We may have seen a wireless revolution in the last decade, but it has been all about wireless data. But when it comes to transmitting power, cables are still vital. That could soon change, however, with wireless power transmission on the agenda for a host of electronic products.
Wireless power is a dream almost as old the generation of electricity itself, going back to the early 20th Century, when Nikola Tesla planned to use huge coils to transmit electrical energy through the atmosphere and ground. In the 1960s, experiments saw mini helicopters powered wirelessly from the ground using microwaves and lasers are being used in a similar way today. 
But it is the wireless powering of small, portable electronic products that is seeing rapid advances and it looks certain that, within a few years, we will not have to remember to regularly plug them into wall sockets. Wireless power could become virtually ubiquitous. The wireless power market consists of three main categories, differentiated by range, power and the technology used.
First is zero range, low power, where induction is used to charge products like mobile phones. A newer version, exploiting resonance, offers somewhat extended range. The second, with a range of tens of feet, is based on the broadcasting of rf energy. And the third, transmission of power using lasers, can cater for a range of power and distance options. One company taking the laser based approach is California based PowerBeam, which fires laser beams at photovoltaic cells, which convert them back to electrical energy.
PowerBeam is involved with a major electronics manufacturing company that, by Christmas 2012, will be launching the first optical, wireless power consumer product. "Initially, both transmitter and receiver will be in a fixed location – a typical product would be a wall mounted led lamp, consuming 4 to 8W," says David Graham, PowerBeam's ceo and cofounder.
An important advance that PowerBeam is exploiting for its optical wireless power systems is safety. "A decade ago, if you had suggested providing wireless power through a laser and photovoltaic receiver combination, people would have said it's possible, but it's not safe. The lasers that were available then were not eye safe. Since then, longer wavelength lasers have become available, making them thermal, rather than optical, devices.
We realised that, by combining such thermal lasers with some customised safety electronics, it is possible to transfer useable amounts of power with reasonable efficiency. "Take a standard optical laser like a red laser pointer. The safe exposure to that light is considered to be about 10W/m2. Operating where we are, in the thermal wavelengths, it is about 1000W/m2.
That is what makes laser based wireless power feasible." PowerBeam will initially target those products consuming less than 10W, but as Graham says, today that power level includes many different product areas. "There has been a remarkably consistent move towards this kind of power requirement. For example, in illumination, incandescent bulbs have been replaced by leds, which are 20 times more efficient.
In audio, amplifiers are far more efficient. In displays, lcds that were backlit by a fluorescent tube are being replaced by leds and soon organic leds. This trend makes the value of what we are doing better and better." Going down the rf broadcasting route for wireless power is Powercast, which is providing low levels of power using the unlicensed 915MHz ISM band, similar to Europe's 868MHz.
There are several potential applications. One is charging batteries over distance, another is to power battery free systems, typically duty cycled devices where activity only occurs periodically. Large sensor networks are a key target market, because Powercast's rf wireless power makes it feasible to create networks with thousands of sensors.
Benefits of this kind of wireless power include cutting operating costs by eliminating wiring and the expense involved in changing batteries. It also helps make systems more reliable because they can be sealed, and potential user damage is eliminated. Placement flexibility is increased and charging becomes automatic, taking place as soon as the device is in range.
Powercast has two commercially available components: its Powerharvester receivers, designed for use by OEMs; and Powercaster transmitters, which send rf power. "Our rf technique is inherently a one-to-many technology, with one transmitter powering multiple receiving devices," says Harry Ostaffe, Powercast's vp of marketing and business development. "We put ourselves into two categories: one being delivery of wireless power; the other is micropower energy harvesting. The latter would compete with energy harvesting techniques like solar, thermal and vibrational. Micropower typically means less than 1W, usually milliwatts or microwatts."
In terms of range, Powercast's systems today reach to around 15m, but the company has just released a new wireless sensor product that has a range of 20m. "Our goal is to increase range by up to four times with every generation of component. It's challenging because, as the distance grows, the available power drops off with the inverse square – you get a quarter of the power at double the distance."
At the heart of Powercast's innovation is the rf sensitivity of the conversion technology itself, which transforms the rf signal into dc. "We have developed an rf to dc technology that works over a fairly wide frequency band and a wide range of input powers, and the secret to what we have done lies in the topology of how we convert the rf into dc, which enables us to maintain the efficiency of the conversion over a wide operating range. Other approaches may achieve high efficiency but it is over a narrow range of operating conditions. The benefit is that someone can take one standard component and design it into a lot of different applications, whether it is high or low power."
Ambient energy harvesting
Powercast's other approach to wireless power is ambient energy harvesting, in which battery powered portable products like phones and notebooks could supply wireless power to other devices near them.
"We have demonstrated powering the sensor featured in our development kit from an iPhone," Ostaffe says. "A lot of phones today have Wi-Fi and the 2G mode is in the high 800MHz band. That could easily be shifted with an app to send out power at 915MHz, for example, or 868MHz."
Magnetic induction is arguably the most commercially advanced of the wireless power technologies and is being used in objects like electric toothbrushes and some mobile phones, with more devices planned. Products like Powermat (from the firm of the same name), and eCoupled, from Fulton Innovation, prove it works.
Recently, Fulton demonstrated a wireless powered kitchen blender, as well as packaging that can light up and flash on a shelf without batteries. Fulton is a founder member of the Wireless Power Consortium, which has developed the Qi standard for interoperability of wireless devices powered through induction. Induction exploits the fact that a fluctuating magnetic field from a coil can induce an electric current in another coil placed very nearby.
It works well with close contact, but there is a drawback – efficiency can fall to zero at even a few millimetres from the transmitter. One way to overcome this is to exploit resonance. In a system developed at MIT, an inducting coil is connected to a capacitor and the energy within this circuit oscillates rapidly between an electric field in the capacitor and a magnetic field in the coil.
If the oscillation frequency in the transmitter's circuit is different from the receiver's, they are non resonant, limiting the build up of energy inside the receiver. But if they are made resonant, the oscillating fields of the two coils would be synchronised, increasing the energy transferred. By exploiting resonance, the MIT team transmitted 60W across 2m with an efficiency of 40%. The idea is being developed by a company called WiTricity, which has powered a 50W tv located 0.5m from the power supply, with 70% efficiency.
In some cases, the improvement in the efficiency due to resonance can be more than 100,000 times that of non resonant induction, MIT says. And, unlike laser based line of sight energy transmission, a magnetic field is not focused and so can pass around or through obstacles between the transmitter and receiver. A similar approach is being developed by Qualcomm, which calls it near field magnetic resonance.
Mark Hunsicker, senior director of product management for Qualcomm's wireless power solutions, explains. "Our technology provides a solution that allows a freedom of placement and design, in that we can power through surfaces. The technology can be used to provide hundreds of watts, if not more, but for our first commercial offerings we are focusing on 20W or less – devices such as tablet computers, smartphones or remote controlled devices.
"It permits a varying antenna size, which means our target devices can be of various sizes, creating a zone based charging approach that could power phones, a tablet, or Bluetooth headsets. Alternative approaches require direct contact or to have a maximum of a few millimetres between transmitter and receiver. We can charge over distances of 50 or 60mm and antennae can be embedded in desktops, furniture, vehicles, with no requirement to position devices in any particular way.
"Qualcomm moved into wireless power because we anticipated the smartphone market would need not only low power semiconductors, but also much better access to charging – making it ubiquitous and natural. We felt it was important users do not have to do something special to charge their devices. They can just drop them on the charging mat, or get in the car and drop them in the trinket tray and they can move around, there is no need to fix them rigidly.
"Initially, when we looked at the market, we felt you would have multiple receivers per charging transmitter. Now, we are thinking of the converse; multiple chargers per receiver – your phone gets charged in the bedroom, the bathroom, the car, the office, the conference room and so on."
As Powerbeam's Graham says, power is now going down the same wireless path that data has already trodden. "A lot of the technology base that has gone into data communication is going into transferring power. For example, the class of lasers that we use was used initially for pumping optical amplifiers so a signal could be transmitted over a great distance. Power is about 25 years behind signals. Clearly, we will obviously continue using wires for heavy power loads. But elsewhere, when a wire becomes a constraint, it will go away," he concluded.
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