Advanced cascade control strategy applied to an indirect hybrid solar-gas dryer: Numerical and experimental investigations

Abstract

In this paper, a cascade control strategy is applied to control the temperature inside a hybrid solar-gas dryer. In order to use the dryer even at night and in unfavorable weather conditions, the drying chamber is heated by two systems. The first one is the solar air heater and the second one is the gas heater. The solar collector heater is relatively slow compared to the gas heating system. Static and dynamic studies were established and have revealed the nonlinear behavior of each dryer subsystem. The cascade control approach was developed to control the drying temperature at different setpoints in a temperature range of [50 °C, 90 °C] by managing the gas flow delivered to burners according to received solar energy. Simulation tests followed by experimental investigations have revealed that the proposed control strategy has been able to fulfill the desired dryer performances at different temperatures level, with a propitious steady-state error, reduced overshoot, and a fast transient settling time. Hence, for controlled temperatures at 60 °C, 70 °C, and 80 °C temperatures, the following performances were respectively obtained: a steady-state error of 0.5%, 0.28%, and 1.28%, an overshoot of 2.6%, 4.1%, and 2.5%, and a settling time of 20 min, 18 min, and 30 min. The developed control approach has shown also an effective perturbance rejection during the dryer operation. The proposed control technique and the manufactured kit can be easily integrated into hybrid dryers and it’s an adequate and compatible solution with solar dryers for a drying operation of wide agro-food products. This solution also makes it possible to optimize the energy consumption of the secondary source.