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Information of Solar Evacuated Tubes

AP Solar Collector

Key Features

  1. Joint British-Australian design
  2. Reliable, efficient, twin-glass solar tubes
  3. Copper heat pipes for rapid heat transfer
  4. Easy plug-in installation
  5. Maintenance Free
  6. Suitable for mains pressure water (up to 8 bar/116psi)
  7. Corrosion resistant silver brazed copper header
  8. All stainless steel frame (304 2B grade SS)
  9. Powder coated (black, brown or silver) aluminium casing
  10. Stable solar conversion throughout the day
  11. The perfect solar collector for domestic solar water heater systems
  12. Ideal for commercial solar water heating applictions

How It Works

The operation of the AP solar collector is very simple.

Solar Absorption: Solar radiation is absorbed by the solar tubes and converted into heat.

Solar Heat Transfer: Heat pipes conduct the heat from within the solar tube up to the header pipe.

Solar Energy Storage: Water is ciruclated through the header, via intermittent pump cycling. Each time the water circulates through the header the temperatures is raised by 5-10oC / 9-18oF. Throughout the day, the water in the storage tank is gradually heated.

About Solar Tubes

Solar tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Solar tubes are also referred to as evacuated tubes, as the space between the two glass layers is evacuated to form a vacuum. Solar tubes have already been used for years in Germany, Canada, China and the UK. There are several types of solar tubes in use in the solar industry. Focus Technology uses the most common "twin-glass tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.

Each solar tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. 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 solar tubes.

Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the solar tube absorbs the radiation from the sun and converts it to heat, we don't won't to loose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that solar tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).

In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the solar tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the solar tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.

The Getter is located at the bottom of the evacuated tube.

Left Tube = Good
Right Tube = Faulty

Solar tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round.

The efficiency of a solar water heater is dependent upon a number of factors, one important one being the level of solar radiation (insolation) in your region.

Evacuated Tube Basic Specifications


Length (nominal)

1500mm /1800mm



Outer tube diameter




Inner tube diameter




Glass thickness




Thermal expansion

3.3x10-6 oC




Borosilicate Glass 3.3



Absorptive Coating

Graded Al-N/Al




>92% (AM1.5)




<8% (80oC)




P<5x10-3 Pa



Stagnation Temperature




Heat Loss

<0.8W/ ( m2oC )



Maximum Strength



AP Solar Collector General Specifications


Manifold Casing Material

Powder Coated Aluminium (grade 3A21)


Frame Material

1.5mm 304 Stainless Steel


Header Pipe Material

99.93% pure Copper & lead free 45% silver brazing



Compressed Glass Wool - K = 0.043W/mK


Rubber Seals and Rings

UV stabilized high temperature silicone rubber


Optimal installation angle

20-70o Vertical, -5o to +5o Horizontal


Maximum Operating Pressure

8bar - 116psi


Pressure Drop

700 Pa @ 3.3L/min for 20 tube manifold (SRCC)


Optimal flow rate

0.1L/min/tube - 0.026G/min/tube


Performance Data ( SPF)

Conversion Factor: ho = 0.717
Loss Coefficients: a1 = 1.52, a2 = 0.0085


Model Specifications







Overall Length (mm / inch) 1

660 / 25.9"

1980 / 77.9"

Overall Width (mm / inch)

376 / 14.8"

796 / 31.3"

1496 / 58.8"

1636 / 64.4"

2196 / 86.4"

Overall Height (mm / inch)

155 / 6.1" (including flush roof mounting frame)

Absorber Area (m2 / ft2) 2

0.08 / 0.86

0.8 / 8.6

1.6 / 17.2

1.76 / 18.9

2.4 / 25.8

Fluid Capacity (ml / ounces)

190 / 6.4

290 / 9.8

510 / 17.2

 550 / 18.6

833 / 28.2

Gross Area (m2 / ft2) 3

0.25 / 2.67

1.57 / 16.95

2.96 / 31.8

 3.24 / 34.8

4.35 / 46.8

Dry Weight (kg / pounds)

8 / 18

35 / 77

64 / 140

71 / 157

95 / 209

Installation Picture

Sunwise Solar

22 Ocean View Avenue
South Portland, Maine, 04106
Phone: (207) 741-2805, (207) 939-0000

Lifetime Energy LLC

15570 SW Village Court
Beaverton, OR, 97007-3529
Phone: 503 643 6007
Fax: 503-641-6055

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