Performance of forced convection evacuated tube solar grain dryer
Osodo, Booker Onyango
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Open sun drying leads to loss of grain as a result of birds and other pests that feed on it. The quality of the grain is also compromised due to contamination by dust, animal droppings and other contaminants as well as due to cracking and discoloration of the grain. Solar grain dryers may be used to address these problems. Direct solar dryers protect the grain from adverse weather, but have limitations, among them being cracking and discoloration of the grain. Natural convection solar dryers, though convenient at places with no mains electricity, perform poorly due to poor ventilation. Forced convection solar dryers, utilizing solar driven fans, may be used to improve ventilation, and to achieve an optimum drying air velocity. However, it is necessary to determine the optimum air velocity for a given dryer. The most common solar air heaters used in crop dryers are the flat plate collectors, whose thermal efficiency is limited due to convective heat loss from the absorber plate to the glass cover. This means that such dryers are only appropriate for small scale drying of maize and are inadequate for large scale farmers. This problem may be resolved by applying evacuated tube solar collectors, in which the only mechanism for heat loss is radiation, leading to greater thermal efficiency. However, few studies have been carried out to investigate the performance of such solar collectors when used in crop dryers. This study aims at simulating and optimizing the design of a forced convection evacuated tube solar grain dryer for large scale drying of maize. Thereafter a prototype of the dryer will be fabricated and tested, with a view to upscaling it for use in large scale drying of maize. The dryer, consisting of three main components: two variablespeed'blowers, an evacuated tube solar collector and drying chamber, shall utilise the solar air heater for pre-heating the air before it is forced into the drying chamber. The optimum air velocity required to maintain a drying air temperature below 60 degrees Celcius, essential for preventing detereoration of grain quality, will be determined. The effects of selected drying process parameters, namely drying air velocity and grain layer thickness and grain moisture content on dryer performance will be determined, both for the simulated solar dryer, and the prototype . The performance of the dryer will be evaluated on the basis of Energy Utilisation Ratio, drying rate and total drying time, and on whether the grains have been discolored and/or cracked after drying or not. In addition, the relationship between grain moisture content on one hand, and drying rate and time will investigated. The drying model that best describes the relationship between moisture content and drying time for the grain will be selected and the model used to predict drying time.