Q & A Do you have a question about hydrogen? Send us an email or submit a question below. Q: I’ve heard that hydrogen is not practical as a fuel because more energy is required to produce hydrogen than can be gotten out of it. Is this true? Show Answer >

A: The statement is half true. Hydrogen is an energy carrier like electricity. About 3 MJ of primary energy is required to produce 1 MJ of electricity; equivalently, the overall efficiency of electricity production is on the order of one-third. Very few people would conclude from this that electricity is impractical as an energy carrier. A high-efficiency steam electrolyzer can produce hydrogen from water and electricity with an efficiency of about 0.9. Thus, hydrogen production can be nearly as efficient as electricity production. The part of the statement “more energy is required to produce hydrogen than can be gotten out of it” is true, but the part of the statement “hydrogen is not practical” is not true for the same reason that such a statement is not true for electricity.

Q: How do you see producing hydrogen on such a large scale as to replace fossil fuels? Show Answer >

A: It is the vision of the Supersonic Institute to produce electricity from nuclear or renewable primary energies, and from this carbon-free electricity, to produce hydrogen. The electricity would carry energy for stationary applications, and the hydrogen would carry energy for mobile applications.

Q: Because of the hydrogen molecule’s low mass, if hydrogen is accidently released into the Earth’s atmosphere, does it ultimately escape our gravitational field and enter into space? Could this explain why there are only trace amounts of hydrogen in the Earth’s atmosphere? Show Answer >

A: All atmospheric molecules, if they are not removed by other processes, will ultimately escape into space. For a given gravitational field and atmospheric temperature, the rate of escape of a gas is proportional to its mean molecular velocity

rate = k √(1/M)

where k is a proportionality constant and M is a gas’s molecular mass (in atomic mass units). Since M = 2 for hydrogen and M = 32 for oxygen, hydrogen will escape 4 times faster than oxygen. If gravitational escape were the only process at work, it would take billions of years to significantly reduce the concentration of a gas. Other processes affect the concentration of a gas in the atmosphere. For example, oxygen is added and removed from the atmosphere by biological organisms. Flammable gases such as hydrogen, carbon monoxide, and methane are removed by air-oxidization; like hydrogen, carbon monoxide and methane also are found in only trace amounts in the atmosphere. Assuming oxidation is a much faster process than gravitational escape, little hydrogen escapes into space because it is oxidized to water before it can escape. The water of oxidation, however, with M = 18 amu, will escape faster than oxygen.

Q: How safe is hydrogen? Would a spark within the supersonic hydrogen tube system cause explosion of the hydrogen and destruction of the entire tube system? Show Answer >

A: No. Combustion of the hydrogen is not possible as long as air in the tube is below 25 % by volume of the tube atmosphere: 25 % air corresponds to 75 % hydrogen, and hydrogen combustion in air is only possible when hydrogen is within the limits of 4 % (its lower flammability limit) and 75 % (upper flammability limit). Prevention of combustion is the reason the tube pressure is kept slightly above air pressure outside the tube. Under these conditions, any leakage of the tube would result in hydrogen escaping to the outside rather than air entering the tube.

Q: Because hydrogen is so tenuous (low density ρ), how can the efficiency of a propeller in hydrogen be equal to one in air? Show Answer >

A: Propulsive efficiency η is the power exhibited by a vehicle divided by the propeller’s input power. In symbols

η = TV / P

where η is the propulsive efficiency, T is the thrust provided to the vehicle by the propeller, V is the vehicle’s velocity, and P is the shaft power required by the propeller. Because both T and P are functions of the first power of gas density ρ, the density cancels and η indeed is independent of ρ. On the other hand, because T increases as the first power of ρ, the thrust provided by a given propeller is much lower when operating in hydrogen than when operating in air. The supersonic hydrogen tube vehicle will require large-diameter propellers, more akin to helicopter rotors than airplane propellers.

Q: How can the supersonic hydrogen vehicle compete on cost with air travel, which has a free infrastructure – the sky? The infrastructure cost will be horrific! Show Answer >

A: The infrastructure of air transport is not free. The true cost includes the cost of airports, access roads and railways required by location of airports distant from city centers, the air traffic control system, VOR beacons and radar antennas covering all flight paths, a portion of GPS, and weather prediction facilities. To meaningfully compare the costs of air transport and supersonic hydrogen transport, one must compare the total cost, which is the sum of infrastructure, operating, maintenance, and social cost. The social costs of air travel are quite large and include lost time due to security screening, lost time due to required early arrival at an airport, lost time and opportunity due to weather delays, and air and noise pollution. A recent study by the University of California-Berkeley's Institute of Transportation Studies found that the social cost of flight delays alone, for the US, was $33 billion in 2007. Social costs are presently borne by passengers, people living near airports, and taxpayers. While the infrastructure cost of the supersonic hydrogen system will likely be higher than air-transport infrastructure cost, its operating, maintenance, and social costs will be lower. To compare the total costs of the two transport modes is the subject of a planned study of the Supersonic Institute.

Q: At what speed is ground-effect large enough to support the vehicle? Show Answer >

A: The proposed levitation method of the supersonic hydrogen vehicle does not employ ground effect, but employs aerostatic gas bearings. Accordingly, the vehicle can levitate at zero forward speed. We have built a small working model to demonstrate this kind of levitation. Further details can be found in the first paper on the supersonic hydrogen vehicle, “Hydrogen tube vehicle for supersonic transport: Analysis of the concept.”

Q: I heard that 75% of the universe is hydrogen. Why is this? Since there is little free hydrogen on Earth, could this be evidence that low-mass hydrogen does in fact escape the gravitational fields of planets? Q: Is hydrogen toxic? What happens if you breathe the gas? Q: What about the Hindenburg? Q: I understand about the Hindenburg. But what about the hydrogen bomb? Q: Is hydrogen fire invisible? If so, does the invisible flame present a safety issue? Q: What’s the difference between hydrogen fission and hydrogen fusion? Q: Would riding in the supersonic hydrogen vehicle be a problem for people with claustrophobia? A similar situation exists for airplanes, but at least airplanes have windows. Q: Will there be “superjetlag” for travel at Mach 2.8, your proposed speed for the supersonic hydrogen vehicle? Q: How serious is the threat of terrorism to the supersonic hydrogen tube system? Q: I realize this is a weird question. Which would be worse: Being burned by hydrogen or being burned by gasoline or propane? Click here to join the discussion, or ask a question