Bioethanol from sugar cane

Where sugar cane can be produced (e.g., Brazil) production is 80 tons per
hectare per year, which yields about 17 600 l of ethanol. Bioethanol has an
energy density of 6 kWh per litre, so this process has a power per unit area
of 1.2 W/m2.

Bioethanol from corn in the USA

The power per unit area of bioethanol from corn is astonishingly low.
Just for fun, let’s report the numbers first in archaic units. 1 acre produces
122 bushels of corn per year, which makes 122 × 2.6 US gallons of
ethanol, which at 84 000 BTU per gallon means a power per unit area of just
0.02 W/m2E – and we haven’t taken into account any of the energy losses in

Cellulosic ethanol from switchgrass

Cellulosic ethanol – the wonderful “next generation” biofuel? Schmer et al.
(2008) found that the net energy yield of switchgrass grown over five years
on marginal cropland on 10 farms in the midcontinental US was 60 GJ
per hectare per year, which is 0.2 W/m2. “This is a baseline study that
represents the genetic material and agronomic technology available for
switchgrass production in 2000 and 2001, when the fields were planted.
Improved genetics and agronomics may further enhance energy sustain-
ability and biofuel yield of switchgrass.”

Jatropha also has low power per unit area

Jatropha is an oil-bearing crop that grows best in dry tropical regions (300–
1000 mm rain per year). It likes temperatures 20–28 °C. The projected yield
in hot countries on good land is 1600 litres of biodiesel per hectare per year.
That’s a power per unit area of 0.18 W/m2. On wasteland, the yield is 583
litres per hectare per year. That’s 0.065 W/m2.

If people decided to use 10% of Africa to generate 0.065 W/m2, and
shared this power between six billion people, what would we all get?
0.8 kWh/d/p. For comparison, world oil consumption is 80 million barrels
per day, which, shared between six billion people, is 23 kWh/d/p. So even
if all of Africa were covered with jatropha plantations, the power produced
would be only one third of world oil consumption.

What about algae?

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 ter-
restrial cousins. But there is one trick that I haven’t discussed, which is

energy density
   – air dried 4.4
   – oven dried 5.5
   – air dried 3.75
   – oven dried 5.0
white office paper 4.0
glossy paper 4.1
newspaper 4.9
cardboard 4.5
coal 8
straw 4.2
poultry litter 2.4
general indust’l waste 4.4
hospital waste 3.9
municipal solid waste 2.6
refuse-derived waste 5.1
tyres 8.9
Table D.3. Calorific value of wood and similar things. Sources: Yaros (1997); Ucuncu (1993), Digest of UK Energy Statistics 2005.