Three-target solar vacuum glass tube
– Key component of solar collector and solar water heater
– Coating material: Al-N/Al or SS-Al/N/Cu.
Evacuated tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass.
The inner tube is coated with a special selective coating (Al-N/Al) which features excellent solar radiation absorption and minimal reflection properties. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures.
This “evacuation” of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes. The vacuum in the evacuated glass tube is less than 5×10-3pa This can only be reached and maintained over a long period of time through a specialised evacuation process in production resulting in an almost total elimination of convection and conduction losses from the tube.
Specification
| Absorbance rate after coating (AM1.5) | α% | ≥94.5 |
| Absorbance rate after evacuating (AM1.5) | α% | ≥ 93.5 |
| Emission rate after coating (80±5 °C) | α% | ≤ 6 |
| Emission rate after evacuating (80±5 °C) | α% | ≤ 6.5 |
| Stagnation parameter | m² °C/kW | ≥ 240 |
| Solar radiance exposure | MJ/m² | ≤ 3.0 |
| Average heat loss coefficient | W/(m²°C) | ≤ 0.70 |
| Selective coating material | AL-N/AL, SS-AL-N/Cu | |
| Impact resistance | Steel Ball Test: Steel ball with diameter of 30 mm fall on the evacuatedtube vertically from 450 mm height. | |
| Vacuum degree | Pa | ≤ 3X10-3 |
| Pressure | Mpa | 0.6 |
| Transparent (AM1.5) | ≥ 0.89 (ISO9806-1: 1994) | |
