Comparative Thermal Performance Study of Indirect and Mixed-Mode Solar Dryers

In the present study, simple performance evaluation method of solar dryers is proposed. The methodology based on steady state heat balance concept applied to the absorber surface of collector-dryer assembly is proposed for identification of dimensionless parameter called effectiveness of dryer, ε. No-load experiments on mixed-mode and indirect solar dryers have been performed for forced air circulation under indoor controlled conditions. Dryers are tested for a wide range of operating conditions of absorbed thermal energy (300-800W/m2) and air mass flow rate (0.009-0.026 kg/s). Based on comparison results of computed values of ε for both of these dryer, it is found that there is significant performance improvement of 48% in case of mixed-mode in contrast to indirect dryer. The study further reveals that for both dryers there is sharp rise in ε values when air mass flow rate is increased from 0.009 to 0.017 kg/s and thereafter a marginal increase can be noticed when air mass flow rate is enhanced to 0.026 kg/s. Another important finding of present study relates to invariable ε values obtained for a wide range of absorbed energy and ambient air temperature data irrespective of air circulation mode adopted for a dryer.