Gravity battery

The earliest form of a device that used gravity to power mechanical movement was the pendulum clock, invented in 1656 by Christiaan Huygens. The clock was powered by the force of gravity using an escapement mechanism, that made a pendulum move back and forth. Since then, gravity batteries have advanced into systems that can harness the power of gravity and turn it into electricity for large scale energy storage.

The first gravity based pumped-storage hydroelectricity (PSH) system was developed in 1907 in Switzerland. In 1930, pumped-storage came to the United States by the Connecticut Electric and Power Company. As of 2019, the total world capacity for PSH is 168 GW (gigawatts).[2] The United States has 23 GW capacity from PSH, accounting for nearly 2% of the energy supply system and 95% of utility-scale energy storage in the US. Gravity based pumped-storage electricity is currently the largest form of grid energy storage in the world.[3][4][5][6]

In 2012, Martin Riddiford and Jim Reeves developed the first functioning prototype of GravityLight, a small-scale gravity battery that is now commercially available in certain countries.[7]

EnergyVault, an energy storage company, is also currently working on research and testing to develop gravity batteries on a larger scale. Founded by Bill Gross, Andewa Pedretti, and Robert Piconi in 2017, EnergyVault is currently in the midst of taking what GravityLight created to a larger scale. EnergyVault is developing a crane that generates electricity from dropping blocks of concrete rather than water. EnergyVault has not specified a release date for its product, but prototypes are in the works and EnergyVault’s stacked blocks concept is being built to be a promising long-duration storage technology.[8][9][10]

Gravitricity, another gravity battery company, is working on another approach to a new energy storage system. Founded in 2011 by inventor Peter Fraenkel, Gravitricity is currently building a 10 meter 250 kilowatt gravity battery prototype in Scotland that plans to start trial operations and grid-connection in 2021.[11][12]

Gravity batteries can have different designs and structures, but all gravity batteries use the same properties of physics to generate energy. Gravitational potential energy is the work required to move an object in the opposite direction of Earth's gravity, expressed by the equation

${\displaystyle U=mgh}$

where U is gravitational potential energy, m is the mass of the object, g is the acceleration of the object due to gravity (9.8 m/s on earth), and h is the height of the object. Using the work-energy principle, the total amount of energy generated can be expressed by the equation

${\displaystyle \Delta E=mg(h_{1}-h_{2})}$

where E is the total amount of energy generated and h1 and h2 represent the initial and final heights of an object. The change of energy directly correlates to the vertical displacement of a mass; the higher a mass is lifted, the more gravitational potential energy is stored. The change in energy also directly correlates to the mass of an object; the heavier the mass, the bigger the change in energy.

In a gravity battery, a mass is displaced, or lifted, to generate gravitational potential energy that is transformed into electricity. Gravity batteries store gravitational potential energy by lifting a mass to a certain height using a pump, crane, or motor. After the mass is lifted, it now stores a certain gravitational potential energy based on the mass of the object and how high it was lifted. The stored gravitational potential energy is then transferred into electricity. The mass is lowered or released to fall back to its original height, which causes a generator to spin and create electricity.

One structure of a gravity battery uses a very tall structure with a heavy mass. This tall structure can be built above ground, such as a tall building or tower, or a deep hole can be drilled into earth's surface to a certain depth necessary for the battery to meet specifications. A mass is lifted to the top of the tower, or the top of the hole, using a system of pulleys. Energy is needed to lift the mass, but this energy is usually surplus energy that is used during times when energy production is greater than the demand. When the surplus energy runs out, the mass is then dropped to generate electricity through the generator.[13]

EnergyVault is working on developing large scale gravity batteries. The gravity battery they are working on developing is an energy storage tower built from concrete blocks. 120 meter cranes use excess energy from the electric grid to lift and stack concrete blocks, each weighing 32 metric tons. Energy is retrieved when bricks fall to generate energy by turning a generator. One commercial unit can store 20 MWh or energy, or enough to power 2,000 Swiss homes a day.[9]

Gravitricity's gravity battery unit consists of a convertible electric winch/generator, cables, a large weight, and vertical shaft going 150 to 1500 meters underground using disused mine shafts initially. The electric winch lifts a weight weighing from 500 to 5000 tonnes to the top of the shaft. When weight is released, it rotates the electric winch within a magnetic field to generate energy. The system generates 10 MWh, enough power for 13,000 homes for two hours. The battery can also be controlled to drop the weight quickly for a small burst of high-power energy.[11]

Another form of a gravity battery is pumped-storage hydroelectricity (PSH), the largest form of grid-energy storage. PSH uses water instead of a solid mass, which is pumped from a lower reservoir to a higher reservoir before being released through turbines to create energy. An alternative proposal uses a proprietary high-density liquid, 2½ times denser than water, which requires a smaller head (elevation) and thus decreases the size and cost of the necessary infrastructure.[14][15]

Gravity batteries have also been developed to work on a smaller scale. GravityLight is a small gravity-powered light that operates by manually lifting a bag of rocks or sand up and then letting it fall by itself to generate energy. The GravityLight was designed to help the almost one billion people in the world who do not have a source of electricity, as it would eliminate the need for people who do not have access to electricity to rely on kerosene lamps, which are expensive, dangerous, and polluting.[7][16][17]

Heavy trains that have run uphill can release their potential energy by using regenerative braking going downhill and act as a gravity battery that way.[18]

Cost of gravity batteries varies by design.

Pumped storage hydropower costs $165/kWh to operate, with a levelized cost of storage (LOCS), of $0.17/kWh.[19][20] The pumps and turbines of PSH systems operate at 90% efficiency.[21]

EnergyVault's proposed gravity battery system range from 7 to 8 million in building price but has a LOCS of $0.05/kWh and a round-trip efficiency of 88-92%. This is 50% cheaper when compared to the LOCS of lithium-ion batteries, which are $0.25/kWh-$0.35/kWh.[22]

Gravitricity's 250 kW demonstrator is expected to be $1.25 million, promising a 50-year lifespan and efficiency of 80-90%. Relative costs of gravity storage installations that would use 2000-tonne weights suspended from winches in disused mineshafts, compared with lithium ion batteries, indicate that although the "up front cost is high" the 25-year lifespan of such equipment—with no degradation of capacity during use—makes a "compelling proposition" for large-scale grid balancing purposes.[23]

Unlike PSH, solar panels, and wind turbines, which can only operate under certain conditions or in certain areas, gravity batteries like those proposed by EnergyVault and Gravitricity can be built anywhere in the world and use materials from the building site.[13][24]

Gravity batteries are designed to be part of a sustainable energy system whose sources (sunlight, wind etc) are frequently variable and do not necessarily coincide with demand. They are expected to react faster to changing energy needs, while having fewer environmental issues, than other traditional storage solutions such as pumped-water storage.[12]

Implementing gravity batteries on a larger scale would therefore decrease the need for fossil fuels, significantly cutting down CO2 emissions.

Gravity batteries are more environmentally friendly than lithium-ion batteries, since lithium-ion batteries have a shorter lifetime and problems arise when they need to be disposed of.[1]

From 1870 to 1930,[25] the term "gravity battery" was used to describe a collection of popular battery types where gravity was used to keep the chemical constituents separate based on their respective densities.[26]

• Energy storage
• Pumped-storage hydroelectricity