Wireless energy

Matthew Tennie and I led a discussion in Sustainable Energy class last week focussing on wireless power. The concept was originally conceived by Tesla, but was halted by the FBI. It has been poked at a number of times since then but a solid technique was never fully realized until it got picked up by a team of theoretical physicists at MIT, lead by Dr. Sljacic. Since their successful development of the technique, a handful of initiatives and ventures have commenced, giving this technology quite the explosive re-birth. Here are the resources Nelson and I used to lead the class conversation.

Thanks to our Prof. Jeff Feddersen for his insight and the comments and discussion generated from the class members (especially Michael Colombo who brought up a number of great points).

Here are the links presented in class, in order in which the discussion was made:

Eric Giler expo of the subject at TED talks

MIT press release on the subject

A very attractive installation regarding wireless energy and overcoming gravity.

Commercial products that are putting the technology “put there”

A Wired link

Another example, a power mat.

Energizers’ presentation which led to the more serious information, regardless of how they present the “cool” and “hip” novelty.

A more serious energizer.

The World Wireless Power Consortium. The worlds organization in charge of standardizing the technology so it can be mass produced. (and consumed)

How it really works

Info on the energy consumption.

A good use for this technology

While it seems very nice to get rid of cables, this technology is not offering a more sensitive use or consumption of energy. It would see that it will make possible to use more energy in more places where it was not possible before (like computing power and internet connectivity, so this would lead into…more energy consumption over all….is that what we “need” in the long run? At this point it is introduced as another COOL thing to buy and plug into the wall. Not a smart thing, really.

Although the idea of materials and building techniques improvements to make the consumption smarter one with the help of this devices is a hopeful one. But then again, it might lead to more consumption overall, just because it is possible to use the energy where it couldn’t be used before.

Never the less, the good applications such as the medical ones, are a big compensator for further developing of this idea. A non invasive procedure to power up or recharge surgically inserted devices in a biological body, which can help relieve or cure diseases or save lives, does give me a good counter weight towards its development.

Some other notes discussed during class:

To understand the effect, it can be compared to mechanical resonances.
Consider a string tuned to a certain tone as mechanical resonator. Even a far away and low level sound generator can excite the string to vibration, if the tone pitch is matched.


electronic products with wireless charging capability are anticipated to increase from 3.6 million units in 2010, to 234.9 million units in 2014.


An estimate of power consumption by wireless chargers.
Let’s first look at the power consumption of a classic mobile phone charger. These chargers are simple so-called “external power adapters”. A good source for data is the ENERGY STAR website. Here you will see that Energy Start compliant AC-DC adapters typically rate:
• Efficiency @ max load: 72% on average for 5 Watt adaptors
• Power consumption @ no load: 0.12W on average for 5 Watt adapters with a few exceptionally good adapters going down to 0.01 W.
Suppose that you use the adapter for 1 hour per day, and that it remains plugged in for the rest of the day. That is not a good practice, but it is quite common to leave power adapters and cradles continuously connected to the mains.
You see that the total energy consumption is:

• charging: 1 hour * 2 W / 72% = 2.8 Wh (this assumes that 5 W charger will supply, on average, 2 W during a complete charging cycle)
• standby (no load): 23 hours * 0.12 W = 2.8 Wh
You see that standby power contributes significantly to the total energy consumption of a mobile phone charger.
Our wireless chargers also contain an AC-DC power adapter. Let’s assume that is has the same efficiency (72%). Let’s also assumes that it has the same standby power (0.12 W). [footnote: Wireless chargers can have a much lower standby power, but this keeps the comparison easier.] The transfer efficiency of the wireless power link is typically 70%. And assume that the wireless charger replaces 2 wired chargers. The total energy consumption is:
• charging: 1 hours * 4 W / 72% / 70% = 7.9 Wh (we are now charging 2 devices simultaneously)
• standby (no load): 23 hours * 0.12 W = 2.8 Wh

Total power consumption of two wired chargers: 2 * ( 2.8 + 2.8 ) = 11.2 Wh
Total power consumption of one wireless charger with two receivers: 7.9 + 2.8 = 10.7 Wh
You see that the total energy consumption is comparable. Although wireless transfer is obviously not as efficient as transport over a copper wire, wireless power transmitters saves standby power energy when the wireless transmitter replaces multiple external power adapters.

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