Gravity Storage of Electricity

Gravity Storage of Electricity

The granddaddy of gravity storage is Pumped Storage, where water is pumped to a higher elevation, using electricity that’s not needed to meet demand, where the water is stored and then released to flow downhill, through generators to generate electricity when electricity is needed to meet a peak in demand.

From this basic tenet has arisen a plethora of storage methods based on gravity.

Storage is essential if renewables are ever to have any possibility of replacing fossil fuels for generating electricity. They are also essential for making renewables more efficient by storing electricity when electricity is produced that can’t be used. Solar, for example, generates excess electricity when demand is lower in the afternoon while the solar output peaks.

Here are a few such gravity based storage proposals.

Energy Vault

This 400-foot-high six-headed crane contraption, featured in the Wall Street Journal’s energy issue, lifts 35-ton bricks using excess electricity, stacking them for storage, and then releasing the 35-ton bricks and allowing them to fall, capturing the kinetic energy to generate electricity when it’s needed to meet demand.

According to the WSJ article, 20 towers would supply power for 40,000 homes for 24 hours.

Diagram from Wall Street Journal

Inclined Rail Car Storage

Using multiple rail cars loaded with stones or concrete, mounted on an inclined track, where the motorized rail cars are moved up the inclined track when there is excess electricity to power the cars, and then allowing the loaded cars to fall freely down the incline to generate electricity.

Underground Hydro Storage

One version of underground water storage is to create two circular pits, a few hundred feet deep, but at different levels, where the water is moved from one level to the other to generate electricity, essentially replicating traditional pumped storage.

Mine Shaft Storage

There has been a proposal to use mine shafts for dropping weights, analogous to the Energy Vault proposal.

Another mine shaft proposal is to use two horizontal mine shafts at different levels to store and move water, analogous to traditional pumped storage.

 

No doubt there will be other gravity storage proposals, but none make any economic or practical sense.

For example, my community is 15 miles long and 4 miles wide, which is roughly the same size as Manhattan Island in New York, and has around 60,000 single family homes. 

It would require 30 Energy Vault towers, each 400 feet tall, roughly the height of a 40 story building, with stacks of 35 ton concrete blocks surrounding each tower, with the towers dispersed throughout my community to provide storage for 24 hours. This would result in two parallel rows of 15 towers stretching the length of the community, with each row about two miles from each other. Towers would be visible from virtually every home in the community.

In addition, assuming the cost estimate from the WSJ is correct, these 30 towers would cost around $270 million.

One has to wonder whether these proposals for storage are merely intended to obtain grant money, which mostly comes from taxpayers.

And, what was the Wall Street Journal’s motivation to give legitimacy to the Energy Vault proposal in its Energy supplement?

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