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Comment by jswanson | Hacker News Reader

This is essentially taking energy from the sun by: - growing a plant in a pool of water - harvesting the plant, treating it with solvents, then extracting the fuel in a crude format - refining the oil

Although solar technologies are still inefficient, it seems like a much more direct path from solar -> electricity -> electric car.

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One way to think about it is that the extra steps allow you to pack solar energy into a much more compact, rapidly transferred energy storage medium. Batteries are expensive, comparatively low energy density, and either take a while to charge or require battery swap stations. Liquid fuel is high energy density and can be pumped rapidly, shipped via tanker to remote locations without any connection to the electric grid, or even carried in a really cheap container such as a plastic bottle.

Yeah, given what sort of stories one reads about science at most internet sites you'd think we'd actually have a way to turn electricity + H2O + CO2 into oil (and sugar, and bread, and proteins, and plastics, and so on and so forth). But we don't.

We do not have anything remotely resembling a reasonable option to do this. There's Fisher-Tropsch, but it's horrifically inefficient.

So we're dependent on plants to do the work for us.

In reality we don't know at all how to efficiently make molecules that have more than 4-5 atoms in them. Oil has hundreds to thousands, depending on the oil. Sugar only has a few dozen (depending on the sugar, starting at 20, going up to several hundred).

Kinda pathetic really.

Yeah, except this does not require a new distribution infrastructure (widespread electric car use would overload current grids) and is a very convenient high energy density fuel convenient for many uses.

There's still the small problem of greenhouse gases.

algae fuel would be carbon-neutral.

in what way is burning plant matter carbon neutral?

It doesn't entrap atmospheric C02, it is grown in a tank. Then you burn it, wherein it -combusts- releasing Co2. Burning dried life is -not- carbon neutral.

...grown under the open sun, in ponds. Taking its CO2 from the atmosphere.

It is carbon neutral in the way that when you grow algae, unless you are feeding it CO2 produced by burning oil, or something, the carbon will come from the biosphere. When you burn it, you return it. Hence it is carbon neutral (on a relevant timescale, like less than a year.) Incidentally, oil is also "carbon neutral" but only when you consider timescales that are so long so as to be effectively infinite compared to human timescales.

Hold on. Isn't the problem here the fact that the CO2 gets dispersed into the atmosphere? How would this be any different for algae than for petroleum?

And how would algae-produced fuel "return" the CO2 to the biosphere any more than petroleum?

The atmosphere is part of the biosphere. Plants growing take their CO2 out of the atmosphere. Algae take it out of the oceans, but CO2 in the oceans is in equilibrium with the atmosphere. (True, the timescale for that is a bit longer, but still pretty short for surface waters where photosynthesizing algae live.)

The difference is not where the CO2 end up, it's where it came from. The carbon in any algae you find today was taken out of the atmosphere within the past ~year. The carbon in petroleum was taken out 50 million years ago. It's that difference in time scale, or equivalently the size of the reservoir, that determines whether something counts as "carbon neutral" or not. The amount of carbon in algae is very small compared to the total carbon budget of the biosphere. The amount in petroleum is not.

Downvotes for not understanding! Woo!

http://en.wikipedia.org/wiki/Photosynthesis

The advantage of algae is that it self-reproduce. You can produce it yourself and sell the algae to factories which will make fuel.

More direct perhaps, but not more efficient. So far nature has far outstripped us in the ability to collect energy from photons.

Using the sun to grow plants and then harvesting those plants for their chemicals is still more efficient than most photo-voltaic cells.

Source of efficiency?

This chart makes me think you're making things up: https://secure.wikimedia.org/wikipedia/en/wiki/File:PVeff%28...

To back that chart up, photosynthesis generally works at around 5% efficiency.

Which agricultural plant gets 5% efficient photosynthesis ? That's huge. 2% is quite a lot.

http://en.wikipedia.org/wiki/Photosynthetic_efficiency

sugarcane?

Perhaps he means if you're using solar cells followed by Fisher-Tropsch to make actual fuel.

I'm pretty sure plants beat that.

But the base efficiency of plants is horrible : 2% is relatively high in plant species (solar energy input -> produced plant matter output). The most efficient plants in the world are close to 4.5% efficient.

I'll dig around and see if I can find the source where I read this, but otherwise it looks like I was wrong. My bad.

From a pure energy efficiency perspective, it's not. A more relevant parameter might be energy return per human energy input, where letting plants do their thing and then using them is pretty darn efficient.

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