## Harvesting energy from the airwaves

Antennae are the most fundamental energy harvesting devices that we know, says Sung Nae Cho at the Samsung Advanced Institute of Technology in south Korea. So why aren’t they more widely used?

Turns out that helical antennae are already used to harvest energy and most of us probably own one already in the form of a transformers. These contain a helical winding that rectifies  AC into DC.

Cho points out that it has recently become possible to build nanohelices and that these might also be used for rectification. He’s designed a device that rectifies, not current, but electromagnetic waves. It consists of a nanohelix layer, a diode  layer and a capacitor layer, all the components of a standard rectifying circuit.

The nanohelix layer consists of an array of  100 million “pixels” which each contain a single nanohelix. That makes the array no bigger than the imaging chips in digital cameras . Cho calculates that if only 10 per cent of the nanohelices harvest energy from ambient electromagnetic waves to the tune of 130 nA, then the device would produce 1.3A.

If he’s right, that’s a handy amount by any standards. Anybody volunteer to prove him right.

Ref: arxiv.org/abs/0901.0769: Energy Harvesting by Utilization of Nanohelices

### 3 Responses to “Harvesting energy from the airwaves”

1. Escape Goat says:

Your text is a little confusing there; unless I’m misunderstanding something, you are conflating the induction coupling that trades off between current and voltage with the diode bridge that rectifies the AC current into a single-polarity current, essentially |sin ϴt|.

(the following should be viewed in a monospace font, and may or may not appear thusly, depending on the acceptance of html markup by the comment box:)
``` Passive AC/DC rectifier. D1-D4 are power diodes; C1 is a large capacitor to smooth out the 'bounce' in the DC current.```

``` 8||6-----o--->|---o------o---o 8||8 | D1 | | 8||8 — — ——— C1 8||8 D2 ^ D3 ^ ——— 8||8 | D4 | | 8||2-----o---|<---o------o---o ```

2. D’oh. This blog entry isn’t impressive. First of all, it’s wrong: transformers don’t rectify AC to DC. And, the distinction that the blog writer makes between “current” and “electromagnetic waves” doesn’t give me confidence. It looks like arxivblog is saving money by farming out the blog entries to non-physicists.

The paper behind the blog entry isn’t as solid as one would like either. It basically describes a variety of solar cell, but the math doesn’t really consider the quantum aspects of rectification at optical frequencies. Essentially, silicon diodes only act like diodes at frequencies where the photon energy is much less than the bandgap. That’s not the case here.

At higher energies, silicon diodes don’t act as a rectifier so much as (drum roll) a solar cell.

So, Sung Nae Cho wants to put helical whiskers on a solar cell. Will that help it work better? I don’t know, and I don’t think the paper really helps answer the question.

3. Sung Nae Cho says:

First off, nowhere in the paper claims transformer alone rectifies AC to DC. Transformer is part of rectifying circuit.

Secondly, this work is not about diodes. The conductivity of nanomaterial can be either measured or theoretically treated using quantum theory. (1) In this work, as it is not on computing conductivity, it is assumed to be measured. (2) Also, the quantization of electromagnetic fields is not needed, as the field here are assumed to be very large, i.e., very many photons. When the above two are satisfied, classical Maxwell equations describe all aspects of electromagnetic theory – at least from engineering point of view.

Lastly, to rectify optical electromagnetic waves, a diode that can handle 100THz or more of frequencies is needed. Such diodes are being developed and one should expect one within 5 yrs from now. But, then, this article is not about designing diodes. And, there are diodes which are fast enough to handle microwaves today. The proposed energy harvesting device can be used to harvest radio to microwave part of EM waves for now.

Hope this clarifies some aspects of the work.

Sung N. Cho,
Feb. 10, 2009.