Model identification and fuzzy control of the temperature inside an active hybrid solar indirect dryer

Abstract

In this paper, an indirect hybrid solar-electric dryer was used to be able to dry a wide range of agriproducts and to ensure a continuous drying operation. The hybrid dryer is a multivariable system made up of a solar collector, a drying chamber, a fan in the outlet duct, and electrical resistance heaters. To preserve the product quality, the temperature should be kept within a restricted temperature range during the drying process. Temperature control is also required to optimize the conventional energy demand and to achieve better use of the available solar energy. This paper presents the thermal behavior study and the control of the temperature inside a hybrid solar dryer. To predict the temperature evolution in a fairly short time, a state-space model based on the nonlinear least square method was developed. Based on the temperature prediction model, Fuzzy Logic Controller (FLC) was developed and its performances were compared with a Proportional Integral (PI) temperature controller. The experimental tests were carried out under an air mass flow rate of 0.025 kg/s and the temperature was controlled at 60 °C and 80 °C. By using this combination, the static temperature inside the drying chamber remains under 0.8 °C and the relative error remains under 1.04%. The developed fuzzy controller could almost recover the dryer performances in less than 8 min after temperature disturbance. Moreover, for continuous use covering day and night, the temperature controller allows reducing the conventional energy consumption by 11.3%.