DEVELOPMENT OF EFFICIENT SOLAR ASSISTED DRYER FOR TROPICAL FRUITS

Abstract

The post-harvest losses of agricultural products can be reduced drastically by using proper drying techniques. As fruits are grown in hot sunny areas and are dried during the period when solar radiation in abundant, the introduction of solar dryers equipped with fan ventilation and supplementary heat source represents an important technical solution and, thus, has a great potential for food preservation in most developing countries. The required amount of thermal energy to dry a particular product depends on many factors, such as, initial moisture content, desired final moisture content, temperature and relative humidity of drying air, and air flow rate. In the present work, an attempt has been made to develop and analyse an efficient tunnel type solar dryer. Theoretical models for the different components of the system have been developed and used with a simulation programme to describe the performance of the system. Since air collector is one of the most important components in a solar food drying system, improvement of design of collector would lead to a better performance of the system. In the present work, a simulation study of three types of solar air collectors, namely, flat plate, finned and v-corrugated, has been performed towards achieving an efficient design of air collector suitable for solar dryers. A series of experiments were conducted according to the ASHRAE standard to investigate the design improvements needed to achieve high performance under wide range of operating conditions. Collectors are also studied in double pass mode to investigate the efficiency improvement without increasing the collector size or cost. Experimental and simulation results indicate a better performance of v-corrugated collector. Double pass operation of the collectors leads to further improvement of the efficiency. A mathematical model for drying of food products undergoing shrinkage has been developed. The model, which includes the influence of both material and equipment, is capable of predicting dynamic behaviour of the dryer. The material model describing the mass and heat transfer during drying is capable of predicting the instantaneous temperature and moisture distribution inside the material. The equipment model, on the other hand, describes the transfer process in the tunnel dryer and predicts the instantaneous temperature and humidity ratio of air at any location of the tunnel. The resulting simulation model becomes an engineering tool for designing tunnel type solar dryer that enables estimating drying characteristics and drying time under various drying conditions. The present drying model has been successfully used to simulate the drying process under different drying conditions. The prediction of the average moisture content and material temperature compared favourably with the experimental results. Predicted air temperature and humidity ratio of a batch type tunnel dryer also agree reasonably with experimental results.