How Far Can We Take Wireless Power Transfer Technology?

Originally posted in EE Time Europe by Lily Young

We’re becoming increasingly used to delivering energy through a wireless interface in applications as varied as earbud chargers and induction hobs, but how far can this technology take us? Will we ever be able to charge a car without plugging in a cable, or take grid power to remote places without planting pylons and digging ditches?

There are two main forms of wireless power delivery. The first involves closely coupling a transmitter and a receiver by forming an electric or magnetic field between them, and then using it to transfer energy from one to the other. Some closely coupled power-transfer schemes use an electric field to couple two electrodes. Many more, such as induction hobs, electric toothbrushes, and wireless phone chargers, create an electromagnetic field in the transmitter and then use that field to induce an electric current in a nearby receiver, which can then take action – like charging a battery.

The second major approach is radiative coupling, which involves directing a beam of energy, often in the form of high-frequency radio waves, to a receiver that is highly tuned to capture as much of that energy as possible.

Alignment between the sender and the receiver is very important for efficient energy transfer. (Image: Molex Ventures)

Each approach has advantages and limitations, with the efficiency of energy transfer and the transmission range being two of the most important measures of merit. For closely coupled schemes, alignment between the sender and the receiver is very important for efficient energy transfer. If you’ve used an induction hob, you’ll know this instinctively, as a pan stops heating immediately when it is moved from the marked center of its ring. You may have also noticed the large numbers of magnets that are embedded into the back of the latest smartphones to ensure that the wireless charging puck is perfectly aligned with the phone’s receiver coil. With charging times being so vital to the perceived utility of new mobile phones, ensuring that the wireless charging is as effective as possible is well worth the engineering effort and per-unit manufacturing costs.

We see similar challenges at a much larger scale with efforts to develop standards for wireless charging of electric vehicles. A recent survey of automotive companies conducted by Molex revealed that 36 percent of respondents believe that, by 2030, wireless charging will be a standard feature. In cell phones, charging rates are measured in tens of watts. Electric vehicles (EVs), though, need charging rates of 50KW to 250KW to become practical alternatives to internal combustion engine vehicles for long journeys. Getting the alignment right between the transmission coil on the ground and the pickup coil under the car is going to be very important. After all, transfer losses of a few percent due to poor alignment could mean hundreds of watts of power being dissipated as useless heat in the interface between the charger’s transmission coils and the vehicle’s receiver.

A concept for electric vehicle wireless charging lanes? (Image: Molex Ventures)

SAE International has already published a standard (J2954_202010) to tackle many of the issues surrounding wireless vehicle charging. It establishes criteria for the interoperability, electromagnetic compatibility, EMF, performance, safety, and testing of wireless power transfer systems for use in light-duty plug-in EVs. The specification is meant for use in stationary charging applications, although dynamic applications may be considered in the future. In its current form, it is limited to above-ground charging pads and does not cover flush-mounted installations.

The SAE J2954 standard also defines an approach to alignment that will help drivers line up their vehicles with the charging pad, to ensure efficient energy transfer, as well as providing the infrastructure for cars to do this autonomously in the future. But it’s going to take good engineering and a lot of user discipline to ensure that wireless charging is as easy and quick as it needs to be to displace the routine user behavior of simply plugging the car in as if it were at a gas pump.

Wireless charging for cell phones in cars is perhaps the best illustration to date of the uncertain promise of wireless power transfer today. Bottom line: It only works when the phone is placed in a particular spot to ensure strong alignment between transmitter and receiver coils.

Various forms of charging pads . Left to right: wireless charging rack, wireless charging pad, and wireless charging cupholder (Image: Molex Ventures)

Cell phone users aren’t that forgiving, which is why the newest smartphones have such strong magnets behind their casing to make alignment a snap. But this kind of wireless charging is still a partially tethered experience – you have to go to where the charging pad is situated. A better user experience would involve being able to charge a device anywhere in a specified volume, without the need for close coupling and precise alignment to a charging coil. A Molex Ventures funded startup called Ossia does exactly this, using a strategy somewhat like the MIMO antenna arrays used in advanced WiFi and 5G systems to enable energy to be beamed to a device even when it is not in the line of sight of the transmitter.

In Ossia’s approach, a power transmitter sends out a regular signal from its antenna to synchronize it with any compatible devices nearby. Each receiver then sends back a beacon signal that announces its presence and its power needs. The power transmitter measures the phase of each beacon signal and uses this to work out the direction in which it should send power for most efficient energy transfer.

This approach works with a single-antenna transmitter, but power transmitters with multiple antennas can measure the slightly different phases of the beacon signal arriving at each of them, to establish the most efficient transmission path more accurately. The power transmitter can then adjust the phase and power output of each of its antennas to steer a coherent beam of energy to the receiver. And this path does not have to be in line of sight – if the beacon signal sent by a power receiver bounced off a wall on the way to the transmitter, the power transmitter will direct its beam back along the same path.

A device enabled with the Ossia Cota power receiver sends a beacon signal to locate the Cota power transmitter which then delivers power wirelessly via the same path. (Image: Molex Ventures)

The transmitter can also support multiple devices within a volume. Each receiver within that volume measures how much power it needs and sends this information to the transmitter as a request. The transmitter then compares all the requests from the receivers it is serving and allocates pulses of wireless power to each of the receivers according to their needs.

The promise of this approach, the company argues, is that once energy can be delivered in this way, all sorts of assumptions about how devices are powered in the environment can be rethought. The paradigm changes from wireless charging to wireless power delivery. For example, ceiling smoke alarms would never need a new battery and robot vacuum cleaners would do their duty without needing to return to a bulky docking station.

Cell phones have taught us that we can access anything from a handheld device, a facility that is only limited by access to bandwidth and suitable levels of battery charge. Wireless energy transfer seems like a useful way to avoid plugging in a phone or a car – but in the end, it is still tethered to the charger’s location. If it becomes practical to power devices wirelessly wherever they are within a defined volume, we may see our opportunities and behaviors change just as they did when we moved from landlines to smartphones.

Ultimately, the change will come with developments in the supporting technologies – such as sensors to assist with alignment issues, or thermal management solutions to facilitate high-power charging. This requires expertise that offers a path to an increasingly connected yet untethered world.