The heat pipe collection tube resides within the single layer glass evacuated tube.  The vacuum stops the transmission of the outside air temperature to the heat pipe, and prevents heat generated by the heat pipe from radiating back out into the air.  Only the radiational energy (Uva, Uvb, infer red, basically all the reasons you wear sun block) of the suns rays pass through the glass and vacuum to the collection fin.   Many other so called evacuated tubes have a double walled glass tube that contains a vacuum and the tube resides in a air pocket inside the double wall system, so the copper collector tube is not truly in a vacuum, but a hybrid.  They are still more effective in winter than flat plate collectors.