Antora Energy develops thermal energy storage for zero-carbon heat and power, with the goal of stopping climate change. Their technology uses excess solar and wind electricity to heat blocks of carbon, which can then be used as a source of heat and light. Antora Energy has received funding from investors such as Bill Gates and Chris Sacca, and has also received support from ARPA-E.
Challenges: thermal energy storage
As the world moves towards a more sustainable future, the transition to renewable energy sources such as wind and solar power is becoming increasingly important. However, with this transition comes the need for large-scale energy storage solutions that can be deployed in the grid. This is because renewable energy sources can be unpredictable, with periods of high energy production that outpaces energy demand, and periods of low energy production that fall short of energy demand.
Energy storage solutions are crucial for capturing excess energy during times of high production and for supplying energy during times of low production. This is important for maintaining a stable and reliable energy supply for homes and businesses. Unfortunately, existing battery technologies have limitations that make them less than ideal for large-scale energy storage. These limitations include challenges with material sourcing, high cost, and performance limitations.
Thermal energy storage is a promising technology that involves capturing excess energy as heat in some type of storage medium, and then using that stored energy to generate electricity or heat on demand. However, despite its potential, thermal energy storage has faced two significant challenges that have hindered its development and widespread adoption.
The first challenge is with the storage medium itself. Conventional thermal energy storage systems use a liquid or gas storage medium that is pumped through a network of pipes and heat exchangers to bring heat to a heat engine. This method of transporting the storage medium is faced with numerous problems and risks. The liquid or gas can leak, corrode the pipes, and degrade the heat exchangers. Additionally, the energy density of the storage medium is relatively low, which means that large storage tanks are required to store a significant amount of energy.
The second challenge is with the solid storage mediums. Research into solid storage mediums has faced other challenges. If heat is extracted from the surface of a storage medium faster than the heat can conduct through the internal volume of the storage medium, a large thermal gradient can develop. This can leave large portions of the storage medium undischarged and creates the potential for a thermal-shock induced mechanical failure.
Antora Energy Technology
Antora Energy develops a thermal energy storage system that combines an inexpensive thermal storage medium, such as carbon, at high temperatures with high-efficiency thermophotovoltaic energy conversion. This system provides a cost-effective and scalable solution to address the technical requirements for large-scale energy storage of renewable resources on the electricity grid. The system uses excess renewable energy electricity to heat carbon blocks to extremely high temperatures. This thermal energy is then stored and can be delivered as heat up to 1500 ºC or as electricity via highly efficient thermophotovoltaics (TPVs).
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Thermal storage system
As schematically depicted in the diagram below, the energy storage system consists of two main components: a thermal storage unit and a power conversion unit.
The thermal storage unit includes an insulated container that houses a thermal storage medium, electrodes, and resistive heating elements. This allows the system to store energy in the form of heat, which can then be used to generate electricity when needed. The energy conversion unit includes thermophotovoltaics (TPVs), cooling plate, throttle, re-radiator, and a binary shutter. These components work together to convert the stored heat energy into electricity, allowing the system to be used for a variety of energy storage applications.
When it comes to energy storage, one of the biggest challenges is finding a material that is both inexpensive and capable of storing large amounts of energy. That’s where carbon comes in. As it turns out, carbon is an ideal thermal storage material:
- It is relatively cheap, with a cost of around $1 per kilowatt-hour;
- It has a high energy density, with a capacity of over 1,500 watt-hours per liter (Wh/L) at 2,200 ºC. This is significantly higher than other materials (such as carbon steel, mullite, Al₂O₃, SiO₂, and Al) commonly used for thermal storage;
- It has a high specific heat, which means it can store a lot of energy per unit of mass. At 2,200 ºC, its specific heat is over 2,000 joules per kilogram-kelvin; and
- It also has a high thermal conductivity, which means it can transfer heat energy quickly and efficiently. This is important for thermal storage systems, as it allows the stored energy to be used quickly when needed.
The design of the storage medium can be just as important as the material itself. Antora Energy has designed a U-shaped thermal storage medium. This unique design has several advantages over traditional thermal storage methods:
- It allows for more heat to be discharged from the carbon storage medium. The increased surface area of the carbon means that more heat can be dissipated, resulting in higher energy efficiency;
- It allows for heat to be extracted more deeply from the carbon medium, avoiding a large thermal gradient; and
- It concentrates heat in the U-shaped chamber, outputting a beam of high-intensity light. This light can then be converted to electricity by thermophotovoltaics (TPVs) or used to make heat up to 1,500 ºC for industrial heat applications.
The volume of the U-shaped chamber is optimized for optimal performance. A large chamber volume results in less carbon material and, consequently, less thermal energy stored.
How does the thermal storage system work?
During active charging, as depicted in the figure below, resistive heating elements are coupled to electrodes to heat the thermal storage medium. To prevent damage to electrodes, water is used to actively cool them.
When the thermal storage medium reaches or exceeds a threshold temperature (e.g., > 1,100 ºC or > 2,200 ºC), the resistive heating elements are idling. The temperature of the thermal storage medium and resistive heating elements are similar. Heat from the resistive heating elements can be conducted back to cooled electrodes that are coupled to the resistive heating elements, resulting in heat leakage. Consequently, when the resistive heating elements and thermal storage medium are idling, the electrodes are detached from resistive heating elements to prevent energy loss, and TPVs are stored in a chamber adjacent to the main chamber.
During discharging, as depicted in the figure below, TPVs are exposed to emitted heat from the thermal storage medium.
A beam of high-intensity light emitted from the hot thermal storage medium heats the re-radiator. The hot re-radiator then emits concentrated light that can be converted into electricity by thermophotovoltaics (TPVs). Because the power conversion efficiency of TPV is dependent on the temperature of the re-radiating heat surface, the throttle between the thermal storage medium and re-radiator controls the amount of heat received by the thermophotovoltaics from the re-radiator. Thereby, by dynamically controlling the throttle, the temperature of the re-radiator can be maintained at the threshold discharge temperature. The cooling plate lowers the temperature of the TPVs to an acceptable level. The power conversion efficiency of TPVs can achieve over 40%.
In the event of safety or emergency actions, the binary shutter can be inserted between the thermophotovoltaics and re-radiator to quickly stop the thermophotovoltaics from absorbing additional heat.
As schematically depicted in the figure below, the TPV comprises a multijunction PV stack positioned between an outer layer (such as a protective layer and/or anti-reflective layer) and a highly reflective back surface reflector. The outer layer is positioned to first receive the light from the hot re-radiator.
Antora Energy’s TPV can reach a record power conversion efficiency of over 40% thanks to following two innovative designs.
- Back surface reflector
The TPV has a highly reflective back surface reflector. As shown in the above figure, sub-bandgap photons that are not absorbed by the PV stack can be reflected back to the re-radiator. The re-radiator preserves the energy of reflected sub-bandgap photons through reabsorption. The reflected and subsequently reabsorbed sub-bandgap photons help to keep the re-radiator hot, thereby minimizing the energy input from the thermal storage medium required to heat the re-radiator.
The hot re-radiator is key because the spectrum of irradiated light redshifts towards longer wavelengths as the re-radiator temperature is lowered, which is why traditional TPVs paired with emitters of less than 1,300 ºC are typically based on 0.74 eV InGaAs or 0.73 eV GaSb, resulting in low open-circuit photovoltage and low power conversion efficiency. The high re-radiator temperature allows the use of III-V semiconductors with a bandgap of at least 1.0 eV instead of low-bandgap InGaAs- or GaSb-based cells traditionally used for TPVs. In addition, the highly reflective back reflector improves recycling of luminescent photons generated by radiative recombination.
- Multijunction PV stack
The spectrum of photon energies thermally radiated by the re-radiator contains a broad range of values. Antora Energy has designed a 1.2/1.0 eV or 1.4/1.2 eV two junction III-V semiconductor stack, as depicted in the figure below, to provide two bandgaps chosen to optimally convert a specific portion of the light spectrum from the hot re-radiator with temperatures between 1,900 and 2,400 ºC.
Thermophotovoltaic technology has reached an efficiency of 40%, making it competitive with turbine-based heat engines. TPVs have the potential for lower costs, faster response times, less maintenance, and easier integration with external heat sources and fuel flexibility, which makes them more desirable than turbines. As turbine costs and performance have reached full maturity, there are numerous opportunities for TPVs to increase their efficiency and decrease their cost.
Antora Energy Patent
- US20220412228A1 Sub-Systems and Methods within a Thermal Storage Solution
- WO2022272098A1 Sub-systems and methods within a thermal storage solution
- US20210143446A1 System and method for a solid-state thermal battery
- WO2022173467A3 Structures and methods for producing an optoelectronic device
Antora Energy Products
Antora Energy has developed a prototype thermal energy storage system called Alpha System, as shown in the image below.
The thermal storage medium can be radiative charging and discharging. It has a capacity of 500 kWh. The system has been demonstrated over 1,000 hours of successful operations and no detectable fouling over longest test runs for 100 hours at 1,500 -1,800 ºC.
Antora Energy’s thermal energy storage system is highly efficient with minimal energy loss during storage and delivery. It’s also highly flexible, it can be tailored to meet the specific needs of different customers and industries. This means that the energy captured from solar and wind power can be used effectively and efficiently, reducing the need for traditional fossil fuels and lowering carbon emissions.
Antora Energy Funding
Antora Energy has raised a total of $51.5M in funding over 5 rounds, including two Grant rounds, a Seed round, a Non-Equity Assistance round, and a Series A round. Their latest funding was raised on Feb 16, 2022 from a Series A round.
Antora Energy Investors
Antora Energy is funded by 11 investors, including
- Fifty Years
- National Science Foundation
- Grok Ventures
- Trust Ventures
- Overture VC
- Impact Science Ventures
- Shell Ventures
- Lowercarbon Capital
- BHP Ventures
- Breakthrough Energy Ventures
- Creative Destruction Lab (CDL)
Antora Energy Founder
Antora Energy CEO
Andrew Ponec is CEO.
Antora Energy Board Member and Advisor
Christina Karapataki is a board member.