Development of a Profiled Multilayer Tube for High Temperature Solar Receivers and Heat Exchangers

In solar tower plants absorber tubes are the main components of various solar receivers, e.g. steam receivers, salt receivers, pressurized air receivers. The solar radiation on the absorber tube causes an inhomogeneous temperature distribution because most of the solar radiation along the tube circumference is one-sided. The resulting internal stresses of the tube and the maximum fluid temperature decrease the lifetime significantly. Within the projects SOLHYCO and FUTUR a profiled multilayer tube (PML) is currently under development in order to reduce this problem. It consists of three metallic layers: a high temperature nickel-based alloy at the outer side, a copper layer as intermediate layer and another high temperature nickel-based alloy at the inner side of the tube. The outer layer provides the structural strength while the copper is used to conduct the heat from the irradiated side to the opposite side. The inner layer protects the copper from corrosion and oxidation at high temperatures. In addition, a wire coil is inserted (profiled) to increase the heat transfer on the inside. The PML is manufactured in a hydro-forming process by deforming the tube composite with water under high pressure. To demonstrate the performance and to determine the heat transfer, the pressure loss and the temperature distribution, a test loop was built to simulate the different loads under laboratory conditions. The thermo hydraulic measurements and finite element calculations show that the temperature gradient and the maximum temperature can be reduced significantly. Based on these studies the advantages of the PML in comparison to common tubes will be presented as well as several possibilities for future improvements.