SOLAR HEAT: Solar thermal technology offers the ability to store energy as well as turn sunlight into electricity. Pictured: Rendering of BrightSource's Ivanpah Solar Power Complex. Image: Flickr/pgegreenenergy
Along with the sun's light, our closest star's heat is an ample source of renewable energy, which generators can harness in ways that overcome one of solar energy's biggest shortcomings.
Using mirrors, developers can focus the sun's rays to produce industrial heat or generate electricity, often using materials that can store the energy, as well. In theory, it is a very simple idea and certainly is not a new one. Legend has it that Archimedes used large, polished mirrors to torch Roman ships during the Second Punic War.
In the United States, concentrated solar power (CSP) manifested in the 1980s as nine power plants in Southern California using trough-shaped mirrors to concentrate energy on a circulating heat transfer fluid. The fluid, often a synthetic oil, then heats up a molten salt to as high as 350 degrees Celsius, which then boils water to drive a turbine.
Interest in CSP waned and development largely idled for more than a decade as low-cost fossil fuels, wind energy and photovoltaics came online. However, in recent years, the demand for renewable energy and solar thermal technology caught up with each other as utilities sought clean and constant power. Now developers in the United States and abroad are building new CSP facilities, spurring research into new methods to make these systems more efficient.
"CSP today offers functionality that [photovoltaic technology] doesn't have, which is dispatchability," said Philip Gleckman, the chief technology officer at Areva Solar.
The liquid salt compound in solar thermal plants can stay hot for several hours, allowing the operator to generate electricity as needed, he explained. This means a CSP plant can produce a steady electron flow as clouds pass overhead and even after the sun sets, unlike photovoltaic panels that only produce electricity when the sun shines.
As a result, CSP stands out among intermittent renewable energy sources as an attractive choice for developers looking for steady power. The National Renewable Energy Laboratory (NREL) released a study last week that showed this flexibility from CSP gives utilities $35.80 per megawatt in value when one-third of their electricity comes from photovoltaics and wind turbines.
"I think the market has spoken very clearly it wants [energy] storage," Gleckman said.
Putting solar thermal through the wringer
Areva Solar is working with Sandia National Laboratories at the National Solar Thermal Test Facility to develop a CSP system that uses molten salt to replace oil in receiver tubes, serving as both the heat transfer fluid and the working fluid, thereby reducing costs and increasing efficiency. The facility was built in 1976 but received $17.8 million under the American Recovery and Reinvestment Act for upgrades.
Gleckman explained that developers need to test solar thermal systems extensively at scale before building a full-size plant, more so than photovoltaic panels.
"It's a pretty hostile environment," he said. "It's not something you just want to convince yourself will work on paper." In a solar thermal system, components can face temperatures higher than 600 degrees Celsius, hot enough to melt glass, along with solar energy swings up to 100 suns in intensity.
At the test site, a 100-yard-by-100-yard patch of parabolic mirrors focuses light on receiver tubes suspended 100 feet above in a design known as a compact linear Fresnel reflector. "This is pretty much a segment of a full-size plant," said Cheryl Ghanbari, a test engineer at the National Solar Test Facility.
Researchers would monitor how well the salt flows, how efficiently the tubes absorb heat and how temperatures change at this site, coupled with Sandia's Molten Salt Test Loop, Ghanbari said. She expects the facility to be up and running by April.