Don Lancaster wrote:
The Ghost In The Machine wrote:
If one assumes 4 atm for a fuel cell one still gets 793 3/4 gallons -- an
energy density by volume of 0.34%, when compared to gasoline,
if my computations are correct.
The supposedly higher energy density by weight of hydrogen is totally
useless for terrestral apps.
You have to consider the CONTAINED energy density by weight, which is
ALWAYS ridiculously less than gasoline.
As to energy density by volume, gasoline offers 9000 watthours per
liter, while STP hydrogen offers 2.7 watthours per liter electrically
recoverable or 3.3 watthours per liter total heat recovery.
At 4 BAR pressure, hydrogen offers 10.8 watthours per liter, or about
1/833rd that of gasoline. About 0.12 percent.
At 100 BAR pressure, hydrogen offers 270 watthours per liter, or about
1/33rd that of gasoline.
There is, of course, more hydrogen in a gallon of gasoline than there is
in a gallon of liquid hydrogen.
See http://www.tinaja.com/glib/energfun.pdf for a detailed analysis.
The problems of converting to a hydrogen economy include the higher
energy of getting it out of easily available sources such as
hydrocarbons than the energy available in the hydrogen and that of
storage and transport.
Of the two I consider the problem of storage and transport to be
solvable near term.
One method being investigated is using glass microspheres to store it
at high pressu
AU researchers looking at hydrogen in tiny glass beads as fuel source
for cars.
http://www.fuelcellsworks.com/Supppage1764.html
This uses microspheres of diameters of a few microns to store it at up
to 100 MPa. It has been found the microspheres will infuse the hydrogen
when heated to high temperatures at high pressure. Then store it at
normal pressure and temperature. The hydrogen can be released again at
high temperature and pressure conditions. The latest research shows
that using lasers can speed up the speed at which the hydrogen is
released.
The Department of Energy has set the ultimate goal for hydrogen energy
storage to be superior to that of gasoline as above 10 MJ energy stored
per kg of total weight and 10 MJ per L of total volume. At an energy
content of hydrogen at 142 MJ per kg, this means about .07 kg of H2 per
kg of total storage system weight and .07 kg of H2 per liter of total
storage system volume.
A material that might be able to reach these criteria is "tetrahedral
amorphous diamond" if used in the form of microspheres.
This report gives an average tensile stength of 7.3 GPa when tested on
micron-scale samples:
Young’s modulus, Poisson’s ratio and failure properties of
tetrahedral amorphous diamond-like carbon for MEMS devices.
J. Micromech. Microeng. 15 (2005) 728–735
doi:10.1088/0960-1317/15/4/009
http://ej.iop.org/links/q03/3NXzoBo,...jmm5_4_009.pdf
The thickness to radius ratio of a spherical pressurized tank is given
by:
h/r = Δp/(2σ)
where h is the wall thickness, r the radius of the sphere, Δp the
overpressure, and σ the tensile strength of the material.
This page gives properties of hydrogen at various pressures and
temperatures (there is deviation from the ideal gas law at very high
pressures):
Hydrogen Properties Package.
http://www.inspi.ufl.edu/data/h_prop_package.html
At a temperature of 300 K, a pressure of 6000 bar gives a density of
72 kg/m^3, or .072 kg/l.
Using a tensile strength of 7.2 GPa = 72,000 bar for the tetrahedral
amorphous diamond and 6000 bar pressure for the hydrogen, the thickness
to radius ratio for a spherical tank would be h/r = 1/24.
The volume for a sphere is V = (4/3)Pi*r^3. For a wall thickness small
compared to the radius, we can take the volume of the wall to be 4*h*
Pi*r^2, which equals (1/6)*Pi*r^3, when h/r = 1/24.
Since the volume of the tank and the wall both have r to the third
power, the radius will cancel when calculating the ratio of the
hydrogen mass to the mass of the tank wall material. So I'll take r =
1. Then the mass of the hydrogen in the tank would be 72*(4/3)*Pi =
301.6 kg.
I'll take the density of tetrahedral amorphous diamond to be that of
diamond, 3500 kg/m^2. Then the mass of the container would be:
3500*(1/6)*Pi = 1885 kg. Then the ratio of the mass of hydrogen to the
container wall mass would be 301.6/1885 = 0.16.
The tetrahedral amorphous diamond is amorphous as is glass. So it may
be that heat and laser irradiation could also allow hydrogen to be
infused and/or released.
Bob Clark