Urenco. the European uranium enrichment company, has adapted its uranium enrichment gas centrifuge technology for uranium enrichment to provide a new concept in energy storage systems based on a light, high speed flywheel.

Urenco once famously claimed that its centrifuges spun a thousand times in the blink of an eye, on their maintenance-free bearings, to separate gaseous uranium isotopes. Now, that same lightweight centrifuge design is the basis of a vertical flywheel energy storage system (FESS) that releases electricity as high-voltage DC power pulses. The company has demonstrated that the same basic mechanical principles can be employed in an energy storage system and this one has 100 kWe capacity. The system is available commercially, delivering electrical energy at up to 750 V for 30 seconds. A demonstrator prototype, now being constructed at Urenco’s laboratories in Capenhurst, UK, is designed to deliver three times the energy: 200 kWe for 44 seconds. Its designers have been careful not to transgress international rules on nuclear technology transfer.

Fusion of technologies

Urenco was created as a tripartite nuclear enrichment company by the governments of Holland, Germany and the UK, under the Treaty of Almelo in 1970. It was a joint venture to exploit research carried out secretly – and separately – by all three nations during the 1960s, following publication of a German engineer’s ideas for a lightweight, high-speed gas centrifuge. Initially, Urenco marketed enrichment or “separative work” carried out by the three individual national technologies.

Gradually, these technologies were fused by R&D into a single Urenco technology, a process completed by 1992. Urenco assembles all its own centrifuges – over 200 000 – and has earned international credibility as a manufacturer of long-lived, highly reliable electrical machines.

Today, Urenco claims a 12 per cent share of a civil world market for enrichment dominated by the huge gas diffusion capacities of three nations: USA, Russia and France. It competes with their economies of scale because its centrifuge is very low in power and maintenance costs. But, a relatively stagnant world market for enrichment persuaded Urenco by the 1990s to diversify some of its R & D effort. Dr Pat Upson, its technical director, began to explore new uses for both its product ideas and its manufacturing technology.

First commercial spin off

FESS is the first commercial spin-off. It extensively uses the company’s proprietary – and still secret – manufacturing technology. Its first commercial demonstration of FESS is in one of its own, and oldest, gas centrifuge plants, to protect machines installed over 20 years ago from power fluctuations. The original design life of this installation was 10 years.

David Kelsall, electrical engineering manager of the New Products Department of Urenco (Capenhurst) Ltd, the UK shareholder, says the idea was born in its laboratories in the mid-1990s. Kelsall told the Pulsed Power Club recently that the high-speed composite flywheel it has developed “is set to revolutionise the field of electrical energy storage.” It offers small size and weight, low losses and a simple interface to electrical systems, he asserts: “The potential for this technology is just beginning to be recognised.”

Flywheels, one of the oldest ways of storing energy, traditionally have been large steel wheels of great mass that rotate slowly and take up much room. Urenco’s lightweight composite flywheel, like its latest centrifuges, is a cylinder made from composite material. Unlike the centrifuge rotor, however, the flywheel is of relatively high mass, wound from carbon and glass-fibre composite, with a bore loaded with permanent magnet powder (a neodyium-iron-boron alloy) wound into the composite. This patented design now has more than 40 000 hours of operating experience.

The dispersed alloy particles are imprinted with two magnetic patterns during manufacture. One end of the rotor is imprinted circumferentially, to form half of a passive magnetic bearing. The rest of the rotor is magnetised axially with 12 poles, creating the rotor of a permanent-magnet electrical machine. “This rotor can run at up to 42 000 rpm and can store 18 MJ of energy”, Kelsall says.

A steel shaft runs down the bore of the rotor, mounting at its upper end the other half of the magnetic bearing which keeps it upright on its bottom bearing. This shaft also carries the stator of the flywheel’s motor generator.

The assembly is mounted in a vacuum-tight steel containment, which both minimises rotor windage losses and contains any “crash”. Bearing seizures, albeit extremely rare, cause the flywheel to come to rest with its rotor intact. Its cylindrical construction is inherently safe compared to conventional flywheels, claims Kelsall.

Interface electronics

The FESS interfaces with control systems through its own electronics. The generator’s output is roughly sinusoidal, three-phase ac which varies in frequency and amplitude with the flywheel’s changing speed. The circuit converts the output to a regulated dc voltage. Power flow to and from the flywheel is controlled by a three-phase voltage source inverter that operates as an inverter when the flywheel is storing energy (building up speed) and as a rectifier when delivering energy. Optical sensors focused on reflectors on the rotor synchronise inverter switching with back emf from the FESS.

The upshot is an energy system for which Urenco sees many and diverse applications in such industries as chemical processing, telecommunications and broadcasting, where power quality is of increasing importance; and traction, where regeneration is also required.

Normally, the flywheel will be coasting at close to full speed (fully charged), but with speed falling slowly through friction. When it has fallen a few hertz the flywheel is automatically restored to full speed. Steady state losses for the 100 kWe model are 1.2 kWe for a top speed of 36 000 rpm.


Among the benefits Kelsall claims for the high-speed flywheel compared with established storage systems, such as batteries, is its robustness. Its operating temperature range is

-20°C to +40°C. It can cope with repeated deep discharges and survive several million cycles. Lifespan, based on centrifuge experience, should exceed 20 years.

Since January, Urenco has been using its first commercial flywheels to protect one of its oldest centrifuge enrichment plants, at Capenhurst, against power quality problems. Supply voltage disturbances can cause the centrifuge drives in these older facilities, which were designed in the 1970s, to trip and results in dumping any gas in process.

A pair of flywheels now protects about one quarter of this plant, set to compensate for about 99 per cent of all voltage dips. The total supported load is 465 kVA.

Higher rated model

As already noted, a more highly rated FESS of the same size is taking shape at Capenhurst, designed to deliver 200 kWe for 44 seconds. Kelsall expects this prototype machine to be ready to run shortly. Its characteristics are more closely aligned with the requirements of the highly competitive UPS (uninterruptible power supply) market. “We believe that, compared to equivalent technology, we’ll have a very competitive product”, Kelsall asserts. “Taken on a life-cycle basis, the system will be significantly cheaper than other types of energy storage”, he says.

Urenco PQ series FESS specification