FEATURE
A BETTER WAY TO STORE THE SUN
At a time when decarbonization efforts are kicking into overdrive, thermal energy storage systems are getting glowing reviews. One specific category, sensible heat storage, looks promising, especially for decarbonizing difficult industrial processes.
Written by Poornima Apte
DECARBONIZATION INVOLVES MOVING AWAY from carbon-emitting fuels, so solar and wind have long been promising alternatives. In the energy industry, it’s widely known that less than 0.01 percent of the solar energy reaching the Earth would be sufficient to meet our energy needs, explained Amir Shooshtari, research professor in mechanical engineering and at the Center for Environmental Energy Engineering at the University of Maryland.
While solar’s potential is immense, it has to be harvested at the right time and stored for use when needed.
Solar and wind power’s inability to always be available when utilities need power contributes to their low effective load carrying capability (ELCC). To overcome this challenge, utilities supplement renewable energy sources with battery energy storage systems like lithium-ion, which provide brief four-hour bursts of power to make up for fluctuations.
Antora thermal batteries, ready for delivery. Photo: Antora
However, Li-ion batteries have been a stopgap solution. Apart from the supply chain challenges involved, the batteries are not ideal candidates when demand for electricity increases—think electric vehicles and the growth of data centers.
To improve ELCC scores for solar and wind, utilities need a mechanism to decouple energy storage from power conversion—i.e., they need to harvest and store energy from the sun or wind when they can and convert that stored energy into power that flows back into the grid as needed.
The positive implications for such a solution are profound:
“If we can develop systems capable of absorbing solar energy and storing it for consistent use, we could significantly reduce the effects of energy source intermittency and mitigate global warming,” Shooshtari said.
Thermal energy storage technology, which provides the ability to store energy as heat (or cold) for later use, underpins such a system.
A thermal battery from U.S.-based Antora on crane. These batteries use solid carbon as the raw material and charge rapidly by resistive heat to reach glowing-hot temperatures up to 2,400 °C. Photo: Antora
Zooming in
Thermal is a subset of a larger group of energy storage systems, which also includes chemical, mechanical, electrical, and electrochemical, each categorized by the way it delivers heat.
Of these, thermal energy storage (TES) systems—they store energy as heat or cold for later use, much like a battery—have seen substantial research and development for diverse applications. Innovations in this field have aimed to enhance efficiency, reduce parasitic losses, lower costs through the use of advanced materials and improved packaging, and incorporate ways to optimize the charging and discharging process,” Shooshtari said.
A key subcategory of TES, sensible heat storage (SHS), uses either solid or liquid materials to store energy. It is this specific subcategory that is coming into the spotlight for a variety of end use cases, including heavy industries.
The basic premise of SHS is fairly simple: Harness solar or any other kind of energy and trap it in a “brick” as heat. Then use the bricks as fuel to drive electricity back to the grid or as direct heat for industrial operations.
Not only do SHS systems capture energy and save carbon emissions, they’re also an economical option, argued Daniel Stack, CEO and co-founder of Electrified Thermal Solutions, a thermal energy storage solutions startup. Using wind, solar, hydro, or nuclear power could be a more effective route to a heat source than fossil fuels in such cases, he said.
Kraftblock pellets are mostly made from upcycled materials like steel slag while a phosphate binder increases its thermal conductivity. Credit: Kraftblock
Kraftblock’s Net-Zero Heat System enables companies to run on green heat instead of gas, without altering existing factory infrastructure. Photo: Kraftblock
How it works
Sensible heat storage companies themselves fall into two broad categories, according to the way they discharge energy: some discharge energy back as electricity to flow into the grid, while others do so directly as heat to be used for industrial processes.
The reasoning behind the latter approach is that melting, drying, and chemical reactions in manufacturing, food processing, and chemical reactions, all run on process heat. They burn fuel to achieve the high temperatures needed to execute specific reactions. So, what if instead of burning carbon-emitting fossil fuels to generate heat, we could simply capture heat from the sun directly in blocks and have these bricks feed heat directly as needed to run a wide variety of industrial processes?
Startup Fourth Power falls into the first category, discharging electricity back into the grid. It uses energy (solar, wind, etc.) to heat liquid tin encased in a series of graphite pipes to the metal’s peak temperature of 2,400 °C. This network of pipes, in turn, transfers heat to carbon storage blocks. Heavy and effective insulation envelopes these graphite blocks that store so much heat they glow, so heat loss during storage is minimal.
To discharge electricity, special thermophotovoltaic (TPV) cells convert the light from the graphite blocks back into electricity for the grid.
U.S.-based Antora also uses solid carbon as the raw material for the thermal battery, which charges rapidly when electricity is cheap by resistive heat and reaches glowing-hot temperatures up to 2,400 °C. “At these temperatures, the carbon blocks transfer energy as light, eliminating the need for complex, maintenance-intensive mechanical systems,” said Nehali Jain, vice president of strategy and growth at Antora. Unlike Fourth Power, Antora discharges energy as heat (not electricity that flows back into the grid). However, an upcoming product will incorporate thermophotovoltaics to convert light from the glowing blocks directly into electricity.
“Getting the rules right for thermal batteries as a new asset class, which is very similar to other storage systems but with just a bit more nuance, needs to be addressed.”
—Daniel Stack, CEO and co-founder of Electrified Thermal Solutions
Addressing roadblocks
While the premise of thermal storage systems is straightforward, the execution is more challenging.
First, the brick needs to be manufactured using materials that have high capacity (the ability to store energy) and heat conductivity (the ability to discharge energy). Fourth Power thermal batteries are 98 percent carbon, which is plentiful and low-cost. Antora also uses solid carbon as the raw material for its thermal battery. “Carbon’s high thermal conductivity enables ultra-fast charging that rapidly soaks up [energy during] the cheapest hours of renewable energy generation,” Jain said.
Germany-based Kraftblock is running with a more “tunable” approach to its heat storage material. Because any given material’s heat capacity and conductivity are fixed, the company changes these properties by varying the composition of the pellets depending on the end use case.
To achieve custom properties, Kraftblock uses a mix of leftover materials like steel slag or copper waste as “capacity fillers.” Key to the composition of the TES pellet is the binder that is responsible for the conductivity. The binder binds the slag powders together and is responsible for transferring the heat from the outer shell deep into the core. Changing the ratio of the binder to the capacity fillers controls the conductivity.
“If the heat needs to be kept around for longer, we reduce the conductivity and vice versa,” said Martin Schichtel, Kraftblock’s co-founder and CEO.
Scalability is another challenge the storage systems have to overcome. What will it take to manufacture bricks on a large enough scale to replace carbon-based fuels? To address this issue, thermal energy storage systems will first need to be low-cost and modular.
Fourth Power achieves the low-cost aspect of the equation by packing a lot of power density into the bricks. It takes advantage of the Stefan-Boltzmann law, which states that the amount of light emitted by an object scales to the fourth power of its absolute temperature. Because the system reaches temperatures of up to 2,400 °C, the amount of light directed to the TPV cells is correspondingly large. High power density also enables the overall system to shrink in size, which makes the systems lower cost, modular, and more adaptable to customer needs.
Another important roadblock that’s hindering thermal energy storage systems like Electrified Thermal Solutions is that they are unable to access wholesale rates for powering their systems. “Getting the rules right for thermal batteries as a new asset class, which is very similar to other storage systems but with just a bit more nuance, needs to be addressed,” Stack said.
E-bricks from Electrified Thermal Solutions Photo: Electric Thermal Solutions
Discharge mechanisms
To get energy out of energy storage systems, companies use a variety of different discharge mechanisms.
Kraftblock delivers what it calls a “storage platform,” using a standalone flexible storage system that clients can connect to different charging and discharging options. None of the devices are integrated into the storage unit. It’s easier to maintain equipment this way, according to Kraftblock, and gives a greater ability to match the system in brownfield applications. The platform approach for KraftBlock means that discharge techniques are also dictated by the customer. The platform can be configured to meet specific discharge temperature requirements so heat is not being wasted.
In Antora, the light from the blocks is controlled using a throttle mechanism and shined onto a heat exchanger. Energy is discharged 24/7 at the scale and temperatures that large industrial operations demand. Antora’s HeatCore heats up a heat transfer fluid that is used to create steam in a central steam generator or is otherwise integrated into the customer’s industrial process. This product integrates with industrial processes below approximately 400 °C usually by hooking the steam output into the customer’s existing steam system.
When heat is being discharged, it’s not just hot air blasted through, said Stack of Electrified Thermal Solutions. “The flow rate and the temperature can be adjusted. If you want to dial it up or down, it’s very simple. By just controlling the flow baffle and the fan in the system, you can increase the temperature or up your flow rate, and vice versa,” he added.
As for what the future holds for thermal storage systems, SHS can alleviate grid stress and help heavy industries decarbonize. “With the rapid expansion of artificial intelligence applications and data center operations, the need for enhanced grid resilience and effective power balancing has become increasingly critical,” Shooshtari said. “Load shifting during peak demand periods is a crucial strategy for alleviating grid stress, and integrating advanced thermal energy storage technologies into systems presents a new, promising pathway to achieving this goal.”
Poornima Apte is a technology writer based in Walpole, Mass.
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