It is not difficult to transmit power through the air. Tesla famously demoed it in 1891, using magnetic induction technology. NASA demoed long distance transport with microwaves. And don’t forget solar power. That is wireless too!
It is pretty obvious why we are not using microwaves to transmit power through the air at home. But why did Tesla’s technology never make it into our homes?
There is a fundamental problem with magnetic induction – the efficiency drops dramatically if the distance through the air is larger than the coil diameter. There are also a couple of practical problems. Getting low standby power is one of those practical issues.
The fundamental efficiency limitation can be seen in the somewhat complicated figure 2 in this article on the transfer efficiency of magnetic induction. What it boils down to is simple:
- Efficiency is reasonable (more than 30%) when the distance between the transmitter and the receiver is less than the diameter of the coils.
- Efficiency drops below 2% if distance is more than two times the diameter of the coils
- It gets worse when the transmitter and receiver coil have different diameters, or when they are not properly aligned.
Look carefully at the distance between the two coils when you see a demo of power transfer through the air. Here is a recent demo by Intel. And another one by WiTricity. The distance between the coils is about the same as the diameter of the coils. That’s not a coincidence. The demo would not work if you move the coils further apart!
This means that, if you want to charge your mobile phone with reasonable efficiency, the phone cannot be more than 5-10 centimeter away from the transmitter. That is not enough to be useful in practice.
The practical issue of low standby power is another reason why ‘power through the air’ does not take off. This is the problem: how can the power transmitter know that your phone needs charging? Your phone’s battery is empty – it is dead and not talking to the transmitter. The only way for the transmitter to know about your phone is to be ‘always on’ and searching for resonating coils in its surroundings. This search need not cost enormous amounts of energy, but it is significantly more than the standby power of a classic wired power adapter.
And I have not even discussed issues with the biological impact on the users. Here is a pointer to information about meeting regulatory EMF limitations. (These regulations limit human exposure to alternating magnetic fields).
For me, the right method to transport power trough the air is with sunlight and wind. Sunlight works great if the solar panel is large enough. But when you integrate the panel in the phone you get 5 minutes talk time for each hour of charging. That is not practical, unfortunately. I would prefer to get 60 minutes talk time with 5 minutes charging. And my phone is usually in my pocket. Not baking in the sun.
