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Cool work! I do research in nonimaging optics, the optics of achieving high concentration ratios (or wide tolerances to errors) in solar concentrators. I like

Cool work! I do research in nonimaging optics, the optics of achieving high concentration ratios (or wide tolerances to errors) in solar concentrators. I like that you are implementing closed-loop control. This is all the rage also in large-scale heliostat fields. Most traditional heliostats are controlled using open loop, which places very strict requirements on both the mechanical structure, the actuators, and on the kinematic model, leading to expensive and very stiff heliostats. People are therefore moving towards cheaper heliostats where the tracking precision is achieved through closed-loop control. Implementing closed-loop control is a little bit more tricky when you have overlapping focal spots from thousands of mirrors, but there are approaches that are being developed, e.g. having cameras around the target looking back out over the heliostat field (developed by Heliogen among others). You mention the challenge of light only being focused for a few hours per day. This is also a problem with large helisotat fields, and is also a field of active research. There's a group at University of Arizona with Professor Roger Angel developing heliostats that actively deform through the day to keep the perfect shape, and there's also an Australian company (Heliosystems) building heliostats that passively deform from gravity to keep as correct shape as possible. When you are only using a single heliostat, as in your project, you could also consider building it as a Scheffler reflector - placing it on a single-axis polar-aligned tracking axis that passes through your target. Then it only requires single-axis tracking through the day, with some (possibly manual) seasonal adjusting. I am very happy to see that you are highlighting the inherent risks in concentrated sunlight. There are lots of stories about people accidentally settings stuff on fire if the tracking doen't track correctly.

remipch 2y ago
Thank you very much for your kind and detailed reply! Indeed, the closed-loop control was the initial idea which convinced me that it would be possible to build the mechanical parts by hand with common tools. In other words, the software "smartness" compensates for the mechanical "ugliness". Another initial idea was to do multi-panels (several orientable panels) with a single camera looking at the target. Indeed, it's not easy, so I finally went back and decided to finish and release something with a single panel. Nevertheless, I have some ideas to do multi-panels with a few more cameras. I would like to work on them in the near future. Thank you for all the references, I will spend time to explore them. There is also a company that uses vacuum to adjust the mirror shape, I'll try to find it and post it here. I wanted to emphasize the inherent risk because my project is not a finished product, but a work-in-progress/proof-of-concept.
- hakonjdjohnsen 2y ago
  Yeah exactly! I like your cable-drive concept by the way. Did you describe it in more details anywhere? Heliogen were also developing a cable-drive system for their commercial heliostats, but I don't know if they are still working on it. How are you getting the right mirror orientation for each mirror (aka canting)? Custom spacers for each mirror? One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. When they look back at the mirrors, they will see the circumsolar radiation (how the sky gets brighter as you get closer to the sun). By comparing the brightness of the sky at different cameras, you can estimate which cameras is "closest" to seeing the real sun, and get an estimate for the real position of the sun.
  - remipch 2y ago
    > I like your cable-drive concept by the way. Did you describe it in more details anywhere? I didn't take the time to describe the cable-bot concept in detail. I modeled almost all the mechanical parts with OpenSCAD, but I struggled to model the cable itself. In the "mechanics" README [0] you can click on any image to view it in an online 3D viewer. The following note in the same README tries to explain how the cable is used : Each cable is actually wrapped around the motor axis, then passed through the pulley and tied to a fixed ring in the corner of the panel. English is not my first language, is this sentence clear enough ? You can see these elements in the main 3D viewer [1] > How are you getting the right mirror orientation for each mirror (aka canting)? I use one bolt that pulls the mirror holder in the center and 3 bolts that push it in the corners. By screwing or unscrewing the corner bolts you can precisely orient each mirror independently. The "panel_board_exploded" view tries to show this [2] > One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. Super clever, thanks for the explanation! I think it might be tricky to calibrate. [0] https://github.com/remipch/solar_concentrator/blob/master/mechanics/README.md https://github.com/remipch/solar_concentrator/blob/master/me... [1] https://remipch.github.io/solar_concentrator/view_3d.html?model=solar_panel_assembled https://remipch.github.io/solar_concentrator/view_3d.html?mo... [2] https://remipch.github.io/solar_concentrator/view_3d.html?model=panel_board_exploded https://remipch.github.io/solar_concentrator/view_3d.html?mo...
foobarian 2y ago
Speaking of risks and fire, is there a known limit to the temperature achievable by concentration? I was wondering if I could melt a piece of tungsten with this method.
- remipch 2y ago
  I think it depends on the mirror area. The more mirrors, the more power. The more power, the higher the temperature will be. Anyway, tungsten melts at 3422°C, I don't know if it's feasible.
  - foobarian 2y ago
    Imagine we built a giant concentrator in space so we can melt tungsten without having to worry about containment!
- hakonjdjohnsen 2y ago
  Yes, and it is super-interesting! The fundamental limit is given by the 2nd law of thermodynamics - you can never reach higher temperatures than the surface of the sun, or around 5800 K. We have the atmosphere that absorbs and scatters some of the light, so on the surface of the earth it is a bit lower, but not by a huge amount. This means that there is a fundamental limit to how small and intense you can make the focal spot in a solar concentrator. The limit is around ~45 MW/m² or 45000 "suns" (which is plenty high, but far from infinite). Concentrators used for eletricity generation use much lower concentration than this, on the order of 25 suns to 1000 suns depending on the type. There are also solar furnaces designed for reaching much higher concentration by using a different type of optics. The most impressive one is the huge Odeillo solar furnace [1]. I would guess that they could melt tungsten, but I have not actually run the numbers. I did a talk last week about a concept we are developing for reaching furnace-level concentration ratios with conventional heliostats [2]. [1] https://en.wikipedia.org/wiki/Odeillo_solar_furnace https://en.wikipedia.org/wiki/Odeillo_solar_furnace [2] https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-freeform-secondary.pdf https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-fr...
  - 2y ago
algo_trader 2y ago
> with some (possibly manual) seasonal adjusting. Any pointers for floating PV seasonal mechanism? With a 5m high prism, and panels along ONE face, how do you get seasonal adjustment of the panels tilt angle. An adjustable/inflatable ballast seems the simplest?

remipch 2y ago

Thank you very much for your kind and detailed reply! Indeed, the closed-loop control was the initial idea which convinced me that it would be possible to build the mechanical parts by hand with common tools. In other words, the software "smartness" compensates for the mechanical "ugliness". Another initial idea was to do multi-panels (several orientable panels) with a single camera looking at the target. Indeed, it's not easy, so I finally went back and decided to finish and release something with a single panel. Nevertheless, I have some ideas to do multi-panels with a few more cameras. I would like to work on them in the near future. Thank you for all the references, I will spend time to explore them. There is also a company that uses vacuum to adjust the mirror shape, I'll try to find it and post it here. I wanted to emphasize the inherent risk because my project is not a finished product, but a work-in-progress/proof-of-concept.

hakonjdjohnsen 2y ago
Yeah exactly! I like your cable-drive concept by the way. Did you describe it in more details anywhere? Heliogen were also developing a cable-drive system for their commercial heliostats, but I don't know if they are still working on it. How are you getting the right mirror orientation for each mirror (aka canting)? Custom spacers for each mirror? One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. When they look back at the mirrors, they will see the circumsolar radiation (how the sky gets brighter as you get closer to the sun). By comparing the brightness of the sky at different cameras, you can estimate which cameras is "closest" to seeing the real sun, and get an estimate for the real position of the sun.
- remipch 2y ago
  > I like your cable-drive concept by the way. Did you describe it in more details anywhere? I didn't take the time to describe the cable-bot concept in detail. I modeled almost all the mechanical parts with OpenSCAD, but I struggled to model the cable itself. In the "mechanics" README [0] you can click on any image to view it in an online 3D viewer. The following note in the same README tries to explain how the cable is used : Each cable is actually wrapped around the motor axis, then passed through the pulley and tied to a fixed ring in the corner of the panel. English is not my first language, is this sentence clear enough ? You can see these elements in the main 3D viewer [1] > How are you getting the right mirror orientation for each mirror (aka canting)? I use one bolt that pulls the mirror holder in the center and 3 bolts that push it in the corners. By screwing or unscrewing the corner bolts you can precisely orient each mirror independently. The "panel_board_exploded" view tries to show this [2] > One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. Super clever, thanks for the explanation! I think it might be tricky to calibrate. [0] https://github.com/remipch/solar_concentrator/blob/master/mechanics/README.md https://github.com/remipch/solar_concentrator/blob/master/me... [1] https://remipch.github.io/solar_concentrator/view_3d.html?model=solar_panel_assembled https://remipch.github.io/solar_concentrator/view_3d.html?mo... [2] https://remipch.github.io/solar_concentrator/view_3d.html?model=panel_board_exploded https://remipch.github.io/solar_concentrator/view_3d.html?mo...

foobarian 2y ago

Speaking of risks and fire, is there a known limit to the temperature achievable by concentration? I was wondering if I could melt a piece of tungsten with this method.

remipch 2y ago
I think it depends on the mirror area. The more mirrors, the more power. The more power, the higher the temperature will be. Anyway, tungsten melts at 3422°C, I don't know if it's feasible.
- foobarian 2y ago
  Imagine we built a giant concentrator in space so we can melt tungsten without having to worry about containment!
hakonjdjohnsen 2y ago
Yes, and it is super-interesting! The fundamental limit is given by the 2nd law of thermodynamics - you can never reach higher temperatures than the surface of the sun, or around 5800 K. We have the atmosphere that absorbs and scatters some of the light, so on the surface of the earth it is a bit lower, but not by a huge amount. This means that there is a fundamental limit to how small and intense you can make the focal spot in a solar concentrator. The limit is around ~45 MW/m² or 45000 "suns" (which is plenty high, but far from infinite). Concentrators used for eletricity generation use much lower concentration than this, on the order of 25 suns to 1000 suns depending on the type. There are also solar furnaces designed for reaching much higher concentration by using a different type of optics. The most impressive one is the huge Odeillo solar furnace [1]. I would guess that they could melt tungsten, but I have not actually run the numbers. I did a talk last week about a concept we are developing for reaching furnace-level concentration ratios with conventional heliostats [2]. [1] https://en.wikipedia.org/wiki/Odeillo_solar_furnace https://en.wikipedia.org/wiki/Odeillo_solar_furnace [2] https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-freeform-secondary.pdf https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-fr...
- 2y ago

algo_trader 2y ago

> with some (possibly manual) seasonal adjusting. Any pointers for floating PV seasonal mechanism? With a 5m high prism, and panels along ONE face, how do you get seasonal adjustment of the panels tilt angle. An adjustable/inflatable ballast seems the simplest?