Environmental Engineering Reference
In-Depth Information
Smith 1993; Wang and Ding 1998). These stoves, oper-
ating with thermal efficiencies of 25%-30%, are credited
with annual savings of close to 2 EJ of fuel wood and
coal (Luo 1998).
Biomass provides only between 1% and 4% of the
TPES in the world's richest countries, with Sweden (at
more than 15%) being a major exception. Woody bio-
mass consumed in the rich countries, mostly by lumber
and pulp and paper industries and only secondarily for
household heating (in old-fashioned fireplaces or high-
efficiency wood stoves), amounted to no more than
about 5 EJ in the year 2000. Nearly all this energy is
used directly as space, cooking, and processing heat (par-
ticularly by the pulp and paper industries), and only a mi-
nuscule part is converted to electricity and liquid fuels.
The world's largest effort to produce fuel ethanol, Bra-
zil's PROALCOOL, began in 1975, and it remains the
most productive solar alternative for converting phyto-
mass to liquid fuels. Cultivation of sugarcane under opti-
mal tropical conditions, with bagasse used to fuel the
distillation of ethanol, results in power production den-
sity of about 0.45 W/m
2
(Macedo, Leal, and da Silva
2004). The U.S. corn-based system has much higher
land requirements because of its inherently lower yields;
during the early 2000s it produced about 7 MJ of etha-
nol, or 0.22 W/m
2
.
The power densities of other biomass energy uses are
similarly low, reflecting the inherently low efficiency of
photosynthesis. Woody phytomass burned in rural areas
is gathered with densities ranging from just 0.02 W/m
2
for twigs, branches, and leaves in arid environments to
about 0.15 W/m
2
for selective stem cutting in moist
forests. Plantations of fast-growing willows, poplars,
eucalypti, leucaenas, or pines yield only 0.1 W/m
2
humid regions or with irrigation; values around 0.5 W/
m
2
are more typical upper rates. Logging residues from
clear-cutting can provide a one-time yield of 1-4 W/
m
2
, but most of them may not be usable (too remote,
too dirty) and will be burned on site. Crop residues used
for household combustion are harvested with densities
ranging from a mere 0.01 W/m
2
from low-yielding cere-
als (assuming straw yield of 1 t/ha, and one-third of it
used for fuel) to 0.4 W/m
2
for sugarcane bagasse (cane
yield of 100 t/ha, all bagasse burned).
Conversion of water's potential energy to electricity is
the second most important nonfossil energy input. Po-
tential energy of global runoff is about 367 EJ, almost
exactly the world's commercial TPES in the year 2000.
If this natural flow were to be used with 100% efficiency,
the gross theoretical capability of the world's rivers
would be about 11.6 TW. Competing water uses, unsuit-
ability of many sites, seasonal fluctuations of flow, and the
impossibility of converting water's kinetic energy with
perfect efficiency at full capacity mean that although the
exploitable capability (share of the theoretical potential
that can be tapped with existing techniques) can be well
in excess of 50%, it is commonly just around 30% for
projects rated above 1 MW. The worldwide total of tech-
nically feasible capacity must be thus constructed by add-
ing specific national assessments. ICOLD (1998) puts it
at about 52 EJ (
>
14 PWh/a) of electricity, or roughly
14% of the theoretical total. Asia claimed 47% and Latin
America nearly 20% of the total, and China has the
largest national total (about 15%).
Not everything that is technically possible is economi-
cally feasible, and projects in the latter category added up
to about 30 EJ worldwide (just over 8 PWh), or roughly
three times the total of 2.6 PWh/year (700 GW, with
another 100 GW under construction) that was actually
in
dry northern climates and 1 W/m
2
in the best stands in
