DIY Solar Concentrators, Made Better (CPCs) — NightHawkInLight

Source: https://www.youtube.com/watch?v=8zlAI32MSsQ (YouTube) Channel: NightHawkInLight (host: “Ben”) Uploaded: 2025-07-10 · ~19.3 min · Type: notes (video transcript) Ingested: 2026-06-26

What it is

A how-to / explainer on compound parabolic concentrators (CPCs) — a class of non-imaging solar reflector that focuses light arriving from a range of input angles onto a finite-size target, rather than a single point. The payoff: concentrated solar heat without active sun-tracking, plus useful gain from diffuse/scattered light (cloudy days).

Core insight: target size changes the ideal mirror shape

An ordinary parabola focuses only light entering at exactly 90° to its aperture onto an infinitesimal point — so it needs precise sun-tracking and fails on cloudy days (like a magnifying glass off-axis).
Real targets (a pipe, a plate) are finite 3-D objects. You don’t need a point focus — light landing anywhere on the pipe still heats it. Relaxing the point-focus requirement lets a mirror accept many input angles at once.
That is the CPC: optimized to intercept 100% of light entering within a chosen acceptance-angle range, concentrating it onto the real target.

String-construction method (no math)

Geometric way to draw the curve for any target shape, using a speed square + a guide rail + a low-stretch string (best: braided fishing line; a plastic-straw sleeve on the pen keeps the pen from jumping the string):

Tubular / 3-D suspended target:

Draw two lines crossing at the target location; the angle between them sets the acceptance range. Rule of thumb 20–40°: tighter = better for direct sun / higher concentration; wider = better for cloudy/diffuse light. (A flat 180° doesn’t work.)
Cut a wooden plug matching the target’s cross-section (e.g. hole-saw a disk for a round pipe) and pin it at the intersection — optimize for any shape.
Set the guide rail at 90° to one of the lines; pin into the target’s side along that line; wrap the string from the square around the pin.
Draw half the curve by unwrapping the string off the target while sliding the square along the rail, keeping the string parallel to the square’s edge (the unwrap + slide combination is what makes it a compound curve). Reverse the board/pin for the other half.

Flat-absorber variant: same two acceptance lines, plus a third line drawn across between them to set the target width; trace from one intersection to the other. This is the ideal CPC shape to focus onto a flat collector.

Building the mirror

Cut the traced curve as a template; replicate for a trough.
Line a wooden frame with aluminum flashing / sheet metal (thumbtacks push through aluminum, no hammer). Aluminum is usably reflective as-is; peel-and- stick mirror sheet boosts performance.
A transparent plastic cover over the trough adds a greenhouse effect and shields the target from wind/rain convective cooling.

Cloudy-day test result

Side-by-side outdoors under clouds, thermal-camera readings inside each pipe, ambient ~60 °F:

CPC (heavy cast-iron pipe, high thermal mass): ~91 °F inside.
Ordinary parabola (small low-thermal-mass aluminum tube): ~72 °F.
~20 °F advantage to the CPC despite its much larger thermal mass — strong evidence of greater real heat capture without tracking. Caveat: cloudy-only test; the host notes a sun-aimed parabola would reach far higher peak temps (“could melt that pipe”). The CPC’s edge is no-tracking + diffuse-light performance, not peak concentration.

Reference links (from the video description)

CPC plotting tool (Desmos, Koen Siteur): https://www.desmos.com/calculator/ocggsmh0b6
Simulation (Matthew Pritchard): http://bit.ly/4lsTm1Y
“A Review of the Compound Parabolic Concentrator (CPC) with a Tubular Absorber” (ResearchGate)
“Mathematical Modeling and Simulation of a CPC Collector with an Absorber Tube” (Energies, MDPI 16(1):287)
Power From The Sun e-book, ch. 9 §9.2 (CPCs): http://www.powerfromthesun.net/Book/chapter09/chapter09.html
Non-imaging optics / CPC concentration limits (ebrary.net)
Wolfram Demonstrations: parabolic-trough off-angle light loss

Relevance to the wiki

A solar-thermal collection source feeding the heat-driven-cooling thread: CPCs are the front-end that turns sunlight into the hot-water/heat input that drives adsorption/absorption chillers — and their no-tracking, diffuse-light capture lowers the BOS cost and complexity of solar-thermal cooling. Complements GSA Honeycomb Solar Thermal Collector and Geometric Blackness — Light Trapping (absorber-side light capture; CPCs are the concentrator-side complement). The string-construction method and the 20–40° acceptance-angle rule of thumb are the reusable engineering takeaways; the cited CPC review papers are good follow-on ingest candidates if the solar-thermal front-end becomes a focus.