On a perfect New Mexico winter day - with the sky almost 10
percent brighter than usual - Sandia National Laboratories and
Stirling Energy Systems (SES) set a new solar-to-grid system conversion
efficiency record by achieving a 31.25 percent net efficiency rate. The
old 1984 record of 29.4 percent was toppled Jan. 31 on SES’s
“Serial #3” solar dish Stirling system at
Sandia’s National Solar Thermal Test Facility.
Solar energy conversion record set
The conversion efficiency is calculated by measuring the net
energy delivered to the grid and dividing it by the solar energy
hitting the dish mirrors. Auxiliary loads, such as water pumps,
computers and tracking motors, are accounted for in the net power
measurement.
“Gaining two whole points of conversion efficiency
in this type of system is phenomenal,” says Bruce Osborn, SES
president and CEO. “This is a significant advancement that
takes our dish engine systems well beyond the capacities of any other
solar dish collectors and one step closer to commercializing an
affordable system.”
Serial #3 was erected in May 2005 as part of a prototype
six-dish model power plant at the Solar Thermal Test Facility that
produces up to 150 kilowatts (kW) of grid-ready electrical power during
the day. Each dish unit consists of 82 mirrors formed in a dish shape
to focus the light to an intense beam.
The solar dish generates electricity by focusing the
sun’s rays onto a receiver, which transmits the heat energy
to a Stirling engine. The engine is a sealed system filled with
hydrogen. As the gas heats and cools, its pressure rises and falls. The
change in pressure drives the pistons inside the engine, producing
mechanical power, which in turn drives a generator and makes
electricity.
Lead Sandia project engineer Chuck Andraka says that several
technical advancements to the systems made jointly by SES and Sandia
led to the record-breaking solar-to-grid conversion efficiency. SES
owns the dishes and all the hardware. Sandia provides technical and
analytical support to SES in a relationship that dates back more than
10 years.
Sandia is a National Nuclear Security Administration
laboratory.
Andraka says the first and probably most important advancement
was improved optics. The Stirling dishes are made with a low iron glass
with a silver backing that make them highly reflective
—focusing as much as 94 percent of the incident sunlight to
the engine package, where prior efforts reflected about 91 percent. The
mirror facets, patented by Sandia and Paneltec Corp. of Lafayette,
Colo., are highly accurate and have minimal imperfections in shape.
Both improvements allow for the loss-control aperture to be
reduced to seven inches in diameter - meaning light is highly
concentrated as it enters the receiver.
Other advancements to the solar dish-engine system that helped
Sandia and SES beat the energy conversion record were a new, more
effective radiator that also costs less to build and a new
high-efficiency generator.
While all the enhancements led to a better system, one aspect
made it happen on a beautiful New Mexico winter day — the
weather.
“It was a ‘perfect storm’ of
sorts,” Andraka says. “We set the record on Jan.
31, a very cold and extremely bright day, a day eight percent brighter
than normal.”
The temperature, which hovered around freezing, allowed the
cold portion of the engine to operate at about 23 degrees C, and the
brightness means more energy was produced while most parasitic loads
and losses are constant. The test ran for two and a half hours, and a
60-minute running average was used to evaluate the power and efficiency
data, in order to eliminate transient effects. During the testing
phase, the system produced 26.75 kW net electrical power.
Osborn says that SES is working to commercialize the
record-performing system and has signed power purchase agreements with
two major Southern California utilities (Southern California Edison and
San Diego Gas & Electric) for up to 1,750 megawatts (MW) of
power, representing the world’s two largest solar power
contracts. Collectively, these contracts require up to 70,000 solar
dish engine units.
“This exciting record shows that using these dishes
will be a cost-effective and environmentally friendly way of producing
power,” Osborn says. “SES is actively engaged in
the commercialization of the system, called the
‘SunCatcher,’ including continuing to prepare it
for mass production, completing project site development and
preconstruction activities, and establishing partnerships with
substantial manufacturing and industrial organizations to develop a
cost-effective manufacturing process and supply chain. The demonstrated
high efficiency means more energy is generated for the given
investment, lowering the cost of the energy delivered.”
SES was formed in 1996 to develop and commercialize advanced
solar technology. The company maintains its corporate headquarters in
Phoenix, Ariz, project and technical development offices in Tustin,
Calif, and engineering and test site operations at Sandia National
Laboratories in Albuquerque.