GRID ELECTRICITY STORAGE: SIZE MATTERS

GRID ELECTRICITY

STORAGE: SIZE MATTERS

IT WOULD BE A LOT EASIER to expand our use of solar and wind

energy if we had better ways to store the large quantities of electricity

we’d need to cover gaps in the flow of that energy. 􀂚 Even in sunny

Los Angeles, a typical house roofed with enough photovoltaic panels

to meet its average needs would still face daily shortfalls of up to about 80 percent

of the demand in January and daily surpluses of up to 65 percent in May. You

can take such a house off the grid only by installing a voluminous and expensive

assembly of lithium-ion batteries. But even a small national grid—one handling

10 to 30 gigawatts—could rely entirely on intermittent sources only if it had

gigawatt-scale storage capable of working for many hours. 􀂚 Since 2007, more

than half of humanity has lived in urban areas, and by 2050 more than 6.3 billion

people will live in cities, accounting for two-thirds of the global population, with

a rising share in megacities of more than 10 million people. Most of those people

will live in high-rises, so there will be only a limited possibility of local generation,

but they’ll need an unceasing supply of electricity to power their homes,

services, industries, and transportation. 􀂚 Think about an Asian megacity hit by

a typhoon for a day or two. Even if long-distance lines could supply more than half

of the city’s temporarily lowered demand, it would still need many gigawatt-hours

from storage to tide it over until intermittent generation could be restored (or use

fossil fuel backup—the very thing we’re trying to get away from). Li-ion batteries,

today’s storage workhorses in both stationary and mobile applications, are quite

inadequate to meet those needs. The largest announced storage system, comprising

more than 18,000 Li-ion batteries, is being built in Long Beach for Southern

California Edison by AES Corp. When it’s completed, in 2021, it will be capable of running

at 100 megawatts for 4 hours. But

that energy total of 400 megawatt-hours

is still two orders of magnitude lower

than what a large Asian city would need

if deprived of its intermittent supply. For

example, just 2 GW for two days comes

to 96 gigawatt-hours.

We have to scale up storage, but how?

Sodium-sulfur batteries have higher

energy density than Li-ion ones, but

hot liquid metal is a most inconvenient

electrolyte. Flow batteries, which store

energy directly in the electrolyte, are

still in an early stage of deployment.

Supercapacitors can’t provide electricity

over a long enough time. And compressed

air and flywheels, the perennial

favorites of popular journalism, have

made it into only a dozen or so small and

midsize installations. We could use solar

electricity to electrolyze water and store

the hydrogen, but still, a hydrogen-based

economy is not imminent.

And so when going big we must still

rely on a technology introduced in the

1890s: pumped storage. You build one

reservoir high up, link it with pipes

to another one lower down and use

cheaper, nighttime electricity to pump

water uphill so that it can turn turbines

during times of peak demand.

Pumped storage accounts for more than

99 percent of the world’s storage capacity,

but inevitably, it entails energy loss

on the order of 25 percent. Many installations

have short-term capacities in excess

of 1 GW—the largest one is about 3 GW—

and more than one would be needed

for a megacity completely dependent

on solar and wind generation.

But most megacities are nowhere

near the steep escarpments or deepcut

mountain valleys you’d need for

pumped storage. Many, including

Shanghai, Kolkata, and Karachi, are

on coastal plains. They could rely on

pumped storage only if it were provided

through long-distance transmission. The

need for more compact, more flexible,

larger-scale, less costly electricity storage

is self-evident. But the miracle has

been slow in coming.