Regenerative braking is an essential part of making any vehicle more fuel efficient. The typical (non-regenerative) braking action involves stopping or slowing forward motion through the application of friction pads to the wheels of the vehicle. Unfortunately, this means that the kinetic energy of the moving vehicle is converted into heat and lost. Regenerative braking seeks to convert some or all of that kinetic energy into potential energy, which can then be used to propel the vehicle forward again. The stored energy can either perform this propulsion by itself, or be used to assist the main powerplant (an internal combustion engine, for example) during the take-off process. In either case, the use of regenerative braking leads to increased efficiency.
This is primarily due to the fact that power requirements are greatest during takeoff, as the powerplant has to overcome the static inertia of the vehicle. By diverting some or all of that task to the regenerative braking system, the primary engine is put under less load, resulting in more efficient operation. Additionally, a vehicle’s powerplant is, in general, far larger than is necessary to accomplish its function for the majority of its lifetime – keeping the vehicle moving by counteracting losses due to kinetic friction.
Except in extreme cases (refuse haulers, transit buses) most vehicles spend a vast majority of their time cruising, which generally requires a minimum of power. However, because the engine must be made large enough to get the vehicle moving from a stop, all vehicles are saddled with an engine that is much heavier, larger, and less efficient than it needs to be. Regenerative braking can alleviate this restriction by removing some of the responsibility of initial take-off from the engine and applying power directly to the drivetrain, resulting in a smaller, lighter, and more efficient engine. It also implies that, provided the capacity of the regenerative braking system is large enough, the stored energy can also provide “boost power” to the drivetrain during, for example, passing situations or emergencies.
In order to fulfill its function in the most optimal manner possible, a regenerative braking system (RBS) needs to fulfill certain requirements.
1) It should be lightweight, as any added vehicle weight will detract from any performance or efficiency gains. Additionally, light weight also simplifies retrofitting the RBS onto an existing vehicle platform – while such a retrofit would not see the efficiency gains of a ground-up design which incorporates the RBS along with the resultant smaller engine, it is still possible for an RBS to have a positive effect on vehicle efficiency. The RBS should also have enough energy capacity to capture all of the available kinetic energy. The single parameter which relates to both of those criteria is the energy density – the available energy storage capacity per unit mass or volume. The energy density of the RBS has a profound effect on its usability for different vehicle platforms. As an example, consider an automobile of average mass (1500 kg), moving at 60 kph (36 mph) – this translates into a little over 208 kJ of energy.
Contrast that to a large garbage truck (13000 kg) moving at 30 kph (18 mph), which has around 450 kJ of kinetic energy. As the average speed of a garbage truck during its daily routine is much slower than that of a passenger car, this is not an unfair comparison. What is immediately apparent is that an RBS which has an energy density of only 1 kJ/kg is sufficient to serve the needs of the garbage truck (implying a total RBS weight of ~ 450 kg – less than 0.5% of the truck weight), that RBS would be useless for the passenger car (as it would weigh over 200 kg – almost 14% of car weight). The added mass of the RBS would nullify any benefit gained from regenerative braking. When considering the energy density of the RBS, it is important to include not only the energy density of the storage material, but also any additional weight in ancillary components required to transmit energy to and from the energy storage material. These additional components add mass/weight without providing any direct energy storage capacity, thus de-rating the total energy density of the RBS.
2) The RBS should also be efficient, in both the absorption and release of energy. The benefits of higher RBS efficiency include: more delivered energy during take-off operations (resulting in less engine load), more reserve energy for passing/emergency situations, and a smaller overall capacity/size of the RBS. The last point comes about because inefficiencies must be taken into account when designing the capacity and size of the RBS. Let us consider the refuse hauler case – in order to accelerate the hauler back up to 30 kph, 450 kJ must be delivered after considering all loss effects. Excluding other loss factors (such as tire rolling resistance) an RBS which is 90% efficient would need to have a capacity of 500 kJ – a 40% efficient unit would need over 1100 kJ. Since losses such as friction are intrinsic to the vehicle platform, it is vital that the RBS be as efficient as possible. One means of accomplishing this is to ensure that minimal energy conversion operations (e.g. mechanical kinetic energy to electrical potential energy) are performed.
3) Related to the efficiency of the RBS is its power density in both charging and discharging operations. As its name implies, power density refers to the amount of power that can be delivered per unit mass or volume. The charge and discharge power densities need not be equivalent; indeed, in most cases they are not the same. For instance, a conventional electrochemical battery is typically only capable of being charged at 1/10th its discharge rate. If the power density is not sufficiently high in the deceleration case, some fraction of the input energy is wasted and the total energy absorbed is less than the maximum amount available. Conversely, if the power density is too low during acceleration, additional power is required from the engine, lowering efficiency. It should be obvious that the most power is required at the beginning of each of the acceleration/deceleration actions, as this is when the inertia (resistance to change in velocity) is highest.
4) Another requirement for an effective RBS is its cycle life – that is, the number of charge/discharge cycles it can perform before a noticeable degradation in performance occurs. Higher cycle life implies that the RBS will be able to perform its function for a longer period of time before needing reconditioning or replacement.
5) For the most optimal performance, the RBS should also have high shelf life – that is, once charged, it should be able to maintain its charge for a long period of time without undue loss. While this is not truly necessary during most operations (the recovered braking energy is only held for long enough to assist in the next acceleration event), it is of vital importance in one instance. When the vehicle is stopped for a long period of time (for instance, at the end of the work day), that final braking event provides energy to the RBS.
This stored energy could then be used when the vehicle is started again (for instance, the next day) to provide acceleration, provided it is held with little or no loss. This is a subtle, but important consequence of shelf life – it plays a role in the ultimate size and power of the primary powerplant. Unless the RBS can maintain its stored energy (or have an independent method of being recharged during a long rest), the primary powerplant will still need to be sized to accelerate the vehicle from a dead stop. So while an RBS with low shelf life will improve efficiency once the vehicle is moving (by reducing the load on the primary powerplant), it will not enable a reduction in overall engine size, which is the most effective means of improving overall engine efficiency.
6) The RBS should also possess temperature stability, at least in the range of temperatures likely to be encountered by the vehicle. Fluctuations in the energy or power density of the RBS as a result of changing temperatures will require that the RBS be oversized with respect to both energy and power capacity – this again increases the size and weight of the RBS, reducing its effectiveness.
7) The RBS should also be durable and reliable, because of the intense stresses associated with the deceleration/acceleration events. These are when the forces on the vehicle are the greatest, and comprise the majority of the RBS’s active life cycle. Therefore, there should be intrinsic durability to the design which allows for repeated and reliable actuation under these most stressing conditions.
8) Finally, the RBS should be effective regardless of the specifics of primary propulsion – it should be compatible regardless of whether the vehicle has a gasoline, diesel, electric, or some other powerplant. This either implies that it has a design which makes the mechanical energy of acceleration and deceleration easily converted into the type of the main powerplant, or that its operation is independent of the main powerplant to such a degree that the two can coexist on the same platform. For instance, an RBS that takes the energy from braking and converts it into heat is not useful unless that heat energy can then be used to accelerate the vehicle or otherwise reduce the load of the primary powerplant during acceleration.