The presentation was discussed in a conference organized by "B+LabNet", at the University of Brescia: Environment, Health, Sustainability, June 5th 2019.
Doping CO2 with an additional fluid to produce a CO2-based mixture is predicted to enhance the performance of the super critical CO2 power cycle and lower its cost when adapted to Concentrated Solar Power plants. A consistent fluid mixture modelling process is necessary to reliably design and predict the performance of turbines operating with CO2-based working fluids. This paper aims to quantify the significance of the choice of an Equation of State (EoS) and the uncertainty in the binary interaction parameter (𝑘𝑖𝑗) on the cycle and turbine design. To evaluate the influence of the thermodynamic model, an optimisation study of a 100MWe simple recuperated transcritical CO2 cycle is conducted for a combination of three mixtures, four equations of state, and three possible values of the binary interaction parameter. Corresponding multi-stage axial turbines are then designed and compared based on the optimal cycle conditions. Results show that the choice of the dopant fraction which yields maximum cycle thermal efficiency is independent from the fluid model used. However, the predicted thermal efficiency of the mixtures is reliant on the fluid model. Absolute thermal efficiency may vary by a maximum of 1% due to the choice of the EoS, and by up to 2% due to 𝑘𝑖𝑗 uncertainty. The maximum difference in the turbine geometry due to EoS selection corresponded to a 6.3% (6.6 cm) difference in the mean diameter and a 18.8% (1.04 cm) difference in the blade height of the final stage. On the other hand, the maximum difference in turbine geometry because of 𝑘𝑖𝑗 uncertainty amounted to 6.7% (5.6 cm) in mean diameter and 27.3% (2.73 cm) in blade height of the last stage.
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 (250-300bar), turbine inlet temperature (550-700Co¯) 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 50Co¯. To verify this potential gain, the most representative pure sCO2 cycles have been optimised at two minimum cycle temperatures (32Co¯ and 50Co¯), proving the superiority of the proposed blended technology in high ambient temperature 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.
Supercritical CO2 (sCO2) power cycles have gained prominence for their expected excellent performance and compactness. Among their benefits, they may potentially reduce the cost of Concentrated Solar Power (CSP) plants. Because the critical temperature of CO2 is close to ambient temperatures in areas with good solar irradiation, dry cooling may penalise the efficiency of sCO2 power cycles in CSP plants. Recent research has investigated doping CO2 with different materials to increase its critical temperature, enhance its thermodynamic cycle performance, and adapt it to dry cooling in arid climates. This paper investigates the use of CO2/TiCl4, CO2/NOD (an unnamed Non-Organic Dopant), and CO2/C6F6 mixtures as working fluids in a transcritical Rankine cycle implemented in a 100 MWe power plant. Specific focus is given to the effect of dopant type and fraction on optimal cycle operating conditions and on key parameters that influence the expansion process. Thermodynamic modelling of a simple recuperated cycle is employed to identify the optimal turbine pressure ratio and recuperator effectiveness that achieve the highest cycle efficiency for each assumed dopant molar fraction. A turbine design model is then used to define the turbine geometry based on optimal cycle conditions. It was found that doping CO2 with any of the three dopants (TiCl4, NOD, or C6F6) increases the cycle’s thermal efficiency. The greatest increase in efficiency is achieved with TiCl4 (up to 49.5%). The specific work, on the other hand, decreases with TiCl4 and C6F6, but increases with NOD. Moreover, unlike the other two dopants, NOD does not alleviate recuperator irreversibility. In terms of turbine design sensitivity, the addition of any of the three dopants increases the pressure ratio, temperature ratio, and expansion ratios across the turbine. The fluid’s density at turbine inlet increases with all dopants as well. Conversely, the speed of sound at turbine inlet decreases with all dopants, yet higher Mach numbers are expected in CO2/C6F6 turbines.
In the last years, several fluids have been proposed to replace steam as working fluid in power cycle for converting thermal power into electricity. This paper describes the procedure to be adopted for the selection of any innovative fluid which can be even mixtures of fluids. The first step consists of the working fluid characterization in terms of thermodynamic properties through equations of state. The equations of state have to be calibrated on experimental Vapour-Liquid Equilibrium measurements while, in the second step, the maximum operating temperature is identified through thermal stability tests. Finally, the impact of the fluid thermodynamic properties on the performance of the power cycle in which it is implemented must be assessed through modelling tools. In this work, the procedure is discussed for the mixture of CO2 and C6F14 as a potential working fluid for gas thermodynamic cycles with liquid phase compression. Results of the application of this mixture in a closed cycle show the benefit of using a CO2/C6F14 mixture which provides 3% points efficiency increase at 400°C with respect to the pure CO2 together with a preliminary design of the expander.
The purpose of this document is to analyze the state-of-the-art of the CSP technology, focused on the solar tower configuration, and the possibility of integrating a supercritical CO2 (sCO2) cycle to substitute the traditional steam cycle. To that end, the document is divided in six sections. The first section presents a description of the fundamentals of Concentrating Solar Power (CSP) technology for the production of solar thermal electricity (STE). In the second section, a review of the solar subsystems is provided, describing the solar field, receiver and Thermal Energy Storage (TES) system. This second section incorporates a detailed description of all the key components present in a CSP facility, familiarizing the reader with the current state of the art of the technology. In the next section, the power block of the CSP plants is reviewed and the usual steam cycles used in contemporary CSP facilities are discussed. After this analysis, a description of the economics of state-of-the-art solar tower plants is carried out, including a discussion about the expected cost reductions and market deployment in the mid and long terms. A study of possible hybridization solutions is performed in the fifth section, under a decarbonization scenario. The last section scrutinizes the supercritical CO2 technology, both its fundamentals and a the potential application to CSP plants. This section approaches the technology from the technical and economic standpoints, akin to the preceding sections about state-of-the-art CSP plants based on steam turbines. Overall, this document provides an insightful review of the Best Available Technologies for CSP applications that are either currently available or foreseen in the near future, along with the pathways to be followed in the short term to improve the performance of this power generation technology
This paper discusses the adoption of CO2 mixtures for improving the thermal-to-power efficiency conversion in solar tower plants and reducing the Levelized Cost of Electricity. Two different fluids are considered for blending the CO2: N2O4 and TiCl4. The main advantage of the innovative mixtures relies in a higher critical temperature with respect to pure CO2, which allows condensing cycles even at relatively high ambient temperatures typical of solar plants locations. Thermodynamic results show that the innovative cycles can achieve conversion efficiencies as high as 43% and 50% at 550 °C and 700 °C maximum temperature respectively, outperforming the reference CO2 cycle by 2 points percent. In addition, the simpler lay-out and the liquid compression reduce the power block capital costs below 700 $/kW. Detailed solar plant annual simulation is performed to assess the overall solar to electricity efficiency which can be around 21% for the innovative fluid, corresponding to 10% increase with respect to state-of-the-art solar plant. The higher performance and lower costs lead to a Levelized Cost of Electricity reduction of 10% with respect to conventional steam cycle power block.
The competitiveness of concentrated solar power technology in the near-future electricity generation scenario, requires a substantial reduction of the Levelized Cost of Energy which can be achieved with an increase of the energy conversion efficiencies while maintaining or reducing the investment costs. This paper discusses the use of pure Dinitrogen tetroxide N2O4, and N2O4/CO2 mixture, as working fluids in supercritical Brayton cycles applied to solar tower power plants. When N2O4 is combined with CO2, the resulting mixture has a compara- tively higher critical temperature than pure CO2, allowing a condensing cycle even at the fairly high ambient temperatures of desert areas, where solar power plants are typically installed. This allows the adoption of simpler cycle configurations than the one used in sCO2 cycles (cost reduction) while achieving very high ther- modynamic efficiency (47% at 700 °C). The N2O4/CO2 mixture with optimized composition, integrated in a solar tower unit, increases the solar-electric efficiency by 1% with respect to commercial plants based on steam cycle with 550 °C maximum temperature (22.3% vs. 21.3%). At 700 °C, the overall solar-electric efficiency can reach 24.5% which is slightly higher than supercritical CO2 cycles, yet with a foreseeable reduction of the investment costs as consequence of the simpler plant lay-out.
Power cycles running on carbon dioxide at supercritical pressure and temperature were introduced in the late ninety-sixties but, after a warm welcome to the theoretical performance announced, they were later abandoned in favour of standard combustion gas turbines. Nevertheless, the technology was brought forward about a decade ago and has since captured significant attention from the scientific and industrial community. The number of publications has risen exponentially and there are several experimental projects under development today. The performances of these cycles have been deeply analysed in literature, proving to be theoretically competitive.