A sun energy collecting unit of a sun field power plant based on the concentrated solar power technique is for instance a parabolic trough collector with parabolic mirrors and a heat receiver tube. The heat receiver tube is arranged in a focal line of the mirrors. By sunlight reflecting surfaces of the mirrors the sunlight is focused to the heat receiver tube, which is filled with a heat transfer fluid, e.g. a thermo-oil or molten salt. With the aid of a solar radiation absorptive coating of a core tube (inner tube) the heat receiver tube absorbs energy from the sun. Energy from the sun is efficiently coupled into the heat transfer fluid. Solar energy is converted to thermal energy.
In order to minimize a loss of thermal energy, the heat receiver tube comprises an encapsulation out glass (glass tube). The glass tube, which is transparent for the sunlight, is arranged coaxially around an inner, central stainless tube of the heat receiver tube. The space between the inner tube and the glass tube is evacuated.