Thursday, October 18, 2012
As a concept, the wireless transmission of energy is not new - Nikola Tesla, the restless genius inventor of AC electricity, devoted a great deal of time and money to establish its efficacy in practice from the late years of the 19th century, though his efforts proved fruitless and funding ran out for the project. The difficulties with wireless power transfer lie in ensuring the efficiency of the energy transmission - quite a different proposition than that involved in information transfer where one only needs the signal to be strong enough to be differentiated from background 'clutter'.
There are several methods by which power can be transmitted wirelessly but they all have limitations depending on the methods used. Electrodynamic induction can only be used over very short distances as it is extremely inefficient - it derives power from the creation of a magnetic field, so the farther away the receiver, the smaller the proportion of the field that it receives. Electrostatic induction was demonstrated by the aforementioned Tesla, who used his principle of AC electricity, alternating rapidly, to create an electric gradient between two plates. Light bulbs between the plates would light up with no attached power source. This was not Tesla's preferred method of achieving this aim.
The main thrust that Tesla made at the problem was by means of current flow, through the Earth and the atmosphere, between two Tesla coils. The transmitter and receiver would need to generate something in the order of 15 million volts to break down the atmosphere and allow the power to flow up up to the conductive troposphere where it could flow around the globe to the receiver. As Tesla put it "You will then see something like an aurora borealis across the sky, and the energy will go to the distant place".
The microwave method of transferring power wirelessly has been demonstrated many times, in some cases delivering 95% efficiency of transfer and has been shown to work over distances of a kilometre or so. Because it can be directed, it has been mooted as a way of powering spacecraft but given the estimated size of the transmitters (1 km) and receivers (10 km) involved, it is not currently practicable. For transfer of power terrestrially this method could theoretically deliver power over an area 10 km in diameter whilst remaining not harmful to humans - this 'not harmful to humans' aspect is quite important and dependent on energy density.
Transfer by laser is a more readily understood prospect - fire the laser at a distant receiver which then converts the laser beam into power. It requires line of sight to be established, and the potential for mishaps is greater due to the need for very tight targeting over distance. This method has been used already to provide a kilowatt of power over several hundred metres.
Since a completely wireless LCD television has already been demonstrated, we can only hope that wireless power transfer is not too far away from being a staple of our domestic lives.