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Advanced Rail Energy Storage (ARES) developed a new storage system for potential energy, which uses gravity, railway lines, shuttles and concrete, and works on a similar concept as a pumped storage hydropower system. The system provides a solution for grid-scale energy storage. The company’s mission is to enable the electric grid to integrate clean, environmentally responsible, renewable energy while maintaining the reliable electricity supply necessary to power growth.
Since its founding in February 2010, the company has developed and filed both domestic and international patents for this advanced method of utility-scale electrical storage. The facilities are designed to provide grid security and reliability, support the increased use of renewable technologies, and to provide an energy storage solution that does not rely on water, as the pumped storage schemes do.
ARES developed the technology in the search for an alternative to using water in a pumped hydro facility. “If we could simulate the basic concept of using gravity to store energy without using water, that would be an innovative idea,” said Francesca Cava of ARES North America. The company has been working on the project for five years, testing and piloting the technology and continues with new configurations of rail technology for different applications. It now has the ARES ridgeline that can operate on much steeper slopes expanding the locations where the facility can be constructed.
The first commercial project which is currently still underway will cost in the region of $55-million. Cost is dependent on the size of the facility, which in the case of the pilot project in Tehachapi, California, has a capacity of 50 MW.
A fleet of electric traction-drive shuttle trains operating on a closed low-friction automated steel rail network, transport a field of heavy masses (such as train wagons carrying large blocks of concrete) between two storage yards at different elevations. During periods when excess energy is available on the grid, ARES shuttle trains draw electricity from the grid to power the individual axle-drive motors, as they transport a continuous flow of masses uphill against the force of gravity to an upper storage yard. When the grid requires energy to meet periods of high demand, the process is reversed. The shuttle trains with their heavy loads return to the lower storage yard with their motors operating as generators, converting the mechanical energy of the movement into electricity.
The facilities integrate significant recent advances in motor/generator traction drive and power control technologies with proven rail technology to produce a reliable and highly capable system that approaches an 80% charge/discharge efficiency.
The facilities are scalable in power and energy ranging from a small installation of 100 MW with 200 MWh of storage capacity up to large 2 to 3 GW regional energy storage systems with 16 to 24 GWh energy storage capacity.
The components of an ARES energy storage system can also be deployed to create a robust ancillary services system which functions as a limited energy storage resource. The fast response ancillary service technology bridges the gap between large-scale battery and flywheel installations and far larger pumped storage hydropower at a lower life-cycle cost than batteries, a higher energy-to-power ratio than flywheels and a greater efficiency and faster ramp-up rate than pumped storage. The ancillary services typically use a single uphill track with a central queue of loaded shuttle trains which travel up and down the gradient in response to an independent system operator’s (ISOs) command to provide frequency adjustments.
The facility will provide the full range of energy storage capabilities generally associated with pumped-storage hydro at approximately 60% of the capital cost and at a significantly higher efficiency, Cava said. Additionally, the system’s features which are not traditional to competing forms of energy storage, include the following attributes:
Reactive power production: The shuttle-train’s on-board Dual 3-Level Active Rectifier/Invertors are capable of supplying 25% of generated system power as reactive power for grid VAR support in full discharge mode and in excess of 100% of system power as reactive power while synchronised to the grid in standby.
Heavy inertia: When in direct grid synchronisation the shuttle-trains provide beneficial heavy inertia, augmenting grid stability against the loss of heavy generating facilities and increasing reliance on solar energy.
High-efficiency regulation: While providing Regulation-Up and Regulation-Down support to the ISO, a separate dedicated pool of loaded shuttle-trains are available to dispatch from mid-system elevation complying with ISO regulation commands without having to overcome the efficiency loss of operating on pre-stored energy. As such, the facility is able perform a round-trip regulation Reg-Up/Reg-Down command at over an 86% operating efficiency.
Variable output at constant efficiency: Unlike compressed air energy storage and pumped-storage hydro, there is no loss of system pressure during discharge. The system efficiency is constant over the full range of discharge and power output.
The advantages of the technology are that it produces no emissions, does not use fossil fuels, does not use water, creates no hazardous waste, requires no harmful extraction of materials, and can be cleanly decommissioned with no lasting impacts.