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#31
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CAFE Electric Aircraft Symposium Set For May 1
Skywise wrote:
Larry Dighera wrote in : Hello Brian, You seem to have firm grasp of the physics involved. Are you able to suggest how efficient an electric power system powered by LH2 would have to be to offset the power density difference from gasoline/kerosene? A quick disclaimer.... I don't have any degrees in this stuff or work with it. I just happen to be very interested in the sciences in general and have taught myself a few things over the years. Having said that, I try very hard to check my facts and figures before I say anything. I have an allergy to foot-in-mouth disease. On to your question.... Per numbers in http://www.tinaja.com/glib/energfun.pdf Liquid H2 has an energy density of 2600 Watt-hours/liter. Gasoline is 9000 Watt-hours/liter. [I used energy density per volume rather than per mass because that's the limiting factor in any vehicle, the volume of the 'gas tank'] If we make an assumption for discussion's sake that an LH2 powered system were 100% efficient, then the gasoline system would only need to be 2600/9000 = 29% efficient to reach parity with LH2. But note that nothing is ever 100% efficient. There are _always_ conversion losses. It's a matter of how much. Per: http://en.wikipedia.org/wiki/Interna...rgy_efficiency Engine efficiency is limited by thermodynamic laws. "Most steel engines have a thermodynamic limit of 37%." Further, "most engines retain an average efficiency of about 18%-20%." Right away we see it's at least potentially possible for gasoline to still beat out 100% efficient LH2. But let's go on the low side and assume a gasoline engine is 18% efficient. Then we need to figure out the efficiency required of an LH2 system to beat gasoline: 9000 * 18% = 1620 1620 / 2600 = 62% Therefore an LH2 system would have to be 62% efficient overall to beat a typical gasoline engine. Per the same Wikipedia article, "Electric motors are better still, at around 85%-90% efficiency or more, but they rely on an external power source (often another heat engine at a power plant subject to similar thermodynamic efficiency limits)." OK. So an electric motor _by itself_ is more than efficient, but as stated it has to get it's electricity from somewhere else. We are assuming an LH2 powered source. Let's go with the high side of 90% on the electric motor. So we have to now figure out what efficiency is required in converting LH2 to electrity so a 90% efficient electric motor produces 1620 Wh/l of LH2... 1620/.90/2600 = 69% Now that leaves us with finding out how efficiently LH2 can be converted to electricity. Per: http://energy.gov/eere/fuelcells/fuel-cells "Fuel cells can operate at higher efficiencies than combustion engines, and can convert the chemical energy in the fuel to electrical energy with efficiencies of up to 60%." So we may be coming up a bit short. However, all my pondering here is surely a gross oversimplification. And it's possible I goofed on my math or went astray with my logic. And I imagine different sources will give different numbers. But I hope it gives you some idea. There are surely other factors that need to be taken into account. Some may make things work out better, others may make things worse. Brian The efficiency numbers look to me to be back of an envelope correct. The real world, however, has very little interest in the energy efficiency of things like airplanes. Some numbers that people care about are endurance, operating cost. initial cost, and life time maintenance cost. Given some reference platform, such as a C-172, what would be the enduraance of a LH2 system for that platform versus gas? How much does 1 hour of LH2 cost versus gas? In what column do we put the typical 1%/day evaporation loss of LH2 and the venting equipment you would have to have in a hanger to get rid of it? We can swag what a LH2 system would cost from commercial stuff, but how much of an adder will aircraft certification cost? LH2 tanks have limited life; inspection and replacement costs? Energy efficiency is nice to talk about, but it is dollars that make things happen. -- Jim Pennino |
#32
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CAFE Electric Aircraft Symposium Set For May 1
wrote in :
The efficiency numbers look to me to be back of an envelope correct. And that's all I guarantee them for! I was about to do some more calculations to try to answer your questions, and in the process of searching for numbers I found the following Wikipedia article. http://en.wikipedia.org/wiki/Hydrogen_economy At a glance this article seems to cover a lot of the issues being discussed. I for one would like to read it before trying to answer. It might be worth other's to read as well. Brian -- http://www.earthwaves.org/forum/index.php - Earth Sciences discussion http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism Sed quis custodiet ipsos Custodes? |
#33
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CAFE Electric Aircraft Symposium Set For May 1
Skywise wrote:
wrote in : The efficiency numbers look to me to be back of an envelope correct. And that's all I guarantee them for! I was about to do some more calculations to try to answer your questions, and in the process of searching for numbers I found the following Wikipedia article. http://en.wikipedia.org/wiki/Hydrogen_economy At a glance this article seems to cover a lot of the issues being discussed. I for one would like to read it before trying to answer. It might be worth other's to read as well. Brian After reading that article, I have come to the conclusion that hydrogen as fuel will become a general economic reality only if at least one of two things happen: The world runs out of petroleum, natural gas, and coal and there is no other choice. There is huge advancement in electricity production, such as cheap fusion. http://en.wikipedia.org/wiki/Hydrogen_economy#Costs -- Jim Pennino |
#35
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CAFE Electric Aircraft Symposium Set For May 1
On Wed, 3 Jun 2015 21:57:44 +0000 (UTC), Skywise
wrote: Larry Dighera wrote in : Hello Brian, You seem to have firm grasp of the physics involved. Are you able to suggest how efficient an electric power system powered by LH2 would have to be to offset the power density difference from gasoline/kerosene? A quick disclaimer.... I don't have any degrees in this stuff or work with it. I just happen to be very interested in the sciences in general and have taught myself a few things over the years. Having said that, I try very hard to check my facts and figures before I say anything. I have an allergy to foot-in-mouth disease. On to your question.... Per numbers in http://www.tinaja.com/glib/energfun.pdf Liquid H2 has an energy density of 2600 Watt-hours/liter. Gasoline is 9000 Watt-hours/liter. [I used energy density per volume rather than per mass because that's the limiting factor in any vehicle, the volume of the 'gas tank'] If we make an assumption for discussion's sake that an LH2 powered system were 100% efficient, then the gasoline system would only need to be 2600/9000 = 29% efficient to reach parity with LH2. But note that nothing is ever 100% efficient. There are _always_ conversion losses. It's a matter of how much. Per: http://en.wikipedia.org/wiki/Interna...rgy_efficiency Engine efficiency is limited by thermodynamic laws. "Most steel engines have a thermodynamic limit of 37%." Further, "most engines retain an average efficiency of about 18%-20%." Right away we see it's at least potentially possible for gasoline to still beat out 100% efficient LH2. But let's go on the low side and assume a gasoline engine is 18% efficient. Then we need to figure out the efficiency required of an LH2 system to beat gasoline: 9000 * 18% = 1620 1620 / 2600 = 62% Therefore an LH2 system would have to be 62% efficient overall to beat a typical gasoline engine. Per the same Wikipedia article, "Electric motors are better still, at around 85%-90% efficiency or more, but they rely on an external power source (often another heat engine at a power plant subject to similar thermodynamic efficiency limits)." OK. So an electric motor _by itself_ is more than efficient, but as stated it has to get it's electricity from somewhere else. We are assuming an LH2 powered source. Let's go with the high side of 90% on the electric motor. So we have to now figure out what efficiency is required in converting LH2 to electrity so a 90% efficient electric motor produces 1620 Wh/l of LH2... 1620/.90/2600 = 69% Now that leaves us with finding out how efficiently LH2 can be converted to electricity. Per: http://energy.gov/eere/fuelcells/fuel-cells "Fuel cells can operate at higher efficiencies than combustion engines, and can convert the chemical energy in the fuel to electrical energy with efficiencies of up to 60%." So we may be coming up a bit short. However, all my pondering here is surely a gross oversimplification. And it's possible I goofed on my math or went astray with my logic. And I imagine different sources will give different numbers. But I hope it gives you some idea. There are surely other factors that need to be taken into account. Some may make things work out better, others may make things worse. Brian Hello Brian, Thank you for your fair and conservative analysis. Very much appreciated. So generally on a theoretical basis, it is within the realm of possibility that using LH2 to generate electric power with a fuel-cell to power an electric motor might be completive in terms of performance with today's General Aviation internal combustion powered aircraft, because the efficiency of the electrically powered system is potentially so much greater than the IC technology, that it compensates for the reduced power density of the LH2 fuel compared to petroleum. The laws of physics don't prohibit it. Of course, for this to be realized, significant engineering remains to be accomplished, but the path to electrically powered aircraft isn't a dead end due to the laws of physics. Larry |
#36
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CAFE Electric Aircraft Symposium Set For May 1
Larry Dighera wrote in
: Of course, for this to be realized, significant engineering remains to be accomplished, but the path to electrically powered aircraft isn't a dead end due to the laws of physics. VERY SIGNIFICANT engineering. None of my rough calculations take into consideration the means by which to store LH2 safely in a vehicle. For one thing, there will certainly be a weight penalty. If enough, it may offset any efficiency gains. Steel pressure tanks aren't the lightest things in the world. I recall work being done in the past in the space program to make composite LH2 tanks, but they just couldn't ever withstand the pressures. http://en.wikipedia.org/wiki/Lockheed_Martin_X-33 But here's another safety thought. Would you want planes flying around that are basically hydrogen 'bombs'? I mean, it's bad enough when any aircraft crashes, but one with a tank of pressurized LH2 on board? hmmm... BLEVE anyone? Brian -- http://www.earthwaves.org/forum/index.php - Earth Sciences discussion http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism Sed quis custodiet ipsos Custodes? |
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