Week 6 Fuel cell powered vehicle model

Matlab Model of Fuel Cell Powered Electric Vehicle

Reference: 6kW 45 Vdc Fuel cell stack taken from matlab model and modelled according to the requirements of Fuel Cell Powered Electric Vehicle.

• Discussion of Model

Block diagram given below represents the working of the Fuel Cell Powered Electric Vehicle

The input to the vehicle which is the fuel flow rate reference in litre per minute is given using a repeating sequence stair. The values are given for a time of 100 s with one pair of acceleration and deceleration. Flow rate selector takes the input from the repeating sequence stair and flow rate regulator.

Inside the flow rate selector block, there is a clock circuit whose output is compared to a constant 10 and is fed through an edge detector whose output is a needle impulse (boolean 1) signal when the specified edge type (Rising) is detected. The constant value is given as 10 because after 10 seconds the flow rate regulator is bypassed and the rate of fuel is changed. The sample and hold circuit samples and holds the signals from the fuel rate regulator and the edge detector. This output of the sample and hold signal is added with the fuel rate reference input and this is the first output from the from the flow rate . the second output is taken from the edge detector and the third output is the flow rate regulator output. All these 3 outputs are given to a switch.

The switch pass through input 1 when input 2 satisfies the selected criterion (second input greater than zero); otherwise, pass through input 3. The output from the switch is given to a satuaration block where the maximum speed value attained during the drive cycle is given as the upper limit value which is 80 m/s here.

The fuel rate selector along with switch and saturation gives a controlled fuel rate signal to fuel cell stack. We have used a PEM Fuel Cell Stack ie Polymer Electrolyte Membrane is sandwiched between anode and cathode having a Stack Voltage of 45Vdc and the nominal Power is 6kW. RL load of 6 kW with a time second of 1 sec is connected to Fuel Cell Stack via 100V dc boost converter.

• Discussion of Steps:

• The input which is the fuel flow rate reference is given as the repetitive sequence stair

Here we can see that the vehicle is started, the slowly it is accelerating, after certain time the vehicle is decelerating.

• The flow rate selector block

The above graph shows the output from the add circuit. The first figure shows the reference fuel flow rate. The second figure is the output from the sampled and hold circuit. These two signals are added and the output is shown in figure 3.

The output from the flow rate selector block is fed to the switch as input 1. the second input is from the clock circuit which is compared to a constant. the third is directly given from the flow rate regulator output.

• Switch

The first 3 are the inputs to the switch and the 4^th graph is the output from switch.

• Whenever the second input is less than or equal to zero; the output follows input 3 ie till time 10 seconds. After that instant the second input is greater than zero; so the output follows first input.
• This output is fed to the saturation block

• Saturation Block

• The first figure is the input to the saturation and the second is the output.
• The maximum speed value attained during the drive cycle is given as the upper limit value which is 80 m/s here.
• So the saturation limits the value to 80
• This signal is given to the fuel cell stack

• Fuel Cell Stack

• The PEMFC-6kW-45Vdc is selected as the preset model because PEMFC is suitable for HEV drives.
• The graph shows the output of the fuel cell.
• The initial voltage spike is due to RL load. So it has an exponential fall in voltage due to RL circuit. As well as there is an initial exponential rise in current.
• After 10 seconds; the fuel rate is increased for acceleration; as fuel rate increases the voltage increases to 55 V but the current falls to 110A due to inherent properties of fuel stack.
• At 30^th second; the fuel rate is again increased. We can see a gradual increase in voltage to 55V and small decrease in current.
• But further if we try to accelerate as per the input reference; the fuel voltage remains 55V as per the saturation limits
• At 70^th second; the vehicle is decelerating; ie the fuel flow rate is decreased. So there is a fall in voltage to 54V. at 80^th second; the fuel flow rate is again decreased. So the voltage falls to 53 V. the current of the fuel stack rises correspondingly
• The current output is given to fuel regulator for regulating the fuel flow rate.
• The fuel cell stack is connected to the RL load via 100 V DC-DC boost Converter.

• Fuel Rate Regulator

• The initial hydrogen utilization is set to 99.56%
• As the fuel rate increases or decreses the hydrogen utilization decreases or increases respectively.

• DC-DC Boost Converter

• The converter is loaded by an RL element of 6kW with a tinvme constant of 1 sec.
• The output of the converter provides a 100V dc to the load.

The complete mathematical model of the Fuel Cell Powered Electric Vehicle is given below.

Analysis:

• Fuel cell voltage, current, DC/DC converter voltage and DC/DC converter current signals are available on the Scope2.

As we have discussed earlier The initial voltage spike is due to RL load. So it has an exponential fall in voltage due to RL circuit. As well as there is an initial exponential rise in current. After 10 seconds; the fuel rate is increased for acceleration; as fuel rate increases the voltage increases to 55 V but the current falls to 110A due to inherent properties of fuel stack. At 30^th second; the fuel rate is again increased. We can see a gradual increase in voltage to 55V and small decrease in current. But further if we try to accelerate as per the input reference; the fuel voltage remains 55V as per the saturation limits. At 70^th second; the vehicle is decelerating; ie the fuel flow rate is decreased. So there is a fall in voltage to 54V. at 80^th second; the fuel flow rate is again decreased. So the voltage falls to 53 V. the current of the fuel stack rises correspondingly. Whatever the change in stack voltage and current the bus dc voltage is maintained to 100V and current to 60 A by dc-dc boost converter. There is a small fluctuation in voltage at 10^th sec; nut the dc-dc converter maintains the voltage to 100V.

• Fuel flow rate, Hydrogen and oxygen utilization, fuel and air consumption, and efficiency are available on the Scope1.

During the first 10 secs, the utilization of the hydrogen is constant to the nominal value (Uf_H2 = 99.56%) using a fuel flow rate regulator. After 10 secs, the flow rate regulator is bypassed and the rate of fuel is increased to the maximum value of 80 lpm during acceleration. We can see that there is a corresponding decrease in the utilization of Hydrogen and decrease in stack consumption and stack efficiency.

After 70 seconds, there is a reduction in the fuel flow rate due to deceleration of vehicle. There we can observe that utilization of hydrogen is increasing. Also the stack consumption and stack efficiency is increasing correspondingly.