Solar collector heat pipe
Heat pipes might seem like a new concept, but you probably use them everyday and don't even realize it. Handheld computers often use small heat pipes to conduct heat away from the CPU, while air-conditioning system commonly use heat pipes for heat conduction.
The principle of heat pipe is actually very simple.
Structure and Principle
The heat pipe is hollow with the space inside evacuated, as much the same as the solar tube. In this case insulation is not the goal, but to change 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 are on the top of a mountain, the boiling point 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, we can achieve the same result by evacuating the heat pipe. 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 water vapour rapidly rises to the top of the heat pipe, which transfers heat. As the heat is lost at the condenser (top), the va pour condenses to form a liquid (water) and then returns to the bottom of the heat pipe and the above process begins again.
At ambient temperature the water forms a small ball, which is much like the mercury reaction when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard of 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! However, 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 influence 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 and then form a pocket of air in the top of the heat pipe, which makes the heat pipe's hottest point (of the heat condenser end) downward away from the condenser. This is obviously harmful to the performance, which therefore indicates the need of only using very high purity copper.
Often heat pipes use a wick or capillary system to help the flow of the liquid. But for the heat pipes used in Sunflower 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 Sunflower heat pipes are not installed horizontally. But the heat pipes can be designed to transfer heat horizontally, which has the relatively high cost.
The heat pipe used in Sunflower solar collectors consists 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 used to provide a large surface area over which heat can be transferred to the header. The copper used is oxygen free one, which thus ensures excellent life span and performance.
Each heat pipe has been tested for heat transfer performance and exposed to 250oC (482oF) temperatures before being applied for use. Under such circumstance, the copper heat pipes are relatively soft. Heat pipes that are very stiff and have not been exposed to such strict 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 one 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). Since the heat pipe is located inside the evacuated glass tube, brief overnight temperatures as low as -20oC (14oF) will not cause the heat pipe to be freezed. But plain water heat pipes will be damaged by repeated freezing. The water used in Sunflower heat pipes still freezes in cold conditions, but it can also freeze in a controlled way that does not cause expansion of the copper pipe.
