Energy Storage Efficiency

There are those who constantly dismiss wind and especially solar as impractical energy sources because it is not available all the time, hence standby power must be available – fair enough. Right now Unit One of Arkansas Nuclear One is offline for refueling.

So is anybody sitting in the dark for a month or two? Of course not. Must there be another nuclear reactor standing idle waiting for Unit One to go down for refueling? Again the answer is no. There is sufficient excess capacity in our electrical grid to make up the difference in electrical energy needs during the outage. The same is true today for our intermittent energy sources, wind and solar. It is estimated that no additional backup will be needed until we reach about thirty percent contribution of these renewable sources to our total electric production.

Right now wind turbines to our west and the solar panels at my home use the grid as storage, just like Unit One. When the wind isn’t blowing, the sun isn’t shining, or Unit one is offline to refuel, the other power sources in the grid make up the shortfall. Electrical energy transmission and storage are hot research topics in science and technology. Numerous ways exist already for grid-scale energy storage, the ongoing research is to find ways to store energy efficiently and affordably.

One option being explored by as Swiss company is a cuckoo clock writ large. Excess energy is used to raise concrete blocks up, just like the weights on a clock. To recover the stored energy simply allow the weight to go down, turning a generator as they come down. This type of energy storage is not very efficient, maybe sixty to seventy percent, but is inexpensive.

At the other end of the spectrum is a high-cost, high-efficiency strategy such as a rechargeable Lithium-ion battery. The efficiency here is about ninety percent. Although expensive, Lithium-ion batteries are quite lightweight so they have value to power portable devices, from hearing aids to electric cars.

One energy storage medium that most don’t recognize as such is Hydrogen. Hydrogen gas is very energy-dense, meaning a small amount of it will store a lot of energy. Hydrogen can be generated by electrolysis of water. The hydrogen can be stored until energy is needed, then the hydrogen can be burned to make steam to turn a turbine to generate electricity. But, burning anything to produce heat to turn a turbine is a quite inefficient process. Because of the laws of thermodynamics, you can only get back about a third of the energy put in.

Better than burning to make heat is another technology known as a fuel cell. Fuel cells convert Hydrogen back to water without as much waste heat. Hydrogen generation via electrolysis, then conversion back to electricity has a “round-trip” efficiency of about fifty percent.

The science and technology of grid-scale electrical energy storage will mature as we expand our reliance on wind and solar, ultimately eliminating the use of fossil fuels.

Dr. Bob Allen is Emeritus Professor of Chemistry, Arkansas Tech University.

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