Extracting heat from a vacuum tube collector
There are two primary methods of extracting heat from the vacuum tube.
One, use a heat pipe or secondly pump the anti-freeze mix down into the vacuum tube.
The second method is konwn as primary flow, or primary fluid. Using a heat pipe introduces
an extra heat exchange surface but in practice there seems to be very little difference
in efficiency between the two extraction methods.
Heat pipes are not exclusively found in solar panels but are commonly used in
laptop computers and air-conditioning systems. The principle behind heat pipe's operation is very
simple and surprisingly efficient.
A heat pipe is simply a copper tube with a small amount of heat conducting fluid inside,
and evaucated. Because of the low pressure, the fluid inside the heat pipe starts to change
state from liquid to gas once the temperature rises above 30C.
As the heat conducting fluid vaporizes, it raises the pressure within the heat pipe,
preventing all the fluid from evoporating, so that more fluid evorporates the hotter it
gets. The vapour rises in the heat pipe bringing heat with it (similar to convection)
until it reaches the manifold. The heat pipe is pushed into pocket in the manifold
so that so that heat passes easily to the antifreeze mix in the solar loop.
| Top of heat pipe with conducting paste...
As the heat is lost at the condenser (top) to the manifold, the vapour condenses
to form a liquid and returns to the bottom of the heat pipe to once again repeat the process.
Slim alumimum sections pushed up against the heat pipe are used to help with the heat transfer process.
Primary Flow / Primary Fluid
The alternative way to extract energy from a vacuum tube is by pumping the anti-freeze mix
within the solar circuit into the inside of the vacuum tube. There are a couple of methods
used. Pipe in Pipe, or U-Pipe.
The U-pipe method is self evident, a double manifold is used where the antifreeze mix is
pumped from the cold manifold to the hot manifold via pipe extending down into vacuum tube.
The pipe in pipe system is similar except that the colder antifreeze mix is
pumped down the inside of the inside pipe and as it is transferred to the outer pipe it picks
up heat from the vacuum tube and is returned to the hotter manifold.
Primary Flow systems have several disadvantages, normally they must be pre-assembled
with tubes before they are brought up to roof level, which makes installing more difficult.
The second more serious drawback, is the anti-freeze mix flows into the middle of the vacuum tube.
Under stagnation conditions this can lead to serious overheating of the polypropelene gychol mixure
which if happens leads to a pre-mature breakdown of the anti-freeze. Thirdly, the narrow pipework is more prone to blockage, which can lead to some tubes being bypassed and overheating.