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Part III: Solar Space HeatingChapter 4. Solar Air Heaters

Screen-Mesh vs. Fin vs. Can Absorbers: What the Tests Say

How to compare absorber types by the measurements that matter, without treating one anecdote as a law.

Difficulty
Intermediate
Cost
Free
published

People argue about absorbers because each kind has a sunny afternoon it can brag about. A fair comparison does not crown the hottest outlet. It asks how much heat the air carried away, which means flow and temperature rise together, on the same day, in boxes that leak about the same. This page is how to read that kind of test, and what the three common shapes are actually doing.

Read how an air collector works first. The absorber is only the dark surface in that path. A leaky box will make a clever absorber look bad, and a stalled fan will make a plain sheet look brilliant and useless.

Difficulty

Level
Intermediate
Time
An afternoon to instrument two boxes, if you build a comparison
Skills
Measuring temperature and refusing to ignore flow

Cost estimate

Free

Choosing is free. Building two collectors to compare them costs whatever the glazing costs, twice. Most people should build one screen or one sheet and spend the second budget on a damper and a thermostat.

Materials

ItemQtyNotes
The same glazing and the same box depthIf you compareChange one thing. Otherwise you measured the weather.
A way to read outlet temperature and room temperature2At the same time.
A way to compare flow1Even a relative reading. Energy without flow is a guess.

Three geometries

A screen absorber is one or more layers of dark metal mesh, with air passing through the mesh. The area in contact with the air is large, the metal is light, and the absorber does not have to get extremely hot to hand the heat off. If the mesh is too tight, the fan works hard and flow falls. If it is too open, streams slip through without warming. Two or three layers, spaced a little, is a practical middle. Screen is easy to cut and easy to replace.

In a fin or sheet absorber, air slides along a dark metal surface instead of through it. A painted aluminum sheet is the window-heater version. Corrugated metal or added fins buy more area. Pressure drop is usually lower than a tight screen, which helps a weak fan or a thermosiphon. Less area means the metal runs hotter for the same heat, so losses through the glazing can be higher if the flow is lazy.

In a can column, air travels through a stack of metal cans. The idea is area plus turbulence inside a long path. The cost is labor, a long list of joints, and leaks at those joints. A leak from the can path into a bypass channel throws away the geometry you just built. Cans also hold dust and are awkward to clean. They are a reasonable experiment after you have already built a simple collector. They are a poor first absorber.

Three absorber pathsScreen mesh lets air pass through the absorber. A fin absorber makes air slide along metal. A can column sends air through a stack of cans.ScreenAir through meshFinAir along metalCan columnAir through cans
Screen: air through the mesh. Fin: air along the metal. Cans: air through a stack. The winner is the one that delivers heat, not the one with the cleverest scrap pile.

What a test is allowed to claim

Run the boxes at the same time, aimed the same way, with the same glazing and the same inlet air. Record outlet temperature, inlet or room temperature, and flow. Useful heat tracks flow times the temperature rise. A box that is 20 degrees hotter and moving a third of the air is behind.

One clear hour is an anecdote. A second day with a different flow will reshuffle a close race. Leaks, a damper that does not open fully, and a fan that is happier on one box will dominate the absorber. If you cannot measure flow, you can still feel whether a real stream is moving, and you should say that your ranking is incomplete.

Published comparisons are worth reading as method: same sun, both temperatures, an attempt at flow. They are not a law that one shape always wins. Copy the method if you experiment. Do not copy someone else's winner into your town and your leaks.

A practical pick

Build a sheet or a screen. Pick screen if you want area and you have a fan that can spare a little pressure. Pick a sheet or a fin if the collector must thermosiphon or the fan is small. Leave cans until a simple box already works and you want a comparison you can instrument. Paint either metal with flat high-heat paint. Gloss reflects. Latex can blister when the box stagnates.

  1. Decide whether you are testing or heating

    One good collector heats the shop. Two half-finished ones produce a debate.

  2. If you test, change only the absorber

    Same box, same glazing, same day, same instruments.

  3. Record rise and flow

    Rank by heat delivered. Keep the hottest-outlet story in a separate column labeled incomplete.

  4. Repeat on a second day

    If the order flips, the race was too close to call.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.