Chapter 4: How Do We Store Energy at Scale?

The Big Idea

There is no single best storage technology — only the right one for a given duration. Batteries rule from milliseconds to a few hours. Pumped hydro rules from hours to a day. For days or seasons, chemical fuels such as hydrogen become the realistic option, even though far more energy is lost along the way.

Did You Know?

Most grid batteries built in India today use LFP (lithium iron phosphate) cells. They store a little less energy per kilogram than NMC cells, but they are safer at high temperatures, last for more cycles, and contain no cobalt — all of which matter more for a stationary battery than weight does.

Story Time — Sravani and the Silent Power Station

Sravani, age 15 · Vijayawada, Andhra Pradesh

When Sravani heard that a “power station” had been built beside the substation near Vijayawada, she expected chimneys. Instead she found rows of white containers, humming quietly behind a fence.

“It generates nothing,” the engineer told her class. “It only moves energy in time. We charge in the afternoon, when solar is cheap and plentiful, and discharge in the evening peak.”

He showed them the two numbers that define it: power in megawatts, and energy in megawatt-hours. “Ours is a two-hour system. Full power for two hours — then it is empty until we charge it again.”

Sravani asked what is lost. “About 10 to 15% in a round trip,” he said. “We put in more than we take out. The trick is that the energy we put in was cheap and would otherwise have been curtailed.”

Inside the control room a screen showed cell temperatures. “Heat is the enemy,” he added. “Cooling costs energy, but it buys years of life.” Sravani wrote her essay that night: “A battery does not make energy. It buys time — and time, on a grid, is worth a great deal.”

Story Time — Roshan and the Second Life

Roshan, age 16 · Ranchi, Jharkhand

Roshan’s uncle runs a small workshop in Ranchi repairing e-rickshaws. Piles of tired batteries used to gather dust behind it — until a recycling company began collecting them.

Roshan learned that a battery is considered “finished” for a vehicle when it holds about 80% of its original capacity, because the range gets too short. “But 80% is still a lot,” he realised. Some packs were tested and given a second life powering shop lights and fans, where weight and range do not matter.

The rest were sent for recycling. “Lithium, cobalt, nickel and copper can be recovered,” the collector explained. “India imports most of these metals, so recycling is not only greener — it is a supply strategy.”

Roshan also saw the dangers. A cracked pack had been left in the sun; the collector handled it with great care. “Damaged lithium cells can catch fire. Never open a pack, never charge with a cheap unbranded charger.”

Roshan started a notice board at the workshop: “Old battery? Don’t dump it. Test it, reuse it, then recycle it.” Within a month, three other workshops asked to join the collection round.

🔢 Numbers Corner

Round-trip efficiency = energy out ÷ energy in × 100. Putting in 100 kWh and getting 88 kWh back is 88%. Over a year, those losses are the “rent” you pay for moving energy through time.

Myth or Fact?

Myth: Electric vehicle batteries become useless waste after a few years.

Fact: They are retired from vehicles at around 80% capacity, which is still useful for stationary storage. After that, lithium, nickel, cobalt and copper can be recovered and reused.

🔬 Try it in the Green Energy Lab

Experiment with these ideas in Lab 2: Battery Chooser.

Brain Buster

Check the battery capacity (in mAh or Wh) of your phone. At 5 W of use, roughly how many hours should it last? Compare with reality.