Transit agencies are increasingly converting their bus fleets to electric under public pressure to reduce emissions and meet legislative mandates. Fleet owners navigating this transition have many options for charging technology, each with unique challenges and advantages. Amid these options, wireless power transfer has been slower to gain traction than some other methods. But the method, also known as inductive charging, can offer several benefits for electric bus systems, including the ability to:
- Extend the range of buses.
- Reduce the cost of fleet conversion.
- Operate without driver assistance.
- Provide durability.
- Improve aesthetics.
These benefits can aid in any potential transition to electric, though it is important to also recognize the challenges associated with wireless power. As transit leaders make this shift and undertake planning studies, feasibility analyses, and route modeling to support their decision making, wireless power transfer should be seriously considered along with other major charging methods.
A proven technology
Wireless power transfer sends power through the air with no cords, cables, or plugs. It is based on using “air-gapped resonant transformer” technology, which has been around for more than a century but has gained traction in recent decades in vehicle technology as electric vehicles have become more widespread. This technology has already been proven in installations across the globe. In Turin, Italy, for instance, it has been in use since 2003, charging buses in the city’s fleet for less than 10 minutes at a time during layover stops on their route. It has also been implemented successfully by bus fleet owners in Utah, Washington, California, and Indiana.
Wireless chargers have the same capabilities as wired chargers. Importantly, wireless power transfer is capable of passing the same high-power levels as plug-in chargers or wired pantograph chargers. This is significant as the power-level for transit system applications can range anywhere from 50kW to 450kW, with even higher power levels coming in the future. Similarly, the efficiency of an inductive charging system is typically within 2% of the efficiency of a corded charger.
Longer routes, lower costs
A major concern for many agencies converting their fleets is maintaining existing routes so as not to impact the availability of service. Keeping routes stable during a transition to electric will help assure customers that service will not be impacted and encourage their continued use of transit. Agencies face a hurdle, however, in that many battery-electric buses cannot complete the same extended routes that their diesel counterparts can without refueling.
One solution — which also can reduce the cost of the transition — is en-route charging, where buses are charged throughout the day at strategic locations along their routes. This “opportunity charging” allows them to drive longer routes without returning to a depot, in turn reducing the need to buy extra buses. With en-route wireless power transfer, a transit agency has a clear path to replacing diesel buses with electric buses at a one-to-one ratio.
Maintaining the same fleet size means spending less for new vehicles, potentially avoiding new facilities, and minimizing deadhead routes of empty buses headed back to the depot to charge. En-route charging can also allow buses to use smaller, lighter, less expensive batteries that are frequently charged rather than large batteries that are more costly and take more space.
Wireless charging systems are well suited for en-route charging, able to quickly deliver a large amount of power, easy to use, and capable of standing up to the repeated use of thousands of buses and charges over time. Detailed modeling can identify which routes and vehicles are best suited for a transition to electric and can help map out a viable incremental transition plan.
These en-route chargers typically provide power faster than a corded depot charger. A bus can drive over a wireless power charger during a scheduled stop in their route, charge for a few minutes, and then continue. Buses with sufficient stops at en-route chargers may be able to forgo depot-based charging altogether, further reducing impacts to existing depot infrastructure.
Benefits and challenges
While wireless charging systems can theoretically be used in any number of configurations, transit installations so far have focused on in-pavement solutions. This means the technology used in wireless power transfer is shielded from the elements under the roadway, and it is unaffected by climate or weather, working in snowy, windy, or rainy conditions. In case of flooding, all the components are sealed. Without exposed mechanical parts or electrical contacts, it is a durable choice, particularly in the salt air of coastal environments or in areas with a lot of moisture such as fog or high humidity. With a low profile, these systems also work well in historical neighborhoods or other areas where maintaining existing visual appeal is a prime concern.
The same characteristics protect wireless chargers from vandalism or accidents. Because most of the infrastructure is installed below ground, collision damage from vehicles is avoided, including the buses themselves. Potential problems such as defacement or intentional damage are also minimized.
The technology can be simple for drivers once trained. Drivers park the bus in a designated area over the transmitter, charging begins automatically, and ends automatically when they drive away. But drivers will need to be trained how to park properly and in the right position, generally to within a tolerance of less than a foot.
As with any charging system, there are challenges as well as benefits. Placed underground, wireless systems are generally more expensive to install than competing options positioned above. Any needed maintenance can be complex, and lifting a unit out of the ground for replacement or repair will require heavy equipment.
A site kept clear of metal obstructions is also needed to ensure the magnetic field is not affected. Most charging systems have built-in foreign object detection capabilities that will interrupt charging to keep people and equipment safe, but they also mean something as simple as a sheet of foil on the ground could disrupt power passed to vehicles.
Starting with detailed analysis
The first step for fleet owners is to determine whether wireless power transfer fits their needs. A detailed exploration of electrification needs and charging solutions will help guide decisions and inform plans. These studies should examine the transition with a holistic, long-range view, looking at how to achieve the goal of transitioning an entire fleet.
Advanced modeling such as HDR’s Zero+ Fleet Optimization tool can combine detailed current system data, such as route alignments, service frequency, travel times, elevation, and more, then layer on data on charging infrastructure options, including cost and power needs. Resulting analyses show detailed power needs to help fleet owners manage charging schedules, determine optimum charging locations, and compare capital, as well as operating and maintenance costs of infrastructure.
In studies done for U.S. transit fleets, we have found that en-route charging, whether with wireless power or another option, makes a significant difference in the feasibility of a transition to an electric fleet. These models can be used to explore different options for charging infrastructure, comparing the financial feasibility of en-route charging versus depot-only charging. For example, on a recent project for a large metropolitan agency, we found that using depot-only charging would require a 24% increase in the number of vehicles and nearly double the non-revenue miles driven, compared to the current diesel fleet. En-route charging with technology such as wireless power transfer would significantly decrease these impacts and was necessary for creating a realistic path to electrification.
A solution to consider
Wireless power transfer is not the only choice for charging technology, and transit owners should weigh each alternative carefully, but wireless power is a valid, viable technology that holds several benefits and deserves consideration by transit owners transitioning to electric. Starting with a customized analysis of your system will lead to decisions driven by data instead of assumptions. Looking at every option can reveal advantages of using different methods, or even benefits of combining multiple methods. With a path to one-to-one replacement of diesel buses, wireless power can minimize the impact to facilities, schedules, and deadhead trips while also creating an aesthetically pleasing, user-friendly emissions-free system that is built to last.
Thomas Stout is Inductive Pavement Engineering Leader at HDR.