Flexible All-metal Pipes and Pipe Couplings For High-Temperature Fluid Transport

Molten salt heat transfer fluid a desirable upgrade to solar plant designs because of its superior temperature, thermal, and pumping properties compared to steam or heat transfer oil. It is non- hazardous to the environment, not flammable, and easy to transport and clean up. It is advantageous for thermal efficiency of engines because it allows higher maximum operating temperatures. It is an excellent thermal storage medium because of these high temperatures and because it can be pumped directly through solar collectors and into storage containers. These advantages make it a clear choice for the next generation of concentrated solar power plants around the world. However, existing solar receiver designs cannot readily be used with molten salt because they require pipe couplers that allow the parabolic troughs to rotate through 180° each day for solar tracking. These couplers do not exist for use with molten salt because of corrosive conditions introduced by the salt and the high temperatures, which ruin industrial seals. All- metal couplers such as bellows with stainless steel pressure jackets, seem like good options except that when the salt cools in a confined space, it forms crystalline solids; upon melting, excessively large pressures can be generated by the expanding liquid salt that becomes trapped in the corrugations of the bellows. It is important to solve these issues because the levelized cost of electricity provided by solar plants will decrease if operating temperatures and storage capacities increase. In turn, electricity produced by clean, renewable solar power will be competitive with electricity produced by other means---an important contribution to our nation's energy independence. The proposed, flexible, rotational pipe coupler addresses these needs. It is all metal, so it is not very susceptible to corrosion or temperature damage from the molten salt and it can be welded to other pipes, preventing leakage problems common with molten salt. There are no corrugations to trap salt and rupture the pipe during freeze/thaw cycles. In phase I, the coupler, which relies upon a novel cross-section, will be analyzed and optimized to ensure proper flexibility and fatigue/creep lifetime. Manufacturing trials will determine how to fabricate the couplers for maximum pressure performance and durability. A final design predicted to operate with molten salt at 550° C for 30 years will be the main result. The molten salt coupler is an enabling technology for molten salt concentrating solar plants, but it is also suitable for use in other piping systems that require flexible couplers with seal-less or high-temperature operation, such as existing tower-based solar plants, conventional power plants, oil and gas piping operations, and supercritical CO2 .