Stack humidification

During start up, when the stack temperature and then performance (voltage) are low the passive BOP devices devoted to stream and stack humidification (enthalpy wheels membrane humidifiers) do not work as they are essentially based on the difference of temperature and then are practically excluded by the FCS management on the other hand, at low temperature the requirements for stack humidification are strongly reduced and membrane could self-hydrate if a sufficient water production (proportional to power requirement) occurs. 

The tests are run at the temperature of 346 K and reactant pressures of 250 kPa. The air flow rate is set for each load value in order to assure the correct stack operation and minimize air compressor power consumption and water flow rate necessary for the humidification. This regulation of oxidant flow rate determines a value of R ranging from 6 at low load to 2 at full load. An excess of air with respect to the stoichiometric requirement is always necessary due to the mass transport limitations on the cathode side (see Sect. 4.3). The stack humidification is performed by saturating the inlet air stream at the temperature value approximately equal to the measured one at the cathode outlet. 

For the following tests, the fuel utilization coefficient is set at the constant value of 0.90, the stoichiometric ratio ranges between 6 and 1.8, for a stack power from 700 W to 12 kW, With regard to the stack humidification, the inlet air is saturated at room temperature (290-300 K) to minimize the amount of energy required for the best membrane hydration and reduce the electric consumption of the humidifier and the purge frequency [4]. As a consequence, the stack temperature is controlled at about 315 K, to avoid any dehydration of the membranes. 

In this section, the experimental results regarding the analysis of the FCS during the warm-up phase are presented. The tests are carried out starting from 18°C, for two different acceleration values (150 and 1,500 W/s). Regarding the stack humidification, the inlet air is saturated at the same temperature adopted for the stack at the beginning of the tests, to avoid membrane de-hydration. 

After humidification, the products are trimmed to size and stacked. The stacks are then moved to the next processing step and many of the secondary treatments of hardboard will take place at the panel production site. These latter may include the following ... 

A fuel cell system also needs ancillaries to support the stack, just as an IC engine has many of the same type of ancillary subsystems. Major subsystems are needed for providing adequate humidification and cooling, and for supplying fuel and oxidant (air) with the correct purity and appropriate c uantity. 

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. 

The primary focus of ongoing research is to improve the performance of the cell and lower its cost. The principal areas of development are improving cell membranes, handling the CO in the fuel stream, and refining electrode design. There has been an effort to incorporate system requirements into the fuel cell stack in order to simplify the overall system. This work has included a move toward operation with zero humidification at ambient pressure and direct fuel use. 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

Operation with low relative humidity of the gases at the stack inlet is preferred because it simplifies the system (humidification of reactant gases and water recovery). PEM fuel cells are operational even at room temperature, but the typical operating temperature is between 60°C and 80°C. In order to reduce the size of the heat rejection equipment there is a lot of reserch and development (R D) on high temperature membranes that would allow operation at 130-140°C. 

Water management can be most simply achieved by providing the gas feed humidification level required to maintain the conductivity of the fuel-cell membrane and of the ionomers in the catalyst layers. Gas feed humidification has been achieved by a variety of methods including, for example, enthalpy exchangers [6] and porous bipolar plates [62]. The two latter approaches rely on utilization of stack-produced water, thereby eliminating the need of frequent water refill . The system in Fig. 29a, can use a condenser to... 

Fig. 29 Schemes of power systems based on a neat hydrogen/air, PEFC stack, for the case (a) where pre-humidification of the incoming fuel feed stream is required and (b) where satisfactory stack hydration is achievable w/o pre-humidification of the incoming fuel feed stream [61]. 

The operative parameters which can be regulated to optimize the stack performance are MEA humidification, reactant pressure, stack temperature, and stoichiometric ratio. While the role of membrane humidification, already partially discussed in Sect. 3.2, is closely considered in Sect. 4.5 and in case studies (Chaps. 6 and 7), the influence of the other parameters is examined here with reference to the stack of Fig. 3.5. These effects have already been described from a thermodynamic point of view (see Sect. 3.1), while kinetic implications are considered in this section for their importance in determining the stack efficiency. 

Thermal management system is necessary because the reaction heat gradually elevates the MEA temperatures. This aspect, related also to humidification issues, requires the development of a sub-system able to control both stack temperature and heat flux inside the overall system. The related discussion is presented in Sect. 4.4. 

The different humidification approaches could be classified as internal or external methods. Internal humidification means that humidification procedure concerns exclusively the inner spaces of fuel cell stack, while external humidification involves modifications in feeding stream humidity ratio outside of the stack [1. 29]. 

A possibility is to saturate at different temperatures the reactants before they enter into the stack [33]. This approach can be accomplished by several procedures based on external dewpoint, external evaporation, steam injection with downstream condensers, or flash evaporation. High temperature values allow to absorb significant water amount in gas streams and then transport it inside the stack compensating the water losses due to internal fast evaporation. However, the main problem with external humidification is that the gas cools after the humidifier device, the excess of water could condense and enter the fuel cell in droplet form, which floods the electrodes near the inlet, thereby preventing the flow of reactants. On the other hand, internal liquid injection method appears preferable for example with respect to the steam injection approach because of the need of large energy requirement to generate the steam. 

The water balance inside FCS requires at least a water condenser located at the outlet of the stack. This device has the dual role to condense a significant part of the water content of the warm and wet cathodic outlet stream (a mixuire of nitrogen, oxygen, and water vapor with the probable presence of small water liquid droplets) for re-cycling in humidification devices, and to transfer/recover a part of significant thermal energy content of the cathode stream. The condenser could also collect, but in minor quantities, the excess water content of the anode compartment. The condenser is then aimed at maintaining the vehicle endurance and to cooperate to efficiency maximization. 

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. 

The control strategies are programmed in Matlab-Simulink, compiled in C-H-and then downloaded into the DSP processor of the d-Space board. The controlled management of the fuel cell system during the dynamic tests operates on hydrogen purge, air flow rate regulation (stoichiometric ratio), external humidification, and stack temperature. 

Other dynamic tests are affected varying the stack power and evaluating the stack response to hydrogen purge, external humidification, and air management... 

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