Fuel cell powered vehicle model

                                                                                                                                Fuel Cell powered vehicle model

Aim:

Executing the MATLAB model of fuel cell-powered electric vehicle and analyzing the result.

Theory:

Fuel Cell:

Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes a negative electrode (or anode) and a positive electrode (or cathode) sandwiched around an electrolyte. A fuel, such as hydrogen, is fed to the anode, and the air is fed to the cathode. In a hydrogen fuel cell, a catalyst at the anode separates hydrogen molecules into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they unite with oxygen and the electrons to produce water and heat.

Fig: Working of PEM fuel cell

Part of PEM Fuel Cell:

PEM fuel cell consists of an electrolyte membrane sandwiched between anode and cathode.

PEM: The PEM is a thin, solid, organic compound, typically the consistency of plastic wrap and about as thick as 2 to 7 papers sheet. The membrane function as an electrolyte, a substance that conducts charged ions but does not conduct electrons. This allows the solution to conduct electricity. The membrane must be kept moist to conduct particles through it.

Anode: The anode is the electrode at which the electrode takes place. In a fuel cell, the anode is electrically negative. The anode is composed of platinum particles uniformly supported on carbon particles. The platinum acts as a catalyst, increasing the rate of the oxidation process. The anode is porous so that hydrogen can pass through it.

Cathode: The cathode is the electrode at which reduction takes place. In a fuel cell, the cathode is electrically positive. The cathode is composed of platinum particles uniformly supported on carbon particles. The platinum acts as a catalyst, increasing the rate of the reduction process. The cathode is porous so that oxygen can pass through it.

Flow Plates: Flow plates perform several functions; 

1. They channel hydrogen and oxygen to the electrodes.
2. They channel water and heat away from the fuel cell.
3. They conduct electrons from the anode to the electrical circuit and from the circuit back to the cathode.

Chemical Reaction:

At anode:

`H_2 rightarrow 2H^+ + 2e^-`

At cathode:

`2e^-  + 2H^+ + frac{1}{2}O_2 rightarrow H_2O`

Cell:

`H_2 + frac{1}{2}O_2 rightarrow H_2O`

Fuel Cell Powered Electric Vehicle

Fig: Part of Fuel Cell propelled vehicle

Like all-electric vehicles, fuel cell electric vehicles (FCEVs) use electricity to power an electric motor. In contrast to other electric vehicles, FCEVs produce electricity using a fuel cell-powered by hydrogen, rather than drawing electricity from only a battery. During the vehicle design process, the vehicle manufacturer defines the power of the vehicle by the size of the electric motor that receives electric power from the appropriately sized fuel cell and battery combination. Although automakers could design an FCEV with plug-in capabilities to charge the battery, most FCEVs today use the battery for recapturing braking energy, providing extra power during short acceleration events, and to smooth out the power delivered from the fuel cell with the option to idle or turn off the fuel cell during low power needs. The amount of energy stored on board is determined by the size of the hydrogen fuel tank. This is different from an all-electric vehicle, where the amount of power and energy available are both closely related to the battery's size.

In MATLAB, the block for FCEV is given as;

Fig: Fuel Cell Stack Block

The Fuel Cell Stack block implements a generic model parameterized to represent the most popular types of fuel cell stacks fed with hydrogen and air.

This model is based on the equivalent circuit of a fuel cell stack shown below:

Fig: Equivalent circuit of a fuel cell stack

Procedure:

In the following model, the ramp block is used to up the flow rate. Using the fuel cell stack, the electrons are produced. The amount of fuel flow is regulated by the flow rate regulator. It gives the input to the flow rate selector block to facilitate the fuel flow. The voltage produce is then stepped up to higher voltage using 100 Vdc Boost DC/DC converter.

Fig: 6 kW 45 Vdc Fuel Cell Stack

The parameter set for the above model are;

The slope is set at 85. PEMFC of 6 kW and 45 Vdc is used as this is more viable in EVs as it takes up less space.

Fig: Input Parameters

We will create a drive cycle giving the flow rate. The flow rate corresponding to speed is given in liter per minute. The following is the drive cycle:

https://docs.google.com/spreadsheets/d/19cbEOS60Qb1OIqcwsz7oA9WrRC-1TkaNz2sUcVNKcZQ/edit?usp=sharing

Fig: Drive cycle in fuel flow rate profile

Then we will connect this to our model using the from workspace block.

Fig: Fuel Cell Stack with added drive Cycle

Then we simulate the model for drive cycle duration i.e 100 sec.

Result:

1. Under default condition at 20 sec

After the simulation of the above model for the simulated time of 20 sec, the result is as follow;

Fig: Fuel Cell Stack Output

In the graph, it can be seen that the fuel flow rate increases steadily until it becomes constant. Due to an increase in slope the fuel flow rate increases in no time i.e. in less time. The increase in slope is due to an increase in the high power output required. The hydrogen utilization remains constant throughout at 60 %. But due to an increase, the utilization of the oxygen goes down. The fuel flow rate is directly proportional to Nernst Voltage. And Nernst Voltage is inversely proportional to Current. Current is directly proportional to stack consumption. All in all, it only means that stack consumption is inversely proportional to the fuel flow rate. This is evident in the graph that both stack consumption goes down due to an increase in flow rate. As the oxygen is underutilized and the load increases the efficiency goes down.

Fuel flow rate `prop` Nernst Voltage

Nernst Voltage `prop frac{1}{Current}`

Current `prop` Stack Consumption  `therefore`   Nernst Voltage `prop frac{1}{Stack   Con.}`

`therefore`   Fuel flow rate `prop frac{1}{Stack   Con.}`

Fig: Voltage and Current plot

The first two figures are the voltage and current of the Fuel cell. During the starting, there is a high demand for voltage due to the starting condition. But after starting the voltage requirement goes down but the opposite happens for current, where it is 0 at the beginning and increases gradually thereafter. When there is a high load condition the voltage increases in slightly less time and thereafter remains stable. But during the same phase, the current demand is reduced, and the current falls down by some margin and remains stable thereafter.  On the output side though, the voltage can be seen higher, but due to the boost converter power operation, there is no fluctuation in output voltage and current. As the load is introduced in 10 sec, there is a slight spike in voltage. During the same time when the load is applied, the current also experiences a minor spike and remain stable after that point.

2. With Fuel Flow Rate profile

Fig: Fuel Cell Stack Output

It is seen that as the fuel flow rate increases the oxygen utilization decreases, and when the vehicle experiences a deceleration the oxygen utilization increases. When there is an acceleration the stack consumption is lowering, it increases during deceleration. The efficiency also takes a hit during acceleration, only to be improved while decelerating.

Fig: Voltage and Current plot

For fuel-cell, due to an increase in speed, the voltage is reduced at the same time there is an increase in current. During acceleration, the DC bus voltage experiences a spike, after that it remains stable. During the same phase DC current increases. In the beginning, the current is low while, the voltage is high. They go opposite after that.

Conclusion:

Executed the MATLAB model of fuel cell-powered electric vehicle as well as analyzed the result.