Solar district heating with evacuated collectors: first year experience of the Knivsta plant

The experience gained during the summer enable some practical demands to be placed on solar collectors, namely: it must be possible to easily replace a solar collector without emptying the system; it must be easy to bleed the system; solar collectors must be able to withstand overheating due to boiling; and obvious leakage risks must be eliminated. There is no doubt that not all types of evacuated solar collectors fulfill these requirements and that further development is essential before large-scale installations with rational operation can be considered.