Humidification active

Humidification is accomplished by two main approaches, passive and direct humidification. In passive humidification, the water generated by reaction is used to maintain a proper moisture balance and humidify the incoming flow without external power. In active humidification, a separate humidifier is used to directly provide the humidification of the incoming flow with stored or recycled water. 

Methods of Active Humidification Active humidification requires a discrete, external humidification system. In a laboratory environment, a sparge-type humidifier, as illustrated in Figure 6.9, is often used. In this system, gas is sparged through a porous rock and into heated water to absorb moisture before entering the fuel cell. This system is not useful outside the laboratory because it is dependent on orientation and almost never 100% efficient. Care must be used to ensure proper humidification is achieved and careful calibration is neccessary. 

If this simple humidification scheme is not sufficient, active humidification of oxygen must be applied, using the liquid water collected from the stack exhaust and the heat from the stack (Figure 9-3). This way it is possible to reach saturation at a temperature closer to the stack operating temperature. Sometimes, humidification is accomplished in a separate section of the stack, also using the stack heat, as shown in Chapter 6. 

There has been an accelerated interest in polymer electrolyte fuel cells within the last few years, which has led to improvements in both cost and performance. Development has reached the point where motive power applications appear achievable at an acceptable cost for commercial markets. Noticeable accomplishments in the technology, which have been published, have been made at Ballard Power Systems. PEFC operation at ambient pressure has been validated for over 25,000 hours with a six-cell stack without forced air flow, humidification, or active cooling (17). Complete fuel cell systems have been demonstrated for a number of transportation applications including public transit buses and passenger automobiles. Recent development has focused on cost reduction and high volume manufacture for the catalyst, membranes, and bipolar plates. 

A typical biofilter setup consists of a blower, humidification chamber and a biofilter unit, and an additional pohshing unit (optional) such as granular activated carbon backup. The biofilter is composed of microbial communities supported on a packing surface material such as wet peat. 

Song et al. [5] explained that for the electrode with 40 wt% PTFE content in the gas diffusion layer, the increase in the size of the kinetic arc was attributable to the substantial decrease in the active Pt area caused by low water content at the interface of the catalyst layer and the gas diffusion layer. This explanation has been verified by cyclic voltammetric results. A possible solution to improve the performance of this particular electrode is simply to raise the humidification temperature in order to increase the water content at the interface. The results at higher humidification temperatures are shown in Figures 6.6 and 6.7. 

Guo et al. [7], as shown by the Nyquist plots in Figure 6.10. In their impedance measurements, different amounts of Nafion ionomer in the catalyst layer, ranging from 0.33 to 1.13 mg/cm2 (dry weight) were examined. The active area of their fuel cells was 1.0 cm2. The fuel cells were operated in H2/air gas feeding mode with a flow rate of 220 cm3/min (at standard temperature and pressure) for both sides. The cell temperature as well as the humidification temperature for both electrodes were controlled at 70°C. The cell s AC impedance was measured using a Gamry PC4/750-DHC2 potentiostat. The perturbation amplitude was set at 5 mV in potentiostatic mode, and the frequency was scanned from 0.01 Hz to 100 kHz with 10 points per decade. 

The shape of particles is normally that of more or less regular spheres, dense or hollow, with smooth surfaces and sometimes cracks. This is related to the composition and the rate of solvent evaporation, with possible existence of internal pressure inside the drops when a rigid surface layer is being formed (Walton and Mumford 1999). All these characteristics will have some effect on handling properties of powders such as bulk and tapped densities, particle density, (mixing with other powders, storage) wettability and solubility, porosity, specific area (rehydration, instantisation) flowability (size, surface asperities), friability and creation/existence of dust, stability in specific atmosphere and medium (oxidation, humidification, active component release) (Huntington 2004). 

Humidification of the gas stream is the preferred method of keeping the filter bed moist. Gas moisture is usually added to the incoming gas stream downstream of the particulate removal APC equipment by either water sprays or steam. Adding moisture directly to the top of the bed in order to maintain filter media moisture is not recommended since this can result in (1) localized drying of the substrate, and (2) cold water addition will reduce the activity of the microorganisms until the water becomes warmed to the steady-state filter-bed temperature. 

Whether humidification of PBI in fuel cells is necessary remains an open question. Short-term fuel cell tests yielded no or small performance losses when dry fuels were used or humidification was reduced [167,168]. However, membrane conductivity does depend on the water activity as can be seen in Figure 27.71 taken from a presentation by Savinell, the conductivity of PBI (3 H3PO4/PBI repeat unit and 6.3 H3PO4/PBI repeat unit) depends considerably on relative humidity [169]. However, Celanese (PEMEAS) in their pubhcations state that no humidification is required for their PBI systems. 

ORR activity as the recast ionomer film gradually dried again while in contact with saturated water vapor at 80 °C. These observations are important in the context of ORR catalytic activity in PEFC cathodes. The sensitivity of the rate of ORR to interfacial water content (Fig. 9) and the tendency of the ionomer to dry up at elevated temperatures when in contact with saturated water vapor, clearly suggest that the level of humidification at the PEFC cathode catalyst should be kept high by maintaining some water in liquid state in contact with the cathode catalyst layer. 

Piyarom S, Yonemochi E, Oguchi T, Yamamoto K. Effects of grinding and humidification on the transformation of conglomerate to racemic compound in optically active drugs. / Pharm Pharmacol 1997 49 384-389. 

Since PEM, like living organisms, need water to function and the amount and state of water are critical for an efficient operation, secondary requirements on this type of fuel cell membranes emerge. These include the necessity of sufficient humidification and the ability to retain water under operation conditions. Associated problems under fuel cell operation include the electroosmotic transport of water to the cathode side accompanied by dehydration at the anode side [45]. In the cathode the accumulation of water at high current densities, typically greater than 1 A cm-2, causes performances losses due to blocking of catalytically active sites and restriction of oxygen transport. 

Studies conducted over the last few decades have shown that active humidity control is required in modern, sealed buildings. These studies investigated humidity control using cooling coils [direct expansion (DX) or chilled water], desiccant dehumidification units, mechanical ventilation devices, and humidification units. Additionally, the use of hygroscopically active materials (such as plaster, brick, and ceramic) to control the relative humidity of the surfaces has been considered. This topic is divided into three sections— cooling coil units, desiccant dehumidification devices, and hygroscopically active materials. 

In designing a microscale fuel cell, there are several considerations that need to be accounted for. These include the following is the fuel cell to be completely active or passive will it operate at room temperature or elevated temperatures will the fuel be at atmospheric pressure or elevated pressure will external humidification be required and finally, will the fabrication techniques... 

Two pressure transducers are located upstream of the stack to monitor anode and cathode pressure during the experimental runs. A spiral heat exchanger, using external water at room temperature as second fluid, is used to control the temperature of the cooling water. The FCS humidification strategy is based on the deionized water injection method (see Sect. 4.5), activating the injection when the outlet air temperature is higher than 60°C. 

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