Improved cascade control structure for water and thermal management in open-cathode direct methanol fuel cells

An undesirable consequence of using standard cascade control scheme for liquid-level management in direct-methanol fuel cells is that the operating temperature becomes variable, and often never reaches the preferred point of operation for optimal device performance. An improved cascade controller is proposed, featuring upper and lower thresholds, and a switching logic that sets the master controller on manual mode whenever possible, and returns it to automatic mode only when a threshold is violated. The approach seeks to ensure that the temperature of the fuel cell, managed by the slave controller, be kept constant at a desired set point during all manual-mode periods. Simulation studies under extreme conditions verify that the design operates as intended, generating electrical power and current that quickly reach constant steady states during manual-mode periods to meet the demands of external electrical loads. Flooding and dry-out states in the tank are avoided, and the scheme is shown to be immune to external perturbations and noisy measurements for long, though intermittent, periods of operations. Observed shortcomings due to the limiting conditions imposed are discussed, and future development needs are suggested.