Composite heat insulation system

A method for the removal of gaseous decomposition products from a high temperature heat transfer fluid (HTF) of an operational solar thermal power plant having an HTF circuit, in which a volume increase of the HTF in the HTF circuit which is caused by incident solar radiation in an HTF-traversed solar field and consequently heating by day takes place regularly in a day-night cycle, comprising:

• collecting from the HTF circuit the additional volume formed by the volume increase in an expansion vessel;
• transferring a portion of the additional volume of the HTF into a drainage vessel operated at relatively low pressure in which gaseous decomposition products and low-boiling constituents escape from the HTF, wherein the low-boiling constituents are condensed;
• and
• recycling a portion of the additional volume of the HTF, resulting from the volume contraction of the HTF occurring during the night time cooling, from the drainage vessel into the expansion vessel and from the expansion vessel into the HTF circuit or directly from the drainage vessel into the HTF circuit, wherein the volumes in the expansion vessel and the drainage vessel becoming vacant as a result of the transferrals of the HTF are filled with an inert gas.