Cheap and plentiful energy is a thing of the past. Concern over climate change is driving ever more stringent regulation to reduce fossil fuel consumption and stimulating the development of cleaner, greener transportation technology to reduce greenhouse gas emissions. But unless you’re one of the tiny minority of car owners who drive a hybrid or electric vehicle, you’re throwing away energy every time you apply the brakes.
A conventional vehicle’s internal combustion (IC) engine operates by converting the stored energy in fossil fuel into the kinetic energy of motion. When it comes time to slow down or stop, you step on the brakes.
Conventional brakes operate by means of brake pads that contact brake rotors in the wheels, producing friction that causes a moving vehicle to decelerate and ultimately stop. In that process, braking friction effectively converts the kinetic energy of motion into heat energy, which then dissipates into the environment—poof, it’s gone.
Well, how about conserving the kinetic energy that your IC engine creates by burning expensive gasoline back into free, clean, stored electrical energy that can then be used for acceleration or to power other onboard electrical functions?
Just as waste recycling conserves natural resources by reusing materials such as glass, aluminum, plastics, and newsprint, an emerging technology called regenerative braking makes it possible to harvest and reuse as much as 30% of the energy that is consumed to propel a vehicle.
This emission-free stored electrical energy is then available to assist acceleration, power the air conditioner, operate power steering, or perform other functions, reducing IC engine fuel consumption and its accompanying emissions.
In a regenerative braking system, the electric motor that is responsible for all or part of an electric or hybrid-electric vehicle’s propulsion (Fig. 1) also does most of the braking. When the driver steps on the brake pedal, instead of activating a conventional friction-based braking process, it sends an electronic signal to the electric motor, directing it to run in reverse mode, which creates resistance to slow the vehicle through a process that is analogous to down-shifting a standard transmission vehicle.
An electric motor running backwards also acts as an electric energy generator or dynamo that can convert the kinetic energy of motion into electrical energy that can be stored for future use. As an added bonus, regenerative braking with an electric motor takes most of the load off mechanical brakes, reducing brake maintenance and replacement expense.
So why haven’t drivers around the world jumped on the regenerative braking bandwagon in larger numbers? Many who are motivated to embrace cleaner, greener cars aren’t willing to sacrifice driving performance.
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Also, to take maximum advantage of regenerative braking, an electric or hybrid vehicle needs a highly efficient energy storage system that can absorb as much electrical energy as possible during the few seconds it takes to stop the vehicle.
Both IC-like driving performance and braking energy recuperation require a large, expensive, rechargeable energy storage system that adds thousands of dollars to a vehicle’s sticker price. Even with today’s high fuel prices, industry analysts calculate that it will take five to 10 years for the owner of a hybrid or electric auto to recoup that additional initial investment through fuel savings.
Despite that unattractive return on investment, more and more early adopters are willing to pay the hybrid premium to be green. But hybrid and electric car owners have found that electrochemical rechargeable batteries based on nickel or lithium chemistries have some inherent characteristics that limit batteries’ efficiency and their suitability for the hybrid or electric vehicle energy storage role. These include low power density, impaired performance at low temperatures, and finite operational life.
Power density can be defined as the rate at which an energy storage device or system can be charged or discharged. If we consider how charging a mobile phone, laptop computer, or any other consumer electronic device requires an hour or more to complete, how much electrical energy can a hybrid or electric vehicle’s rechargeable battery be expected to absorb during a braking event that lasts only a few seconds? Some, but certainly not all of the clean electrical energy a kinetic energy-driven dynamo is capable of producing during braking.
On the flip side, rapid, deep discharges to power acceleration stress batteries and shorten their life. To overcome these charge-discharge rate limitations and make rechargeable hybrid and electric vehicle batteries last longer, they typically are oversized, adding to the volume, weight, and cost of the energy storage system.
Batteries generate and store electrical energy by means of chemical reactions that operate most efficiently within a fairly narrow temperature range. Therefore, the rate at which rechargeable batteries charge and discharge declines sharply as temperature drops, impairing their ability to absorb regenerative braking energy or supply energy to power the vehicle’s electric motor.
That’s why current hybrid autos’ fuel economy and electric vehicles’ range deteriorate sharply in cold climates. The high temperatures common in engine compartments also impair battery performance, shorten battery life, and create serious safety hazards, so batteries need protection from overheating as well.
Every battery, irrespective of chemistry, has a finite operational life and wears out after hundreds to a few thousand charge/discharge cycles. To extend battery life and put off costly replacement, system designers build in power electronics that limit charge rate and depth of discharge. That means only a portion of the battery’s full capacity is available to absorb recuperative braking energy and power the electric motor.
Here again, to compensate for those limitations and provide acceptable performance, the battery must be oversized, so hybrid or electric vehicles must sacrifice efficiency by hauling around very large, heavy, expensive battery systems. Given those cost and efficiency limitations, how can hybrid and electric vehicles become more practical and affordable and allow drivers around the world to stop throwing away energy every time they brake?
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The Hybrid Energy Storage Solution
A lower-profile energy storage device known as an electrochemical double-layer capacitor, or ultracapacitor, appears to be the perfect complement to rechargeable batteries for greener, more efficient, and affordable hybrid and electric vehicles (Fig. 2). Ultracapacitors fully charge or discharge in as little as fractions of a second, operate normally down to –40°C, and perform reliably for 1 million or more charge/discharge cycles.
In a regenerative braking system, ultracapacitors can absorb virtually all of the recycled energy each braking event produces. They then feed that stored energy back to the electric motor just as quickly for acceleration or recharge the battery at whatever rate is most conducive to battery health and longevity.
Ultracapacitors already are performing that role as the standalone energy storage technology in nearly 2000 hybrid transit buses and electric rail vehicles currently operating in the U.S., Europe, and Asia. Also, scientists at the Argonne National Laboratory have demonstrated that an integrated system combining batteries with ultracapacitors will dramatically improve braking energy recuperation efficiency and eliminate the need for battery oversizing, reducing the weight and cost of the entire system.
Such a hybrid energy storage solution would enable carmakers to provide an economic as well as environmental rationale for owning a hybrid or electric car. It also would eliminate the current unattractive choice between overpaying for an oversized battery system or throwing away energy.