The Solar II Project
A 300 ft tower rises from the valley floor just east of Daggett. At the top of this
tower a cylindrical region glows with an intense light that is eerily brilliant. Some
fifty feet away, hovering 300 ft. in the air on either side of the tower are two strange
balls of light. This is the Southern California Edison Company Solar II Generating Plant.
Located a few hundred yards north of Route 66, this phenomenon is visible for miles and
is a local landmark. Solar II is undergoing final testing and is designed to generate 10
megawatts of electricity, enough to power 10,000 homes.
How It Works
Surrounding the base of the tower are located some 2000 large, motorized mirrors called
heliostats. Each heliostat is computer controlled to track the sun and focus reflected
energy onto a receiver atop the tower. There are 24 receivers each 24 feet tall
surrounding the top of the tower in a cylindrical arrangement. Piping in each of these
sections of the receiver carries molten salt, which acts as a heat collector to store the
sun's energy. The molten salt is heated to 1050 degrees Fahrenheit and pumped to a storage
tank where it is further pumped to a system that produces super heated steam used to drive
a turbine generator thus transforming solar energy to electrical energy.
Molten salt, consisting of 60% sodium nitrate and 40% potassium nitrate, is used
because its thermal properties allow energy to be stored for long periods of time. Thus
the facility continues to generate electricity when the sun is behind clouds and even at
night. A disadvantage to the use of molten salt is that all pipes, and of course the
storage tank, must be highly insulated. If the temperature of the salt falls below
approximately 450 degrees fahrenheit it crystallizes to form a solid.
Operation Results in Strange Phenomenon
Each morning as the sun rises the Solar Energy Plant comes to life. The mirrors
surrounding the tower all move toward the east so as to capture the sun's rays. Instead of
immediately focusing the energy onto the receiver at the top of the tower, the computers
controlling the field, direct the motorized mirrors so as to reflect light towards one of
four "aim points," located some distance from the tower. A small number of
mirrors are then moved from the aim points so as to focus light on the tower. As the
receivers absorb light energy from the sun, more mirrors are moved so as to provide
additional heat. The highly controlled process continues, with more mirrors contributing
reflected energy onto the receivers until the temperature of the molten salt reaches
predetermined limits. The computers then cause the field of mirrors to track the sun's
trajectory across the sky making motorized corrections every minute or so.
It is the light passing through the aim points, before the mirrors are focused on the
tower, that gives rise to the strange phenomenon seen from the highway. Some 50 yards from
the top of the tower and on either side a ball of light hangs suspended in midair. There
are actually four "balls of light" but from the highway they appear to the
untrained eye as two. The light from all those mirrors passing through an aim point is
reflected off dust particles, water vapor and the superheated air itself to produce the
phenomenon.
Manual Adjustment Mirrors
The receivers are 24 feet in length and are suspended lengthwise (vertically) in a
cylindrical pattern around the top of the tower. Each mirror in the field of 2000 is
assigned the top, middle or bottom eight- foot segment of one of these receivers. The
mirrors themselves are glass and are mounted to a motorized frame by bolts. Field
calibration of each mirror is accomplished manually by adjusting the mounting bolts so as
to bend the glass into a shallow parabolic curve that focuses light reflected by the
mirror onto a specified cross section at the position of its designated target.
Calibration is checked by focusing the mirror onto the white, trapezoidal, screen-like
region just below the receivers. Visual examination of the shape of the spot of light thus
created provides a means of determining correct curvature of the mirror. Examination of
this spot of light also reveals defects in the mirrors that may indicate that replacement
or some other maintenance action is required.
STACEE - Solar II Used to Study Universe
An exciting scientific experiment is currently being conducted at the Solar II
facility. The project, known as the Solar Tower Atmospheric Cherenkov Effect Experiment
(STACEE) will use the Solar II heliostats at night to study very short duration flashes of
blue light that result from gamma ray air showers (gamma rays are high energy
"particles" similar to X-rays). The flash of light lasts only a few nanoseconds
(billionths of a second). The gamma rays originate from astrophysical sources at galactic
and extragalactic distances. When they enter the earth's atmosphere, the gamma rays
interact with air molecules, creating a shower of high energy particles that travel faster
than the speed of light in air. The speed of light in air is slower than the absolute
limit of the speed of light in a vacuum and particles traveling faster than this threshold
produce a "shock wave" of blue light analogous to the shock wave produced by an
airplane traveling faster than the speed of sound. These flashes of light impact the
surface of the earth over an area some 200 meters in diameter. The phenomenon is known as
Cherenkov radiation.
The light is collected by the heliostats at Solar II and reflected to an optical
receiver atop the tower. From various measurements of the signals thus collected the
position in the sky of the source of radiation can be determined. The most intriguing
possibility to scientists is that the Cherenkov radiation arises from interaction of the
atmosphere with gamma radiation in an energy range which has not been explored before. It
is believed that observations of sources in this energy range will yield important new
evidence concerning the acceleration mechanisms of the most energetic objects in the
Universe, including rotating neutron stars (pulsars), remnants of exploded supernova
stars, gamma ray bursts and distant, but intense, active galactic nuclei (quasars).
Observations may also provide a direct probe of the diffuse intergalactic infrared
radiation, which in turn may have profound implications for the cosmological structure of
the Universe.
The most expensive part of a Cherenkov observatory is in the construction of a field of
directional mirrors used to collect the flashes of light. The mirrors at Solar II have
been characterized by scientists as having sufficient optical quality and pointing
accuracy to allow the experiment to be conducted. Thus we have a very clever and cost
effective dual use of the facility that allows it to be utilized during the day to
generate electricity and at night to study the universe.
Site Manager Gives Tour
During the last caravan along Route 66 to Needles, CA we stopped to observe the light
phenomenon described in this article. Knowing a little about the facility (very little), I
attempted to explain the balls of light that hovered in the sky near the tower. Of course,
I had seen the phenomenon before and had developed an elaborate theory to explain it. The
explanation I offered was totally wrong but imaginative if nothing else.
I determined to verify my theory and contacted the site manager, Mr. Paul Speidel, who
invited me out for a tour of the facility. During the tour, Mr. Speidel explained the
phenomenon, the operation of the plant, a little of the history of the facility and drew
my attention to the STACEE project. He also most graciously extended an invitation to our
next tour group to visit the facility. Anyone interested in such a tour please contact the
author. If there is sufficient interest we will try to schedule a tour of the facility as
part of a bus tour or caravan style event.
