How Much Energy Can A Gravity Battery Produce?
What is a gravity battery?
A gravity battery, also known as a gravitational energy storage system, stores energy by lifting a heavy weight and holding it against gravity. The system uses the principle of gravitational potential energy, which is the stored energy an object possesses due to its height above the ground.
Here is a simplified explanation of how a gravity battery works:
- During off-peak electricity hours when extra energy is available on the grid, an electric motor uses this spare electricity to lift a heavy weight up high inside a tall shaft or tower.
- At times of peak electricity demand when more energy is needed, the weight is allowed to gradually descend, turning a generator to produce electricity.
- The heavier the weight and the higher it is lifted, the more gravitational potential energy that can be stored and later released to generate electricity.
So in summary, these systems effectively store energy by using surplus electricity to lift a weight, and then recapture that stored energy later by lowering the weight to generate electricity. The battery is “recharged” by using cheap, off-peak energy to lift the weight back up the shaft.
History and development
Gravity batteries originated in the early 2000s as researchers explored ways to store energy using gravity. An early concept was developed in Scotland using abandoned mineshafts. Weights would be raised to store potential energy, then lowered through the mineshafts to convert that potential energy into electricity.
In the 2010s, companies like Gravitricity and Energy Vault began developing full-scale prototypes of gravity batteries. These used tall towers and cranes to lift concrete or sand weights. As the technology matured, engineers optimized the lifting and lowering mechanisms to maximize efficiency.
Recent innovations allow modern gravity batteries to achieve round-trip efficiencies of 80-90%. This makes them competitive with lithium-ion batteries while leveraging low-cost materials. Today, gravity storage is seen as a promising large-scale energy storage solution.
How Much Energy Can Gravity Batteries Store?
Gravity batteries store potential energy based on lifting a weighted mass. The energy is stored by lifting the mass and then recovered when lowering it. The amount of energy that can be stored depends on the mass of the weight and the height it is lifted to.
The potential energy stored is calculated using the equation PE = mgh. Where m is the mass, g is the acceleration of gravity (~9.81 m/s2), and h is the height the mass is lifted. The weight (e.g. a container full of sand or rocks) is first lifted up to a platform at height h. Then when the stored energy needs to be recovered, it is lowered, and the potential energy converts to kinetic energy to spin a generator and produce electricity.
For example, if a 100-ton concrete block was lifted 500 meters high, it would store approximately 490 megajoules of potential energy using the equation PE=mgh. This energy could then be recaptured when lowering the block.
So in summary, the energy storage capacity depends on the mass that can be lifted and the maximum height. Gravity batteries can store anywhere from kilowatt-hours to megawatt-hours of energy depending on their scale and design.
Case studies
There are a few notable real-world installations of gravity energy storage systems. One example is a system installed in 2021 at Heerlen, Netherlands by Energy Vault. This system uses 35-meter tall concrete blocks weighing 35 tonnes each that are lifted and lowered by cranes to store and release electricity. It has a power capacity of 2 MW and storage capacity of 20 MWh.
Another installation is a gravity-based system by Gravitricity at Edinburgh, UK. This system uses a large weighted piston that is winched up inside a deep vertical shaft during charging. When electricity is needed, the weight is lowered, turning generators on the way down. This demo system has 250 kW power capacity and can store 500 kWh.
In Santa Barbera, California, Advanced Rail Energy Storage (ARES) has developed a gravity-based storage system using rail cars loaded with concrete blocks. During charging, the cars are pushed up a grade and then allowed to roll back down through motor generators when discharge is needed. This system has a power capacity of 1 MW with 10 hours or 10 MWh energy storage.
Advantages
There are several advantages to gravity batteries that make them an appealing energy storage option compared to alternatives:
Low Cost
The structure and components of gravity batteries are relatively inexpensive compared to other storage methods. They primarily consist of a weight using commonly available materials like sand, rocks, steel, that can be cheaply sourced at scale. No expensive chemicals or metals are required.
Long Lifespan
Gravity batteries are noted for having an extremely long operating life. Since they have minimal moving parts and do not degrade chemically over time, they can reliably store and discharge energy for several decades with practically no loss in capacity.
Sustainability
Most gravity energy storage designs utilize renewable materials and sources, with potentially recyclable components. They do not require any elemental inputs like lithium as in batteries and provide clean hydroelectric power output. Overall, gravity batteries are an environmentally sound storage mechanism.
Disadvantages
While gravity energy storage offers some major benefits, it also comes with some notable drawbacks that limit its use. Two key disadvantages are geographic constraints and low power density.
First, the locations where gravity batteries can be built are severely limited. The technology requires specific terrain and geologic formations in order to create the “height” difference in elevation needed. Suitable sites have to have the right combination of a mountaintop location, access to water reservoirs/dams at lower elevations, and geography allowing stable construction of water pipes/tunnels between the upper and lower reservoirs. This means only certain mountainous regions are candidates for gravity storage systems.
Secondly, gravity batteries suffer from low power density compared to other large-scale storage solutions like lithium-ion batteries and pumped hydroelectric power stations. This means that for the physical footprint and space they occupy, gravity batteries cannot store as much total energy or generate as much power capacity as rival technologies. So more land area and geographic space is needed per unit of energy produced by gravity batteries.
Comparison to other energy storage
Gravity batteries offer some key advantages and disadvantages compared to other common large-scale energy storage technologies like lithium-ion batteries, pumped hydro storage, compressed air energy storage (CAES), and flywheel energy storage.
Compared to lithium-ion batteries, gravity batteries have a longer lifespan, lower environmental impact, and potentially lower costs over the full lifetime. However, lithium-ion batteries have faster response times to charge and discharge. They also currently have more real-world examples to demonstrate their capabilities.
Versus pumped hydro storage, gravity batteries do not require specific terrain or negative environmental impacts related to dams and reservoirs. But pumped hydro can currently store larger total amounts of energy more cost effectively.
In relation to CAES systems, gravity batteries again avoid geographic constraints and have a smaller surface footprint. Though CAES benefits from more commercial operating history to date.
Finally, flywheel systems offer faster response times for energy smoothing services compared to gravity-based solutions. Though flywheels store relatively small amounts of total energy and require replacement more frequently.
Future outlook
Researchers are working to improve the efficiency and viability of gravity energy storage through various means. Efforts are underway to develop advanced materials that can maximize energy density and reduce energy losses. Additionally, companies are exploring gravity batteries with lower height requirements, which would allow them to be deployed in more locations.
One area of focus is electricity generation during discharge. By integrating technologies like turbo-generators, the released kinetic energy can be more efficiently converted to electricity. There are also studies examining alternative lifting media besides water, such as sand or gravel, that could increase system efficiency.
With many of these innovations still in early phases, commercial viability remains a challenge. However, if breakthroughs can be achieved in critical areas like energy density and discharge efficiency, gravity batteries have immense potential as a sustainable, large-scale energy storage solution. Many experts predict major advances in the next 5-10 years that will accelerate adoption.
Main applications
Gravity batteries are well suited for large-scale energy storage applications like stabilizing electricity grids and enabling greater renewable energy penetration. Some of their key uses are:
Grid storage – Gravity batteries can store massive amounts of energy to help balance electricity supply and demand on the grid. During periods of low energy demand, excess electricity can be used to lift a weight. When demand spikes, that weight can then be lowered to generate electricity.
Renewable integration – The intermittent nature of renewable power sources like wind and solar requires energy storage to smooth out fluctuations. Gravity storage provides bulk storage capabilities to capture excess renewable energy when available and dispatch it when needed.
Power reliability – Gravity batteries can supply backup power if conventional generators fail or if a grid blackout occurs. Stored energy in the battery can be released to stabilize the grid and prevent outages.
Load levelling – Gravity storage allows shifting energy usage to off-peak times. Energy can be stored when demand is low and utilized during high-demand periods, reducing peak loads.
Frequency regulation – Gravity batteries can rapidly adjust their output to maintain stable grid frequency in response to fluctuations in electricity supply and demand.
Conclusions
In summary, gravity batteries are an innovative and promising form of energy storage that leverage height differentials and gravity to store and release energy. While the technology is still in early stages, pilot projects have demonstrated their ability to provide large-scale, long-duration energy storage at a competitive cost. Key advantages of gravity batteries include their long lifespan, minimal geographic constraints, and use of simple mechanical principles rather than reliance on scarce materials.
While gravity batteries are not suitable for all applications, they can play an important role in efforts to increase grid stability and integrate more renewable energy. Going forward, they have the potential to enable greater balancing of electricity supply and demand across days, weeks or even months. However, more research and development is still needed to improve roundtrip efficiency, refine optimum configurations and scale up to larger installations. Overall, gravity batteries represent a promising energy storage solution that can make a meaningful contribution to a sustainable energy future if their current limitations can be addressed.
