produce 300 kg of hydrogen per day. Hydrogen contains 39 kWh per kg,
so this algae-to-hydrogen facility would deliver a power per unit area of
4.4 W/m2. Taking into account the estimated electricity required to run
the facility, the net power delivered would be reduced to 3.6 W/m2. That
strikes me as still quite a promising number – compare it with the Bavarian
solar photovoltaic farm, for example (5 W/m2).
Grain crops such as wheat, oats, barley, and corn have an energy density
of about 4 kWh per kg. In the UK, wheat yields of 7.7 tons per hectare per
year are typical. If the wheat is eaten by an animal, the power per unit area
of this process is 0.34 W/m2. If 2800 m2E of Britain (that’s all agricultural
land) were devoted to the growth of crops like these, the chemical energy
generated would be about 24 kWh/d per person.
We found a moment ago that the power per unit area of a biomass power
station burning the best energy crops is 0.2 W/m2. If instead we grow
crops for food, and put the left-overs that we don’t eat into a power station
– or if we feed the food to chickens and put the left-overs that come out
of the chickens’ back ends into a power station – what power could be
delivered per unit area of farmland? Let’s make a rough guess, then take a
look at some real data. For a wild guess, let’s imagine that by-products are
harvested from half of the area of Britain (2000 m2 per person) and trucked
to power stations, and that general agricultural by-products deliver 10% as
much power per unit area as the best energy crops: 0.02 W/m2. Multiplying
this by 2000 m2 we get 1 kWh per day per person.
Have I been unfair to agricultural garbage in making this wild guess?
We can re-estimate the plausible production from agricultural left-overs
by scaling up the prototype straw-burning power station at Elean in East
Anglia. Elean’s power output is 36 MW, and it uses 200 000 tons per year
from land located within a 50-mile radius. If we assume this density can be
replicated across the whole country, the Elean model offers 0.002 W/m2.
At 4000 m2 per person, that’s 8 W per person, or 0.2 kWh/day per person.
Let’s calculate this another way. UK straw production is 10 million
tons per year, or 0.46 kg per day per person. At 4.2 kWh per kg, this straw
has a chemical energy of 2 kWh per day per person. If all the straw were
burned in 30%-efficient power stations – a proposal that wouldn’t go down
well with farm animals, who have other uses for straw – the electricity
generated would be 0.6 kWh/d per person.