Energy saving in a convective dryer by using novel real-time exergy-based control schemes adjusting exhaust air recirculation

This study was aimed at developing and comparing three different novel real-time exergy-based control schemes for adjusting exhaust air recirculation in a convective dryer for the first time. These control schemes were based on output–input warm exergy ratio of drying cabinet (Scheme I), output–input warm exergy ratio of drying cabinet considering a fixed upper limit for the output wet exergy rate (Scheme II), and output–input warm exergy ratio of drying cabinet considering a varying upper limit for the output wet exergy rate (Scheme III). A continuous time-based control approach (Scheme IV) was also developed for comparison purposes. The exergetic effectiveness improvement and energy saving of the process achieved using the developed control schemes were assessed with respect to the base operation mode (without exhaust air recirculation) in order to make decisions on the most efficient approaches. As a case study, poplar wood chips were dried at two temperatures (55 and 70 °C) and two volumetric flow rates (360 and 450 m3/h) of drying air. The exergetic effectiveness of the drying process varied from 1.4–3.1%. The experimental results showed that the exergetic effectiveness of the drying process could be improved by employing all the developed control schemes. Among the developed control schemes, the control scheme III showed the highest exergetic effectiveness improvement in the range of 46.5–75.9%. This control scheme could also reduce the overall energy consumption of drying system in the range of 30.7–34.5% compared with the base operation mode. The selected control scheme could be implemented on various drying systems in order to improve their efficiency, productivity, and sustainability by recuperating the waste exergy from exhaust air.