When incorporating regenerative braking, you need to set up systems to fully realize the benefits. The addition of IoT and the right controls can help accurately evaluate the true economic value.
By Jay Schultz and Patrick Berkner
Without a doubt, efficiency has become the big buzzword of the decade. Everyone is talking about ways to make their fleets greener, use less power, and waste less energy.
Regenerative braking is one of those ways, especially for operations like those in the refuse industry that involve a lot of starting and stopping. It is an efficient process by which the kinetic energy created by vehicle motion is converted into electrical energy through braking, which can be returned to the battery for future use to effectively extend the range of the vehicle.
The concept for this type of energy harvesting is by no means new, having originated back in the late 1960s. Until recently, however, it has largely been reserved for use on electric cars. The practice of using regenerative braking on heavy-duty trucks, like refuse trucks, is still very much in its infancy of adoption.
An Ideal Option for Refuse Trucks
Regenerative braking makes a lot of sense for trash collection operations, especially in densely populated neighborhoods where trucks are stopping every 100 feet or so. The more frequent the stops, the more fuel savings that can be recognized. With regenerative braking, you draw energy from the motion of the truck and put it back into the battery pack for future use. Without it, the kinetic energy converted into heat during braking events gets wasted into the environment. The goal is to put the greatest percentage of kinetic energy back into stored energy, so you are essentially using much less electrical “fuel” to power work functions, as well as to propel the truck.
Yet, the benefits of regenerative braking go beyond energy savings, even though the promise of substantial fuel savings has been a driving force in creating initial interest and demand in the refuse industry. Regenerative braking also creates less wear and tear on brake pads because you are not using the brake pads fully at every stop. The regenerative braking is essentially a brake, slowing and stopping the vehicle electrically. With all the stopping involved in most trash pickup routes (especially in the city), fleet owners go through brake pads very quickly. So, the introduction of regenerative braking can result in significant maintenance savings.
How Does Regenerative Braking Work?
When the truck is propelled, the motors are in positive torque mode to move the vehicle forward (see Figure 1, Power Out). When the truck slows down, there is negative torque applied with no mechanical interference. As the driver presses down on the brake, the truck switches into a slight regenerative mode and then keeps going into that mode the harder the brake is pressed, eventually transitioning from regenerative braking to a mechanical stop if the brake is pressed too hard.
With regenerative braking, you are reversing the polarity of the current inside the electric motor such that it acts as a generator and routes that energy as electrical energy back to the battery (see Figure 1, Power In). Some experts in the industry have estimated that regenerative braking can recapture upwards of 70 percent of the kinetic energy that would otherwise have been lost during braking, although the exact amount is completely dependent on the vehicle itself. A larger vehicle, for example, has more energy it can put back into the battery. However, it takes more energy to get the vehicle up to speed. So, while it is true you are regenerating a lot more energy, it depends upon your battery size as to the time you can operate.
If designed properly, it is possible to fully stop a truck through regenerative braking. Exactly how well regenerative braking works and how much energy and wear and tear it saves depends largely on driver behavior. Thus, there is a need for driver education so that drivers learn to start slowing down sooner to avoid having to go into mechanical stop mode. With gentler stops, it is possible to ensure that all the energy is used with little-to-no waste.
Drivers do not need to drastically alter the way they drive to optimize the benefits of regenerate braking. Rather, more intuitive changes are recommended so that they are only making a small adjustment to the rate of release or not fully releasing the accelerator. The trick is to know exactly the level of pressure that triggers the transition from friction braking to regenerative braking.
Regenerative braking reverses the polarity of the current inside the electric motor so it acts as a generator and routes that energy as electricity back to the battery (Power In), recapturing up to 70 percent of the kinetic energy that would otherwise have been lost during braking.
Images courtesy of Parker
Why is Regenerative Braking Uncommon?
It comes down to cost and some lingering misconceptions in the market. Despite advances that have brought down the cost of batteries, they remain expensive, especially considering the larger battery size that is necessary to accelerate a larger vehicle like a refuse truck.
Government grants have had some impact, making it more affordable for fleets to make the large upfront investment in batteries as part of longer-term electrification efforts. If a truck is already battery electric, regenerative braking can be leveraged at almost no cost.
There are, of course, some potential disadvantages to consider, such as what to do when the battery is full since there is nowhere for the excess energy to go. That is when you need to use a mechanical brake. It is like overfilling a gas tank. When a battery is fully charged, it cannot accept any more charge, so all regenerative braking power is wasted. You need to think what will happen in these scenarios. This is more likely a problem at the beginning of the day if the battery was fully charged the night before.
What About Hybrids?
Hybrids, although somewhat rare in the refuse industry, can use regenerative braking. However, it is incrementally more cost effective to go all-electric versus a hybrid.
The push in the market is for fully electric chassis, at least for the time being. This drives up the cost over what a diesel chassis would traditionally cost, but has many advantages. At this point, governments around the world are strongly pushing municipalities and refuse collection companies to grow the number of BEVs in their fleets as part of growing green initiatives.
The Need for an ePTO
Once the chassis has transitioned to BEV, and regenerative braking is now possible, the vehicle still has to perform work to collect the trash. In traditional refuse trucks, this was accomplished with a mechanical PTO connected to the engine. The PTO allowed the engine to power all the hydraulics to pick up, compress, and dump the trash. With BEVs, this mechanical PTO is no longer possible to use. So, an ePTO is the only solution. An ePTO uses mostly the same components that are in the electric powertrain, but instead of providing propulsion, they provide a means to run the hydraulic pump from the battery.
The greatest potential for energy savings comes from re-architecting the hydraulic circuits for maximum efficiencies (think reducing the oil flowing over relief valves). Eliminating/reducing these hydraulic losses is an essential task when transitioning a refuse truck from diesel to all electric—almost as important as implementing regenerative braking.
Incorporating Regenerative Braking into Your Existing Electric Trucks
If you have a battery electric truck, you have the capability to incorporate regenerative braking. But you need to set up systems to fully realize the benefits. The addition of IoT and the right controls can help accurately evaluate the true economic value of regenerative braking. IoT also helps to optimize routes for maximum fuel efficiency. Such calculations are important when determining your return on investment.
Future technologies in regenerative braking will include new types of motors which will be more efficient as generators, new drive train designs that will be built with regenerative braking in mind, and electric systems which will be less prone to energy losses. | WA
Jay Schultz is a Business Development Manager for the Motion Systems Group – Vehicle Electrification for Parker Hannifin. A degreed engineer, he has been working at Parker for more than 21 years and, in his current role, is responsible for advancing and applying Parker’s SMART Electrification technologies (vehicle duty motors, inverters and ePumps) on heavy-duty vehicles and mobile equipment. He has written and contributed to numerous articles and has spoken around the world at various industry events on the topic of vehicle electrification. Jay can be reached at [email protected].
Patrick Berkner is a Mobile Systems Engineer for Parker Hannifin where he has worked for nearly 38 years. In his current role, he is responsible for helping customers adapt to the latest electrical technologies by assisting with Parker’s SMART Electrification Design, Application, and overall system considerations. He holds a degree in electrical engineering technology and can be reached at [email protected].