The absorber is a black metal plate with water passages: a salvaged car radiator is the quickest, copper pipes clamped or soldered to sheet metal at 10-15 cm spacing is the most controllable, and two corrugated sheets sealed face-to-face is the cheapest. Whichever you choose, test it for leaks under pressure before painting, and check that it can survive the temperature it reaches when no water is flowing.
Part of Flat Plate Collectors
The absorber is the plate that converts sunlight to heat and hands it to water through channels bonded to it.
Heat Travels Along The Plate To The Water
Sunlight lands on the whole plate, but water only touches the pipes. The metal between pipes acts as a fin: it conducts heat sideways to the nearest pipe. The wider the gap between pipes, the farther heat must travel and the hotter the middle of the plate runs, which wastes heat to the air. The parent page’s spacing of 10-15 cm puts the farthest point 5-7.5 cm from a pipe. A thin sheet needs closer pipes than a thick one; copper conducts heat far better than steel, so copper allows wider spacing or a thinner sheet.
What You Need
- Option 1: a salvaged car radiator, garden hose or pipe connections, clamps
- Option 2: 12-15 mm copper pipe, a sheet of copper or steel, clamps or solder
- Option 3: two sheets of corrugated metal, silicone or crimped edges
- Matte black heat-resistant paint (Absorption vs Reflection)
- A pressure or leak test: a hose, a plug and a means of closing the outlet
Method: Build And Prove The Absorber
Option 1: Car radiator. Drain and flush it repeatedly; it will have held coolant, which is poisonous (Fluids: Oil, Coolant, Brake Fluid). Paint it matte black, connect inlet and outlet. Radiators are built for low pressure and some have plastic end tanks, so test with water at the pressure you will use and watch for softening in the sun. Use a radiator only in a loop that stays out of drinking water, or put a heat exchanger between them.
Option 2: Copper pipe on sheet. Run the pipes in a serpentine pattern with 10-15 cm between runs. Bond the pipe to the sheet over its full length by solder or tight clamps; a gap between pipe and plate is an insulator. Solder drinking-water joints with lead-free solder; the 60/40 solder on Soldering Basics contains lead and belongs to electrical work only.
Option 3: Corrugated roofing sheets. Lay two sheets face-to-face so the corrugations form channels. Seal the edges and the ends. The seal is the hard part: silicone seals can fail at stagnation temperatures, so a crimped or soldered edge is better. Pressure-test.
All options: Join headers at top and bottom, water in at the bottom and out at the top (parent page). Test at working pressure for an hour, then paint.
Common Mistakes
- Mixing metals in one loop (copper with steel or aluminium). Water makes a battery and the less noble metal corrodes.
- Loose contact between pipe and sheet. The plate stays hot, the water stays cool.
- Painting before testing. A leaking radiator is easier to repair bare.
Safety
The absorber reaches boiling heat and holds pressure it did not have in a car
Stagnation takes an absorber to 120-220°C (The Greenhouse Effect in Collectors). Water in a sealed absorber at that temperature is steam at several times atmospheric pressure, so keep a vent or relief path open at the top. Never fill a hot absorber with cold water. Coolant residue and a second metal are both poisoning and corrosion risks for drinking water.
What’s Next
- Learn next: Glazing (Glass or Plastic) — the cover that traps the heat
- Combine with: Plumbing Connections — joining the absorber to the system