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by Eric Gunnerson.
Original Post: Hydrogen from Sunlight
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John Read wrote in this ABlog post (I hate group blogs):
The Aussies have done it. And not a moment too soon! Using special titanium oxide ceramics, they can now use sunlight to split water, and produce hydrogen fuel for unlimited energy. The device rivals still-unrealized “controlled fusion”
This is an interesting development in the research area of Photoelectrochemical water splitting, but it's not about whether you can get hydrogen from sunlight. You could already do that, if you were willing to go from photovoltaics to electrolysis, or use solar energy for thermal water splitting. So, I think John is a bit over-enthusiastic.
The real question is whether you can produce energy at economical rates, and I haven't seen any information on that. Given a competitive efficiency, it's likely that you could create such a system that's quite a bit cheaper than photovoltaics, but you still have to deal with the fact that solar energy is a fairly diffuse energy source. Time for a little math - how much space would it take to support an average household. Feel free to check my numbers...
Picking a spot in California as a test case, we find that it averages about 5 KWh / sq meter / day.
So, if our system were 100% efficiency, we'd need about 6.5 meters of collector area. That sets a lower bound. Picking some more reasonable numbers:
Light to hydrogen efficiency = 10%
This seems like a fairly reasonable guess. Photovoltaics are in the same realm, and photosynthesis clocks in at around 8% for efficient plants.
Hydrogen to electricity efficiency = 75%
This is a harder number to come up with. There are claims of up to 80% here, so I'm going to choose 75% as a reasonable number.
Power inverters (to go from DC to AC) are approximately 90% efficient.
So, that makes our sunlight to AC power efficiency =
0.1 * 0.75 * 0.9 = 0.0675
Dividing our 6.5 meters by this factor gives us around 96 square meters of collection area required to take a California house off-grid. That's a collector that's properly aligned with the sun - being at the wrong angle reduces the efficiency.
That's a pretty large amount of space.
If, however, you can do this in large desert areas, you can get a megawatt of daytime hydrogen out of a collector that's about 150 meters square.
There are also some interesting space applications. Presuming you can get water from asteroid sources, you could perhaps generate hydrogen and oxygen fairly simply.
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