Solving the major problem of renewable energy: intermittency

from the concentrators could be stored either chemically or thermally. Chemical systems might be based on the reversible dissociation of ammonia or on dissociated metal hydrides.

Thermal ones might store the heat directly in concrete or in molten salt.

Compressed air and pumped hydro
Pickard says, however, that if you look at the website of the Electricity Storage Association, only two energy storage technologies stand out as truly massive. They are compressed air energy storage and pumped hydro.

Compressed air energy storage is really quite simple, he says. “When you have energy you don’t know what to do with, you simply compress air into a cavity under the Earth and when you need the electricity, you blow this air through a high-speed turbine, spinning a generator, and you’ve got your energy again.”

There is a catch, though. As anyone who has pumped up a bike tire knows, when you compress air, it heats up and when you allow it to expand, it cools down. To avoid thermally cycling your storage chamber, he says, you would need to compress the air in stages, with counterflow heat exchangers between the stages.

“If you believe the design figures, you can get 60 or 75 percent turnaround efficiency, which isn’t bad. The only problem is that nobody has ever built a functioning adiabatic compressed-air energy-storage system,” Pickard says.

Pickard prefers pumped hydro, but with a twist. To achieve the goal of 2 gigawattdays of stored power, you’d need a reservoir that would have roughly the volume of 10 Great Pyramids, and to minimize losses and maximize power, this reservoir would have to be several hundred meters above a lower reservoir and yet close to it horizontally.

“The solution is to excavate an underground reservoir many hundreds of meters below surface level and to exchange water between it and a surface reservoir created immediately above it and diked using spoil from the excavation. This variation of hydro storage is called underground pumped hydro,” Pickard says.

Such a facility could be put almost anywhere that there was low-quality land underlain with competent rock — in industrial brownfields, for example.

Pickard questions, however: “if underground pumped hydro is so great, how come it does not yet exist?”

Perhaps because to displace an entrenched technology, the new technology must be clearly superior under present conditions, he says. But the superiority of pumped hydro may become “starkly manifest” only in the future.

Pickard says it is important to remember that there are moral as well as economic and technical dimensions to the intermittency challenge. If our generation lets the matter slide, “our descendants will be saddled with the detritus of a wastrel lifestyle.”