This is an update of some numbers from an older posting
http://ergobalance.blogspot.co.uk/2007/10/ulf-bossel-platinum-and-hydrogen.html in light of the larger commercial wind turbines that are now available
http://ergobalance.blogspot.co.uk/2012/01/shaky-foundations-for-offshore-wind.html. There is some improvement in the overall figures, but the task of switching from oil to hydrogen remains stupendous.
At the outset, we should note that hydrogen does
not occur free in nature but must be freed from other elements, such as
oxygen in water, with which it is naturally combined, and the separation
of elements requires other forms of energy. Almost all the hydrogen
used currently in the world - principally as a chemical feedstock e.g. for oil
refining and making artificial fertilizers - is made by steam-reforming
natural gas, and there is a CO
2 budget that must be costed-in, hence
hydrogen from this source is not clean but contributes to CO
2 emissions.
Furthermore, it consumes natural gas, and so there is a further demand placed
on another resource, in accord with the indisputable fact that it takes
resources to extract resources. Ideally therefore, that hydrogen should
be produced by e.g. water electrolysis using electricity made from
renewable sources.
Some while ago, Ulf Bossel pointed out
http://www.fuelcellforum.com/reports/E21.pdf there are losses at each stage in the chain of production, storage and distribution for hydrogen.
There is obviously a loss of 50 - 60% incurred when the material is oxidised in a fuel cell, but in its favour is the fact that an
efficiency of even 40 - 50% is substantially above the Carnot-cycle
limit (Thermodynamics again) of around 35% for a typical internal
combustion engine. The losses may be summarised as follows: 90%
efficiency for rectifying alternating current to DC to run the
electrolyzer; 75% overall efficiency (ideal) for the electrolyzer
itself; and then the storage of the bulky hydrogen gas either as a
highly compressed gas, which takes about 20% of the energy content of
the hydrogen to compress it (or as a cryogenic liquid, which takes 30 -
40% to produce); 10% for distribution and say 50% efficiency for the
fuel cell itself, which amounts to about a 25% efficiency overall.
There
are electrolyzer units
http://www.nrel.gov/docs/fy04osti/36705.pdfthat can produce high pressure hydrogen and if
each gas-station were to make its own hydrogen by electrolysis, much of
the distribution losses (probably 30%) might be avoided. A report has
been published by a firm of independent analysts in Germany which is
critical of some of Bossel's figures
http://mpfc.de/pdf/LBSTonBossel.pdfespecially in regard to storage and
transmission, particularly across large distances say from sunny north
Africa (if the hydrogen were produced using PV technology which would be
much more efficient there) by pipeline to Europe. However, an
in-situ
arrangement as I allude to would surely get around that, presuming we
could make enough renewable electricity, or if there were a grid of
electrons (rather than of hydrogen) including north African PV, European
wind-power, North Sea wave energy and so on, such power might be
supplied to run local electrolysis equipment, which would avoid actual
hydrogen transmission. But if Bossel is right, why not use these
electrons in a more direct manner?
On a tit-for tat basis, we can make the following calculation:
The
heat of combustion of hydrogen is -285 kJ/mol, and so 1 kg of hydrogen =
1000 g/2 g/mol x -285 kJ= -142,500 kJ = 1.425 x 10^8 J.
We get
through 82 million tonnes of oil altogether annually in the UK and we
use 60 million tonnes of that for fuel. The energy content of oil is
rated at 42 GJ/tonne and so that 60 million tonnes "contains" 60 x 10^6 x
42 x 10^9 Joules = 2.52 x 10^18 J of energy.
Hydrogen can be
produced at a pressure of up to 10,000 psi by electrolysis at a rate of
60.5 kW/kg of H
2. Hence the equivalent H
2 to match that amount of oil
is:
2.52 x 10^18 J/1.425 x 10^8 J/kg = 1.768 x 10^10 kg H2.
Bossel has used the conversion factor of 1.5, i.e. that H2 can be used
with 1.5 times the recoverable energy efficiency of gasoline. Since
gasoline gives an approximately 14% well-to-wheel efficiency that would
make about 21% overall for hydrogen, which seems a bit low and I would
think that say 59% for the electrolysis system x 90% for rectification x
50% for the fuel cell = 26.6% is more like it.
However, let's
consider the generating capacity the whole enterprise would need. To
make 1.768 x 10^10 kg of H2 over a year, i.e. 8760 hours, would require:
1.768
x 10^10 kg x 60.5 x 10^3 (W/kg H2)/8760 = 122.1 GW. But this figure
is mitigated according to the efficiency with which hydrogen may be
used. If Bossel is right, this becomes 81.4 GW or let's call it a factor
of two (which seems more reasonable), making it 61.0 GW.
Either
way, we would need a colossal installation of renewables, e.g. 5 MW
wind-turbines, with a rated capacity of 5 MW - but an actual output of
say 30% if placed offshore, which amounts to 1.5 MW per unit. Hence we
would need 61 GW/1.5 MW = 40,667 of them. Probably these could be
accommodated in the North Sea in a 202 x 202 square of turbines, and at
an average spacing of "ten rotor diameters", i.e. 1.23 km,we are talking about an area of 247 kilometers squared (= 61,113 km^2),
which doesn't sound too bad, albeit that the weather in the North Sea is
some of the roughest in the world, and so maintenance might prove a
problem. If the turbines were placed around the coast of the U.K. mainland (assumed to be 2,500 km in length), at a mutual separation of 1.23 km, 2,033 turbines could be so accommodated in a single strand, and to contain all of them, a band would be created, 40,667/2,033 = 20 turbines deep. Assuming that same 1.23 km separation, this would be 25 km (15 miles) wide. So, how quickly might this farm of 40,000+ wind turbines be created? The question really is one of "how long is a piece of string?" but assuming that one turbine could be fabricated and installed every day, the process would take at least 111 years, and probably far longer, in reality.
As an alternative, around 60 new nuclear reactors could
be installed to make the electricity for hydrogen, and on top of the new
generation required to replace the decommissioned current 31 reactors,
actually equal in output to about 14 1 GW reactors, and so it would be
necessary to quadruple this capacity by which means to install a
"Hydrogen Economy" in the UK. I have been told that hydrogen could be
made more efficiently using the thermal power from a nuclear reactor to
run the iodine-sulphur cycle, rather than by electrolyzing water (50%
compared to 35%) , but the installation capacity needed remains huge. If
Bossel is right and electrons can be used with three times the
efficiency than will be recovered (hydrogen actually re-generates
electrons in the fuel cell, to turn wheels, in a chemically-fuelled
electric car) by turning them into hydrogen, the installation capacity
immediately falls to 20 new nuclear power stations, or about 13,555
turbines, which is still enormous but appears more achievable.
I
am not ruling out hydrogen altogether but simply making the point that
when oil supplies begin to wane, it is not a simple matter of switching
from oil to hydrogen, but a new and vast infrastructure must be
implemented first, to both produce and use hydrogen. The question looms:
is it worth it, or might there not be better ways to deal with our
impending transportation problems, such as relocalising society to use
less transport? Even those who are profound advocates of the "Hydrogen
Economy" need to address the problem that the PEM (Proton Exchange
Membrane) cell relies on an electrode consisting partly of platinum
(about 50 - 100 g worth), which is a metal so rare than only 200 tonnes
of new platinum are produced each year, and well below the current and
growing demand for it.
Admittedly, the 40% of world platinum that
is presently put into catalytic converters could be fabricated into PEM
cells, were the putative conversion from oil-power to H
2-power to be
made, but this is only sufficient to put around: 200 tonnes x
1000 kg/tonne x 1000 g/kg x 0.4/50 g/cell = 1.6 million new "vehicles"
on the road each year, out of a world total of about 1,000 million. Hence over a period of 15 years we could replace just 2.4% of the current number. Thus, unless
more platinum is recovered on a huge scale (from sources as yet unknown
to geology), or some alternative fuel cell technology is brought to a
commercial level of development on some similarly immediate timescale,
the enterprise looks set to fall at the last fence, in this, the last
race that humankind will ever have to place bets on.
