Preventing reactant starvation of a 5 kW PEM fuel cell stack during sudden load change 2,3
Mircea Raceanu, 3Adriana Marinoiu, 3 Mihai Culcer, 3Mihai Varlam
3
National Center for Hydrogen and Fuel Cell National Research and Development Institute for Cryogenics and Isotopic Technologies ICIT Rm. Valcea, Romania,
[email protected] Abstract – Reactant (hydrogen and air) starvation during PEMFCs operation can produce serious irreversible damages. To preventing reactant starvation and, at the same time, to allow a dynamic operation of the fuel cell system, both stoichiometric ratio λH2 and stoichiometric ratio λAir of the input gases need to be adjusted rapidly during sudden load change. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of pressure, flow rate, temperature, current density, and relative humidity in an experimental PEMFC stack have been performed during both air and hydrogen starvations. This study is based on the real-time control of Nedstack P5.0-40 bench PEM fuel cell test system and presents a suitable control strategy for hydrogen/air supply, based on PID feedback and feedforward control of the reactant feeds for preventing the starvation and/or obtaining the maximum net power of PEMFC stack to sudden changes in load. Keywords-component: PEM fuel cell stack, reactant starvation, feedback and feed-forward control
I.
INTRODUCTION
The efficiency of polymer electrolyte membrane fuel cells (PEMFCs) known as a very attractive power source for vehicle applications, is higher than internal combustion engines, but for the commercialization of fuel cell vehicles, long-term performance and long lifetimes are required. The major elements affecting the life and performance of PEMFCs are fuel, oxidant starvation, poor water management, carbon support degradation, sintering and migration of Pt, chemical reactions of cell components, and the contamination of cells [1-7]. The dynamic performance of the PEMFCs is time dependent and involves several typically transient processes: start-up, shut-down and load changes. The transient responses of the operating conditions are an indication of the complex interactions between the related parameters and the dynamic performance of PEMFCs [8-10]. The most damaging source of cell reversal is reactant starvation, an inadequate supply of fuel. Many factors can lead to starvation in a fuel cell: poor cell design or machining with uneven mass distribution in flow fields, poor stack design or assembly with uneven flux distribution between cells, poor water management with channel block by flooding; poor heat management in cold start-up with ice blocking, and detrimental operation conditions with sub-stoichiometric gas feeding. Moreover, a
1,2
Nicu Bizon
1
University of Pitesti, 1Targu din Vale, Arges, 110040,Pitesti, Romania 2 University Politehnica of Bucharest, 313 Splaiul Independentei, 060042,Bucharest, Romania
[email protected] sudden change in oxygen demand, such as at start-up and load change, and water accumulation during longterm operation can cause an air starved condition. Significant reactant starvation during PEMFC operation, especially fuel starvation in PEMFC stack, can cause severe and irreversible damages, such as catalyst surface area loss, corrosion of carbon based catalyst support, and even cell failures [11-13]. In the case of fuel starvation, hydrogen is no longer enough to be oxidized to maintain the current, and anode potential will increase high enough to make alter oxidized in anode with oxygen produced. At the same time, oxygen is reduced in cathode, so the net effect is analogous to a bilateral selective “pump” - oxygen is pumped from cathode to anode while water from anode to cathode simultaneously. Oxidant starvation, usually occurring under harsh operating conditions such as sub-zero start-up, rapid load change and water accumulation during long-term operation, during transients, if reactants are consumed in the fuel cell faster than they can be supplied, etc. is one of the potential factors to result in the degradation of PEMFCs. In a fuel cell stack, if the oxygen supplied is not enough to maintain the stack current, the oxidant starvation will occur. In this case, a reversal of cell voltage could happen. In the absence of oxygen, protons pass through the membrane and combine with each other. Thus hydrogen is produced to provide the compensatory current [14-15]. To prevent reactant starvation and to allow for a dynamic operation of the fuel cell, the excess ratio of hydrogen and oxygen needs to be adjusted rapidly by increasing the mass flow into the anode and cathode. This increase is limited by the inertia of the actuators. Especially at fast load changes the risk of starvation is high. This problem can be faced by limiting the dynamics of load changes or by decoupling the desired load from the effective load [16]. Many researchers have studied air flow control to prevent starvation. A big number of control strategies and methodologies for fuel-cell systems have been proposed in specialty literature, which range from simple proportional–integral–derivative (PID) controllers to advanced control strategies such as fuzzy controllers and Model Predictive Control (MPC) methodologies. There are only few papers which report specifically the regulating of PEM fuel cell output voltage or air excess ratio, conventional proportional integral derivative (PID) control 17], LQG control [18-20], fuzzy control [22], neural
2
Mircea Raceanu, Adriana Marinoiu, Mihai Culcer, Mihai Varlam, Nicu Bizon
network control [23], time delay control [21] or model predictive control (MPC), and an even more limited number of studies exist on real plant validation. In this paper, we utilized our test bench to study the starvation behaviour of a Nedstack P5.0-40 fuel cell stack in detail. The control objective is to protect the PEMFC from damage and reduce the difference between the demand load and the stack current. Double feedback controller and feed-forward controller are used to regulate the hydrogen and air stoichiometric ratios to prevent starvation and to obtain maximum net power of the PEMFC stack. Feed-forward controller adjusted rapidly the stoichiometric ratios at fast load changes. The transfer function between voltage fluctuation on fuel cell and hydrogen and air stoichiometric ratios is determined via experiments to write lookup table configuration. It is shown experimentally that the controller structure can improve the voltage and current performance of the system when the load changes. II.
EXPERIMENTAL
A. Experimental setup A.1 PEM Fuel Cell Stack. In order to investigate the behavior of the fuel cell stack it is important to measure a variety of variables. For this purpose a Nedstack P5.0 - 40 fuel cell stack was used. The stack consists in a 40 cells allowing for a nominal maximum power of 5 kW. The operating voltage of the stack is a function of current and decreased with increasing power. The PEM fuel cell stack can operate from atmospheric pressure to about 3 bars. The main operating parameters of the Nedstack P5.0-40 are shown in Table 1. TABLE1. Parameters of the Nedstack P5.0-40.
Parameter
Value
Unit
Rated Power
5000
W
Voltage (OCV)
39
V
Voltage (maximum power)
22
V
Maximum current
230
A
Hydrogen humidity
>50
%RH
Air humidity
80
%RH
Operating temperature
65
°C
Conductivity of coolant
