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September 19, 2021


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Bruce Reid MIMechE

Wow, this blew me away...
Normal vehicles burn hydrocarbons. A hydrocarbon is a molecule consisting of hydrogen and carbon. During burning, the carbon reacts with Oxygen to create carbon dioxide (and carbon monoxide), and the hydrogen creates H2O (water). In addition, there are huge additional pollutants - most fuels are contaminated by sulphur-compounds and, as it breaths air and not pure oxygen, the temperatures and pressures involved split the Nitrogen molecules and forms oxides of nitrogen (NOx) which are highly toxic (and also deep red in colour and the source of the red-brown layer that sits above cities and can be clearly seen when flying).
They hydrogen and carbon in this instance has been sequestered underground at some point over the last 3.5 billion years (since the first photosynthetic life appeared on the planet and sequestration began).
Since we started using coal, oil and gas (all hydrocarbons) we have been releasing the effects of that sequestration. Including that water!
Hydrogen vehicles utilise pure hydrogen in combination with a fuel cell driving an electric motor. the output is ONLY water with ZERO toxins as the fuel cells are both low pressure and temperature: The Nitrogen molecule is never split and and the contaminants do not exist. (An IC engine running on Hydrogen has far worse NOx emissions than hydrocarbon fuel)

John Dunn

If I understand then, the water vapor emissions of a hydrogen fueled vehicle would not pose a new threat. That might depend on relative amounts of water vapor emissions, but the point is understood.

What about the 10% hydrogen dispersal into the upper atmosphere?

Bruce Reid MIMechE

Apologies, I did not receive a note of the reply:
I think the 10% figure sound exceptionally high - and somewhat skewed to me...(possibly political...?). It also depends on the transport medium utilised for fueling the hydrogen - gas/supercooled/molecular.
Either way: whilst capable of up to 37% efficiency in "ideal conditions", a petrol(gas) engine generally runs at about 20%. Take into account a gearbox efficiency for the drive train of 85% (slightly generous perhaps..?) and for the petrol(gas) a refinery to pump efficiency of ~80% (I could find no information on the cost of extraction and delivery to the refinery, unfortunately, but that is not likely to be insignificant!)
This gives a petrol(gas) IC engine [0.2*0.85*0.8 = 0.136] a 13.6% efficiency at the wheels - remembering the pollutants.
With a hydrogen vehicle:
Fuel cell efficiency - 60% (and improving), electric motor 70-96% efficient - take 85% as a middle of the road figure (though - I suppose I must acknowledge it - the tesla motor uses some very "clever" engineering to get an exceptionally efficient motor, in the 90+% I believe!), and the PEM efficiency of electrolysis for green hydrogen of 85% (with a 'theoretical optimum' of 92%) - 0.6*0.85*0.85 = 0.4335 - 43.34% efficiency at the wheels and zero harmful emissions.

When you take into account the amount of water emitted from a conventional petrol (gas) IC engine and compare it to a hydrogen vehicle, due to enhanced efficiencies, the volumetric water output is SIGNIFICANTLY LOWER! This is a surprise and only discovered in researching (and calculating) to formulate this reply. In addition, if obtained from "green" sources (e.g. non-potable water) and not "blue" sources (reformed gas/hydrocarbon) we are not reintroducing historically sequestered sources, merely circulating the environmentally present stuff!

John Dunn

Getting back to hydrogen loss to the atmosphere though, I believe that the interviewee on the radio was referring only to a 10% loss in the process of transferring hydrogen from the filling fixture to the vehicle. He was not discussing sequences of events.

I think that his point was that there appears to be a greenhouse downside to hydrogen which is not being addressed to the general public nor to regulators.

That points up a larger problem that there is a seemingly insatiable desire for panaceas, for simple one-step answers to any problem that will simply make that problem vanish.

Electric car batteries are very much an example of that. Please see:


There are other URLs as well that take up this topic.

To my view, some lithium-ion batteries are fire-bombs waiting to go off. Recently reported electric car fires and now Chevrolet Bolt fire hazards illustrate the point. Advice is given to not park an electric car where a fire might erupt and burn down your house.

Every technology has its up side and its down side, but proponents seem to keep the down sides out of discussions much to everyone's peril.

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