Off-design performance of CSP plant based on supercritical CO2 cycles (original) (raw)
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Applied Energy, 2021
Supercritical Carbon Dioxide (sCO2) cycles can achieve higher efficiency compared to steam-Rankine or Air-Brayton cycles, therefore they are promising for concentrated solar power applications. In the present work a recompression sCO2 cycle is connected to a central-tower solar field with two-tank thermal storage delivering molten chloride salt at 670°C. Although sCO2 cycles show higher design efficiency, the off-design efficiency is highly sensitive to the ambient conditions, impacting the power block net-power and heat input. The temperature of the molten-salt exiting from the power block and returning to the cold storage tank increases by 46°C with respect to the design value when the compressor inlet temperature is raised by 13°C relative to the design condition of 42°C, which implies that the capacity of the thermal storage reduces by 25%. The main focus of this work is to investigate the off-design performance of a sCO2 recompression cycle under variable ambient temperature, molten-salt inlet temperature and molten-salt flow rate. Multi-objective optimisation is carried-out in off-design conditions using an in-house code to explore the optimal operational strategies and the Pareto fronts were compared. Since the power cycle can either be operated in maximum power mode or maximum efficiency mode, this study compares these two operational strategies based on their annual performance. Results indicate that the capacity factor of the concentrated solar power can be increased by 10.8% when operating in maximum power mode whilst the number of start-ups is reduced by about 50% when operating in maximum efficiency mode.
Preliminary Assessment of sCO 2 Power Cycles for Application to CSP Solar Tower Plants
Energy Procedia
This work presents a preliminary thermodynamic assessment of three different supercritical CO2 (sCO2) power cycles integrated in a high temperature solar tower system, working up to 800°C. An indirect cycle configuration is considered with KCl-MgCl2 molten salt as heat transfer fluid (HTF) in the solar receiver and a two tanks thermal energy storage (TES) system. The most promising cycle configuration is selected, optimizing the cycle turbine inlet temperature to achieve the best compromise between cycle and receiver efficiency. An estimate of the yearly energy yield of the proposed power plant is finally performed, indicating the possibility of reaching solar-to-electric efficiency of about 17.5%.
Energy Conversion and Management, 2020
A superstructure-based method is applied to optimize the design of supercritical carbon dioxide cycle for concentrated solar power systems. A superstructure is designed for the cycle featuring various options in cold-end and hot-end configurations. Process integration and cycle variables are simultaneously optimized for the superstructure based on the integrated thermoeconomic model of the overall system. Optimizations are performed for eight cases with different designs of maximum and minimum temperatures. Sensitivity analyses are conducted on the design parameters and related capital costs of subsystems of the overall system. 1.5-13.5% reduction in levelized cost of electricity is obtained through the superstructure-based optimization relative to the corresponding base cases. The cost of thermal storage system at optimal condition is significantly surged by 36.3-111.6% when the maximum temperature surpasses 600°C, which consequently leads to the variation in configuration for the optimal cycle. The minimum levelized cost of electricity is obtained at 600°C with a reduction of up to 4.1% compared to that at 550°C. The further increase in the maximum temperature beyond 600°C will be followed by 8.6%-20.2% rise in the levelized cost of electricity. The improvements in maximum pressure and turbomachinery performance can lead to significant reduction in levelized cost of electricity, especially at high maximum and minimum temperatures. However, these improvements have relatively uneventful effects on the configuration of the optimal cycle. The capital cost of thermal storage system has significant effects on the design of the optimal cycle configuration whereas the variation in the capital cost of cycle system has less eventful effects. The effect of capital cost of cycle system is more dominant than that of the thermal storage system at 550°C and 600°C maximum temperatures in terms of levelized cost of electricity, while this situation is reversed as maximum temperature rises higher due to the soaring capital cost of the thermal storage system. The configuration with main compression intercooling or partial cooling design in the cold end and single-turbine design in the hot end is finally suggested for the supercritical carbon dioxide cycle applied in state-of-the-art and next-generation concentrated solar power systems.
A number of studies have been performed to assess the potential of using supercritical carbon dioxide (S-CO 2) in closed-loop Brayton cycles for power generation. Different configurations have been examined among which recompression and partial cooling configurations have been found very promising, especially for concentrating solar power (CSP) applications. It has been demonstrated that the S-CO 2 Brayton cycle using these configurations is capable of achieving more than 50% efficiency at operating conditions that could be achieved in central receiver tower type CSP systems. Although this efficiency is high, it might be further improved by considering an appropriate bottoming cycle utilizing waste heat from the top S-CO 2 Brayton cycle. The organic Rankine cycle (ORC) is one alternative proposed for this purpose; however, its performance is substantially affected by the selection of the working fluid. In this paper, a simple S-CO 2 Brayton cycle, a recompression S-CO 2 Brayton cycle, and a partial cooling S-CO 2 Brayton cycle are first simulated and compared with the available data in the literature. Then, an ORC is added to each configuration for utilizing the waste heat. Different working fluids are examined for the bottoming cycles and the operating conditions are optimized. The combined cycle efficiencies and turbine expansion ratios are compared to find the appropriate working fluids for each configuration. It is also shown that combined recompression-ORC cycle achieves higher efficiency compared with other configurations.
International Journal of Energy Research, 2020
The off-design characteristics of an improved recompression supercritical carbon dioxide cycle integrated with a two-stage intercooled main compressor are investigated with a focus on the concentrated solar power application. An off-design model is established for each crucial component of the cycle system of 100-megawatt scale. Four cycle control schemes with different main compressor configurations or/and cycle maximum pressure modes are evaluated and compared. A sensitivity analysis is performed on the parameters related to the cycle thermal input and ambient condition to predict the off-design characteristics due to the plant dispatch and ambient condition change in a solar power plant. The off-design results regarding the cycle thermodynamic performance and operational issue prevention are presented. The effect of the design-point value of the main compressor inlet temperature on the off-design characteristics is evaluated with the comparison among the results at three design points. The results reveal that the compressor surge may occur to the main compressor with basic configuration as the main compressor inlet temperature decreases to a certain value beneath the corresponding design point. By contrast, the surge risk can be prevented with the modified main compressor configuration by activating the recirculation system and the cycle can thus operate normally in the entire off-design range of main compressor inlet temperature. The off-design change in thermal input has overall limited effects on the cycle system control. No operational compressor issues occur for the main compressor with either basic or modified configuration as the thermal input deviates from the design points and varies in the studied ranges. The cycle maximum pressure mode has slight effects on the cycle thermodynamic performance as the thermal input deviates from the design point. The flexible cycle maximum pressure mode has slightly lower sensitivity to the thermal input variation in net output power due
E3S Web of Conferences, 2019
The supercritical carbon dioxide (S-CO2) cycle is regarded as a potential option for the next generation power conversion system. Concentrated solar power (CSP) plant is one of the promising scenarios to adopt the S-CO2 cycle due to the appealing thermal efficiency and the ability to integrate thermal storage and dry cooling. Among various cycle configurations of S-CO2 cycle, the recompression S-CO2 cycle with intercooled main compressor is one of the optimal choices that can provide superior efficiency and a large enough temperature differential for thermal input, which together contribute to the minimization of the overall levelized cost of electricity (LCOE) of the whole CSP plant. The off-design performance and the associated control scheme have important effects on the CSP plant. This paper develops an off-design model for the recompression S-CO2 cycle with intercooled main compressor for the commercialized hundred-megawatt CSP plant. The effects of different off-design conditi...
2019
Supercritical CO2 cycle, due to its potential to reach high thermal efficiency and high flexibility, is a promising approach to increase the competitiveness of concentrated solar power. By taking into account thermal energy storage utilization, this paper provides a meaningful comparison of different supercritical CO2 cycles for application in concentrated solar power. Regenerative, recompression, pre-compression and partial cooling cycles are considered as four fundamental cycles. By combining these fundamental cycles with intercooling, preheating and reheating, it results in a wide range of cycle candidates for comparison and analysis. Each cycle is modeled and optimized with the objective to maximize the specific power output for different thermal energy storage utilization. The results show that, with a high thermal energy storage utilization, in order to maximize the cycle efficiency, it is not optimal for most of the studied cycle to reach its upper limit of temperature. Besides, with the thermal storage utilization as a constraint for optimization, intercooling, preheating and reheating show different efficiency enhancement behavior on different region of thermal energy storage utilization. This text is made available via DuEPublico, the institutional repository of the University of Duisburg-Essen. This version may eventually differ from another version distributed by a commercial publisher.
Peculiar thermodynamic properties of carbon dioxide (CO 2) when it is held at or above its critical condition (stated as supercritical CO 2 or sCO 2) have attracted the attention of many researchers. Its excellent thermophysical properties at medium-to-moderate temperature range have made it to be considered as the alternative working fluid for next power plant generation. Among those applications, future nuclear reactors, solar concentrated thermal energy or waste energy recovery have been shown as the most promising ones. In this paper, a recompression sCO 2 cycle for a solar central particles receiver application has been optimized, observing net cycle efficiency close to 50%. However, small changes on cycle parameters such as working temperatures, recuperators efficiencies or mass flow distribution between low and high temperature recuperators were found to drastically modify system overall efficiency. In order to mitigate these uncertainties, an optimization analysis based on recuperators effectiveness definition was performed observing that cycle efficiency could lie among 40%e50% for medium-to-moderate temperature range of the studied application (630 Ce680 C). Due to the lack of maturity of current sCO 2 technologies and no power production scale demonstrators, cycle boundary conditions based on the solar application and a detailed literature review were chosen.
Energy, 2017
In this paper, we perform a comprehensive parametric study for supercritical CO 2 (sCO 2) Brayton cycle integration with a concentrated solar thermal (CST) plant. The main focus is to develop operational strategies for the cycle to adapt to fluctuations in solar energy availability. Several cycle layouts are analysed and a 'combined' cycle comprising recompression, reheat and intercool is found to be the most efficient cycle. Two key cycle parameters (sCO 2 circulation rate and splitting fraction) are sensitized while net shaft power (NSP) output is controlled at 11.5 MW. By manipulating these two parameters, the cycle can adapt to heat input variations without affecting NSP output. This finding leads to two operational modes: flexible temperature mode (FTM) and constant temperature mode (CTM). A solar-assisted case study is used to test the proposed strategies, and an auxiliary fossil-fuelled backup (AFB) unit is utilised when solar energy is insufficient. In general, the proposed solar-assisted cycles (CTM and FTM) were able to achieve the highest fossil fuel savings of 28.9% and 31.2%, respectively compared to a conventional cycle without solar. It is found that both FTM and CTM can deal with fluctuations in solar thermal energy supply. However, FTM is more effective in tolerating drops in solar energy supply and considerably outperforms CTM in lowering contributions from AFB unit. Thus, even at the most efficient conditions, CTM still underperformed FTM by 4.5% in term of fossil fuel saving.