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By Mark Jones

Are smart rocks on the horizon?

“Dumber than a box of rocks” is a colorful idiom. Suppose it is wrong. Suppose a box of rocks is actually smart.

I will admit I checked the cover of Power magazine when I first read about thermal energy storage using a box of rocks. I checked to make sure it wasn’t the April Fool’s Day edition. The article indicated a box of rocks was an intelligent solution to the vexing problem of storage of renewable energy.

Renewable energy immediately conjures images of windmills and photovoltaic panels and disproportionately means renewable electricity. Windmills and photovoltaics share one shortcoming, intermittency. Power grid operators love dispatchable power sources, plants they can summon when needed. Grid operators can’t summon solar energy at night or wind energy when the wind isn’t blowing — so storage is the obvious answer.

Electricity storage conjures images of batteries, large banks of batteries collecting electrical energy and making it dispatchable. That image is o target. Electricity is being stored today, but not very much by batteries. Pump hydro provides most of the electricity storage, by some accounts, 96% of the 4.67 TWh of total storage capacity. The world currently uses around 28 PWh of electricity. We don’t currently store much electricity, relying on dispatchable fossil generation to keep the lights on.

Battery storage capacity is estimated to be ~56 GWh, needing to grow by 15-17x to yield stable electricity supply by 2030 due to decarbonization. Most battery storages use lithium-ion batteries and, while other chemistries are being deployed, most estimates are for lithium-ion to be the dominant battery chemistry for stationary, transportation, and portable applications. Costs have dropped significantly, but lithium-ion batteries are still the most expensive commonly used batteries. Lithiumion batteries seem to be the smart solution for energy storage, but there are drawbacks, their performance degrades both with number of cycles and calendar age. Plus, they don’t last forever, and overheating and fires can occur. A plain old box of rocks, dumb as it may sound, may prove to be a smarter solution. Siemens Gamesa Renewable Energy is one of the companies developing electric thermal energy storage (ETES), currently in operation in a large pilot. 1,000 tonnes of volcanic rock are the storage medium. Resistance heaters make hot air, heating the rock to 650° C when electricity is in surplus. When electricity is needed, steam is made to power a turbine. This is a new spin on simply storing and using the heat. It allows the use of existing power generation capacity as fossil plants are decommissioned. The technology has been under development since 2014 and piloted starting in 2019.

There are disadvantages, for sure. Battery systems are more e cient, with roundtrip e ciency around 80%. It is only about 50% for ETES, over 95% to heat. The advantages are cost and longevity: rocks are cheap and don’t age. The estimates are that the initial capital will be well less than half of a battery system. Rocks also don’t catch fire. There is no concern about there being su cient rocks, while there are concerns about supplies of battery materials. Recycling, a concern in batteries, is not with rock. It is quite possible a box of rocks is an intelligent solution. DW

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