Heat Pipe Technology
You may be unfamiliar with Heat pipes but chances are you already own something that uses them. Laptop computers use heat pipes to conduct heat away from sensitive electronic components. Air Conditioning systems also commonly use heat pipes for heat conduction.
A Heat pipe is a thin metal rod with a hollow core. Inside the core is a small amount of purified water and a few special additives.
The space inside the heat pipe is evacuated much the same as the tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside.
At sea level water boils at 100°C but if you were to climb a mountain, you would see the boiling temperature fall to less than 100°C this is because of the decreased air-pressure at altitude. The higher you go the lower the boiling temperature.
Based on this principle of water boiling at a lower temperature with decreased air-pressure, by evacuating the tube the same result can be achieved. The heat pipes used in Westech collectors have a boiling point of only 30°C. So when the heat pipe is heated above 30°C the water inside vaporizes. The vapor quickly rises to the top of the heat pipe transferring heat. As the heat is lost into the manifold the vapor condenses back into liquid form falling to the bottom of the heat pipe ready to repeat the process.
Inside the vacuum the water forms the shape of a small ball much like mercury when you pour it onto a flat surface at room temperature. When the heat pipe is shaken the water ball can be heard rattling inside just like a piece of metal.
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 results more than 20 manufacturing procedures are required and with strict 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 affect the performance. The purity of the copper itself must also be very high, containing only trace amounts of oxygen or 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 WesTech Solar’s 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 WesTech 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 WesTech Solar collectors comprises of 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 250°C 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.
Even though the heat pipe is a vacuum and the boiling point has been reduced to only 25-30°C, the freezing point is still the same as water at sea level, 0°C. Because the heat pipe is located within the evacuated glass tube, brief overnight temperatures as low as -20°C will not cause the heat pipe to freeze. Plain water heat pipes will be damaged by repeated freezing. The water used in WesTech Solar heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe.