Structure and Principle

The heat pipe is hollow with the space inside evacuated, much the same as evacuated solar collector tube. Inside the heat pipe there is a few purified water and some special additives. 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 our 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 and transfers the heat. As the heat is lost at the condenser (top), the vapor condenses to form liquid (water) and returns to the bottom of the heat pipe at once and then repeats the process.

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 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.

Often heat pipes use a wick or capillary system to aid the flow of the liquid, but for the heat pipes used in our solar collectors, no such system is required since the interior surface of the copper is extremely smooth, allowing efficient flow of the liquid back to the bottom. Also our 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 our solar collectors comprises two copper components, the shaft and the condenser. Prior to evacuation, the condenser is brazed to the shaft. The condenser has a much larger diameter than the shaft, which can provide a large surface area where heat transfer to the header can occur. The copper used is oxygen free copper, thus ensuring excellent life span and 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 out heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe.