How Energy Is Produced - The Renaissance of Renewable Energy - page 155

year in the Earth's crust alone) is huge if compared with current global primary energy
supply (530 EJ per year). Clearly, we are sitting on a vast potential resource, yet the
estimated technical potential varies depending on technology development and the depth at
which the resource is expected to be exploited.
Figure 4.57. Ranges of technical potentials of geothermal energy for direct (heat) uses
at a depth of 3 kilometres, and for electricity generation (at 3-, 5-, and 10-kilometre
depths). Source: Goldstein et al. ( 2011 ) (modified).
For electricity generation, assuming drilling to a depth of 3 kilometres, the technical
potential is estimated at 118 exajoules per year, one-fifth our current global energy
production. But 3 kilometres is still relatively shallow. Geothermal wells are currently
drilled to depths of 5 kilometres using methods similar to those used for oil and gas
exploration. If advances in technology enable drilling down to 10 kilometres,
geo-electricity potential could rise to 1,100 exajoules per year, more than twice global
primary energy production. Most of this potential is ascribed to EGS (Biello 2013b ), while
the technical potential of direct thermal uses is estimated to range from 10-312 exajoules
per year (Dickson and Fanelli 2004 ; Goldstein et al. 2011 ) .
4.9 Ocean Energy
Between two-thirds and three-quarters of our planet's surface is covered by contiguous
seas and oceans. This huge mass of salt water has always played a central role in human
development, acting both as a conduit and a constraint. The Mediterranean region became
the cradle of Western civilization precisely because that body of water facilitated exchange
betweenthepeoplesthatsettledonitscoasts.Bycontrast,theoceanswereformillennia the
great unknown, a place of unfathomable depths and terrible monsters, the inspiration for
innumerable sailors' yarns. Even in the modern age, when satellite images map out almost
every inch of landmass, the oceans are the least known parts of the world.
In terms of energy, oceans represent an immense and largely untapped resource. For
centuries, basic mechanical techniques to generate energy from the tides and currents have
been known, but these have only been pursued consistently since the 1970s. Ocean power
languished in the post-oil-crisis period of the 1980s, but at the turn of the millennium, amid
growing concern about peak oil and climate change, research and development of a wide