Desktop Assessment

Desktop Assessment of the process efficiency of electrolysis and fuel operation

Introduction

The experiment is outlined in Appendix A.

The key part of the unit is a proton exchange membrane (PEM). A layer of catalyst material has been applied to both sides of the PEM to form the anode and cathode.

Reaction in the electrolyser: 2H2O ® 2H2 + O2

* On the anode side oxygen, electrons and H+ ions are formed when voltage is applied.

2H2O ® 4e- + H+ + O2

* The H+ ions pass through the membrane to the cathode and (with the electrons flowing through the circuit) form hydrogen.

4H+ + 4e- ® 2H2

Reaction in the fuel cell: 2H2 + O2 ® 2H2O

* Hydrogen supplied to the anode is oxidised into protons and electrons.

2H2 ® 4H+ + 4e-

* The H+ ions pass through the membrane to the cathode. The electrons pass through the electrical circuit to the cathode producing electrical work. The oxygen supplied to the cathode is reduced to produce water.

4e- + 4H+ + O2 ® 2H2O

The potential difference between the two electrodes is temperature dependant and its theoretical value can be calculated from the free enthalpy of reaction DG.

Eo =

The quantity of substance n liberated at an electrode can be calculated using Faraday's law.

n =

The volume of a substance quantity n can be determined using the general gas equation.

V =

The efficiency of the fuel cell is a function of the gas pressures, the amount of gas produced, the moistness of the membrane and the temperature. (PHYWE, 1998)

Chapter: Discussion

Summary of results

Efficiency of the electrolyser = = 64.5%

The table in Appendix C shows that the electrical power consumption of the electrolyser is approximately constant with time. The theoretical hydrogen production is also constant as can be seen in the graph of gas volume against time. The observed gas volumes differ from this linear function most likely due to the accuracy of the reading.

The energy efficiency of the experimental electrolyser is 64.5%. This means that 64.5% of the electrical energy which is used to operate the electrolyser is stored in the hydrogen gas. Electrolyser efficiencies can be as high as 94%.

Losses arise as the combined result of;

* Overvoltage due to electrodes.

This is the deviation of the theoretical value from the actual.

* Internal resistance of the cell.

· Diffusion losses of the gases within the cell.

A proportion of the gases diffuses through the membrane of the electrolyser and reacts in contact with the catalyst to form water. A small proportion of the gases produced in reconverted without escaping from the cell.
Efficiency of the fuel cell = = 41.9%

Overall unit efficiency = Electrolyser efficiency × Fuel cell efficiency = 27.0%

The table shown in Appendix C shows the electrical power output of the cell is approximately constant with time. The theoretical hydrogen consumption is also constant, as shown in the graph of hydrogen consumption. The deviation of the observed value from the theoretical is most likely due to the accuracy of the reading. The drop in voltage between approximately 150-300 seconds is due to the formation of a bubble of water in the gas pipe. Ideally this should not happen and unfortunately reduces the accuracy of the results.

The energy efficiency of the fuel cell is 41.9%. This means that 41.9% of the energy stored in the hydrogen is output as electrical energy. The maximum theoretical efficiency of a fuel cell is 83% at 298K. (Donca, 2008)

Similar to the electrolyser, losses arise from the combined result of overvoltages, internal resistance and diffusion losses. As a result the acheievable terminal voltage will never reach the ideal value of 1.23 volts. Voltage losses, manifested as heat further reduce the efficiency.

Thermal management of this type of system should be investigated to improve performance. The fuel cell efficiency is directly proportional to the cell potential. The fuel cell potential difference is thermally dependant therefore the system efficiency should improve when maintained at a higher constant temperature. (Shapiro, 2005)

The efficiency of the fuel cell is dependent on the power. If the load has a high electrical resistance, the efficiency of the cell is high but it only operates under part load and the power extracted is less than it can produce. The experiment can be repeated with different resistances to determine the load resistance at which the energy efficiency is the greatest. (H-tec, 2002)

Discussion

How might the system efficiency of the fuel cell be improved?

* Water Management

The membrane must be hydrated so that water is evaporated at the same rate that it is produced. If the water evaporates too quickly, the membrane dries and resistance across it increases causing it to crack. If the water evaporates too slowly, the electrodes flood causing the reaction to stop.

* Flow Control

The hydrogen and oxygen is necessary to keep the fuel cell operating efficiently.

* Temperature Management

The same temperature must be maintained throughout the cell in order to prevent damage to the cell.

* Power Management

The power maintained between two limits in order to get maximum efficiency.

(Donca, 2008)

How useful is the system?

This type of system could provide reliable, environmentally friendly power to remote installations. The design used in this experiment would need improvement for these conditions. Under normal operation, water is consumed only on the oxygen side of the system, creating a volume imbalance. This has the potential to reduce the run time of the electrolyser for two reasons. Firstly at some point the oxygen side will run out of water. Secondly, the increasing pressure imbalance will cause damage to the electrolyser. By allowing the relative volume of the two gases to equalize, it will extend system run time and will even allow operation under system problems such as a slow oxygen leak.

What is the economic feasibility of large scale use of the technology?

Economic analysis of the system must consider the cost of the technology components, operating costs and maintenance costs. For large scale stand- alone system, a hydrogen storage tank would be required to provide energy for seasonal demand. The sizing of the photovoltaic (PV) array is an important cost factor of the system. The electrolyser may be sized to receive all the power from the array and therefore would operate at the same capacity factor which is determined by the availability of the sun. However the hydrogen storage tank is a major cost factor. (Zoulias, 2006)

If the grid is available, the output from the photovoltaic (PV) panel can be combined with the required input from the grid to provide the fuel cell with constant power. This would significantly improve economics by eliminating the cost for a hydrogen storage tank.

How does the energy retrieval of the system compare with other methods?

This system is technologically feasible, reduces emissions, noise and energy consumption from fossil fuels.

References

Appendix A - Experiment outline and apparatus setup

1. Aim/Objectives:

* To develop an understanding of the usefulness of electrolysis and fuel cells in allowing renewable energy to be delivered exactly when it is needed.

* To develop an understanding of the efficiency of fuel cell systems connected to devices that produce electrical energy from renewable energy resources.

* To understand the effect of cell losses on performance.

2. Theory

There are two fundamental problems with renewable energy.

Firstly, some renewable energy resources are intermittent.

There is a difference between the characteristic of intermittency and predictability.

Some renewable energy is entirely predictable, like tidal energy. They are governed by the laws of celestial, mechanics, which were formulated by Newton centuries ago.

But other renewable energy like wind or solar energy are not predictable.

The second fundamental problem with renewable energy is load matching:

Renewable electricity may be available in abundance on occasions when little is required, or alternatively, there may be little renewable energy available when it is most needed.

The use of electricity from renewable energy sources to produce hydrogen could allow effective storage of renewable energy for use when it is needed, thus better matching renewable electricity generation to demand. Hydrogen could also be used in fuel cells vehicles, allowing renewable sources to further displace fossil fuels.

The efficiency of this process is fundamental to its likelihood to serve as a useful addendum to the deployment of renewable energy generating plant.

This experiment uses an electrolysis cell to produce hydrogen and oxygen and a fuel cell to convert hydrogen and oxygen gas into electricity. The electrolysis is powered by a PV cell.

3. Apparatus

* Photovoltaic cell / Electrolysis cell / Fuel cell

* Bright desk lamp

* Temperature sensor probe with digital readout

* Multimeters for voltage and current measurements

4. Procedure

1). Produce a neat, clear, good-sized labelled diagram of the apparatus.

2). Shine the light from the desk lamp at the PV cell. Maintain a distance of around 400 mm between the lamp and the PV cell. Use white paper or card to isolate the electrolyser from the lamps radiation.

3). Connect the PV cell to the electrolysis cell. Make sure that both cylinders have sufficient water (topping up with the distilled, deionised water dispenser if necessary — Do Not Use Tap Water). Turn the lamp on and start recording multimeter readings, equipment temperatures and gas at 1 minute intervals for 10 minutes.

4). Switch off the lamp, disconnect the electrolyser from the PV cell and transfer the instruments to the fuel cell.

5). With sufficient gas now accumulated in the cylinders, open the screw valves connected to the fuel cell briefly to allow water to be purged from the fuel cell. This will allow a new dose of gases into the fuel cell.

6). Connect the fan to the fuel cell. Take observations of the voltage and current and carefully monitor the level of gases in the cylinders over a period of 5 minutes. Also record the temperature of the fuel cell.

7). Make any further observations necessary to drawing conclusions as to process efficiency.

Appendix B - Calculations

Standard Enthalpies and Entropies of Formation

Enthalpy

H20 (l)

-285.83

kJ/mol

H2 (g)

0

kJ/mol

O2 (g)

0

kJ/mol

Entropy

H20 (l)

69.91

J/mol K

H2 (g)

130.68

J/mol K

O2 (g)

205.14

J/mol K

Reaction

1 H20 + 2e ® 1 H2 + 0.5 O2

Reaction conditions

Ambient pressure

P

101300

Pa

Ambient temperature

t

25

T

298.14

K

Gibb's Free Energy calculation

Enthalpy change (electrolysis)

dH

285.8

kJ/mol

Entropy change (electrolysis)

dS

163.3

kJ/mol K

TdS

48.7

kJ/mol

Gibbs free energy (electrolysis)

dG

237.1

kJ/mol

Max theoretical efficiency (electrolysis)

83.0

%

Cell potential calculation

Faraday's constant

F

96485

C/mol

Number of electrons transferred

z

2

Ideal gas constant

R

8.314

J/(mol K)

Gibbs free energy

dG

237.1

kJ/mol

Standard cell potential

Ecell

-1.229

V

Chapter: Discussion

10

Appendix C - Electrolyser

Time

Temp

Voltage

Current

Power

Electrical Energy

Charge

H2 produced

H2 produced

Chemical

Cumulative

O2

Increment

Cumulative

Increment

Cumulative

(Faradays Law)

(Observed)

Energy

Efficiency

secs

degrees

V

A

W

J

J

C

C

mol

cm^3

mol

cm^3

J

%

cm^3

0

23.0

1.562

0.120

0.187

0.0

0.0

0.0

0.0

0.00E+00

0.0

0.00E+00

0.0

0.0

0.0

0.0

30

23.2

1.564

0.120

0.188

5.6

5.6

3.6

3.6

1.87E-05

0.5

0.00E+00

0.0

0.0

0.0

0.5

60

23.3

1.565

0.120

0.188

5.6

11.3

3.6

7.2

3.73E-05

0.9

2.04E-05

0.5

4.8

43.0

0.5

90

23.4

1.565

0.120

0.188

5.6

16.9

3.6

10.8

5.60E-05

1.4

2.04E-05

0.5

4.8

28.7

0.5

120

23.4

1.566

0.120

0.188

5.6

22.5

3.6

14.4

7.46E-05

1.8

4.09E-05

1.0

9.7

43.0

1.0

150

23.5

1.566

0.120

0.188

5.6

28.2

3.6

18.0

9.33E-05

2.3

6.13E-05

1.5

14.5

51.6

1.0

180

23.5

1.566

0.120

0.188

5.6

33.8

3.6

21.6

1.12E-04

2.7

8.17E-05

2.0

19.4

57.3

1.0

210

23.5

1.566

0.120

0.188

5.6

39.4

3.6

25.2

1.31E-04

3.2

1.23E-04

3.0

29.1

73.7

1.0

240

23.6

1.566

0.120

0.188

5.6

45.1

3.6

28.8

1.49E-04

3.7

1.43E-04

3.5

33.9

75.2

1.0

270

23.7

1.566

0.120

0.188

5.6

50.7

3.6

32.4

1.68E-04

4.1

1.43E-04

3.5

33.9

66.9

1.5

300

23.8

1.565

0.120

0.188

5.6

56.4

3.6

36.0

1.87E-04

4.6

1.63E-04

4.0

38.8

68.8

1.5

330

23.8

1.565

0.120

0.188

5.6

62.0

3.6

39.6

2.05E-04

5.0

1.84E-04

4.5

43.6

70.3

2.0

360

23.9

1.565

0.120

0.188

5.6

67.6

3.6

43.2

2.24E-04

5.5

2.04E-04

5.0

48.5

71.7

2.0

390

23.9

1.565

0.120

0.188

5.6

73.3

3.6

46.8

2.43E-04

5.9

2.25E-04

5.5

53.3

72.8

2.0

420

23.9

1.565

0.120

0.188

5.6

78.9

3.6

50.4

2.61E-04

6.4

2.25E-04

5.5

53.3

67.6

2.5

450

24.0

1.565

0.120

0.188

5.6

84.5

3.6

54.0

2.80E-04

6.8

2.45E-04

6.0

58.1

68.8

2.5

480

24.0

1.565

0.120

0.188

5.6

90.2

3.6

57.6

2.98E-04

7.3

2.66E-04

6.5

63.0

69.9

3.0

510

24.1

1.565

0.120

0.188

5.6

95.8

3.6

61.2

3.17E-04

7.8

2.66E-04

6.5

63.0

65.8

3.5

540

24.1

1.565

0.120

0.188

5.6

101.4

3.6

64.8

3.36E-04

8.2

2.86E-04

7.0

67.8

66.9

3.5

570

24.2

1.564

0.120

0.188

5.6

107.1

3.6

68.4

3.54E-04

8.7

2.86E-04

7.0

67.8

63.4

4.0

600

24.3

1.564

0.120

0.188

5.6

112.7

3.6

72.0

3.73E-04

9.1

3.07E-04

7.5

72.7

64.5

4.0
Appendix D - Fuel Cell

Time

Temp

Voltage

Current

Power

Electrical Energy

Charge

H2 consumed

H2 consumed

Chemical

Cumulative

O2

Increment

Cumulative

Increment

Cumulative

(Faradays Law)

(Observed)

Energy

Efficiency

secs

degrees

V

A

W

J

J

C

C

mol

cm^3

mol

cm^3

J

%

cm^3

0

22.5

0.738

0.06

0.044

0.0

0.0

0.0

0.0

0.00E+00

0.0

0.00E+00

0.0

0.0

0.0

4.0

30

22.5

0.734

0.06

0.044

1.3

1.3

1.8

1.8

9.33E-06

0.2

2.04E-05

0.5

-4.8

27.3

4.0

60

22.5

0.709

0.06

0.043

1.3

2.6

1.8

3.6

1.87E-05

0.5

2.04E-05

0.5

-4.8

53.6

4.0

90

22.5

0.243

0.03

0.007

0.2

2.8

0.9

4.5

2.33E-05

0.6

4.09E-05

1.0

-9.7

29.1

4.0

120

22.5

0.120

0.01

0.001

0.0

2.9

0.3

4.8

2.49E-05

0.6

4.09E-05

1.0

-9.7

29.4

4.0

150

22.4

0.004

0.01

0.000

0.0

2.9

0.3

5.1

2.64E-05

0.6

4.09E-05

1.0

-9.7

29.4

3.8

180

22.4

0.002

0.06

0.000

0.0

2.9

1.8

6.9

3.58E-05

0.9

4.09E-05

1.0

-9.7

29.5

3.8

210

22.4

0.001

0.06

0.000

0.0

2.9

1.8

8.7

4.51E-05

1.1

4.09E-05

1.0

-9.7

29.5

3.8

240

22.4

0.001

0.06

0.000

0.0

2.9

1.8

10.5

5.44E-05

1.3

4.09E-05

1.0

-9.7

29.5

3.8

270

22.4

0.000

0.06

0.000

0.0

2.9

1.8

12.3

6.37E-05

1.6

4.09E-05

1.0

-9.7

29.5

3.8

300

22.4

0.000

0.06

0.000

0.0

2.9

1.8

14.1

7.31E-05

1.8

4.09E-05

1.0

-9.7

29.5

3.8

330

22.3

0.745

0.06

0.045

1.3

4.2

1.8

15.9

8.24E-05

2.0

1.02E-04

2.5

-24.2

17.3

3.5

360

22.3

0.744

0.06

0.045

1.3

5.5

1.8

17.7

9.17E-05

2.2

1.02E-04

2.5

-24.2

22.9

3.5

390

22.3

0.742

0.06

0.045

1.3

6.9

1.8

19.5

1.01E-04

2.5

1.02E-04

2.5

-24.2

28.4

3.0

420

22.3

0.739

0.06

0.044

1.3

8.2

1.8

21.3

1.10E-04

2.7

1.23E-04

3.0

-29.1

28.2

3.0

450

22.3

0.735

0.06

0.044

1.3

9.5

1.8

23.1

1.20E-04

2.9

1.43E-04

3.5

-33.9

28.1

3.0

480

22.3

0.732

0.06

0.044

1.3

10.8

1.8

24.9

1.29E-04

3.2

1.43E-04

3.5

-33.9

32.0

2.8

510

22.2

0.728

0.06

0.044

1.3

12.2

1.8

26.7

1.38E-04

3.4

1.63E-04

4.0

-38.8

31.4

2.5

540

22.2

0.724

0.06

0.043

1.3

13.5

1.8

28.5

1.48E-04

3.6

1.63E-04

4.0

-38.8

34.7

2.5

570

22.2

0.719

0.06

0.043

1.3

14.8

1.8

30.3

1.57E-04

3.8

1.84E-04

4.5

-43.6

33.8

2.0

600

22.1

0.714

0.06

0.043

1.3

16.0

1.8

32.1

1.66E-04

4.1

1.84E-04

4.5

-43.6

36.8

2.0

630

22.1

0.708

0.06

0.042

1.3

17.3

1.8

33.9

1.76E-04

4.3

2.04E-04

5.0

-48.5

35.7

2.0

660

22.1

0.703

0.06

0.042

1.3

18.6

1.8

35.7

1.85E-04

4.5

2.25E-04

5.5

-53.3

34.9

2.0

690

22.1

0.695

0.06

0.042

1.3

19.8

1.8

37.5

1.94E-04

4.8

2.25E-04

5.5

-53.3

37.2

2.0

720

22.1

0.686

0.06

0.041

1.2

21.1

1.8

39.3

2.04E-04

5.0

2.25E-04

5.5

-53.3

39.5

1.5

750

22.1

0.677

0.06

0.041

1.2

22.3

1.8

41.1

2.13E-04

5.2

2.45E-04

6.0

-58.1

38.3

1.5

780

22.1

0.661

0.06

0.040

1.2

23.5

1.8

42.9

2.22E-04

5.4

2.66E-04

6.5

-63.0

37.3

1.5

810

22.1

0.650

0.05

0.033

1.0

24.4

1.5

44.4

2.30E-04

5.6

2.66E-04

6.5

-63.0

38.8

1.0

840

22.1

0.634

0.05

0.032

1.0

25.4

1.5

45.9

2.38E-04

5.8

2.66E-04

6.5

-63.0

40.3

1.0

870

22.1

0.615

0.05

0.031

0.9

26.3

1.5

47.4

2.46E-04

6.0

2.86E-04

7.0

-67.8

38.8

1.0

900

22.0

0.586

0.05

0.029

0.9

27.2

1.5

48.9

2.53E-04

6.2

2.86E-04

7.0

-67.8

40.1

1.0

930

22.0

0.565

0.04

0.023

0.7

27.9

1.2

50.1

2.60E-04

6.4

2.86E-04

7.0

-67.8

41.1

1.0

960

22.0

0.541

0.04

0.022

0.6

28.5

1.2

51.3

2.66E-04

6.5

3.07E-04

7.5

-72.7

39.3

1.0

990

22.0

0.518

0.04

0.021

0.6

29.2

1.2

52.5

2.72E-04

6.7

3.07E-04

7.5

-72.7

40.1

1.0

1020

22.0

0.498

0.04

0.020

0.6

29.7

1.2

53.7

2.78E-04

6.8

3.27E-04

8.0

-77.5

38.4

0.5

1050

22.0

0.481

0.04

0.019

0.6

30.3

1.2

54.9

2.84E-04

7.0

3.27E-04

8.0

-77.5

39.1

0.5

1080

22.0

0.466

0.04

0.019

0.6

30.9

1.2

56.1

2.91E-04

7.1

3.27E-04

8.0

-77.5

39.8

0.5

1110

22.0

0.451

0.04

0.018

0.5

31.4

1.2

57.3

2.97E-04

7.3

3.27E-04

8.0

-77.5

40.5

0.0

1140

22.1

0.441

0.04

0.018

0.5

32.0

1.2

58.5

3.03E-04

7.4

3.27E-04

8.0

-77.5

41.2

0.0

1170

22.1

0.427

0.04

0.017

0.5

32.5

1.2

59.7

3.09E-04

7.6

3.27E-04

8.0

-77.5

41.9

0.0

1200

22.1

0.010

0.03

0.000

0.0

32.5

0.9

60.6

3.14E-04

7.7

3.27E-04

8.0

-77.5

41.9

0.0

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