Shaft-end sealing for sCO2 turbomachinery is more challenging than typical applications in centrifugal compressors for Oil & Gas. Due to strong influence of specific heat capacity near the critical point and low dynamic viscosity of sCO2 on machine efficiency and cycle performance, the lowest leakage rates are crucial for the technical feasibility of many applications. These requirements mandate the use of contactless sliding face seals (Dry Gas Seals – DGS). One of the most demanding applications is within radial expanders for power cycles, where process gas temperatures and pressures exceed 500 °C and 80 bar, with tangential speeds of rotating ring higher than 180 m/s. Current designs use a thermal management zone to limit the temperature on DGS (especially for secondary sealing elements of polymers or elastomers) to 200 °C. Such thermal zones utilize hot and cold flows to create an optimal thermal gradient on the shaft while not resulting in excessive thermal stress. A high-temperature resistant DGS with low leakage rates as a replacement for the thermal management zone would increase machine thermal efficiency, simplify the gas inlet and outlet flows, reduce the axial length required for shaft-end sealing (making possible the addition of new machine stages or shortening machine shaft) and reduce thermal stresses on machine shaft.

This paper presents the preliminary results of the development of a prototype DGS for turbomachinery in sCO2 applications with leakage rates no higher than 1,5 Nl/min for each bar sealing pressure operating with sealing gas at 510 °C and 89 bar and rotating ring tangential speed of 192 m/s. The machine shaft diameter is 110 mm.

The first project stage comprises the development of one DGS component: a high-temperature resistant balance sealing element, which seals the radial gap between stationary ring and balancing sleeve. Subjected to a dynamic reciprocating motion it is one of the most critical components in any DGS. The prototypes were tested with air/helium at room temperature and with CO2 at material design temperature (600 °C). The second project stage consists of upgrading two DGS (one tandem and one single with identical core parts) to a high-temperature resistant design. This was done by replacing usual temperature limited components by their respective high-temperature versions and doing the required design adaptations each at a time. After every upgrade each DGS was statically and dynamically tested to the operating conditions with air/helium mixture supplied at room temperature (the highest recorded temperature inside DGS was about 220 °C). Tests allowed the evaluation of equipment performance by means of leakage rates and leakage stability after changing operating conditions (speed and pressure). Visual inspection of internal parts after each test were performed. The test bench for CO2 supplied at 510 °C is still being commissioned to the time of this publication.

It was verified during the dynamic tests with air/helium supplied at room temperature that the leakage rates remained stable and around 1 to 1,3 Nl/min/bar for operating conditions. Design is still to be validated with dynamic tests with CO2 supplied at design temperature.

Supercritical carbon dioxide (sCO₂) Brayton power cycles offer higher efficiency and more compact component sizes than traditional Rankine power cycles. These attributes make sCO₂ technology particularly appealing for future concentrated solar power (CSP) applications, as the increased cycle efficiency could lead to smaller CSP components, such as the mirror field. One of the major factors affecting the efficiency of these cycles is the design of the turbomachinery. It has been shown that marginal increases in the efficiencies of the compressors and turbines can significantly reduce the size of the costly recuperators, thereby lowering cycle costs and further enhancing overall cycle efficiency.

The focus of this work is on the preliminary design of high-pressure compressors (HPC), low-pressure compressors (LPC) and recompression compressors (RCC) for a 50 MWe partial cooling sCO₂ Brayton power cycle. For the current work the scope is limited to the design of centrifugal compressors. The design calculations utilise an in‑house developed real‑gas mean-line analysis approach along with aerodynamic and parasitic loss models accounting for phenomena such as incidence and disk friction losses. Initially the study sets out to design the compressors using typical values for flow coefficients, work coefficients, rotor tip diameter ratios and meridional velocity ratios. Once the reference compressors have been designed, a sensitivity study is conducted to assess the impact of various design parameters on compressor geometries and thermodynamic performance. The sensitivity study includes sets of design variables selected to comprehensively cover the design space using Latin-Hypercube sampling. For each of the generated samples, the mean-line analysis code, developed in the Engineering Equation Solver (EES) software, is utilised to calculate compressor geometries and thermodynamic performance of the machines. The impact of the various design variables are therefore determined, and finally a compressor train is designed for the partial cooling power cycle.

The design of the compressor train for the 50 MWe power cycle is completed using flow coefficients between 0.1-0.55, work coefficients between 0.26-0.9, meridional velocity ratios between 0.23-0.99 and rotor tip diameter ratios between 0.6-0.75. This resulted in a five compressor system with a single HPC and RCC together with three LPCs in parallel, all with a design rotational speed of 20000 rpm. For the LPC reference case, the requirement for three machines in parallel is dictated by the low pressure-ratio combined with the high volume flow rate. The sensitivity analysis yielded compressor design configurations with isentropic efficiencies ranging from 75% to 90% and rotor tip diameters between 210 mm and 250 mm, demonstrating the potential for high efficiency and compact designs in sCO₂ Brayton power cycles.

As a consequence of global warming caused by greenhouse gases, there has been an increase in interest in improving energy efficiency and developing carbon-free energy sources. In particular, there is a considerable amount of unused waste heat and unused energy globally. It is anticipated that greenhouse gas emissions could be significantly reduced if energy conversion technologies capable of utilising this waste heat were developed. Among the aforementioned waste heat-based energy conversion technologies, supercritical CO₂ power generation is attracting considerable attention.

Carbon dioxide is attracting attention as an environmentally friendly working fluid due to its non-flammable, non-toxic, high thermal stability and economic characteristics. The supercritical CO2 power generation system utilises a compact configuration, low pressure ratio, small turbine size and the ability to be applied to various heat sources by exploiting the unique physical properties of CO2. This system has significant potential as a next-generation energy conversion technology, as it is capable of efficiently converting power from a range of energy sources, including solar, geothermal, waste heat and nuclear. In particular, the utilisation of waste heat is beneficial for improving energy efficiency and reducing greenhouse gases. Consequently, KEPCO has focused on the development of waste heat utilisation technology due to the abundance of potential resources. The supercritical CO2 power generation system is comprised of two principal components: compressors and turbines. In particular, compressors are known to be challenging to operate due to the rapid alteration of numerous physical properties in the vicinity of the critical point. In order to enhance the likelihood of success of the demonstration at the nascent stage of the development of the supercritical CO2 power generation system utilising waste heat, and to expedite its commercialisation, KEPCO facilitated the advancement of the system through the utilisation of units that were nearing commercialisation. At that time, the inlet conditions of the compressor were selected to be extremely low in order to minimise the inlet temperature, thereby increasing the efficiency and output of the system, while the integrated gear-type compressor was selected to reduce the system cost. This paper presents the initial attempts and results related to the design, fabrication and testing of the compressor and turbine during the development of a 2 MW supercritical CO2 power generation system using waste heat, which has been under development by KEPCO since 2016. The paper also discusses the challenges encountered during this process.

Supercritical Carbon Dioxide (sCO₂) turbines, modeled on traditional axial-flow steam turbine design practices, often rely on empirical loss models developed by experimental campaigns on steam and gas turbines blades with aspect ratio greater than one. This study evaluates the applicability of these models for designing sCO₂ turbines featuring low aspect ratio blades (under 1). A low fidelity modeling tool (zTurbo), originally formulated based on Craig and Cox (CC) and Traupel (TR) loss correlations, was extended by integrating loss models proposed by Kacker Okappu (KO) and Aungier (AG). zTurbo was applied for Mean Line Design (MLD) of a five-stage axial-flow low aspect ratio sCO₂ turbine (236.5 bar, 893.2 K inlet; 81 bar outlet). The four MLDs conceived using CC, KO, TR, AG loss models achieved flowpath total-to-total efficiency of 87.5%, 87.5%, 90.8%, and 91.9%. A detailed comparative analysis of four MLDs led to selection of AG loss model coupled with TR tip loss correlation to effectively capture all the losses within sCO₂ turbine flowpath. zTurbo was linked with the Non-linear optimization algorithm to generate optimized AG based MLD flowpath exhibiting a total-to-total efficiency of 92.2 %. The optimized AG based MLD was then modeled to form full 3D flowpath, and high fidelity numerical simulations were performed to evaluate the turbine performance and losses encountered along the flowpath. Both modeling approaches showed strong agreement for estimating turbine flowpath total-to-total aerodynamic efficiency, differing by less than 0.1%. A detailed loss analysis comparison between two modeling techniques showed AG loss correlation reliability for estimating profile losses; however it overestimated the secondary losses, highlighting the need for detailed investigation into secondary flow development in sCO₂ turbines featuring low aspect ratio blades.

The Brayton Loop facility at Sandia National Laboratories (SNL) has been instrumental in identifying technology gaps and developing solutions critical for the commercialization of supercritical carbon dioxide (sCO2) power cycles. The benefits afforded by these new conversion technologies are required to fully leverage the potential from the next generation of nuclear reactors: increased efficiency, smaller footprint, and the ability to use dry cooling. While the recompression closed-loop Brayton Cycle (RCBC) test loop test program has been successful, its full potential has been stymied by the poor reliability of the turbine-alternator-compressor (TAC) bearings. While many of the early events were due to poor controllability issues, resulting from the first-generation power electronics, it demonstrated their lack of robustness necessary for these high-power density machines.

Despite contactless operation at speed, foil air bearings are in contact with the shaft at all speeds that are below that required for lift-off; around 8,000 RPM. Therefore, surface to surface contact occurs during start and shutdown, leading to wear. These unpreventable events contribute to loss of low friction surface treatment and are life limiting. While fully developed foil bearings are commercially applied to air cycle machines and small gas turbines (microturbines), rotors of low mass, this failure mechanism is amplified for large commercial sCO2 systems with heavy rotors and larger loads, rendering them unsuitable for future commercial small modular reactors (SMRs). It was therefore decided to identify and test more robust bearing solutions that would not only improve run time accumulation and facilitate performance and cycle configuration testing, but also demonstrate solutions which are scalable and better suited for large commercial systems.

As part of the TAC improvement program, and after the control system upgrade, the bearings were identified as the main technology to increase TAC reliability and efficiency. TACs currently employ aerodynamic foil type bearings. This report considers the retrofit design, acceptance testing, and the initial testing of a TAC retrofitted with externally pressurized porous (EPP) gas bearings. Both radial and axial constraint are required to support the rotor and react the axial thrust load. This was accomplished by replacing the compressor journal and thrust bearings with a single EPP bearing assembly, and a simple replacement of the journal on the turbine side.