The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 143

250

GW

194

200

159

150

120

100 94

74

59

48

50

39

31

24

17

14

10

8

6

0

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Fig.6.2Globalcumulativeinstalledwindcapacity1996-2010.DataobtainedfromGWEC(2011)

6.6.4 Ocean energy

The sun is responsible for three effects occurring in the oceans: an ocean thermal
gradient, from which thermal energy could be eventually extracted; the thermoha-
line circulation, which is caused by the thermal and salinity gradient and causing
vast volumes of water to move around the globe; and ocean waves, indirectly gene-
rated by the sun through the blowing of winds.

6.6.4.1 Ocean thermal gradient

The sun heats the sea's surface to around 22 C, generating a thermal gradient
given that the temperature drops to 2 C in the deep ocean. This temperature
difference represents a specific exergy of about 800 J/kg seawater (Hermann, 2006).
Considering that the mass of the oceans is equal to 1:37 10 23 kg, this gives an
absolute exergy of 1:1310 8 Gtoe (Table 6.8). Theoretically, this thermal gradient
could be used for drawing energy from the oceans. However, the small temperature
variation involved makes ocean thermal power extraction impractical and as of yet
no commercial plant exists. However, if this source of energy could be used even
at an e ciency of less than 1%, the potential would exceed that of all fossil fuels
(Skinner, 1986).
Another consequence of the thermal gradient is the so called thermohaline cir-
culation (THC) or “the great ocean conveyor belt” (Broecker, 1991). The THC
transports enormous volumes of cold, salty water from the North Atlantic to the