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*[http://en.wikipedia.org/wiki/Solar_water_heating Solar water heating Wikipedia]
Evacuated tube collectors (ETC) are a way in which heat loss to the environment,[22] inherent in flat plates, has been reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes.[28] Since two flat sheets of glass are normally not strong enough to withstand a vacuum, the vacuum is rather created between two concentric tubes. Typically, the water piping in an ETC is therefore surrounded by two concentric tubes of glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent.[29] Life of the vacuum varies from collector to collector, anywhere from 5 years to 15 years.

Flat plate collectors are generally more efficient than ETC in full sunshine conditions. However, the energy output of flat plate collectors is reduced slightly more than evacuated tube collectors in cloudy or extremely cold conditions.[22] Most ETCs are made out of annealed glass, which is susceptible to hail, breaking in roughly golf ball -sized hail. ETCs made from "coke glass," which has a green tint, are stronger and less likely to lose their vacuum, but efficiency is slightly reduced due to reduced transparency.

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