Thermo-economic assessment of supercritical CO2 power cycles for concentrated solar power plants

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Potential of Supercritical Carbon Dioxide Power Cycles to Reduce the Levelised Cost of Electricity of Contemporary Concentrated Solar Power Plants

This paper provides an assessment of the expected Levelised Cost of Electricity enabled by Concentrated Solar Power plants based on Supercritical Carbon Dioxide (sCO2) technology. A global approach is presented, relying on previous results by the authors in order to ascertain whether these innovative power cycles have the potential to achieve the very low costs of electricity reported in the literature. From a previous thermodynamic analysis of sCO2 cycles, three layouts are shortlisted and their installation costs are compared prior to assessing the corresponding cost of electricity. Amongst them, the Transcritical layout is then discarded due to the virtually impossible implementation in locations with high ambient temperature. The remaining layouts, Allam and Partial Cooling are then modelled and their Levelised Cost of Electricity is calculated for a number of cases and two different locations in North America. Each case is characterised by a different dispatch control scheme and set of financial assumptions. A Concentrated Solar Power plant based on steam turbine technology is also added to the assessment for the sake of comparison. The analysis yields electricity costs varying in the range from 8 to over 11 g/kWh, which is near but definitely not below the 6 g/kWh target set forth by different administrations. Nevertheless, in spite of the results, a review of the conservative assumptions adopted in the analysis suggests that attaining costs substantially lower than this is very likely. In other words, the results presented in this paper can be taken as an upper limit of the economic performance attainable by Supercritical Carbon Dioxide in Concentrated Solar Power applications.Unión Europea (Programa Horizonte 2020) 81498

Influence of Working Fluid Composition on the Optimum Characteristics of Blended Supercritical Carbon Dioxide Cycles

Proceedings of ASME Turbo Expo 2021: Turbomachinery Technical Conference and Exposition GT2021 June 7-11, 2021, Virtual, OnlineThe supercritical Carbon Dioxide power cycle technology has attracted growing interest from the scientific community, becoming one of the most important options currently considered for CSP applications. This is thanks to its high thermal efficiency, even at moderate turbine inlet temperatures, and small footprint. Nevertheless, sCO2 power cycles require a fairly low compressor inlet temperature to exploit their full thermodynamic potential. When this cannot be achieved, as it is usually the case for Concentrated Solar Power plants where ambient temperatures are high, the interest of the technology is compromised. To compensate for this effect, the SCARABEUS project is working on the development of certain chemical dopants that could be added to the raw CO2, obtaining new working fluids with the same or even better performance than pure CO2 even at higher minimum cycle temperatures. This paper studies the impact of using CO2 mixtures blended with Hexaflurorobenzene (C6F6) and Titanium Tetrachloride (TiCl4). It is found that these mixtures enable thermal efficiencies that are higher than if pure CO2 were used. The efficiency gain can be as high as 3 percentage points, depending on the dopant used and the operating conditions considered. In addition to this absolute performance gain, the paper reveals that there are additional degrees of freedom that enable more effective cycle optimisation. These are the dopant molar content, not only its composition, and the cycle layout used. When this is studied, it is found that the optimum molar content ranges from 10 to 20% and that the layouts of interest when using mixtures are simpler than if plain CO2 were used. These results open the way for a significant performance enhancement of Concentrated Solar Power plants.Unión Europea, Horizon 2020, Nº81498

Thermal efficiency gains enabled by using supercritical CO2 mixtures in Concentrated Solar Power applications

Supercritical Carbon Dioxide (sCO2) power cycles have been proposed for Concentrated Solar Power (CSP) applications as a means to increase the performance and reduce the cost of state-of-the-art CSP systems. Nevertheless, the sensitivity of sCO2 systems to the usually hot ambient temperatures found in solar sites compromises the actual thermodynamic and economic gains that were originally anticipated by researchers of this innovative power cycle. In order to exploit the actual potential of sCO2 cycles, the utilization of dopants to shift the (pseudo)critical temperature of the working fluid to higher values is proposed here as a solution towards enabling exactly the same features of supercritical CO2 cycles even when ambient temperatures compromise the feasibility of the latter technology. To this end, this work explores the impact of adopting a CO2-based working mixture on the performance of a CSP power block, considering hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4) as possible dopants. Different cycle options and operating conditions are studied (250-300 bar and 550-700ºC) as well as molar fractions ranging between 10 and 25%. The results in this work confirm that CO2 blends with 15-25%(v) of the cited dopants enable efficiencies that are well in excess of 50% for minimum cycle temperatures as high as 50 or even 55ºC. It is also confirmed that, for these cycles, turbine inlet temperature and pressure hardly have any effect on the characteristics of the cycle that yields the best performance possible. In this regard, the last part of this work also shows that cycle layout should be an additional degree of freedom in the optimisation process inasmuch as the best performing layout changes depending on boundary conditions.Unión Europea SI-1900/10/201

Analysis of the Thermodynamic Potential of Supercritical Carbon Dioxide Cycles: A Systematic Approach

After the renewed interest in supercritical carbon dioxide cycles, a large number of cycle layouts have been proposed in literature. These works, which are essentially theoretical, consider different operating conditions and modeling assumptions, and thus, the results are not comparable. There are also works that aim to provide a fair comparison between different cycles in order to assess which one is most efficient. These analyses are very interesting but, usually, they combine thermodynamic and technical restrictions, which make it difficult to draw solid and general conclusions with regard to which the cycle of choice in the future should be. With this background, the present work provides a systematic thermodynamic analysis of 12 supercritical carbon dioxide cycles under similar working conditions, with and without technical restriction in terms of pressure and/or temperature. This yields very interesting conclusions regarding the most interesting cycles in the literature. Also, useful recommendations are extracted from the parametric analysis with respect to the directions that must be followed when searching for more efficient cycles. The analysis is based on efficiency and specific work diagrams with respect to pressure ratio and turbine inlet temperature in order to enhance its applicability to plant designs driven by fuel economy and/or footprint

Capital Cost Assessment of Concentrated Solar Power Plants Based on Supercritical Carbon Dioxide Power Cycles

Previous work by the authors has shown that broader analyses than those typically found in literature (in terms of operating pressures allowed) can yield interesting conclusions with respect to the best candidate cycles for certain applications. This has been tested for the thermodynamic performance (first and second laws) but it can also be applied from an economic standpoint. This second approach is introduced in this work where typical operating conditions for concentrated solar power (CSP) applications (current and future generations of solar tower plants) are considered (750 °C and 30 MPa). For these, the techno-economic performance of each cycle is assessed in order to identify the most cost-effective layout when it comes to the overnight capital cost (OCC). This analysis accounts for the different contributions to the total cost of the plant, including all the major equipment that is usually found in a CSP power plant such as the solar field and thermal energy storage (TES) system. The work is, thus, aimed at providing guidelines to professionals in the area of basic engineering and prefeasibility study of CSP plants who find themselves in the process of selecting a particular power cycle for a new project (set of specifications and boundary conditions)

Thermal efficiency gains enabled by using CO2 mixtures in supercritical power cycles

The present paper explores the utilisation of dopants to increase the critical temperature of Carbon Dioxide (sCO2) as a solution towards maintaining the high thermal efficiencies of sCO2 cycles even when ambient temperatures compromise their feasibility. To this end, the impact of adopting CO2-based mixtures on the performance of power blocks representative of Concentrated Solar Power plants is explored, considering two possible dopants: hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4). The analysis is applied to a well-known cycle -Recuperated Rankine- and a less common layout -Precompression-. The latter is found capable of fully exploiting the interesting features of these non-conventional working fluids, enabling thermal efficiencies up to 2.3% higher than the simple recuperative configuration. Different scenarios for maximum cycle pressure (250–300 bar), turbine inlet temperature (550–700 °C) and working fluid composition (10–25% molar fraction of dopant) are considered. The results in this work show that CO2-blends with 15–25%(v) of the cited dopants enable efficiencies well in excess of 50% for minimum cycle temperatures as high as 50 °C. To verify this potential gain, the most representative pure sCO2 cycles have been optimised at two minimum cycle temperatures (32 °C and 50°C), proving the superiority of the proposed blended technology in high ambient temperature applications.European Union's Horizon 2020 - SCARABEUS project - N 814985University of Seville (Spain) Internal Research Programme (Plan Propio de Investigación) No 2019/0000035

Effects of pre-operative isolation on postoperative pulmonary complications after elective surgery: an international prospective cohort study an international prospective cohort study

We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care. We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care