OXTO Energy: A New Generation of Flywheel Energy Storage

Image: OXTO Energy


The Inertia Drive technology is based on the flywheel mechanical battery concept that stores kinetic energy in the form of a rotating mass. Our innovations focus on design, assembly and manufacturing process.

Solar and wind power only produce when the wind is blowing or the sun is shining. This causes grid instability due to loss of system “inertia”, which ultimately impacts energy supply to consumers. Inertia Drive is a flywheel. This Solution will lead the stability of inertia of the system, fostering renewable integration and electrification of transport. A flywheel is considered as a mechanical battery that stores kinetic energy in the form of a rotating mass. It is a truly sustainable solution to the challenges of decarbonising power generation and transport industries.

The stored energy depends on the moment of inertia and speed of the rotating shaft:

Energy = ½ * Inertia * Speed²

  • Speed matters more than mass
  • Ratio of material strength and density determines the maximum energy which can be stored


OXTO´s technology was born from previous successful deployments in satellite and spacecraft applications. Its founder, Dr Prassinos, brought his know-how in order to develop the current earth-based technology.

OUR MISSION is to assist industries by stabilising electricity from renewable intermittent sources of energy and reducing the cost of power consumption, with a truly sustainable & cost effective solution.


The way we store energy is changing. The global demand for a low carbon economy is bringing rapid changes to energy networks and large energy companies. Renewable integration and rapid deployment of Electric Vehicle charging stations are reshaping the energy networks loading scenarios and are creating more and more grid losses and power cuts. Our hardware solution provides stabilisation for electricity from renewable intermittent sources of energy thus allowing for greater integration of solar and wind energy within the grid and supporting your company’s sustainability goals.

The unique inherited design allows for working alongside any power application from wind turbines, heavy-industry to EV-charging infrastructure.


OXTO’s mechanical battery has outstanding technical performances & low cost

  • 95% round-trip efficiency, 4 ms response, 100% DOD & unlimited cycles
  • Modular system: Standard size of 65 kW / 5 kWh used for each flywheel unit
  • Long lifetime: Similar lifetime to most power plants (25 years)
  • Low vacuum: avoiding Paschen’s law issue (plasma created from residual gas and electrons at very low pressure)
  • Low costs: No degradation of performance, very limited maintenance, no cooling system.
  • Simple & robust design: Flywheel and motor/generator are same part. Few components and moving parts reducing risks of failure.
  • Full metallic system: No use of magnets or magnetic bearings.

Flywheel units are organized in clusters. Each flywheel unit has its power electronics, including power converter, motor controller, FPGA.

The flywheel size (4-foot/1.2m diameter) is perfectly optimized to fit a cluster of 10 units inside a 20-foot container.

Cables run from each flywheel unit to the associated power electronics rack. Power Electronics racks are stored in an electrical cabinet.

A DC bus of 585-715V links the units (650V nominal). The overall constitutes the DC System, a cluster of flywheels.

There is no limit in the scale up of the system; as many units as required may be connected as clusters of 10 to 16 flywheels per cluster. Each cluster is connected to a AC/DC converter.


OXTO’s flywheel may be installed above ground in a container or directly above ground outdoor installation on a concrete slab. This makes it an easy, fast and cheap installation.


The DC bus is the backbone of the entire system.

Communication between all DC bus connected devices is vital to ensure correct operation and synchronisation. The flywheel energy storage systems all communicate with a cluster master controller through EtherCAT. This protocol is used to ensure consistent low latency data transfer as is required for fast response times, which is <4ms to bus load changes.


There are 3 modes:

  1. Regenerate mode where energy is converted from the kinetic motion of the flywheel into energy capable of charging the capacitor banks. During this operating mode, the flywheel acts similar to a pulsed constant current source, charging the capacitor banks at a constant rate.
  2. Motoring mode where energy is absorbed from the DC bus and converted back into kinetic energy. To ensure a good level of efficiency and ensuring the flywheels are not switching between both modes around the nominal voltage, a hysteresis controller is used to reduce changes around the nominal voltage.
  3. Freewheeling mode where no acceleration or forced deceleration occurs.

UK Patent GB2489523

International Patent PCT/GB2019/052636

  • USA
  • Europe
  • China
  • India
  • Australia
  • Brazil

The move of electricity markets towards decentralised bi-directional systems requires a higher level of power quality and control. Power systems are not operating at their full potential. With the decarbonisation of the global economy, it’s proving challenging to balance grids on a second-by-second basis due to a drop of inertia with the integration of intermittent renewable energy sources.

We target several markets where our product is able to provide high power with very fast response and unlimited cycles.

Power quality for critical industrial processes

Power failures and voltage variations import equipment and productivity of industrials. Conversely, some industrial processes create disturbance on the grid with inrush current.

Flywheels provide:

  1. Peak load capping – save on energy bill
  2. Power conditioning – UPS, voltage sags
  3. Allow transition to renewable energy source for critical loads 4.Reduce impact of industrial process on grid

Grid stability with renewable integration

Flywheels manage both active and reactive power, providing support for frequency and voltage control. Due to their fast response, flywheels may also help on inertia compensation.

Flywheels may be installed as standalone, for grid ancillary services, or in colocation with a renewable power plant for power smoothing.

Energy recovery from braking

Electrical systems or equipment that requires brakes may gain energy efficiency using flywheels. When a train brakes, an elevator lifts down or a crane brings down a container, part of the energy otherwise lost by heat may be stored in flywheels and reused later. Flywheels allow reusing captured energy from trains with flexibility for any usage.

For elevators and seaport cranes, 1 trip up every 3 trips down is operated using energy stored in flywheels.

EV charging anywhere anytime

Electrical systems were not designed to charge EVs, which power demand is highly variable. Charging stations cannot always be installed with a good grid connection. Operators of such stations have high peaks of demand, challenging their power capacity and increasing their electricity expenses.

Flywheels provide a power booster, allowing a decent rate of charge anytime and anywhere, wherever a charging station is located. Flywheel save also on electricity cost by reducing peak power demand.


Our route to market is to work with key partners, namely engineering firms, project developers, utilities, or system operators that have projects and end-users; our system may also be integrated within a larger energy solution.

Our aim is to be top-of-mind for all these stakeholders so as soon as an energy storage challenge occurs, they think of OXTO as their first port of call:

  • Project developers/operators, utilities. They will integrate technologies to projects; therefore, the objective is to have the proposed product integrated in these projects.
  • System operators (grid, train systems). Although, depending on the regulations, they usually do not own or operate such assets, they may be interested in pilot projects and their adoption would validate the solution.
  • Engineering Procurement & Construction. These companies advise and design systems for energy project owners. OXTO’s aim is to be included in their list of preferred and recommended technology providers. They generally operate in several industries.


OXTO is delivering four (4) projects in Europe, North America and Africa, related to different commercial applications:


Our flywheel will be run on a number of different grid stabilization scenarios


OXTO will install an 800kW flywheel energy storage system for a tea manufacturing company in Kenya.

The OXTO flywheel will operate as UPS system by covering both power and voltage fluctuation and diesel genset trips to increase productivity. The system will also create power system stability and enable less diesel fuel consumption.


OXTO Energy will be integrating our flywheels into a regenerative braking system which will recover energy from the slowing trains by converting its kinetic energy into electricity.


The microgrid will utilize flywheel as part of a hybrid wind/diesel/solar/storage system. This microgrid will also be linked to the distribution network. Our flywheel will be run on a number of different grid stabilization scenarios.

Published by PQBlog

Electrical Engineer

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