A flat plate collector is a matte black metal plate with water passages, set in an insulated box under a pane of glass, and it heats water to 50-80°C for bathing, laundry and washing. Build it from four parts (absorber, glazing, insulated frame, pipework) and give it a vent or relief path from the first day, because with no water flowing it heats far past boiling.
Part of Solar Thermal Collectors
A flat plate collector gathers sunlight over its whole face with no mirrors or tracking and passes the heat to water running through or across a dark plate.
Four Parts, Each Doing One Job
- Absorber — the black plate that turns sunlight into heat and passes it to the water (Absorber Plate Construction).
- Glazing — the pane that lets light in and keeps heat from leaving (Glazing (Glass or Plastic)).
- Insulated frame — the box that holds the others and slows heat loss from the back and sides (Insulation & Frame).
- Plumbing — the pipes that bring cold water in at the bottom and take hot water from the top (Plumbing Connections).
Nothing moves. The collector needs no tracking: a fixed tilt facing the equator collects through the day (Insolation & Angle).
What Operating Temperature Means
The parent page gives 50-80°C as the working range, enough for bathing, dishwashing, laundry and space heating. That is a working temperature with water moving. With the water still, the same collector is far hotter: the stagnation temperature is 120-220°C by the loss-coefficient estimate in The Greenhouse Effect in Collectors. Parts must survive the higher number, not the lower one.
What You Need
- An absorber — car radiator, copper pipe on sheet, or corrugated metal
- One or more panes of glass — about 85-90% light transmission (parent page figure)
- A shallow box — 5-10 cm deep (parent page figure)
- Insulation that tolerates heat (see the frame page)
- Pipe, fittings and a vent
- Matte black heat-resistant paint
Method: Build In This Order
Step 1: Absorber first. Build and test the absorber with water at working pressure before anything else is fitted. A leak found later means taking the collector apart.
Step 2: Frame and insulation. Line the box, set the absorber in it with an air gap of 2-3 cm below the glass.
Step 3: Paint. Matte black over the whole face, including the bare metal between pipes.
Step 4: Glaze and seal against rain with weep holes at the bottom for condensation.
Step 5: Plumb and vent. Bottom in, top out, an open vent or relief valve at the high point, and a drain at the low point.
Step 6: Aim. Face the equator at the tilt in the insolation page, then run water through and feel for hot and cold spots.
Common Mistakes
- Closing the top outlet. A valve between collector and vent turns the collector into a closed boiler.
- Mounting flat. Rain and dust sit on a flat pane, and the low winter sun glances off.
- Leaving the collector uncovered and empty in sun. An empty collector overheats; cover it or drain it only when you will not fire it, and refill slowly because cold water on a hot absorber flashes to steam.
Safety
A flat plate collector in the sun is a heater with no off switch
With no flow it passes the boiling point of water. A closed loop with no pressure relief is a pressure vessel heated by the sun, and a relief path is not optional: use an open vent or a relief valve, discharged to a safe place (Safety Valve). Do not fill a hot, empty collector with cold water; steam and hot water can erupt from the outlet. Antifreeze in a stagnating loop degrades and can become acidic. Water leaving the collector can also scald in seconds, so temper it before it reaches a tap.
What’s Next
- Learn next: Absorber Plate Construction — the heart of the collector
- Combine with: Thermosiphon Systems — a pump-free way to connect it to a tank