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Solar Collector Tube with Heat Pipe
  • Solar Collector Tube with Heat Pipe
All-glass Evacuated Solar Collector Tube with Heat Pipe
  • SFVB series
Every tube on an evacuated tube collector is like a thermos bottle. It holds on to the heat it collects from the sun, even when the heat is small, thus allowing your hot water to be heated or your home.
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What is a Heat Pipe?

Heat pipes might seem like a new concept, but you are probably using them everyday and don't even know it. Laptop computers often using small heat pipes to conduct heat away from the CPU, and air-conditioning system commonly use heat pipes for heat conduction.
The principle behind heat pipe's operation is actually very simple.
Structure and Principle:
The heat pipe is hollow with the space inside evacuated, much the same as the solar tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside. Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100oC (212oF), but if you climb to the top of a mountain the boiling temperature will be less that 100oC (212oF). This is due to the difference in air pressure.
Based on this principle of water boiling at a lower temperature with decreased air pressure, by evacuating the heat pipe, we can achieve the same result. The heat pipes used in AP solar collectors have a boiling point of only 30oC (86oF). So when the heat pipe is heated above 30oC (86oF) the water vaporizes. This vapor rapidly rises to the top of the heat pipe transferring heat. As the heat is lost at the condenser (top), the vapor condenses to form a liquid (water) and returns to the bottom of the heat pipe to once again repeat the process.
At room temperature the water forms a small ball, much like mercury does when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard rattling inside. Although it is just water, it sounds like a piece of metal rattling inside.
This explanation makes heat pipes sound very simple. A hollow copper pipe with a little bit of water inside, and the air sucked out! Correct, but in order to achieve this result more than 20 manufacturing procedures are required and with strict quality control.
Quality Control:
Material quality and cleaning is extremely important to the creation of a good quality heat pipe. If there are any impurities inside the heat pipe it will effect the performance. The purity of the copper itself must also be very high, containing only trace amounts of oxygen and other elements. If the copper contains too much oxygen or other elements, they will leach out into the vacuum forming a pocket of air in the top of the heat pipe. This has the effect of moving the heat pipe's hottest point (of the heat condenser end) downward away from the condenser. This is obviously detrimental to performance, hence the need to use only very high purity copper.
Often heat pipes use a wick or capillary system to aid the flow of the liquid, but for the heat pipes used in Sunflower solar collectors no such system is required as the interior surface of the copper is extremely smooth, allowing efficient flow of the liquid back to the bottom. Also Sunflower solar heat pipes are not installed horizontally. Heat pipes can be designed to transfer heat horizontally, but the cost is much higher. The heat pipe used in Sunflower solar collectors comprises two copper components, the shaft and the condenser. Prior to evacuation, the condenser is brazed to the shaft. Note that the condenser has a much larger diameter than the shaft, this is to provide a large surface area over which heat transfer to the header can occur. The copper used is oxygen free copper, thus ensuring excellent life span and performance. Each heat pipe is tested for heat transfer performance and exposed to 250oC (482oF) temperatures prior to being approved for use. For this reason the copper heat pipes are relatively soft. Heat pipes that are very stiff have not been exposed to such stringent quality testing, and may form an air pocket in the top over time, thus greatly reducing heat transfer performance.
Freeze Protection:
Even though the heat pipe is a vacuum and the boiling point has been reduced to only 25-30oC (86oF), the freezing point is still the same as water at sea level, 0oC (32oF). Because the heat pipe is located within the evacuated glass tube, brief overnight temperatures as low as -20oC (14oF) will not cause the heat pipe to freeze. Plain water heat pipes will be damaged by repeated freezing. The water used in Sunflower solar heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe.

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Evacuated Solar Collector Tube:

Length 1500mm 1800mm
Outer tube diameter 47mm 58mm
Inner tube diameter 37mm 47mm
Weight 1.3kg 2.2kg
Glass thickness 1.6mm 1.6mm
Material Borosilicate Glass 3.3 Borosilicate Glass 3.3
Absorptive coating Graded AL/N/AL or SS-ALN-CU Graded AL/N/AL  or SS-ALN-CU
Vacuum degree P<5*10 -3 Pa P<5*10 -3 Pa
Thermal expansion 3.3*10 -6 /°C 3.3*10 -6 /°C
Insolation Temperature >200°C >200°C
Absorptance >93% >93%
Emittance <8% <8%
Heat loss <0.8W/(m2°C) <0.8W/(m2°C)
Maximum strength 0.8Mpa 0.8Mpa
Resist cold -35 °C -35 °C
Resist hailstone Ø 25mm Ø 25mm
Resist wind 30m/s 30m/s
Start-up temperature <=25°C <=25°C


Heat Pipe:

Item No. Ф47mm×1.5M Ф58mm×1.8M
Copper Material TP2 Non-oxo Copper TP2
Medium Inorganic Medium
Pipe diameter Φ8mm
Dimension of heat exchange end Ф14mm×64.5mm
Weight 196g 240g
Length 1370mm 1680mm
Pipe thickness 0.75mm
Start-up temperature ≤25℃
Insolation Temperature >200°C
Power 125W


Absorber (Heat transfer) Fin: 

Material Aluminum
Thickness 0.15mm
Shape (Flat or Cylindrical) Cylindrical
Length 350mm 420mm
Quantity 6 pcs / 3 pairs 8 pcs / 4 pairs


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Heat pipe Specification:

Item No. Specification
Diameter of solar tube Length of solar tube
SFVB4715 Ø47mm 1.5M
SFVB5818 Ø58mm 1.8M