depth and initial moisture content. The study of drying parameter profiles showed that a faster and more complete drying is observed using solar-natural air drying, which was also reflected in the width of drying front being greater when solar-natural drying than when using solar drying alone. The solar-natural air drying regime appeared to be superior to that of solar drying alone with respect to the required total drying time and in achieving complete drying within the safe storage time, with a saving of actual
drying hours. A solar heated dryer was developed by Muller et al., taking a plastic film greenhouse as superstructure to lower the initial costs, consisting of a black absorber tissue, which is placed between the transparent cover of the greenhouse and an insulating air-bubble foil. A prototype was tested in Yugoslavia, drying mint, sage and hops. Compared to conventionally dried crude drugs, the percentage of active ingredients was up to 40% higher and the low investment and operating cost and the high quality of the crude drugs permit the solar dryer to be used in agriculture. A bin-type solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed by Janjai and Tung. To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002-2003. The products being dried in the dryer were completely protected from rains and insects and the dried products were of high quality and the solar air heater was found to have an average daily efficiency of 35% and it performed well both as a solar collector and a roof of a farmhouse. Janjai et al. presented the experimental performance of solar drying of rosella flower and chilli using roof-integrated solar dryer and also the modeling of the roof-integrated solar dryer for drying of chili. Field-level tests for deep bed drying of rosella flower and chili demonstrated that drying in the roof-integrated solar dryer resulted in significant reduction in drying time compared to the traditional sun drying method and the dry product is a quality dry product compared to the quality products in the markets. Economical investigations revealed that the payback period of the roof-integrated solar dryer is about 5 years.