Storing energy: from fuel stocks to batteries

By the end you will be able to

• Distinguish power capacity, energy capacity and duration, and explain why headline gigawatt figures can mislead
• Compare the energy held in oil, gas and coal stocks with the world's electricity storage
• Explain how a lithium-ion cell works and how LFP, NMC and sodium-ion chemistries differ
• Compare pumped hydro, compressed air, thermal storage and hydrogen on round-trip efficiency and duration
• Interpret battery pack prices and levelised cost of storage figures
• Assess the safety, wear and critical-mineral risks attached to storage

Contents

1. Part 1 Why storage matters and how much the world holds

   Energy is produced and used on different timetables. The sun shines at noon while Indian demand peaks after dark; gas is produced all year but burned hardest in winter. Storage bridges those gaps. Measured by energy held, fuel stocks still dwarf every other store, but electricity storage is growing faster than any other part of the system.

   • Storage bridges time, as transport bridges distance
   • Fuel stocks dwarf every electricity store
   • Pumped hydro's long lead meets the battery surge
2. Part 2 Batteries: chemistry, cost and wear

   Batteries are now the fastest-growing store of electricity and the heart of the electric car. They are efficient, modular and quick to build, but they hold little energy per kilogram compared with fuels, they wear out with use, and they depend on a handful of minerals. Understanding the main chemistries explains most of the trade-offs.

   • A lithium-ion cell moves ions between two electrodes
   • LFP, NMC and sodium-ion trade energy for cost and safety
   • Battery prices fell by about 90% in little more than a decade
   • Batteries wear out and, rarely, catch fire
3. Part 3 Mechanical, thermal and chemical stores

   Batteries are not the only way to keep energy for later, and for long durations they may not be the cheapest. Water raised uphill, air squeezed into caverns, heat held in water, rock or salt, and hydrogen made by electrolysis all store energy. Their round-trip efficiencies range from over 90% to under 40%, and their best uses differ accordingly.

   • Pumped hydro is the old workhorse
   • Compressed air, liquid air, gravity and flywheels
   • Heat is the cheapest energy to store
   • Hydrogen and other fuels for long gaps
4. Part 4 Choosing a store: efficiency, duration, cost and minerals

   No single storage technology does every job. The right choice depends on how long energy must be held, how often the store cycles, how much loss is acceptable and what materials it needs. Putting the options side by side shows why batteries dominate short durations, why long-duration storage is still unsettled, and why minerals have become a matter of state policy.

   • Duration and cycling decide the right store
   • Storage hype meets arithmetic
   • Critical minerals tie storage to a few countries
5. Wrap-up Key terms, try it in the table, final quiz, sources