Comments URL: https://news.ycombinator.com/item?id=48005432

Points: 477

# Comments: 65

Comments URL: https://news.ycombinator.com/item?id=46723224

Points: 1

# Comments: 0

The idea is to use a photovoltaic cell (“solar cell”) to convert thermal radiation to electricity. A regular solar cell has limited efficiency because the sun has a wide spectrum and a single material is not efficient across the whole spectrum. With thermophotovoltaics, the hot surface is so close to the cell that you just reflect the “bad” photons back to the hot surface to recycle them instead of losing their energy.

In theory, a more efficient alternative to a traditional solar cell is to use the sunlight to heat a surface to ultra-high temperatures and then run a thermophotovoltaic cell on that hot surface, but this is easier said than done.

As an outsider I do think it looks like the competitor Antora Energy has a simpler approach: instead of pumping the heat using high-temperature liquid (with lots of moving parts), they just use thermal radiation to transfer the heat inside their battery.

You want to count particles per volume of air, so conventional sensors use a fan to have a constant volumetric flow and then count particles per second to infer particles per volume.

The way I interpret the above marketing language is that they use the optical sensor not only to count particles but also to measure the particle movement and infer airflow. So as long as there is some natural movement in the air, they can measure both particle count and volumetric flow, and thus infer particles per volume.

When I came across this amazing project and wanted to share it to HN, I was debating whether to post the youtube link or the project page. I decided to post the project page and mention the youtube link in the description for those who prefer video, but somehow that description got posted as a comment instead (not sure how that happened?). Anyway as you said the video is embedded in the project page so it wasn't really necessary

Comments URL: https://news.ycombinator.com/item?id=44661846

Points: 1073

# Comments: 182

Last year the presenter also wrote a blog post about the attack, which received some discussion at the time:

https://news.ycombinator.com/item?id=41608949

Comments URL: https://news.ycombinator.com/item?id=43586102

Points: 2

# Comments: 1

If I understand correctly, your two main benefits are broader spectrum and lack of PWM flicker. Did you measure the spectrum of the light from the prototype monitor? The light goes through several filters - first I assume the daylighting system has an IR filter to prevent overheating. Then it goes through the LCD itself, and the color filter array in front. Are you still left with a lot of IR (or the frequencies are considered beneficial) after all this?

Let's say you go to one of the illuminated areas that paid for reflectorbital-light and look up. What would you see? You would see a tiny bright spot flying past, with an angular size of about 10^-10 steradians [1].

This tiny spot has the same "brightness" (radiance) as the sun, because a mirror preserves radiance. However the mirror looks about 10 000 times smaller than the sun from your perspective (the sun has an angular size of about 10^-5 steradians). This means that the satellite would only give you 0.01% of the light compared to the real sun.

If you could somehow take 10k satellites and use them to illuminate the same spot, you could technically get it to resemble real sunlight. But imagine what this would look like: These satellite would need to be many enough / huge enough to cover a very significant portion of our sky, on the order of the apparent size our actual sun. They would be spread out in a sun-synchronous orbit, so they would be visible at dusk with this size, from all points on the earth. Would we really want that?

The founder has been thinking about using mirrors to collimate the sunlight to get around this problem, but it won't work. The collimator design he showed in a 2022 article [2] would decrease the focal spot from a 5km diameter to some smaller diameter as intended, but it would do so by throwing away light, not by increasing the brightness in this smaller spot. This is given by conservation of ètendue, one of the fundamental laws of nonimaging optics (where I do research).

[1] They are planning a 100sqm mirror at 600km altitude, which gives a solid angle of (100 m^2)/(600e3 m)^2

[2] https://www.vice.com/en/article/this-man-is-trying-to-put-mi...

Unfortunately, this is not how the size of the spot on the ground is decided. Sunlight, even if reflected by a perfectly shaped mirror, spreads by approx 1 meter every hundred meters. At the "edge of space" at 100km, your spot already has a 1km diameter, in reality with a higher orbit and imperfect mirror & tracking it will be much larger. The size of your (ideal) mirror decides the brightness of the spot, not the size of the spot.

Liquid mirrors in space seem like a cool concept though!

By the way, I’m happy to find someone with CSP knowledge on HN. Are you working in the field?

[1] https://doi.org/10.1364/OE.515422

I do research in a subfield of optics called nonimaging optics (optics for energy transfer, e.g. solar concentrators or lighting systems). We typically use these optical design applications, and your observations are absolutely correct. Make some optical design software that uses GPUs for raytracing, reverse-mode autodiff for optimization, sprinkle in some other modern techniques you may blow these older tools out of the water.

I am hoping to be able to get some projects going in this direction (feel free to reach out if anyone are interested).

PS: I help organize an academic conference my subfield of optics. We run a design competition this year [1,2]. Would be super cool if someone submits a design that they made by drawing inspiration from modern computer graphics tools (maybe using Mitsuba 3, by one of the authors of this book?), instead of using our classical applications in the field.

[1] https://news.ycombinator.com/item?id=42609892

[2] https://nonimaging-conference.org/competition-2025/upload/

I help run an academic conference on Nonimaging Optics (optics for optimal energy transfer in solar concentrators or illumination systems). This year, we are hosting a design challenge, and I thought it could be fun for some in the HN crowd to participate. Maybe some of you have ideas for completely different design approaches than what we typically see on the field.

The challenge: Design an efficient "solar concentrator" for a sun shaped like the letters "XX" (instead of our regular round sun). The challenge is set up so that the ideal geometry would be 100% efficient*. However, we don't know how to design such an ideal geometry, and the simulated mirrors are only 95% reflective so that all submissions will be <100%. Highest simulated efficiency wins.

I made it possible to test any design directly in the submission portal, so you can participate without buying expensive licenses for optical design software.

In general, if you want to make a difference to solar concentration or illumination optics, there are many low-hanging fruits in the field and a lot of room for crossover from the HN and CS crowd**.

Stack:

- Astro.js for the main static website (I've been away from frontend for about a decade, and was pleasantly surprised to see what can be done with a static site generator nowadays)

- SolidJS for the competition portal

- Three.js for visualization

- Three-mesh-bvh for raytracing and evaluating the submissions (currently a bit slow because I haven't moved it off the main render thread on CPU)

- OpenCascade.js for loading and meshing of uploaded step-files

* See https://news.ycombinator.com/item?id=41391715 for an explanation of why there is a limit to how strongly you can concentrate sunlight

** As an example, the commercial design tools in the field support optimization-based design, but they all do raytracing on the CPU and use finite differences instead of reverse mode autodiff. Therefore, the conventional wisdom is that optimizing systems beyond a few hundred parameters is hopeless.

Comments URL: https://news.ycombinator.com/item?id=42609892

Points: 10

# Comments: 0