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Hydrogen vs Fossil Fuels
Name: Tim D
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 11/15/2004
Question:
The current design of Hydrogen cars seems to include relatively
massive radiator systems built into the cars to eliminate from the car the
heat
byproduct of the engine.
How do the heat emissions from a Hydrogen engine compare with the heat
emissions of a fossil fuel engine?
Replies:
You raise scientifically complicated and politically charged issues.
Hydrogen is presented as the "clean burning" alternative for fossil fuels,
but the dream is neither so simple nor clear. Put aside for the moment
that the technology of producing, storing, distributing, and burning
hydrogen (in air) is not in place and the associated engineering problems
and economics are daunting. A few examples fished from a large school of
problems. Hydrogen has very large flammability limits. That's good news in
some respects, but it also places tight constraints on engine design,
because "hot spots" in the cylinders (assuming an internal combustion
engine) will result in pre-ignition, "pinging" as it is known in the
hydrocarbon world. The flame temperature of the combustion of hydrogen is
higher than hydrocarbon fuels. This high temperature (in principle) is a
benefit, but since air is 80% N2 the high temperature can result in the
formation of higher levels of nitrogen oxides. That is the bad news. Just
how enough H2 can be manufactured economically, and the infrastructure to
distribute it nationally, even globally, is unsolved. If you compare H2 to
CH4 on an equal mass basis it appears that there is a 16/2 advantage (the
molecular weights) in favor of hydrogen; however, on an equal energy
basis: H2 + 1/2O2 = H2O compared with CH4 + 2O2 = CO2 + 2H2O all
components in the gaseous state the advantage is only ~ 3.6 because
considerable energy is derived from the formation of CO2. But I am saving
the best (or worst), and most neglected issue for last. One that has been
neglected, like the proverbial elephant in the living room. WATER IS A
POTENT GREENHOUSE GAS. I have searched in vain for climatological studies
that have factored in the effect of the release of millions of tons of
water vapor into the atmosphere. We may be trading smog for fog! The
hydrogen age is politically correct these days, but the scientific and
engineering realities are being swept under the carpet.
Vince Calder
Tim- not having seen what vehicle you are referring to, I could use a picture or reference.
There may be more than one kind of hydrogen-fueled car, since there are several distinct advantages to hydrogen:
1) directly reactable to make electricity (fuel cells). Hence more energy-efficient.
2) industrially regenerable, given sufficient electrical power. Hence less environmental burden to provide it.
3) no CO2 green-house gas is generated when it runs. Hence environmentally tolerable to greater levels of world-wide use.
4) inherently less smog potential, because only 3 elements (O,N,H) instead of 4 (O,N,H,C) in the flame.
A hydrogen-powered piston-engine is conceivable, as are other heat-engines such as turbine or steam or Sterling,
because these would lose only advantage (1) above.
These kinds of thermal engines have the same 2/3 waste as heat that our present gasoline engines have.
No reason for larger radiators there.
Steam engines have 5th advantage, the 3rd element N is never forced to react with O,
eliminating NOx components of smog.
A high-efficiency steam engine requires a good heatsink at a pretty low temperature,
preferably lower than boiling water.
Cooling the same heat-flow to a lower temperature requires a larger radiator.
If the exhaust was to be cooled to keep the newly made steam (2H2+O2-> 2H2O) as water,
that would require large radiators to take out sufficient heat in a short enough time
for the high flow rate of the gaseous exhaust.
You'll notice that "moonshine stills" or chem-lab condensers always have noticeable or
large cooling coils;
it takes a surprisingly large apparatus to pull the large heat of condensation out of a
flowing vapor.
In these cases the radiator is larger than an ordinary car's, not because the heat
output is more than an ordinary car,
but because the goal temperature is cooler and closer to ambient than the 230F
temperature of a car's radiator.
If it's a fuel-cell electric car you refer to, then the total heat output should be
significantly less than present cars.
You should look to see what component of the vehicle the radiator serves:
- the fuel tank (may actually emit heat while filling, or need to run warm while
emptying),
- the fuel cell (must run warm, may avoid heatsinking),
- the exhaust system (see exhaust steam above?),
- the electric motor(s),
- the combustion engine (if it's a hybrid car),
- the batteries (if it's a hybrid car),
That component might, in that particular vehicle, require a lower temperature than
boiling water.
Jim Swenson
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