Design and Analysis of Solar Dryer for Mid-Latitude Region

In this study, an indirect natural convection updraft solar dryer was designed and fabricated with objectives to; suit mid-latitude applications, determine its performance efficiency, analyze air properties by use of psychrometric chart and assess the quality of dried product. The dryer was fabricated using low cost and locally available materials. For mid-latitude regions, zenith angle is dependent on latitude, solar declination angle, time of the year and of the day. Based on these changing parameters, the solar dryer was designed to suit the conditions. The main parts of the dryer are; hinged flexible angled solar collector, drying chamber consisting of three drying trays, updraft chimney with metallic absorber plate. The experiments were carried out to dry sliced apples of 2.5 mm thickness spread over the drying trays. Temperature and relative humidity of airflow were taken at the collector inlet, drying chamber inlet and drying chamber outlet to chimney using data loggers. Airflow inlet to solar collector and from drying chamber was determined using hot wire anemometers, irradiation in W/m2 was measured by radiation meter. Drying characteristics such as the collected solar energy to evaporated moisture, average drying rate, moisture content, etc., were determined. Natural convectional flow was adopted with an advantage of utilizing updraft air chimney to induce thermo-siphoning effect hence improving efficiency. In our current study, fresh apples of 886.64 grams with moisture content of 86% were dried to moisture content of 8.12% (wet basis) within 9 hours 20 minutes at an average irradiance of 534.45W/m2. The overall dryer efficiency was estimated to be 17.89%. The possibility of formation of water droplets within the drying chamber, which is a challenge to most direct solar dryers was considered to be minimal. This was based on analysis of the dew-point of drying chamber inlet air in comparison to outlet air properties. This design is simple; it can easily be replicated elsewhere in the world, products can be dried in one day and has capability to dry products at commercial scale while retaining the color, flavor and nutrients.