An evacuated tube collector is a solar thermal collector built from parallel glass tubes, each enclosing its absorber in a vacuum that practically eliminates convective and conductive heat loss. With heat loss coefficients around a1 ≈ 1–2 W/m²K versus 3–4 for flat plates, it holds its efficiency at large temperature differences — at the price of higher cost and stagnation temperatures up to 300 °C.
How does an evacuated tube collector work?
A vacuum is the best thermal insulation available, and the evacuated tube wraps one around every absorber strip. Each tube is a glass envelope — usually a double-walled "Sydney" tube or a single glass wall around a metal fin — evacuated to roughly a hundred-millionth of atmospheric pressure. With no air inside, convection and conduction from the hot absorber to the cold outer glass essentially stop; only radiation remains, and the selective coating suppresses that to a few percent. The result is a collector whose losses barely grow as it runs hotter, which is precisely what the low a1 and a2 values in its efficiency parameters express.
Heat pipe or direct flow: what is inside the tubes?
Two constructions dominate. In a heat-pipe tube, a sealed copper pipe holds a small charge of fluid that evaporates at the hot absorber, rises, and condenses in a bulb plugged into the manifold — a "dry" connection, so a single tube can be swapped without draining the circuit. Heat-pipe tubes need a minimum installation tilt, typically 20–30°, for the condensate to flow back. In a direct-flow (U-tube) design the system's own heat-transfer fluid circulates through each tube, which permits flat-roof and façade mounting but makes tube replacement a wet job.
When does an evacuated tube outperform a flat plate?
Whenever the collector must run far hotter than the outdoor air. Because losses are so low, tube collectors keep working at temperature differences of 60–100 K, where a flat-plate collector's output has fallen away sharply.
| Property | Evacuated tube | Flat plate |
|---|---|---|
| Heat loss a1 | ≈ 1 – 2 W/m²K | ≈ 3 – 4 W/m²K |
| Optical efficiency η0 (gross) | ≈ 0.45 – 0.65 | ≈ 0.75 – 0.82 |
| Stagnation temperature | Up to ≈ 300 °C | 160 – 200 °C |
| Strengths | Process heat, solar cooling, cold climates, 70 °C+ targets | Domestic hot water, combi systems, cost, robustness |
For standard domestic hot water at 45–60 °C in most of Europe, both technologies deliver similar annual yield per gross square metre and the flat plate usually wins on price; the full comparison is in flat-plate vs evacuated tube collectors. Globally, evacuated tubes dominate installed capacity — the bulk of the world's 544 GWth of solar thermal capacity (IEA SHC, 2024 data) is Chinese-market tube collectors — while European installations remain predominantly flat plate.
What are the practical drawbacks?
Stagnation is the big one. Up to 300 °C at the manifold stresses the glycol fluid and demands high-temperature components and careful expansion vessel sizing — see stagnation temperature. Snow is another: because the outer glass stays cold (a consequence of the excellent insulation), snow does not melt off tube arrays the way it slides from a warm flat plate. Individual tubes can break under impact, though this cuts both ways — a broken tube is replaceable in minutes, whereas flat-plate glazing damage means panel replacement. Finally, the vacuum itself is a component that must last.
How long does the vacuum last?
Quality tubes hold their vacuum for 15–20 years and more. Each tube carries a barium "getter" — a metallic flash deposited inside the glass that absorbs residual gas molecules and doubles as an indicator: silver-mirror means the vacuum is intact, milky-white means it is lost and the tube should be replaced. Vacuum durability and performance are verified in EN ISO 9806 testing, and certified parameters for every model are published in the Solar Keymark database.
Frequently asked questions
Do evacuated tubes work better than flat plates in winter?
At sub-zero ambient temperatures their low heat loss preserves more output at a given irradiance, so yes for high delivery temperatures. Persistent snow cover can reverse the advantage, since tube arrays shed snow poorly compared with flat plates.
Can I replace a single broken tube?
Yes. Heat-pipe tubes plug into the manifold with a dry connection and swap individually without draining the solar circuit. Direct-flow tubes are also replaceable but require the loop to be drained and refilled.
How do I know a tube has lost its vacuum?
Look at the getter patch at the tube's end: an intact vacuum keeps it a silver mirror; exposure to air turns it milky white. A white getter or visible condensation inside the tube means that tube no longer insulates and should be replaced.
Are evacuated tubes worth it for domestic hot water?
For 45–60 °C tap water in most European climates, flat plates deliver comparable annual output per gross square metre at lower cost. Tubes earn their premium where target temperatures exceed roughly 70 °C, in severe winters, or for process heat and solar cooling.