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Solar Thermal Vacuum Tubes

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Evacuated tube solar panels are solar collectors consisting of a highly insulated manifold in which a row of glass solar vacuum tubes are inserted. It is through these vacuum tubes that solar energy is harnessed and converted into heat. This Heat energy can then be used for the purpose of heating your hot water cylinder, and can provide up to 70% of your annual hot water demand. The Evacuated Tube Solar Panels are on average 20% more efficient than flat plate panels. It is their vacuum technology that gives them this efficiency gain over flat plat solar panels. The vacuum inside the tubes is the perfect insulation, preventing heat loss from the tube to the outside environment. In addition evacuated tubes will absorb solar energy not only quicker, but will also perform better in cooler cloudier weather making the vacuum tube solar system ideal for the Irish Climate.

What is a vacuum tube?

A vacuum tube is a glass tube that has had the air evacuated from it. This is done to eliminate heat loss. Inside the glass tube there is a metal (usually copper) pipe with an attached aluminium fin, the fin (known as the absorber)  is covered in specially selected coating that will maximise its solar heat gain. It is through this fin that the solar energy is absorbed and transferred to the fluid circulating within the pipe.

Direct Flow Evacuated Tube Solar Panels

Direct flow systems are ideally suited to circumstances that prevent the solar collector from having an angle less 25° or more than 70°. For example a solar collector is mounted directly onto a flat roof, for aesthetics the solar collector must not be visible from the ground. With the direct flow evacuated tube collectors the heat transfer fluid is pumped into and out of each vacuum tube. This means that the direct flow system is not reliant on heat transfer through convection currents, as is the case with the heat pipe solar collector. Although direct flow systems do have their drawbacks, in direct flow solar panel system the vacuum tubes are effectively on the wet side of the solar system, meaning if a tube required removal for replacement or maintenance purposes the entire solar system would require draining. A heat dump will also be required, to prevent boiling over of the heat transfer fluid.

Heat Pipe Evacuated Tube Solar Panels

With heat pipe evacuated tube system the connection between the vacuum tube and the manifold is a dry fit connection meaning quicker installation and straight forward maintenance. If a vacuum tube requires replacing this can be done without the need for draining the system. Another advantage of a heat pipe solar panel system is they are available with a built in overheat protection. This not only prevents the heat transfer fluid from boiling but also over removes the need for a heat dump, meaning a less costly straight forward install. When installing a heat pipe evacuated tube solar panel system careful consideration must be given to the angle of placement of the solar collector. Inside the heat pipe vacuum tube is a copper pipe with an attached absorber strip. Inside the pipe is a small amount of liquid, as sunlight falls on the surface of the absorber the liquid inside the pipe turns into hot vapour and rises to the top of the heat pipe vacuum tube which is inserted into the collector manifold. The heat transfer flows through the manifold and collects this heat. The liquid inside the heat pipe cools and flows back down the heat pipe where it is heated, repeating the process. It is for this reason that heat pipe solar panels must be mounted with a minimum tilt angle of 25°.

 Solar Thermal System Components

(A) Solar Collector (B) Pump Station (C) Hot Water Storage Tank (D) Back-up Heat Source (E) Hot Water Distribution System (F) Solar Controller (G) Expansion Vessel

How Solar Thermal Systems Work

Evacuated tube solar thermal systems work by absorbing direct and diffuse solar radiation and transferring into a heat energy than can be used to heat your hot water. The solar collectors should be positioned so that they face as close to south as possible.

(1) When Sunlight hits the solar collector the heat is absorbed.

(2) A high temperature solar circulation pump circulates the heat transfer fluid from the solar collector to the hot water cylinder where the heat is transferred via a high output coil.

(3) The cooled heat transfer fluid is then returned to the solar collector for reheating.

(4) When required a back-up heat source boost the hot water temperature to the required temperature for use