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A high-capacity energy storage solution is needed to capture clean energy and release it when demand exceeds supply. Massive electrochemical batteries are one possibility, but battery technology has limitations related to cost and durability.
British start-up company Gravitricity has developed a system to store excess electricity by using the power of gravity.
The company's power supply system uses a series of winches to store electrical energy as potential energy by raising large weights in a vertical shaft. When energy is needed, the weights can be released and the winches can be turned into electrical generators.
Gravitricity was developed by inventor Peter Franco, who also produced the first full-scale tidal energy turbine.
What is Gravity-Based Energy Storage?
Gravity-based energy storage is an evolution of pumped hydro storage (PHS) technologies, which can store large quantities of energy using the mass of water at different elevations.
PHS systems are only economically viable as massive operations due to installation costs. For instance, the cost of a PHS tunnel doubles when it doubles in diameter. However, a tunnel twice the width quadruples the quantity of water that can pass through it, boosting the storage capacity of the system.
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How does the Gravitricity System Work?
Rather than a series of tunnels and pumps, the Gravitricity system is based on a vertical shaft up to 1500 meters deep with weight configurations ranging from 500 to 5000 tonnes.
Raising the weight charges the system while lowering the weight discharges the electricity back to the grid.
The weight system is guided with a network of thick, tensioned wires that keep the weights from swinging into the sides of the shaft. The winch structure has also been designed to keep the weights stable.
The Gravitricity system is designed to last at least 50 years without cycle limits or degradation. The company claims it has an efficiency of 80 to 90%, with costs below that of a comparable lithium battery system.
A Gravitricity system can be set up to create a peak power between 1 and 20 MW, with an output time of 15 minutes to eight hours.
Even though the weight system works exceptionally well by itself, the system's storage capacity can be augmented by pressurizing the shaft, as this creates a compressed-air energy storage (CAES) system that can function in tandem with the weight system.
Pressurizing the shaft involves adding an airtight cap on top of the mine shaft and lining it with an airtight material. The winch system can be designed so that only electrical cables pass into the pressurized section of the overall system.
The Overall Positives of the Gravitricity System
According to the company, the system could power more than 13,000 homes for two hours.
Importantly, the system can lower the weights very quickly to deliver bursts of very high power. This flexibility makes the Gravitricity system adept at solving current storage challenges. For example, electricity networks need to stabilize the grid at 50 Hertz. Gravity is well-suited to this because it can respond with full power in less than one second.
The Gravitricity system's response times are similar to those of lithium-ion batteries, but, unlike batteries, it can be charged and discharged many times a day for more than 25 years with no loss of performance.
In a recent application, a multiple weight system was used to store large quantities of energy at low cost and deliver it for long durations.
Analysts at Imperial College London calculated that a commercialized Gravitricity system would be cheaper than the current leading energy storage solutions, including lithium-ion batteries.
Within 20 years, renewables will be the world's single largest source of power generation, and finance estimates have found the global energy storage market will be worth $620 billion by 2040.
Gravitricity’s Demonstrator Rig
Gravitricity is currently building a 250 kW, 16-meter demonstrator rig in Scotland, which will be connected to the existing grid. If successful, this will be followed by a full-scale commercial prototype in a disused mine shaft.
The company is also planning systems for Europe, South Africa, and Australia. Once the technology is proven in mine shafts, the company plans to build single-purpose systems wherever superfast, long-life energy storage is required. For example, near to towns, factories and renewable energy installations.
In October 2019, Gravitricity raised more than £750,000 capital to fund its systems. In early-2020, the company received £300,000 from Innovate UK to analyze disused mines in South Africa for suitability. However, fundraising has been severely impacted by the COIVD-19 pandemic and economic downturn.
Other Gravity-Based Storage Systems
In addition to the Gravitricity system, there are several other gravity-based storage systems in development.
Mountain Gravity Energy Storage (MGES) is one gravity-based storage system that is based on lifting solid mass during charging and lowering a mass during discharging. Rather than placing weights in an old mine shaft, this storage system uses sand, gravel or other material located on top of a mountain.
The international team behind the system has pointed out that sand, gravel and mountains are low-cost source materials that lend themselves to a long-term storage system.
Another system from the company Energy Vault uses a six-arm crane on top of a 33-story tower to store energy by raising and lowering massive blocks of concrete.
The key innovation behind this approach is the use of automation software that operates the crane and oversees the management of power.
Energy Vault has said its system costs about $7 million to $8 million. This system is also based on the "levelized storage cost of electricity" (LCOS), which is a measure of the break-even price used to charge and discharge power.
Video Credit: Energy Vault Inc/YouTube.com
Some consider LCOS to be a more precise way of measuring energy costs. The LCOS takes up-front capital expenditure and operating costs, maintenance expenses and replacement cost into consideration. According to Energy Vault calculations, PHS has a LCOS of $0.17/kWh, while the Energy Vault system is less than $0.05/kWh.
References and Further Reading
Gravitricity. [Online] Available at: https://www.gravitricity.com/ (Accessed on 10 June 2020).
Holder, M. (2020) Gravitricity to pilot £1m gravity-based energy storage system in Edinburgh. [Online] Business Green. Available at: https://www.businessgreen.com/news/4015015/gravitricity-pilot-gbp-gravity-energy-storage-edinburgh (Accessed on 10 June 2020).
Hunt, D. J. (2020) Mountain Gravity Energy Storage: A new solution for closing the gap between existing short- and long-term storage technologies. Energy. Available at: https://doi.org/10.1016/j.energy.2019.116419
Husseini, T. (2019) Tower of power: gravity-based storage evolves beyond pumped hydro. [Online] Power Technology. Available at: https://www.power-technology.com/features/gravity-based-storage/ (Accessed on 10 June 2020).