Sky Power Evacuated Solar Tube Water Heating Systems

SKYPOWER evacuated tubes are made of borosilicate glass. Each tube consists of a glass outer and an inner tube or absorber, covered with a selective coating (aluminium-nitrogen on an aluminium base layer AI-N/AL) that absorbs 93% of solar energy, but inhibits radioactive heat loss.

The air is withdrawn (evacuated) from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss.

A copper heat rod is installed inside each tube. The copper heat rod transmits energy to its tip, which is plugged into the collector's heat transfer manifold. As water is pumped through the manifold the heat is transferred from the copper to the water.

The copper heat rod is hollow and contains a special type of refrigerant. The hollow centre of the heat rod is a vacuum so even at temperatures of 40oC to 50oC the refrigerant will vaporize (boil). The vapor rises to the tip (condenser) of the heat rod where the heat is transferred to the water flowing through the manifold. The loss
of heat causes the vapor to condense and flow back down the heat rod where the process is once again repeated.

This method of transfer is thousands of times more efficient than a solid copper rod. Heat is therefore very efficiently transferred from the solar collector to the water flowing through the manifold.

Evacuated tubes are round and the amount of solar radiation striking the collector is relatively constant from mid morning to mid afternoon. This feature maximizes the total amount of solar radiation the collector is exposed to each day. Furthermore, the sun is always striking the tubes at an angle that is perpendicular to their surface thus reducing reflection. Any sunlight that is reflected off the glass surface or that passes between the tubes is reflected back to the underside of the tube by the reflective backing panel.

The tubes exposure to, and subsequent absorption of, solar radiation is therefore maximized.

The evacuated tubes are extremely strong and can withstand hail stones of up to 25mm in diameter. Evacuated tubes are generally kept clean by rainfall, but may have to be cleaned down occasionally to ensure maximum performance is not impeded.

Evacuated tubes are more efficient than flat-plate collectors for several reasons. First they perform well in both direct and cloudy conditions and diffuse solar radiation. This characteristic, combined with the fact that the vacuum minimises heat loss to the outdoors, makes these collectors particularly useful in areas with cold, cloudy winters.

Second, because of the circular shape of the evacuated tube, sunlight is perpendicular to the absorber for most of the day. For a comparison, in a flat pate collector that is in a fixed position, the sun is only perpendicular to the collector at noon.

Evacuated tube collectors achieve both higher temperatures and higher efficiencies than a flat plate collector

EVACUATED TUBES GENERAL SPECIFICATIONS

1) Tubes are 1.5 metres in length.

2) The inner glass tube diameter is 37mm, the outer glass tube diameter is 47mm.

3) The evacuated tubes are made from borosilicate glass 3.3, which has a thermal expansion coefficient of 3.3 x 10-6 K-1.

4) The 5 x 10-3 Pa vacuum effectively inhibits out-gassing from the glass, and provides a high degree of insulation preventing convected heat loss from the inner tube.

5) The selective absorbing surface is graded aluminium-nitrogen on an aluminum base layer (AL-N/AL). The absorbance is up to 0.93 (AM1.5), and the emittance is around 0.07.80 (80oC).

6) Inside the evacuated tube is a copper rod filled with a low boiling point refrigerant (R22). The refrigerant vaporizes as solar energy is collected and rises to the tip of the rod transmitting the energy into the manifold.

7) The manifold is fully insulated and encased into a stainless steel frame work with a reflective backing plate.

8) Each tube is capable of producing 7.5 litres of hot water at between 50oC and 70oC over six hours.

9) The manifolds come complete in two forms - 10 tubes and 14 tubes for ease of handling and installation.

10) Each system is managed by a differential temperature controller, which controls the pump and back-up energy source. This ensures effective transfer of energy from the collector to the storage cylinder.