Hydrogen Pressure Control in Hydrogen Fuel Cell System

Abstract: The hydrogen supply system is an important part of fuel cell system, and dynamic control of pressure difference between air side and hydrogen side is particularly important for the reliability of entire fuel cell system. This article will focus on problems exist in hydrogen pressure control of hydrogen fuel cell system, also recommend the use of a precision proportional flow control valve, and introduce characteristics and technical parameters of proportional flow control valve in detail.

1. The formulation of question
As an important part of fuel cell system, hydrogen supply system collaborate with voltaic pile, air supply system, water and heat management system and electronic power system to ensure the stable supply of hydrogen flow and pressure, and to realize hydrogen recycling. The simplified structure of fuel cell hydrogen supply system is shown in Figure 1-1. The high-pressure hydrogen storage tank is source of hydrogen for system. Hydrogen passes through pressure reducing valve, and pressure is reduced to a suitable range for system. It’s usually about a few bars. The hydrogen intake valve is used to control the amount of hydrogen entering voltaic pile, and then control the pressure of hydrogen circuit of the voltaic pile. Now, the most common hydrogen intake valve is proportional control valve, switching valve or multiple switching valve groups.



As the thickness of membrane electrode of fuel cell gradually reduces, its mechanical strength decreases accordingly. Therefore, the dynamic control of the pressure on the air side and the hydrogen side is particularly important for the reliability of the entire fuel cell system. The general requirement is that the pressure on the hydrogen side should be equal to or slightly higher than that air side. When adjusting pressure on both sides, making sure that pressure rises and falls at the same time to reduce the damage to proton membrane. However, in the current hydrogen fuel cell power system, the control of pressure difference between two sides has the following problems:
(1) The switching valve is used to control hydrogen intake, which makes fluctuation in the entire hydrogen circuit too large and difficult to control.
(2) Although proportional solenoid valve can be used for pressure control which is similar to PID mode according to a certain proportion.
However, proportional solenoid valve will bring serious problems of unstable control due to large hysteresis phenomenon. This article will focus on problems exist in hydrogen pressure control of hydrogen fuel cell system, also recommend the use of a precision proportional flow control valve, and introduce characteristics and technical parameters of proportional flow control valve in detail.

2. Proportional flow control valve
Proportional flow control valve is shown in Figure 2-1.

Model	FC-20	FC-120	FC-300	FC-1000
Valve Type	Proportional Needle Valve
Spool Drift Diameter	0.9mm	2.25mm	2.75mm	4.10mm
Actuator	Bipolar stepper motor control
Response time	0.8sec(Open to close)
Standard size	G1/8”			G3/8”
Fluid	Inert gas and liquid
Contact Materials	Stainless Steel
Pressure Range	-1 ~ 7bar			-1 ~ 5bar
Maximum Flow	50L/min @7bar	240L/min @7bar	290L/min @7bar	600L/min @7bar

 

Linearity	±2%	±0.1 ~ 1%	±0.2 ~ 5%	±11%
Repeatability (Full scale)	±0.1%
Flow Resolution (Step length)	0.1L/min	0.1 ~ 0.2L/min	0.2 ~ 0.75L/min	1L/min
Shift Resolution (Step length)	12.7um		25.4um
Operating Temperature Range	0 ~ 84ºc
Seal	Standard FKM or other choices of seals
Control Signal	DC: 0 ~ 10V (or 4 ~ 20mA)
Power Supply	DC: 24V (12W)

Figure2-2 proportional flow control valve technical indicators.
Please visit https://www.genndih.com/proportional-flow-control-valve.htm

2.2. Drive Module
The proportional flow control valve is equipped with a stepper motor drive circuit module to provide the required power and control signals, and to convert DC signal into stepper control of bipolar stepper motor, and can also provide direct control of RS485 serial communication.



2.3. Features
New proportional flow control valve of digitally controlled motorized for proportional flow regulation combines the precision and repeatability advantages of a stepper motor with the linearity. The resolution is in less than 2% hysteresis, 2% linearity, 1% repeatability and 0.2% adjustable flow control with high performance. It is an upgraded product of commonly used solenoid proportional valve. Combining with various PID control algorithms and pressure controllers, it can form a fast and accurate hydrogen pressure control device.
Proportional flow control valve has the following characteristics:
(1) Multi-specification throttling area: from the low-flow diameter 0.9mm (0~50L/min gas) to the high-flow diameter 4.10mm (0 to 660 L/min gas) needle valve throttling area, can meet different application needs.
(2) High linearity: The linearity of less than 2%. It simplifies surface or matching of external control hardware and software. It also simplifies the relationship between command input and flow output. (3) High repeatability: By reaching the same flow rate of 0.1% each time, it can provide long-term stable consistency.
(4) Wide pressure range: With a pressure of 5 or 7 bar, depending on the size of hole, the inlet environment can cover wide pressure range. The stiffness and power of motor ensures that valve opens with the same input command, not about pressure.
(5) Low Hysteresis: Hysteresis of less than 2% makes integration and programming simple, providing consistent flow when increasing and decreasing to the set point.
(6) High resolution: 0.2% resolution allows proportional flow control valve to perform minimum flow adjustment according to small changes in adjustment command, providing excellent controllability.
(7) Quick response: the whole operated time is less than 1 second, which can provide rapid flow regulation and control.