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The Greenhouse Effect in Collectors

4 min read

Glass over a collector lets visible sunlight in but blocks the longer-wavelength infrared radiation that the hot absorber emits, and it also stops air from carrying heat away. With no water flowing, an absorber under glass climbs until its losses equal its gains, which with typical loss figures is 120-220°C, well above the boiling point of water. That temperature, called the stagnation temperature, is the one every collector’s plumbing and glazing must survive.

Part of Solar Energy Fundamentals

A glazed collector heats until the heat leaving it equals the sunlight it absorbs; the glass lowers the loss, so the equilibrium temperature rises far above ambient.

Two Wavelength Bands, One Pane Of Glass

Every hot body radiates, and the wavelength of the peak follows Wien’s law: peak wavelength = 2,898 µm·K ÷ T (absolute temperature, constant from standard physical tables). The sun’s surface at about 5,772 K peaks at 2,898 ÷ 5,772 = 0.50 µm, in the visible band. A collector absorber at about 350 K (77°C) peaks at 2,898 ÷ 350 = 8.3 µm, in the thermal infrared.

Window glass transmits the 0.5 µm light and is opaque to the 8.3 µm radiation. Light goes in, heat radiation cannot come out. The parent page describes this effect, and the second job of the pane is easy to overlook: it also holds still air against the absorber, which stops wind and convection from carrying heat off. Both effects matter in a collector; in an unheated greenhouse, an early experiment by R. W. Wood (1909, recalled) suggested the convection effect dominates.

How Hot Does A Bare Black Plate Get?

The radiation law (Stefan-Boltzmann, constant σ = 5.67 × 10⁻⁸ W/m²·K⁴) gives the heat a surface emits as σT⁴. Ignore all air cooling and take a black plate (absorbs and emits fully) in 1,000 W/m² sunlight that loses heat only by radiating:

  1. Balance: absorbed 1,000 W/m² = emitted σT⁴.
  2. T⁴ = 1,000 ÷ 5.67 × 10⁻⁸ = 1.76 × 10¹⁰ K⁴.
  3. T = (1.76 × 10¹⁰)^(1/4) = 364 K = 91°C.

That is the ceiling for a bare plate with no air cooling at all, and a real bare plate sits below it because air carries heat away. It agrees with the parent page’s 80-100°C for a plate with no concentration (Concentration Ratio & Temperature extends this to mirrors).

With Glass: Use A Loss Coefficient

With glass, losses are described by an overall loss coefficient U_L in W per square metre per kelvin of difference between the absorber and the outside air. At equilibrium without flow, the absorbed power equals U_L × (T − T_ambient), so:

T_stagnation = T_ambient + absorbed power ÷ U_L

Inputs: absorbed power 800 W/m² (the glazed-black figure in Absorption vs Reflection, rounded); T_ambient 20°C; U_L assumed between 4 and 8 W/m²·K, the range typical of single-glazed collectors in solar-engineering textbooks (recalled, not measured here; your own collector will differ).

  • U_L = 8: T = 20 + 800 ÷ 8 = 120°C.
  • U_L = 6: T = 20 + 800 ÷ 6 = 153°C.
  • U_L = 4: T = 20 + 800 ÷ 4 = 220°C.

So the stagnation temperature of a well-made single-glazed flat collector is 120-220°C. The better the collector, the lower its U_L and the hotter it stagnates. A collector that works well under load is hottest when there is no flow.

Method: Check A Collector’s Stagnation Behaviour

Step 1: Fill the collector, close it to flow, and leave it on a clear noon with a thermometer touching the absorber.

Step 2: Read the temperature every 10 minutes. It rises quickly and levels off. The level-off value is your stagnation temperature (measured at that day’s sun and air temperature).

Step 3: Compare with the softening or melting point of every part: hose, plastic pipe, glazing film, paint, sealant. Replace any part that fails at that temperature.

Safety

Stagnation: a closed, sunlit collector is a pressure vessel with no flow to cool it

A collector left in the sun with no water moving through it passes the boiling point of water, as the figures above show. Steam from the water inside can burst a closed loop; a closed loop with no pressure relief path is a pressure vessel heated by the sun. A relief path is not optional: use an open vent that discharges where steam and hot water can hurt no one, or a relief valve sized for the weakest component. Never close a valve between the collector and its vent. Antifreeze also suffers: glycol held at high temperature degrades, and degraded glycol can turn acidic and attack the metal around it.

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