Reflecting hope: Concentrating solar power can feed the grid and perhaps even remove carbon from Earth's atmosphere
In California’s Mojave Desert, about a 30-minute drive from the Las Vegas Strip, sits an electric power plant resembling few others in the US that are currently plugged into the grid. To make electricity, the concentrating solar power (CSP) plant’s circular arrays of tens of thousands of mirrors—aka heliostats—begin by directing sunlight to receivers atop three 459-ft tall towers, inciting them to glow with a jarring, otherworldly light, even against the harsh midday Sun.
When the plant, the Ivanpah Solar Electric Generating System (ISEGS), opened in 2014, California officials and others expressed concern that drivers on the nearby I-15 might be distracted by the power plant’s decidedly sci-fi-looking setup. Airline passengers, one story goes, have looked down at Ivanpah’s vast heliostat arrays from cruising altitude convinced that a new type of settler has found their way to this iconic, Wild West landscape.
While the solar towers at ISEGS may look like science fiction, says Guangdong (GD) Zhu, a CSP expert at the US National Renewable Energy Laboratory (NREL), they are in fact a variation on simple boiler technology. They use the Sun’s thermal energy, as reflected by the heliostat mirrors, to turn water into high-pressure steam, which in turn can be used to drive a turbine and create electrical power.
There’s nothing fancy about the optics of the 75.6 sq-ft mirrors at ISEGS, which are mounted two reflectors per heliostat. The heliostats are computer controlled to track the Sun’s movement through the day. There are no photovoltaic (PV) cells or panels converting sunlight directly to electricity.
In each of ISEGS three solar plants, a Rankine-cycle reheat steam turbine is fed steam from the solar collector located in the receiver at the top of a tower. There are two natural gas-fired steam boilers onsite: a nighttime preservation boiler, and an auxiliary boiler used during a morning start-up cycle to bring the plant to operating temperature. It’s also needed on cloudy days to maintain the steam turbine. The ISEGS system uses dry cooling to conserve and recycle water, and the solar plants are limited to a combined 100 acre-ft-per-year of water for operations. That’s about how much water 100 families would use in one year.
The CSP concept dates to the 1980s, says Zhu, who is also executive director of the Heliostat Consortium for Concentrating Solar-Thermal Power—HelioCon, for short. The consortium aims to advance heliostat technologies and workforce development, “by integrating academia, industry, government—all the stakeholders—to really advance CSP technologies for next-generation CSP deployment.”
HelioCon, under the broad umbrella of the US Department of Energy (DOE), brings together experts from NREL, as well as Sandia National Laboratories (SNL), the Australian Thermal Research Institute, and others. Zhu says the consortium is always looking for new partners, regularly issuing requests for project proposals from interested parties in industry, academe, or government.
And the interest in CSP is heating up as the push toward renewable energy grows more urgent. Around the world, Zhu says, countries like China, India, and the United Arab Emirates are wasting no time bringing megawatt-capacity CSP plants online, particularly in desert climates or anywhere there is abundant sunshine.
There are many variations on the CSP concept, but they all use heliostats to direct solar radiation to manmade structures, including troughs and towers, to heat materials that can capture, store, and release solar-thermal energy. Besides water, thermal-energy storage media can also include nontoxic oils, molten salts, and even sand to sequester usable energy from the Sun and keep it for days at a time or longer.
Solar receivers, surrounded by thousands of heliostats, glow eerily bright in the desert sky. Photo credit: William G. Schulz
The captured thermal energy can be used to make carbon-free electricity or to power industrial processes like desalination, food processing, chemical production, mineral processing, and more. Unlike solar photovoltaics, CSP systems can provide energy after sundown or on days that are too cloudy. No (toxic, cumbersome, hazardous) battery required.
According to the DOE, in the past decade, the cost of electricity produced by CSP has dropped by more than 50 percent, from about $0.21-per-Kw-hour to a little more than $0.09-per-Kw-hour, thanks to more efficient systems making wider use of thermal energy storage, allowing that energy to be dispatched around the clock. DOE says it is working hard to make CSP even more affordable, with the goal of reaching $0.05 per-Kw-hour for solar-thermal plants with at least 12 hours of thermal-energy storage.
In the US, Zhu says, capacity building for CSP has lagged in part because energy production is big business, and the technology hasn’t been able to compete with cheaper forms of energy. Zhu says comparisons with other renewable forms of energy are dynamic and complex, but he offers the following: For CSP with no thermal energy storage or less than four-hour storage, solar PV plus some battery storage is less expensive renewable energy. With more than four hours of thermal energy storage, CSP at 0.09 cents-per-kWh would be less expensive.
The original owner/investors of ISEGS, now known as Solar Partners Brightsource, received backing for the venture via US government loans, and they had to get permission to build the sprawling plant on otherwise-protected government property. After 10 years of operation, they have demonstrated that CSP can work reliably to supply energy to the grid, without undue harm to desert plants and animals, including the region’s endangered desert tortoise.
Today, ISEGS is listed as a 386 MW-capacity plant that generates electricity for California’s Pacific Gas and Electric utility. It does not store thermal energy and so is only providing energy to the grid when the Sun is shining. Wildlife studies have shown its environmental impact to be minimal, including concerns that the two-mirror heliostats, which stand on metal posts driven in to the desert floor, would block sunlight and damage habitat. But that doesn’t seem to be causing problems, biologists who have studied the area say, beyond some local critters finding new shady spots to take a nap.
For their safety, fencing was erected to keep tortoises out of the plant, and the animals are relocated whenever they are found inside the plant perimeter. The fence keeps out other curiosity seekers, too: “Every now and then you see cars and they slowly drive up here. And I will say, ‘Can I help you?’ And they ask, ‘Can I get a tour?’ But we don’t do that,” explains Ivanpah’s plant manager Michael Munroe. “And they’ll say, ‘But I’m an engineer and I really think this is neat. Can I come back tomorrow?’ But no, I’m sorry. We don’t give tours. If we did, that’d be all I do!”
But Zhu would encourage curiosity about CSP, and he says more widespread interest, familiarity, and public support for the technology is critical to its development. The technical challenges for CSP are many, Zhu says, including better and cheaper heliostats, optical metrology for better mirrors and mirror arrays to focus and concentrate light from the Sun, as well as education and workforce training, to name a few.
Solar power entrepreneurs are needed to invest in, invent, and manufacture better heliostats and CSP support systems, like robots or drones that can clean mirrors when they get too dusty, for example, or monitor heliostat arrays for damaged or broken mirrors that need to be fixed to maintain operational solar flux. Zhu says the permitting process for new facilities also needs to be speeded up if CSP is to attract greater investor interest.
In recent years, HelioCon and the promise of CSP has attracted the interest of two of the best mirror and optics experts on Earth: Roger Angel and Daewook Kim of the University of Arizona and its Wyant College of Optical Sciences, and Department of Astronomy and Steward Observatory. Angel is the current and founding director of the UA’s famed Mirror Lab, whose large-mirror innovations have revolutionized ground-based astronomy. Kim’s expertise in optical engineering has ensured precision and the best-possible views of the Universe from many existing world-class instruments, including the coming Giant Magellan Telescope with its seven 27-foot diameter mirrors.
A full disk image of the Sun is reflected to a target (distance right) from a twisting heliostat. Photo credit: Roger Angel
Now, extrapolating from their work with astronomical telescope mirrors, Angel and Kim propose a new paradigm for heliostat design that stands to blow performance past previous technical limits for CSP, including delivery of solar thermal energy at very high temperatures that promise game-changing new applications—including planet-saving removal of atmospheric carbon.
“I had this incredible opportunity, to work with Roger so that we can use large optics to save the planet,” Kim says.
Kim and Angel have introduced the so-called twisting heliostat (patent pending), a design that automatically warps the heliostat mirror. The shape of the mirror changes with the Sun’s angle of incidence throughout the day, so as to reflect a well-focused disc image of the Sun toward the solar receivers.
Their prototype reflector, which will be tested shortly at the SNL National Solar Thermal Test Facility, uses a single glass mirror, 2.4 m ´ 3.3 m, attached to a steel frame. Four diagonal back struts extend from the central mechanism out to the rectangular frame corners. The glass mirror is attached to the steel frame by 58 screw actuators. Before twisting the frame, the glass shape is adjusted to the bi-conic shape needed to form a 1-m-diameter disc image on a 113-m distant target when sunlight is incident at 60 degrees.
To form full images (in contrast to concentrated distributions) over the Sun’s range of angles of incidence from 0 degrees (normal) to 70 degrees, the struts push the corners up and down by up to 17 mm. A deflectometry metrology system was used to set the initial shape, and to measure the accuracy of the different twisted shapes. Angel is careful to emphasize that, “We are not just changing the focal length, we are imaging.”
They have also developed a new, tracking camera that exploits the target-oriented mirror mount. It uses a beamsplitter to superpose images of the directly transmitted Sun and the reflected target. By using simple closed loop control of tracking speeds to keep these two images overlapped, the heliostat light is kept centered on the target.
Angel is also devising field arrangements for full disk imaging heliostats, the point being to achieve the highest possible concentration of solar thermal energy. He believes it is possible to achieve solar thermal power at temperatures of up to 1,500 degrees C.
Kim likens the twisting heliostat concept to baking a delicious pizza. “Instead of just baking one pizza and then selling it throughout the day,” he says, “we are baking a really delicious pizza every moment of the day.”
A lot can be done, it turns out, with a ‘pizza’ that delivers a sizzling 1,500 degrees C of solar thermal energy. It is hot enough for industrial chemical processes to make syngas fuels and hydrogen from CO2 and water, for example, and through another chemical reaction called calcination, CSP technology could provide the energy needed for direct carbon capture from the air. Angel and Kim stress that investors can and must be lured toward CSP to build a carbon-free future for the sake of humankind.
The twisting heliostat is purely mechanical: No computer required. Photo credit: Roger Angel
Given the urgency to address climate change as its impacts rapidly accelerate, Angel says, he can imagine CSP facilities set up around the world for the sole purpose of pulling the already accumulated excess carbon out of the atmosphere, though the calcination reaction can be used in other industrial processes.
And Angel has done the math. “If we can fix the planet at 100 bucks per ton of CO2 extracted,” he says, “the total bill is somewhere around $100 trillion.”
Compared to the cost of climate inaction, $100 trillion to heal the planet with sunshine seems like a bargain.
William G. Schulz is the Managing Editor of Photonics Focus.
