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How do evacuated tubes work?

Evacuated solar tubes were first designed in Australia over 30 years ago and the basic design has not changed much over that period. 

They work by absorbing the suns energy in the form of UV rays and convert that energy  into thermal energy using  conduction. 

Each tube works independently and is over 93% efficient in capturing the suns energy.  This extraordinary efficiency is only possible due a vacume within the tube that acts as an insulation barrier between the internal tube and the external air.  The internal layer of glass has three coatings to help with heat absorption and transfer and the external layer of glass is clear to let the heat pass through it. The insulation layer ensures that the heat entering the tube does not escape again. An evacuated tube can be over 200 Deg C on the inside but the external layer of glass will be within a few degrees of the ambient air temperature. 

 

The 2 types of evacuated tube solar hot water:

Wet Tubes/flooded tubes: 

A 'wet' or 'Flooded' evacuated tube is full with water and the suns heat is directly transferred to the water within the tube. That heat is then thermosyphoned into a storage tank.

Heat pipes/Dry tubes:

A dry tube uses a heat exchange within the tube to transfer the heat from the internal layer of glass to the water. Within the tube there is a aluminium fin and a copper pipe. When the inner glass layer warms up, the heat is transferred through the aluminium fin to the copper pipe. The copper pipe is sealed and has a small amount of liquid inside it that boils at around 40 Deg C. When the liquid boils, steam is created within the pipe and the steam rises to the top of the copper pipe. The end of the copper pipe has a bulb that fills with steam and then transfers the heat to a copper manifold that is holding the liquid to be heated (usually water). As the water extracts the heat from the bulb, the steam condensers and the liquid runs back down the copper pipe to be reheated again. Once the water in the manifold is 8 deg C hotter than the storage tank, the pump will turn on and bring the solar heater water down to the storage tank.

 

Evacuated tube specification details.