at the same irradiation of 700 W/m2 over the bag. The mean radiation over the collector of 533 W/m2, however, was lower than that over the bag implying that the results were not perfectly comparable. The same accounts for the indirect drying, i.e. without direct exposure to sunlight, where the drying rate was 31 % lower than in the open air sun drying measurement. The mean radiation over the solar collector contributing to the indirect drying was only 533 W/m2, giving a drying rate possibly lower than if it had the same irradiation as that over the bag (700 W/m2). Even though the direct drying showed the highest drying rate, it is not obvious that it is the better choice for drying fruit. While drying, the surface temperatures stayed below critical levels, however within hours after drying out, they reached levels higher than the 70 °C recommended when drying fruit, possibly resulting in vitamin C degradation. Also, the radiation on the top of the membrane bag could make the drying rate uneven, as the upper layer of the bag dries first, inhibiting the evaporation from the inside of the bag. Due to large uncertainties in the measurement range of the wind anemometer, no conclusions could be drawn regarding effect on dry rate related to specific wind flows over the membrane bags. However, the measurements showed that, for the particular boundary conditions, the highest drying rates were achieved with the ventilation openings closed. This is likely due to that the air exchange resulted in lowered temperatures which affected the drying rates negatively. A solar dryer not only enhances the drying rate, it also protects the membrane bags from contamination such as rain, dust and bird litter. Strong winds that could inhibit the drying rate are also prevented. Most passive solar dryer designs are relatively easy to build, and consist of materials abundant in most countries. The design build in this project was relatively complex however did not need any previous experience in carpentry to construct.