Spurred on by the energy crisis of the 1970s, scientists searched for alternatives
to fossil fuels in order to supply our energy needs. As mankind has done
for thousands of years, they looked to the sun-the natural provider of energy
to our planet. Solar radiation can provide more energy than we could ever
possibly need. Energy reaching the earth from the sun has an intensity of
about 1.2 kilowatts per sq m or about 1.3 hp per sq yd. But harnessing this
energy has always been difficult.
From the earliest days solar energy helped dry crops and produce salt from
brines. In the 1950s, MIT built an experimental house that derived its energy
from the sun through solar panels on its roof. However, early attempts to
harness the power of the sun with solar cells and batteries proved expensive
and not very practical.
Continued experimentation resulted in small advances, and slowly solar-powered
products became available. Pocket calculators, hot-water heaters and battery
rechargers for backpackers were some of the new products making use of solar
power. However-for solar power to make an impact upon the energy needs of
the country-other, more significant applications required development.
Another area where solar power began to make inroads was arrowboards and
changeable message signs (CMS). In the late 1980s the first solar-powered
arrowboard made its appearance followed by solar-powered CMSs in the early
1990s. David Wilfong, marketing representative for Precision Solar Controls
explains," Arrow signs were introduced in 1987 and changeable message
boards were introduced in 1991."
Off to a slow start, solar-powered arrowboards and CMSs have gained in popularity
and are increasing in dominance over diesel-powered signs. Now, according
to Wilfong, 90% of the signs used are solar powered. Ken Smith, national
sales manager for American Signal Co. agrees that solar- powered signs will
dominate the market; however, he does not feel they will totally replace
diesel-powered signs. Smith says, "Solar CMS will replace about 95%
of all diesel signs. It won't be 100% until there are rgulations in place
to require their use." However, some states may grandfather the use
of diesels and contractors who already own diesel may not purchase new solar
equipment until the diesels wear out. For contractors who hold on to their
diesel signs, conversion kits are available from some manufacturers, which
allow a switch over to solar. Allmand Brothers, for instance, offers a kit
to convert gasoline- and diesel-powered arrowboards to solar-powered.
Improved technology is the main factor in solar signs' rising popularity.
"Solar wasn't used sooner because the technology was not there,"
explains Nathan Batson, president of American Electronic Sign. "The
capability to charge the batteries has improved. A display that does not
use a lot of power also was needed."
Since the mid-1970s great strides have been made in photovoltaic (PV) cell
technology. PVs are devices that generate voltage when exposed to radiant
energy. A PV solar cell is usually a thin wafer made of silicon, gallium
arsenide, cadmium sulfide, or cadmium telluride. Solec International, a
manufacturer of commercial PV, has concentrated its energy on improving
crystalline silicon PV technology. Through its fast-pull crystal growth
process, Solec has cut the growing time by 60% resulting in lower cost PVs.
This improves the practicality of PV devices allowing for more widespread
use. It also makes solar power more affordable.
Improvements in commercial PVs has allowed manufacturers to decrease the
size of their panels without a corresponding loss of energy produced. Smith
said that American Signal Co. first produced solar-powered signs with an
array of solar panels that was about 6 ft ¥ 8 ft and now they are about
half that size.
Another technological advance that helped the rise of solar-powered signs
is high-intensity light-emitting diodes, commonly known as LEDs. An LED
is made of a special semiconductor material usually gallium arsenide, gallium
arsenide phosphide or gallium phosphide. This material gives off light when
an electric current passes through it. LEDs are often used in pocket calculators
and digital clocks, watches and radios. While LEDs are more expensive to
produce than incandescent lights, they offer many advantages. They operate
on very low voltages and can be designed to emit a specific light color.
In contrast, an incandescent lamp emits a white light with a broad range
of light frequencies. LEDs also are very rugged and more durable than incandescent
devices. However, excessive voltage and heat will damage them. Resistors
are used with LEDs, to limit the voltage to acceptable values, because of
the LEDs' susceptibility to even a slight increase in voltage.
The combination of more efficient energy consumption, through the use of
LEDs, and improvements in solar cells, have allowed solar-powered signs
to gain an upper hand over diesel-powered signs and it appears that solar
will eventually replace diesel. "Diesel power is fading. The new signs
that we sell are all solar powered," says Batson. "Most states
are now providing specs for solar CMSs. In a few years they will replace
diesel boards," adds Wilfong. Bob Heinz, marketing manager for Addco
Manufacturing Co. agrees, "Solar will most certainly replace diesel.
The diesel market may not exist one year from now."
Solar-powered signs posses many advantages over diesel- or gasoline-powered
signs. Diesel signs use an engine and a generator to produce its power;
thus requiring more manpower in refueling, as well as fixing break-downs
and performing maintenance to the engine and generator. Solar, on the other
hand, requires a minimum amount of maintenance. "Solar only requires
a periodic check of the battery banks," states Wilfong.
With more moving parts, and a reliance on consumable fuel, diesels have
a greater potential for power failure, thus resulting in possible tort liability
concerns. Relying on the sun, solar-powered signs have less chance for power
failure, because their source of energy is endless. Their use of battery
banks ensure that the sign will remain lit at night. Depending on the model
most units can last 18 to 21 to 30 days without sunlight.
Contrary to some beliefs, overcast weather does not effect the signs to
the degree some would believe, because solar energy is made up of visible
light, infrared radiation and ultraviolet (UV) rays.
Heinz explains, "Overcast weather is not a factor because enough sunlight
gets through to allow the system to charge the batteries." Addco's
model also allows the user to tilt and rotate the solar panel in order to
receive the full benefit of the free energy from the sun. In addition to
recharging the batteries with the sun, solar signs also can be charged by
plugging them into an AC outlet.
Some manufacturers provide fail-safe modes for their solar signs in case
of a power failure. American Electronic Sign, for example, provides a retroreflective
flap that displays a preprogrammed default message in the event of a power
failure. This flap can also be used to conserve energy stored in the batteries.
Solar also has environmental benefits over diesel. It does not produce air
pollution and it is quiet running. Smith comments, "Solar is socially,
politically and environmentally more correct quieter and cleaner."
As its popularity increases, solar-powered CMSs are moving beyond the work-site
to play a role in intelligent transportation systems (ITS) (see Portable
Traffic Management System Shows Promise for Easing Congestion, October,
p 36). The Kentucky Department of Highways worked with Addco to use solar-powered
CMSs in its ITS. This ITS project will alleviate congestion and air pollution
on I­p;64 and I­p;75, through Lexington. CMSs will inform travelers
of upcoming incidents and guide them through route changes. The signs also
are equipped with radar guns to warn motorists to slow down as they approach
Georgia DOT also is using CMSs in its ITS. American Signal Co.'s signs were
used in the recent high-occupancy vehicle-lane construction in Atlanta.
Now that the construction is complete the CMSs will be used for incident
management and traffic control during the Olympics. "What was once
a tool used exclusively for work-zones is now used as an incident and congestion
management tool," said Smith.