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EV WORLD EXCLUSIVE ARTICLE |
Toyota ES3 concept diesel-electric hybrid is experimentally rated at 47km/liter or 103 mpg. Weighing 700 kilograms, the has a greater relative fuel efficiency than the best automotive fuel cells, according to the author. In addition to the concept cars fuel-sipping 1.4 direct-injection diesel turbo engine and hybrid drive, it boosts a slick 0.23 Cd ratio and a continuously variable transmission (CVT).
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Fuel cell proponents point to the device's high efficiency as one of the major advantages of the technology, but just how efficient are they really?
Open Access Article Originally Published: August 05, 2004
The dream of converting fuels such as coal, alcohol and hydrogen directly and with great efficiency into electricity date back to the middle 1800s-indeed this is no longer a dream as evidenced by the variety of fuel cells now under development-but sadly, reports of efficiency are often times far in excess of both the theoretical and practical realities claimed of this technology. This paper will attempt to clear up some of the common misconceptions that abound in any discussion concerned with fuel cells in general and PEM (proton exchange membrane) fuel cells specifically.
The PEM fuel cell has been the focus of much research and enthusiasm; its intended role as a superior prime mover in automotive applications has been the subject of many popular articles and talk show discussions. It has even been prophesized that “the reign of the internal combustion engine is coming to an end”-but there are good reasons for taking these kinds of statements with a grain of salt. While the perceived advantages of a fuel cell power source seem immense-low or zero pollution, quiet operation, and high efficiency to name a few-it is the last claim that deserves some measure of reassessment.
It has often been said that a fuel cell is a very efficient energy converter, that its efficiency can “approach 83%”. This is actually quite true-in theory. However, when one considers all of the parasitic losses and ancillary subsystems necessary to make a practical fuel cell-powered vehicle, the real world efficiency plummets from 83% to something like 40% or lower-a number that is surprisingly close to the actual efficiency of some Diesels and other high-compression engines! In fact, the 83% theoretical efficiency is not unique to the fuel cell-it is equally applicable to heat engines, thermoelectrics and even human muscles as we shall see later in this paper-and this is a point that is often overlooked by some overzealous proponents of fuel cell technology.
The first step necessary in evaluating the efficiency of any system is to define efficiency explicitly: It is the ratio of the useful work output to the heat energy contained in the fuel being considered. Using hydrogen as our fuel, we find that there are actually two values of heat to be considered: The higher heating value (HHV) and the lower heating value (LHV). Normally, this heat content is given in terms of a quantity called the enthalpy, or heat content as measured in joules per mole of reactants, but it can also be expressed quite nicely in terms of the voltage and electric charge, as would be the case when considering any electrochemical power source. The difference in these two numbers stems from the fact that the HHV represents all the heat that is possible to recover, including the heat that is derived from condensing the water which is an inevitable product of the reaction; the LHV does not include this heat, consequently the two values differ by about 18%.
As it turns out, European engineering practice favors the use of HHV while Americans prefer the LHV when evaluating efficiency and this leads to some interesting consequences, not the least of which is that some devices, like heat exchangers and boilers, can achieve efficiencies exceeding 100% if one chooses to use the LHV instead of HHV-clearly greater than the conservation of energy principle should allow! Moreover, when comparing hydrogen to other fuels like methane and various alcohols that contain a significant fraction of carbon, the difference in the HHV and LHV artificially favors hydrogen (hydrocarbon fuels differ by only about 10% between the HHV and LHV).
The implications of this practice provide for some illuminating revelations: As indicated a moment ago, the heat content, and consequently the efficiency, can be expressed in terms of the voltage-specifically, the ratio of output voltage per cell divided by the theoretical voltage possible for any such combination of fuel and oxidizer.
A typical automotive fuel cell operates in the region of 0.55-0.75 volts per cell; the theoretical values for the HHV and LHV are 1.48 and 1.23 volts respectively. Using the values just given return efficiencies of 37% -51% for the HHV and 44%-61% for the LHV-but this is only the chemical-to-electric conversion efficiency; a car ultimately requires mechanical energy, not electrical, to move it and this is where the highly-touted fuel cell efficiency begins to take a turn for the worse.
As mentioned earlier, there are a number of other systems that must be included in the overall efficiency analysis of a fuel cell-powered vehicle-the electronic inverter, motor, air compressor (fuel cells needs oxygen that must come from the air) and the energy needed to get the hydrogen fuel stored in the tank of the vehicle. These subsystems have their own efficiency conversions that calculate out as follows: Inverter 90%, motor 90%, air compressor 80% and hydrogen storage (compressed gas) at about 85%. Multiplying these efficiencies by the earlier values given for the HHV and LHV efficiencies gives us something like 14%-28% and 24-34% respectively.
Notice that even the overly-optimistic LHV efficiency falls short of some state-of-the art Diesels, natural gas and high-compression alcohol engines (40% LHV); this is a point that is often glossed over when hydrogen automotive fuel cells are discussed: Proponents like to use the highest chemical-to-electric conversion efficiencies (when the loads are smallest) of the fuel cell and compare them to the average brake thermal efficiency (chemical-to-motive power) of a conventional automobile engine. This sort of analysis is really misleading since it uses the most ideal operating characteristics of a fuel cell (and the most unrealistic in everyday driving) and compares them to a vehicle that is not optimized for maximum efficiency. In fact, when an internal combustion engine is designed to use a high-compression fuel like methane or alcohol in a hybrid configuration, the perceived differences in overall efficiency begin to blur. And this leads us to the final point alluded to earlier in this paper: Just what is the maximum theoretical efficiency of any prime mover using hydrogen and oxygen as an energy source?
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25 comments so far...
19-Sep-2006
33259
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I find the direct methanol conversion fuel cells to be very attractive. Although the efficency of present devices is low, the new Toshiba designs are showing improvement. The really attractive part is that the methanol can be produced from waste biomass; corn stocks, corn cobs, soybean stocks, wheat straw, mowed grass, wasteland grass, tree waste, paper waste and garbage... In other words, the raw feed stock does not directly compete with other resources like food, ethanol vegtable oil or natural gas. I think a methanol fuel cell program would be complementry to a SVO Straight Vegtable Oil diesel engine program. Each energy source could be used for the most suitable application. We need to harvest the largest energy per acre of land. You must consider, vegtable oil and ethanol only use the small plant seed. Methanol would use the entire plant while preserving the seed for food or as polymer feedstock.
Posted by: alan hulsebus
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08-Aug-2004
4266
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'...efficiency of ... (UTC Fuel Cells PC 25) ... only 35% chemical-to-electric; a natural gas, combined cycle generating facility ... pushes the envelope at 55% chemical-to-electric conversion efficiency.'
Combined cycle power plants have a final stage expansion turbine to recover lower-grade heat energy often lost to the atmosphere.
To be fair, there are combined cycle fuel-cell based powerplants which use a final stage turbine to recover energy from the waste heat of (I believe) a molten carbonate fuel cell.
In short, it's probably not fair to compare an automotive PEM and a stationary combined-cycle powerplant. I agree the diesel-electric comparison is fair game."
Posted by: Ron Fischer
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08-Aug-2004
4269
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This paper points to the position I have held for quite a while. We will improve fuel cells, and eventually fuel cells will be the basis for our cogeneration plants (at hospitals, factories, hotels, and eventually large commercial developments). Putting a fuel cell into a car currently only makes sense if you need to heat the passenger compartment to 800 degrees. We might eventually have fuel cell vehicles, but it doesn't seem likely to happen in the next 20 years. Who knows where battery and capacitor technology will be in 20 years?"
Posted by: Steve Erlsten
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08-Aug-2004
4270
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There are some good issues raised in this article.
It's a good thing to take any and all claims and counter claims with a grain of salt as the author suggests.
The devil is always in the details.
For instance the voltage per cell numbers referenced are not a fixed value. They go up or down based on power demands, just exactly as an internal combustion engine wastes more and more heat(translated efficiency)in the exhaust,coolant and increased internal friction when higher power is demanded.Some of the new nano science and advanced polymer and catalyst work I am seeing will continue to bring the FC efficiency numbers up; possibly dramatically ,while the internal combustion engine has had over 100 years of development time and it is unlikely they will see any dramatic increases.Also a great deal of the quoted efficiency of the ICE is available at only a narrow RPM range or load factor. Hybridization can help here but electric/electric will always be more efficient overall than mechanical/electric.
The efficiency quoted for the hydrogen tank is also misleading as this number only applies if some form of external compression is needed. Not so with the new generation of monopolar electrolyzers that have little if any parasitic losses for compression. Yes for reformer based h2,but then I cannot for the life of me understand why we would go there anyway. That's for another debate.
Also the efficiency of the electric motor and inverter are quoted on a low side. Higher efficiencies are being demonstrated currently and can still move higher."
Posted by: Larry Elliott
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08-Aug-2004
4271
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Stationary fuel cells have the advantage of being able to capture both the electrical and heat energy from the original fuel stock, typically natural gas. From my conversations with our local utility,which runs the UTC PC 25 unit at the Henry Doorly Zoo. Here's a quote from the article I wrote back in late January for EV World entitled, Jungle Fuel Cell Safari.
"Roberts gave me a copy of a Powerpoint presentation that includes many of the numbers found in this report, but one of the more interesting graphs is a bar chart showing the relative thermal efficiencies of a conventional gas-fired electric generator (20%), a microturbine (24%), a diesel-electric generator (30%), and the PC 25 (40%). If you added to this heat recovery from the fuel cell -- and Roberts estimates he's only capturing about 10% of the potential heat available -- the overall thermal efficiency of the fuel cell would be closer to 80%."
The key to tapping the potential of fuel cells is not just the electricity, but both the heat and the electricity, which is true for other co-gen systems, as well. In an automotive application, heat is a problem -- look at the size of the radiators in the Honda FCX. In a stationary application it's an added bonus.
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Posted by: Bill Moore
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08-Aug-2004
4272
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Bill. Your comment on the size of the radiators on the FCX illustrates my point of devils in the details.
The size of the radiators in the FCX and other FC vehicles is not so much a function of a large amount of heat needed to be dumped versus the ICE, but more the fact that the PEM fuel cell must operate at a temperature of ideally about 160 f. An ICE can be as high as 200+.
Since both will operate in the same ambient temps the delta T is lower on the fuel cell raditor so a larger surface area is needed to achieve an equal amount of cooling.
As to the cogen aspects, bear in mind that real world vehicles need both cooling and heating and a heat pump utilizing FC waste heat can supply both with essentially no additional loss in overall efficiency.The hvac unit would be larger and perhaps heavier than an equal refrigerant unit,again due to the lower heat values,but that design is doable.
Not so with batteries."
Posted by: larry elliott
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08-Aug-2004
4276
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Heaters in cars? That's for wimps! Put some gloves on! (chuckle)
Seriously, though, vehicles must have heaters, even if just for defogging purposes (for someone in Florida, for example). Some people have gotten around the extreme drain on the battery packs by installing propane heaters on their BEVs. Ultimately, all ICE vehicles should have block heaters capable of heating the engine to 200 degrees before start-up. Every time I start the Insight cold, the mileage drops while the engine heats up and the catalytic converter gets hot. The 2004 Prius has its thermos, which seems to work pretty well.
BEVs could easily be programmed to heat up while plugged in, with a small electric heater to maintain the temperature while driving and provide defogging capability."
Posted by: Steve Erlsten
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09-Aug-2004
4297
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position statement of American Society of Mechanical Engineers, March 2004
"A new program in the EERE budget is the FreedomCAR, which is R&D to develop a fuel cell vehicle. FreedomCar is funded through crosscutting areas and by funds previously allocated for the Partnership for a New Generation of Vehicles. A two-year study, conducted under the auspices of ASME, showed that hydrogen fuel cells are very efficient -- once hydrogen is available. However, the inherent inefficiency in hydrogen production from fossil fuels results in a well-to-wheel efficiency of only 10% compared with 30% for current hybrid (engine/battery) vehicles. This reduced efficiency carries with it an equivalent increase in the production of greenhouse gases unless hydrogen is produced at large-scale central plants that employ carbon sequestration technologies, at nuclear power plants, or at smaller installations that employ renewable technologies. While the COE Energy Committee supports the long-term development of a wide range of transportation technologies, we believe that the FreedomCar should not be funded for the economic and environmental reasons delineated above and that the $150M funding be re-allocated to other R&D programs."
http://www.asme.org/gric/ps/2002/02-14.html
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Posted by: Dursun Sakarya
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23-Aug-2004
4450
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Larry:
I must point out that the radiator question is really due to a much larger temperature gradient in an ICE between the wall of the cylinder and the cooling fluid (on the order of several hundred degrees F); the fuel cell's membrane must never exceed 180 degrees F and it is this temperature gradient that comes into play. It's a subtle difference but one that plagued the Stirling engine designers as well. Unfortunately, in order to achieve high power outputs, automotive fuel cells need to push the operating temp of the membrane right up to the limit (180 F) and it is this upper temperature that really taxes the fuel cell, especially when operating in the Mohave desert. Remember that the combustion process is actually extremely high in temp ( over 2,000 degrees F) and it is this gradient that ultimately determines the heat transfer rate.
Also, I'm constrained to point out that the use of the waste heat in a process can be theoretically employed to operate a heat pump for heating or cooling, the low delta T's of the fuel cell preclude it's use in such an application (this is why solar cooling using flat plate collectors is so difficult). I would be only too happy to discuss this matter in greater detail with you if you would be so inclined since it is very interesting conceptially but insurmountably difficult in practice."
Posted by: Dominic Crea
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23-Aug-2004
4451
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A valid point!
In all fairness, I think we will find that overall, a fuel cell and advanced engine hybrid combination will shake out very close in efficiency. Certainly the "2-3 times more efficient" number needs to be exposed as misleading and I believe the Toyota ES-3 demonstrates this with stunning reality.
"
Posted by: Dominic Crea
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23-Aug-2004
4463
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BSFC or thermal efficiency data for car motors in production today aren't all that easy to find. Lots of proponents of engines that exist in very small numbers, or like my boron ones, only on paper, like to estimate real ones' efficiencies as below 20 percent, below 15 percent, some bold ones even say below 10 percent, but I think they are all liars. Where is some accessible truth?
--- Graham Cowan
How individual mobility gains nuclear cachet (.DOC file)
"
Posted by: Graham Cowan
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24-Aug-2004
4473
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Efficiency of any electric motor based car could be improved by mechanically coupling a flywheel to each wheel. When the car stops the flywheel would continue to rotate and convert the accumulated rotational energy to electric by charging the on-board battteries.
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Posted by: John Boyd
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24-Aug-2004
4485
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boooo"
Posted by: bob bob
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25-Aug-2004
4495
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Graham,
BSFC numbers can be found in the seminal work on ICEs: The Internal Combustion Engine in Theory and Practice by Charles Fayetted Talyor. This book is used by many universities and is an excellent source for data. Another source is the SAE papers (you'll probably have to go through a library to access these for free). Good luck!"
Posted by: Dominic Crea
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25-Aug-2004
4496
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John,
the electric motor's rotational energy is actually very small--hardly worth recovering in practice. As a matter of fact, the kinetic energy that's recovered in a conventional hybrid vehicle (far higher than the rotational motion of the electric motor)works out to be something like only 10% of the total energy consumed--good for braking but not a major contributor to good efficiency unless you're talking about city driving."
Posted by: Dominic Crea
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25-Aug-2004
4497
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Dominic: Excellent article. The flywheel system
I am proposing would be a flywheel that would
be the size of the four axle driven wheels
and would be rotating as the car moves. Not just
during breaking. A mechanical method would have
to be devised to de-couple the flywheel from
the wheel when the car approaches a stop. At
stop, the flywheel's rotation would charge the
batteries. Another source of lost energy is the heat from the engine. Thermocouples could be used to charge the batteries."
Posted by: John Boyd
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25-Aug-2004
4499
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Thanks. But hasn't there been big progress
in new vehicles' overall average thermodynamic efficiency in a typical driving cycle,
something like 0.33 ---> 0.36, in the past 15 years?
--- Graham Cowan
How individual mobility gains nuclear cachet (.DOC file)"
Posted by: Graham Cowan
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25-Aug-2004
4506
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Mr D
Flywheels are wonderfull for nonmoving applacations, and in moving applactions there called a gyroscope. They dont want to change there axis of rotation without horrible presesion forces. I worked at ford in the 80' on a flywheel hybred car for a while. To store enough enegery to be usefull a huge gyroscopic loads are imposed on the bearings. Chysler made a prototype fly wheel/engine demo race car that the flywheel was supended in a complex supension gymble with the axas of rotation up/down to try to answer this problem. This made a great show demo but the flywheel tended to rip it's self and the car apart with acumulated bumps and jogs of driving. Its what keeps a motorcycle up, and the wt. of the wheels turning is why we have powerstering. My grandfathers old pontaic was realy hard to turn the wheel at 60 mph. Its an intersting ave. of investagation with the modern carbon composite rotors . and people are."
Posted by: Lin Higley
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26-Aug-2004
4522
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Just finished reading August 13 Science -- 21 pages on fuel cells.
What about good ol' 'lectricity. We already have the infrastructure and distibution system.
Here's a guy with some new storage-battery technology:
http://visforvoltage.com/main/modules.php?op=modload&name=News&file=article&sid=9&mode=thread&order=0&thold=0"
Posted by: Josh Levin
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27-Aug-2004
4527
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unfortunately (and this is the "fly-in-the-ointment" that kills so many really cool ideas), until we do the actual calculations, many very promising ideas grind to a very sobering halt (this happened to me after building my first Stirling engine!)
The problem with flywheels mounted on the wheels or axles becomes one of energy storage and something called the "moment of inertia". There is simply too little energy available in these sorts of devices at speeds any less than several TENS of THOUSANDS of RPM and SEVERAL HUNDRED KILOGRAMS OF MASS (if you give me the particulars, I'll do the calculations for you if you'd like). Thermoelectrics also come up against a block: the efficiency is, at best, only a few percent and this is when we're talking about temperature differences that are in the realm of 400-1500 degrees F. What also kills us is the fact that the waste heat from an engine must be tampered with as little as possible (unless we utilize this heat at a VERY low temperature). This is a result of the Carnot restriction which also comes into play with the absorbtion air conditioning idea that you or one of the other readers had mentioned.
You can bet your bottom dollar that if there are efficiencies to be gained, engineers will "squeeze blood from turnips" to get 'em! This can be seen in the Honda Insite where aluminum bodies, ball-peened pistons with short skirts, low viscosity oil (5W as opposed to 10W in conventional cars), low drag (.23) and dozens of energy saving techniques were implemented in the design of this amazing car.
One device that is being given serious consideration since it tops the list in terms of enrgy recovery, is the venerable exhaust turbine--while it will only increase mileage by about 10%, it is without a doubt, the most tantalizing in terms of cost and weight.
Keep on with the dream of hybrids--there's more up their sleeves than many imagine!
"
Posted by: Dominic Crea
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27-Aug-2004
4528
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Hello Grahm,
yes indeed, there has been some progress (the Honda and Toyota utilze a very interesting cycle in their engines to achieve high efficiencies of 31%). Most noteworthy are the high compression OTTO cycle methane and alcohol engines achieving 40%-42%--while obtaining very low pollution levels. "Things be a changin' in the world these days".
The really big winner will be the plug-in-hybrids that will allow for extended trips on batteries alone. The benifits of this technology are too many to list here but I would love to do an extended article on the topic."
Posted by: Dominic Crea
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27-Aug-2004
4529
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howdy Josh,
try checking out the July series in SCIENCE on hydrogen and it's not-so promising future. Good reading and I also recommend David Suzuki's piece in EEN (Environmental New Network) online. David goes through some very good points in his article on the dubiousness of hydrogen and offers up solutions that are much more enticing."
Posted by: Dominic Crea
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14-Sep-2004
4709
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When talking about efficiency one should continue to compare apples to apples. If you are going to add in the efficiency of compressing the fuel to store on the vehilce, then you need to talk about the efficiency of pumping and delivering the other fuels too.
This the same compliant us EVers had with comparing the dyno fuels to electric.
Its like trying to quote from the Bible you can make an case you like look good and make your own idea stand out.
I can NOT produce oil in my back yard. H2 is possible in a lot less same the corn based fuels."
Posted by: Rick Reinhars
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30-Apr-2008
61445
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Re Fuel cell efficiency , the article is great and looks very real world.
However, the conclusion that the present day car engine is not dead, is misleading.
Because a car engine be it petrol or diesel, is one of the major fossil fuel consumers and thus generates CO2 and all those climate woes.
The future energy systems will be based on non-fossil fuels and the most abundant , from rocks or from water is hydrogen.
The solar or atomic energy sources as the power source are already with us. The fly in the ointment was how to keep hydrogen liquid in your tank for at least a month- Now thanks to the MOF-74 type of compounds, the Hydrogen evaporation
temperature has been raised to 77ºK.
Meaning at last a practical hydrogen power car is here.
The electrolisis of water to produce Hydrogen using sun derived energy from solar towers using the liquid salt systems and then coupled steam turbines and generators at last offer a competitor for small site power production of NON Fossil fuel power in industrial quantities without the massive hardware and infrastructure investment for alternatives like wind and wave.
Getting back to the Fuel cell, its efficiency doesn't really matter if it is comparable to the present vehicle motors.
Here we have a choice a classical car but with a hydrogen fuel for the motor, or the same hydrogen in a fuel cell producing electricity for the electric motor or motors.
Both work. Both can be bettered for losses, maybe the fuelcell being younger has more system loss components but that can be addressed.
The important thing in all of the above is the storing of hydrogen in a tank long enough to move around and not have to refill the tank every day because your deposit ( and dollars) evaporated in the heat of the night.
Thanks to MOF-74 we can do this and thanks to hydrogen fuel for transport we can enter the hydrogen based energy era.
What do we do with the huge deposits of oil?
Well maybe make aspirins?? :-))
Bye
Fred Madrid spain
Posted by: Fred mah
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07-Sep-2004
4633
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I can't argue with anything in the article, but can raise one point in favor of fuel cells.
Global oil production will peak, somewhere between now and 2010 at the latest, while we all know demand only increases. In not very long, demand will outstrip possible production, and the prices of oil will be likes of which we've yet to see and have only been imagined by arab terrorists. Gasoline-powered vehicles will be nice, highly-expensive luxuries...
Unless, of course, you've got yourself a hydrogen powered vehicle, be it fuel cell or a converted ICE using compressed Hydrogen from either a compressed can or hydride tank. As long as the gods let the sun keep shining, the wind blowing, and tidal forces churning, then the power needed to liberate hydrogen from water or natural gas will flow. Flow, unliked Saudi oil. It's worth noting most of Saudi's largest fields now pump almost as much water as they do oil; an indication that the fields are almost half depleted, and once they are, oil production will steadily decline.
Fuel cells, and hydrogen technology, will either make the weaning process from the oil age an easy one, or an Easter-Island style one."
Posted by: Michael Ringold
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