Study of the Drying Behavior and Performance Evaluation of Gas Fired Hybrid Solar Dryer
Authors: Anum Rabia , Ghafoor Abdul , Munir Anjum 
Published: February 2016

The conventional drying methods are energy intensive and are not cost competitive. The severe energy crises in Pakistan during the last decade has decreased the interest in drying of farm produce which is resulting huge postharvest losses due to product spoilage and nonavailability of storage facilities. This study has been carried out for the performance evaluation of newly developed hybrid solar dryer for the drying of onion, ginger and cabbage under local climatic conditions. The hybrid solar dryer has a total cross‐sectional area of 0.122 m2 out of which 34% is the collector while 64% is drying section. The dryer uses solar energy for drying during daytime while assisted with backup source during night for continuous product drying. The average initial moisture content of ginger, onion and cabbage batches was measured to be 88.45, 55.76 and 72.51%, respectively, while the final moisture content after drying was 10.27, 15.69 and 15.02%, respectively. The result also shows that the initial high moisture content of the products was removed in early hours of sunshine without any expense of primary energy cost. However, the remaining small amount of hygroscopic moisture was removed using backup energy source but with lower costs as compared with conventional drying. The final moisture content was achieved between 10 and 16 h for all batches of the onion, ginger and cabbage. On an average, greater than 70% moisture of the products was removed on the expense of solar energy. The overall average efficiency of the dryer was found to be 42.79%.

Practical Applications

The agro‐climatic conditions of Pakistan are ranging from tropical to temperate, thus, allowing to grow 40 different kind of vegetables and 21 type of fruits. Due to lack of processing facilities, the farmers have to sell their products at very low prices. Moreover, nearly 40–50% amount of perishable products are spoiled all over the world due to lack of farm gate processing facilities. In this research, the experiments have been conducted on onion, ginger and cabbage and successful results were obtained for all the products. In the conventional solar dryer, the major problem faced was to maintain the drying rate at uniform level but these types of dryer gave unsatisfactory results during the adverse weather conditions when the solar radiation are not in the limit to dry the products. Additionally, in winter, when the day time duration is comparatively shorter and solar dryer were not capable to dry the product in a single day. The objective of the hybrid solar dryer was to use the solar energy as a primary heat source during the favorable environmental conditions and the backup system is only taken into circuit when the solar system is not capable to maintain the desired drying rate. So, this type of dryer, not only maintain the drying rate but also vanishes the inversion rate of the perishables and hence ensures the quality of the dried products. So this hybrid feature built in the dryer is more attractive option for the farming community to dry their products without any harmful effects on the products. The hybrid solar dryer is demand base technology and will find a place in the manufacturing circle to transfer this technology to the end users, stakeholders and farming community for on‐farm processing of variety of perishable products for value addition. Furthermore, using hybrid solar dryer does not cause any kind of environmental pollution. On the contrary, solar dryers can help the rural and farming community to improve their living standard by earning profit through drying their on‐farm perishable and agro‐products while, at the same time, protecting their environment. At the same time the use of solar dryers on a large scale could be beneficial to reduce carbon emissions and the associated global warming issues.