Long Duration Storage Using Gravity

In the future, will we pump water up a hill or have heavy blocks being raised and lowered to store large amounts of energy?  Doesn’t sound very futuristic but as renewable energy sources like solar and wind become more prevalent, the need for large-scale, long-duration energy storage is rapidly increasing. Let’s look at some basic physics.

Gravitational batteries store energy by raising a mass a height, increasing the mass’s potential energy.  The mass is then lowered and energy is extracted by a device, like a generator that converts the potential energy into electricity.  Let’s look at some numbers to see what this would take and put things into perspective.

Potential Energy = mass*gravity*height

Gravitational Acceleration = 9.8 m/s^2

Mass = 1 kg

Height = 1 m

PEgravitational = (1 kg)*(9.8 m/s^2)*(1 m) = 9.8 Joule = 0.002722 Wh

So the potential energy associated with raising 1 kg a height of 1 m is 0.002722 Wh.  To put this into perspective a AAA battery stores around 1 Wh, or 367 times the amount of this energy.

Utility Scale Long Duration Storage

Let’s look at this on a mass scale such as what a utility is interested in storing, say for this example 100 MWh.  To get 100 MWh we need to drastically increase the mass and change in height.  

100 MWh = 360 billion Joules

Gravitational Battery

Consider storing this energy using a gravitational battery.   Assume an elevation change of 100 meters, the roundtrip efficiency is an excellent 85% and the mass is concrete with a density of 2000 kg/m^3.  Estimate the required mass and volume.

mass = 360 billion Joules / [(9.8m/s^2)*(100 m)*85%] = 432,000,000 kg

volume = 216,086 cubic meters = 60 m x 60 m x 60 m

Perhaps we want to split this volume into more manageable size blocks.  Let’s say 10,000 independent blocks.  The ensuing blocks would still each be 21.6 cubic meters (2.8 m x 2.8 m x 2.8 m) and weigh 43,217 kg. 

Supporting this kind of weight in a free standing building 100 meters tall would require a massively strong structure.  If you have 10,000 blocks each 2.8m x 2.8m, and they were placed side-by-side (ignoring space for machinery/structure), that would be a footprint of 78,400 square meters (843,898 square feet).  

Check out the company Energy Vault exploring these kinds of solutions.

Pumped Hydroelectric Power

Let’s compare this to pumped hydroelectric power where we can take advantage of a natural elevation change, potentially making this idea more practical.  Assume the mountain has an elevation change of 1000 meters, which is quite significant but not out of the realm of possibility.  Lets also assume that the process is 85% efficient, density of water is 1000 kg/m^3 and we can drain either reservoir down 50%. How big do the top and bottom reservoirs need to be?

mass = 360 billion Joules / [(9.8 m/s^2)*(1000 m)*85%] = 43.2 million kg

volume of reservoir = 43,217 cubic meters / 50% = 86,435 cubic meters

Assuming an average depth of 10 meters each reservoir would be 93 meters x 93 meters

Case Study: Check out Ireland’s Turlough Hill pumped hydroelectric storage project for a comparison.  The picture above shows Turlough Hill. Here are some stats:

Year Fully Operational: 1974 

Round-trip Efficiency: 75%

Elevation Change (Head): 285.75 m

Peak Power: 292 MW
Storage Capacity: 1,590 MWh Upper Reservoir Size: 40 acres = 160,000 square meters = 1,722,225 square feet Upper Reservoir Dimensions: Built an embankment nearly 1 mile long and nearly 30m high

Battery Energy Storage Systems (BESS)

In further comparison, a utility scale BESS unit stores about 5 MWh, so it would take 20 BESS units, roughly each the size of a shipping container, to have a capacity of 100 MWh. A 4-hour BESS has about a 1.25 MW power output, resulting in a power output of 25 MW for the whole facility.