Rondo Energy, an American clean tech startup founded in 2020, has developed a low-cost thermal storage system to store intermittent wind and solar energy at grid scale and outputs heat, steam, and/or electricity for various industrial applications. Such a thermal storage system is known as a heat battery. The Rondo heat battery has a capacity to store 300 MW and continuously delivers a hot gas stream at a constant temperature between 800 and 900 ºC, as well as superheated steam when coupled with a steam generator. The hot gas flow and steam provided by the Rondo heat battery help energy-intensive industries, such as cement production and steel manufacturing, reduce carbon emissions.
Challenges: renewable energy storage
Renewable energy storage
Renewable energy sources like wind and solar power are becoming more important as the world moves toward sustainability. These renewable energy sources are unpredictable. They might make more energy than is needed during the day, but not enough energy to meet needs during the night. Therefore, this transition requires grid-deployable large-scale energy storage solutions to deliver a stable and reliable energy supply for homes and industrial applications.
Renewable energy storage solutions include pumped hydroelectric storage, compressed air energy storage, flywheel energy storage, stacked blocks, liquid air, underground compressed air, and flow batteries. Lithium-ion battery technology for grid-scale energy storage has limitations of material sourcing, high cost, and performance, which make them less than ideal for large-scale energy storage. Most of these storage solutions only deliver electricity on demand, which might not be suitable for industrial applications that need heat or steam.
This book 100% Clean, Renewable Energy and Storage for Everything discusses the renewable electricity and heat generating technologies needed, how to keep the electric power grid stable, and how to address non-energy sources of emissions. (see on Amazon)
The thermal storage system is a promising way to store energy on a grid scale. It involves putting heat converted from extra renewable energy into a storage medium and then using that heat to make heat, steam, or electricity when it is needed. The thermal storage system is called a “heat battery”. So, the heat battery can be used in residential and industrial locations that need heat, steam, and/or electricity.
Many heat batteries have problems with uneven heat distribution, brick wear from heating and cooling cycles, and safety and maintenance issues.
They have issues of “thermal runaway” or “heat runaway” which happens when there are even small imbalances between how local heating is done with resistive heating elements and how local cooling is done with heat transfer fluid flow. Changes in how fast things heat up and cool down can cause temperatures to get out of control, which can cause heaters to break and refractory materials to wear out. Any such failure would require the powering down of heaters, disassembly, and replacement of cracked bricks, which would be time-consuming and expensive.
They also have severe limitations in terms of the temperatures that are practically achievable, as well as material usage and storage capacity.
Rondo Energy Technology
Rondo Energy (Rondo) has developed a low-cost heat battery that efficiently stores thermal energy generated from intermittently solar and wind energy and delivers continuous flows of hot air (or hot CO₂) and steam at a controllable temperature for many industrial processes, such as making cement, steel, hydrogen, and ethanol. By integrating the Rondo heat battery with these industrial processes, we can use less fossil fuels and reduce carbon emissions.
The Rondo heat battery’s key innovation is that it prevents “thermal runaway” or “heat runaway” by cooling thermal storage bricks well below their operating temperature in alternate discharge cycles. In addition, the Rondo heat battery can be heated by electrical energy while delivering convectively heated air and steam. That is, the heat battery can simultaneously charge and discharge.
Rondo heat battery
The diagram below depicts the Rondo heat battery system.
The heat battery includes an electrical energy supply input, a container that houses thermally conductive bricks, resistive heating elements, sensors and devices, and blowers, an output gas flow of steam and/or heat, and a control system.
- Electrical energy supply
The container’s height is about six meters. It has two sections that are fluidically connected. A thermal barrier separates the two sections. The first section contains thermally conductive bricks that are charged or heated by resistive heating elements. The second section contains blower and steam generation units.
- Thermally conductive bricks
There are radiative cavities and air passages in each brick. The air passages are open to the cavities on top and bottom surfaces. They are internally exposed to a radiating surface heated by resistive heating elements. In the cavities, heat is transferred by thermal radiation from relatively hotter surfaces to relatively cooler surfaces.
The bricks are stacked to form structures that facilitate the horizontal movement of thermal energy through cavities. This approach avoids hot spots. Simultaneously, thermal energy is discharged to the top of the stack along the vertical axis. Thus, the thermocline can be maintained by permitting radiation to move freely in the horizontal plane but not substantially along the vertical axis.
Bricks are made of materials with high thermal conductivity and absorption capacity. These materials reduce temperature differences for given heat flux and enable the use of fewer, larger bricks. The materials include alumina, aggregates such as magnetite and olivine, and binders. Bricks can be heated to temperatures of at least 1,200 ºC.
- Resistive heating elements
The resistive heating elements are wires, ribbons, or rods that pass through channels in the brick stack. Each of the heating elements is electrically connected to an electrical power generation source and generates thermal energy to heat bricks.
The resistive heating elements are composed of SiC, MoSi₂, or FeCrAl. FeCrAl is coated with aluminum by processes including hot aluminizing, aluminum electro-plating, sol-gel processing and aluminum plating followed by anodizing. The surface treatment increases the surface’s emissivity.
The blower regulates the air flow (or CO₂) between the first and second sections of the container and the outside environment. It facilitates the air flow through the air passages of the bricks, from the bottom to the top of the brick stacks, at a suitable flow rate. The thermal energy stored in the bricks heats the air flow from 250 ºC to over 800 ºC.
The hot air flow enters the second section of the container. The hot air (or CO₂) can be delivered for industrial applications. It can also heat the liquid in the steam generator’s conduits, cools down, and then recirculates through the first section’s bricks.
- Steam generator
Water in the steam generator’s conduits is heated by the hot air flow and becomes steam. The steam can be used for industrial applications. The steam generator can be a Once-Through Steam Generator (OTSG) or a Heat Recovery Steam Generator (HRSG).
- Control system
The control system automatically manages heat battery charging, discharging, and maintenance based on sensor feedback and external information. Sensors include voltage and current sensors, wind sensors, sky cameras that detect passing clouds, and solar radiation sensors. External information includes weather forecasts and market conditions such as the availability of electricity, its cost, and the presence of alternative energy sources.
As the heat battery ages, the resistance of the heater elements, the airflow behavior, and heat transfer in bricks all change. The control system uses the changes in physical properties of the components and airflow patterns to inform an artificial intelligence (AI) system in order to maintain high performance for years by optimizing heating and cooling rates, as well as peak temperature duration.
How Rondo heat battery works
The Rondo heat battery’s key innovation is its ability to prevent thermal runaway by periodically cooling the thermal storage bricks to temperatures well below their operating temperature. This is accomplished through successive charge and discharge cycles in which each storage brick stack is alternately deep-cooled and the output airflow is maintained at 800 ºC.
The following diagrams show how each storage brick stack is deeply cooled through successive charge and discharge cycles. Here, we’ll look at two arrays of thermal storage brick stacks.
- Stage 1
When renewable power sources are available, both thermal storage brick stacks are charged (or heated). The airflow in stack 1 is topped. The stack 2 heats the airflow and delivers hot gas at 800 ºC to the steam generator and/or directly to industrial applications. The heat battery charges and discharges simultaneously.
- Stage 2
When charging stops at the end of a solar day or windy period, stack 2 discharges continuously and provides a constant 800 ºC airflow. Stack 1 is idle.
- Stages 3
When 65% of thermal energy of stack 2 is discharged, the temperature of the hot airflow from stack 2 will begin to drop. As discharge continues, stack 2 no longer delivers 800 ºC airflow. Stack 2 achieves “deep cooling”.
At this point, the airflow of stack 1 is open, allowing stack 1 to discharge. Its hot outlet airflow is now mixed with the cool airflow from stack 2 to maintain an outlet airflow of 800 ºC.
Stack 2 continues to be cooled, and the temperature of its outlet airflow continues to fall below 800 ºC.
- Stage 4
As stack 1 discharges, its outlet airflow temperature begins to drop. To maintain 800 ºC outlet gas, the airflow of stack 2 is closed. At this time, the peak brick temperature in stack 2 is significantly lower than in stack 1.
The heat battery is fully discharged when the stack 1 outlet airflow temperature falls below 800 ºC.
- Stage 5
The next charging cycle begins.
Stack 1 is charged and provides hot airflow at 800 ºC. Stack 2, which has been deeply cooled, is charged without airflow.
The heat battery charges and discharges simultaneously.
- Stage 6
At the end of this charging period, stack 1 discharges continuously while stack 2 is idle.
- Stage 7
When 65% of thermal energy of stack 1 is discharged, the temperature of the hot airflow from stack 1 will begin to drop. As discharge continues, stack 1 no longer provides 800 ºC airflow. Stack 1 achieves “deep cooling”.
At this time, stack 2 begins to discharge. The cool and hot airflow are mixed to maintain outlet gas flow temperature of 800 ºC.
Charging stops. Both stacks discharge to provide hot gas flow.
- Stage 8
As the outlet gas flow temperature stack 2 falls, stack 1 stops discharge. Stack 2 continuously discharges to provide 800 ºC outlet gas flow.
The heat battery is fully discharged when the stack 2 outlet airflow temperature falls below 800 ºC.
Rondo Energy Patent
- US11603776B2 Energy storage system and applications
Rondo Energy Heat Battery Applications
Cement production contributes significantly to CO₂ emissions, accounting for about 8% of global CO₂ emissions. Typically, cement is made from limestone and clay. These raw materials are processed in a rotary kiln at about 1,400 ºC to produce calcium oxide, magnesium oxide and CO₂. The heat required for cement production is supplied by the combustion of coal or natural gas, which also produces CO₂.
The Rondo heat battery powered by renewable energy can provide clean heat other than burners in the kiln.
The exhaust CO₂ gas from the kiln can be captured and circulated through the heat battery which provides hot CO₂ flow at a controlled temperature. The output hot CO₂ flow can be used to dry and preheat raw materials to desired reactor operating conditions. Therefore, CO₂ is not released into the atmosphere. In addition, the use of Rondo heat battery in cement production can decouple the gas flow between the kiln and precalciner. Thus, gas flow and heating rates can be optimized independently for each process.
The conventional solid oxide electrolyzer (SOE) receives an input of heated gas and superheated steam. The gas is heated by an electric resistive heater or a fuel heater. Fuel heaters consume fossil fuels, such as natural gas, and emit CO₂. Electric heaters powered by renewable energy sources have unstable temperatures and limited operating periods.
The Rondo heat battery powered by renewable energy can be integrated into the SOE and provides a continuous hot gas flow at a constant high temperature of 850 ºC to the SOE.
The application of the heat battery avoids the use of an electrical resistive heater or a fuel fired heater, reducing the cost of operation and carbon emissions. In addition, the heat battery can improve the SOE’s power conversion efficiency and hydrogen production.
Many Direct Air Capture (DAC) technologies require heat to regenerate solid or liquid adsorbent materials that capture CO₂ from air flow through chemical reactions. For example, solid calcium oxide (CaO) absorbs CO₂ from the air, forming limestone (CaCO₃). Amine solvent captures CO₂ to form ammonium carbonate. Thermal energy is required to regenerate CaO or amine for more cycles of CO₂ capture.
The Rondo heat battery can be integrated into the DAC system to provide hot gas flow and steam for regeneration of CO₂ capture adsorbent.
A portion of CO₂ gas that is captured is used as heat transfer fluid in the Rondo heat battery. Thus, the heat battery can provide a steady flow of hot CO₂ to the calcine to regenerate CaO. The heat battery can also provide steam by a heat recovery steam generator (HRSG) to power an amine solvent reboiler for amine adsorbent regeneration.
The Rondo heat battery powered by renewable energy can be applied in oil extraction.
By coupling with an Once-Through Steam Generator (OTSG), the heat battery can provide a heat flow to the OTSG, which generates steam for oil extraction at a controlled temperature and flow rate. The OTSG feed water can be recovered from a water/oil mixture.
The production of ethanol as a fuel from starch and cellulose involves processes of hydrolysis, fermentation, and distillation. Ethanol plants have substantial electrical energy demand for pumps and other equipment, as well as substantial heat requirements for hydrolysis, cooking, distillation, and dehydrating. Using conventional electric power and fuel-fired boilers and cogeneration would result in significant CO₂ emissions.
Carbon emissions can be reduced by integrating Rondo heat battery power from renewable energy into an ethanol production facility. The heat battery provides a continuous hot airflow to dry grain and supplies steam for plant operations.
Desalination processes traditionally run continuously, and a significant portion of the world’s desalination is currently performed by membrane systems. The vast majority of the desalination in the Middle East uses older thermal desalination technology that is coupled to a combined cycle power station.
The combined cycle power station has a combustion turbine and a steam turbine, which drive either a multi-stage flash or a multi-effect distillation production system. This reduces the steam turbine’s electricity output by a few percent, but significantly reduces the electricity required to desalinate seawater. Moreover, even when there is no other demand for the electricity, the power station continues to operate to keep desalination operational.
The Rondo heat battery powered by renewable energy can solve these issues. The heat battery can provide a hot gas flow at a higher temperature than the combustion turbine’s outlet temperature. Thus, the Rondo heat battery can be integrated into existing desalination systems.
Glass production typically requires high temperatures of 1,500-1,700 ºC to transform raw materials into molten glass in a melting furnace. This process consumes over half of the energy used in glass production. The heat is provided by fossil fuel combustion. The container and flat glass industries emit over 60 megatons of CO₂ per year.
The Rondo heat battery, which is powered by renewable energy, can supply the glass production with the necessary heat to reduce carbon emissions. The Rondo heat battery uses nitrogen gas as a heat transfer fluid, supplying a hot flow of nitrogen gas to the float tank. This reduces the cost of air separation and the production of harmful nitrogen oxides (NOₓ) caused by thermal reaction between nitrogen and oxygen in air.
Steel production requires temperatures around 1,600 ºC. Every ton of steel produced emits 1.4-1.85 tons of CO₂. The steel industry accounts for about 30% of the global industrial CO₂ emissions.
Using direct reduction processes with an electric arc furnace substantially reduces CO₂ emission in the steel industry. For example, using natural gas as the reducing agent reduces CO₂ emissions by approximately ⅓ compared to the traditional blast furnace method. Using renewable H₂ as a reducing agent reduces carbon emissions even further. However, the process is thermally unfavorable because the reaction between hydrogen and iron oxide is endothermic.
The renewable energy-powered Rondo heat battery can be integrated into the blast furnace of the steelmaking process to provide hot reducing gas flow. The heat battery uses reducing gas of natural gas or hydrogen as heat transfer fluid and supplies a hot flow of reducing gas to the blast furnace. This improves production efficiency and reduces carbon emissions.
Rondo Energy Products
The Rondo Heat Battery comes in two ready-to-use models, the RHB100 and RHB300. Both are scalable to reduce CO₂ emissions and operating costs in various industries. Both products can operate continuously 24/7, have a lifespan of 40 years or more, and have zero carbon emission.
Storage: 100 MWh
Discharge: 7 MWt
Charge: Up to 20MWac
Storage: 300 MWh
Discharge: 20 MWt
Charge: Up to 70MWac
Rondo Energy installed a 2 MWh Rondo Heat Battery at Calgren Renewable Fuels’ facility in Pixley, California, in 2023. Calgren Renewable Fuels produces the world’s lowest carbon intensity ethanol, biodiesel, and renewable natural gas at their Pixley facility.
Rondo Energy Funding
Rondo Energy has raised a total of $82M in funding over 2 rounds:
Their latest funding was raised on Aug 16, 2023 from a Series B round.
Rondo Energy Investors
Rondo Energy is funded by 10 investors:
- Energy Impact Partners
- Breakthrough Energy Ventures
- Microsoft Climate Innovation Fund
- Siam Cement Group
- Rio Tinto
- Sdcl Energy Efficiency Income Trust
- Aramco Ventures
- John Doerr
Rondo Energy Founder
Rondo Energy CEO
John O’Donnell is CEO.