How Energy Is Produced - The Renaissance of Renewable Energy - page 145

Sources: IEA website, IEA Bioenergy ( 2009 ), Chum et al. ( 2011 ) .
Assuming that efficiency increases in agricultural productivity keep pace with food
demand, so that the share of land devoted to foodproduction remains the same, the best
chance of increasing the share of energy from biological sources rests with the ability to
extend crop cultivation to less fertile lands that do not compete with food production and
to exploit residues from forestry and wastes.
All in all, an optimistic estimate is that 100-300 EJ per year can be sustainably derived
from bioenergy by 2050. It is reasonable to assume that biomass could sustainably satisfy
no more than one-third of future global energy demand. However, there are large
uncertainties inthispotential,suchasmarketandpolicyconditions,anditstronglydepends
on the rate of improvement in the production of food and fodder as well as wood and pulp
products (Andrews et al. 2011 ; Chum et al. 2011 ; Eisentraut 2010 ; IEA Bioenergy 2009 ) .
4.8 Geothermal Energy
Jules Verne was responsible, perhaps more than any other writer, for stimulating popular
interest in science. His adventure stories have not only enthralled generations of children
and adults; they also anticipated several major scientific innovations. Verne's tales of space
and undersea travel may have prefigured scientific discovery, but his most popular novel,
Journey to the Center of the Earth , was definitely more fantasy than science fiction.
Published in 1864, it describes how three men descend into a volcanic crater in Iceland,
travel beneath the Earth's crust at a depth of more than 150 kilometres, and emerge through
another volcano in Sicily. If someone were to try to replicate this adventure they would
experience rapidly increasing temperatures in the outer crust (30°C per kilometre). At a
depth of 150 kilometres, they would encounter sweltering temperatures of 600°C. Were
they to make it to the core, as Verne's heroes intended, things would get very torrid
indeed, with immense pressure and temperatures slightly higher than the surface of the
sun (more than 6,000°C). While many of Verne's other fictional adventures, from deep-sea
exploration to space travel, have been replicated, humankind's journey into the Earth has
not gone very deep - so far, just 12.3 kilometres. 25