Structure and PrincipleThe 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 va pour rapidly
rises to the top of the heat pipe transferring heat. As the heat is
lost at the condenser (top), the va pour 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 Newfriend 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 Newfriend heat pipes are not
installed horizontally. Heat pipes can be designed to transfer heat
horizontally, but the cost is much higher.