Energy from gravity

By this I don't mean pumped hydro storage, where water is pumped into a raised reservoir, ready to be released through turbines, back into the lower reservoir.

I mean energy stored in solid mass, which is raised when energy is cheap - much like the water in pumped hydro - and dropped to spin a generator during events of peak demand or network stress.

Let's look at two recent grid-scale examples.

First up is Energy Vault, based in Lugano, Switzerland. This project is entirely above ground, and uses a six-armed crane to lift and descend 30 ton weights. Here's their wonderful short video, which fans of Minecraft will especially enjoy:

The funding for Energy Vault is certainly impressive: in summer 2019 they obtained $110m of investment from the mighty Softbank, their first energy storage investment. According to the FT (paywall), Softbank themselves will utilise one deployment, and Tata Power have also ordered another.

The video shows the crane supporting a remote onshore windfarm, which seems the most feasible deployment. Aside from the planning issues that would mar more urban (especially on crowded islands like UK and Japan), and high winds surrounding offshore windfarms, it could solve the economic barrier of materials transportation. Whilst some of their videos suggest concrete blocks being used, that would seem an expensive option. I wonder whether desert sand or rural soil could be mixed with the special binding agent that investor Cemex has developed. Other videos show nearby PV solar panels, though this could present the issue that the crane would cast shadows. This could be less of an issue with concentrated solar power farms, and those near the equator.

You can read more on Energy Vault's website. They appear to be building upon an earlier pioneer, Energy Cache, that Bill Gates confirmed that he invested in. That used a modified ski lift to haul buckets of gravel up and down a mountain, seemingly a solid mass equivalent to pumped hydro. Though Energy Cache now looks to be defunct (according to LinkedIn), it has personnel and at least one investor (Idealab) in common with Energy Vault.

Second, we have a British company, Gravitricity. Quite hard to type, but easy to grasp. Almost the opposite to Energy Vault, this design is almost entirely below ground. It's uses giant mass suspended in a sunken shaft, which could be a disused mineshaft or one built specially. Here's a short video on it:



A purpose built shaft would seem like an expensive way to achieve this, especially given the number of abandoned mines in countries like the UK. I suppose it depends on the state of an existing shaft, which would presumably affect the insurability and thus financing costs. Still, it's a great to see a British firm pioneering this idea, and that in Feb 2018 InnovateUK awarded them £650k to progress the idea. You can read more on their website.

All of these projects are novel, massively scaled, implementations of an ancient technology, the clock pendulum. Or a more contemporary invention, the regenerative braking lift, such as the 68 Otis Regen models retrofitted to the Empire State Building in 2017. These which use up to 30% less electricity than standard models, a bit like a vertical Prius hybrid.

Given the limited geographic suitability of pumped hydro, and the drawbacks of grid-scale batteries, it's great that such a historically proven technology is being given a fresh look. Solid mass-based energy storage has the advantage of potentially being able to utilise existing infrastructure (mine shafts) and materials (sand, soil), and the capacity doesn't degrade over time. The more energy storage technologies that we can pursue, the better.