Monday, March 11, 2013

Wind for Hydrogen - An Update.

This is an update of some numbers from an older posting in light of the larger commercial wind turbines that are now available 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 CO2 budget that must be costed-in, hence hydrogen from this source is not clean but contributes to CO2 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 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.









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.



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 H2-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.


Trim said...


More articles like this one please! Outstanding presentation of reality.



S/V Trim

Chris Rhodes said...

Hi Ken,

I wrote quite a number of "quantitative" articles like this particularly back in the early days of this blog. Maybe it's time to revisit some of them (as I have done here), or/and once more apply the approach to some of the newer factors such as fracking, shale gas/oil etc.



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