Week 6 Fuel cell powered vehicle model
Modelling a Fuel cell powered vehicle
Aim:
* To model a vehicle which is powered by a fuel cell and to analyse the fuel cell's characteristics.
* To analyse the behaviour of the fuel cell used to power an electric vehicle in areas like,
1. Fuel flow rate
2. Hydrogen and oxygen utilization
3. Fuel and air consumption
4. Efficiency of the fuel cell stack.
5. voltage behaviours of the fuel cell
6. current behaviours of the fuel cell
7. Boost converter voltage
8. Boost converter current.
Fuel cell:
* Fuel cell is a type of energy source device which produces electrical energy by electrochemical reaction
* Similar to a battery, a fuel cell is comprised of many individual cells that are grouped together to form a fuel cell stack.
* Each individual cell contains an anode, a cathode and an electrolyte layer. When a hydrogen-rich fuel such as clean natural gas or renewable biogas enters the fuel cell stack, it reacts electrochemically with oxygen (i.e. ambient air) to produce electric current, heat and water.
* While a typical battery has a fixed supply of energy, fuel cells continuously generate electricity as long as fuel is supplied.
Working:
* Every fuel cell has two electrodes, the anode which is positively charged and the cathode which is negatively charged. The reactions that produce electricity take place at the two electrodes.
* Every fuel cell also has an electrolyte, which carries electrically charged particles from one electrode to the other, and a catalyst, which speeds the reactions at the electrodes.
* Anode reaction:
- 2H2 → 4H+ + 4e−
* Cathode reaction:
- O2 + 4H+ + 4e− → 2H2O
* Overall cell reaction:
- 2H2 + O2 → 2H2O + heat + electrical energy
* The Hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons. These atoms are now ionized and carry a positive electric charge.
* The negatively charged electrons provide the current through wires to do work.
* If alternating current (AC) is needed, the DC output of the fuel cell must be routed through a conversion device called an inverter.
Polymer Electrolyte Membrane:
* There are many types of fuel cell based on the elctrolyte they have and the type of fuel cell used to power electric vehicles is Polymer Electrolyte Membrane (PEM).
* The polymer electrolyte membrane fuel cell (PEMFC) uses a water-based, acidic polymer membrane as its electrolyte, with platinum-based electrodes. PEMFC cells operate at relatively low temperatures (below 100 degrees Celsius) and can tailor electrical output to meet dynamic power requirements.
* Due to the relatively low temperatures and the use of precious metal-based electrodes, these cells must operate on pure hydrogen. PEMFC cells are currently the leading technology for light duty vehicles and materials handling vehicles.
* Hydrogen fuel is processed at the anode where electrons are separated from protons on the surface of a platinum-based catalyst.
* The protons pass through the membrane to the cathode side of the cell while the electrons travel in an external circuit, generating the electrical output of the cell.
* On the cathode side, another precious metal electrode combines the protons and electrons with oxygen to produce water, which is expelled as the only waste product.
* This is the working of PEM fuel cells that is used to power the automobiles.
Fuel cell vehicle's Powertrain:
* The fuel cell powered electric vehicle wiil be of a combination of Battery and a fuel cell stack.
* The powertrain of the fuel cell powered vehicle is given in the above figure.
* To model the fuel cell behaviours when used for an electric vehicle's application we are using the Inbuild model available in simulink.
Inbuild Fuel cell model:
* The nominal Fuel Cell Stack voltage is 45Vdc and the nominal power is 6kW. The converter is loaded by an RL element of 6kW with a time constant of 1 sec.
* 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 85 lpm in order to observe the variation in the stack voltage. That will affect the stack efficiency, the fuel consumption and the air consumption.
* Fuel flow rate, Hydrogen and oxygen utilization, fuel and air consumption, and efficiency are available on the Scope1.
* Fuel cell voltage, current, DC/DC converter voltage and DC/DC converter current signals are available on the Scope2.
Simulating the Fuel cell to power an electric vehicle:
* To know the effects of a Fuel cell in an electric vehicle's characteristics like, voltage, current, DC Bus voltage, DC Bus cuurent, Fuel flow rate, Fuel utilization, Stack efficiency etc, we need to do the simulation using the inbuild Fuel cell model with some modifications.
* In the Inbuild model, fuel flow rate (Litre per minute) input can be fed with the fuel flow rate data related to an actuall vehicle driving over a drive cycle. It is done using a signal builder block.
* In the signal builder block the current values related to the drive cycle is given as an input.
* If you can see the inbuild model's Flow regulator block it actually takes the ouput current values as a feedback to calculate the input fuel flow rate and so the signal builder block is given with the current signals as the input.
* The Fuel flow rate (LPM) ramp that gives the load reference to the fuel cell stack is replaced by the signal builder block.
* The simulation is done for first 100 sconds of an UDDS Drive cycle to know the characteristics of the fuel cell.
Results:
Scope 1
* The scope 1 shows the behaviour of the fuel cell in areas like,
1. Fuel flow rate
2. Hydrogen and oxygen utilization
3. Fuel and air consumption
4. Efficiency of the fuel cell stack.
1. Fuel flow rate:
* The fuel flow rate is displayed in the first plot which shows the rate of fuel supply into the fuel cell in Litre per minute (LPM).
* This rate of fuel supply is based on the drive cycle condition, as we have fed that as the input.
* Based on the speed data at that particular time of the UDDS Drive cycle, the fuel flow is supplied.
* The highest flow rate of 54.3 LPM is observed at t=30second in this 100 seconds sample drive cycle simulation.
2. Hydrogen and oxygen utilization:
* This plot shows the amount of hydrogen and oxygen is consumed as per the drive cycle inputs.
* The yellow line indicates the Oxygen consumption and the magenta line indicates the Hydrogen consumption in Percentage %.
* The Oxygen consumption level is constant at 60% throughout the 100 seconds of the drive cycle and hydrogen consumption varies based on the drive cycle data.
3. Fuel and air consumption:
* This plot indicates the amount of the hydrogen fuel and oxygen consumed by the fuel cell stack.
* The yellow line indicates the oxygen comsumption by the fuel cell stack and the magenta line indicates the hydrogen consumption by the fuel cell stack.
* Based on the acceleration and decceleration occuring in the drive cycle, the consumption of the fuel and oxygen varies and it is seen in Litres per minute (LPM).
4. Efficiency of the fuel cell stack:
* This plot shows the overall efficiency of the fuel cell stack.
* This efficiency is based on the drive cycle data and it is given in percentage %.
* The efficiency reaches upto 78% initially and it remains around 60% for some duration and when there is deceleration till rest, the stack efficiency also drops down to 0% instantly and then it raises as per the drive cycle.
Scope 2
* The scope 2 shows the behaviour of the fuel cell in areas like,
1. Voltage
2. Current
3. DC Bus voltage
4. DC Bus current
1. Voltage:
* The voltage level in the fuel cell stack is shown in this plot.
* The voltage level fluctuates as per the drive cycle data, when there is acceleration in the drive cycle the voltage increases and when there is deceleration the voltage level decreases in the fuel cell stack.
* The voltage level fluctuates between 46V and 53V most of the time and also at sudden deceleration it is obeserved to reach 0V.
2. Current:
* This plot shows the currrent fluctuations at respective time periods based on the input drive cycle acceleartion, deceleration and rest.
* The current values remain stable around 117A throughtout the sample of 100 seconds of the UDDS Drive cycle with only one massive shoot up during when the decceleration and immidiate acceleration occurs.
3. DC Bus voltage:
* This plot shows the voltage variations happening in the DC-DC Boost converter.
* The boost value of voltage is set to 100V and most of the time the voltage level is maintained.
* When the deceleration, rest and immidiate acceleration occurs in the drive cycle the voltage in the boost converter is observed to have fluctuations.
* The volatage level goes till -410V and then in shoots upto 850V in deceleration and immidiate acceleration part of the drive cycle and after some time the stability is achieved to 100V.
4. DC Bus current:
* This plot shows the current variations in ampere occuring in the DC-DC Boost converter.
* The boost converter is set to deliver 60A throughout and it is also observed to deliver 60A in most of the time.
* Based on the drive cycle at t=38 to 40 seconds there is a deceleration and sudden acceleration occuring and in that part there is a fluctuation of current observed in the boost converter from 60A to 19.5A and then it attains the stability of 60A again in a few seconds.
* Thus the fuel cell powered vehicle is modelled using the inbuild 6KW 45Vdc fuel cell model from simulink.
* The fuel cell behaviour, while it is made to power a vehicle to run on a UDDS Drive cycle is analyzed in different aspects and the resuts are discussed.
* For further research in the fuel cell powered vehicles, the inbuild fuel cell model can be modified as per our desire to model and observe its behaviour in powering our own designed vehicle.