**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

**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.

**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.

**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.

**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.*