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Study on the Operation of the LUTsCO2 Test Loop with Pure CO2 and CO2 + SO2 Mixture Through Dynamic Modeling
Abstract
Within the framework of the Horizon 2020 DESOLINATION (DEmonstration of concentrated SOLar power coupled wIth advaNced desAlinaTion system in the gulf regION) project, CO2-based mixtures will be used as the working fluid of the power cycle coupled with the desalination plant. Desalination technologies require temperatures above 50 ℃, therefore a fluid with a critical temperature above 70 ℃ is required to perform the compression step in the liquid phase, minimizing compression work and increasing cycle efficiency. Blending CO2 with other fluids, such as SO2, leads to an increase in the critical temperature of the mixture and makes it suitable for transcritical cycles in concentrated solar power applications, while preserving the advantages of pure CO2 over steam cycles. Throughout the DESOLINATION project, CO2 blends will be tested in the LUTsCO2 test loop to provide validation data for the design of heat exchangers. In this work, the adaptation of the test loop to the CO2 blend is investigated. A dynamic model of the test loop is built in MATLAB-Simulink, which allows each component to be modeled independently and to realize a closed-loop cycle model coupled with a controller and real gas property tables. Steady-state simulations of the system are performed and the dynamic model of each component is verified with the design values. As a result, the study highlights the key performance and fluid dynamic differences which arise from the use of a CO2 blend instead of pure CO2.KeywordsCO2 mixturesTranscritical refrigeration cycleDynamic modelSimulink
This paper focuses on the use of the CO2 + SO2 binary mixture as innovative working fluid for closed transcritical power cycles with a minimum temperature above 50 °C. Starting from a literature review of the available experimental data on the mixture, the PC-SAFT EoS is identified as a suitable model to characterize the mixture behavior. Once the proper thermodynamic model is selected for this mixture, a comparison between the innovative transcritical cycle and the sCO2 cycle is proposed for various plant layouts in order to find out the advantages of the innovative mixture. The analysis is presented fixing the cycle maximum temperature at 700 °C and the maximum pressure at 250 bar: the results depict an increment in cycle electric efficiency and cycle specific work, along with a lower temperature of heat introduction in the cycle for any considered configuration of transcritical CO2 + SO2 cycle, when compared to pure sCO2.
An economic analysis of the power block is then performed to support the selection of the innovative working fluid. Two of the most promising plant layouts are evidenced: the recompression layout is selected for highly efficient power blocks, while the dual recuperated layout works effectively in applications characterized by higher hot source exploitation. The recompression layout adopting the CO2 + SO2 mixture presents a power block electric efficiency of 48.67% (2.33% higher than the respective sCO2 cycle) and a reduction of the power block CAPEX from 1160 /kWel when compared to the sCO2 configuration for a 100MWel size, while the dual recuperated layout exploiting the CO2 + SO2 mixture shows a power block electric efficiency of 39.58% (0.69% above the same sCO2 cycle), a decrease of power block CAPEX from 795 /kWel and 70 °C of additional heat recovery from the hot source with respect to the analogous sCO2 cycle.
The supercritical carbon dioxide (SCO2) Brayton cycle, as a substitute for the steam cycle, can be widely used in a variety of power generation scenarios. However, most of the existing SCO2 cycle studies are restricted to basic thermodynamics research, parameter optimizations, system design in different application fields, and even economic analysis. Considering the load variability and control flexibility of the power generation system, the dynamic performance research of the SCO2 cycle is also crucial, but the work done is still limited. Based on the previous studies, Simulink software is used in this paper to develop a dynamic model of the 20 MW-SCO2 recompression cycle, which specifically includes component models that can independently realize physical functions and an overall closed-loop cycle model. A series of comparative calculation are carried out to verify the models and the results are very positive. The SCO2 recompression power system is built with the developed models and the dynamic model runs stably with a maximum error of 0.56%. Finally, the simulation of the dynamic switching conditions of the 20 MW-SCO2 recompression cycle are performed and the analysis results supply instructive suggestions for the system operation and control.
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.
The interest in Supercritical Carbon Dioxide (sCO2) power cycles has grown exponentially in the last decade, thanks to distinctive features like the possibility to achieve high thermal efficiencies at intermediate temperature, small footprint and adaptability to a wide variety of energy sources. In the present work, the potential of this technology is studied for Concentrated Solar Power applications, in particular Solar Tower systems with Thermal Energy Storage.
Further to a previous thorough sensitivity analysis of twelve sCO2 cycles assessing the impact of turbine inlet temperature and pressure ratio on thermal efficiency, specific work, solar share and temperature rise across the solar receiver, the present paper investigates the features of two of these cycles in more detail. The most important conclusions of this section are that: a) the peak values of these thermodynamic figures of merit are obtained at different pressure ratios; b) specific work and temperature rise across the receiver seem to follow parallel trends whilst this is not the case for thermal efficiency; c) for a given turbine inlet temperature, higher pressure ratios increase the temperature rise across the receiver strongly, but the effect on thermal efficiency is uncertain as this can either increase or decrease, depending on the cycle considered.
A deeper analysis of thermal efficiency and receiver temperature rise is therefore mandatory, given that these parameters have a very strong effect on the capital cost of CSP power plants. On one hand, a higher thermal efficiency implies a smaller solar field, the largest contributor to the plant capital cost; on the other, the temperature rise across the receiver is inversely proportional to the size of the thermal energy storage systems, as it is also the case for state of the art steam turbine based CSP plants. In order to quantify these trends, an economic analysis is developed using an in-house code and the open-source software System Advisor Model to evaluate the trade-offs between these two effects. As a result, the Overnight Capital Cost is estimated at some 5000 $/kW, with the individual contributions of solar field, thermal energy storage and power block being given in the paper.
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.
Significant amount of energy is wasted in engine systems as waste heat. In this study, the use of supercritical Brayton cycles for recovering exhaust gas heat of large-scale engines is investigated. The aim of the study is to investigate the electricity production potential with different operational conditions and working fluids, and to identify the main design parameters affecting the cycle power production. The studied process configurations are the simple recuperated cycle and intercooled recuperated cycle. As the performance of the studied cycle is sensitive on the turbomachinery design and efficiencies, the design of the process turbine and compressor were included in the analysis. Cycles operating with CO2 and ethane resulted in the highest performances in both the simple and intercooled cycle configurations, while the lowest cycle performances were simulated with ethylene and R116. 18.3 MW engine was selected as the case engine and maximum electric power output of 1.76 MW was simulated by using a low compressor inlet temperature, intercooling, and high turbine inlet pressure. It was concluded that working fluid and the cycle operational parameters have significant influence not only on the thermodynamic cycle design, but also highly affects the optimal rotational speed and geometry of the turbomachines.
Supercritical carbon dioxide cycles are a promising power conversion option for future nuclear reactors operating with a reactor outlet temperature in the range of 550 to 650°C. The recompression cycle version operating with ∼ 20-MPa turbine inlet pressure achieves similar cycle efficiencies as helium Brayton cycles operating at ∼ 250°C higher turbine inlet temperature. The simplicity and high efficiency of the recompression cycle makes it a prime option from among the family of supercritical carbon dioxide cycles. The elimination of the need for intercooling due to the small required compressor work (because of the high density close to the critical point) makes the recompression cycle even simpler than helium Brayton cycles, which require intercooling to achieve attractive efficiencies. The high operating pressure reduces the size of the plant components significantly, making it a promising power cycle for low-cost modularized electricity-generating nuclear systems. However, the real gas behavior that improves the cycle efficiency presents a challenge for part-load operation. The traditional inventory control used for helium Brayton cycles may not be feasible. Bypass control is thus the prime option for part-load operation, making the cycle less efficient than during base-load operation. Since nuclear power plants are operated almost exclusively in base load, this drawback is not a disqualifying blemish.
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