## Energy Storage with Bricks – a Really Bad Idea

Friends, the intermittent nature of many renewable energy resources makes energy storage critical for any future renewable electricity network.

But the amounts of energy that need to be stored are immense.

In round numbers, to store one day of electricity for the UK requires 1 terawatt-hour of storage.

• 1 kWh is the unit of electrical energy used by electricity companies on our domestic electricity meters – typically it costs about 20 p for us to buy and about 5 p for the companies to buy.
• 10 kWh is roughly the amount of electricity my wife and I use in a day.
• 1000 kWh is 1 Megawatt hour (MWh)
• 1000 MWh is 1 Gigawatt hour (GWh)
• 1000 GWh is 1 Terawatt hour (TWh)

And multi-day storage is just a multiple of that.

This requirement – and the business opportunities available to those who can compete in this market – have driven people to consider all kinds of off-beat ideas.

This article is about one idea which is really stupid, which will never work, but which is apparently worth over a billion dollars.

Existing Energy Storage

The hot money in energy storage is in electrical batteries of all kinds.

Tesla (for example) can supply a collection of batteries that occupies a football field or so with the following (rounded) specifications

• 130 MWh of storage (0.01 % of 1 TWh)
• 100 MW charge and discharge rate
• Efficiency ~ 90% – some energy is lost in the charge-discharge process.
• 100 million US dollars in 2020
• \$0.8M is the cost per MWh

Over the course of time I would expect this kind of facility to get cheaper and better.

The biggest storage facility in the UK is the Dinorwig Power Station in North Wales.

• 9.1 GWh of storage (0.9 % of 1 TWh)
• 1.7 GW discharge rate – charges more slowly
• Efficiency ~ 75% – energy is lost in the charge-discharge process.
• 500 million US dollars in 1984 – about 2 billion US dollars now
• \$0.2M is the cost per MWh

Dinorwig works by pumping water between two lakes with a height different of 500 m. The mathematics is easy to do.

The stored energy (in joules) is the calculated by multiplying three numbers which school students learn as mgh

• m is the mass of water stored in the upper lake (in kilograms)
• g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
• h is height difference (in metres)

The discharge rate (in watts) is also calculated by multiplying three numbers

• The mass of water per second flowing through the turbines (kilograms per second)
• g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
• h is height difference (in metres)

Compare and contrast

Dinorwig is massive – storing almost 1% of the UK’s daily electricity requirements and and the storage is cheap per unit of energy stored.

But there are – as far as I know – no other sites in the UK with similar potential.

Tesla batteries can be placed anywhere but they are relatively expensive.

We can foresee that there will be technological innovation, and mass-production effects that will reduce the costs and improve the performance of batteries in coming decades.

However there is nothing we can do to substantially improve the performance of Dinorwig. The simple formula mgh limits all gravity-based storage systems.

To get good performance, one needs a big mass (m) lifted up, and then dropped from, a great height (h).

No technological innovations can beat mgh.

• But is there a gravity-based storage system which doesn’t need a unique geography?
• Something which could be built out in modular form like Tesla’s battery farms?

Here’s the stupid idea: Project Jenga

The idea – from a company called Energy Vault –  is to store energy by building a pile of bricks.

Their videos make it seem a superficially clever idea, but I can’t get the visions of Jenga out of my head.

Basically a robot crane system uses electricity to build a tower out of very large bricks. This is equivalent to ‘charging’ a battery.

To ‘discharge’ the tower, the crane lets the bricks down onto a lower tower, and as the bricks fall they turn a generator.

Fortunately, because all gravity-storage systems are limited by the mgh equation I mentioned above, it’s possible to work out its performance parameters.

Based on information gleaned from their videos and web site I conclude that:

• Brick size ~ 6 m x 1 m x 2.5 m – mass ~36 tonnes
• Tower in charged state – 40 layers tall with ~ 100 bricks per layer
• Tower in discharged state has ~ 500 bricks per layer and so is ~ 8 layers tall

Still from an Energy Vault video showing their concept for their Jenga-like tower. Notice they imagine this free-standing pile of bricks being built near wind-turbines

The total stored energy is mgh where:

• m is the total mass of the tower, and
• h is difference between the heights of the centres of mass in the two configurations, which must have the same basic volume and number of bricks.

Calculation of stored energy in the tower system. Each state (charged and discharged) is modelled as a hollow cylinder, with the discharged cylinder being 1 metre outside the charged cylinder. The volume is conserved between the two shapes. The stored potential energy is mgh where h is the difference in height between the centres of mass in the two configurations

So my estimate for the system they describe is:

• 37 MWh of storage
• 6.8 MW charge and discharge rate (assuming (optimistically) it takes 10 seconds to move 2 bricks simultaneously)
• Efficiency ~ 85% is claimed.
• Energy vault claim \$18M, but I find it hard to believe it will cost less than 100 million US dollars: The bricks alone will cost around \$5M in raw materials.
• ~\$3M is the cost per MWh

So the system costs more per MWh than a battery-based system, with no potential for future technological improvements.

Why it won’t work

For a 37 MWh Energy Vault device, charging and discharging requires building, and then dismantling, a structure the height of Canary Wharf Tower, at 240 metres tall, the UK’s third tallest building.

A 37 MWh Energy Vault store would be 240 m tall: the height of Canary Wharf Tower. Charging would involve building such a tower from free-standing bricks in about 6 hours. Mmmmm.

The 37 MWh of stored energy in such a structure, when sold as electricity at 20p/kWh, would be worth – optimistically – around £10,000. The company profits would then be the difference between the sale and the purchase price of the electricity – let’s guess £5,000 per dis-assembly/assembly cycle.

• Nominally the system would take a few hours to build (charge) and dismantle (discharge)
• Can you imagine building anything the size of Canary Wharf in a few hours for £5000?

The charged structure would be free-standing with no reinforced concrete or steel beams to hold it together.

But this tower is envisaged to be deployed in open country, perhaps  near wind turbines – i.e. where its often windy!

Later versions of the Energy Vault concept have a different format – with mass movements taking place using some clever un-revealed geometry inside a building which looks like it is only about 40 m tall, but spread out over a much larger area.

A still from another video showing a newer version of EnergyVault enclosed in a frame inside a building. It appears to be only – maybe – 40 metres tall.

But no matter how clever they are, they can’t escape mgh.

If the building is 40 m tall, then the centre of mass is at most 20 m off the ground.

For the same 4000 bricks they used in the ‘Jenga’ design, the uncharged area of all bricks on the ground would be 10,000 m^2 i.e. 100 m x 100 m.

If this whole 144,000 tonne structure were raised by 20 m (a likely overestimate) then the stored energy would now be just 8 MWh, storing only £1,600 worth of energy in the charged state.

But in the charged state this would now be supported by an immense (= expensive) reinforced concrete frame capable of lifting and moving these large loads.

8 MWh of storage is tiny: the equivalent of 600 Tesla PowerWall batteries (like I have in my house) which would cost around £6M but which could be bought ‘off the shelf’ with no risk.

What’s going on?

It’s not just me that has noticed that this idea is a non-starter. The video above calls out the project for its ridiculousness at great length.

But if you look at the Energy Vault website you will see story after story about investment by banks and grand plans to establish a company worth billions of pounds.

What’s going on? I have no idea: it is simply madness.

### 8 Responses to “Energy Storage with Bricks – a Really Bad Idea”

1. Simon Duane Says:

For a _much_ better idea, have a read about this. I’d like to think you are already aware of it. If not, I wish I’d shared it with you before.
Best wishes
Simon

• protonsforbreakfast Says:

This is an example of pressurised gas energy storage. I have seen a number of variants of this.

The key problem here is that compression of gases produces large amounts of heat.

If this heat is lost – then when the gas expands – it needs to gather heat from some other source of low-grade heat.
If this heat is stored – for example in insulated tanks of water – then there is added complexity to the system.

For this instance I could not see how his issue is dealt with. But there are many variants.

Overall, this is a good candidate for energy storage over periods of hours to days.

• Simon Duane Says:

I have developed a great respect for the engineering design in heat exchangers. I think the issues you raise are straightforward to handle. They sound like potential difficulties but are not as serious as they might appear (to us physicists). I would be interested in your style of thinking applied to the carbon dioxide version. The more I thought about it the more enthusiastic I became.
BW

• protonsforbreakfast Says:

Simon

The Thermal Energy Storage (TES) does feature in this plan – I didn’t see what TES stood for initially.

But while it can be “handled”, the way in which it is implemented limits round trip storage efficiency and duration – and adds complexity. Basically the enthalpy terms in the compressed gas are stored separately (PV) in a pressurised container and CT in an insulated container, and then re-combined to “discharge” the system.

I haven’t looked at any of these systems in detail, but they seem to be feasible but very large. Perhaps we will be bringing back gas-holders to our cities in the near future! Or maybe batteries will eventually just eat every other storage technology.

• Simon Duane Says:

Ah yes, haha – I found myself making a very similar suggestion only this week:
“More information about a pilot project here, but the technology is not remotely pushing-back-the-frontiers stuff. Once upon a time, our country was littered with gasometers, storing town gas. This is could be a kind of 21st century update.”