May 2011 - Illuminating Engineering Society

ENERGY ADVISOR
Edison’s Revenge
Classrooms run 700 to 1,000 sq ft in
size, enough for PV panels on the roof
above them to generate 7 to 10 kilowatts
of power—more than enough for all the
classroom’s electric needs. Schools can
cover their parking lots with PV panels, like
the panels that shopping malls are erecting,
to generate more power, even enough
for low-rise multi-story schools.
Imagine a school with roof-mounted PV
panels, generating DC power for lighting
the spaces directly below, and like a DC
battery pack, inverting some to AC for the
plug load and the utility reserve. The roof
capacity of 10 watts per sq ft plus that of
the covered parking lot can provide all of
the school’s electric power needs. The
system is backed up by the local utility on
rainy days and dark nights, cashing in on
the excess power generated during the
day and “banked” by the net meters. The
back-up AC wiring could be run on the roof
with a transfer switch, transformed down
and rectified. Such systems are now in
development by the EMerge Alliance, the
LRC and others with a low-voltage DC
track that LEDs, OLEDs, computers and
other low-voltage DC devices can plug
directly into, thereby contributing to creatby
Willard L. Warren
Is it possible that
Steve Jobs, during
his long absences,
has found a way to
commune with Edison
and then return each
time to introduce a new
low-voltage DC device?
In the late 1800s Thomas A. Edison, our
nation’s most prolific inventor, perfected
the incandescent lamp and the
“electric dynamo“ (generator) to run it,
and built an electrical distribution system
in lower Manhattan for light and power.
Edison’s electrical devices worked on direct
current (DC) and his DC distribution system
was limited in size because it could not be
transmitted over long distances. On the
other hand, Nicola Tesla and his employer,
George Westinghouse, developed a competing
high-voltage alternating current (AC)
system capable of being transmitted for
miles—which could be transformed back
down to a safe working voltage for lighting
and motor loads. Edison’s system was
eclipsed and abandoned and he never forgave
the two of them.
Fast forward to the present: LEDs, OLEDs,
computers, cell phones, iBooks and iPods
operate on low-voltage DC, while power
sources such as photovoltaic (PV) panels,
fuel cells and wind turbines generate lowvoltage
DC which has to be inverted to AC,
transformed to a higher voltage for transmission,
then transformed back down to a
lower AC voltage for distribution to loads
and then rectified to low-voltage DC to operate
these new DC loads. Is it possible that
Steve Jobs, during his long absences, has
found a way to commune with Edison and
then return each time to introduce a new
low-voltage DC device?
MORE POWER TO YOU
PV panels generate 10 watts of DC power
per sq ft of panel area. Lighting systems
need only 1 watt per sq ft to meet recommended
illuminance levels under the lighting
power density (LPD) allowances. Plug
loads (receptacles) consume only 0.5 watts
per sq ft, leaving plenty of spare capacity
during daylight hours. Some cities, like San
Diego, get only 10 days of rain a year and
can easily “bank” power by inverting the
excess DC power generated by the PV panels
to AC and driving a “net” electric meter
backwards, building up a reserve with the
utility. It obviously takes longer to bank the
reserve in other climate zones.
HIGHER LEARNING
30 May 2011 | LD+A www.ies.org