The vacuum tube solar collector consists of a set of cylindrical tubes. The tubes are made up of a selective absorber, located on a reflective seat and surrounded by a transparent glass cylinder.
A vacuum has been created between the transparent outer tube and the inner absorber. Thus, thermodynamic losses by conduction and convection from the absorbent surface are avoided. This characteristic makes it possible to reach temperatures of more than 100ºC and to take much more advantage of solar radiation.
This type of solar panel has a higher performance than flat collectors.
Solar vacuum tubes can be used to obtain sanitary hot water, heating systems, swimming pool heating, etc.
Types of Vacuum Tube Solar Collectors
Direct flow vacuum solar collectors.
Solar vacuum heat pipe collectors.
Direct Flow Vacuum Solar Collectors
Direct flow vacuum solar collectors have a main heat collector from the primary circuit in the upper part of the tube. Each tube is connected to this collector by means of an outward and a return circuit (sometimes they can reach 20 tubes).
Through this circuit, the fluid of the primary circuit is circulated, which heats up during the journey and transports the thermal energy to a heat exchanger.
The inner circuits of the tubes have two basic configurations: the concentric circuit or the separate circuit.
Heat Pipe Vacuum Solar Collectors
In this system, as in the previous case, the thermal solar collectors have a main solar collector of the primary circuit in the upper part. In this case, the vacuum tube has a central copper tube sealed and filled with an alcoholic mixture.
This tube is connected to the absorber of the collector by means of a condenser pipette. The pipea carries out a heat exchange towards the primary circuit.
Solar radiation makes the absorber heat up and evaporates the alcoholic mixture inside. The evaporated mixture rises to the condenser located in the solar thermal panel. At this point, the heat will give up in the primary circuit and it condenses again, returning to the bottom of the collector under its own weight.
One of the specific characteristics of this type of collectors is that some models allow each tube to be tilted individually. This feature is very useful in locations where, due to building requirements, it is necessary to mount the solar collectors disoriented from the south.
How Does a Vacuum Tube Solar Collector Work?
In solar vacuum tube collectors the insulating effect is achieved by a vacuum in a glass tube or in the space of two concentrically arranged glass tubes. This technique greatly reduces the thermodynamic transfer of heat to ambient air through reduced convective losses.
Especially in winter, vacuum collectors, due to their good insulation, obtain a significantly higher thermal performance than flat-plate solar collectors. Very low temperature resistance is given as an average of -30 degrees Celsius.
Individual manufacturers in the field of solar energy cite the limit for tubes of 70mm at -50 degrees Celsius. However, frost protection must be provided at temperatures below -10 degrees Celsius for the lines.
To carry out this protection there are several possibilities. The most common solutions are antifreeze additives to heat the medium. For systems running on 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 alternative heating system is pumped into the solar system line system.