Drying is simply the process of moisture removal from an agricultural product, and can be performed by different methods. The main purpose of this research work was to study the thermal performance of two different solar driers (active and passive dryers) based on the energy balance equations for the drying air and the drying product during dehydration process of seedless grapes in August 2009 under the climatic conditions of El-Mansoura city (31.05ºN and 31.37 ºE), Egypt. Three identical solar tunnel greenhouse driers (STGD) were functioned as an active solar drier, and an open sun drier (OSD) under direct solar radiation without cover (as a passive solar drier) to dry seedless grapes. The three active solar driers were operated under three different mass flow rates of 0.122, 0.183, and 0.259 kg/s. Three different shapes of grapes (clusters, parts, and grains) with total quantity of 33 kg, were spread on wire mesh tray with floor surface area of 2.0 m2. The results of greenhouse solar driers were compared individually with open sun drying. During the drying process, it was observed that, the hourly average air drying temperatures inside the three active solar dryers were increased to 49.9, 45.5, and 40.1ºC, respectively, whereas the hourly average ambient air temperature was 31.6ºC. The seedless grapes could be dried from initial moisture content of 79.1%wb to 17.1, 18.5, and 20.8%wb within drying time of 61, 61, and 71 hrs, using the three active solar dryers, respectively, while it was taken 96 hrs under open sun drying method to reach the final moisture content of 21.2%wb. The daily average thermal efficiencies for the four different solar drying modes, respectively, were 22.37%, 21.57%, 19.12%, 1and 10.81%. The quality of raisin (dried grapes) in terms of physicochemical parameters such as colour retention and water activity were evaluated.
The overall study concluded that good quality dehydrated raisin could be produced by using active solar dryers as compared with passive sun drying method. The produced raisin of grapes under different drying methods had various colours. They, respectively, were light brown and dark brown when the grapes dried under active solar dryers and passive sun drying method. The experimental work showed that, the solar tunnel greenhouse drying (active solar dryer) was satisfactory and competitive to a natural convection solar drying process (passive dryer).