There has been quite a bit of buzz about gravitational energy storage recently. That is, storing energy by lifting weights. This energy can be released by lowering the weights and using the kinetic energy to run a generation. That is, pump hydro but with concrete blocks instead of water.

A key attraction of these concepts – compared to pump hydro – which has been in operation for well over a hundred years, is the flexibility of a modular structure that isn’t dependent on terrain/geography. That is, a storage system, that can be put where it is needed.

The challenge with gravity energy storage is that gravity isn’t very strong. It takes an enormous amount of weight raised to a large height to store large amounts of energy.

Let’s put some numbers on it. The weights in a grandfather clock – which is a long-standing form of gravity energy storage – might store around 100 Joules of energy (say 10kg x 1 metre).

To compare that with a battery. A CR2032 coin cell battery (commonly used in a watch or heart rate monitor strap) – can provide 150 milliamp hours at 3 volts – which is 0.45 Wh or 1620 joules of energy. That is, it would take 16 grandfather clock weight systems to replace one tiny coin cell battery.

To put this in utility scale terms, a Tesla Megapack 2 can store 3 MWh or 10,800 MJ. To store the equivalent in a gravitational storage system with a head of 50 meters (ie something that is the height of a 20 storey building), would require a mass of 22,000 tonnes!

What that represents is hard to comprehend. I like to think of large weights in terms of blue whales (140 tonnes according to Wikipedia) and so the weight would need to be equivalent to 150 blue whales! Alternatively, this is around 9,200 cubic metres of concrete or a solid block 20 metres by 20 metres by 20 metres.

For me the sheer material cost of the weights and the system to hold them up represent a huge challenge for land based gravitational storage. That is, large head provided by terrain of traditional pump energy, is a key advantage, despite the site selection limitations it imposes, that is hard to overcome.

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