inside ases | chair’s corner
another roadside attraction
The interstate highway system can accommodate
both photovoltaic arrays and new transmission lines.
Photovoltaics (PV) demand huge swaths of real estate
to generate a significant portion of the nation’s electricity supply. Solar opponents ask, “Do we really
want our farms and forests replaced by all that glass?” We
answer, “There’s enough roof area to meet the need.” And
we also have the nationwide highway right-of-way.
At the close of 2008, Oregon became the first state to
utilize a stretch of interstate highway for a PV array, as
reported in SOLAR TODAY (“Oregon PV Array to Offset Highway Lighting,” November/December). The busy
intersection of Interstate 5 and Interstate 205 in Tualatin,
a suburb south of Portland, is now home to a 104-kilowatt
array. Placed on racks at the northern edge of the wide and
treeless right-of-way, the 600-foot-long, 5-foot-wide array
faces due south at a tilt angle of 30° (latitude minus 15°).
Funded by a public-private partnership among the
Oregon Department of Transportation (ODOT), Portland General Electric (PGE) and U.S. Bank, the array
feeds power into the PGE grid. The project offsets about
28 percent of the power used to run the interchange’s
lighting, despite Oregon’s gray winters and low electricity
rates (ODOT pays about 6 cents per kilowatt-hour). The
modules were made locally by Solar World (solarworld-usa.
com) and the inverters by PV Powered ( pvpowered.com)
in Oregon. Some incentive funds came from the Energy
Trust of Oregon ( energytrust.org).
In Europe, solar arrays have been built along highways
and railroads for many years. This is particularly evident
in Switzerland and Germany, countries farther from the
equator and with lower annual solar resources than most
American states. Europe’s high population density means
that more electricity consumers are located near the roadway arrays, so distribution costs are lower. Open land is at
By JoHn reynolds,
Faia
John Reynolds, FAIA, is
chair of the American
Solar Energy Society
board. Contact him at
chair@ases.org.
this 104-kilowatt roadside array in tualatin, ore.,
went live late last year.
a premium, and a large share of energy is imported at a high
cost. These factors make roadside land more valuable for
energy production and help to offset the cost of the array.
The U.S. interstate highway system offers a huge land area
with largely unimpeded solar access. Outside cities, perhaps
half the right-of-way corridor is unpaved. Few medians are
planted with trees. The typical corridor is wide enough that
shadows from trees and buildings on the southern edge
wouldn’t reach an array along the northern edge, even in
winter. In urban areas, PV arrays can double as traffic-noise
barriers, which will help to justify their cost and will help
utilities meet the energy loads of neighborhoods adjacent
to freeways. There is some tension between the need for
vertical sound barriers and the near-horizontal arrangement
of PV arrays; in Europe, this has been resolved by tilting PV
arrays at a compromise angle of about 45˚. Whatever the
design, the array needs to be placed to avoid shadows from
passing trucks. The design might use a vertical sound barrier
topped with PV arrays.
For highways that run north-south, opportunities for
PV arrays are less continuous. Overhead highway signs,
where PV arrays can be added to the signs’ existing racks,
are good spots for unimpeded solar access. Bridges can be
roofed with PV arrays that would shelter the roadway from
snow, reduce corrosion from winter salting and extend the
life of the substructure. As in the case of PV sound barriers, it is easier to justify PV when it can fulfill more than
one purpose.
In either case, lining the northern edges of east-west
highways or signs and bridges above north-south highways,
there is the question of reflected glare. PV researchers work
continually to improve the percent of incident sunlight that
is absorbed rather than reflected; increased efficiency of
capture equals decreased glare.
The continuous interstate right-of-way also offers a
potential corridor for electricity transmission. This could
be a distinct advantage, as we need more transmission lines
to get renewably-generated electricity to urban markets.
If underground, the transmission system is expensive but
weather-protected. Advances in superconductors will make
underground distribution more attractive, helping to maintain the required low temperatures. Using the interstate
corridor will also reduce the pressure to carve new rights-of-way through national forests and other rural lands.
gary weBer, odot
Someday, solar-powered pod cars may zip above the
interstate highway system. In the meantime, let’s use the
modernization of our transport infrastructure to make our
interstate system serve more than just cars and trucks.
