Comparison: Fuel cell – combustion engine

Hydrogen generator with metal hydride storage

Solutions 1 Fuel cell application II: Fuel cell car C.9

5 Comparison: Fuel cell – combustion engine

5.1 The fuel cell, in connection with an electric motor, represents a feasible replacement for the combustion engine (e.g. in the automobile).

Describe the advantages of the combination of fuel cell and electric motor as against the combustion engine on the basis of energy transformation chains.

Discuss the relevant efficiencies, to which the respective transformation chain is subject. Which advantages concerning the operating temperature result for the fuel cell?

In both cases the chemical energy of the fuel is to be converted into mechanical energy to the drive of the vehicle. In the combustion engine this takes place through the intermediate step of the thermal energy (warmth), with the fuel cell this occurs through electricity.

The energy transformation chains are subject however to different efficiencies:

The transformation of the chemical energy into electrical with the help of the fuel cell is subject to the Gibbs efficiency (theoretical electrical efficiency of a fuel cell), which indicates the relationship of free reaction enthalpy ^RG to the reaction enthalpy ^RH:

R R

G η = ^∆ H

Gibbs ∆ .

With the usual operating temperature of 50 °C – 120 °C this theoretical efficiency is above 90%

(for gaseous product water) and decreases linearly with rising temperature.

The energy transformation in the combustion engine is subject to the Carnot efficiency, which is computed from the upper process temperature T1 and lower process temperature T2:

2 1

1 T η_Carnot = −T .

The higher this temperature difference, the higher is the efficiency. However the breakdown of materials limits the temperature spread and a maximum Carnot efficiency of 40 % – 50 % results.

The Carnot efficiency is like the Gibbs efficiency only a theoretical value, thus a maximally attainable upper limit. Still all losses arising in the system must be taken off from this maximum value. There the Gibbs efficiency like the Carnot efficiency, lies in the use of the fuel cell technology that has the potential of about twice as large an initial efficiency.

Additionally the materials used by fuel cells lie in a more favorable operating temperature range, substantially reducing the thermal stress.

Solutions 8 Fuel cell application II: Fuel cell car C.9

5.2 With the use of fuel cells in motor vehicles the required fuels will change.

List different storage possibilities for hydrogen and compare these with conventional fuels for combustion engines using characteristic data and suitable graphics.

Hydrogen can be stored in its pure form (compressed gas, liquid gas, metal hydride storage), or stored chemically, in technically reproducible hydrogen-rich substances (e.g. methanol).

Depending upon storage type different gravimetric and volumetric energy densities are reached.

We should always consider expenditure on production and storage. For example with the storage of the hydrogen in liquid form about 30% of the stored energy is needed for cooling. In the following table the gravimetric and volumetric energy densities of the specified hydrogen reservoirs are compared to the corresponding values of gasoline and diesel. The following graphic shows the differences clearly.

Fuel Volumetric energy density (kWh/L) Energy density (kWh/l and kWh/kg) Volumetric energy density

Gravimetric energy density

Solutions 9 Fuel cell application II: Fuel cell car C.9

Gasoline and diesel have the highest energy densities, thus the most energy per volume and/or per mass. Methanol compares favorably, however demands a complex reformation to obtain the hydrogen. The disadvantages of the energy-intensive storage of liquid hydrogen have already been mentioned. A possibility of storing hydrogen in a simple manner is metal hydride storage. Because of its high weight it is suitable however only for small applications; too heavy for use in the automobile. In the end compressed gas storage, which does not have a

particularly high power density, remains a technically simply and developed method.

5.3 On a fuel cell vehicle a fuel storage is to be specified so that the vehicle with a single tank filling can travel the same distance as a vehicle with combustion engine (gasoline) with 30 L of fuel in its tank . The overall efficiency of the fuel cell vehicle is 40 %, that of the vehicle with combustion engine 20 %.

Compare the mass and volume of compressed gas, liquid gas and metal hydride storage for hydrogen, as well as a methanol tank, and the equivalent values of a gasoline tank.

Discuss your result and then choose a suitable storage. Under which simplifications, related to the hydrogen reservoirs, did you perform the calculations and how does this affect the result?

Assume the volume of the gasoline tank itself is negligible:

Vgasoline = Vtank = 30 L

:LWKWKHHPSW\ZHLJKWRIWKHWDQNDUELWUDULO\FKRVHQWREHNJDQGWKHGHQVLW\ ^gasoline = 0.78 kg/dm³ a total mass results:

mtotal = mtank ^gasoline • Vgasoline = 25.9 kg.

Using the lower heat value LHV = 42 MJ/kgDQGWKHGHQVLW\ ^gasoline the energy content of the gasoline can be determined simply as:

Egasoline ^gasoline • Vgasoline • LHVgasoline = 273 kWh

Considering the efficiency differences of both propulsion principles the masses and volumes of the storage can be computed using the power densities specified in 5.2. For the methanol tank, similar to the gasoline tank, the volume of the tank itself is ignored and an empty weight of 2.5 kg was used.

Solutions 10 Fuel cell application II: Fuel cell car C.9

Storage volumes and mass are lowest for the vehicle with a combustion engine. Due to the significantly better efficiency of the fuel cell vehicle, the methanol tank is however only a little larger and heavier. Also liquid storage of hydrogen represents a practical alternative. In the case of compressed gas storage the range of the vehicle would have to be reduced or a much larger storage chosen. The relatively small volume of a metal hydride storage is favorable, but the mass of the system is so high as to make it impractical.

With this comparison the energies needed by the respective storage methods were neglected.

To maintain liquid hydrogen storage significant energy expenditure is necessary, for example to guarantee cooling. When using methanol, energy is needed to loosen bound hydrogen from the molecular lattice (reformation). Therefore additional fuel is necessary, in order to meet such power requirements.

In summary to select a suitable storage system, you would consider volumes and mass for the methanol method. If one wants to avoid the reformation, the choice is clearly for the liquid gas storage. But because this is achieved at high technical cost, it may be appropriate to return to the larger, heavier, but more simple compressed gas storage.

Comparison of fuel storage methods

Solutions 11 Fuel cell application II: Fuel cell car C.9

5.4 Compare the structure of the drive train of a fuel cell vehicle with that of a combustion engine vehicle.

Which advantages result for the fuel cell vehicle?

To operate a vehicle with a combustion engine a clutch as well as a transmission is necessary, since with the combustion engine only a limited range of engine speeds can be used. Since at rotational speed 0 no torque is delivered, at least an idling speed must always be maintained. In order to move in the optimal speed range of the engine, a complex transmission is necessary.

The power transmission to the wheels must be through shafts. Different distribution of forces or different numbers of revolutions of individual wheels can be realized only through power-absorbing differentials.

In a fuel cell vehicle the drive is achieved by electric motors. Since these produce high torque over the entire speed range of the motor, a clutch and transmission are not necessary. Thus wear-intensive units can be omitted, and the structure of the engine will be significantly simpler and more durable. An additional advantage is that each wheel can be propelled individually, thus shafts and differentials in the power transmission would be unnecessary, the distribution of forces being regulated electronically. Many years of experience already exist, e.g. in building rail-mounted vehicles.

The larger volumes and masses for hydrogen storage could be compensated by savings in the power train.

5.5 A fuel cell vehicle is to be propelled with an asynchronous engine.

How would you control the rotational speed and how could the power be delivered?

To control the number of revolutions of asynchronous machines the frequency voltage control is best suited. With this control it is possible to accelerate away from a stop in minimal time up to the rated speed. A further increase of the number of revolutions is possible, however with a reduction of the torque.

To implement this control a variable DC voltage is necessary, over which the working force level can be stepped. In addition a DC—DC transducer is necessary, which converts a variable input voltage into a variable output voltage. The output voltage should always correspond to a pre-set value. For the frequency control an inverter must be used, which sets the desired frequency and thus the number of revolutions of the motor.

Work sheet 1 The basic functions of the fuel cell system D.1

Voltage converter VC100 (optional) 621 Traffic light TL10 (optional) 622

Arrange the devices as in the following diagram:

Elektronische Last

Work sheet 2 The basic functions of the fuel cell system D.1

Name Grade / Course Date

Task:

Learn about the FC50 Fuel Cell System and its components by stepping through their operation. Notice how the system reports operation errors and learn how to correct them.

Execution:

Note: This procedure shows you the operating modes of the individual components and later helps you to easily recognize and correct errors. You should follow the sequence step by step as indicated here. If you notice a mistake or omission in the procedure steps, you should nevertheless do the steps as indicated in order to learn the behavior of the system in the event of an error.

To solve the following problems and answer the questions it will be necessary to refer to the Component Descriptions of the devices used.

1 Installation and start-up of FC50, EL200 and hydrogen supply:

When setting up and starting the equipment, follow the instructions provided in Part A: Component Descriptions for the individual components, especially the safety instructions.

During experiments, ensure the area has adequate ventilation and keep away from sources of ignition.

1.1 Place the modules into the mounting frame arranged as shown in the above illustration.

Use the AC power cord to connect the EL200 Electronic Load to the source of AC power. (Connection is on the right side behind the front panel.) Ensure the toggle switch on the EL200 front panel is OFF.

Use two short test leads to connect the FC50 with the EL200, paying attention to the polarity.

1.2 On the FC50, set the main (toggle) switch to ON and press the START button.

Which problem occurs and how can it be corrected?

Work sheet 3 The basic functions of the fuel cell system D.1

1.3 After you have corrected the problem, press the START button again.

Which problem now occurs and how can it be corrected? Use the error list in A.3 Fuel Cell Module FC50 to explain.

1.4 Press the START button again. For approx.10 seconds a system test is performed. If this is successful, the displays are illuminated. The FC50 is now ready for use.

1.5 Turn the main power switch located behind the EL200 front panel on. The ‘Power’

display is illuminated.

Turn the 10-turn potentiometer, in order to apply a load current.

What does this show?

Work sheet 4 The basic functions of the fuel cell system D.1

Name Grade / Course Date

1.6 The load current previously set on the potentiometer is drawn from the Fuel cell and can be read on the appropriate display. The power Pload absorbed by the electronic load is shown in the EL200 display window.

1.7 Cooling fans supply air necessary for the operation of the fuel cell. The speed of the fans can be adjusted to suit the load current automatically or manually. Use the knob beside the display ’ Fan Power ’, to set a fan power between 5 % and 100 %.

Try setting different operating points on the EL200 and try to set an appropriate fan power. Watch how the system reacts when you change these settings.

1.8 Now apply a load current of 9 A and reduce the fan power slowly to 5 %. Watch the stack voltage display.

What did you observe? Use the “Error messages” list (see A.3 Fuel Cell Module FC50) in order to explain why the FC50 shut off.

1.9 Switch the FC50 off. Ensure that the potentiometer of the EL200 is set to zero and the toggle switch on the front panel is OFF.

1.10 If you are not making further measurements with the system, proceed to shut down and switch off the system as follows:

• On the EL200, turn the potentiometer to zero, set the toggle switch to ^OFF, and turn off the main power switch behind the front panel.

• On the FC50, turn the fan control knob to ^AUTO and turn the main switch OFF.

• Follow the correct procedure to shut down your hydrogen supply, as described in sections A.8, A.9 or A.10 as appropriate.

• Remove the hydrogen supply from the FC50 by opening the quick-coupler.

Work sheet 5 The basic functions of the fuel cell system D.1

2 Installation and start-up of COMPUTER-SUPPORTED Operations

To operate the FC50 in the COMPUTER-SUPPORTED mode, it is necessary to have a computer with RS232 interface on which you have installed the provided software. Refer to operation of the software in the section A.7 "Control

Software".

2.1 Connect the port ’ RS232 ’ of the FC50 to the appropriate interface on your computer using the provided long 9-pin data cable.

Start the program ’FC50 software’ on your computer selecting the menu option ’user Interface’ and click the ’START’ button. Follow the instructions in the reporting window of the control software.

2.2 When you are requested from the software, switch on the FC50 and start it.

Which problem occurs and how can it be corrected?

2.3 The measured values of the FC50 are now shown on both the module and on your computer. But you can adjust the fan power only through the software.

Work sheet 6 The basic functions of the fuel cell system D.1

Name Grade / Course Date

2.4 Also, setting the load current is only possible through the software. Set a value of Iload = 2 A

Why doesn’t the EL200 react?

2.5 In the ’user Interface’ of the FC50 software click the label ’data display’. Observe the behavior of the different fuel cell parameters when you change the load current.

2.6 When you are through with the system, proceed to shut down and switch off the system as follows:

• Terminate the FC50 software. The FC50 sees the interruption of communication and displays an error.

• Turn the potentiometer of the EL200 to zero, set the toggle switch to OFF and switch off the main switch behind the front panel.

• Turn the knob for the fan power to AUTO and turn the FC50 main switch OFF.

• Put the hydrogen supply out of operation correctly.

• Remove the hydrogen inlet to the FC50 by disconnecting the quick-coupler.

Work sheet 7 The basic functions of the fuel cell system D.1

3 Installation and start-up of VC100 and TL10 (optional)

This part can only be performed if the voltage converter VC100 and the traffic light TL10 is available. It does not matter if the FC50 is operating in COMPUTER-SUPPORTED mode or in manual mode.

Follow the safety instructions provided in section A: Component Descriptions for the individual components.

3.1 In the following the fuel cell system is self-powered. Switch the FC50 off and remove the AC power pack. Instead connect the 12 V DC input of the FC50 to the “Parasitic Load” output of the VC100 using the provided 3-pin cable.

From the “Available Power” output of the VC100, the traffic light TL10 or other loads can be supplied.

Use the provided short 9-pin RS485 data cable to connect the VC100 to the unused interface port of the EL200, to provide communication in the

COMPUTER-SUPPORTED mode.

Start the FC50. Which error occurs?

Work sheet 8 The basic functions of the fuel cell system D.1

Name Grade / Course Date

3.2 Restart the FC50 and wait for the system test to complete. In the VC100 display

‘parasitic load' see the power consumed by the FC50. In the display 'available power' see the power consumed by the attached load. Briefly try out the traffic light TL10 and observe the ‘available power’ display:

At switch position ON all lamps shine; at position AUTO, a normal traffic light sequence occurs. In the middle position the device is off.

3.3 The electronic load EL200 can be operated in parallel with the traffic light. Gradually increase the load current of the fuel cell using the EL200 potentiometer. Try to reach the maximum EL200 load current.

Explain why the FC50 switches off. What has to be considered when restarting?

3.4 When you are through with the system, proceed to shut down and switch off the system as follows:

• Turn the potentiometer of the EL200 to zero, set the toggle switch to OFF and switch off the main switch behind the front panel.

• Turn the knob for the fan power to AUTO and turn the FC50 main switch OFF.

• Put the hydrogen supply out of operation correctly.

• Remove the hydrogen inlet to the FC50 by disconnecting the quick-coupler.

Work sheet 9 The basic functions of the fuel cell system D.1

4 Summary

Considering the problems and the associated error messages again, look at the error list in section A.3 Fuel Cell Module FC50. Explain why it is useful to divide the errors into two groups: start-up errors and operating errors. Give at least one example of each group.

Work sheet 1 The characteristic curve of a fuel cell D.2

Required devices:

Description Item

I 630

II 642

Hydrogen supply (alternates)

III 652

Fuel cell FC50 610

Electronic load EL200 620

Arrange the devices as in the following diagram:

Electronic Load EL200

Fuel Cell FC50 Hydrogen supply

Work sheet 2 The characteristic curve of a fuel cell D.2

Name Grade / Course Date

Task:

In this experiment we determine the voltage-current characteristic of a fuel cell and plot a power-current diagram. This provides a basic knowledge of the behavior of a fuel cell. The results can be used to size and design fuel cell stacks.

Execution:

1 Set-up

When setting up and starting the equipment, follow the instructions provided in section A: Component Descriptions for the individual components, especially the safety instructions.

During experiments, ensure the area has adequate ventilation and keep away from sources of ignition.

1.1 Connect the AC power pack cable to the 12V= DC power input on the FC50 Fuel Cell.

Connect the other end of the AC power pack to a source of AC power.

On the front panel of the EL200 Electronic Load ensure the toggle switch is OFF. Use the AC power cord to connect the EL200 to a source of AC power; then turn on the main power switch located behind the EL200 front panel.

1.2 Use two short test leads to connect the FC50 with the EL200, paying attention to the polarity.

1.3 Attach the hydrogen supply quick-coupler to the FC50. Connect the 9-pin plug of the

In document Instructor Experiment Guide (Page 151-200)