Gravitricity goes underground for energy storage

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Using gravity and solid weights to store energy makes perfect sense, but only if you do it underground, says Gravitricity Commercial Director Robin Lane.

The idea of using gravity to store energy is not new.

Clockmakers have relied on it for centuries and in many countries pumped storage hydro has been a feature of mature electricity grids for decades.

In the UK, for example, we have four pumped storage schemes totalling 2.8 GW, and whilst it is ideal for large-scale storage, the very specific geographies (not to mention huge capital cost) required means such schemes will always be relatively rare.

At Gravitricity we believe that a world of distributed energy generation will require distributed energy storage, so we have been working on taking the intuitive simplicity of gravity-based energy storage and adapting it to develop a system which can be located anywhere – alongside renewable generation, at the transmission or distribution level, in off grid locations or within urban centres.

So how is this best achieved?

The energy a gravity-based storage system can store and discharge is a function of mass, gravity (which is constant) and the distance of the drop: this formula,  Energy = mass x gravity  x height, or E = mgh, will be familiar to physics and engineering students everywhere.  

This simple science informs the two guiding principles which underpin the design of the Gravitricity system:

  • The first is that you need heavy weights in order to deliver interesting amounts of energy. Really heavy weights. Weights of a few tonnes, or even a few tens of tonnes, are not going to generate the power requirements which will be needed by tomorrow’s energy grids. In order to achieve this we need to be raising and lowering weights in the hundreds of tonnes each.
  • The second is that you need to be raising and lowering those weights through a significant distance. Heights of tens of metres are again, not going to cut it. This insight is fundamental, because it means man-made structures will simply never be tall enough (or frankly strong enough) to be viable.

To put this in context, you have to drop 500 tonnes around 800 metres to generate 1MWh.

These two principles have led us inescapably in one direction: underground. By deploying our systems in existing mine shafts (and in the future, sinking our own shafts) we are able to use weights significantly heavier than anything which could be cost-effectively supported by above ground structures.

And we can drop those weights over longer distances: the sites we are evaluating for our initial deployments have mine shafts of 1000m deep, allowing a much greater ‘drop’ than anything which could realistically be achieved above ground.

Already we are advancing plans to build a full-scale single-weight project in a disused mine shaft in mainland Europe, to commence this year.

And just last month, BEIS awarded us £912,000 to investigate the feasibility of building a purpose-built, multi-weight energy store in the UK.

In the future, we plan to build multi-weight systems raising and lowering weights totalling up to 12,000 tonnes in shafts up to 750 metres deep – offering almost 25 MWh of flexible storage.

Analysts at Imperial College calculate such a system will offer long duration energy storage at a lower levelized cost than alternative technologies, including lithium ion batteries.

But whether we build future systems in existing or purpose-built shafts, the only way to build cost-effective long-term gravity energy storage is to go underground.

The principles which have led us to our design are laws of physics, applicable equally in all parts of the world, in all seasons, and in all conditions. They are inescapable.

Understanding them is fundamental to what makes Gravitricity different.