Gravity Energy Storage Technology Verification Accelerates
Energy storage technologies are generally categorized into mechanical energy storage, electrical energy storage, electrochemical energy storage, thermal energy storage and chemical energy storage.
(Source: WeChat Official Account “GaoGong Energy Storage”, ID: weixin-gg-ess)
Recently, scientists at the International Institute for Applied Systems Analysis (IIASA) have proposed a fascinating solution: transforming skyscrapers into giant gravity batteries. This concept leverages elevators and vacant apartments inside high-rise buildings to store energy.
The concept is simple and follows the principle of gravity energy storage, a branch of mechanical energy storage. Surplus renewable energy can be stored as potential energy by lifting heavy masses to higher elevations. Energy is then released when gravity drives a generator. The scientists named this concept Lift Energy Storage Technology (LEST).
Lift Energy Storage Technology is one type of gravity energy storage. Researchers also analyzed the economics of LEST. The analysis assumes that the existing elevators are equipped with regenerative braking and that there is zero cost for renting space to place containers at upper and lower storage locations. Therefore, the only capital expenditure covers containers, mass-adding materials for the containers and automated trailers. For a demonstration building with 5,000 storage containers and an average height difference of 100 meters, the estimated energy storage cost is 64 US dollars per kWh. A larger vertical gap between upper and lower storage sites reduces the LEST cost. Another advantage of LEST is that containers and materials (accounting for 79% of total costs) have a service life exceeding 30 years, while automated trailers (21% of total costs) last for 5 years.
Gravity Energy Storage Poised to Enter the Year of Technology Verification
On May 16, 2022, China Tianying signed a strategic cooperation agreement with State Grid for gravity energy storage technology research. The two parties will set up a joint team to push forward the construction of the 100 MWh user-side gravity energy storage demonstration project in Rudong.
As early as December 13, 2021, China Tianying Inc. formally signed a strategic cooperation agreement on China’s first 100 MWh gravity energy storage project with the Energy Investment Professional Committee of China Investment Association, US-based STERA Energy and Skytower (Beijing) Energy Storage Technology Co., Ltd. This marked the launch of China’s first gravity energy storage demonstration project.
Notably, on February 14 of the same year, Energy Vault, a gravity energy storage enterprise in which China Tianying holds investment, was listed on the New York Stock Exchange, raising a total of 235 million US dollars.
Energy Vault focuses on gravity energy storage technology. It uses surplus electricity to lift 30-ton “mobile modules” to an elevated position; when power is needed, the modules are lowered to generate electricity, operating on a principle similar to pumped hydro energy storage. These mobile modules are mainly manufactured from waste materials including combustion residues, mine tailings, waste fiberglass and concrete fragments.
Omar Aoun, Vice President of Sales at Energy Vault, stated in an interview that the round-trip efficiency of this gravity energy storage technology stands at approximately 80%. The technology can deliver 4 hours of storage at 90% capacity and is scalable. Energy Vault’s energy storage project is designed with a 25 MW power capacity and 8-hour duration. Calculations show that a 100-meter-tall building covering two acres can deliver 100 MW of power, meaning only 20 acres are required for 1 GW capacity. From the perspective of scale, this technology offers better cost-effectiveness than battery energy storage.
Energy Vault already operates a gravity energy storage demonstration project in Switzerland, which was connected to the grid in July 2020. The company claims its technology costs 51% of lithium-ion batteries. It is negotiating cooperation with various clients including independent power producers, large utilities and major mining firms, and has signed a memorandum of understanding with BHP. The company’s first commercial project is scheduled to start operation in Texas by the end of next year.
Globally, the gravity energy storage industry is still in its early stage of application. The technical feasibility and economic viability of hundred-megawatt-scale deployments remain to be verified.
Only a small number of startups worldwide are commercializing gravity energy storage, mainly Switzerland’s EV, Gravity Power from California in the United States, and Gravitricity in Scotland. Among them, EV attracts the most attention after its SPAC listing.
Founded on January 1, 2017, EV is a Swiss developer of renewable energy storage technologies and products. It has completed multiple rounds of financing, including USD 110 million from SoftBank Vision Fund in 2019 and investment from Saudi Aramco Energy Ventures in 2021, and launched its first energy storage crane product EV1.
As a new type of mechanical energy storage, gravity energy storage is expected to enter its year of technology verification in 2022.
Gravity Energy Storage Outperforms Pumped Hydro in Efficiency
According to expert estimates, gravity energy storage achieves higher efficiency than pumped hydro energy storage. As an emerging technology, its principle is straightforward: electric energy is converted into potential energy via electric motors to lift gravity masses to a preset height. When the power system requires electricity, the gravity masses descend to drive generators, converting potential energy into mechanical energy, and the rotor cuts magnetic field lines to produce electricity. Control systems constitute the core technology.
For pumped hydro energy storage, a lower reservoir is first built with a dam along a river channel. An upper reservoir is constructed on mountains with a vertical height difference of more than 500 meters, usually excavated from mountain hollows. Pump-turbines pump water upward to convert electric energy into potential energy. An inclined water conveyance tunnel with an inclination angle above 60 degrees connects the upper and lower reservoirs. When the system needs power, water from the upper reservoir flows down the tunnel; potential energy turns into kinetic energy to strike the turbine runner, and the generator produces electricity.
Gravity energy storage shares a similar underlying principle with pumped hydro energy storage. However, gravity storage skips the kinetic energy stage, following the energy conversion path: potential energy → mechanical energy → electric energy. Pumped hydro follows potential energy → kinetic energy → mechanical energy → electric energy. Fewer energy conversion links yield higher efficiency, hence gravity energy storage outperforms pumped hydro. Reportedly, pumped hydro generally reaches around 75% efficiency, while the gravity energy storage technology introduced by China Tianying achieves over 85% efficiency.
Gravity energy storage has no special requirements for site selection. Pumped hydro demands favorable geological conditions: a river for dam construction at the lower elevation and mountain valleys for building upper reservoirs. For gravity storage, a framed structure of roughly 120–150 meters can be built at the target location, with fewer site constraints.
Popular Science of Gravity Energy Storage
Theoretically speaking, gravity energy storage is one of the simplest energy storage methods. Similar to pumped hydro, it stores energy using gravitational potential energy. When there is surplus electricity, motors lift heavy masses to high positions. When power is needed, the descending weight drives generators to produce electricity. According to overseas references, major gravity energy storage technologies include piston-type gravity storage, suspended gravity storage, concrete block energy storage towers and mountain-based gravity storage.
Advantages: Simple principle and relatively low technical barriers. Since energy is stored in physical media, its energy storage efficiency can reach up to 90%. It only takes 2.9 seconds to ramp output power from zero to 100%, with a service life exceeding 30 years. Unlike pumped hydro stations which impose strict site constraints, its construction cost is only one-third of pumped hydro, and its levelized cost of electricity equals two-thirds of pumped hydro.
Disadvantages: Low energy density and large footprint. Gravity energy storage towers are commonly over 100 meters high, yet their output power is equivalent to a wind turbine of the same height. Moreover, the technology imposes extremely high precision requirements on tower cranes. For dozens-of-meters-long cables, the positional error of each of the 5,000 concrete blocks must be kept below several millimeters. Each tower requires thousands of concrete blocks, and concrete pouring generates massive carbon dioxide emissions. If poorly managed, the carbon footprint may exceed the emission reductions brought by higher penetration of new energy generation.