This article is a copy from my recent assignment which is for the subject EET3196: Energy Technologies. I think I should share this article online. Thus, other online researcher can have a little information from my work. This research had been done by extracting information and data from several books and web pages related to the subject. Currently the price of petroleum has been increased to USD60/Barrel. I hope in several years soon, Malaysia’s government and Proton will take Micro fuel-cells as an alternative to fossil fuel since it can be used in automobile. In addition, it is portable and can produce up to 5kw-25kw of electrical energy in a time. I would like to show my appreciations to Dr. Lim Wee Kuan who is the supervisor for this project paper and as the lecturer for this subject.
Background/Theory of Micro Fuel-Cells:
Fuel cells have been known about more than 150 years. It was operated first by W.R.Grove in his laboratory in 1839 which produced electrical energy by combining hydrogen and oxygen, 17 years before G.Plante built the first lead-acid accumulator. W.R.Grove based his experiment on the fact that sending an electric current through water splits the water into its component parts of hydrogen and oxygen. So, W.R.Grove tried reversing the reaction - combining hydrogen and oxygen to produce electricity and water. This is the basis of a simple fuel cell.
Fuel cells are based on an electrochemical cell. They are devices which convert hydrogen and oxygen into electricity and water. But in fuel cell the reacting substances are supplied from outside and the electrodes only provide the surface for the reaction and the connection to current flow. For this reason, fuel cells do not store electric energy, but are converters of energy, and storage parameters. In a battery, the electrodes are chemically dissimilar, which causes an electric potential difference between them. In fuel cell, the electrodes are chemically similar but one is supplied with fuel and the other with an oxidant, generating potential between them. Therefore, fuel cells cannot directly be compared with batteries.
In this particular paper project, we focused on Micro Fuel-Cells which regarding about fuel-cells mainly in portable energy source. This means Micro Fuel-Cells are understood to be all fuel-cells that can be used in portable applications. The first mass market portable micro fuel-cell application is likely to be the direct methanol Fuel cell applied to cell phones. A tiny cartridge of wood alcohol can power a modern cell phone for about a month. However it does emit some CO2 in the process. This should be considered as Micro fuel-cell. But this technology hasn’t discussed well since it is still under development for mass market.
Figure1.
As described in the Figure1, by closing the electric circuit external to an electrochemical cell, a current maybe drawn from the cell, generating electrical power. The electrical energy consumed in the external circuit is generated by chemical changes between the cells. The electrical energy produced to the load is not greater than the reduction of free energy of the accompanying chemical changes between the cells.
Referring to figure1, two porous metal electrodes are separated by a space filled with an electrolyte, a fluid or solid in which the fuel or oxidant can dissociate into ionic components. Fuel and oxidant are supplied to separate electrodes, diffusing through the porous material to the electrolyte. At the anode (the negative electrode), electrons are transferred to the electrode from the electrolyte as positive ions are formed, at the cathode (the positive electrode), electrons are emitted to the electrolyte to form negative ions or to neutralize positive ions. If the electrodes are connected by electrically conducting wires to an external load, an electric current will flow and electric work will be expended on the load because the cathode electric potential is greater than that of the anode. Inside the fuel cell, the electric current completing the circuit is carried by ions moving through the electrolyte.
For instance, from figure1, hydrogen and oxygen are supplied to porous electrodes separated by an electrolyte in which the electric current is carried by hydrogen ions. In the external electric circuit, current is carried by a matching electron flow. The product of oxidation, water, evolves from anode. Let the electrolyte be Potassium Hydroxide, so the cell reaction will be:
At Anode: 2H2 + 4OH- => 4H2O + 4e-
At Cathode: O2 + 2H2O + 4e- => 4OH-
Basically, micro fuel-cells can be described as primary cells since they are designed to convert their fuel into electrical energy only once. Thus fuel need to be supplied in order to get continues electrical energy producing by the fuel-cell. Below is the table that describing the review and survey that has been done about few types of fuel-cells according to the electrolyte ionic conduction and composition of fuels.
(Table1: Courtesy of ‘Battery Technology Handbook Second Edition’ by H.A. Kiehne, published by Marcel Dekker Inc. 2003)
SOFC stands for Solid-oxide fuel cells. MCFC stands for Molten-carbonate fuel-cells. AFC stands for alkaline fuel-cells. PAFC stands for phosphoric-acid fuel cells. And PEM stands for polymer electrolyte (membrane) fuel-cells. According to the data, the lowest temperature is alkaline fuel-cell. This means the lowest heat transferred by the reactions. Thus, little energy waste as heat transferred to the environment. So, AFC is the most efficient to convert the fuel to electrical energy.
Previously work and applications on fuel-cells had been done by NASA. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. And fuel-cells are being rediscovered nowadays because they offer the possibility to convert chemical energy to electricity directly unconstrained by the Carnot efficiency. For the past few years, the main effort of previous development work and investment was concentrated on the industrialization of PEM and SOFC type fuel-cells.
Now days, regenerative fuel cells produce electricity from hydrogen and oxygen and generate heat and water as by products, just like other fuel cells. However, regenerative fuel cell systems can also use electricity from solar power or some other source to divide the excess water into oxygen and hydrogen fuel, this process is called “electrolysis.” This is a comparatively young fuel cell technology being developed by NASA and others.
Fuel cells can convert energy with up to 83% efficiency and be totally emission free. But cost and durability are the major challenges to fuel cell commercialization. However, hurdles vary according to the application in which the technology is employed. Size, weight, and thermal and water management are barriers to the commercialization of fuel cell technology. In transportation applications, these technologies face more stringent cost and durability hurdles. In stationary power applications, where cogeneration of heat and power is desired, use of PEM fuel cells would benefit from raising operating temperatures to increase performance. The key challenges include:
1. Cost: The cost of fuel cell energy systems must be reduced before they can be competitive with conventional energies.
2. Durability and Reliability: The durability of fuel cell systems has not been established. For transportation applications, fuel cell power systems will be required to achieve the same level of durability and reliability of current automotive engines
3. System Size: The size and weight of current fuel cell systems must be further reduced to meet the packaging requirements for automobiles.
4. Air, Thermal, and Water Management: Air management for fuel cell systems is a challenge because today’s compressor technologies are not suitable for automotive fuel cell applications. In addition, thermal and water management for fuel cells are issues because the small difference between the operating and ambient temperatures necessitates large heat exchangers.
5. Improved Heat Recovery Systems: The low operating temperature of PEM fuel cells limits the amount of heat that can be effectively utilized in combined heat and power (CHP) applications.
Books:
1) ‘James A.Fay, Dan S.Golomb’, Energy and The Environment, Oxford University Press. 2002.
2) ‘H.A. Kiehne’, Battery Technology Handbook Second Edition, Marcel Dekker, Inc. 2003.
Online:
1) ‘http://www.fuelcellcontrol.com’, Fuel Cell Control. ltd
2) ‘http://en.wikipedia.org/wiki/Fuel_cells’, Wikimedia Foundation, Inc.
