Gravity block energy storage

Gravity block energy storage is too expensive

Simple calculations suggest that “Gravity block energy storage” will be far more expensive than battery or pumped hydro storage.

The planned NSW Waratah battery will store 2,800 Megawatt-hours (MWh) of electricity. To hold that energy, a gravity block energy storage would need:

25.7 million tonnes of blocks,
734,688 blocks, each weighing 35 tonnes,
hoists to lift these blocks 50 metres,
generators to produce electricity as the blocks descend,
an enclosed building 1 kilometre wide and 3.3 kilometres long, and
with a height of 61 metres, i.e., like a 17-storey building.

The vast expense of this building alone would be prohibitive.

The strange thing is that some big companies are investing in this block energy storage, e.g.:

BHP,
Sun Metals in its Townsville zinc refinery, &
Atlas Renewable & China Tianying.

(Company Energy Vault signs a deal with a Chinese company: Renew Economy: 2 February 2022)

Below I consider “gravity block energy storages” of different sizes, and they also support this conclusion.

If I have missed something in my calculations, please let me know.

On this Page:

Block storage of 2,800 MWh: Waratah Battery

Here are detailed calculations showing the size of a “gravity block energy storage” holding 2,800 MWh, the same energy as the Waratah Super Battery in New South Wales, Australia. Planners expect this battery to be in service by the end of 2024.

(Waratah battery: NSW government site)

An enclosed purpose-built structure

An artist’s impression of a block storage facility shows an enclosed building. So, here I consider the proposal to move the blocks between ground level and the top of an enclosed purpose-built structure. The building would exclude wind, ice, snow, dirt, and animals which could interfere with the automated placement of the blocks.

(Sun Metals taps gravity energy storage in a quest for green zinc: Renew Economy: 10 January 2022)

I do not consider moving blocks around in the open, stacking blocks on top of one another. This form of storage would involve many more movements of the blocks, many over short distances, exposed in the open air.

(BHP taps giga-scale gravity energy storage that may use recycled wind turbine blades: Renew Economy: 20 December 2021)

(Vaulting into global energy storage markets: Forbes: 2 December 2021)

The density of the blocks

When picking a density for the blocks, I’ve considered the following materials:

balsa wood density 160 kg/cubic metre (An article mentioned making the blocks from recycled wind turbine blades, and these blades can include balsa wood to keep them light),
water density 1,000 kg/cubic metre,
rammed earth density 1,750 kg / cubic metre (The articles also mention making the blocks from compressed earth mixed with resins),
concrete 2,400 kg/cubic metre
iron density 7,900 kk/cubic metre (Making the block from iron would be too expensive, but iron gives an upper limit for the block density), and
lead density 11,300 kg/cubic metre (another upper limit for the block density).

I’ve chosen a density between water and rammed earth, 1,500 kg/cubic metre.

Size of the blocks

Assume each block is a cube with:

density 1,500 kg / cubic metre,
weight 35 tonnes = 35,000 kg, and
sides of length L.

Then:

the volume of each block = L cubed = 35,000 / 1,500 = 23.33 cubic metres,
the length of each side of the cube (L) will be 2.85 metres, making each block the size of a small bedroom, &
the area of each block on the ground = L squared = 8.15 square metres.

Energy losses in a gravity block energy storage

Some of the energy losses would occur when the storage facility:

lowers each block and lands it gently,
generates heat by coiling and uncoiling the cables carrying the blocks, &
uses energy to move each block horizontally to a hoist (each block will not have its own hoist).

The required energy input

Energy output = 2,800 MWh

For the storage facility to generate 2,800 MWh of electricity, the electrical hoists lifting the blocks to the top of the building will use more electricity than 2,800 MWh to allow for energy losses.

Assume the block storage energy efficiency = 80%
Required energy input = 2,800 / 0.80 = 3,500 MWh = 12,600,000,000,000 Joule

The needed total mass of blocks

Here is a standard formula:

The gravitational potential energy of a mass =
(Earth’s gravitation acceleration) x (mass moved) x (vertical movement)

From above, the required “energy” input = 12,600,000,000,000 Joule.
Earth’s gravitation acceleration = 9.8 metres per second each second.
Assume that the building allows each block a “vertical movement” of 50 metres.

So, the total mass needed = 12,600,000,000,000 / (9.8 x 50) = 25.7 million tonnes

(Note, I have not given exact figures for some calculated numbers shown here, e.g., this “total mass needed” is 25.714,080,001 million tonnes which is about 25.7)

The number of gravity blocks

Number of blocks needed = Total mass needed / block mass = 734,688 blocks

The block storage building size

Assume you stack the blocks two high,
then the number of blocks on the ground = 734,688 / 2 = 367,344

The ground area of all the blocks = 367,344 * 8.15 = 3.0 square kilometres

There needs to be room for the structure of the building, maintenance, and block movement, so allow 10% extra area.
The building area = 367,344 * 8.15 * 1.10 = 3.3 square km

Assume the building length = 1.0 km
So, the building width = 3.3 km

The height of the building has to allow:

blocks stacked two high: 2 x 2.85 = 5.7 metres,
the top of the top block to rise 50 metres, and
hoisting equipment, access space and roofing: 5 metres.

So, the building will be about 60.7 metres tall.

Each storey in an ordinary building adds about 3.5 metres in height.
So this building will be as tall as a 17-storey building.

The storage facility could be:

as high as a 17-storey building,
1 kilometre wide, and
3.3 kilometres long.

When the storage holds its maximum energy, all the blocks would be at the top level, so the building would have to be strong enough to carry the winches/generators, winch cable, and the 25.7 million tons of blocks on the top level.

Building this enormous structure would be like creating a mountain. Why construct this mountain when you could store this energy in a battery or a pumped-hydro facility?

The power of the block storage facility

The power of the block storage facility depends on the number of hoists and generators. The more hoists it has, the more rapidly, i.e., powerfully, it will be able to store energy. Also, the more generators, the more powerfully it will be able to generate electricity.

Considering only the amount of energy stored has shown how expensive the block storage facility would be. I’ve not examined the power of this block storage facility.

Block energy storage of other sizes: 1,400 MWh

I put these calculations on an Excel spreadsheet, so I can easily change the block storage “energy output” and immediately see the details of the block storage for any “energy output”.

For you, there is a simple way to do this. If you multiply the energy output of 2,800 MWh by a factor, e.g., multiplying by 0.5 to halve the energy output, then you change key storage attributes by that same factor,

Energy output = 2800 * 0.5 = 1,400 MWh
Total mass needed = 25.7 * 0.5 = 12.85 million tonnes
Number of blocks needed = 734,688 * 0.5 = 367,344
Width of the building = 3,2926 * 0.5 = 1,646 metres

These results depend on keeping other attributes the same, e.g.:

Building length = 1.0 km, and
Building Height = 60.7 metres (a 17-storey building)
Block lift height = 50 metres

Block storage of 450 MWh: Victorian Big Battery

Consider a “gravity block energy storage” holding 450 MWh. This energy storage is the same as the Victorian Big Battery, which began operation in December 2021.

This storage would need:

4.13 million tonnes of blocks,
118,075 blocks each of 35 tonnes,
Building width 529 metres

(The Victorian Big Battery website)

Block storage of 2,000 MWh: Kidston pumped hydro

Consider a “gravity block energy storage” holding 2,000 MWh.

This energy storage is the same as the Kidston pumped hydro scheme in Queensland, which should be running by 2025.

This storage would need:

18.4 million tonnes of blocks,
524,777 blocks each of 35 tonnes,
Building width 2.4 kilometres

(Genex begins building the Kidston pumped hydro storage: Power Technology: 24 Jan 2022)

Block storage 5,000 MWh: Wivenhoe pumped hydro

Consider a “gravity block energy storage” holding 5,000 MWh.

This energy storage is the same as the Wivenhoe pumped hydro:

Upper reservoir holding 28,600 mega-litres, i.e., 28.6 million tonnes.
Water drops of 76 metres to generate electricity.
Installed capacity of 500 MW which can generate for 10 hours,
Energy storage is 5,000 MWh,

This storage would need:

45.9 million tonnes of blocks,
1,311,943 blocks each of 35 tonnes,
Building width 5.9 kilometres

(Wivenhoe Power Station: Wikipedia)

Other criticism of block storage

Other people have also criticised gravity block energy storage, e.g.,

(The Energy Vault is a dumb idea: A YouTube video by Adam Something: Sounds like an invented name)

Conclusion

There are difficulties with gravity block energy storage as it involves:

constructing an enormous building,
the automatic, precision movement of millions of tonnes of blocks around the building,
many hoists/generators, and
many moving parts which can wear out and fail, including the blocks themselves.

Pumped hydro schemes move these large tonnages of water masses efficiently.

At the moment, batteries do not hold large amounts of energy. However, you would need an enormous block storage facility to store the same energy as the Waratah battery. Also, batteries have the advantage that their cost is decreasing rapidly, and they provide more than just energy storage.

The capital cost of the gravity block solution alone should make this form of storage uneconomic compared to pumped hydro and batteries.

Concerns about hydrogen efficiency

Updated: 7 July 2022