Carnot b attery ( m olten salt cycle)
Power plant
Molten salt cycle
Water/steam cycle
Hot storage tank
Turbine + generator
G
Boiler
Electric heater
Steam generator
Cold storage tank
Condenser
Figure 2 Carnot thermal storage (heat batteries) – molten salt
● Best use: Medium-duration storage (roughly eight hours to a few days). ● Pros: Very low $/kWh storage cost (using cheap thermal media); can build large capacity for multi-day storage economically; long asset life (20+ years); minimal energy loss over time when storing heat. ● Cons: Low conversion efficiency; requires bulky, high-temperature equipment and significant space (not as modular as batteries); power conversion (heat engines/turbines) adds complexity and cost; slower to respond than batteries (ramping a thermal system takes time). Hydrogen ‘power-to-gas’ storage Hydrogen offers a way to store electricity in the form of a fuel. In a power-to-gas-to-power system, surplus electrical energy is used to run electrolysers, which split water into hydrogen gas (H₂) and oxygen. The hydrogen can then be stored in large quantities, and later converted back to electricity by feeding it into a fuel cell or burning it in a generator or turbine. Essentially, electricity is transformed into hydrogen (an energy-dense gas) for storage, and then reversed back into electricity when needed. The appeal of hydrogen storage lies in its massive potential scale and duration. Hydrogen gas has an extremely high energy content by weight, and if stored in the right conditions, it can preserve energy indefinitely without self-discharge. The most economical
way to store huge amounts of hydrogen is in underground geological formations like salt caverns or depleted gas fields. In regions with suitable salt geology, a single cavern could hold terawatt-hours of energy – enough to supply whole countries for days or weeks. This makes hydrogen uniquely suited for seasonal storage, such as saving excess renewable energy from a windy spring and using it during a calm winter period. Unlike batteries or thermal stores, hydrogen in a cavern does not ‘leak’ energy over time; it can sit for months and still be there when needed. However, hydrogen storage is quite inefficient and requires extensive infrastructure. Round-trip efficiency is only around 35-40%, meaning roughly 2.5 units of renewable electricity are needed to get one unit back via hydrogen. This is much less efficient than using a battery for storage. Above-ground hydrogen storage (in tanks or as liquid) is extremely expensive per kWh and can lose energy over time. By contrast, if salt caverns are available, hydrogen can be stored in bulk very cheaply. This means hydrogen storage is far more viable in regions with the right geology than in those without. Implementing hydrogen storage at scale also requires significant infrastructure investment. Large electrolyser facilities must be built to generate hydrogen, and new or adapted turbines or fuel cells are needed to convert it back to power. Safety and materials
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