A Carnot battery helps store energy in the form of heat. Named after French physicist Nicolas Léonard Sadi Carnot — considered the ‘father of thermodynamics’ — the battery converts electrical energy into heat, stores the heat and, when needed, converts it back to electrical energy.

An overview of Carnot batteries cited the use of carbon dioxide as the optimum choice of working fluid. Published in the Renewable and Sustainable Energy Reviews journal, the paper states that the battery can store large amounts of renewable energy.

The problem with renewable energy is its intermittency — solar panels and wind turbines yield energy only when the sun shines or the wind blows, respectively. They idle at other times, forcing users to draw coal-based power.

Storing such energy would require very large batteries.

A Carnot battery works like a thermal storehouse. On a sunny afternoon, when a solar unit generates more power than what is needed, the battery system conducts a thermal cycle by using the extra electricity to work a heat pump to compress carbon dioxide and store it under high pressure. During discharge, the stored carbon dioxide is expanded through a turbine to generate electricity (the storage medium could be some salts or rocks), according to an earlier paper in the Energies journal.

Typically a Carnot battery’s efficiency is 30–70 per cent. If you store 100 units of solar power in a Carnot battery, it will give back 30–70 units. The lower end is below that of lithium ion cells, but a Carnot battery is far less expensive, besides obviating the need for critical minerals such as lithium.

Carbon dioxide is a good choice as ‘working fluid’ as it is non-toxic and non-inflammable. Compared with air or hydrogen, it enables higher efficiency in a Carnot battery since it is better at heat transfer and can store more heat since it has a higher energy density. The components used are also smaller in the case of carbon dioxide.

A review published in the Journal of Energy Storage in 2022 indicates that the levelised cost of storage for a form of pumped thermal energy storage system (such as Carnot) is €70–110 per MWh, with an eye on achieving 72 per cent efficiency.

The cost for a pumped hydro project would be about €110 per MWh and for lithium-ion batteries about €300 per MWh. The levelised cost of storage refers to the total cost of the storage system per unit of electricity discharged, including initial capital outlay and charging costs.

‘Long storage’

The scale of the models studied for carbon dioxide Carnot batteries went up to 100 MW. The authors say this “shows great promise... for large-scale, long-term energy storage application”, with real efficiency of 40–80 per cent, depending on operating conditions.

Dr Satya Seshadri, Associate Professor, IIT-Madras, says the technology is being piloted across the world, including in India. NTPC had, in January 2025, a 160-MW carbon dioxide-based Carnot system installed by the company Energy Dome.

He points out that carbon dioxide-based Carnot batteries are good for long-duration energy storage — anywhere from 24 to 72 hours. “Lithium-ion batteries are great for short periods, that is 4-6 hours, but become expensive beyond that.”

On the other hand, carbon dioxide-based Carnot batteries require heavy capital expenditure and hence work best for large-scale industrial or grid use.

Seshadri also refers to the lifetime of the technology. “Many turbines and power plants run with those systems for 20-30 years.” Over time, he says, ‘round-trip efficiency does not matter as much as the lifetime reliability of the technology’.

Also, while India has greatly stepped up its renewable energy capacity, it still constitutes only 15 per cent of all energy capacity. “When we get to 50 per cent, then there will be need for storage of all durations,” he says.

Asked if such technology also has the potential to cut carbon dioxide levels and aid in achieving net-zero emission goals, he explains that these batteries are “closed-loop” — namely they circulate the same carbon dioxide within the system.

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Published on March 9, 2026