Environmental Engineering Reference
In-Depth Information
The maximum theoretical efficiency of PV cells is lim-
ited by the range of photon energies, and large efficiency
gaps remain between theoretical, laboratory, and field
performances (Perlin 1999; Markvart 2000; Archer and
Hill 2001). Thin PV films are made of amorphous Si or
of GaAs, CdTe, and CuInSe
2
. Theoretical single-crystal
efficiencies are 25%-30%. Lenses and reflectors can boost
these efficiencies, and stacking cells sensitive to different
parts of the spectrum could push the theoretical peak to
50%. Actual efficiencies of new commercial single-crystal
modules are 12%-14%. Thin-film cells can convert 11%-
17% in laboratories but as little as 3%-7% after several
months in field operation. Multijunction amorphous Si
cells convert as much as 11% in large modules. With an
eventual 17% field efficiency PV cells would be averaging
about 30 W/m
2
.
Actual field performance of PV cells has been, so far,
much lower. Generation at the world's largest PV site—
Bavaria's Solarpark, with installed capacity of 10 MW,
peak power of 6.3 MW, and area of 250,000 m
2
divided
among three sites (PowerLight Corporation 2005)—
prorates to only about 3 W/m
2
. The site's high cloudi-
ness is the obvious reason for this poor performance; the
location was chosen because of generous subsidies and
not because of its suitability for PV conversion. These
subsidies also explain why by the end of 2005 eight out
of ten of the world's largest PV projects were in cloudy
Germany and only two in the United States (in Tucson
and in Rancho Seco, California).
The use of mirrors to concentrate solar radiation in
order to heat water for electricity generation has seen
only limited trials. The world's largest solar tower proj-
ect, Solar Two in Barstow, California, with a solar field
of 81,000 m
2
and annual generation of 17.5 GWh, had
overall power density of less than 25 W
e
/m
2
CES 2005). So far, direct solar conversions using flat
plate collectors for space and water heating have been
much more important than PV cells. Flat plates operate
at up to 100
C and transfer up to 60% of insolation to
water at 40
C-50
C. Well-designed systems rate 30-
100 W
t
/m
2
and in peak noon-time as high as 500 W
t
/
m
2
. Globally, these installations, ranging from home
rooftop heaters to industrial arrays, count in the millions,
but it is impossible to offer an accurate estimate of their
aggregate power. Finally, the world's sole tidal plant, the
240-MW Rance plant near St. Malo, Brittany, which was
completed in 1966, has power density of about 14 W
e
/
m
2
. Plans to harness the world's highest tides in Nova
Scotia's Bay of Fundy have been repeatedly laid aside.
The Bay of Fundy tides would deliver 15-16 W/m
2
.
9.3 Global Consumption Patterns: Growth and
Inequality
Perhaps the single most prominent characteristic of fossil-
fueled civilization is the exponential increase in per capita
energy consumption. Typical annual wood and charcoal
consumption in richer preindustrial societies ranged be-
tween 20 GJ and 40 GJ per capita. In forest-rich, thinly
populated North America of the eighteenth and nine-
teenth centuries, it peaked at atypically high rates of 70-
100 GJ per capita (the latter was the U.S. rate in 1860).
If the e
1
of fireplaces, stoves, and simple furnaces averages
10%, such consumption rates would prorate to no more
than 2-4 GJ, and exceptionally to 7-10 GJ, of useful en-
ergy per capita. During the nineteenth century countries
with rich resources of coal increased their rates of gross
fossil energy consumption rapidly, from just 20 GJ in
1800 to 116 GJ in 1900 in Britain, from a negligible
amount to 105 GJ during the same period in the United
States. Per capita consumption of coal in countries that
(SolarPA-
