When a vehicle decelerates, a large amount of the kinetic energy converts to heat in the vehicle´s friction brakes. In a kinetic energy recovery system or regenerative brake system the kinetic energy is stored during deceleration and can be used for acceleration.
This concept is well known in the automotive industry and was used as early as 1894 in the Krieger electric landaulet, which was an electric horseless carriage equipped with an electric motor in each front wheel with additional bifilar coils for regenerative braking.
There are currently several different concepts for kinetic energy recovery in vehicles. Some have already reached the market, while others are on the way. The most common application is electric and based on the vehicle's existing generator which is controlled to utilize its full capacity when the vehicle decelerates during engine braking giving rise to further retarding power while the battery is recharging a bit more than usual. Such a system recovers a very small part of the potential energy recovery, due to the low power density of car batteries and their inability to efficiently store energy during short periods of time. Other concepts are using more advanced batteries or SC in combination with powerful generators and electric motors mounted in the vehicle's powertrain, with a possible theoretical fuel saving of up to 25% in city traffic.
An other approach is to recover kinetic energy using flywheel technology, in this way you avoid converting the kinetic energy into electrical and then chemical energy. This technology has previously only been used in motorsport but is today developed towards production vehicles. Flywheel technology is an effective concept since advanced batteries and SC are large, heavy and still very expensive.
Today, it is of course difficult to say which type of system will be dominant in the future when different systems have different advantages and disadvantages. Different types of kinetic energy recovery systems is listed further down on the page.
There are several important aspects to be considered during kinetic energy recovery. One of the foremost is the vehicle brake balance. The kinetic energy recovery system must be mounted in such a way that the vehicle's brake balance, and therefore safety when braking, do not change during fully regenerative braking. Weight, volume and flexibility are three other important aspects, but also for the system to be easily installed and where it is most beneficial. A flexible kinetic energy recovery system could also be developed for use in various industrial vehicles to take advantage of both the kinetic energy and any potential energy. For example, the load lowering for fork lifters, cranes, or wheel loaders. If the systems are integrated with each other, the potential energy of a load drop used to accelerate the vehicle or vice versa.
The various types of KERS do not only differ in the energy storage device, but also in power transfer between energy storage and drive wheel.
Flywheel Technology in Kinetic Energy Recovery systems
The flywheel KERS stores the kinetic energy directly in a flywheel. This can be done by with an integrated transmission in the vehicle drivetrain spinning up or slowing down the flywheel to achieve deceleration or acceleration of the vehicle. This concept provides a system efficiency of about 70% with continuously variable transmission. Energy Storage capacity is relatively high, as it relates to both the square of the rotational speed and the flywheel radius and linearly follows the flywheels mass. A flywheel can spin up and slow down very rapidly resulting in potentially high power density.
Flywheel technology, however, contains many mechanical moving parts and is still in its beginning so knowledge about durability in the harsh automotive environment is yet unwritten. Today's technology has presented flywheels with rotational speeds up to 60,000 rpm in vacuum chambers. This puts extreme demands on the bearing and the advanced transmission, while the high speeds of rotation creates friction heat corresponding to an energy loss of 2% per minute of the total stored energy. The gyroscopic forces related to flywheel puts limitations in both energy storage and power density, these are in currently small for small systems but should be considered in larger applications to ensure that it does not affect the vehicle's handling ability.
Flywheel technology is well suited for applications where you want to store and reuse only kinetic energy to and from the vehicle drivetrain. If the kinetic energy should be used for other energy consumers in the vehicle additional components will be needed.
Pneumatic Technology in Kinetic Energy Recovery systems
Pneumatic KERS uses a motor capable of generating compressed air during deceleration (engine braking). The compressed air is stored in pressure tanks and can then be used to drive the motor. Storage of kinetic energy in the form of compressed air requires first relatively large pressure tanks and / or very high pressures. Compression also produces a considerable amount of heat, which is difficult to retain and the system might be rather inefficient. This type of system is still at a very early stage and publications on the function, efficiency and durability are rare or nonexistent.
Electric Technology in Kinetic Energy Recovery systems
Electric power and the ability to convert kinetic energy into electrical energy is a very established technology, you can find generators and electric motors in everything from consumer electronics and automotive to power plants and large industries. The technology is robust, proven, effective and inexpensive.
Electric KERS use generators and electric motors connected to the vehicle's powertrain, either completely integrated or retrofitted and connected via a belt drive or similar. The energy is stored electrochemically in a battery or electrostatically in a SC. An electric kinetic energy recovery system is flexible to the extent that the electric motor and generator can be the same device and does not need to be placed next to the energy storage device. This allows the units to fit where it is most favorable. With an electric kinetic energy recovery system is simple to control the transmission and several generators and electric motors may be used together with one energy storage device. The stored energy can be used for all vehicle electronics or to charge the battery and not only to accelerate the vehicle. The problem with today's electrical energy storage, in the kinetic energy recovery point of view, is that they have low power density. Today there are advanced energy storage commercially suitable for the application, but these are still very expensive. Our paper-based SC have the possibility to be cost effective and have high performance which would suit the application excellent.
SC Technology in Kinetic Energy Recovery systems
Electric motors and generators along with SC enable a very efficient system. SC has high power density and can be charge and discharged rapidly with an efficiency of over 95%. On the market today there are electric motors suitable for the application with efficiencies of over 90%, giving a total system efficiency of about 80%, which is comparable to (or even better) than the flywheel technology. SC has a long life, is easy to integrate with the vehicle's existing electrical system and its design is very flexible, making it easy to place the SC anywhere in the vehicle.
SC has of course also disadvantages, one is the relatively low energy density, which leads to high weights and volumes for high storage units. Another negative aspect is that all the SC has a certain leakage current, which causes slow energy consumption even while the capacitor is not in use. The balancing of cells may also be a problem. Despite these negative aspects electric power in a mature technology with many users. All hardware, including SC, is tested and already commercially mass-produced which provides a wide range of components for a low price. It is only the SC unit that is still to expensive and to that problem we have a potentially environmentally friendly solution.