Gravity To the Rescue

By
Branko Terzic

The simple use of gravity may be one answer to the problem of large scale storage of energy from electricity.  Pulling a weight from a lower height to a higher elevation results in energy stored as potential energy. Energy Vault Holdings Inc (NYSE: NRGV) is one new company with a long duration energy storage option based on lifting weights of concrete by crane in a special built structure.

How does it work?

Energy is stored when a weight is lifted by an electric motor. As the weight is released it pulls a cord around an electric generator producing electricity. The round-trip efficiency can be 80%.

The change of energy from lifting a weight directly correlates to the vertical rise (the higher a mass is lifted, the more gravitational potential energy) and to the mass of an object (the heavier the mass, the bigger the change in energy.) So, we want heavier weights moved over greater vertical distances.

My understanding is that Energy Vault uses local composite materials for weights, existing lifting technology and standard building construction. They estimate a 35 life for the system which exceeds the life of the wind and solar energy resources supplying the power.

That seems like a simple elegant solution to the energy (from electricity) problem.

But why is it necessary to build a structure to provide the height. The vertical distances can be manmade or natural:

A. Manmade height differences

1. Existing construction

• • Buildings
  • Mining shafts
  • Wells
  • Railroad inclines
  • Bridges
  • Elevator shafts
  • Excavated open pit mine

2. Special purpose structures

B. Natural height differences

• Deep water
• Cliffs
• Hills
• Elevated lakes

The sites can use natural height differences saving on construction costs and the weights can come from any dense and cheap material such as concrete, water, scrap metal, rocks etc.

Ideally the Gravity batteries to be economic should not require new transmission lines either to receive energy for storage or to deliver energy to the grid. This means that locations with natural height differences will be usable only if the location is suitable with respect to additional costs of transmission construction.

Special built structures such as those used by Energy Vault can be located at sites with the most economic characteristics including use of existing transmission assets.

As gravity battery technologies attract more investors and inventors the new companies will most likely start looking at locations with unused natural height differentials suitable for development.

Stay tuned.