Thanks to discussions/links at Instapundit and Jerry Pournelle's site I got interested in how much space would you need to replace the world's crude oil consumption with algally produced oil.
From this US DOE graph we see that world crude demand is c. 82M bbl/day. which equates to some 30 Billion bbl/year (365.25*82 = 29950.5). According to various places (google search) one barrel of oil makes 28 gallons (US) of gasoline/petrol and contains 42 gallons of crude. Hence depending on how you do the calculation we need either 840B (28*30) or 1260B (42x30) US gallons of algal fuel per year.
So how much algae is needed for that?
[There was a reason why I converted to gallons. You see when you search for algal biofuel yield it tends to be given in gals/year] This article on cnet reports that:
A single hectare can generate 8,000 gallons of oil, 2,400 gallons of ethanol a year and 2.6 tons of glycerin,...
1 sq km = 100 Hectares can therefore generate 800,000 gallons of oil.
1 million sq km can therefore generate 800 B gallons of oil (approximately the same amount of refined crude 800B vs 840B gal). By simple maths (1260/800 = 1.575) this means that 1.575m sq km can therefore generate enough to replace the entire crude oil output. And produce all that ethanol and glycerin too.
OK so what does 1.575m sq km look like? roughly 60% of the Mediterranean (total 2.5 m sq km) or under 0.5% of the 361 m sq km of oceans on this planet.
Looked at one way that is a lot of algae. Looked at another way, since algae grows just fine in oceans (all those troublesome algal blooms), if we can develop a way to harvest oceanic algal blooms then dedicating a small amount of tropical or semitropical ocean* (say part of the Sargasso sea or somewhere a little further south) to algal exploitation would give us all the oil we'd need without needing to pay those troublesome OPEC nations for their crude. We also wouldn't need to disturb Ted Kennedy or anyone else with wind farms, wouldn't need to pollute the desert with solar panels etc. etc.
I must note that this is merely a back of the envelope calculation and makes the critical assumption that oceanic algal yields would be the same as those in a small pond. This is probably not a correct assumption but I suspect that it is correct to within an order of magnitude and that therefore a significant proportion of world oil needs could be met from oceanic algae.
*why tropical is left as an exercise for the reader (hint where does the sun shine most?)
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