Search

Compared to the current state of the art, power cycles based on supercritical CO2 (sCO2) offer the potential for significant increases in power density and efficiency and are thus seen as a promising element for a more sustainable and flexible heat utilization in the future. There is a particular need for experimental work in order to bring this technology to market maturity. For instance, it is necessary to validate theoretical approaches and to test and develop components. As part of the supercritical carbon dioxide laboratory (suCOOLab), a test facility was set up at TU Dresden. The aim is to provide a flexibly usable and expandable test infrastructure for basic research, validation work, as well as small-scale component tests and development. Key components of the system are an electric heater, two compressors and a powerful cooling system. Temperatures up to 300 °C and pressures up to 20 MPa allow covering of a wide range of parameters in the trans- and supercritical fluid region of CO2. The integration of a capable measurement and data acquisition system ensures the continuous availability of measurement data at all relevant points in the rig. Experimental setups can be flexibly integrated into the process at various locations, which additionally allows the simultaneous use of different parameter ranges in one or more experimental setups. Furthermore, special care was taken within the selection of the components itself. All parts, including the compressors, are designed for a lubricant free operation ensuring high fluid purities. This is beneficial for experiments with pure CO2 and also enables investigations of CO2-mixtures. This paper describes the design and the current status of the suCOO-Lab test facility at TU Dresden. Special emphasis is placed on the targeted parameter range, characteristic design aspects of the rig, and on the capabilities of measurement and data acquisition., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 78

The investigation of favorable operating conditions for sCO2 cycles for waste heat recovery is a process that requires a proper analysis of operating parameters, component design, and economic performance. In this aspect, examining the heat exchanger has preponderant significance. In this study, as part of the Carbosola project, a thermoeconomic multiobjective optimization of the sCO2 cycle is analyzed based on a onedimensional design of the heat exchangers, focusing on the recuperator. The main objective is to investigate the effect of the non-linearity of the CO2 properties in the heat exchanger geometry and on the cycle performance. The results highlight the significance of the one-dimensional investigations in the optimization process, while sensitivity analyses indicate the recuperator as key equipment for the optimal cycle operation. Furthermore, the examination reveals the influence of the CO2 properties characteristics at different operating ranges and its effects on the system's equipment design and thermo-economic performance. Finally, the results indicate a prominent potential of the sCO2 Preheating architecture for heat recovery from gas turbines., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 140

The favorable location of the critical point close to common ambient temperatures makes carbon dioxide (CO2) highly attractive to be used as working fluid for supercritical power cycles. The combination of the thereby wide usable range of temperatures with the special fluid properties close to the critical point, e.g. high densities and low viscosities, holds a distinctive potential for significant efficiency increases as well as smaller component sizes compared to the actual state of the art. However, due to the highly non-ideal behavior of the fluid properties in the regions of interest, especially at near-critical conditions, reliable equations of state (EoS) are needed to correctly predict the fluid behavior. This concerns all steps in design and development of supercritical power cycles, from the preliminary modeling of the cycle up to tasks of detailed engineering of individual components. If, in addition, mixtures or impurities are considered instead of a pure substance, the deviation of the EoS of each component is also included in the mixture calculation, which underlines the importance of accurate EoS. Therefore, a certain sensitivity is required to what extent the selection of the EoS may influence the expected results. In this work, the influence of different equations of state on the thermodynamic design of CO2 power cycles is investigated. Within this context, five different equations of state were compared to each other by calculating a selection of power cycle configurations, which are typically considered for sCO2 applications. Aside characteristic process parameters such as relevant fluid properties at each state point of the cycle and the thermal efficiency, differences in the sizes for the internal heat exchangers are considered. The results show, with some exceptions, a largely good agreement in the cycle efficiencies for most of the considered EoS. However, it can also be seen, that the thermophysical properties can differ significantly between the EoS, which is also reflected in notable variations in the heat exchanger performance parameters and furthermore may lead to non-negligible deviations in subsequent evaluations., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 66

Compared to existing technologies, thermodynamic cycles based on supercritical carbon dioxide (sCO2 ) are leading to higher efficiencies and a more compact design of the components. However, it is possible to improve the performance of sCO2-based power cycles by using mixtures of CO2 with suitable additives, as also discussed in the literature for some applications such as concentrated solar power plants or the usage of geothermal heat. The large variety of possible fluid combinations makes an experimental investigation of all conceivable additives considerably difficult. Therefore, a more viable alternative is to set up a model for the power cycle and conduct a screening in order to identify promising candidates for working fluid mixtures. In a next step these could subsequently be experimentally verified. In order to carry out a screening, a preferably accurate and likewise predictive mixture model is needed. This work investigates the potential to optimize the characteristics of sCO2 power cycles by selectively adding different substances in varying amounts to CO2. For the theoretical screening, the reference equation of state for CO2 was applied in combination with a multi fluid mixture model. In the literature studies were mainly limited to mixtures for which adjusted mixture models are available. In contrast, in this work the use of a predictive mixture model allows a screening of additional fluids for which multi parameter equations of state are available (e.g. alkanes, alkenes, alcohols, and hydrofluorocarbons). The predictive model, which was recently developed at our institute, allows the use of the excess Gibbs energy model COSMO-SAC in combination with the multi fluid mixture model. Applied to an exemplary thermodynamic cycle, changes in efficiency compared to the use of pure CO2 have been evaluated. Several pro mising mixture candidates have been identified. Additionally, shifts of the critical point have been investigated and are discussed., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 212

Cycle architecture, fluid parameter selection, and component design of an exhaust/waste heat recovery cycle require an integral approach. The exhaust/waste heat shall be utilized to a maximum but at minimum costs. The bottoming cycle needs to be aligned with the topping cycle regarding operational behavior, especially for a part load. To analyze potentials of exhaust heat recovery in a combined gas turbine sCO2 cycle, the bottoming cycle's optimum cycle architecture and fluid parameters must be determined. A thermo-physical model of the sCO2 bottoming cycle, including knowledge of component design, component behavior, and costs, is based on the multiobjective optimization procedure. As part of the CARBOSOLA project, techno-economic optimizations for a use case of exhaust heat recovery have been carried out. The paper aims to present the optimization methodology followed by the specific use case's boundary conditions, investigated sCO2 cycle architectures, and results of optimum cycle architecture and fluid parameters for maximum heat recovery and minimum costs. Attention will also be paid to accurate modeling of heat exchangers operating near the critical point., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 113

This study investigates the performance of a centrifugal compressor stage operating with supercritical CO2. The candidate geometry comprises a channel diffuser and is based on main dimensions of a test-loop compressor operated by Sandia National Laboratories. A non-dimensional performance curve is derived through three-dimensional RANS calculations performed with an in-house compressible CFD solver and is compared to experimental data as well as a meanline analysis that is conducted applying a single-zone modelling approach, including internal and external loss models. Within the CFD simulations, real gas thermophysical properties of carbon dioxide are assessed through an accurate and efficient tabulation procedure, the Spline-Based Table Look-Up Method (SBTL), which is optimised for the density-based solver architecture. The conducted RANS calculations show an ideal head rise of up to 41%through the channel diffuser compared to an ideal head assessment considering the impeller. The applied meanline methodology provides good agreement with the impeller performance characteristics derived from RANS calculations over the entire investigated flow range. Except for flow coefficients exceeding values of φ ≈ 0.043, where an abrupt decrease of the ideal head recovery is identified in the CFD assessments, which is not resembled by the meanline diffuser model, also satisfactory agreement of the meanline stage performance characteristics with the CFD results is obtained. Hence, applicability of the corresponding meanline loss models for sCO2 compressor analysis is indicated., Conference Proceedings of the European sCO2 Conference4th European sCO2 Conference for Energy Systems: March 23-24, 2021, Online Conference, p. 230

In solarthermischen Kraftwerken (Concentrating Solar Power (CSP)-Kraftwerken) dient die solare Direktnormalstrahlung (DNI: Direkt Normal Irradiation) als Wärmequelle für einen thermodynamischen Kreisprozess. Somit handelt es sich um Kraftwerke für Regionen mit hoher solarer Direktstrahlung (> 1.500 kWh/m²a). Aufgrund hoher Konzentration der Strahlung in den Solarkollektoren erreichen Solarturmkraftwerke höhere Systemwirkungsgrade als Solarkraftwerke mit Parabolrinnen. Das Maximum des Systemwirkungsgrades eines Solarturmkraftwerks liegt bei einer maximalen Prozesstemperatur von rund 1.000 °C, z. B. [1]. Da dieses Temperaturniveau bei Dampfkreisprozessen nicht erreicht werden kann, erscheint es sinnvoll, die Hochtemperatur-Solarwärme primär in einen Gasturbinenprozess einzuspeisen. Dies geschieht durch externe Aufheizung der aus dem Verdichter austretenden Luft einer Gasturbinenanlage (GTA). Durch die seriell oder parallel zur Solarwärmeeinspeisung angeordnete Brennkammer (Bild 1) kann ein derartiges solar-hybrides Gasturbinensystem eine bedarfsgerechte Stromerzeugung ohne Back-up-Erzeugereinheiten auch bei jahres- und tageszeitlichen Schwankungen des Solarwärmeangebots gewährleisten.

Die Erhöhung der Prozessparameter Druck und Temperatur bietet das größte Potenzial für die Effizienzsteigerung von Kraftwerksanlagen. Sie ist jedoch eng an die Werkstoffentwicklung gebunden, da die thermischen und mechanischen Beanspruchungen für eine geforderte Bauteillebensdauer bis an die Grenzbeanspruchbarkeit des Werkstoffes gehen. Um eine sichere Auslegung zu gewährleisten und dennoch eine optimale Werkstoffauslastung zu erreichen, wird weltweit an der Weiterentwicklung der Berechnungsverfahren für komplexe Beanspruchungen gearbeitet, so auch im Institut für Energietechnik der TU Dresden. Ein Forschungsschwerpunkt liegt dabei auf dem Gebiet der thermisch-mechanischen Ermüdung. Aber auch der Betrieb von Anlagen der Hochtemperatur-Energietechnik beeinflusst deren Sicherheit und Verfügbarkeit. Diesbezüglich werden im Beitrag einige Aspekte angerissen. Raising of the process parame- ters pressure and temperature offers the greatest potential for increased power station effi- ciency. However, this is closely dependent on materials devel- opment, as thermal and mechanical stresses over the required service life approach the stress limits of the materi- als. In order to ensure safe designs, but at the same time to optimise material utilisation, the permanent enhancement of design procedures for complex stresses is a subject for R&D worldwide, including the Institute of Power Engineering of the TU Dresden. Here, ther- mal-mechanical fatigue at high temperatures is the main focus of research. But power station operation similarly influences safety and availability. Corresponding aspects are also touched upon in the paper.