The coefficient of determination (R2), and root-mean-square error (RMSE) were applied to determine the best mathematical drying model for rubber sheet drying under mixed mode and indirect solar drying as:
where Mex,i is the experimental value, Mp,i is the predicted value, and N is the number of observations.
Performance indicator
The most common indicator for dryer performance is thermal efficiency. It is defined as:
where, ηsolar dryer is the thermal efficiency of the solar rubber sheet dryer, mL is the mass of removed water (kg), L is the latent heat of vaporization (kJ/kg), I is the total solar insolation (kJ/m2), and At is the total area exposed to solar radiation (m2).
In the present study, both dryers used solar collectors as a source of indirect heat. Therefore, it is important to know the efficiency of the collectors under operating conditions. The solar collector efficiency (ηsolar collector) can be calculated as: