Second law analysis of flat membrane humidifiers for PEM fuel cell applications

The efficient operation of PEM fuel cells requires humidifying the supply air. This can be achieved by integrating a membrane humidifier into the cathode subsystem. In this paper, a second law analysis of a flat membrane humidifier is carried out using a detailed dynamic simulation model that is discretized along the channel. The newly developed modeling and analysis approach enables a detailed calculation of entropy production in membrane humidifiers under steady state and transient operating conditions. Suitable equations are introduced and implemented to model the entropy production rates inside the humidifier due to mass and heat transfer as well as friction. With the model, inevitable entropy production due to water transfer can be distinguished from entropy production by heat and momentum transfer. The model is verified and validated using literature data. In addition, the effect of discretization on accuracy and calculation speed of our model are discussed. It is found that the model is accurate and offers fast calculation times. With the model, the effects of geometry and operating conditions are analyzed by simulation. It is found that water transfers in the membrane and the dry side channels are the dominant entropy production mechanisms in most conditions and geometries investigated. In an analyzed start-up procedure starting from ambient temperature it is found that entropy production caused by each mechanism dominates at some point in time. Finally, a new analytic approach based on the mass transfer efficiency is proposed that allows to calculate the entropy production caused by water transfer in a balanced counterflow humidifier using inlet quantities and the mass transfer efficiency. • A new model is proposed to calculate entropy production in membrane humidifiers. • Entropy production by heat, mass and momentum transfer are considered in the model. • For water transfer, a new relation of efficiency and entropy production is found. • Water transfer is found to be the dominant source of entropy production. • The effect of geometry on individual entropy production mechanism is investigated.