WC Archive

Thanks for posting this. It looks promising, but beyond facing technical hurdles, there are severe limits. The world currently uses about 12 to 13 terrawatts (trillion watts - TW) of power. Current photosynthesis collects about 0.7 watts per square meter. Theoretically, with optimal fertilizers, water, temperature, etc., photosynthesis could perhaps collect 4.7 W/M2. Sticking with the 0.7 W/M2 figure, we'd need up to 1.86 billion hectares of land, and to harvest all of its photosynthetic conversion, to provide 13 TW. There are currently about 1.5 billion hectares of agricultural land. Water to irrigate all this cropland is becoming increasingly scarce.

Your related post, about the need for vast improvements in energy efficiency, is strengthened when you consider these facts.

Is the efficiency really that low, David? Incident solar radiation is about 1 kW/m^2, so your figure of 0.7 W/m^2 means end-to-end efficiency of less than 0.1%. A casual search brings up this report which quotes 3% to 6% as typical -- that's a big difference. Maybe they don't consider all the factors your figure takes into account?

Besides, I'm sure not all of your 12 TW really needs to be generated with biomass. It would probably be most effective in producing biodiesel for vehicles, where the concentrated liquid form of the fuel really is an advantage over, say, photovoltaics. Other carbon-neutral sources can pick up electricity, heating, etc.

Nevertheless, you and Jamais are certainly correct about decreasing energy use, presumably through both efficiency improvements and conservation.

Well in america nuke and coal will handle between 80-95% of our energy needs for the next century so we only need to boost wind solar and so on up a factor of 5-20 to fill in the gap.

Other places its gona be grim.

Japan will likely be forced to generate 70% from nuke alone and somehow manage 30% from renewables within the next 30 years. Thats gona be nasty hard.

Europe is in for a bad century thats for sure. While this sort of thing might have let em fram some amount of enrgy to keep some things running.. climate change prediction point to a nasty weather setup over europe and we dont realy know how good renewables will fair after the shift. Its gona get ugly.

Richard, thanks for the citation. I'm not sure, but I think the difference in efficiency is because the report you cite measures only parts of the spectrum affecting photosynthesis, while my figure was for visible light and infrared. My figure is from a peer-reviewed article in "Science", and no one challenged that figure, yet your citation is solid too. Either way, photosynthetic efficiency is lower than photovoltaic conversion of sunlight at 10% to 12% and climbing. I think we agree on the power of efficiency improvements.

Oh a vital point on this issue is that the primary boon from finding a good way to convert wood to fuel is that trees grow very well in NON arible lands. As in you can add maybe .5 or more BILLION hectars of farmland beyond what we currently use without the need of water sources and arible land.

Also the same bugs can be used to convert other plant matter and those plants could expand the farmland regions of the world far beyond whats traditionaly concidered arible land.

I'm not the biggest fan of using land to create fuel even if it's carbon-neutral. It seems to me that we'll need this land to produce food for the 9billion of us who are going to need it in 50 years. I'm fine with using the 'waste' biomass, but I'm guessing the real juice is in the crop. I don't want to compete with the local utility for the food on my table.

On the more helpful side, I would suggest not fixating on the dry land for producing biomass. You could use the oceans to grow biomass. Kelp can grow really fast in the right envionment.

Thats the point if you can convert any wood fiber to fuel you can convert scrub brush to fuel and that grows where little if any food grows.

I hope everybody keeps in mind that there exists right now technology that will convert ANY biomass, untreated, dirty, wet and green, into electricity. Its the same old stirling engine that people discount as an automotive power plant, except that it eliminates the unreliable crank mechanism and its lubrication that gums up the heat exchangers and replaces it with things sliding on gas bearings that last forever.

I have run a free piston 1kW stirling engine-alternator on rough wood, and it got about 15% overall conversion efficiency (crediting air dried wood with 14mJ/kg). Not great, but it was a prototype, and almost no money had been invested in it. Then its development was dropped, and the engineers all went off to space and military stuff where they are doing really super duper whizz bang things-- for space isotope power. Well?

I also agree that there is no hope unless we somehow restrict our population and do a lot of smart things with energy efficiency- which we well know how to do but don't.

Oh we dont have to worry about restricting our pop after all a big war is bound to happen soon.

As for the other thing the main issue is how effectively you convert the biomass. Making things more effective and thus able to do more is alot of what r &d is about.

You raise effienecy as much as affordable thus raising effectiveness.

if you want biomass, phytoplanktons the key. Just start fertlisiing the oceans then scoop the stuff up

Solar radiation that makes it to the ground averages about 240 W/m2, averaged over the globe, day and night.

Of the solar radiation that makes it to the surface, roughly 50% of this is "photosynthetically active radiation" (or "PAR" -- the stuff that plants can utilize in photosynthesis).

So the global biosphere can use about 120 W/m2 of energy from the sun, averaged over the whole planet, averaged for entire year (day and night).

Now photosynthesis can achieve some pretty high efficiencies, maybe as high as 3-5%, but only when the growing season is perfect, water is abundant, and nutrients are totally available.

So, at *maximum*, you might expect to get something like 4-6 W/m2 of biological energy available. This is close to the optimum that David quoted earlier.

However, the biosphere is far from that ideal. It is, averaged over the whole world, something more like 0.5-1.0 W/m2.

Typically this called the global "Net Primary Productivity". On land it is typically estimated to be about 55 billion tons of carbon (or about 110 billion tons of dry organic matter) per year. Over the oceans, it's probably about 40 billion tons of carbon (or 80 of dry matter) per year.

Does this help?