The parabolic trough (PT) standard receiver is integrated by an inner steel pipe surrounded by a glass tube to reduce convective heat losses from the hot steel pipe. The steel pipe has a selective high-absorptivity, low-emissivity coating that reduces radiative (emission) heat losses. Receiver tubes with selective coating achieve higher thermal efficiency at high operating temperatures (400°C). Receiver tubes without selective coating are usually for working temperatures below 250°C, because thermal losses are less critical at these temperatures.
The glass pipes are coated with an antireflective (AR) coating to minimize reflection losses. Due to manufacturing constraints, the length of single receiver pipe is 4 m, so the complete receiver tube of a parabolic trough assembly is composed of a number of single receiver pipes welded in series up to the total length of the assembly (150 m).
The outer glass tube is attached to the steel pipe by means of flexible metal differential expansion joints (bellows) which compensate for the different thermal expansion of glass and steel when the receiver tube is working at nominal temperature. The glass-to-metal welding used to connect the glass tube and the bellow is protected by aluminum shield, placed over the welding.
Two chemical getters (for each tube) are placed in the gap between the steel receiver pipe and the glass cover to absorb gas molecules from the fluid that get through the steel pipe wall to the annulus. The getters are protected by radiation shields. | 
 "The Receiver Layout")
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