Concentrating optics use a curved or segmented mirror to collect sunlight over a large area and deliver it to a much smaller one, and the ratio of the two areas, the concentration ratio, decides how hot the target gets. A trough focuses on a line and tracks the sun in one direction, a dish focuses on a point and tracks in two, and a Fresnel array approximates either with flat strips. The focus is dangerous: it ignites wood at once and damages eyes permanently, without pain.
Part of Concentrated Solar Power
A solar concentrator is a mirror that redirects the sun’s parallel rays so they all cross a small target, raising the power per square metre there.
Why A Mirror Can Beat A Flat Collector
A flat collector can only receive the sun’s power over its own area, about 1,000 W/m² at peak, and its temperature is limited by how fast it loses heat (The Greenhouse Effect in Collectors). A mirror gathers sunlight over a large area and aims it at a small receiver. If the mirror area is 100 times the receiver area, the receiver sees about 100 times the power per square metre, and a hot receiver with little surface to lose heat from can reach hundreds of degrees.
The ratio of mirror area to receiver area is the concentration ratio, C. The parent page gives approximate values: a trough 30-80x, a dish 100-1,000x, a Fresnel array 10-40x. Temperature climbs with C but not in proportion to it (Concentration Ratio & Temperature derives the limits).
The sun is not a point: its disc spans about half a degree, so even a perfect mirror forms a small spot, not a point. A flat mirror gives no concentration at all; it throws back a patch as large as itself, which is why a flat pocket mirror reflects light but does not ignite anything.
Three Shapes
- Parabolic trough: a curved channel focusing on a pipe along its length; one-axis tracking (Parabolic Trough).
- Parabolic dish: a bowl focusing on a point; two-axis tracking; the hottest (Parabolic Dish).
- Fresnel reflector: flat strips each tilted to aim at a fixed receiver (Fresnel Reflector).
What You Need
- A reflective surface — mirror glass, polished aluminium or foil (Building Reflectors from Salvage)
- A support that holds the shape while it moves
- A way to follow the sun (Tracking Systems)
- A receiver — a blackened pipe, pot or work-piece
- Eye protection and a cover for the reflector
Common Mistakes
- Expecting a flat mirror to heat anything. Flat mirrors give about 5x at most with several together (parent page) and no focus.
- Ignoring the sun’s size. The spot cannot be smaller than the sun’s image.
- Building before planning the stow position. A concentrator left aimed at the sky sweeps its focus across whatever is nearby.
Safety
Concentrated sunlight ignites materials at once and blinds without pain
The focus of a mirror ignites paper, wood and cloth immediately, and the eye focuses a bright spot onto the retina, where it causes permanent injury painlessly: you do not feel retinal damage as it happens. A tracking concentrator left unattended sweeps its focus across whatever is nearby, including fences, roofs and people. Never look at the focus or at a mirror reflecting it; keep it covered when not in use; and stow it face-down or covered (Glass, Mirrors, Lights & Reflectors).
What’s Next
- Learn next: Parabolic Trough — the easiest shape to build and track
- Combine with: Concentration Ratio & Temperature — what ratio buys