Mechanical Stress Optimisation in a Directly Illuminated Supercritical Carbon Dioxide Solar Receiver

Abstract

CSIRO is constructing a solar thermal driven high temperature supercritical carbon dioxide (sCO2) Brayton cycle test facility, which includes an absorber tube based solar receiver, at the CSIRO National Solar Energy Centre in Newcastle, Australia. The sCO2 facility is capable of producing high temperature CO2 at 700° Celsius. In this paper the mechanical challenges of designing a sCO2 solar receiver are presented. The design is an optimization of the impact of having a high thermal gradient through the absorber tube wall as a result of high heat transfer and the effect of containing sCO2 under high pressure inside the absorber tube. The first drives the design toward needing a thin tube wall to reduce the thermal gradient stress, whereas the high pressure drives the absorber tube toward needing a thick wall. It is worth noting that the thermal stress being considered here are those within the tube wall thickness only and not about the effects of thermal expansion along the length. The contradictory nature of these two drivers resulted in an iterative approach to choosing the receiver’s optimal absorber tube size with nine options investigated and compared for optimal mutual design conditions for pipe wall stresses cause by internal pressure and thermal gradient. This led to choosing the smallest available pipe size of 3/8 inch considering the available high heat resistant and high strength materials and conventional construction techniques for seamless pipe. Going to smaller diameters would require eliminating pipe sizes from the selection and restricting the availability to standard tube sizes, and also eliminating the high heat resistant and high strength materials from the selection. The other factor in the design decision was to consider the constructability of an absorber tube based receiver that will allow conventional pipe fit up and welding techniques. Ultimately, the sCO2 operating conditions still produced pipe wall stress high enough to limit the life of the solar receiver in the order of 1,000 hours due to high temperature material creep. This outcome is suitable for short lived experimental work but may not be suitable for commercial long lived projects. Building a commercially viable sCO2 solar receiver would require the selection of costly very high strength and creep resistant materials, or a receiver design that allows economic replacement of parts during the plant life, or the development of alternative receiver designs and construction techniques.