Thursday, August 25, 2005
Chemical could improve PEMFC efficiency
The study was published in the Journal of the American Chemical Society. From the article:
Further investigation into 1H-1,2,3-triazole and its derivatives, especially copolymers with suitable acidic groups, may lead to new membranes with excellent properties for a new generation of PEM fuel cells to be operated at temperatures above 100C with much higher energy efficiencies.
Sunday, August 21, 2005
Current status of hydrogen storage
Hydrogen storage is the biggest technical challenge that we have to overcome if we want to be driving hydrogen fueled cars. The problem with hydrogen is that it’s the smallest molecule in the world, with 2 protons and 2 electrons, and so it’s very difficult to store. You can’t pump hydrogen through a natural gas pipeline for instance because it will diffuse through the steel and you’ll end up losing most of your hydrogen before you can use it. You have to be very careful what materials you use to store hydrogen too because, just say you’re storing it as a gas, the hydrogen molecules in the gas cylinder are continuously moving around and hitting the sides of the cylinder and so it actually structurally weakens the gas cylinder. So there’s a lot challenges in storing hydrogen. There are a couple of ways of doing it though. Of course there’s the compressed hydrogen gas storage or you can liquefy hydrogen and store it as a liquid, or you can store hydrogen in carbon nanotubes or you can store it in a solid like sodium borohydride for example, which has a chemical formula of NaNH4, and when you react NaBH4 with a base, you end up with hydrogen gas and some other things.
Here's a podcast about the current status of hydrogen storage.
As explained in the podcast, the Department of Energy is focusing on compressed hydrogen (5000-10000psi) and liquefied hydrogen for near term hydrogen fueled vehicles. The long term plan is to develop solid state hydrogen storage, which means metal hydrides and chemical hydrides. The overall goal is to allow vehicles to travel for 300 miles without refueling.
More specifically, by 2010, the DOE wants to develop and verify on-board hydrogen storage systems achieving 2 kWh/kg (6 wt%), 1.5 kWh/L, and $4/kWh.; by 2015, 3 kWh/kg (9 wt%), 2.7 kWh/L, and $2/kWh.
Here's a quick overview of some of the hydrogen storage technologies that the D.O.E. is working on:
The simplest way of storing hydrogen is to compress hydrogen and stuff it into a gas cylinder at very high pressures. Right now we can store hydrogen gas pretty easily at a pressure of 5000psi. And obviously the higher the pressure, the more hydrogen you can store. But at 5000psi, we don’t meet that 300 mile range that the D.O.E is targeting, and so the solution is to use higher pressures. Right now there’s work going towards 10000psi compressed hydrogen gas tanks, which is progressing very nicely and will get us up to the 300 mile range, and it’ll reach the 6 wt% hydrogen requirement, but there are extra costs associated with the materials that are needed to get up to those pressures and the fabrication methods don’t seem to be very flexible so meeting the cost target is the big issue. Another problem with compressed hydrogen is that it’s very expensive to fill up. Compressing hydrogen to 10000psi is not cheap, energy wise, for anybody interested, going from atmospheric pressure, which is 14.5psi to around 10000psi requires 2.21kWh/kg of H2, that’s the equivalent of 10 cents Canadian per kg of hydrogen, for me at least, which would add about 50 cents to the cost of a fill up because 5kg of hydrogen will do you about 300 miles these days. An extra 50 cents doesn’t sound that bad, but it would be about 25% of the cost of filling up with hydrogen, which is targeted to cost $1.5/kg.
A lot of people are also concerned of having compressed gas cylinders in their cars though, because they think that if they get into an accident, then having 5000psi or 10000psi gas cylinders would be just like having a rocket in their car, if the gas cylinder ruptures, then there is trouble. I think it’s probably a valid fear, the gas tanks are pretty strong in some locations but at the neck they can be very weak, I’ve heard stories of these gas cylinders going through huge concrete walls and actually there’s a scene from the last James Bond movie where he’s chasing somebody in a hospital in Cuba and blows a hole through the wall with a gas cylinder, that’s what it would be like in your car. But on the other hand, hydrogen leaking through the gas cylinder isn’t really a problem.
Current cost status:
5000psi tanks: $15/kWh (73% reduction in costs)
10000psi tanks: $18/kWh (78% reduction in costs)
Hydrogen can be stored as a liquid in a cryogenic container. But first you have to cool it down to -240C, which is -400deg F, which is 33 Kelvin. That’s cold. This has two problems, first of all the amount of energy required to liquefy the hydrogen, and also what’s known as the boil-off effect. The energy required to cool hydrogen from room temperature to -240C is somewhere between 1/3rd to 1/2 of the energy content per unit weight of hydrogen. So that means that if you’re going to be liquefying hydrogen, you’ll only be able to use half of the potential energy of hydrogen, which doesn’t start you off on the right foot. The idea of liquefying hydrogen is pretty much dead. To make things worse, at those low temperatures, hydrogen will boil off, even if the cryogenic container has the best insulation, hydrogen will boil off and the gas will leak out of the container. So it’s not an option for anybody except NASA.
$6/kWh (33% reduction in costs)
Solid State Systems: Metal Hydrides and Chemical Hydrides
Ok, this type of hydrogen storage can get pretty complicated and I’m not up to speed on it, I know there’s at least one listener out there who used to work on metal hydrides, maybe he could explain it to us. So from what I gather, there are metal hydrides and complex hydrides, and I think the complex hydrides are sometimes also called chemical hydrides. Basically what happens for metal hydrides is that the metals will absorb hydrogen, and in the process of absorbing hydrogen, lots of heat is released. If you want to get that hydrogen back, then you have to supply heat and the hydrogen will be released for you to use in your car. So you would have some type of metal chunk in your car supplying hydrogen when it’s needed. The big challenge here is to find the right combination of metals that will release hydrogen by supplying the minimum amount of heat. For example, MgH2 requires 25% more energy to release the hydrogen than what’s contained in the hydrogen, so we can’t use that. Then there’s a group called the complex metal hydrides, which are looking more promising. The reason why they’re better involves a lot of chemistry which I don’t want to learn about or talk about, but one promising complex metal hydride is lithium boro-hydride, or LiBH4, which can hold about 18% hydrogen by weight. So remember the goal by 2010 is to hold 6% hydrogen by weight, but that’s 6% by weight of the entire system and this 18% is only by weight of the complex metal hydride, it doesn’t include any balance of plant components that might be needed. But nevertheless, it sounds promising. Another promising material is NaBH4, my friend Flora Lo does research on NaBH4, what happens is you react it with a base and you get hydrogen and some other stuff that has sodium and boron in it. The big problem is getting the right rate of hydrogen and that depends on how fast the NaBH4 reacts with the base. So, as far as I know, NaBH4 is one of the most promising long hydrogen storage methods in the long term because it can be recycled and it can hold lots of hydrogen.
There are lots, mostly because these systems are still very much in the research and development stage. They include hydrogen capacity, right now they just can’t store enough, the regeneration of these hydrides and also the lifetime of these hydrides isn’t very long right now, you can’t go through many cycles. Also, people just don’t understand how they work, there aren’t any test protocols or independent testing facilities, and it’s still a little tricky to envision how these hydrides would work exactly, how would you refill your chemical or metal hydride, it would have to be a lot different concept than going up to a gas station and filling up with hydrogen, it might involve taking the spent hydride out, physically, and trading it in for a hydride full of hydrogen.
Metal Hydrides Current status:
$16/kWh (75% reduction in costs)
Chemical Hydrides Current status:
$16/kWh (50% reduction in costs)
Figure 1 and Figure 2 below show the improvements over today's technology that is required for each of the hydrogen storage techniques to meet the 2010 and 2015 targets respectively.
Figure 1 and Figure 2 below show the improvements over today's technology that is required for each of the hydrogen storage techniques to meet the 2010 and 2015 targets respectively.
Figure 1. Improvement over today's technology that is required to meet DOE hydrogen storage targets by 2010.
Figure 2. Improvement over today's technology that is required to meet DOE hydrogen storage targets by 2015.
Liquefied hydrogen looks good in these two figures but the problem is the energy associated with cooling the hydrogen to -240C (-400F), which is between 1/3rd - 1/2 of the hydrogen's higher heating value (142 MJ/kg). The cost/kWh doesn't include the cost of cooling the hydrogen, just the cost of the materials.
Figure 3 and 4 show the current status of hydrogen storage technologies in absolute terms.
Figure 3. The current status of today's hydrogen storage technologies in voulmetric and gravimetric terms.
Figure 4. The cost of today's hydrogen storage technologies.
Hydrogen storage is definitely a big concern. But all of these targets that have been set by the DOE are to make hydrogen fuel competitive with gasoline so that the end user will have a minimal behavioral change. What if our behavior now is wrong though?
Friday, August 19, 2005
Fuel cell powered fork-lifts move ahead
Lead acid batteries used today typically last 4 to 8 hours and large distribution centers often change batteries over 300 times per day resulting in lost productivity and increased operational cost. Cellex’s fuel cell power units eliminate battery changing, run longer than batteries, maintain consistent power delivery to the lift truck and refuel in one minute.
There is also a strong economic case for using a PEM fuel cell in a fork lift because they replace about 3 lead-acid batteries each. Ballard Power systems will be providing 27, 4.8kW PEM fuel cells to Cellex, which Cellex will use in their Beta fork lift version trials with Wal-Mart. Cross posted to theWatt.com.
Hydrogen in your future?
I think $1.99/kg for H2 is very cheap. A recent project done by yours truly (and others here at theWatt) shows that it's probably more like $14/kg, of course it depends on how the H2 is made.
The efficiency of fuel cells
First of all, we have to define the Gibbs free energy. This is defined as the "energy available to do external work, neglecting any work done by changes in pressure and/or volume". In a fuel cell, external work involves moving electrons around an external circuit and any "mechanical work" is not used by the fuel cell directly, it is possible to combine a fuel cell with a turbine in a combined cycle system though.
This post isn't a thermo lesson, there are plenty of resources from plenty of people who know plenty more about thermodynamics than I do. But to calculate the Gibbs free energy of the reactions occuring in a fuel cell, you have to first know what reactions are occuring in the fuel cell. We'll take the PEMFC as an example:
|1/2O2+2H++2e- <--> H2O|
|H2 <--> 2H++2e-|
|H2+1/2O2 <--> H2O|
Gibbs free energy is defined as: dG = dH - TdS
To calculate the Gibbs free energy (dG), we do the old:
dG = n*gf products - n*gf reactants
trick, where n is the number of moles and gf is the Gibbs free energy of formation.
So for the PEMFC, we'll have dG = 1 * gf, H2O - 1/2 * gf,O2 - 1 * gf,H2
gf changes with temperature and state, which means that in order to calculate it, you'd need to do some integrals and find some heat capacities as a function of temperature and it's not stuff you'd want to do by hand (it's not too hard but it's just long) and probably not stuff that most people who might possibly be reading this would ever want to know about. Lots of thermodynamics text books will have tables of these values as a function of temperature.
So, for convenience sake, here is the Gibbs free energy for the PEMFC reaction:
H2 + 1/2O2 <--> H2O
|Phase of water||Temp (C)||dGrxn (kJ/mole)|
If there are no losses in the fuel cell, or if the processes are reversible, then all of the Gibbs free energy is converted into electrical energy, but this never happens.
The efficiency of the fuel cell is defined as:
Eff = dG/dH or Eff = 1 - TdS/dH
The maximum energy from any fuel is achieved by burning it, this is the "enthalpy of formation", dH. This is found in a similar way for the Gibbs free energy of reaction, I'd recomend looking it up in tables though because it depends on temperature and you'd have to do some more integration.
So, what I'm really leading towards is a comparison of the maximum theoretical efficiencies between a heat engine (using the carnot cycle) and a fuel cell. Of course, the Carnot efficiency is (T1-T2)/T1 where T1 is the operating temperature of the heat engine and T2 is the temperature of the exhaust (assuming exhaust is at 25C):
So then, at low temperatures, fuel cells have a high "theoretical" efficiency compared to a heat engine, as the temperature increases though, the "theoretical" fuel cell efficiency decreases and the "theoretical" heat engine efficiency increases. At around 750C, the Carnot efficiency and the fuel cell efficiency intersect.
Remember though that these are maximum theoretical efficiencies. They are very hard to reach. It gets even a little more complicated as well. In practice, high temperature fuel cells are actually more efficient than low temperature fuel cells because the losses (explained in another post) are smaller at higher temperatures.
So, there you go, that's all about fuel cell efficiencies. Real world efficiencies for fuel cells are around 35-45%. This can be bumped up to 80% for high temperature SOFC fuel cells when recycling the waste heat.
Honda FCX (fuel cell vehicle) review
The NY Times has a review of the Honda FCX (for Fuel Cell Experimental) car which is the first zero-emission (this is debatable), hydrogen-driven vehicle to be certified by both the Environmental Protection Agency and the State of California for everyday commercial use. The journalist was given a week of unsupervised fun with the car. Honda says that the car cost about $1-2 million to develop, there are only 20 of them in existence. The engine is an 86 kilowatt (107 horsepower) fuel cell coupled with an ultracapacitor which is charged from regenerative brakes, two 5000psi hydrogen tanks are used in the car which gives a travel range of about 190 miles (305km). The vehicle has a top speed of 93 miles an hour and takes about 11 seconds to accelerate to 60 miles an hour. The car can start up at -20C (-4F), but the startup time under these conditions was not given. Fuel efficiency of 57 miles per kilogram of hydrogen (this would be roughly equivalent to 57 miles per gallon of gasoline since 1 gallon of gasoline has about the same energy content as a kilogram of hydrogen). Also, hydrogen is expected to cost $3/kg by 2008 which would mean that in 2008, the operating cost of this car would be about $0.05/mile.
Thursday, August 18, 2005
DaimlerChrysler aims for 100,000 FC cars by 2015
ENV hydrogen bike about 1 year from commercialization
Intelligent Energy is finalizing development of it's ENV Bike ("envy"), a motorcycle built around IE's state-of-the-art fuel cell, "The Core." According to specs (1.7 MB pdf), the bike can go up to 80 km/h (50 mph), and a single tank lasts 100 miles and takes, can run continuously for four hours before the fuel cell needs re-fueling less than 5 minutes to refuel. "The Core," the fuel cell powering the bike, is fully removable and can function at temperatures up to -40°C. In the future, Intelligent Energy is considering creating a suite of remote-power products running from the core. An article from The Toronto Star (not available online) suggests it may be available as early as next year, and for as little as $16,000 CAN.
The BBC is also reporting that the motor bike is so quiet that it might be fitted with an artificial "vroom" because of worries its silence might be dangerous. The fake engine noise device would help alert road users. Cross posted to theWatt, and here.
Wednesday, August 17, 2005
Evaluating the performance of fuel cells
At first I was planing to make this post very long and technical, but then I thought: "neh...". I think it would be much more useful if I keep it simple, and to the point. Basically, when we want to compare the performace of a fuel cell, we want to know the voltage that the fuel cell generates at a given current load. The load by the way would be whatever the fuel cell is powering, something like a computer, or a house, or the grid. The reason we want to know the voltage and the current is so that we can calculate the power being generated by the fuel cell:
The figure on the right is a cartoon of a typical fuel cell performance curve (sometimes called a polarization curve). The blue line is the maximum theoretical voltage that can ever be generated by the fuel cell. But because nothing is 100% efficient, the fuel cell can never actually reach that voltage. The red line is the voltage that the fuel cell generates, and it depends on how much current is required by the load (the load would be whatever your fuel cell is powering, like a computer, a house or the grid). The more current that the load requires, the higher are the voltage losses. The reasons why voltage decreases can get a little tricky, I'll explain it quickly.
There are 3 main causes:
- The electrochemical reactions are too slow (technical term: activation losses)
- The resistance of the fuel cell is too big (Ohmic losses)
- The gasses (oxygen and the fuel) can't get to the reaction site fast enough (technical term: mass transfer losses)
Causes 1 and 3 are sometimes bulked together and are called the "overpotential". In the first figure, you see the η+iR term. η is the overpotential, iR is the Ohmic loss. So, the operating voltage of a fuel cell can be expressed by the following equation:
The cathode and the anode have overpotential losses (the overpotential of the anode is different than the overpotential of the cathode) and the entire fuel cell has the iR loss. E0 is the maximum theoretical voltage, typically around 1.2V for fuel cells.
Looking at figure 2 with the red and green line now, the fuel cell that generates the green performance curve will have a higher power output than the fuel cell that generates the red line because for every current, the voltage is higher. Each of the 3 causes mentioned above contributes to the voltage loss for every current load, but they do not all have the same contribution to voltage loss at all current loads. So I'll explain based on this second polarization curve.
The electrochemical reactions are too slow (Region #1):
When current loads (same thing as current density by the way) are small, then the voltage losses are dominated by the slowness of the electrochemical reactions. Comparing the fuel cell that generates the red performance curve to the fuel cell that generates the green performance curve in region #1, the green curve has faster electrochemical reactions than the red curve, this could be caused for many reasons like: a) the green curve is operating at a hotter temperature, b) the green curve has a better catalyst to promote the electrochemcial reactions, c) the green curve is operating on a fuel that is "easy" to react.
The resistance of the fuel cell is too big (Region #2):
Region number 2 also has "activation losses" but it is dominated by the fact that the fuel cell has a high resistance. If a fuel cell has a high resistance, then electrons will travel more slowly through that fuel cell and more heat will be lost (heat generated = i2R). That's bad. This is called Ohmic loss because the loss can be described by Ohm's law (V=iR). The difference between the fuel cell with the green curve and the fuel cell with the red curve is that the green curve has a lower resistance. So, how can we minimize the resistance? Well we can: a) make the fuel cell thinner (the thicker the material, the more resistance it offers) or b) try and find another material that has a higher electronic conductivity
The gasses can't get to the reaction site fast enough (Region #3):
If we can't supply the gass molecules to the reaction site fast enough, then the rate of the electrochemical reactions is limited. Gasses have to diffuse through a porous network (that makes up the electrode) until they reach the reaction site. At very high current loads, then a lot of gas has to be supplied so that a lot of electrons can be generated, but sometimes the gas can't get to the reaction site fast enough, and so the performance of the fuel cell is reduced. There could be a couple of differences between the fuel cell that generates the green curve and the fuel cell that generates the red curve: a) the green curve has an electrode that is more porous than the red curve, and so the gasses can travel through the electrode more easily b) the green curve could be using 100% oxygen for example, instead of air which is composed of oxygen and nitrogen.
So then, ready to see what a "real" performance curve looks like?
This comes from a solid oxide fuel cell from the company previously known as Global Thermo Electric (they were bought out by Fuel Cell Energy). The red curve is the best performance, the blue curve is the worst performance. On this graph you also see the power density as well, that's just the voltage * current density.
And that's it! Take home message: You want the highest voltage possible for any given current density.
ExecutivesCorner interviews CEO of Ballard Power Systems
Campbell said that Ballard's main strategy is to focus on the global automotive market and the residential cogeneration market in Japan because of high volume potential, government support and "strong socio-economic forces". Potential markets for near term sales are the industrial lift truck (fork-lift) and pallet jack markets, which even at today's pre-market volume, Campbell says fuel cells offer cost benefits.
This is cross posted to theWatt.com.
The various types of fuel cells
When most people hear the word "fuel cells", they automatically think "hydrogen" and "cars", which means that they automatically think of a polymer electrolyte membrane fuel cell (PEMFC), whether they know it or not. There are a couple of different types of fuel cells though, which operate under the same fundamental principles but vary in the materials that they are made from, the ionic species that is transported through the electrolyte, their optimal operating conditions and the types of fuel that they can use. The table below (click to enlarge) lists the electrochemical reactions that occur in each type of fuel cell (Src: The Fuel Cell Handbook, 5th Edition). All fuel cells are named based on the nature of their electrolyte. The following is a list of the most popular types of fuel cells.
Polymer Electrolyte Fuel Cell (PEMFC)
Operating temperature: 80-120°C
Fuels: Pure hydrogen only
Electrode materials: Electrically conductive, porous carbon cloth, carbon fiber layer
Catalyst layer: Platinum impregnated carbon and Nafion membrane
Electrolyte material: Solid perfluorinated sulfonic acid polymer membrane (Nafion®)
Ionic species: H+
Application: Cars/buses, APU, UPS, small power plants
Companies: Ballard Power Systems, Hydrogenics
Other configurations: Direct Methanol Fuel Cell (DMFC) which uses a methanol/water mixture as the fuel
Alkaline Fuel Cell (AFC)
Operating temperature: 120-240°C
Fuels: Hydrogen (not as pure as PEM)
Electrode materials: Platinum impregnated porous electrodes
Electrolyte material: Concentrated (85wt% for high temp) KOH (liquid)
Application: NASA space shuttles, 1-4kW applications
Companies: Astris Energi
Phosphoric Acid Fuel Cell (PAFC)
Operating temperature: 190-205°C
Fuels: Hydrogen (purity less than PEM)
Electrode materials: Platinum/Carbon
Electrolyte material: 100% H3PO4 (liquid)
Diffused ionic species: H+
Application: Power plants (200kW)
Companies: UTC Power
Molten Carbonate Fuel Cell (MCFC)
Operating temperature: 650°C
Fuels: Hydrogen, natural gas
Electrode materials: Anode: Ni-Cr/Ni-Al Cathode: lithiated NiO
Electrolyte material: 50wt% Li – 50wt% Na or 62wt% Li – 38% K (liquid)
Ionic species: CO3-
Applications: Power plants
Companies: Fuel Cell Energy
Solid Oxide Fuel Cell (SOFC)
Operating temperature: 800-1000°C
Fuels: Hydrogen, alcohol, natural gas, CO, paint fumes and others
Electrode materials: Ceramics at cathode, composite Ni/Cu and ceramics at anode
Electrolyte material: Solid ceramic materials
Ionic species: O2-
Applications: UPS, small power plants, residential/commercial/industrial power supply
Companies: Siemens Power Generation, Fuel Cell Technologies, Fuel Cell Energy
Here is an overview (click to enlarge) of various fuel cell types from the Fuel Cell Handbook, 5th Edition (printed in October 2000 so it is a bit outdated).
Tuesday, August 16, 2005
Ceres Power, outsmarting the competition
Ceres Power Chief Technology Officer, Nigel Brandon, said: “We have beaten critical industry benchmarks on four fronts at once – power output, integration, durability and manufacture. Our customers will recognise these dramatic results as key achievements on the road to unlocking global mass markets.”And I think it's true. I am familiar with the work done at Imperial College (it's my job to read through their papers) and I know what they are capable of. I also saw some of their cell test results at a fuel cell conference and they were very, very good. Their focus is to get the temperature of their fuel cell down to around the 500C level, no easy task, but they've been doing a fantastic job at it. For all of those who want more technical info, their cells are metal supported and they use low temperature cathode ceramic catalysts (LaSrCoFe) and a low temperature GDC electrolyte material. I'm not exactly sure what they use for their anode, I assume some type of Ni/GDC cermet.
One problem with SOFCs is that they are poisoned by sulfur. I know of one company which actually sells SOFC systems (although at a very expensive price) and they have sulfur problems because sulfur is spiked in natural gas. Whenever I don't hear of companies complaining about this, it just means that they're still using simulated fuel.
Monday, August 15, 2005
The need for H2PEC, the OPEC of the hydrogen world
So I would not advocate the use of fossil fuels to make hydrogen. The solution then? Well, making hydrogen from electrolysis of water using power generated from a renewable source or nuclear is the answer. But, keep in mind that if we were to go to a hydrogen economy relying on nuclear power, something that Geoffrey Ballard himself advocates, then we'll run out of uranium faster than your mom can say "don't touch that", yes, perhaps we can regenerate the depleted uranium, but it's still unclear to me what the energy requirements are to do this, and yes, new nuclear reactor technology could make the situation better, and yes, maybe we could use thorium instead of uranium, but the fact of the matter is, to go to a hydrogen economy entirely based on nuclear power, we'll have to build THOUSANDS of new nuclear reactors (source: the book Fueling The Future).
So then, how do we make our hydrogen? Well, we need to find locations that can have a niche in making hydrogen. Iceland is one example, they can make hydrogen from their geothermal energy, another place is the Island of Unst, in Northern Britain. They have very high and consistent windspeeds and claim that they can make hydrogen from electrolysis and wind power for only $2.58/kg (this is equivalent to $2.58/gallon of gasoline because 1kg H2 and 1 gallon of gasoline have about the same energy content). It ends up being very expensive to lay electricity lines from the island to the mainland ($100/meter) and so making hydrogen makes economic sense so that all of the wind resources can be used to its full potential.
So, here we have the makings of certain regions in the world where it actually makes sense to make hydrogen. This is the start of H2PEC, "hydrogen producing and exporting countries". By the way, just in case this idea takes off, I've reserved the domain name h2pec.com.net.org (hey, why not? they only cost $8 each).
Next thing we have to consider: How the heck to we store and ship that hydrogen? Unst wants to liquefy it, maybe that is the solution, but it does take a whack load of energy. I have another idea. One word: BLIMPS!
Sunday, August 14, 2005
What is a fuel cell anyway?
A fuel cell is an energy conversion device, the internal combustion engine is also an energy conversion device. Basically, an energy conversion device takes a fuel, and fuel is just a way of storing energy, and converts the energy in the fuel into something that we can use. Hydrogen for example can be made by passing a current through water and splitting water into H2 and O2. The current can be generated from a wind turbine, a nuclear reactor, solar power or coal power. So H2 is a way of storing energy the same way oil stores the sun's and the earth's energy. The big difference between a fuel cell and an internal combustion engine is that a fuel cell has no moving parts, no explosions, just chemical reactions (actually electro-chemical reactions). Fuel cells require the fuel to be a gas though, although it's fairly easy to convert liquid fuel into a gas since fuel cells have lots of waste heat. One more thing, fuel cells generate direct-current electricity, internal combustion engines generate mechanical energy which can be converted into electrical energy. So, that's the basics of what a fuel cell is: fuel in, electricity out.
The figure at the top right is how a hydrogen fueled polymer electrolyte fuel cell (PEMFC) works. Every fuel cell has 3 main components: 1. Cathode 2. Anode 3. Electrolyte. The cathode and the anode are known as electrodes. In all fuel cells, the cathode breaks down oxygen (electrochemical reduction), the anode breaks down the fuel, such as H2 (electrochemical oxidation). Electrons are produced at the anode, travel around an external circuit to the cathode, the movement of electrons is electricity. There's one more piece in the puzzle, and that's an ion, which travels through the electrolyte. The type of ion and what electrode it is generated in depends on the type of fuel cell. These ions can be H+, OH-, O2-.
That's all I'm going to say about how fuel cells work here. Read more in the wiki.
I do want to mention though that there are a couple different types of fuel cells. The main difference between these fuel cells are the materials that they are made from and the temperature that they operate at. Both of these factors determine what type of fuel can be used. Fuel cells (like the PEMFC) operate at around 100C (low temperature) and because they are at low temperatures, they need a good catalyst, platinum and therefore high purity hydrogen since impurities can deactivate the platinum catalyst. High temperature fuel cells (like the SOFC) don't need platinum and they so they can use hydrocarbons as fuel, usually natural gas.
These are the most popular types of fuel cells, starting with low temperature and going up to high temperature operation. Except for the DMFC, They are all named by the material that their electrolyte is made from:
- Direct methanol fuel cells (DMFC)
- Polymer electrolyte fuel cells (PEMFC)
- Alkaline fuel cells (AFC)
- Phosphoric acid fuel cells (PAFC)
- Molten carbonate fuel cells (MCFC)
- Solid oxide fuel cells (SOFC)
Overpotential - A blog about the world of fuel cells
So then, why am I doing this? Well, for a couple of reasons. First of all, I'm interested in fuel cells, how they work, their applications. Secondly, I think that there are some common misconceptions about fuel cells. I've spoken to many people who outright dimsiss fuel cells for one reason or another. The most common reason is that they require hydrogen to operate, and hydrogen is expensive to make. Well, no. Come back to this blog and you'll learn why, but not to leave you hanging, not all fuel cells require hydrogen, some actually run quite hapily directly off of natural gas, or even biomass. There are also a number of niche applications for fuel cells, which of course I'll discuss in more detail in other blog posts, but these niche applications mean that hydrogen fueled fuel cells are cost competitive today and in some cases, even save money!
Ok, next up, the name of the blog. Why is it called overpotential? Well, for a couple of reasons. First of all is the technical reason. Overpotential is a voltage loss from an electrochemical device, such as a fuel cell. The higher the overpotential, the higher the voltage loss, which is not good! To increase the efficiency of a fuel cell, we must reduce the overpotential. It can get a bit complex actually, so I'll save it for another post, probably a couple posts actually. There is also a lot of people who think that the potential of fuel cells are over stated...so that could be an overpotential, and then there are certain applications that fuel cells can performace over the potential originally thought. I know, I know, those are long shots.
So who am I? Well, I'm a grad student in Canada working towrads my PhD researching solid oxide fuel cells. I'm also pretty interested in the world of energy. I run another website called theWatt, check it out at theWatt.com! theWatt is for all types of energy news and discussion, from alternative energy to the dirtiest of fossil fuels. Also part of theWatt is a weekly podcast that I do, called theWatt Weekly (pretty originial huh?). So my podcast talks about all types of energy news and discussion.
Ok, that's what this blog is about. Send me an email: ben@theWatt.com.