standard practice in the algae-to-biodiesel community: they grow their
algae in water heavily enriched with carbon dioxide, which might be col-
lected 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 4 W/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 Chap-
ters 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 212 tons per year per person), it could service 230
square metres
per personE of the algal ponds described above – roughly 6%
of the countryE. 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.

### What about algae in the sea?

Remember what I just said: the algae-to-biodiesel posse always feed their
algae concentrated CO2. If you’re going out to sea, presumably pumping
CO2 into it won’t be an option. And without the concentrated CO2,
the productivity of algae drops 100-fold. For algae in the sea to make a
difference, a country-sized harvesting area in the sea would be required.

### What about algae that produce hydrogen?

Trying to get slime to produce hydrogen in sunlight is a smart idea because
it cuts out a load of chemical steps normally performed by carbohydrate-
producing plants. Every chemical step reduces efficiency a little. Hydrogen
can be produced directly by the photosynthetic system, right at
step one. A research study from the National Renewable Energy Laboratory
in Colorado predicted that a reactor filled with genetically-modified
green algae, covering an area of 11 hectares in the Arizona desert, could

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