Tackling KERS in Formula One
The importance of developing hybrid technology has been recognised by the FIA and from the start of the 2009 season, Formula One cars are permitted by regulation to collect, retain and re-use kinetic energy that would otherwise have been dissipated as heat under braking. Regulations permit the Kinetic Energy Recovery System (KERS) fitted to a Formula One car to collect and store energy during braking at a rate of 60kW. Up to 400kJ of this stored energy can then be re-introduced into the drivetrain each lap at a rate of up to 60kW; an increase in overall power of about 10%. Drivers will have a ‘boost’ button allowing them to deploy this extra energy tactically during a race. Therefore on the track KERS will not only increase the efficiency of Formula One cars but could contribute to more exciting racing. Once these technologies have been validated in the highly demanding environment of Formula One, wider application beyond the race track should advance the progress of mainstream hybrid vehicles.
Energy storage systems
A hybrid vehicle’s recoverable energy storage system (RESS) is the primary limiting factor in short term efficiency gains and long term benefits over its lifetime. This component is also one of the most expensive elements of a hybrid system. Therefore the choice of the specific RESS technology used in a hybrid vehicle will largely determine the extent of its performance, economic and environmental benefits.
RESSs that can be used in a hybrid vehicle broadly fall into three categories: namely chemical batteries, supercapacitors (also referred to as ultracapacitors) and flywheels. The three technologies all have widely different benefits and advantages; defining their applicability in different contexts.
In the short term, the two most important characteristics of any RESS are specific energy and specific power. The former refers to the amount of energy per kilogram that the system can store and the latter to the rate at which energy can be put into or taken out of the system per kilogram.
Chemical batteries for one have the advantage of unrivalled specific energy. As such these system can store most energy per Kg and are hence very interesting for compact RESS designs.
However; the specific power - the rate to charge or release energy - of flywheels far exceeds that of battery technology. Lithium-ion batteries — an advanced chemical battery type found in many mobile phones and laptops — take one or more hours to fully charge due to their low specific power. By contrast a flywheel with its high specific power can be fully charged in about 10 seconds. Therefore chemical batteries work well in situations where they can be charged slowly at low power but in applications demanding high power — such as a KERS in a hybrid vehicle — the amount of batteries required increases the weight and cost dramatically.
Supercapacitors are the opposite; possessing high specific power but such low specific energy that storing a useful amount of energy in a hybrid context requires a heavy and bulky system.
When choosing the correct RESS design, it is important to keep in mind in which situations it will be used. Chemical batteries for instance perform differently depending on the ambient temperature; in particular their function rapidly degrades in low temperatures. This presents problems for battery-based hybrid vehicles operating in cold locations whereas flywheels are not affected by ambient temperatures. The latter system is however not suited for long term energy storage, something that batteries can cope well.
In the medium term, it is important how the usage profile of an RESSs affects its long term performance. Performance of a chemical battery degrades irreversibly as it is used, with degradation accelerating under heavy or deep discharging conditions common to hybrid vehicle applications. Certain types of flywheels are immune from any such degradation over their life regardless of the usage profile.
The lifetime of a RESS is a key determinant in its long term viability. Chemical batteries can typically be cycled (charged and discharged) about a thousand times before they must be disposed of or recycled. Supercapacitors have a lifetime of around a million cycles but it is flywheels that excel in longevity with lifetimes upward of ten million cycles.
When considering a RESS in a long term context, it is informative to look at the number of charge/discharge cycles over its life multiplied by the energy exchanged per kilogram. This figure illustrates the amount of energy that the system handles over its life. For a chemical battery this is in the region of 1000 MJ/kg, for supercapacitors about 10,000 MJ/kg and for flywheels it approaches 1,000,000 MJ/kg. To put this in context, a single 40kg flywheel system would recycle 40 terajoules over its lifetime which is the same amount of energy as the electricity used by 2.2 million UK households in a year.
The above advantages of a flywheel also fuel the FIA's desire to move away from chemical batteries as energy storage device. It is more and more important to keep costs under control while keeping in mind the friendliness to the environment - flywheels to not require the use of toxic materials and can be safely recycled after usage. As such, the FIA makes no secret that it might push for a standard KERS device. When such design is proposed, it looks highly probable that the flywheel will win the challenge.
The most important aspect of any RESS, especially in a mobile context, is safety. By definition any energy storage device represents a safety risk should it fail and rapidly release its energy. Batteries can explode if they overheat and flywheels can explosively shatter. Any system must therefore be able to safely withstand an internal failure but also be resistant to external events such as a high speed crash.
In order to ensure the safety of the KERS systems, the drivers and teams have repeatedly sit together to find solutions to problems that arose with the new technology. Worries have been reported by several drivers and teams after both BMW Sauber and Red Bull suffered from failures. In the end, the FIA have issued measures such as rubber gloves for team personnel and danger courses for the track marshals in case of an on-track failure.
Systems used in Formula One
No matter how the energy is stored in a KERS system, all Formula One teams have chosen for an electrical link between the car's drivetrain and its KERS system. The teams therefore connect a motor to the driveshaft at the front of the engine, close to the fuel tank. A cabling system then transports the energy to and from the RESS system, a point where there is great difference between the teams' designs.
Flywheel technology by Williams Hybrid power
WHP has taken the electrically powered integral motor - effectively a flywheel with integrated motor - design and radically improved its performance characteristics by incorporating Magnetically Loaded Composite (MLC) technology. The MLC technology was developed in the nuclear industry by Urenco and has been licensed by WHP.
In WHP’s Magnetically Loaded Composite Flywheel Energy Storage System (MLCFESS), the permanent magnets of the integral motor are incorporated into the composite structure of the flywheel itself. In the event of a burst failure, the containment has to withstand only the crushing force of the composite material, which is less than the load of discrete metallic fragments. The reduced containment requirements minimize the overall weight of the system. The magnetic particles in the composite are magnetised after the rotor is manufactured which means that it can be magnetised as a Halbach Array; avoiding the need for backing iron to direct the flux.
As the magnets in an MLC system are comprised of tiny magnetic particles and there is no additional metal in the structure, the eddy current losses of the machine are significantly reduced. This can result in one-way efficiencies of up to 99%. The ultra-high efficiency means thermal management of the system is easier and it can be continuously cycled with no detriment to performance or reduction in life.
Its final version for Formula One will have a high specific power of more than 5kW/kg which is possible by spinning the wheel to more than 50000rpm. Even more so, because the system is fully contained and doesn't require an external motor, it is probably the most compact F1 KERS solution and proves suitable for volume production to other appliances.
Except for Williams, all Formula One teams have taken the route of chemical batteries to store the recoverable energy. Despite the flywheel's advantages, the short life cycles in Formula One and the limitless search for miniaturisation have made teams decide on batteries. The best known supplier of such a system is Magnetti Marelli who presented their system to the public in November 2008. It features an electric motor that can work as an alternator to convert motion energy to eletrical as well as the other way around to release previously stored energy to the wheels.
Additionally, the motor also has a built in water cooling system. It is then up to the teams to design an effective cooling system with the least possible drag penalty. Not such an easy task considering the stringent rules on cooling apertures. The Italian firm is however sure that in the medium term it will be able to increase the motor's efficiency and hence reduce its cooling requirements.
Several teams will be using this system, including Ferrari and Scuderia Toro Rosso. Because of Magnetti Marelli's expertise with electronics, the resulting motor and accompanying KERS control unit weighs as low as 4kg.
The actual energy storage device is left up to the teams to design. Magnetti Marelli had initially planned to design such package but quickly realised that teams had very different demands in this area, hence leaving it up to them to create the energy storage systems. It appears that most teams will be using lithium-ion batteries as supercapacitors weigh far too much to be useful in Formula One - estimated at 300kg to equal the 40kg flywheel solution of Williams Hybrid drive.
A troublesome road
Just ahead of the start of the first KERS season, it is evident that some teams are much further along with their KERS development than others. Despite the high level of secrecy around the new system, we take a look at who is where:
- McLaren have already tested its KERS system extensively during pre-season testing but have completed the Barcelona test without it. The team look focus on their performance issues first but will nonetheless be using KERS at Melbourne.
- Ferrari is using the Magnetti Marelli system but have only tested it at Mugello and Sakhir. Later winter tests at Jerez and Barcelona were run with ballast. While it initially appears unlikely to see a KERS introduction at Melbourne, the team confirmed one week ahead of the first Grand Prix that its KERS was ready and would be used.
- BMW Sauber has started 2009 development very early on and have never made any suggestions that its KERS system would not make it to the Melbourne grid. All winter tests have been completed with KERS, so it'll be at Melbourne too.
- Renault F1 have experienced some trouble with its solution but declared in Februari that there was sudden and unexpected progress. Recent testing was done with the system on-board, and given the team's announcements we may expect the Renault R29 to be fitted with KERS.
- Red Bull Racing have not had much KERS testing but are going for the Renault solution after they had trouble developing their own system. Because of the lack of testing with the system, it is expected that the team will wait for Renault to validate its reliability.
- Toyota have declared they will not fit KERS any time soon.
- Scuderia Toro Rosso will use Ferrari's system and won't be able to introduce it ahead of Ferrari.
- Williams F1's Kazuki Nakajima stated KERS hasn't been tested on track yet and will not be fitted in the first 6 races of the season.
- Force India are awaiting McLaren's system.
- Brawn GP is also still unsure about KERS, no introduction plans at the start of the season.
Which ever the outcome, it is at this time still unclear if KERS currently provides enough of an advantage to really need it in Formula One. Leaving it out of the car allows for the engineers to have an additional 60kg of ballast to move around and ensure of perfect weight distribution. However, as some teams appear to be ready, hopefully they can display the overtaking advantage it potentially offers.