Cache Energy, an American energy storage startup founded in 2022, develops a low-cost heat battery for renewable energy storage. The heat battery converts renewable electricity to heat, stores heat, and releases heat or electricity as needed. This is achieved through the reversible chemical reactions of Ca(OH)₂ dehydration and CaO hydration. Both thermal storage materials are cheap, earth-abundant, non-toxic, non-flammable, and non-explosive. Cache Energy’s heat battery is capable of providing cost-effective, long-duration renewable energy storage.
Challenges: renewable energy storage at a low cost
By 2022, the world had about 1,185 GW of installed solar capacity and about 906 GW of installed wind capacity. Solar power has grown at a 24% annual rate over the last decade. The US solar industry installed 6.1 GW of capacity in the first quarter of 2023, 47% more than in the first quarter of 2022. On the other hand, the average power of new wind turbines installed in the US in 2022 was 3.2 MW, 7% higher than in 2021.
As more and more solar cells and wind turbines are installed, electricity storage systems with long durations would significantly increase the efficiency and reliability of these intermittent renewable energy sources. Although current technologies such as lithium-ion batteries are suitable for a number of applications on the grid, they are not suitable for longer-duration energy storage.
Heat battery
Heat batteries are a promising technology for renewable energy storage due to their efficiency, environmental friendliness, and cost-effectiveness. They are particularly useful for storing energy from intermittent renewable sources like wind and solar, and can play a significant role in the transition towards a more sustainable and renewable energy system. The heat battery technology converts renewable electricity into heat, stores heat, and releases heat or electricity when the sun is not shining or wind is weak.
The heat battery based on reversible thermochemical reactions of slaked lime (Ca(OH)₂) dehydration and quicklime (CaO) hydration has attracted much attention.
During heat storage, Ca(OH)₂ absorbs heat and decomposes to solid CaO and steam, storing the heat as chemical energy. Under atmospheric pressure, the Ca(OH)₂ heat storage temperature ranges from 400 to 600 ºC.
In the exothermic process, CaO reacts with steam to form Ca(OH)₂, releasing stored chemical energy as heat. The released heat temperature ranges from room temperature to 500 ºC. These temperatures allow for the integration of this heat battery with the steam Rankine cycle to generate electricity.
The energy process of this heat battery is depicted by the diagram below.
Ca(OH)₂/CaO thermal storage materials are cheap, earth-abundant, non-toxic, non-flammable, and non-explosive, however, their low thermal conductivity (~ 0.1 W/mK) results in a high heat transfer resistance. This causes large temperature gradients within the material bed. Consequently, some parts of the bed materials can reach much higher temperatures than favored for thermodynamics. This exposure to higher temperatures over repeated cycling causes sintering, which reduces thermal storage material’s reactivity and the heat battery’s energy capacity.
Cache Energy Technology
Cache Energy develops a Ca(OH)₂/CaO heat battery based on a well-designed honeycomb fixed bed reactor. Its fixed bed reactor is able to maintain temperature uniformity within the Ca(OH)₂/CaO bed, provide stable power output over a sustained period, and alleviate sintering problems. Since the cost of the fixed bed reactor accounts for about 70% of the total reactor cost, Cache Energy uses cost-effective aluminum or steel material for the reactor to reduce the cost of the heat battery, realizing a cost-effective renewable energy storage solution.
Cache Energy heat battery
The primary components of the Ca(OH)₂/CaO heat battery are heat storage materials and a reactor. As depicted in the diagram below, Cache Energy uses a honeycomb structure filled with CaO powders as the reactor bed.
The use of honeycomb structure reactor has many advantages:
- the lowest structural material density
- a high compression strength
- easy to fabricate and readily available commercially
- made of cost-effective aluminum or steel
- a high thermal conductivity (238W/mK for aluminum and 38 W/mK for steel)
The cost of the heat battery is significantly reduced by the use of aluminum or steel for the reactor. The honeycomb arrangement is repeated to make a modular reactor, whose energy and power capacities are scalable depending on the application.
How Cache Energy heat battery works
Ca(OH)₂ dehydration and CaO hydration can be represented by the chemical reaction:
Ca(OH)₂(s) + ∆Hr ⇄ CaO(s) + H₂O(g) ΔHr = 104.4 kJ mol⁻¹
The equilibrium temperature at 1 atm steam pressure is 505 ºC. Above this temperature, Ca(OH)₂ dehydration is thermodynamically favored, whereas below this temperature, CaO hydration is favored.
- Charging
During Ca(OH)₂ dehydration (charging), the renewable electricity is converted to heat and the Ca(OH)₂ bed is heated. Ca(OH)₂ dehydration begins when the bed temperature exceeds 505 ºC and produces CaO and steam. The renewable electricity is converted to chemical and thermal energy.
- Discharging
Below 505 ºC, CaO hydration is thermodynamically favored. During CaO hydration (discharging), steam flows through the bed, initiating hydration and releasing heat. The heat is transferred to the fluid at the bed’s outer walls. The battery’s hot fluid output can be used for industrial heating or to generate electricity.
CaO hydrates rapidly between 120 and 450 ºC. Beyond 450 ºC, the time for complete hydration increases significantly as the temperature approaches the equilibrium temperature of 505 ºC. Maximum hydration is always found close to the wall of the honeycomb cells because of the better heat transfer and, consequently, the lower temperature.
The heat released during this initial hydration raises the temperature at the core of the honeycomb cells to near or even beyond equilibrium temperature (505 ºC), especially in large cells, due to the increased heat transfer resistance between the core and the outer walls. This high temperature slows further CaO hydration at the core of cells. After a long period of heat diffusion out of the cells, the temperature at the core of cells falls below the equilibrium temperature and the hydration reaction restarts. The duration of power discharged largely depends on the size of the honeycomb cells.
Cache Energy Heat Battery Applications
Long-duration electricity storage
Cache Energy’s heat battery provides a safe, efficient, and scalable solution for long-duration renewable energy storage, offering a promising pathway towards a cleaner and more sustainable energy future. Cache Energy’s heat battery is cost-effective compared to lithium-ion battery and flow battery technologies.
Supplying process heat/steam to Industries
Cache Energy’s heat battery can convert renewable electricity to heat, store heat, and release heat and steam for various industrial applications.
Commercial and residential heating
Cache Energy’s heat battery can convert renewable electricity to heat, store heat, and release heat for commercial and residential heating.
Cache Energy Products
Cache Energy is developing heat battery systems.
Cache Energy Funding
Cache Energy has raised a total of $4.5M in funding over a Seed round on Aug 1, 2023.
Cache Energy Investors
Cache Energy is funded by 4 investors:
Unshackled Ventures and Cantos are the most recent investors.
Cache Energy Founder
Arpit Dwivedi is Founder.
Cache Energy CEO
Arpit Dwivedi is CEO.
