Every day, substantial energy is lost on the world’s roads by friction brakes dissipating the kinetic energy from decelerating vehicles as waste heat. Only a small proportion of the road fleet has regenerative brakes, which drive a generator that converts the kinetic energy into electricity for use during subsequent acceleration.
Regenerative braking offers the obvious benefit of reduced fuel costs, while the lower emissions could enable the vehicles to enter the growing number of low-emission zones. Governments worldwide are under pressure to improve urban air quality amid growing awareness of the effects of pollution on public health.
Electric trains have used regenerative braking for decades, returning power via the supply cable or third rail. Equipping the supply system to receive the power is a relatively complex task for DC systems such as the commuter railways south of the Thames, where Network Rail invested significant funds to enable trains to recover up to 20% of their energy.
Trains have a relatively smooth energy demand profile, with schedules, signalling and segregated right of way enabling them to cruise for long distances. Road vehicles, on the other hand, frequently alternate between acceleration and deceleration in response to other road users, traffic signals, hills and bends. Sustained congestion-free cruising is a rarity for most British drivers, and will become even scarcer if the UK government’s forecasts of major traffic growth come to pass.
Key obstacles to mass adoption of regenerative braking in road vehicles are storing the energy generated during coasting and braking, and coupling the storage device to the transmission in a cost-effective way.
Effective energy storage needed
There are mechanical methods of storing energy from braking. Ford and others have trialled hydraulic accumulators, but Isobel Sheldon, business manager for hybrid and electric systems at consultancy Ricardo, says she has yet to see such a system that would be commercially practicable.
She is more optimistic about using flywheels to store braking energy. This is perhaps best suited to buses, since the energy decays rapidly. She says: “Flywheels work in cities where you’ve got less than half a kilometre between stops. You can deploy the energy back into the drive system, and the bus moves off. That really does work.”
Regenerative braking can be retrofitted to vehicles powered by internal combustion engines. Tim Shallcross, head of technical policy at the Institute of Advanced Motorists (IAM), believes this holds significant potential for road hauliers, until regenerative braking becomes commonplace in new lorries. Although hybrid lorries are available, they’re likely to remain a minority interest for the foreseeable future. Shallcross says: “For heavy-duty vehicles, a liquid fuel – which doesn’t necessarily have to be diesel – is the best option for long-distance heavy haulage.
“It’s a very, very condensed form of energy. When you have an articulated lorry doing 7mpg, if you can increase that to 8mpg with technologies such as regenerative braking it’s a big saving.”
Sheldon says Ricardo’s work on business cases for retrofitting regenerative braking has identified operators of large lorry fleets in the US for whom the option looks attractive, partly because nine states have introduced low-emission standards. “In Europe the case for making those changes appears to be not so clear cut,” she says. “The efficiency of the diesel engines we have here is quite high.”
Shallcross, who represents the IAM on Britain’s Low-Carbon Vehicle Partnership, doubts whether the economics of retrofitting regenerative systems would stack up for cars, even for taxis. Another issue is shortage of space within cars for the equipment.
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Hybrids lead the way
However, he believes energy recovery systems will become commonplace in new cars in the next decade, partly because of the spread of hybrid technology and its range-extension cousin. The latter features electric batteries and traction motors, plus a petrol engine to generate additional electricity if required. Since these technologies already involve batteries and traction motors, regenerative braking is a simple addition. The usual friction brakes are also provided, and take over whenever the electric brakes have fully charged the battery.
Sheldon predicts that regenerative braking will become a standard feature of cars, because of demand for greater efficiency and regulation of emissions. “There’s a natural percolation of this kind of technology to the mainstream,” she says. “The cost will reduce and it will be widely adopted. We’re starting to see that now.”
She sees regenerative braking as an integral part of the electrification of vehicle drive systems. Ricardo’s work in this area has included, for example, implementation of electrical superchargers. Turbochargers, typically driven by the flow of exhaust gases, and superchargers, generally driven by the engine crankshaft, compress the air flowing into the engine. Electrical superchargers spool up rapidly and can be used on demand, provided there’s enough energy available from the battery, instead of waiting until exhaust gas pressure has built up sufficiently.
She predicts that the traditional 12V electrical system in cars will be replaced with higher voltages such as 32V, 36V or 48V, to match the growth in electrical loads. One benefit will be a reduction in the thickness and weight of copper cabling. Systems using those higher voltages are better able to accept the electricity generated by brake applications.
Shallcross says many modern non-hybrid cars embrace the basic principle of regenerative braking by using the engine to recharge the battery only when engine braking comes into play, and not when the engine is providing traction.
Mazda takes the principle further, with an energy recovery system that sidesteps the complexities of transmitting recovered energy to the wheels. Many of its cars include a supercapacitor, which involves no chemical reactions and can be charged faster than a battery. The supercapacitor weighs only 6kg and there is practically no limit to the times it can be charged and discharged, whereas a battery degrades.
At first glance, the potential energy savings might appear insignificant where regenerative braking is not used to assist acceleration. But Shallcross says: “You would be surprised at how much of a drain on a vehicle the electrical system is.”
Professor Peter Wells, automotive industry expert at Cardiff University, says: “Batteries are running many more systems – more entertainment, navigation aids, climate control and so on. There’s a case for some of that demand to be met by regenerative braking.
“There’s a long-run trend towards the electrification of the car, whether or not it has an internal combustion engine on board. The internal combustion engine will become more of a generator and less of a direct drive provider.”
He says motor manufacturers are keenly exploring ways to reduce emissions, partly to anticipate future mandatory standards and tax incentives. There’s also growing recognition that vehicle test cycles pay too little attention to periods when some systems might come under extreme loads, for example a car starting from cold on a winter morning with the heater, window heaters, in-seat heaters and windscreen wipers running. When vehicle performance in such situations is included in tests, innovations such as energy storage could count for more.
Shallcross, who is involved in plans for the introduction of the Worldwide Harmonised Light Duty Vehicle Test Procedure, says a new EU test regime is due to be introduced in 2017, partly in response to evidence that the existing regime understates vehicles’ real-life emissions. “It will make regenerative braking much more attractive to manufacturers,” he predicts.
Regenerative braking inevitably introduces further complexity and new maintenance requirements to vehicles. Last year Mazda recalled 88,000 cars worldwide because of a software problem relating to its energy recovery system in heavy rain or floods. No mechanical changes were needed, only reprogramming of the power control module.
Savings on maintenance
On the plus side, however, there are potential maintenance savings elsewhere. For example, the friction brakes should have a longer maintenance periodicity where regenerative brakes handle routine deceleration. Mazda says its supercapacitor system prolongs the lifespan of the car’s lead-acid battery.
Sheldon says Ricardo’s HyBoost and Adept research programmes developed intelligent electrification systems, which included regenerative braking and produced the same performance from a three-cylinder 1-litre car engine as from a conventional 2-litre unit. One result is a reduction in the size and weight of engine components. It’s easy to imagine also that such vehicles would require smaller fuel tanks, reducing the weight carried.