Abstract future improvements in control strategies have been

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Last updated: September 10, 2019

AbstractIn recent years,fuel cell technology has found wide applications in automotive sector. Therehas been successful implementation and production of light duty fuel cellvehicles and lot of research is going on in using the technology for heavy-dutyvehicles such as army trucks and in large scale applications like railway. Fuelcell hybrid vehicles (FCHV) are considered to be the future of non-conventionalautomobiles which can possibly replace the conventional vehicles in the market.

With the hybridization of fuel cells with batteries, super-capacitors etc. thereis a paramount need of an efficient and economic control strategy that can cruciallymanage the power distribution and provide an optimum fuel economy for theautomobile. In this paper, Ihave qualitatively compared two control strategies used in an FCHV to controldifferent parameters and discussed their applicability in the automotivesector. To include a wide spectrum of driving styles and to get a better understandingof dynamics and controls of FCHV, I have reviewed researches conducted on threedifferent drive cycles. Dynamic behavior of the components of FCHV has beenstudied by splitting it into three major components and evaluating theirperformance individually. Model of each components have been presented andsimulation has been simulated for FTP-75 cycle to analyze their variation duringload changes.

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Finally, the conclusions and future improvements in controlstrategies have been expressed.IntroductionThe continuous increase of greenhouse gases around the globe has ledto detrimental effects like global warming, acid rain, air pollution etc.Moreover, the fast depleting reserves of fossil fuels and high oil prices hasbecome a major issue in the automobile sector.

To overcome these drawbacks ofIC engine vehicles, it is paramount to change the source of power of transportsector to an environment friendly alternative fuel. Many alternative fuel vehicles are consistently being developed in therecent years for replacing fossil fuel vehicles such as hybrid electricvehicles (HEV), battery electric vehicles (BEV), fuel cell vehicles (FCV),natural gas vehicles etc. The FCV are the most promising technology inautomobile applications owing to their fast startup even at low temperatures,solid state electrolyte, high power density and a good tank-to-wheel efficiencythan other fuel sources.

One of the limitations of FCV is that they’re unableto respond to the dynamic load demand. To overcome this issue, they’resupplemented with a battery (as it has a high-power rate) to provide a stableresponse throughout the drive cycle. With use of a battery, we’re also able tostore the regenerative energy procured during braking. This has led to thedevelopment of fuel cell hybrid vehicle (FCHV).A control strategy governs the power management of FCHV which iscrucial in determining the fuel economy.

In the recent years, several controlstrategies have been developed and proposed like optimal control theory,equivalent consumption minimization strategy, rule based algorithm, fuzzycontrol logic etc. power manage To compare the applicability, economicalityand performance two different control strategies have been considered: (1)optimal control based on minimization principle and (2) EMS. The optimum control strategy optimizes the power distribution betweenthe sources. It does this by introducing a costate parameter between fuelconsumption and battery energy consumption. The upper and lower limits of SOCare denoted by a penalty function. ems paper A performance variable (J) isintroduced which is a function of battery SOC and fuel usage which in case ofPEMFC is hydrogen gas.

Maximum fuel economy is obtained when the slope of J iszero i.e. ?J=0. Zhenget el. demonstrated the simulation results of this theory on threedifferent drive cycles viz.

NEDC 2000, FTP-75 and Japan 1015. They found theoptimum fuel consumption by considering both constant and variable costates. Asboth costates predicted the same result, they concluded that using a constantcostate greatly shortens the calculation duration and simplifies the controlstrategy.

Energy management system (EMS) is a control strategy that focuses onfinding the optimum balance between multiple power sources. EMS uses ruledbased feedback control algorithm to determine proportion in which power shouldbe divided from the sources. EMS Hannan et el. demonstrated theimplementation of EMS on light vehicle for the ECE-47 test cycle. Theyconsidered a three-source energy model having fuel cell (FC), battery andsuper-capacitor (SC) as energy sources. The controller determines which powersources are to be actuated depending upon the pedal acceleration. The FC helpsin regeneration of battery SOC as well as supports the acceleration. The SC isactivated when both the sources are not able to meet road load conditions.

Feedback control parameters used are: (1) acceleration pedal, (2) battery SOCand (3) vehicle speed. They also showed that vehicle tracked the ECE-47 cyclesatisfactorily, except for some inaccuracies caused due to PI controller. Theyconcluded that multi-source vehicles with EMS technology provide an optimaltrade-off between fuel consumption and pure electric driving cost.

This term paper focuses on the study of Kang et el. on dynamicsimulation of fuel cell vehicle (FCV) during FTP-75 driving cycle whichinvestigates the dynamic performance of crucial components in FCV system.Scrutinizing the dynamic behavior of various components, we can identify somecritical points which can improve the efficiency of fuel cell system. Aone-dimensional dynamic model of the PEMFC in Matlab-Simulink had been used forthe study.

The areas of primary interest are water management and thermalmanagement as they critically affect the performance of fuel cell during severeload changes. The model is divided into three major systems: (1) PEMFC stack,(2) air feeding system and (3) thermal management system (TMS). In the nextsection mathematical and theoretical models are developed for the system aboveto capture their transient response. Then the dynamic results obtained from thestudy are discussed. The principle findings of the dynamic behavior have beenenlisted in the conclusion.    Model descriptionFuel-cellhybrid vehicle A hybrid vehicle model consisting of a PEMFCstack and a supplemental battery source is developed on AMESim shown in fig().The specifications of the selected vehicle are given in table 1. The V-ECUdetermines the power balance between PEMFC and the battery depending upon therequested torque by the vehicle.

As the current obtained from PEMFC depends onthe stack voltage, the voltage required by the battery changes with time. Inthe event of sudden acceleration, the battery backup should be able tocomplement the PEMFC due to its relatively slow response. The PEMFC systemrecharges the battery when the SOC drops below lower limit of SOC. ·       PEMFC systemThe key components of PEMFC system considered are: PEMFC stack,air-feeding system and thermal management system (TMS). Due to the highcomputational time of multi-dimensional fuel cell model, one-dimensional PEMFCmodel is used. The detailed specifications are given in table 2. The PEMFCsystem has hydrogen storage tank along with anode gas recirculating pump on theanode side. The air is supplied to the stack through an air blower and then viahumidifier to the cathode side.

fig().Heat generated in fuel cell is obtained by subtracting enthalpy ofwater formation from the electric work generated. The heat transfer in the stacktakes place by the following modes: 1) convection- between gas in the channeland GDL; 2) conduction- between collecting plate and GDL. The coolant helps inmaintaining fuel cell temperature by extracting the heat through convection. The water content in the PEMFC stack membrane is an importantparameter for determining the efficiency of the system as it affects theelectrochemical reactions taking place in PEMFC.

The water content in the stackvaries with water activity according to  The electrochemical reactions occurring in the fuel cell determine thecell voltage and power available for driving the vehicle. The Nerst potentialof each cell is given byThe dynamic activation overpotential of the cell is studied during theFTP-75 cycle to indicate the effect of water in GDL on the performance of PEMFCand is denoted byThe ohmicoverpotential is given by ·       HumidifierTo study the dynamic behavior of the humidifier, it is discretizedinto three control volumes: (1) Shell, (2) Membrane and (3) Tube. As the dryair flows along the tube, the water vapor from the shell volume is transferredto the membrane which finally diffuses in the tube control volume. Theconcentration gradients in all these control volumes are responsible fordriving the water diffusion. The heat transfer takes place by convection,initially from shell to membrane and later from membrane to tube.   ·       Heat exchangerHeat exchanger is critical in maintaining the temperature of air at anoptimal level to derive better efficiency of the PEMFC. The flow configurationwas selected to be cross flow.

Diving the heat exchanger intoquasi-two-dimensional control volumes enables us to obtain local states inparallel & perpendicular direction if coolant flow. The primary areas of interestin the heat exchanger are: (1) tube, (2) wall and (3) fin. ·       Air blower The blower maintains the air supply system to provide sufficientreactant flow to keep the fuel cell working at optimum efficiency. (ref.

airblower paper) The important equations governing the blower are as follows:The desiredair flow rate of PEMFC stack-Temperatureat which air exits blower is-        System Dynamic ResultsFCHVEnergy efficiency of a vehicle is function of its driving cycle as itis governed the necessary torque to drive the vehicle. In case of urban citydriving (like FTP-75) there are frequent starts and stops due to varioustraffic regulations. Therefore, quick dynamic response of power source isnecessary to meet the high-power requirement and rapid variations in speed ofthe vehicle. As PEMFC is unable to satisfy this dynamic requirement due tolimitation of supply rates of reactants, a battery back-up is provided tofulfil additional power requirement.

The variations in current, voltage andpower produced in the PEMFC stack during the cycle have been captured anddisplayed in fig(9a). The input current, being proportional to the air-flowrate, varies analogous to the velocity during the cycle as depicted in the fig(9a).Humidifier- A proper membrane humidity in PEMFC should be always maintained foroperating at optimum performance. The shell-and-tube humidifier is used toobserve the dynamics in this study. ref dynamic As the air passes through theblower, a sharp decrease in its relative humidity (RH) is found at thebeginning.

The reason for this is increase in the magnitude of saturationpressure with increase in temperature is much higher than increase in partialpressure with increase in pressure. The sharp decrease in RH indicates thatthere is a very small amount of water left in the air at the blower exit. Thisdehydration of air can result into degradation of fuel cell and thereforedwindle the performance of fuel cell. Hence, a humidifier is necessary, postthe blower to maintain the performance of fuel cell. The air gains moisturecontent and becomes almost saturated at fractional distance of 0.6 as ittravels through the length of the humidifier. The relative humidity variation obtainedfrom the FTP-75 cycle has been plotted in fig.(11b)Air blower should be operated to meet dynamic requirements of air-flowrate which varies proportional to the current in the PEMFC.

1 The results of simulation ofblower are shown in fig(10). The temperaturedifferences in PEMFC stack (becomes higher) and air after blower (relativelylower) causes the dry air to absorb heat from wet air. Hence, dry air tempgradually increases along air flow direction. Thedifference in vapor and heat transfer rate between dry air and wet air resultsin variations of air temperature and relative humidity in the humidifier asshown in fig(12). In this way, the dry air entering the FC stack is heated andhumidified. Thus, the use of humidifier improves the fuel economy of FCHV byhelping the system to reach its efficient operation zone in terms oftemperature and humidity quickly.

Water coolant pump-The TMS comes into action when temperature of stack coolant at outletexceeds 70 °C. the coolant is then passed through the heat exchanger whichremoves the excess heat from the coolant and reduces its temperature down to 65°C. The variations in coolant flow and heat rejection rate through TMS havebeen displayed in fig(14) For minimizing ohmic overpotential in the stack, the membrane ishydrated initially. Later, to avoid the flooding in the channel due toincreased humidity, the temperature is increased to 70 °C and is maintainedconstant by the TMS. Power curves-As stated earlier, we have coupled fuel cell stack with battery topower the hybrid vehicle. The power obtained from battery is dependent on thepower extracted from PEMFC stack. The power from PEMFC stack depends on the hydrogenlower heating value (LHV), heat generated in the stack and the usefulelectrical energy produced. The electrical energy generation is a function ofvehicle velocity and the road conditions.

The total electric power and heat generatedduring the cycle has been shown in fig(17). From the simulation results we canverify that H2 usage rate is proportional to the vehicle speed inthe FTP-75 cycle. (fig.18)      ConclusionsIn thisstudy for the term paper on various control strategies and theirimplementation, I have assimilated the knowledge on the following:1.     A smart controlstrategy is vital in deciding the tradeoff between multiple energy sources and deliveringa good fuel economy. 2.

     In a PEMFC, air-flowrate is proportional to the input current of the fuel cell. Hence, air flowrate is found to vary rapidly during FTP-75 cycle similar to that of thecurrent.3.     To avoiddehydration of air at the exit of air blower, a humidifier is used after theair blower to moist the air and provide a stable performance of PEMFC.4.     TMS is used to maintainoptimum working temperature of the fuel cell.

A coolant is used to extractexcess heat from PEMFC which is recirculated through a heat exchanger to removethe heat.5.      Heat generation in fuel cell increases proportionally with the load onthe vehicle.     Future Scope/Improvements1.

      The dynamic performance of fuel cell vehicle can be compared with anequivalent conventional IC engine vehicle. A statistical comparison can be donefor economicality and applicability of fuel cell in transportation technology.2.

      The amount of water generated in the PEMFC stack can be recirculatedand used in the humidifier to rehydrate the air flowing into the stack.3.      The dynamic performance of FCV should be evaluated for different typesof driving cycles to understand the operation and anomalies for large spectrumof driving conditions.          

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