Boeing Calls Hydrogen UAV "Game Changer"

[allardjd 1,853]

Boeing Calls Hydrogen UAV "Game Changer"

 

http://www.avweb.com/eletter/archives/a ... tml#202873

 

I should explain why I'm posting this here in General rather than in Real World Aviation. The really interesting and exciting part of this is the engines and fuel. This post is much more about energy and ecology than aviation.

 

"The aircraft is powered by two converted Ford Ranger truck engines that burn liquid hydrogen and turn four-blade props. The only byproduct is water."

 

"The hydrogen propulsion system will be the key to Phantom Eye's success. It is very efficient and offers great fuel economy."

This is a really interesting and exciting development and my reaction on reading it was, "At last..."

 

I react with excitement, not to the new aircraft or its capabilities, but to the details of how it's powered. I've been a strong proponent of using hydrogen fuel in conventional combustion engines for a long time and it appears Boeing has built a working version with the intent of doing a real-world job with it.

 

All this business about using very expensive, heavy and complex fuel cells in automobiles in order to covert hydrogen into usable energy with no harmful emissions seems silly to me. The very same hydrogen can simply be burned in a conventional engine with relatively small modifications. The only thing that comes out the exhaust pipe is water vapor.

 

This is where automotive technology should be heading. If we're worried about carbon emissions, we should be charging down this road, but for some reason the tree-huggers don't seem to favor it. The vehicles and engines would not need to be much different from those manufactured now. It would not be necessary to re-design from the ground up for radically different new components, electric drive, etc. The vehicles could be much the same as now, with a different kind of fuel storage system and a different method of metering the fuel into the engine. All else would be very much the same, with no need to somehow incorporate a fuel cell that costs more than the rest of the car, is the size a young refrigerator and weighs even more.

 

The only carbon emissions occur at the power plant that burns fossil-fuel to make the electricity that is used to extract the hydrogen (and concurrently, oxygen) from water. If the power plant is nuclear, the carbon emissions for the whole process are essentially zero. Even if coal or oil fired, power plants are significantly more efficient and cleaner than producing the same amount of energy in a million automobile engines burning gasoline or diesel.

 

This methodology, if broadly applied, would go a very long way toward solving the dual problems of carbon emissions and dependence on foreign oil. I hope that Boeing's work sparks further applications that use hydrogen combustion engines to replace conventional engines. Everyone wins except OPEC.

 

John

[mutley 4,498]

Hi John,

I am with you about the use of Hydrogen ignited rather than used in cells, it seems like a no-brainer. I wonder if the oil companies try to stifle its development?

[allardjd 1,853]

To quote an infamous Englishwoman, "Well they would, woudn't they?"

 

John

[ddavid 149]

allardjd said:

To quote an infamous Englishwoman, "Well they would, woudn't they?" :biggrin:

John

 

Actually, John, she said He would - wouldn't he and, in fact, she was welsh - born in Carmarthenshire, near our first farm (although she moved to Solihull before we got there!)

 

As to burning hydrogen, there are a couple of minor difficulties:

 

1. It is possibly the most flammable substance that we know, and

2. You'd use up one heck of a lot of carbon equivalent in extracting it from water - even if you used nuclear powered electricity.

I'd love it to be possible, but I'll carry on hugging trees!

 

Cheers - Dai.

[allardjd 1,853]

Quote

1. It is possibly the most flammable substance that we know...

 

As opposed to, say, gasoline, diesel fuel, propane or natural gas, all of which are used successfully to fuel vehicles?

 

NASA seems to have done alright with the Shuttles burning tons of it too, if you're willing to overlook one launch accident that has no bearing on automotive uses of H2. Burn a hole in any large fuel tank with solid rocket exhaust and you'll probably have a spectacular result, so I'll submit that we can discount that one. That aside, NASA handles and burns it quite effectively and safely.

 

Quote

2. You'd use up one heck of a lot of carbon equivalent in extracting it from water - even if you used nuclear powered electricity.

 

How so?

 

If nuclear power is used, there are essentially zero carbon emissions - none, zip, nada, zilch.

 

If the power for electrolysis of water comes from a coal or oil fired plant, the equivalent energy in H2 will come at a much lower carbon cost than if that same amount of energy is produced by a million gasoline or deisel engines. The efficiencies and economies of scale of large power stations come into play and the myriad of mobile polluters come in a distant second.

 

The carbon emitted by the power plant powering the electrolysis process will be more than offset by the hydrocarbon fuel not burned by the vehicles powered by burning the hydrogen.

 

References

 

How much energy is required to produce hydrogen via electrolysis of water?

 

The energy required to produce hydrogen at atmospheric pressure via electrolysis (assuming 1.23 V) is about 32.9 kWh/kg. A kilogram is about 2.2 lb. For 1 mole (2 g) of hydrogen the energy is about 0.0660 kWh/mole. Compressing or liquefying the hydrogen would take additional energy. One company produces hydrogen through electrolysis at about 7,000psi at an energy usage of about 60kWh/kg H (2).

 

Because a Watt is Voltage x Current, this is equivalent to Power x Rate x Time. The power in this case is the voltage required to split water into hydrogen and oxygen (1.23 V at 25?C). The rate is the current flow and relates directly to how fast hydrogen is produced. Time, of course, is how long the reaction runs. It turns out that voltage and current flow are interrelated. To run the water splitting reaction at a higher rate (generating more hydrogen in a given time), more voltage must be applied (similar to pushing down on the accelerator of a car; more gas is used to make the car go faster.) For commercial electrolysis systems that operate at about 1 A/cm2, a voltage of 1.75 V is required. This translates into about 46.8 kW-hr/kg, which corresponds to an energy efficiency of 70%.

 

Lowering the voltage for electrolysis, which will increase the energy efficiency of the process, is an important area for research.

 

Sources

U.S. Department of Energy

National Hydrogen Association

 

Doesn't it take too much energy to make hydrogen? Is it worth doing?

 

Like all fuels, it takes energy to produce hydrogen and deliver it to a vehicle. The amount of energy required depends on how the hydrogen is made. Some methods require more energy than others.

 

While it may take more energy to produce and deliver hydrogen than it takes to produce and deliver gasoline or natural gas, the hydrogen fuel is used more efficiently in hydrogen vehicles. Most hydrogen internal combustion engines (ICEs) are about 25% more efficient than their gasoline counterparts and fuel cells are 100-200% (2-3 times) more efficient. In many cases, the overall "well-to-wheels" energy usage can be much lower for hydrogen vehicles than for gasoline or natural gas vehicles using a conventional internal combustion engine.

 

Source

National Hydrogen Association

 

Industrial rates for electricity in the US are about $06.5 per kw-hr. Using the stated 60 kw-hr per kg cost of producing H2 at 7,000 psi (included the cost of compressing it for storage and transport), that's about $3.90 per kg to produce. I don't know the energy equivalent of similar amounts of H2 vs. gasoline or diesel, but that sounds pretty reasonable off the top.

 

If you also take into account the stated benefit of the better engine efficiency burning H2 and the positive effects on use of foreign oil and carbon emissions, this sounds like a winner. Large scale conversion might also lead to further economies in the production of H2 going forward.

 

John

[stu7708 244]

As opposed to, say, gasoline, diesel fuel, propane or natural gas, all of which are used successfully to fuel vehicles?

John

The difference from gasoline and diesel is that they aren't really flammable in a liquid form, and at normal temperatures liquid is the natural state for those fuels. It's the correct mixture of air (oxygen) and fuel that is highly flammable..

I recall a Mythbusters episode where the tried this out. A line of gasoline will burn, but at a very slow rate (you could easily outrun it on foot, and it's really the vapour burning, not the liquid. Diesel, and even jet-fuel, poured on to the ground couldn't be lit at all with a torch. They also tried to fire a tracer round in to a gasoline tank, and nothing happend, a part from the tank starting to leak of course...

With liquid Hydrogen, if you did get a leak I'm guessing it would turn back to a gas state right away, and be extremely flammable. Making a crash with cars running on it a potentional nightmare. So extremely durable fuel tanks would be needed...

But as you say, NASA has made it work on a much larger scale....

[allardjd 1,853]

I heartily agree, of course, that the fuel tanks would need to be durable and well protected, but don't see that as any more than an engineering problem.

 

A fundamental decision would be whether to go with liquid H2 (cryogenic) or high pressure gas bottles - there are arguments for and against both.

 

I don't see the use of H2 as auto fuel being significantly more hazardous than anything else - it's fuel after all, and by definition is flammable and therefore hazardous. It's not absolutely safe - nothing is.

 

Burning hydrogen in conventional engines is a uniquely simple, easily available way to wean ourselves from oil-guzzling automobiles in the relatively short term. The only hurdles are engineering, logistics, economics and public acceptance. Unlike some other things being proposed, there's nothing needed in the way of scientific research. It's down to the nuts and bolts now and Boeing seems to have made it work fairly easily by modifying standard light truck engines.

 

I'm a strong proponent of going this way. It may not be a perfect solution, but if we don't do something we'll be paying $50 a pint for gasoline one of these days and the OPEC guys will be laughing all the way to the bank - which they will own.

 

John