Evacuated tubes are a newer design of collector, offering a slightly more efficient solar gain.  The collectors consist of a set of evacuated glass tubes (similar to a vacuum flask) with concentric borosilicate glass collector pipes inside to carry the heat transfer fluid.  The outer surface of the inner tube is selectively coated for maximum solar heat gain and an aluminium circular mirror ensures maximum energy collection.  These tubes are one of the most efficient on the European market, and a proven design with again little to go wrong.  They are sold as part of the Vaillant solar system, which we are trained to install.  Obviously state of the art technology costs a little more.