Hydrogen Rate Manipulation of Proton Exchange

Membrane Fuel Cell (PEMFC) Stack using

Feedback Control System

*R.E.Rosli, E.H.Majlan, W.R.Wan Daud, S.A.A.Hamid

Fuel Cell Institute

Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor, Malaysia

Email: rosemilia86@gmail.com

Abstract - Control system design of PEMFC systems has been

focused mostly on the electrical power side, where auxiliary

battery or supercapacitor are connected either by direct parallel

integration or with single, multiple or multi-input single output

DC-DC converters, with the PEMFC left running at constant

predetermined hydrogen flow rates. The control strategy is

simply adapted to balance the voltage and power between the

PEMFC stack and auxiliary power during start-ups and sudden

power demands and the PEMFC stack is effectively left out of the

control loop or in open loop. Peak power above the rated PEMFC

power cannot be supplied on demand because of the inherent

inability to supply more hydrogen fuel to the PEMFC stack. This

paper presents a study on the design of the control system of an

open cathode PEMFC stack that closes the loop of the feedback

process control of the PEMFC by manipulating hydrogen flow

rate to produce the required peak power and to reduce fuel

wastage during low power demand. The Proportional Integral

Derivative (PID) feedback control systems are implemented using

National Instrument (NI) Data Acquisition (DAQ) devices

powered by Laboratory Virtual Instrument Engineering

Workbench (LabVIEW) because of their simplicity and

customization flexibility for measuring, processing and recording

of data. Test results on the control of a small PEMFC system

show that the new control system performs better and reduce

wastage compare to the previous open loop system.

Keywords- Fuel cell, Power Controller, PID, LabVIEW,

National Instrument (NI).

I. INTRODUCTION

Fuel cell converts energy from chemical and efficiently produces electrical energy direct from an electrochemical reaction between fuel and oxidant as the reactant and they require minimum maintenance due to less moving parts. There are many types of fuel cell being studied today and the most reliable fuel cell for stationary, portable electronics and vehicle application is (PEMFC). This is due to its low operational temperature, low noise, light weight, low corrosion, small volume and fast start up capability result in high energy density.

For PEMFC, hydrogen and oxidant (pure oxygen or air) is use as the fuel where hydrogen input is supply towards the anode of the fuel cell using hydrogen reformer or directly using

pure hydrogen and oxidant input to the cathode of fuel cell using pure oxygen, blower or air compressor. The pressure and flow rate amount to be applied is very complicated that an increase in flow rate will boost the kinetics of electrochemical reactions, resulting in higher power density and stack efficiency. On the contrary this also reduces the net available power from the fuel cell system.

Usually, this fuel is supplied at fix amount but problem will occur during the load varying where the fuel cell output power and current will increase. This problem needs serious attention towards vehicle application and load variation application because with the less hydrogen supply, it will cause power transient issue that leads to performance degradation where this causes unsatisfactory response at higher load demand and damaging MEA. But with full or excess hydrogen supply, it will issue the hydrogen wastage at lower current demand. In order to overcome this problem, a controller needs to be implemented to control the flow rate of hydrogen supplied to the fuel cell that will be included in this system.

To have an efficient and impressive performance, fuel cell needs to have a system where its function is to deliver the correct amount of power generated on demand. The four important types of systems to be controlled are humidity, reactant pressure and flow rate, water management and temperature. These parameters play their own role; where the controller will determine the correct amount that needs to be supplied, resulting in better generation of electricity.

Studies in reactant control conducted by other researchers resulted in better performance where they presented an application of online self tuning PID controller to prevent momentary drop during transient response cause by load variation [1-2]. And [3] implemented the Proportional Integral (PI) control on air flow rate to improve slow transient response of a fan. Our contribution in this paper focused on the application of controller function to control the reactant being fed to the fuel cell system to improve system response during load change.

In present studies, changing the parameter sometimes proved to be of no significance since it can work even without a controller. However, when it was being controlled many

2012 IEEE International Conference on Power and Energy (PECon), 2-5 December 2012, Kota Kinabalu Sabah, Malaysia

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improvement and advantages are noted [4-5]. Some researchers construct a controller to control input reactant where it will adjust the rate continuously according to the output load [6]. In this paper, we will focus on implementing a reactant controller, where a design of a feedback control system will be done to manipulate the flow rate of reactant supply to the fuel cell, where the required amount of hydrogen will be supplied to have a better PEMFC durability, capability and as prevention from reactant waste and MEA damage. This implementation can be widely used in many applications that involve load variation like an electric vehicle where the power demand is impulsive rather than constant.

Here in section II, we will discuss on fuel cell power system and the technology being used. While in section III, the strategy of the controller will be analyzed and in section IV the proposed controller design will be presented to prove the result and the conclusion for this paper will be discussed in section V.

II. SYSTEM SETUP

The 100W open loop PEMFC stack being used in this experiment consisted of 24 cell of MEA with the electrode area of 24cm². This fuel cell can generate 100 watts at its maximum and a rated performance of 14.4V at 7.2A, the hydrogen humidity being set at constant 80% and temperature at 50°C. Hydrogen gas used was of 99.99% purity that was stored in compressed gas cylinder under 100 atm and was reduced to 0.5 atm using a pressure regulator before supplying it to the Mass Flow Controller (MFC) that in turn will be controlled using PID controller. These controllers will use LabVIEW program with National Instrument devices where the program is used to measure and record values for the fuel cell stack on a real time basis. A programmable electronic load was used to generate a pulse current load profile.

Conventionally, measurement is done by various types of standalone instrument like oscilloscope, digital multimeter, counters and etc. These devices cannot fulfill all that is necessary, required to save the measurement and the process of gathering the gather data for visualization. Using NI technology, all required needs can be fulfilled where it can measure and monitor voltage, current, power, temperature and also in controlling the gas flow and fan speed. LabVIEW software is used for data logging. LabVIEW and cFP are standard products of NI that provide precise data acquisition. This module is placed between computer and fuel cell system to convert analog input to digital output for the field point measurement.

Figure 1. Flow diagram of proposed control algorithm.

There are four flows of control algorithms as shown in figure 1, where it starts from fuel cell system, DAQ hardware tools (read), LabVIEW programme and DAQ hardware tools (write). These sequences were kept running and continue to analyze as the system was powered.

The setup of fuel cell system consisted of PEMFC stack, microprocessor, cooling fans, hydrogen purging valve, single stage pressure regulator, power connector, MFC for hydrogen gas, and DC electronic load as depicted in figure 2.

DAQ hardware tools were used to acquire the data so that the signal can be sent out (control) and received (feedback). It consisted of 8-channels analog voltage, 4-channels analog voltage or current input, 4-channels analog current output, 16-channels digital output, 8-channels PWM output where in turn were connected to a fuel cell power system.

LabVIEW 8.6 programme was installed to a computer where a graphical programming language that used icons instead of lines of text to create application. This was used for data analyzing, monitoring, result presenting, data recording and sending implement signal to be processed. Next, the design of PID controller was done using LabVIEW and downloaded to Field Point to run the application, which will be discussed later on in detail.

III. DESIGN CONCEPT

Fuel cell practices open loop system where the hydrogen is supplied with a direct supply to the stack from compressed gas cylinder through a pressure regulator without being controlled. A major problem in this research concerns the load current variation where this will cause the voltage drop in fuel cell stack at low hydrogen supply or wastage at higher hydrogen supply. A controller is the best solution to this problem where PID controller will be used to vary the flow rate of fuel cell to its hydrogen supply. In this study, a control method of reactant will be determined where the controller will give a sufficient amount of reactant to the fuel cell system. Through this, a more reliable operation, one that prevents excessive fuel consumption and of high system efficiency will be determined.

Figure 2. Experiment setup of fuel cell system.

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A. Open Loop System

At open loop system, hydrogen fuel is directly supplied form hydrogen tank and regulated to a specific value of pressure amount by the pressure regulator as shown in figure 3. Before entering the fuel cell stack, the hydrogen gas is humidified to 80% and at 50°C. Oxygen, on the other hand is supplied using an axial fan fixed at 17V. The excess hydrogen is purged because of the accumulation of inert or water. The frequency and duration of purging depend on the purity of the hydrogen gases being used and water net transport through the membrane. These parameters are controlled by the PEMFC controller circuit. The output voltage and power generated are monitored using DC electronic load.

Figure 3. Diagram of open loop system for 100W fuel cell.

B. Control System

Here the fuel supplied is controlled using MFC, the control signal strategy that analyzed the PID controller designed in order to fulfill the fuel cell demand, where the controller will adjust the hydrogen fuel flow rate to maintain and improve stack output voltage. This implementation focuses to apply the sufficient amount of hydrogen needed by increasing and decreasing the amount of reactant based on power demand. This implementation is vital in order to provide efficient amount of power and reduce hydrogen wastage. The setup diagram is shown in figure 4. The controller implementation will be discussed accordingly.

Figure 4. Diagram of feedback control loop.

C. PID Controller

Proportional Integral Derivative or PID controller is used for a better fuel cell performance [7-9]. The concept of PID is where it calculates the difference between a measured and desired value (error value). With this error, the controller will adjust the process control input to minimize the error. Every value of PID term determines different reaction where P, (proportional) determines the current error, its function is to ensure the stability and it depends on present error. I, (integral) determines the reaction based on the sum of recent errors and its function is to permits the rejection of a step disturbance and it is an accumulation of past errors. D, (derivative) determines the reaction based on the rate at which the error has been changing, its function is to provide damping or shaping of the response and it is a prediction of future error. The sum of these three actions is then used to adjust the process via control element, so the controller can provide control action designed for specific process requirement [10-11] and helps the system to be more accurate and stable. The construction of PID is shown in figure 5. The general form of PID is:-

∫ ++= )t(edt

dKdt)t(eK)t(eK)t(u DIp (1)

Where:-

u (t) = control signal sent to the system

y (t) = measured output

r (t) = desired output

And the tracking error,

)t(y)t(r)t(e −= (2)

The idea of implementing the PID feedback loop in fuel cell system is to organize the dynamic behavior of the system. The mechanism of above activity is that the fuel cell system output will be supplied back through a sensor or measurement output to the reference value. Error from the difference between reference and measurement output will be received by the NI read controller, and the LabVIEW in turn will change the inputs using NI write controller and pass it back to the fuel cell system. This whole process will be repeated continuously to ensure the desired value set to be achieved. The path of signal is shown in figure 5 below and NI cFP-PWM-520 module from National Instrument used to control the amount of hydrogen flow rate. This implementation is more precise than the on/off controller because the change of mass flow meter rate is based on power output, so a multiple range can be set.

Figure 5. PID fuel controller implemntation for fuel cell system.

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The PID controller programmer development using LabVIEW program is shown in figure 6. The front panel (Figure 6) shows the data measured and the graph that indicates the PID reaction towards the change of set point (current load).The detail of the structure is shown in block diagram, represented in Figure 7.

Figure 6. The front panel of LabVIEW.VI

Figure 7. The block diagram of the real time target control.

IV. EXPERIMENTAL RESULT AND DISCUSSION

In this section, the real time implementation of new

controller system were tested and compared with the

uncontrolled system to validate the design proposed in this

paper. The system was tested with different flow rate supply

to observe the performance. Based on figure 8, the pressure

(0.5bar) and temperature (50ºC) being applied at different

flow rate. From the polarization curved plotted, it was shown

that loading can be satisfied with adequate amount of fuel

supplied but a decrease is noted when fuel supplied was not

sufficient as the current was continually loaded. This proved

that with insufficient amount of fuel it will result in depletion

of fuel cell performance and this situation can cause fuel

starvation that later will damage the MEA inside fuel cell. Figure 9 demonstrates the condition at controlled (figure

9a) and uncontrolled (figure 9b) flow rate tested with stepping current loading. The result proved that the proposed controller system can fulfill the load demand same as uncontrolled system but with improved consumption of hydrogen especially at low current loading where around 80% hydrogen consumed can be preserved. These strategies effectively reduce the hydrogen wastage problem.

(a)

(b)

Figure 8. Fuel Cell performance at different flow rates a) Higher fuel supply

b) Low fuel supply

a. Voltage at incresing loading..

b. Hydrogen volume at fully open and control valve.

Figure 9. Different hydrogen flow rate at the same current loading yield

same output but different hydrogen consumption.

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Figure 10. Incresing loading with different valve condition.

Figure 11. Hydrogen consumption on two different condition.

To observe the potential difference at different valve

condition, the current loading was varied as plotted in figure

10. The result showed that the output was approximately the

same for both conditions. However, there were different

consumptions of hydrogen gas used, where at fully open valve

condition, the hydrogen consumption was higher compared to

PID control valve that can be examined in figure 11. These

results proved the benefits gained from the new controller

system proposed in this paper.

V. CONCLUSION

The new proposed fuel cell control system can follow the load demand even in transient response. Where when with sufficient amount of hydrogen, fuel cell performance is improved, fuel wastage is reduced and MEA damage is prevented. New control system performs better than the previous open loop system and PID implementation is very effective to control hydrogen flow for active current load variation. This implementation can be widely used in many applications with large load variation such as fuel cell vehicle where the power demand is impulsive rather than constant. For future applications, this fuel cell system controller will be implemented in a fuel cell powered golf buggy.

ACKNOWLEDGMENT

The authors thank the Institute of Fuel Cell, Universiti Kebangsaan Malaysia under the Arus Perdana Grant Number of UKM-AP-TK-08-2010 and National Science Fund (NSF) by MOSTI for their financial support.

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