currently devoted to agriculture. So the maximum power available, ig-
noring all the additional costs of growing, harvesting, and processing the
0.5 W/m2 × 3000 m2 per person = 36 kWh/d per person.
Wow. That’s not very much, considering the outrageously generous as-
sumptions we just made, to try to get a big number. If you wanted to
get biofuels for cars or planes from the greenery, all the other steps in the
chain from farm to spark plug would inevitably be inefficient. I think it’d
be optimistic to hope that the overall losses along the processing chain
would be as small as 33%. Even burning dried wood in a good wood
boiler loses 20% of the heat up the chimney. So surely the true potential
power from biomass and biofuels cannot be any bigger than 24 kWh/d per
person. And don’t forget, we want to use some of the greenery to make
food for us and for our animal companions.
Could genetic engineering produce plants that convert solar energy
to chemicals more efficiently? It’s conceivable; but I haven’t found any
scientific publication predicting that plants in Europe could achieve net
power production beyond 1 W/m2.
I’ll pop 24 kWh/d per person onto the green stack, emphasizing that I
think this number is an over-estimate – I think the true maximum power
that we could get from biomass will be smaller because of the losses in
farming and processing.
I think one conclusion is clear: biofuels can’t add up – at least, not in
countries like Britain, and not as a replacement for all transport fuels. Even
leaving aside biofuels’ main defects – that their production competes with
food, and that the additional inputs required for farming and processing
often cancel out most of the delivered energy (figure 6.14) – biofuels made
from plants, in a European country like Britain, can deliver so little power,
I think they are scarcely worth talking about.
38... compensate for the tilt between the sun and the land. The latitude of
Cambridge is θ = 52°; the intensity of midday sunlight is multiplied by
cos θ ≈ 0.6. The precise factor depends on the time of year, and varies be-
tween cos(θ + 23°) = 0.26 and cos(θ - 23°) = 0.87.
–In a typical UK location the sun shines during one third of daylight hours.
The Highlands get 1100 h sunshine per year – a sunniness of 25%. The best
spots in Scotland get 1400 h per year – 32%. Cambridge: 1500 ± 130 h per
year – 34%. South coast of England (the sunniest part of the UK): 1700 h per
year – 39%. [ ] Cambridge data from [ ]. See also figure 6.16.