This is a fine post, one of the best I have seen in a while.
Thank you.
I have a small query about the sentence underlined.
If the temp in the cylinder is > than 15 degrees then what heat gets transferred.
In terms of energy transfer, I know from my mountaineering days that while the water might 'boil' on the side of K2, it took much longer to cook the egg as the water was boiling at 50 or so C
Am a bit puzzled here, have no issue with the post, just on the science in the last piece
Flat plate panels are easier to keep clean especially if u are in an area prone to lots of leaves.
I believe easier to replace a broken tube in the evac tube ones
Having said that neither make sense $$$ wise in the standard configuration 'sold' in Ireland.
There are mainly two types of tubes. The Sydney tube (typical Chinese manufactured tube in tube) where the vacuum is between a thin gap between an inner tube and an outer tube. The absorber is within normal atmospheric pressure and open to normal losses. The second is one whole tube where the absorber itself is within a vacuum. All heat pipe evacuated tubes contain two vacuums. One to stop heat loss from the absorber and the other is located within the heat pipe itself.
The fluid within the heat pipe section of the tube is within a vaccum and not the heat transfer fluid (Glycol). This is a very small amount of fluid. When the solar rays hit the absorber and the absorber coating (Tinox, which is also there to increase temperature), because the fluid is within a strong vacuum, it boils at approx 15
°C. Once boiled, it turns to steam and rises to the top of the heat pipe and into the condenser of the heat pipe. If you touched the condenser during this cycle, you would most certainly burn yourself. The condenser section of the heat pipe is plugged into the solar manifold which the glycol passes it, thus gaining the heat from the steam within the condenser. As steam it has a touch temperature of that similar to boiling water and beyond. Hence, temperatures of nearly 200
°C can be achieved but unadvisable due to as temperature increases so does pressure and in addition the glycol mix within the system will stagnate, loosing its beneficial properties. Once the heat is taken from the condenser and transferred into the Glycol, it therefore cools and turns back into a liquid and runs back to the bottom of the heat pipe for the cycle to be repeated. This is why heat pipe tubes cannot be installed on roof pitches less than 20
° and more than 70° as the steam maybe blocked by the liquid returning and vica versa. Only direct flow systems can be used down to 1° and 89°. Before you ask, the 1° is stop air locking.
The 15
°C has nothing to do with the cylinder temperature. Kingspan's HP200 has a temperature limiter built into the condenser to limit the manifold temperature to 95
°C, whilst the HP250 (commercial tube) limits to 135
°C. Stagnation occurs at approx 179
°C. This manifold temperature is the temperature reached in the Glycol at the manifold and is the temperature that is used to be pumped through the cylinder coil, thus heating your domestic hot water or what have you.
Generally speaking, tubes are self cleaning whilst flat retain dirt for longer periods, however, I agree in areas prone to leaves, they can get caught up in between the tubes, however, a rare occurance.
Only replacing a heat pipe tube is easier as it is a plug and play system. The heat pipe is a dry connection. You can have a direct flow tube that will require a system drain down to change a tube, or at least to drain down below the level of the manifold.