There are a variety of evacuated tubes designed, manufactured and used commercially for water heating purposes. Single wall tube as per the manufacturer has a life span of minimum 15 years. Double wall glass tube is more efficient and reliable with a life of more than 20 years.

the lifetime of evacuated tubes being used for solar commentators is measured by the mean time to failure (MTTF) or mean time between failures (MTBF). These reliability measures are documented in many textbooks and reliability databases. So all what you have to do is to do literature search on the two figures of merits MTTF or MTBF for this component.

The MTTF or MTBF for the evacuated tubes are based on reliability accelerated life tests (ALT) or sometimes highly accelerated life tests (HALT).

Hope this helps answer your question.

Thanks for your answers, are very useful.

The vacuum inside the tube remains all these years? or we need to generate from time to time as maintenance?

Regards!

Yes, the vacuum created during manufacturing will remain for its life of 15 -20 years.   At present the tubes which are being used in our research centre are working effectively. As the tubes are factory sealed, it will not be possible to create vacuum when ever it is lost. at our end.

Jesus - the vacuum in the tubes must be sustained for the lifetime of the tube. Loss of vacuum is indicative of leaks in the tube and most likely such leak or leaks cannot be repaired. Moreover it would be more costly to do so compared to just replacing the failed tubes.

Typically, the manufacturers give a warrantee period for their products and, thus, would repalce failed tubes during the warrantee period.

Hopes this answer your question.

Depend on the end control in production lines from different manufacturer we can expect a life time of evacuated tube over 20 years (average) from our experience. 

its depens on manufacter data, some papers explaint the life time period are 15 or 20 years