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The SCARABEUS project investigates the use of CO2–based mixtures as working fluid in power cycles for nextgeneration Concentrated Solar Power plants. These fluids exhibit a critical temperature higher than pure CO2, enabling dry condensation of the working fluid even at the high ambient temperatures typical of sites with a high solar radiation. As a consequence, the SCARABEUS power cycle achieves higher thermal efficiency than standard sCO2 cycles, whose performance deteriorates significantly with ambient temperature. In any case, the actual feasibility of this concept is still to be confirmed by a complete techno-economic assessment. To that purpose, it is critical to accurately estimate the power consumption of the Heat Rejection Unit (HRU), which is one of the most important parasitic loads of the system. Bearing all this in mind, this manuscript presents the design of a horizontal, direct air-cooled condenser (ACC). The bundle geometry proposed is comprised of seven tubes in three passes, with a staggered arrangement. The complete thermal model, developed in MatLab, has been already disclosed by the SCARABEUS consortium in a previous paper, and validated both experimentally in a dedicated test rig and against results obtained by the commercial software Xace®. The novelty in the present manuscript lies in the integration of this thermal model of the tubes with a complete design and integration tool of the whole heat rejection sub-system, including the design of a rotoronly axial fan and supporting frame. The impact of several design parameters (i.e., air temperature rise, acceptable hot pressure drops, tube length) is studied, taking into account auxiliary power consumption, footprint and cycle efficiency as main figures of merit. Two candidate mixtures are taken into account, identified in previous works by the same authors (85%CO2-15%C6F6 and 80%CO2-20%SO2), and a pure sCO2 case is also considered for the sake of comparison. The results show that, for a given gross cycle output, using pure sCO2 yields the smallest ACC with the lowest fan power consumption. Moreover, tube length and air face velocity are found to be the key-parameters driving the design process of an ACC, for which increasing tube length is always beneficial as far as the ACC design is concerned. Finally, various considerations regarding the role played by the optimum design of the ACC within the global optimisation of the power plant are made. It is found that the rationale employed for the design of the ACC may be in conflict with that used from an overall plant optimisation standpoint. It is hence concluded that the definition of the optimal design space of an Air-cooled Heat Exchanger (ACHE) must be included in the global optimisation of the power plant., Conference Proceedings of the European sCO2 Conference5th European sCO2 Conference for Energy Systems: March 14-16, 2023, Prague, Czech Republic, p. 283

This paper focuses on the thermodynamic comparison between pure supercritical Carbon Dioxide and blended transcritical Carbon Dioxide power cycles by means of a thorough exergy analysis, considering exergy efficiency, exergy destruction and efficiency losses from Carnot cycle as main figures of merit. A reference power plant based on a steam Rankine cycle and representative of the state-of-the-art (SoA) of Concentrated Solar Power (CSP) plants is selected as base-case. Two different temperatures of the energy (heat) source are considered: 575 ◦C (SoA) and 725 ◦C (next generation CSP). Compared to SoA Rankine cycles, CO2 blends enable cycle exergy efficiency gains up to 2.7 percentage points at 575 ◦C. At 725 ◦C, they outperform both SoA and pure CO₂ cycles with exergy efficiencies up to 75.3%. This performance is brought by a significant reduction in the exergy destruction across the compression and heat rejection process rounding 50%. Additionally, it has been found that the internal condensation occurring inside the heat recuperator for those mixtures with a large temperature glide improves recuperator exergy efficiency, supporting the use of simpler layouts without split-compression. Finally, CO₂ blends exhibit lower cycle exergy efficiency degradation than pure sCO₂ in the event of an increase in the design ambient temperature. University of Seville Internal Research Programme (Plan Propio de Investigación) under contract No 2019/00000359

The SCARABEUS project, funded by the European Commission, is currently investigating the potential gains brought about by the utilization of carbon dioxide mixtures in supercritical power cycles of Concentrated Solar Power plants, in lieu of the common Rankine cycles based on steam turbines or even pure carbon dioxide cycles. The analysis has already confirmed that it is possible to attain thermal efficiencies higher than 51% when ambient temperatures exceed 40°C, which is unheard of when conventional technology or standard CO₂ technology is used. Additionally, this extraordinary performance is achieved with simpler cycle layouts, therefore with lower capital costs. The additives considered include organic and inorganic compounds which are added to the raw carbon dioxide in a variable proportion, depending on the composition of the additive and on ambient temperature. Regardless, it is important to assess whether or not there is an additional environmental advantage in terms of carbon dioxide and other potential hazards brought about by the new chemicals in the system. This is presented in this paper where the results obtained so far by the consortium for the carbon footprint from a Life Cycle perspective are discussed. Along with the assumptions and methodology, the results are compared for three reference plants: state-of-the-art CSP plant based on steam turbines, innovative CSP plant using pure supercritical CO₂ technology, and the SCARABEUS concept using supercritical CO₂ mixtures. The results are promising as they suggest that it is possible to reduce the carbon footprint of a 110 MWe CSP plant to be significantly less than 27kgCO₂/MWh from the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR5).

The adoption of CO2-based mixtures as power block working fluid for CSP plant can turn supercritical CO2 cycles into efficient transcritical cycles even at high ambient temperature, with significant performance improvement and potential power block cost reduction. In this work, the use of CO2+C6F6 mixture as working fluid for a power cycle coupled with a solar tower is analyzed. Two different cycle maximum temperatures (550°C and 650°C) are considered and for both configurations the overall plant design is performed. The yearly energy yield is computed with hourly data and the LCOE is minimized varying storage and cycle recuperator sizes. Results show comparable results for the innovative working fluid and for the sCO2 cycles European Union’s Horizon 2020 No 814985

This paper investigates the interest and potential of using working fluids based on Carbon and Sulphur Dioxide mixtures (CO2-SO2) in a transcritical Recompression cycle. In order to assess the actual thermodynamic potential of the concept proposed, the influence of dopant (SO2) content is assessed for two different turbine inlet temperatures (550 °C and 700 °C). The results obtained are compared with other CO2 mixtures already proposed in literature (CO2-C6F6 and CO2--TiCl4) and for two alternative cycle layouts (Recuperated Rankine and Precompression). The results pf the analysis reveal that, at high ambient temperature, the Recompression cycle operating on CO2-SO2, with Sulphur Dioxide content between 20% and 30%(v), is a very interesting option for Concentrated Solar Power plants, able to achieve thermal efficiencies of approx. 45% and 51% at 550 °C and 700 °C respectively. At a minimum cycle temperature of 50 °C, the proposed configuration leads to thermal efficiency gains of 6% and 2% with respect to the Brayton and Recompression cycles working on pure CO2. This performance enhancement of the Recompression cycle with CO2-SO2 is comparable to or higher than that enabled by other CO2 mixtures proposed in literature, but with significantly higher specific work (smaller footprint) and temperature rise across the solar receiver (lower installation costs). Unión Europea H2020-814985 University of Seville - Internal Research Programme (Plan Propio de Investigación) contract No 2019/00000359 Junta de Andalucía - Programme for Youth Employment 2014–2020

This manuscript investigates the supercritical carbon dioxide (sCO2) power cycle employed in the power block of concentrated solar power (CSP) plants—solar tower—as an alternative for solar desalination, developed with either distillation or reverse osmosis. This concept is investigated as a possible up-scaling of the SOLMIDEFF project, originally based on a hot-air micro gas turbine combined with a solar dish collector. For the upscaled concept, five different sCO2 cycles are considered, chosen amongst the best-performing configurations proposed in the literature for CSP applications, and modelled with Thermoflex software. The influence of ambient conditions is studied, considering two minimum cycle temperatures (35 °C and 50 °C), corresponding to Santa Cruz de Tenerife and Abu Dhabi, respectively. The results show that the low temperatures at the inlet of the heat rejection unit compromise the viability of distillation technologies. On the other hand, the high thermal efficiency achieved by these cycles, especially with the recompression and partial cooling layouts, reduces the specific energy consumption when combined with reverse osmosis (RO), below that of photovoltaic (PV)+RO. Feed-water preheating is explored as a solution to further reduce energy consumption, concluding that its actual interest is not clear and strongly depends on the location considered and the corresponding water quality standards. Spanish National R+D+i Programme RTI2018-102196-B-100 University of Seville Internal Research Programme (Plan Propio de Investigación) contract No 2019/00000359 Junta de Andalucía Programme for Youth Employment 2014–2020 (Phase 4)

The digital transformation is an important driver of long-run productivity growth and, as such, it has the potential to promote a more inclusive and sustainable growth. However, digital capabilities, crucial to develop and govern new digital technologies, are unevenly distributed across European regions increasing the risk of divergence and polarization. By taking advantage of a set of original indicators capturing the level of digital skills in the regional workforce, this work analyzes the factors shaping the process of digital skill accumulation in the EU over the period 2011-2018. Relying on transition probability matrices and dynamic random effects probit models, we provide evidence of a strong and persistent regional polarization in the adoption and deployment of digital skills. Further, we investigate whether European Funds (European Regional Development Fund, Cohesion Funds, and European Social Funds) are capable to shape the digitalization process and to favor regional convergence.

The potential of supercritical Carbon Dioxide power cycles to supersede subcritical steam turbine technology in Concentrated Solar Power applications is widely acknowledged. Some differential features of the former are higher efficiency at similar temperatures (in the range from 600 to 750ºC), smaller footprint, higher flexibility and lower cost. Several theoretical and experimental R&D projects are currently working on aspects such as component development (turbomachinery and heat exchangers), system integration into the solar subsystem (receiver and thermal energy storage system), operability, materials… Nevertheless, whilst progress is being made at a very high pace, there is still a great deal of uncertainty regarding how much sCO2 technology will be able to reduce the cost of solar thermal electricity with respect to contemporary CSP technology. This is mostly caused by the sensitivity of cycle performance to ambient temperature, bringing about a large efficiency drop when this temperature exceeds 35ºC. The root cause for this performance drop is the unfeasibility of compression near the critical point, where the very high density of the fluid reduces density and, therefore, compression work. The SCARABEUS project is based on the addition of certain dopants to carbon dioxide in order to yield a working mixture with higher critical pressure and temperature. As a consequence of these modified critical properties of the fluid, compression near the critical point is enabled even at ambient temperatures as high as 40-45ºC. Moreover, at these high temperatures, condensation and compression in liquid state are still possible. The characteristics of the new working fluids have been proved to enable thermal efficiencies higher than 50% for minimum cycle temperatures as high as 60ºC, hence boosting the performance of CSP plants well beyond of the capabilities of systems based on steam turbines. This implies a substantial reduction of the cost of the plant. Nevertheless, whilst the thermal and economic performances are more favourable for CO2-mixtures, new technical challenges must be faced if the technology is to be mature: thermal stability and potential hazards of the dopants, new turbomachinery and heat exchanger designs adapted to the composition of the mixture, phase separation, materials (selection, compatibility and degradation) and others. This paper introduces the main advantages and technical potential of the SCARABEUS technology along with a discussion of the main challenges faced by the consortium in order to demonstrate the technology and beyond. Unión Europea H2020-814985