The Resources of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 145

Fig. 6.3 A simplified summary of the path of the Thermohaline Ocean Circulation (Kuhlbrodt
et al., 2007)
substantially reduced by wave break and internal friction which means only 3 TW
hits the world's coastlines (Hermann, 2006).
The best wave energy climates have deep water power densities of 60-70 kW/m
but fall to about 20 kW/m at the foreshore. Around 2% of the world's 800,000
km of coastline exceeds 30 kW/m giving a technical potential of around 500 GW,
assuming a 40% e ciency for offshore wave energy devices (IPCC, 2007b).
Various wave energy conversion schemes have been developed but none are op-
erating on a large-scale. In 2006, the only two commercial wave power projects
totalled 750 kW (IPCC, 2007b). According to REN21 (2011), at the end of 2010,
an estimated total of 2 MW capacity had been installed, most of which in Europe.
6.6.5 Biomass
Biomass is a term used for plant and animal derived material and includes wood,
energy crops, crop residues and animal dung. It predominately consists of cellulose,
lignin, protein and ash. Specific exergy of biomass ranges from 15 to 20 MJ/kg on
a dry basis depending on carbon and ash content (Hermann, 2006) (see Table 6.3).
The photosynthetic e ciency of converting solar radiation into energy-rich or-
ganic compounds averages around 1%. Only 2.9 TW of exergy is transformed into
the chemical exergy of plants (Szargut, 2003). Estimates of the dry weight of all
living terrestrial plant matter vary but do average about 210 12 metric tonnes
(Skinner, 1986). Considering a specific exergy of biomass of 17 MJ/kg, the total