Comment by gene-h | Hacker News Reader

Comment by gene-h | Hacker News Reader

One of the biggest issues with space based solar power(SBSP) is RF interference. In order for SBSP to be used for baseload power, you need to be able to beam power through clouds which means transmitting in the sub-10 GHz range.

Unfortunately, many of these frequencies are used by our tech. 2.45 GHz is one of the best frequencies to transmit at, however, this now used by wifi and bluetooth. According to this paper[0](see figure 55) it is reasonable to expect degradation of 2.45 GHz communication thousands of kilometers from the transmitter. This is due to the fact that microwaves spread out. [0] also points out that harmonics of the transmit frequency, which can't be ignored because SBSP transmits gigawatts of power, will interfere with licensed satellite transmission bands.

In short, frequency really needs to be allocated, otherwise SBSP may really be illegal. There are some good options, at least in the US, in the sub-10 GHz range used by few people.

[0]https://ieeexplore.ieee.org/document/9318744

> Unfortunately, many of these frequencies are used by our tech.

So what? Microwave frequencies aren't entirely line-of-sight, but they aren't going to diffuse very far off-target. Unless you have houses in between elements of the receiving antenna array, the disruption of comms in the immediate area isn't going to actually affect anybody. Perhaps there's some risk to jets that choose to fly through the energy beam.

Yeah, but that directionality is bumping up against the vastly different levels of power: you don't just need a little attentuation between your target and the wifi, you need something like 12 orders of magnitude attenuation. That's not so easy over long distances. See the paper OP cited which does actually do the math instead of handwaving.

The transmitter would be located in geostationary orbit ~35,000 km away from the receiver, so spread due to diffraction becomes significant. With 6.7 GW of transmitted power at 2.45 GHz, ~790 MW ends up in the side lobes[0]. So it is reasonable expect RF interference not only near the receiver, but thousands of kilometers away from it. Figure 55 in [0] shows a plot of incident power with distance from the receiver. As this spread is due to diffraction, the only way to lessen it using the same frequency is to build the transmitter larger. In the case considered here, the transmit aperture was already 1 km.

[0]https://ieeexplore.ieee.org/document/9318744

From the article:

> Phased array of transmitters focuses gigawatt-power beam anywhere on Earth in line of sight.

I wonder if it's possible to steer the power beam around clouds, so you transmit power to whatever ground-station has the best view. (Unclear if it makes economical sense to overbuild ground-stations just to be able to pick one with a clear view of the sky)

Beam steering is trivial, but unnecessary. You can pick a frequency that goes through clouds, and you're wasting one of the few real benefits over normal PV if you don't.

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