Vacuum tube solar collector
The solar collector of vacuum tubes consists of a set of cylindrical tubes. The tubes are formed by a selective absorber, located on a reflector settlement and surrounded by a transparent glass cylinder.
Between the transparent outer tube and the inner absorber, the vacuum has been made. With this, conduction and convection losses from the absorbent surface are avoided and this fact allows reaching temperatures of more than 100ºC and making much more use of solar radiation.
Currently, there are two types of these collectors, with quite differentiated techniques in terms of heat transmission from the vacuum tube to the primary circuit.
On the one hand, vacuum collectors with direct fluid, have a primary sensor of the primary circuit in the upper part of the collector; Each tube is connected to this sensor by means of a round circuit and a return circuit. This circuit circulates the fluid of the primary circuit that will heat up during the journey.
The inner circuits of the tubes present two basic configurations: the concentric circuit or separate circuit.
The other heat transfer trend in vacuum tubes is called heat pipe.
In this system, as in the previous case, the solar thermal collectors have a primary sensor of the primary circuit in the upper part, but in this case, the vacuum tube has a sealed central copper tube and is filled with a alcoholic mixture.
At this point, the heat in the primary circuit will decay and the alcoholic mixture will be condensed, which will return to the lower part of the collector by its own weight.
One of the specific characteristics of this type of collectors is the possibility, depending on the model, to individually tilt each tube. This feature is very useful in locations that, due to building requirements, it is necessary to mount the disoriented sensors of the south.
Operation of the vacuum tube solar collector
The insulating effect is achieved in the vacuum tube collectors by a vacuum in a glass tube or in the space of two concentrically arranged glass tubes, which greatly reduces the thermodynamic transfer of heat to the ambient air through reduced convective losses.
Especially in winter, vacuum collectors, due to their good insulation, obtain significantly higher yields than flat plate collectors, but thaw slightly in snow or ice cover. Resistance to very low temperatures is given as an average of -30 degrees Celsius.
The diameter of the vacuum tube, however, already. Individual manufacturers cite the limit of 70 mm tubes at -50 degrees Celsius. In principle, however, frost protection should be provided at temperatures below -10 degrees Celsius for the lines. There are several possibilities. The most common are antifreeze additives to heat the medium. For systems that work with pure water, the solar controller will control the temperature limits. If the temperature in the return drops to approximately 4 degrees Celsius, the heat from the storage tank or the alternative heating system is pumped to the solar system line system.
Last review: May 30, 2018