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Fresnel Reflector

4 min read

A Fresnel reflector replaces a curved mirror with rows of flat strips, each tilted so it throws the sun’s light onto one fixed receiver above them, giving 10-40x concentration from mirrors that are easy to salvage. The tilt of each strip is half the angle between the vertical and the line to the receiver: for a strip 1 m to the side of a receiver 2 m up, that is 13.3°, and 22.5° for a strip 2 m out.

Part of Concentrating Optics

A Fresnel reflector is a row of flat mirror strips, each at its own angle, that together approximate the focusing of a parabolic trough.

Each Strip Aims At The Receiver

A flat mirror turns a ray through twice its own tilt. With the sun overhead and a receiver a height h above the strip row, a strip at horizontal distance x must send light along the line of angle α = atan(x ÷ h) from the vertical. A mirror tilted by θ turns light by 2θ, so θ = α ÷ 2.

Inputs, assumed: receiver height h = 2 m.

  1. Strip at x = 1 m: α = atan(0.5) = 26.6°, so tilt θ = 13.3°.
  2. Strip at x = 2 m: α = atan(1.0) = 45°, so tilt θ = 22.5°.
  3. Strip at x = 0 (directly beneath): flat, θ = 0°.

When the sun is not overhead, every strip must be re-tilted. A linkage that rotates all strips together by the same amount follows the sun roughly, with a small correction per strip; hand-adjusting each strip once per hour is practical for small arrays.

Why Strips Must Be Narrow

A flat strip does not focus. It throws a patch the size of itself, plus a spread from the sun’s own width (about 0.0093 of the distance to the receiver, because the sun’s disc is about half a degree wide). A strip 10 cm wide at 2.2 m from the receiver (the slant distance for the 1 m strip, √(2² + 1²) = 2.24 m) throws about 10 + 0.0093 × 224 = 12 cm. So to keep the patch on a receiver a few centimetres wide, the strips must be narrow, or the receiver needs a secondary reflector above it.

The parent page’s 10-40x follows: an array 1 m wide of strips onto a 5 cm receiver is a ratio near 1 ÷ 0.05 = 20 before losses.

What You Need

  • Mirror strips — salvaged mirror glass cut to long bars, or polished metal (Glass, Mirrors, Lights & Reflectors)
  • Pivots at each strip’s centre
  • A fixed receiver on a stand above the array
  • A frame that keeps strips parallel and level
  • A cover for the strips when idle

Method: Lay Out An Array

Step 1: Fix the receiver pipe at height h above the middle of the array.

Step 2: Space strips across the base, each on its pivot.

Step 3: At solar noon, set each strip’s tilt from the formula, and check with a card at the receiver: all bright bars should land on it.

Step 4: Repeat the adjustment every hour (or link the strips so one lever moves them together).

Common Mistakes

  • Wide strips. The patch overshoots the receiver.
  • Uneven pivots. One strip tilted a degree too far sends its light two degrees away, 7.8 cm at 2.24 m.
  • Receiver shading the strips. Keep its support thin.

Safety

Many beams cross at a single point

Every strip sends its full beam to the receiver, so combustibles near it ignite and the glare off the array can reach eyes some distance away. Strip mirrors left at noon angles reflect onto walls, roofs and people. Cover or lay the strips flat and face-down when idle, never look at the receiver or the strips toward the sun, and keep dry vegetation and fuel away.

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