PRINCIPIOS Y DIRECTRICES PARA LA APLICACIÓN DE LA EVALUACIÓN DE RIESGOS MICROBIOLÓGICOS
4. DIRECTRICES PARA LA APLICACIÓN
In this simulation the transient response to a step demand of 10% of rated current to 23A.
250 280 310 340 370 400 0 46 92 138 184 230
85kW Hydrogen PEMFC IV Characteristic
Fuel Cell V
oltage (V)
Figure 5.13 - Fuel Cell System Output Current Step Response
5.6.4 System Losses & Efficiency
Figure 5.14 shows how the efficiency of the fuel cell system varies across the rated system output power range.
The dominant source of loss at the low end of the power range is the auxiliary load of the fuel cell system that maintain the operation of the fuel cell once it is started. The auxiliary load comprises a) turbo compressor, b) reactant humidification system, c) hydrogen flow control valve and d) fuel cell heating/cooling system. Together with the fuel cell stack and hydrogen tank these components make up the complete fuel cell system, sometimes referred to in the press and literature as the fuel cell engine and shown earlier in Figure 3.4.
Current limitations in materials technology limit the lifespan of the fuel cell stack. One key limitation is the finite number of times the fuel cell stack can be started up and shut down. For current fuel cells this is in the order of 4000 start-up and shutdown cycles and so once the system has been started on a journey it must remain on. The stack cannot be shut down during the journey like an ICE can be turned on and off in a stop-start mild hybrid. The fuel cell stack also takes considerably longer to start than an ICE, typically around a minute though the exact time varies with ambient temperature e.g. in cold conditions the stack must be warmed before the fuel cell can be started to prevent damage to the membrane from frozen water vapour that was not completely purged from the stack at the end of the last operating period.
The auxiliary load is therefore an additional parasitic load on the fuel cell system and present for the entire duration of operation and is modelled as such. The main source of loss within this auxiliary load is the air compressor [91, 193]. To provide the volume of air sufficient to sustain the reaction rate required ambient air must be pressurised and fed into the stack. This can be done with a blower or turbo compressor however as previously discussed using a turbo compressor results in a significantly higher power density in the cell. Most automotive fuel cell systems require the maximum possible power density for a given system volume and all employ turbo compressors to pressurise the air supply.
Humidification is one of the key control processes in the fuel cell system. If the membrane is allowed to become too dry the reaction rate decreases and the cell internal resistance increases, decrease the efficiency of the reaction. Hot spots can also occur on the membrane that can ultimately cause it to break down and degenerate. If there is too much water in the cell it can block the gas channels and slow down the reaction, again decreasing the efficiency of the stack [212, 213]. The reactant humidification system therefore carries out three processes key to ensure the efficient and safe operation of the stack a) Humidification of the reactants to prevent the membranes drying out b) Heating of the
reactants to ensure the stack operates at an optimum temperature c) Removal of waste heat and water vapour from exhaust gases.
The integration of the turbo compressor and humidifier into the reactant delivery system is shown below in Figure 5.15. The hydrogen control valve is presumed to be ideal and can deliver the exact flow rate of gas required in an ideal laminar flow upon demand.
Figure 5.15 - Fuel Cell Reactant Delivery System
Some studies model the turbo compressor loss as a fixed value, Ogburn et al, for instance measured a power loss to the system of 3.65kW at full load and to reduce the simulation complexity used this value for the entire range of stack loads[193]. The turbo compressor however requires careful consideration. The stoichiometric ratio of air to hydrogen is not constant throughout the full range of operation and therefore the amount of air the compressor supplies in the real system changes. Since the turbo compressor is the dominant source of loss in the fuel cell system auxiliary load ensuring that the losses associated with it are represented accurately is vital. Otherwise if a fixed value of full compressor load power is used the losses may be far higher or lower ay any given point than they are in reality, devaluing the fuel cell model and compromising the whole analysis.
The power consumed by the turbo compressor can be related to the gas flow rate of the system, which in turn can be related to the system current demand. The power map of the auxiliary load related to system demand current was obtained from the manufacturer and the electrical power consumed by the auxiliary systems relative to the fuel cell output power is shown in Figure 5.16. The power at low loads is dominated by the very low efficiency of the turbo compressor at low mass flow rates [214].
H2
Humidification System H2 Flow Control Valve
Turbo Compressor
WASTE DRY AIR & HEAT WASTE WATER PEMFC Stack H2 AIR EXHAUST
Figure 5.16 - Fuel Cell System Output Power vs. Auxillary Load Power Loss
The final characteristic determined by the fuel cell model is the mass flow rate of oxygen and hydrogen in the cell. The rate of oxygen then determines the mass flow rate of air that the turbo compressor must supply to the stack. The manufacturer measured the mass flow rates across the full range of system demand currents (as shown in Figure 5.17) and a linear constant derived for each to relate the flow of gases to the system current:
mHydrogen =KHydrogenIFC
mAir=