Environmental Engineering Reference
In-Depth Information
photosynthesis. Moreover, the highly unequal distribu-
tion of human NPP use means that phytomass appropri-
ation ratios are more than 60% in East Asia and more
than 70% in Western Europe (Imhoff et al. 2004).
Claims that simple and cost-effective biomass could
provide 50% of the world's TPES by 2050 or that 1-2
Gt of crop residues can be burned every year (Breeze
2004) would put the human appropriation of phytomass
close to or above 50% of terrestrial NPP. This would fur-
ther reduce the phytomass available for microbes and
wild heterotrophs, eliminate or weaken many ecosys-
temic services, and reduce the recycling of organic mat-
ter in agriculture. Moreover, the average power densities
of NPP are minuscule (about 450 mW/m
2
of ice-free
land). Even the most productive fuel crops or tree planta-
tions have gross yields of less than 1 W/m
2
, and subse-
quent conversions to electricity and liquid fuels prorate
to less than 0.5 W/m
2
.
No other renewable energy resource can provide more
than 10 TW. Generous estimates of technically feasible
maxima (economically acceptable rates may be much
lower) are less than 10 TW for wind, less than 5 TW for
ocean waves, less than 2 TW for hydroelectricity, and less
than 1 TW for geothermal and tidal energy and for ocean
currents (fig. 12.9). These flows can be tapped with den-
sities generally no higher than 10
0
-10
1
W/m
2
and as
low as 10
1
W/m
2
. In order to energize the existing
residential, industrial, and transportation infrastructures
inherited from the fossil-fueled era, a solar-based society
would have to concentrate diffuse flows to bridge power
density gaps of 2-3 OM (fig. 13.8).
The mismatch between the low power densities of
renewable energy flows and the relatively high power
densities of modern final energy uses means that a solar-
based system will require a profound spatial restructuring
13.8 Mismatch of typical power densities of renewable
energy conversions and common energy uses in modern
societies.
with major environmental and socioeconomic conse-
quences. Most notably, there would be vastly increased
fixed land requirements for primary conversions, and
some of these new arrangements would also necessitate
more extensive transmission rights-of-way. Efficient and
economical means of hydrogen production from renew-
ably generated electricity (or by direct microbial conver-
sion) would be an effective high-energy-density solution.
Only solar energy transformed into phytomass
through heavily subsidized photosynthesis (in order to
eliminate any water and nutrient shortages and to protect
the harvest from pest attack) can be harvested and used
predictably. Without massive storage none of the pro-
spective renewable kinetic flows could provide the large
base loads required by modern electricity-intensive soci-
eties. Yet voluminous water reservoirs have been the
