By Alex Kimani
Silicon panels pretty much rule the solar energy sector, with more than 90% of panels manufactured using the versatile element.
This is not by chance; Si PV cells are cheap, robust, relatively easy to install, and perform reliably for decades.
Unfortunately, they also come with a major drawback: Silicon PV panels are quite inefficient, with the most affordable models managing only 7%-16% energy efficiency depending on factors like placement, orientation, and weather conditions. Indeed, solar cells have been around for more than six decades, yet commercial silicon has barely scraped into the 25% range.
Even the much-hyped perovskite solar cells have just barely managed to break the efficiency glass ceiling, with scientists recently setting a new efficiency record for a single-junction perovskite solar cell at 25.6%. To complicate matters, sunlight is a diffuse form of energy.
This makes solar panels unsuitable for hard-to-decarbonize sectors such as steel, heavy industries, marine, and aviation.
Fortunately, concentrated solar power (CSP) is proving to be a viable solution for the solar power and green energy conundrum.
CSP technology employs large revolving mirror arrays, also known as heliostats, to reflect and concentrate sunlight onto a receiver. The mirrors are angled to reflect the sunlight onto a large solar receiver. This heat–also known as thermal energy–can be used to spin a turbine or power an engine to generate electricity, and also power a variety of industrial applications, including enhanced oil recovery, mineral processing, water desalination, chemical production, and food processing far from the harvesting point.
Concentrating solar-thermal power systems are generally used for utility-scale projects that can be configured in different ways, such as Power tower systems that arrange mirrors around a central tower that acts as the receiver; Dish-engine systems whereby mirrors are distributed over a parabolic dish surface to concentrate sunlight on a receiver fixed at the focal point, Linear systems that have rows of mirrors that concentrate the sunlight onto parallel tube receivers positioned above them or Parabolic- trough systems that use curved mirrors to focus the Sun’s energy onto a receiver tube that runs down the center of a trough.
CSP comes with major advantages: The thermal energy concentrated in a CSP plant can be stored and used to produce electricity when it is needed–day or night–with bedrocks usually used to store the thermal power to be used to power industrial processes when the Sun goes down.
Energy 101: Concentrating Solar Power (Video)
Source: U.S. Department of Energy
CSP might sound quite quixotic, but many readers might be surprised to find that the idea isn’t particularly new–the first commercial CSP plant was developed in the 1960s. Indeed, there are ~1,815 megawatts of CSP plants in operation in the United States today, enough to power about 1.5 million homes.
CSP plants in the United States:
- Ivanpah Solar Electric Generating System (Brightsource Energy/NRG Energy, Inc.)
- Mojave Solar One (Abengoa Solar, Inc.)
- Solana (Abengoa Solar, Inc.)
- Crescent Dunes (SolarReserve, LLC)
- Genesis Solar (NextEra Energy Sources, LLC)
- Solar Energy Generating System (NextEra Energy Sources, LLC)
- Nevada Solar One (Acciona)
- Kimberlina Solar Thermal Power Plant (Areva)
- Sierra SunTower (eSolar)
- Martin Next Generation Solar Energy Center (FL Power & Light)
- Stillwater Solar Geothermal Hybrid Project (Enel Green Power)
That revelation naturally begs the question: If CSP tech is so hot, why has it failed to achieve mainstream adoption the way solar panels have?
CSP comes of age Actually, the simple answer to that question is that fossil fuels have, for decades, remained much cheaper than CSP when deployed at scale. The fact of the matter is that in the past, CSP has not been cheap enough to implement on a massive scale.
A CSP plant operates most efficiently, and thus most cost-effectively with built-in sizes of 100 MW and higher. A typical CSP plant requires 5 to 10 acres of land per MW of capacity, with the larger acreage accommodating thermal energy storage.
Luckily, as with other conventional renewables energy technology such as solar panels and wind, CSP is now pretty close to reaching a tipping point where it will become competitive with fossil fuels in power generation costs thanks to major advances in technology.
Bill Gates-backed renewable energy outfit Heliogen is perhaps the most famous CSP startup. Heliogen has a mission to completely replace fossil fuels with solar thermal energy. What makes the company unique is that it’s making the process of reflecting and storing sunlight more predictable, controllable, and streamlined.
Previously, CSP companies were able to generate heat anywhere from 400 to 500 degrees centigrade. Heliogen has more than doubled that output by successfully building a solar thermal system that’s capable of producing temperatures up to 1,500 degrees centigrade.
To achieve this feat, the Heliogen team employs machine learning to get the angle of the mirrors as precise as possible, down to a twentieth of a degree. All of the collected heat gets funneled down an insulated steel tube to a bed of rocks where the heat is retained as thermal energy well after the Sunsets.
Heliogen has an award-winning test facility in Lancaster, California, with 400 heliostat mirrors but says it needs to scale that up to a system with 40,000 mirrors. A few weeks ago, Heliogen managed to reel in $108m from two funding rounds to push its ‘sunlight refinery’ concept through to commercialization. The construction of its giant sunlight refinery will be highly automated, with robotic tractors deployed to place the heliostat foundations and set the mirrors efficiently. The company’s dream is to have thousands of sunlight refineries operating across the southwest United States, Australia, and the Middle East-North Africa region by the turn of the decade.
CSP: Making Fuel From Sunlight and Air
The aviation industry is one of the worst offenders as far as GHG emissions go. In fact, a one-hour flight on a twin-engine jet aircraft adds almost 19,000 pounds of carbon dioxide to the atmosphere, with the global aviation industry emitting so much CO2 and other harmful greenhouse gases that if it was a country, it would rank among the top 10 emitters.
Aviation biofuels have been touted as a viable solution to curb this runaway pollution.
However, 13 years since Virgin Atlantic flew a Boeing 747 between London and Amsterdam partly powered by a biofuel made from Brazilian babassu nuts and coconuts, aviation biofuels still account for less than 1% of the 1.5 billion barrels of aviation fuels (15% of global oil supply) that commercial airlines burn through in a typical year.
Luckily, scientists have now developed a carbon-neutral fuel that uses the Sun’s energy to pull carbon dioxide from the air and turn it into fuel.
Researchers at the Swiss Federal Institute of Technology have developed a solar technology that is able to produce liquid fuels using just two ingredients: solar energy and ambient air, with the resulting hydrocarbon fuels releasing only as much carbon dioxide during combustion as was previously extracted from the air thus making them carbon neutral.
It may initially seem like alchemy, but the Swiss Federal Institute of Technology has developed an elegant CSP technology whereby heliostats track the Sun, boosting the sunlight’s intensity by a factor of 2,500 to 2,700 degrees Fahrenheit while reflecting it onto a 50-foot-high tower.
The thermal energy heats a reactor with a core made of cerium oxide, an inexpensive compound often used to polish glass. The high temperatures lead to oxygen being liberated from the cerium, after which it’s mixed with water and carbon dioxide captured from the air in the reactor. As the reactor cools, the reduced cerium claws back oxygen molecules from the added material, leaving a mixture of hydrogen and carbon monoxide called syngas. This is funneled into a second reactor, where the syngas is converted into kerosene molecules. Two years ago, the Móstoles refinery announced its first trickle of solar kerosene.
The Swiss Federal Institute of Technology researchers believe that with a modest boost in current efficiency, solar refineries with a heliostat array the size of Indiana could supply the entire world’s jet aviation fuel demand. At the moment, solar kerosene is likely to ring up around $9 per gallon, about 3x more expensive than gasoline in the United States. But you can expect costs to fall as the technology improves in efficiency and grows in scale.
Solar kerosene will probably find a ready market.
Last year, aircraft manufacturers and other aviation organizations committed to a net-zero emissions target by 2050, effectively cutting CO2 emissions from 30 million tonnes per annum to zero despite a projected 70% increase in passenger numbers over the timeframe.
To achieve this target, they plan to use a mix of cleaner aircraft, sustainable fuels, and better air traffic management.
For aviation fuel to be considered renewable, about half of its contents must be derived from biofuels such as ethanol made from corn or wood chips.
The biggest reason why most airlines continue giving biofuels a cold shoulder is due to their higher costs. Fuel costs constitute the biggest line item for airlines, typically accounting for ~22% of their overheads.
Using renewable air fuel would likely necessitate passing the extra costs to customers by increasing ticket prices, something that would not work well unless everybody did it at once because airline-specific fare changes are highly price elastic. On average, renewable jet fuels would need oil prices ~$65-per-barrel oil for them to become cost-competitive, a level lower than current WTI oil prices of $72 per barrel.