5 In 2015, under a US Department of Energy/National Energy Technology Laboratory programme, a cooperative grant funding solicitation was issued to design, build and operate a 10 MWe supercritical carbon dioxide demonstration loop – the STEP Demo project. The project was won by GTI Energy, a technology development and training organization, who became project lead working in partnership with GE Vernova, and fellow non-profit organisation SwRI, along with power cycle specialists including Heatric and others. Fundamental to the power cycle is the need for large amounts of heat recuperation and among the key components for the STEP Demo project are three high efficiency heat exchangers based on PCHE technology. These exchangers are the High Temperature Recuperator (HTR), the Low Temperature Recuperator (LTR), and the Main Cooler (MC). Heatric won the contract for the design, engineering, manufacturing, and supply of all three heat exchangers in August 2018. In terms of design, the MC is a relatively conventional PCHE similar to those widely used and well-proven in the oil and gas industry over many decades. The LTR design is slightly more customised for the duty but still close to conventional PCHEs in its layout, whilst the HTR is a heavily customised PCHE to manage more aggressive process conditions while still using regular austenitic stainless steel. This HTR design led to many collaborative work activities and iterations between GTI Energy, SwRI and Heatric to facilitate its integration within the STEP Demo process loop. Design conditions for the main cooler are 150°C temperature and 175 barg pressure. For the low temperature recuperator, design conditions are higher than those found in the MC at 290 bar and 250°C, although still easily managed by conventional PCHE technology. However, for the high temperature recuperator, a design temperature of 600°C and pressures of the order of 290 bar make for much more challenging conditions that had to be addressed and required some design changes for the plant even post-contract. The difficulty of this sCO2 cycle is the combination of high pressure and high temperature; and whilst that is advantageous from a thermodynamic point of view, it is a major challenge from a thermal mechanical design point of view. To manage this high-temperature and high-pressure CO2, the necessary pipework needed to use thick walls leading to very high thermal stress / loads on the recuperator as the thermal expansion of thick sections can deliver substantial mechanical forces. The design of the pipework to bring the working fluid into and out of the heat exchangers and turbines thus became a complex problem. Secondly, a fully bonded and fully welded compact heat exchanger with a large heat gradient results in high bending stresses. This thermal expansion issue presented substantial engineering challenges that had to be overcome. Image 4 – Heatric “HTR” (High Temperature Recuperator) PCHE in a support frame at the 10 MWe STEP Demo project in San Antonio, Texas in 2023. STEP INTO THE FUTURE “We are delighted with the project achievements to date. The STEP Demo facility is a driving force in power cycle technology development, establishing the groundwork for future sCO₂ applications. GTI Energy is proud to be leading this collaboration enabling future low carbon power generation processes.” John Marion, Senior Director for Carbon Management & Conversion, GTI Energy
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