of indirect and mixed mode solar dryers with olive pomace in passive, active, and hybrid operation modes for different operating conditions namely temperature, relative humid- ity, air mass flow and confirmed better performance of mixed type dryer with hybrid operation in terms of effi- ciency. In another comparative performance study, dou- ble-pass solar drier (DPSD) exhibits higher efficiency to an extent of 109% with respect to cabinet drier (CD) for red chilli (Banout et al. 2011). Since operating conditions and food characteristics have significant influence on dry- ing efficiency, even a given dryer tested with same food product for different operating conditions or different food products with same operating condition may not give con- sistent efficiency results.
It can be inferred from foregoing literature review that despite several experimental studies reported on perfor- mance evaluation of different types of solar dryer; not much attention has been given for development of stan- dard test method that can be utilized for performance comparison on equitable basis. The main concern that needs to be addressed is how to obtain test results indepen- dent of process variables influencing performance of dryer. Thus, there is an immediate need to carry out detailed experimental investigations on the most common solar dryer design with load for different drying process vari- ables to evolve such test method that can provide perfor- mance test results of solar dryer in more rational and meaningful manner. Therefore, laboratory model of mixed-mode solar dryer is specially designed and con- structed to perform indoor drying experiments with cylin- drical potato samples as test food material for 16 different test conditions. The major objectives of present study are (i) to develop standard test method and identify dimen- sionless parameter that can characterize the effectiveness of a dryer system through a single generalized drying char- acteristic curve representing 16 different test conditions and (ii) to validate the indoor results with those obtained for real climatic conditions for different food products namely potato (vegetable), banana (fruit) and wheat (grain).
2. Materials and methods
2.1. Experimental arrangement
In present study, the mixed-mode dryer comprising of air-heater connected in series with drying chamber was chosen for experiments due to its superior performance in terms of faster food drying. The dryer system has an arrangement of pre-heating the air through solar flat plate air heater of size 0.92