What about algae

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Algae are just plants, so everything I've said so far applies to algae. Slimy underwater plants are no more efficient at photosynthesis than their terrestrial cousins. But there is one trick that I haven't discussed, which is standard practice in the algae-to-biodiesel community: they grow their algae in water heavily enriched with carbon dioxide, which might be collected from power stations or other industrial facilities. It takes much less effort for plants to photosynthesize if the carbon dioxide has already been concentrated for them. In a sunny spot in America, in ponds fed with concentrated CO2 (concentrated to 10%), Ron Putt of Auburn University says that algae can grow at 30 g per square metre per day, producing 0.01 litres of biodiesel per square metre per day. This corresponds to a power per unit pond area of 4W/m2 - similar to the Bavaria photovoltaic farm. If you wanted to drive a typical car (doing 12 km per litre) a distance of 50 km per day, then you'd need 420 square metres of algae-ponds just to power your car; for comparison, the area of the UK per person is 4000 square metres, of which 69 m2 is water (figure 6.8). Please don't forget that it's essential to feed these ponds with concentrated carbon dioxide. So this technology would be limited both by land area - how much of the UK we could turn into algal ponds - and by the availability of concentrated CO2, the capture of which would have an energy cost (a topic discussed in Chapters 23 and 31). Let's check the limit imposed by the concentrated CO2. To grow 30 g of algae per m2 per day would require at least 60 g of CO2 per m2 per day (because the CO2 molecule has more mass per carbon atom than the molecules in algae). If all the CO2 from all UK power stations were captured (roughly 21/2 tons per year per person), it could service 230 square metres per person of the algal ponds described above - roughly 6% of the country. This area would deliver biodiesel with a power of 24 kWh per day per person, assuming that the numbers for sunny America apply here. A plausible vision? Perhaps on one tenth of that scale? I'll leave it to you to decide.

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