Bottling the Sun

by Andy Zhao

As a Ph.D. student in Materials Science, I spend my days in lab similar to how Justin Timberlake spends his days in the studio—pondering the intricacies of solar thermal energy storage. Here is Justin on the mechanics of how a solar thermal power plant works in his song Mirrors:

            And now it’s clear as this promise

            That we’re making two reflections into one

            ‘Cause it’s like you’re my mirror

In a solar thermal power plant, mirrors are used to reflect and concentrate sunlight to heat up a storage material. I like to think of it like a gigantic thermos. In the morning, you fill up your thermos with hot coffee, and whenever you need a boost in energy, the hot coffee is waiting for you. Here is the thermos outside of Las Vegas that powers 75,000 homes all day and night:

Crescent Dunes Solar Energy Plant. Photos courtesy of Solar Reserve

Like I said—GIGANTIC. This plant outside Las Vegas came online in 2015 and is the current state of the art in grid-level thermal energy storage. It works by first pumping up nitrate salts to the top of the tower. There, the concentrated sunlight heats the salts up to a blistering 550 C (1,022 F). The hot molten salts are then pumped down into storage tanks, awaiting the sun’s departure to ignite the lights of Vegas (actually the suburban areas off The Strip, but igniting suburban lights doesn’t sound as hot).

Solar thermal power plants typically use nitrate salts because they have extremely high heat capacities, which means they can store loads of energy in a small volume. Also, mixtures of nitrates have low melting points, making them easy to melt and pump around. And because nitrates are stable up to 550 C, they can efficiently convert heat to electricity. Above this temperature, nitrates break down into other chemicals and lose their energy storage abilities.

That’s basically how solar thermal power plants work. Interestingly, the technology has been criticized for killing a bunch of birds accidentally caught in the mirrors’ crossfire. But there is one bird in particular that energy storage has actually been aiming to take down—the Duck.

To explain our Duck problem (I promise there will be more pictures of ducks soon), first let me show you a graph depicting how much energy the entire state of California used on July 4, 2018, where zero on the x-axis signifies the start of the day at midnight:

And here is how much of that energy was provided by clean and renewable energy, mostly from solar (yellow line) and wind (blue line):

Now, if you subtract the energy provided by solar and wind from the total demand, you obtain the net demand trend, better known in the energy community as the fabled “Duck Curve”:

I know what you’re thinking because I thought the same thing when I first saw it—“Where is the duck?!” Let me help you with my phenomenal Photoshop skills:

I don’t know whose bright idea it was to name this the Duck Curve, but the Duck signifies the energy provided by natural gas and other fossil fuels. As California builds more solar panels and wind turbines, the Duck becomes smaller and smaller.

Solar panels are widely thought of as the silver bullet that will kill the big bad fossil fuel industry, represented here as the “Mighty Duck.” It makes sense since there is enough sunlight that strikes the Earth every 2 hours to power the world for an entire year. But there is a persisting problem—the sun sets every night. Hurling more solar panels at the problem does not kill the Duck, it just dodges the incoming projectiles by stretching its creepy neck, lingering through the night. #DuckDodgers #MightyDucks

To successfully cut off the Duck’s head, we need a way to store excess solar energy during the day so we can use it at night. Enter my (and Justin Timberlake’s) favorite technology—solar thermal energy storage. Solar thermal power plants, similar to the one outside Las Vegas, are currently under construction around the world and are expected to be cost competitive with natural gas. Grid level batteries are also being heavily researched and developed, but they are still much more expensive than solar thermal energy storage.

Scientists and engineers are exploring new materials other than nitrates that could increase solar thermal energy’s operating temperature, energy density and storage time, which could further decrease the cost of energy storage. For example, metal fluorides are being studied for their ability to store energy as latent heat—the energy it takes to change a material from one phase to another. To put latent heat into context, let’s look at the energy you can extract from one liter of liquid water before turning it into ice. When you cool water to exactly its freezing point (0 C), it will remain a liquid. You can squeeze out an additional 333,550 joules of energy before it transforms into ice—enough to power a 60 Watt lightbulb for one and a half hours. In comparison, fluorides have twice this latent heat and can be used at much higher temperatures than water.

Researchers are also studying thermochemical energy storage to increase the energy density and storage time of solar thermal power plants. In this process, concentrated sunlight heats up a chemical, driving a reaction to create fuel that stores the thermal energy as chemical potential energy. When the energy is needed, the chemical reaction is reversed. The chemical fuels that drive this reaction retain the sun’s energy much longer and more densely than either nitrates or fluorides.

I am currently working on a project in collaboration with Los Alamos National Laboratory that uses metal sulfides as a potential thermochemical storage material. We are designing and building prototypes of reactors that heat up metal sulfides to separate them into their metal and sulfur constituents to store energy. When this energy is needed, sulfur and metal are recombined to cause an extremely exothermic reaction. At UC San Diego, we are also developing new techniques to understand how thermal storage materials (nitrates, fluorides and sulfides) transport and store heat at such high temperatures.

While next generation solar thermal power plants that run on latent heat or thermochemical energy are far from commercialization, solar thermal plants that run on nitrate salts have already begun competing with fossil fuels around the world. And as California begins its journey towards 50 percent clean and renewable energy by 2030, solar thermal energy storage will play a key role in eating the Duck.

Personally, my preferred way to deal with a duck is dominating the tea-smoked duck at VillageNorth restaurant in San Diego.