High-temperature heat pumps are gaining increasing interest as a means for electrifying process heat. While a number of systems for steam production has been reported, there are only a few publications on heating air e.g. for drying processes. Due to its high temperature glide in trans-critical cycles, as well as its beneficial physical and environmental parameters, CO₂ is an interesting candidate working fluid for such applications. We have developed a relatively simple cycle that allows the heating of air or other sensible media with few components, where the most complex one (besides the compressor) is a three-stream heat exchanger for simultaneously heating the air as well as allowing internal heat exchange. Simulations show that ambient air can be heated up to 200 °C with a COP of 2.61 and evaporating temperatures of 10°C, allowing for economic provision of process heat, for example for milk powder drying.

Thermal-power cycles operating with supercritical carbon dioxide (sCO2) could have a significant role in future power generation systems. This includes applications like nuclear power, concentrated-solar power, fossil fuel, and waste-heat recovery. However, there remain challenges which needs to be resolved that relate to the design and operation of the turbomachinery components and heat exchangers, material selection considering the high operating temperatures and pressures, in addition to characterizing the behavior of supercritical CO2. This working fluid may be incompatible with thermohydraulic components of device, such as the heat exchangers, where issues around material strength and corrosion may be significant. Therefore, innovative solutions are required.

Brazing process as a coating on a base material offer such a solution. The aim of our contribution was to verify the functionality of the coatings, which were applied to base materials by using of the brazing process. The coatings were on base of aluminum oxide and BNi-5. Two types of steel were used as the base material - 316Ti and 253 MA. These specimens were then exposed to 1000 hours of loop operation at a maximum operating temperature of 550°C and a maximum operating pressure of 25 MPa, with a constant flow rate of sCO2 medium of 0.1 kg.s^-1.

The EU funded Horizon Europe SCO2OP-TES project aims to develop a Thermally Integrated Pumped Thermal Energy Storage (TI-PTES) system based on sCO2 components and test it in an operative environment valorising the waste heat coming from a combined cycle power plant.

The test-rig, hosted in the joint laboratory by University of Genova and Tirreno Power, will consist of two purposely designed charging (Heat Pump) and discharging (Recuperated Power Cycle) cycles integrating different sCO2 enabling components and having as common component an innovative Solid Media Thermocline TES inter-exchanging heat with the sCO2 cycles via a Molten Salt loop to facilitate its controllability. In specific relation to the different enabling components (Heat exchangers and machines), the project aims to validate he reliability and the technical performance of all key components present in both the cycles. Ultimately, the overarching goal of the project is to serve as a stepping-stone towards the evaluation of the viability of sCO2 based TI-PTES and also of high temperature heat pumps based on sCO2 components . Besides the specific validation objectives, and related component development and verification, the project aims to investigate the techno-economic performance of the proposed sCO2 TI-PTES also looking at alternative layouts and operating temperature/pressure levels (also in terms of waste heat sources to be valorised) as well as the electric market strategicity.

Supercritical carbon dioxide (sCO₂) has emerged as a transformative fluid in energy systems, offering promising improvements in efficiency, performance, size of the equipment, and environmental impact. Recent developments in sCO₂ applications highlight its potential in various energy sectors, including power generation, waste heat recovery, and concentrated solar power. The key advantages of sCO₂ systems include higher thermal efficiency, reduced footprint, and lower operational costs compared to traditional steam cycles. Innovations in sCO₂ cycle designs, such as the Allam Cycle and various recompression cycles, have demonstrated significant improvements in efficiency and environmental impact. Recent developments include novel sCO₂ cycle configurations such as the recompression, partial cooling, and dual-fluid cycles, which optimize efficiency and operational flexibility. Additionally, cutting-edge control technologies and integration techniques are addressing previously challenging operational issues. Advances in materials science and component technologies, including high-temperature alloys and compact heat exchangers, have further enhanced the viability of sCO₂ systems. This study reviews the latest progress in sCO₂ key technological breakthroughs and their implications for the future of energy production, evaluates their integration into existing and new energy systems, and explores ongoing research aimed at addressing technical challenges and optimizing performance. The continued evolution of sCO₂ applications holds the potential to significantly influence the future landscape of energy systems, contributing to more sustainable and efficient energy solutions.

A new and innovative method for visualizing the sub-processes of various thermodynamic cycles used by heat-to-power and power-to-heat processes in heat pumps, cooling systems, and various heat engines will be introduced. Specific attention has been paid to carbon dioxide (CO₂) as a working fluid, whose thermodynamic properties are particularly relevant in modern thermal systems, being able to utilize heat sources around ambient temperature. Due to the unique characteristics of CO₂, an accurate and detailed understanding of the processes is of paramount importance, especially in cycles that start from saturated states and end in saturated final states, such as liquid-liquid, liquid-critical, liquid-gas, gas-liquid, gas-critical, and gas-gas transitions.

In our research, we have focused on the detailed analysis of isothermal (constant temperature), isobaric (constant pressure), and isenthalpic (constant enthalpy) processes. The visualization method we have developed allows a transparent representation of these processes, thus facilitating the understanding of the thermal cycle context.

In particular, our visualization technique is helpful in clearly labeling isobaric, isentropic, and real adiabatic (non-isentropic) expansion processes that occur during CO₂-cycles. These processes are critical in CO₂-based systems because real expansion processes often deviate from ideal conditions, especially in the high pressure and temperature ranges typical of transcritical CO₂-cycles.

Our method also has significant educational benefits by allowing the audience to better understand the complex structure and operating principles of CO₂-cycles. 3D diagrams allow for a clearer understanding of the relationship between the different state changes and a deeper understanding of how CO₂-based power-generating, cooling and heat pump systems work. Furthermore, the method provides the opportunity to explore in more detail the specific characteristics of CO₂ cycles and analyze the differences and similarities between them in more depth, thus facilitating more effective learning and a better understanding of practical applications.

Overall, our work demonstrates how this visualization method can make CO₂-based thermodynamic processes more transparent and understandable and how it can contribute to a better understanding and appreciation of thermal cycles in both practical and educational applications.

Given the increasing demand for eco-friendly refrigeration technologies, the use of natural refrigerants in thermal management systems for High Energy Physics (HEP) detectors is becoming more and more significant. While the application of boiling CO2 for cooling purposes is well-established at CERN, particularly in scenarios requiring low temperatures, this study explores the heat transfer potential of carbon dioxide under supercritical conditions. In this context, fluids above their critical point exhibit key properties such as high thermal capacity and low density and viscosity. Additionally, due to their inherently single-phase-like behaviour, supercritical fluids may facilitate simpler fluid management in complex, multi-branched circuits. These characteristics, coupled with a critical temperature of 31 °C, position supercritical carbon dioxide (sCO2) as an ideal candidate for thermal management systems where electronics are operated above 32 °C. Nevertheless, several aspects of this topic remain unresolved in existing literature. Specifically, the variability in the effects of certain parameters on the heat transfer coefficient creates challenges in establishing reliable correlations, highlighting the need for further precise experimental data.

This study presents the development and implementation of a specialized test rig to evaluate the thermodynamic performance and efficiency of sCO₂-based systems. The rig is designed to measure the heat transfer coefficient and pressure drop in a single small pipe under various operating conditions. It operates at pressures up to 120 bar, with mass fluxes ranging from 500 kg/m²s to 1200 kg/m²s, and uses pipes with inner diameters from 1 to 3 mm. By providing different heat fluxes and controlling inlet temperatures between 20 to 40 °C, the rig ensures a thorough analysis of system behaviour, offering valuable insights for the optimization of sCO₂-based thermal management systems.

The rig is now under commissioning phase. Due to unexpected delays, sCO2 data could not be included in this article. However, the first operation commissioning step, showing the ability of the system to stably and precisely regulate the working pressure at a desired level, is shown at the end of the manuscript.

CO₂ is commercially available in various purity grades, typically containing impurities such as air components: nitrogen, oxygen, and water. This study investigates the impact of impurities on the performance of a supercritical CO₂ (sCO₂) system operating in a sequential heating architecture under both supercritical and transcritical regimes. A multi-fluid mixture model was used to model the thermophysical properties of the investigated mixtures. The parametric analysis indicates that impurities strongly alter the thermodynamic properties of CO₂, such as specific heat, viscosity, and density, and significantly impact cycle performance. By analyzing twenty CO₂-based mixtures with varying purity levels and chemical compositions, the study reveals that impurities particularly affect the density of CO₂, leading to performance degradation. The results show that system performance is highly sensitive to CO₂ purity levels. Mixtures over 99.8% purity exhibited minimal performance losses, with net power reductions limited to 2%. Mixtures with 99% purity experienced losses ranging from 5% to 11%, while mixtures with 96% purity suffered reductions of up to 30%. These findings highlight the importance of analyzing impurity levels and compositions in CO₂ supply for sCO₂ systems, as they significantly influence efficiency and operational feasibility. The results also highlight the need for impurity analyses tailored to specific system architectures and operating regimes

This paper proposes a new formulation of the momentum balance equation for real gases applied in the thermofluid network modelling methodology, and then systematically compares it with the conventional incompressible flow and ideal gas approaches. The case studies address a range of phenomena including isentropic flow through diffusors and nozzles, and flows with and without work, losses and heat transfer. The results confirm that near the critical point the real fluid behaviour is more like that of an incompressible liquid, than that of a gas. Therefore, the ideal gas model provides very inaccurate results in this area. At some distance away from the critical point, the results from the ideal gas model compares well with that of the real gas model and both models satisfy the entropy balance equation. Here, the incompressible model does not compare well with the real gas model and shows notable imbalances in the entropy balance equation. The proposed real gas methodology properly treats all cases with or without work input or output, with or without losses, with or without heat transfer, as well as any combination of these, while fully satisfying entropy balance requirement. This means that it can be applied with confidence in all the regimes of interest in sCO2 power cycles.

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