“Optical rectennas” (or sometimes “nantennas”) are a technology that is sometimes advertised as a path towards converting solar energy to electricity with higher efficiency than normal solar cells. I looked into them extensively as a postdoc a decade ago, wound up concluding that they were extremely unpromising, and moved on to other things. Every year or two since then, I run into someone who is very enthusiastic about the potential of optical rectennas, and I try to talk them out of it. After this happened yet again yesterday, I figured I'd share my spiel publicly!

(For some relevant background context, check out my write-ups on the fundamental efficiency limit of single-junction solar cells, and on the thermodynamic efficiency limit of any solar energy conversion technology whatsoever.)

1. What is a rectenna?

Rectenna is short for “rectifying antenna”, i.e. a combination of an antenna (a thing that can transfer electromagnetic [...]

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Outline:

(00:58) 1. What is a rectenna?

(02:07) 2. If RF rectennas can turn RF electromagnetic waves into electrical energy, why can't optical rectennas turn sunlight into electrical energy?

(02:52) 3. The easy problem: antennas

(03:48) 4. The hard problem: diodes

(06:31) 5. But what if we combine the power collected by many antennas into a single waveguide, to increase the voltage?

(06:58) 6. But what if we track the sun?

(08:03) 7. But what if we track the sun virtually, with a phased array?

(08:15) 8. But what if we use an impedance converter?

(09:17) 9. But what if ... something else?

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First published:
September 11th, 2025

Source:
https://www.lesswrong.com/posts/gKCavz3FqA6GFoEZ6/optical-rectennas-are-not-a-promising-clean-energy

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Narrated by TYPE III AUDIO.

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Images from the article:

Just like in Calculus 101, if you take a curve and zoom into a very narrow range of x-values, it looks like a line. Hence, if you take a device which functions as a diode for ±1V signals (left), that same device probably functions as a resistor for ±1mV signals (right).

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“Optical rectennas are not a promising clean energy technology” by Steven Byrnes

LessWrong (30+ Karma)