Kinetic Energy Storage Takes Off

Written by: Tom Shelley | Published:

Tom Shelley reports on the long awaited commercialisation of flywheel energy storage for motor vehicles



Flywheel based energy storage coupled with a continuously variable transmission (CVT) has, at last, reached the stage of finding its first commercial customers.
It has been a long story. Eureka’s November 1985 edition first described a flywheel development to conserve energy from a bus, which was being developed by a BP research facility in Sunbury. Sadly, neither still exists.
Despite borrowing much of the innovation, the present collaboration between Torotrak, Flybrid and Xtrac is showing much more promise and has now gone commercial. It too runs in a vacuum chamber but rotates at a rather frenetic 64,500rpm, compared to the modest 16,000 rpm of the BP device.
The present development is a collaboration between three companies: Torotrak that invented the CVT, Flybrid Systems that makes the flywheel and designed the rest of the transmission system, and motorsport gearbox company Xtrac that makes some of the gears.
But a large number of technical problems have had to be overcome along the way. The mechanical seal around the shaft that turns the flywheel sits in a vacuum chamber. At the recent Autosport Engineering show, Flybrid design engineer Dan Jones explained that this consists of double lip seals, with a “Special fluid” between them. The vacuum, which is “fractions of a millibar,” is effectively sealed by oil. The flywheel shaft bearings are outside the vacuum to avoid problems that might be caused by outgassing.
For F1 use, the vacuum chamber is pumped down before the race, and remains under sufficient vacuum for the duration. For a road car, we were told, the flywheel speed would be reduced to 50,000 rpm for durability and the chamber would be pumped down by a Flybrid designed onboard vacuum pump, “about every couple of weeks.” The flywheel shaft is bolted directly to the sun gear in a planetary gearbox, connected to the Torotrak patent CVT through a centrifugal clutch.
The overall performance figures are: 60kW of power with 400kJ of energy storage in an assembly that weighs less than 25kg. Absolute energy storage of the flywheel at full speed is 590kJ. The 60kW and 400kJ comes from the F1 2009 regulations, which state that: “The maximum power, in or out, of any KERS must not exceed 60kW. Energy released from it may not exceed 400kJ in any one lap. Measurements will be taken at the connection to the rear wheel drive train.”
Mechanical round trip efficiency, spinning up the flywheel during braking, and using it to boost acceleration subsequently is more than 70%. These figures come from tests of a prototype system that has been, “fully calibrated”, on a dynamometer system.
Jones told us that the system is involved in four projects, two of which he could reveal, and two of which he couldn’t. He said that the company had signed up with a, “supercar manufacturer just before Christmas.” There has also been an agreement to supply units for a demonstration project for First Group Buses last year. This is expected to result in both retro fitting old buses and fitting them onto new buses.
It has also recently announced a joint project with Jaguar and Land Rover to put the system into a more fuel efficient luxury car. The project backed by the Technology Strategy Board is said to making, “good progress.”
One of the strong applications is still F1. This has been the driving force that has got the system into a production form. Initially, the competition has been between hybrid electric systems and the mechanical flywheel system.
FIA president Max Mosley supports the mechanical option, saying in a letter to the Formula One Team’s Association, “We are increasingly of the view that the use of chemical storage (in particular batteries) should be prohibited in Formula One owing to the unsuitability of the batteries currently available.”
But the Automobile Club de l’Ouest (ACO) that governs sports car racing, and in particular the Le Mans series, has the opposite view. It says only electric hybrids are going to be allowed in the 2009 season and Le Mans race. Although team will not be able to earn points it will allow them to test the system against non-hybrid teams to clearly assess any advantage, or indeed, disadvantage. In 2010, the electric hybrid cars will be classified in races and so far, it is known that Peugeot intends to run a hybrid electric version of their 908 HDi FAP (908 HY) diesel sports car.
This is a pity and not untypical Anglo-French chauvinism. There are features of both systems that offer benefits, and it would be useful to find out which is best in competition. Simon Shaw of Xtrac told us at the show that the KERS system occupies a more, “precise” package space. However, while electric hybrids take up more space the batteries can be distributed around the car to improve balance. He also observed that most enquiries being received by Xtrac are not from motorsport, but for more general “transportation systems” such as trains and buses.
So what F1 team will choose what systems? All Shaw would say is, “we have undertaken an initial programme with an F1 client.” But it is no great secret that a dummy KERS unit could be seen poking out of the side pod of the Ferrari driven by Luca Badoer at the second day of F1 testing in Barcelona last November. The presumable aim was to evaluate its effect on the car’s balance. It is believed that towards the end of 2008, BMW, Sauber, McLaren, Toyota and Williams all tested KERS systems, but details are understandably unavailable at the present time.
Mosley believes that, “Formula One would benefit from systems with more capacity than the present (for example maxima of 2MJ stored, 150kW in, 100kW out) but still be very small and light, as is essential in Formula One. These figures are theoretically possible with mechanical devices, but not feasible in the foreseeable future using batteries and/or capacitors.”
The only other possible technology that could be used is fluid power, using braking force to compress nitrogen in an accumulator through hydraulic pressure. Such systems are proving to be of considerable interest to makers of heavy goods vehicles, especially for military markets, but we have at the present time, no information about anyone developing such a system for motorsport.

Pointers
* Kinetic energy storage systems based on flywheels, combined with the Torotrak continuously variable transmission seem to have a last reached the stage of becoming a commercial product
* The first non motorsport application is to be in buses, as was originally intended
* The main competitor is the electric hybrid system, but the relative merits of the two alternatives seem to have been muddied by national interests promoting their own agendas, British favouring the use of British technology and French interests favouring their own solutions


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