Vol. 5 Num 1046 Sat. May 12, 2007  

Wireless Power

Hassle free transmission

Wow! At last the power transmission goes wireless. Yes, it is not a rumour but an astonishing fact that will embellish the existing power distribution technology in your premise. Basically, this technology will give you freedom from wires and at the same time ensures safe power transmission.

I believe most of us are familiar with the terms alternative current (AC) and direct current (DC). AC changes direction over the time or its polarity changes. On the other hand DC does not change its polarity.

AC is suitable for long distance power transmission and for this reason in our households we get alternating current from power outlets. But all most all devices require direct current to conduct their functionalities. As a result, we need to convert the AC power into DC.

Naturally, when we need to run any device we connect its plug to the wall outlet. The device receives alternating current and a special arrangement inside the device converts the AC into DC. But the scenario is slightly different in case of wireless power technology.

Wireless power technology is based on the concept of a transformer. In a transformer current induce from one coil to another through magnetic field. When electrical current passes a wire it creates circular magnetic field around the wire. Bending the wire into coil amplifies the magnetic field. As the loop increases it will create bigger magnetic field. If a second coil of wire is placed in the magnetic field, the field can induce a current in the wire.

Nicola Tesla unveiled the idea of wireless power transmission in the late 1800s and early 1900s. The innovation was exciting but it did not initiated wide spread implementation of this technology.

The recent instance of wireless power transmission is electric toothbrush. This device follows the method of transformer to recharge the battery. The charger unit draw the electric current from the wall outlet and generates magnetic field. In a transformer, this coil is called the primary winding.

When the toothbrush is installed on its charger unit, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery. In this way the current recharges the battery.

Nowadays there are several devices available in the market that are based on the same principle but recharge more devices at once. For example, the Splash Power recharging mat and Edison Electric's Power Desk both use coils to create magnetic field. Electronic devices use corresponding built-in or plug-in receivers to recharge while resting on the mat. These receivers contain compatible coils and the circuitry necessary to deliver electricity to the devices' batteries.

At this moment, distance limitation is the major drawback of wireless power technology. For this reason, device and its charger unit maintained a minimum distance to induce a current, which can only happen if the coils are close together.

But researchers at Massachusetts Institute of Technology (MIT) subvert this setback and came out with an updated solution. In November 2006, the team, led by Marin Soljacic, theorised that by considering resonance it is possible to transmit power between coils separated by a few meters.

Resonance is a tendency of a system that usually means that, the system oscillate at maximum amplitude at a certain frequency. This frequency refers to the system's own resonant frequency. Resonant frequency is approximately equal to natural frequency of the system in the case of small dumping and it is the frequency of free vibrations.

If two coils resonate at the same frequency and located within a few meters of each other, the power moves from the transmitting coil to the receiving coil. According to the theory, one coil will be able to send energy to several receiving coils, as long as they all resonate at the same frequency. This kind of set up could power or recharge all the devices in a single room.

Beyond home accessories wireless power technology has also been employed to supply electricity to the aircraft. The unmanned plane, called the Stationary High Altitude Relay Platform (SHARP) is such a lucid example. Canada's Communications Research Centre created this small aircraft in 1980s and this plane could fly in circles two kilometres in diameter at an altitude of about 13 miles (21 kilometres).

This plane is powered from the ground and a massive microwave transmitter controls all functions. The SHARP's circular flight path kept it in range of this transmitter. Consequently, a disc-shaped on board antenna known as rectifying antenna located just behind the plane's wings and it receives microwave signal and converts it into direct current.

Another implementation of microwave based wireless power transmission is transmitting electricity to earth from solar power stations on the moon. Special antennas would capture the energy beam from moon and converts it into electricity.

Till now the idea of power stations on the moon has not come into practice. But it is apparent to us that, in near future we must discover the alternative source of energy to meet the uprising demands of fast growing population. In this regard, wireless power technology could emerge as a potential alternative for us.