Operating principle
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Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don't want to lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).
In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the evacuated tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.
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Left Tube = Vacuum Present |
The efficiency of a evacuated water heater is dependent upon a number of factors, one important one being the level of evacuated radiation (insolation) in your region.
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 Progress-XXI 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 Progress-XXI 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 Progress-XXI heat pipes are not
installed horizontally. Heat pipes can be designed to transfer heat
horizontally, but the cost is much higher.
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The heat pipe used in Progress-XXI is 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 Progress-XXI heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe.