Decarbonisation Technology August 2025 Issue

Carnot b attery ( f ixed bed)

Power plant

Charge

Discharge

Fixed bed

Water/steam cycle

Turbine + generator

Air

Boiler

Electric heater

HRSG

Storage module

Condenser

Figure 1 Carnot thermal storage (heat batteries) – fixed bed

● Pros: Highly efficient; fast to respond; modular and easily scaled (from kW-scale up to GW- scale systems); well-commercialised. ● Cons: Expensive for large energy capacities (high $/kWh for long durations); practical limit of a few hours’ storage per installation; limited lifespan (~10-15 years) before replacement. Carnot thermal storage (heat batteries) ‘Carnot’ thermal storage – often dubbed a thermal battery – stores electricity in the form of heat and later converts it back to electricity. In a typical Carnot storage system, surplus power from the grid (for example, excess solar at noon) drives a heat pump or resistance heater, which raises the temperature of a thermal storage medium. Common media include molten salts, sands, or other solids that can hold heat. The stored thermal energy is kept in insulated tanks or chambers. When the grid needs power, the stored heat is used to run a heat engine or turbine (such as a steam turbine or a high- temperature gas turbine) to generate electricity. This concept is similar to how concentrated solar power (CSP) plants store heat from the sun in molten salt and later use it to produce power. The big advantage of thermal storage is its low cost per unit of energy stored. Heat can be stored in inexpensive materials (like nitrate salts, concrete, or rocks) inside large tanks, which is far cheaper per kWh than manufacturing battery cells. This makes it practical to build

storage for many hours or even several days of energy. Furthermore, the hardware can last for decades, and a well-insulated thermal store loses only a few per cent of heat per day, so energy can be held for days with minimal loss. The trade-off comes in efficiency. Converting electricity to heat and back involves significant losses. A Carnot battery might only return around 30-50% of the electrical energy that was used to charge it. In practice, about 40% efficiency is common in pilot projects. Advanced designs could improve this somewhat, but thermal storage will never be as efficient as electrochemical batteries. This means it is mainly useful when surplus renewable power would otherwise be wasted; in those cases, a 40% recovery is better than nothing. Carnot thermal systems also require significant infrastructure – large hot and cold tanks, as well as heating and power-generation equipment – so they benefit from economies of scale. They would be large, stationary installations. They are well suited to ‘middle’ durations: bridging one long night or a couple of cloudy days in a row. For example, a thermal storage plant might charge up during a sunny day and discharge over the night and into the next morning. It could also store a few days’ surplus and release it during a short spell of bad weather. Key points – Carnot thermal: ● Efficiency: ~40% round-trip (much lower than batteries due to heat-to-power losses).