Environmental Engineering Reference
In-Depth Information
ited to areas no larger than 10
2
m
2
higher than the convective heating experienced by the
space shuttle's nose during reentry into the Earth's atmo-
sphere (Laub and Venkatapathy 2003).
Heat dissipation in microprocessors takes place in very
tightly confined areas where even relatively modest tem-
perature increases can slow down and eventually shut
down the electronic transfer process (temperature should
be kept below 45
C for optimal functioning). Future
microchips will integrate billions of transistors and may
draw more than three times as much power as today's
top designs, and their heat rejection rates of 10
6
-10
7
W/m
2
would be equal to 10%-30% of the flux through
the Sun's photosphere (64 MW/m
2
). Low-power de-
signs will help, but it is clear that standard fan-cooled
heat sinks (with heat conducted into metal fins cooled
by relatively bulky and noisy fans) have reached the limit
of their practical performance and that new cooling
designs are needed to cope with higher heat fluxes.
Cooling the microelectronic circuitry is an extraordi-
nary engineering challenge, but the total power involved
per processor is too small to have any environmental
effects. Similarly, other concentrated heat rejection pro-
cesses, ranging from hot plates and car exhausts to tall
power plant stacks, are limited to relatively small areas.
As the power density of anthropogenic energy conver-
sions increases, the spatial extent of the more intensive
heat rejection fluxes declines at a considerably faster
rate. As a result, heat rejection phenomena that consti-
tute a significant share of solar inputs (10
1
W/m
2
) are
limited to areas no larger than 10
8
m
2
, to large cities, ex-
tensive industrial regions, and busy transportation corri-
dors. Those heat rejection rates (10
4
-10
5
W/m
2
) that
greatly surpass average insolation rates are restricted to
areas smaller than 10
4
m
2
(100
100 m), and the high-
est anthropogenic heat fluxes (10
6
-10
7
W/m
2
) are lim-
(tall stacks) and as
small as 10
4
m
2
(microprocessors).
Excessive heat degrades the performance of microcir-
cuits, but the flux is negligible against the overall thermal
background. Even many extensive high-density heat
releases have no serious environmental impacts. Cooling
towers and tall stacks generate considerable clouds and
some local fogging and icing, but only infrequently do
they create precipitation anomalies. But persistent heat
rejection by large cities is a major factor in creating
clearly discernible urban heat islands (UHI). The other
reasons for this phenomenon are higher thermal capacity
and lower albedo of built surfaces, a smaller sky view that
hinders radiative cooling, and higher Bowen ratios due to
more of the sensible and less of the latent heat flux (Taha
2004). UHIs are on average about 2
C warmer, and
their cores may temporarily have temperatures up to
8
C higher, than the surrounding countryside (Taha
2004). UHIs are most readily identifiable at night when
their effect on air temperature may be 1 OM greater than
during the day.
Daytime heating of paved and built-up areas generate
much higher surface temperatures than the heating of
vegetated countryside (this difference is much smaller at
night), but it will also engender much stronger urban
convective flows, that is, more vigorous mixing within
the atmospheric boundary layer, which minimize daytime
temperature differences between city cores and the
surrounding countryside (Camilloni and Barros 1997).
Peterson (2003) did not find any statistically signifi-
cant impact of urbanization on annual temperatures for
the contiguous United States. His subsequent analysis
showed that leaving 30% of the highest population sta-
tions out of the data set resulted in no statistically signif-
icant UHI impact, and that even the entire set could
