Relative humidity effects water balance

Figure 13. Effect of relative humidity on solvent balance upon evaporation of sec-butyl aIcoho1-2-butoxyethano1-water blend. Reproduced with permission from Ref. 62. Copyright 1978 Federation of Societies for Coatings Technology.

As briefly mentioned in Section 4.3.S.2, Atiyeh et al. [152] performed water balance measurements and calculations to determine the effect of using DLs with MPLs (on either or both cathode and anode sides). In their fuel cell test station, water collection systems were added in order to be able to collect and measure accurately the water leaving both anode and cathode sides of the fuel cell. Based on the operating conditions (e.g., pressures, temperatures, relative humidities, etc.) and the total amount of water accumulated at the outlets of the test station, water balance calculations were performed fo defermine the net water drag coefficient. Janssen and Overvelde [171] used this method to observe how different operating conditions and fuel cell maferials affected... 

The most obvious effect of changes in a solid API s water activity can be formation or loss of waters of hydration from the API crystal lattice. In some cases, loss of waters of hydration ean lead to the complete conversion of the API to an amorphous form, which in turn can lead to greater chemical instability. Eortunately, such effects are generally well-known with a given API based on moisture sorption isotherms (i.e., water uptake and loss measured using sensitive balances as a function of relative humidity). 

When used for automotive applications, fuel cells must respond to changes in the load. Changing the load alters the water production, changing the balance between water produced and water removed, resulting in a change of the membrane water content. The effect of the load resistance on the water activity can be seen in the polarization curves for the STR PEM fuel cell shown in Figure 3.9A [23]. The STR PEM fuel cell was operated in the autohumidification mode. The STR PEM fuel cell was equilibrated at 80C for 12 h with a fixed load resistance (either 0.2 or 20 fi). After equilibration, the polarization curve was obtained by sweeping the load resistance between 0.2 and 20 ft in 100 s. The relative humidity in the anode and cathode streams... 

This initial stage of droplet formation deserves a careful explanation. Over a flat, pure water surface at 100% relative humidity (saturation with respect to water), water vapor is in equilibrium, which means that the number of water molecules leaving the water surface is balanced by the number arriving at the surface. Molecules at water surfaces are subjected to intermolecular attractive forces exerted by the nearby molecules below. If the water surface area is increased by adding curvature, molecules must be moved from the interior to the surface layer, in which case energy is required to oppose the cohesive forces of the liquid. As a consequence, for a pure water droplet to be at equilibrium, the relative humidity has to exceed the relative humidity at equilibrium over a flat, pure water surface, or be supersaturated. The flux of molecules to and from a surface produces what is known as vapor pressure. The equilibrium vapor pressure is less over a salt solution than it is over pure water at the same temperature. This effect balances to some extent the increase in equilibrium vapor pressure caused by the surface curvature of small droplets. Droplets with high concentrations of solute can then be at equilibrium at subsaturation. 

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