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Evacuated tubes are the most modern way of collecting energy from the sun for heating water.
Each evacuated tube consists of an inner and an outer tube made of extremely strong high impact transparent borosilicate glass ,Pyrex.
The inner tube is coated with a special coating that has excellent solar heat absorption and minimal heat reflection properties. The air is withdrawn (evacuated) from the space between the two glass tubes to form a vacuum, which eliminates heat loss, similar to a Thermos Flask.
Evacuated Tube versus Flat Plate technology
Evacuated Tube technology captures sunlight better than Flat Plate systems because tubes maintain a greater surface area during the movement of the heater in relation to the sun. The net benefit is more hot water over a longer daily period. (refer to diagram on right)
Evacuated Tubes are more efficient at transferring heat as there is considerably less heat loss because of the vacuum between the tubes. This acts on the same principle as a Vacuum Flask, and ensures a good standard of performance on cold windy days.
Evacuated Tube collectors can be used in subzero temperatures without the system sustaining damage. Flat plate systems often require expensive and complicated "antifreeze" systems to be installed.
Evacuated Tubes are strong, long lasting, and should one be broken, inexpensive and easy to replace.
Due to the high efficiency absorption of solar radiation even during overcast conditions, combined with excellent insulate properties of the tube, Evacuated Tube collectors will heat water all year round (backup required for particularly overcast or rainy weather).
Due to the various advantages of Evacuated Tube collector over flat plate collectors, a smaller collector can be used to provide the same heating performance. For example, a standard household of 4-5 people would usually require a 250-300L water storage tank. Depending on your location, only 2.1 - 2.8 square metres of Evacuated Tube surface area would be required to efficiently heat this volume of water. Another advantage is evacuated tubes do not freeze.
To reduce the cost of flat plate prices various manufacturers have changed the material of the collector from copper to steel. They then need to use corrosion inhibitors to stop rusting.
Another major disadvantage of flat plate is they suffer from condensation collecting on the inside of the glass and mould growth after a time. Most of the flat plate systems that I have seen after a few years are completely opaque.
This is the typical lay out of piping which is all generally 20mm copper.
If the tank is higher than the collector the thermosyphon system can be used ie no pump required. If the tank is lower a pump must be used as hot water does not fall, it rises.
It pays to keep the tank as close as possible to reduce cost and maintain efficiency.
Get the tank above the collector to use the thermosyphon principal to get rid of the pump also for the same reasons.
The flat plate systems that are prone to freezing use anti freeze and the water therefore cannot go directly into the tank but must pass through a coil inside the tank. This increases cost.