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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 |
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