Why This Liquid That Stores Solar Energy for Years Matters

Storing solar energy cheaply and efficiently is a key component for the future of renewable energy. Even though lithium batteries are great, they can still be costly and, depending on the chemistry, there can be safety concerns. There are ways we can store solar energy more directly though … and one of those is heat. For instance, concentrated solar energy plants can use that heat for producing electricity, cement, steel, green hydrogen, or anything else that needs high temperatures. A recent breakthrough could allow us to store solar energy directly into a liquid for up to 18 years. How’s it work? And could this be a viable path forward for solar energy storage? Let’s see if we can come to a decision on this.

Solar panels are great! I’ve got solar panels on my home, which are converting sunlight into electrons that end up getting used in my home or shared out onto the grid. However, energy storage is key for renewables due to their intermittency. In order for me to supply my own energy at night, I need to store my excess solar production into my Tesla Powerwall. Photons are converted into electrons, which get shuttled into a chemical battery and back out again … and then the conversion from DC into AC electricity for my house and the grid … there’s a lot of conversions happening throughout this process, each losing a little more energy. That’s just for my home. Scale that up to utility scale and it gets a lot more complicated. Many of the battery technologies available today are still quite expensive and large, and when it comes to lithium, there are also safety concerns. ¹ Harnessing solar energy with solar panels and storing it in traditional lithium batteries isn’t the only way to go.

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Between some tried and true methods, as well as recent R&D projects, there are interesting ways to directly store the sun’s photons as heat or in a chemical form, like that liquid I mentioned earlier that can store it for up to 18 years. Before getting to that one, let’s start with some of the tried and true methods to see how they compare and where this could potentially go.

The first approach is concentrated solar power (CSP). It uses mirrors to reflect and concentrate sunlight onto a specific point to produce heat…and a lot of it! This incredible amount of heat can be used to drive a steam turbine or engine to produce electricity, but they can also store thermal energy to provide power when the sunlight reaches low levels. However, it’s not limited to just that. The heat can be used for water desalination, enhanced oil recovery, cement production, steel production, and even to create green hydrogen. ^{2 3 4} CSP is primarily used for grid-scale power generation and is classified into three general types.

Parabolic trough systems

In parabolic trough systems parabolic reflectors angled toward the sun focus sunlight onto a central receiver pipe — usually called an absorber tube. The parabolic, curved shape focuses the parallel light rays onto a central point, which boosts their reflection capacity 30 – 60 times. That makes the tube, which usually contains thermal oil, heat up to 750°F (about 390ºC) and that heat can be used to boil water to power traditional steam turbines and generators. ^{5 6}

Solar parabolic trough systems are the most advanced and widely used CSP technology. The Genesis parabolic trough power plant, which is located in Riverside County, California, is one of the largest CSP plants in the U.S. with a generation capacity of 250MW and has been operating since 2011.⁷ This system, like many power plants, takes advantage of the Rankine cycle, where a fluid flows from a heat source to a heat sink, performing mechanical work along the way. In the case of the CSP, this fluid is steam that’s generated at the boiler and flows through a turbine, just like many other power plants. The CSP and other parabolic trough power plants can also be used to create hybrid systems with thermal-fired power plants that use fuels like coal, natural gas and biofuel. However, of all the CSP technologies, trough systems have the lowest efficiency (about 15%) since the fluid doesn’t achieve as high of a temperature as the other systems. That’s right … 750°F isn’t considered that hot. I guess it’s a dry heat.[^brighthub] ^{8 6}

Dish/Engine Systems

Then there are dish systems, which kind of look like a satellite dish. They’re composed of parabolic disk mirrors that use sun tracking to focus and concentrate sunlight onto a power conversion unit that’s located along an extended arm from the disk’s center. Inside the power conversion unit are thermal receivers, which can be a bank of tubes with a cooling fluid. That absorbs the heat reflected from the mirror and transfers it to the heat engine, which is usually a Stirling engine. That’s an interesting topic on its own and I actually have a video on Stirling engines if you’d like to watch it, but it’s that engine that drives an electric generator to produce power. ^{9 10}

Dish systems can usually produce between 10-25kW per dish and have a conversion efficiency of about 30%. However, the combination of the reflector with a Stirling engine doesn’t make the technology a great fit for storing energy. ^{11 12 13 14 15 16}

Power tower systems

And finally the towering giants of CSP are the power tower systems, which use flat, sun-tracking mirrors known as heliostats to focus and concentrate sunlight onto a receiver on the top of a tower … kind of obvious with “tower” in their name. It’s there that a fluid moves to transfer heat to a boiler to produce steam, which is then utilized to generate electricity in a turbine. While some power tower systems use water as the heat-transfer fluid, other advanced designs have been using molten salts due to their superior heat transfer and energy-storage capabilities. Because molten salt can effectively retain heat, it can be kept for days before being used to generate power. Even on overcast days, or several hours after sunset, electricity can be generated during times of high demand.^{17 18}

The largest concentrated solar thermal plant in the United States is the Ivanpah Solar Electric Generating System. The project, which is situated in the Mojave Desert of California, can generate 392MW with its 173,500 heliostats. It started commercial operations in 2013 and it’s still running.^{17 19 20 21 22}

The Norwegian company Yara has been working on a new ternary mixture of molten salts based on Calcium-Potassium-Sodium-Nitrate that reduces the risk of molten salt freezing and solidification. Molten salts are intended to be used at high temperatures as a liquid – usually from 270ºC to 565ºC – so if its temperature gets lower than that minimum it can freeze and solidify. The frozen salt can clog pipes, cause damage and stop the power plant operation, which increases risks and maintenance costs in current molten salt technologies.²³

The mixture has a low melting point of 131°C and a wide temperature variation (438ºC), which increases the energy yield/efficiency, requires less of the molten salt blend, and the composition of the salt reduces corrosion. These features increase the lifecycle of the plant and lower costs due to the lower amount of material needed, the competitive price of potassium calcium nitrate, and reduced corrosion maintenance costs. ^{24 25 26}

But molten salt isn’t the only way to go with solar energy storage in CSP. Heliogen, a California-based company, is developing a concentrated solar solution that stores energy in rocks and uses advanced computer vision/AI to precisely align an array of mirrors. The position of the mirror edges and the angles of reflection are evaluated and adjusted 30 times per second.

While most power tower systems usually produce heat anywhere from 400 to 500ºC, this system is capable of reaching 1500ºC due to AI and machine learning. The heat is directed down an insulated, steel tube to a bed of rocks, which can stay hot for days or even up to a week in a properly insulated storage unit.^{27 28 29} Heliogen’s technology provides higher efficiency and reduced water usage compared to traditional steam turbines. In addition, when it comes down to costs, the company’s founder, Bill Gross, mentioned that its technology is targeting delivering heat at $0.01/kWh. ³⁰

In March 2022, the company signed a project agreement with Woodside Energy to develop a concentrated solar energy project at a site in Mojave, California, with a capacity of 5MW. A couple months later, the company announced that the project had moved from design into the testing and implementation stage.³¹ [^project_heliogen_test]

Beyond CSP

Innovations in storing solar energy as heat aren’t limited to CSP. This is on a very different scale than what CSP provides for utility scale energy generation, but it’s a sign of where research is heading for storing that solar energy in even more areas of our lives. Scientists from Chalmers University of Technology in Sweden have been developing a fluid that’s potentially able to store solar energy for up to 18 years. The fluid contains a molecule that’s composed of carbon, hydrogen and nitrogen — called norbornadiene. ³²

You might have flashbacks to high school science classes here, but just bear with me for a second. Two or more atoms make up molecules and, through the bonds that hold them together, the atoms share electrons. Different kinds of molecules have unique three-dimensional forms. Methane, for example, is shaped like a tetrahedron. When energy is added to the molecule, it can change its structure/shape, and its atoms can join together in new bonds that could store varying amounts of energy.

In the case of the Chalmer University’s molecule, it was modified by the researchers to absorb more of sunlight’s different wavelengths. It can harness energy from UV and the blue and green light spectrum. When hit by sunlight the molecule undergoes a transformation into an isomer with high energy, which is a molecule made up of the same atoms but bonded together differently — a new shape. ^{33 34}

In order to manage the storage and release of energy from the molecule, the research team created a catalyst to act as a filter for the liquid, which puts the molecule back into its original state. This change in shape raises the temperature of the fluid by 63ºC. Once it’s back into its original state it’s ready to capture more solar energy. This new technology is named Molecular Solar Thermal Energy Storage System (MOST).^{35 36 37}

I had the chance to talk to Kasper Moth-Poulsen, who’s leading the research, and he provided me with a full high-level explanation of the cycle:

“We have a liquid system flowing through a panel. It’s basically two glass plates with a liquid in between, and the molecules are flowing through and being exposed to light and then converted. Then, they’re going into a small tank, and that small tank is where the molecules with the high energy form are stored. Later, they can flow over a catalyst that is sitting in a little device that is then triggering the heat release and sending out the stored energy. Finally, the molecules can go back into the panel and go and capture energy again. So, it’s a close cycle where the input is solar energy, and we store it as chemical energy and we release it later as heat on demand and recover the original molecule. So, in this way, it’s like a cycle that can operate several times, hundreds of thousands of times…” -Kasper Moth-Poulsen

They sent the molecules with energy absorbed from Swedish sunlight to China, where researchers from Shanghai Jiao Tong University released and converted the energy into electricity using a generator developed by them. I’d say this sounds MOSTly promising. The fact that Swedish sunshine was sent to the other side of the world and converted into electricity is kind of crazy. ³⁸

Zhihang Wang, a researcher from Chalmers University of Technology said:

“…The generator is an ultra-thin chip that could be integrated into electronics such as headphones, smart watches and telephones. So far, we have only generated small amounts of electricity, but the new results show that the concept really works. It looks very promising…” -Zhihang Wang ³⁴

When it comes to efficiency, Kasper said:

“…The best systems operate between 30 to 50% efficiency at the wavelength that they receive. But then we have the full solar spectrum. It’s many wavelengths. So, in reality, our best system captures about 3% of the incoming solar energy right now…The theoretical max is between 12 to 16% of the incoming energy. It’ll not be as much as a photovoltaic, and that is because you have to pay something for the storage in the molecules and therefore it cannot be as efficient as that… -Kasper Moth-Poulsen

He also mentioned that the molecule has been the hardest part to optimize.

“…The heat recovery is going fairly well. We extract the heat that we are storing. There are quite some losses in the storage process, and we would like to expand to a bigger part of the solar spectrum and at the same time maintain the high inner density and so on. There are like three or four parameters we are trying to optimize at the same time and they’re pointing a little bit in different directions. The difficulty is to create systems that are good in all aspects…” -Kasper Moth-Poulsen

This is still ongoing research and they’ve been testing new molecules and improving them along the way. The next step, according to Kasper, is to scale up the system from just a few watts to possibly a thousand watts. Although the molecule may not achieve efficiencies similar to photovoltaics or CSP, it could be layered into other existing things to make them even better. Imagine layering this with a photovoltaic system on your home. Not only would you be converting photons into electricity for your home, but you’d also be storing it as heat for use later … like helping to produce hot water in the middle of the night. In my chat with Kasper he mentioned some possible applications for their system, including heating your hot water without needing electricity or burning gas, heating a car’s cabin, and more.

As we move forward towards a greener future, storing solar energy for later use is essential. Harnessing sunlight, converting it into electricity, and then converting it again to store it into batteries isn’t the only path. While storing heat in CSP power plants using molten salts has been around for a while, newer approaches like storing solar energy directly into molecular bonding is still at the early stage. Scaling it up and finding the best applications for it will still take time, but the benefits that it can offer, like storing solar energy for 18 years, are very promising.