Evacuated tubular or classical flat plate solar collectors?

Evacuated tubular solar collectors are increasingly used all over the world due to their low coefficients of heat losses to the environment. They are presented as devices that collect much larger quantities of solar energy than are usually obtainable from typical flat collectors. However, they suffer from the poor radiation transmissivity characteristics of the transparent shield covering the absorber. This makes the profits in terms of energy gain in the operating conditions of a typical solar power system in Poland only slightly dependent on the nature of the solar collectors used. This article seeks to explain this phenomenon through theoretical considerations.