In this perspective paper, the research and development of S-CO2 cycles are analyzed from two aspects: (i) the system design and analysis and (ii) energy transfer/conversion mechanisms and key ...components development. Based on the analysis, barriers for further promotion of S-CO2 cycles are summarized, including the lack of system design and analysis methodology, not well understood mechanisms of energy transfer/conversion, and technic barriers such as seal, leakage and roto-dynamics stability of key components. To overcome these issues, perspectives on three aspects are proposed. First, S-CO2 cycle adapting to the distinct characteristic of a heat source should be optimized to promote the global system efficiency. Second, new theoretical/numerical works are suggested emphasizing the real gas effect of S-CO2 to improve the accuracy, convergence and stability of numerical simulations, fine experiments should be expanded to verify the correctness of the numerical simulations. Third, the integrated solution strategies for key components such as intermediate heat exchanger, recuperator heat exchanger and turbomachines should be developed and verified in large-scale test loop for reliable and efficient operation. This critical review is hopefully to present readers a clue to promote the development of S-CO2 cycles driven by nuclear energy, renewable energy and fossil energy.
•A comprehensive review has been performed for S-CO2 cycle system and key components.•S-CO2 cycle adapting to each heat source shall be optimized to improve efficiency.•Investigations should be expanded to improve the design accuracy of key components.•Package solutions shall be proposed to address the technic barriers of key components.
A novel recompression supercritical CO2 Brayton cycle integrated with an absorption chiller (RSBC/AC) is proposed for air-cooled concentrated solar power (CSP) plants. The residual heat of CO2 in the ...cold end of the supercritical CO2 cycle is utilized to drive the absorption chiller, which chills the CO2 exiting the precooler further before it enters the main compressor. Parametric analyses and optimizations are performed for the RSBC/AC. Energy and exergy analyses and comparisons are conducted to illustrate the mechanisms of RSBC/AC performance improvement. Sensitivity analyses of pressure drop and ambient temperature are performed to investigate RSBC/AC performance under various working conditions. Economic evaluations of a CSP plant integrated with RSBC/AC are performed to investigate its feasibility as an alternative to the stand-alone supercritical CO2 cycle. Results show that the optimized thermal and exergy efficiencies of RSBC/AC are 5.19% and 6.12% higher, respectively, than those of the stand-alone supercritical CO2 cycle. The exergy destruction/loss in the high-temperature recuperator and precooler of RSBC/AC are significantly reduced. The levelized cost of electricity and payback period for the plant integrated with RSBC/AC are reduced by 0.46–0.77 ￠/kWh and 0.67–5.27 years, respectively, with an annual full-load hour ranging from 5000 to 8500.
•A novel combined cycle is proposed for the air-cooled concentrated solar power plant.•Energy and exergy efficiency of the proposed cycle increase by 5.19% and 6.12%.•The exergy destruction/loss in the HTR and precooler are significantly reduced.•LCOE and payback period of the plant is reduced by 0.46–0.77￠/kWh and 0.67–5.27 year.
Peculiar thermodynamic properties of carbon dioxide (CO2) when it is held at or above its critical condition (stated as supercritical CO2 or sCO2) 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 sCO2 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%–50% for medium-to-moderate temperature range of the studied application (630 °C–680 °C). Due to the lack of maturity of current sCO2 technologies and no power production scale demonstrators, cycle boundary conditions based on the solar application and a detailed literature review were chosen.
•Mathematical modelling description for recompression sCO2 cycle.•Split fraction and recuperators effectiveness effect into sCO2 cycle performance.•Optimization methodology of sCO2 cycle for an innovative solar central receiver.•Power generation using particles central receiver.
•A review on recuperators for micro gas turbines is presented.•Different types of recuperators and material selection are given and compared.•Research on heat transfer and pressure drop ...characteristics is summarized.•Optimization methods used to improve recuperator performance are reviewed.•Future development of recuperators is discussed.
Micro gas turbines are a promising technology for distributed power generation because of their compact size, low emissions, low maintenance, low noise, high reliability and multi-fuel capability. Recuperators preheat compressed air by recovering heat from exhaust gas of turbines, thus reducing fuel consumption and improving the system efficiency, typically from 16–20% to ∼30%. A recuperator with high effectiveness and low pressure loss is mandatory for a good performance. This work aims to provide a comprehensive understanding about recuperators, covering fundamental principles (types, material selection and manufacturing), operating characteristics (heat transfer and pressure loss), optimization methods, as well as research hotspots and suggestions. It is revealed that primary-surface recuperator is prior to plate-fin and tubular ones. Ceramic recuperators outperform metallic recuperators in terms of high-temperature mechanical and corrosion properties, being expected to facilitate the overall efficiency approaching 40%. Heat transfer and pressure drop characteristics are crucial for designing a desired recuperator, and more experimental and simulation studies are necessary to obtain accurate empirical correlations for optimizing configurations of heat transfer surfaces with high ratios of Nusselt number to friction factor. Optimization methods are summarized and discussed, considering complicated relationships among pressure loss, heat transfer effectiveness, compactness and cost, and it is noted that multi-objective optimization methods are worthy of attention. Moreover, 3D printing and printed circuit heat exchanger technologies deserve more research on manufacturing of recuperators. Generally, a metallic cost-effective primary-surface recuperator with high effectiveness and low pressure drop is a currently optimal option for a micro gas turbine of an efficiency of ∼30%, while a ceramic recuperator is suggested for a high efficiency micro gas turbine (e.g. 40%).
•Sensitivity analysis on two Supercritical CO2 cycles has been done.•Component quality considerably affects to Supercritical CO2 cycle efficiencies.•Recompression Cycle is optimal in most cases.•Most ...exergy is destroyed in heat recovery process.•Viability of Supercritical CO2 cycles depends on component quality.
Supercritical CO2 cycles have been said to be a good alternative to the Rankine Cycles for Concentrating Solar Power plants of the future. The next generation molten salts will be able to achieve 700 °C, which is a suitable temperature for Supercritical CO2 cycles. However, there is a big uncertainty about the efficiencies of the cycle components, which could make these cycles unviable. A sensitivity analysis of the energy efficiency of the Recompression Cycle and Partial Cooling Cycle, regarding turbomachinery isentropic efficiencies and Recuperator effectiveness variations, has been carried out to show that the Recompression Cycle’s energy efficiency is considerably more sensitive than the Partial Cooling Cycle’s. From the sensitivity analysis, it can also be concluded that the Recompression Cycle is the best performing cycle for most of the studied cases, with energy efficiencies in the range between 32.97% and 51.91%. Exergetically, the Recompression Cycle is also more suitable in most situations, and the exergy analysis on cycle components shows that irreversibilities occur mainly in the Recuperators, which means that future research should focus on methods to reduce irreversibilities in these components.
The state-of-the-art of Supercritical Rankine Cycle plant net energy efficiencies currently reach 45.60% for fossil fuel plants. Although Supercritical CO2 cycles are a simpler and more compact alternative, this work concludes that only the optimized Recompression Cycle with turbomachinery isentropic efficiencies over 92% and Recuperator effectiveness over 95% are able to obtain similar or higher efficiencies than actual Supercritical Rankine Cycles. Furthermore, the sensitivity analysis plots permit the areas to be mapped where each of the optimized two-cycle efficiencies can compete with the Supercritical Rankine Cycles regarding the turbomachinery isentropic efficiencies and Recuperator effectiveness.
In this paper, a complete mathematical model is developed to carry out the thermodynamic analysis and comparison for different direct-heated S-CO2 Brayton cycles (simple, pre-compression, ...recompression, partial-cooling, and intercooling) integrated into a solar power tower (SPT) system. Based on the model, the effect of turbine inlet temperature (TIT) on the thermodynamic performances of the receiver, the thermal energy storage unit, the S-CO2 power cycle blocks and the integrated SPT systems is investigated respectively for these cycles. Additionally, a comparison of cycle efficiencies and overall integrated SPT system efficiencies is performed for five S-CO2 cycles at a series of total recuperator conductance (UAtotal) values. The results reveal that the TIT exhibits a parabolic effect on the overall efficiencies for each S-CO2 cycle, and the intercooling S-CO2 cycle achieves the highest overall efficiencies followed by the recompression, the partial-cooling, the pre-compression, and the simple cycles at different TIT values. Furthermore, the partial-cooling cycle possesses the highest overall specific work at each TIT and offers higher overall efficiencies than the recompression cycle at a constant TIT (650 °C) as the UAtotal is rather low, having the potential to reduce the costs of integrated SPT systems with limited UAtotal values.
•The integration of five different direct-heated S-CO2 Brayton cycles into a solar power tower system is studied.•A complete mathematical model is developed for the direct integrated SPT systems.•A comprehensive thermodynamic analysis and comparison are carried out among five S-CO2 Brayton cycles.•The effect of UAtotal on the cycle efficiencies and overall system efficiencies is investigated.
As an advanced power cycle, supercritical CO2 (sCO2) Brayton cycle has been considered as a promising alternative of conventional steam Rankine cycle for coal-fired power plants. The sCO2 power cycle ...must be improved to deal with coal-fired system integration constraints since coal-fired boiler is significantly different from nuclear and CSP heaters. The inlet temperature of the working fluid entering coal-fired boiler in sCO2 cycle is much higher than that in steam cycle, as it can be preheated more sufficiently in recuperators. Hence, the exhaust heat of coal-fired boiler flue gas cannot be fully utilized itself. It is of great importance to study on how to make good use of the exhaust heat of flue gas. An in-house code of sCO2 Brayton cycle tailored for coal-fired power plant was developed at first. Then, three improved cycle layouts for better utilization of the exhaust heat of flue gas were proposed, which were assessed in depth based on comprehensive analyses of both sCO2 boiler and cycle layout. The improved cycle with a second split flow to the boiler was proved to be the most effective one. With parameters of 31MPa/600 °C/620 °C, the maximum net efficiency was improved from a based value of 45.96% to an optimized value of 50.71%. It was also higher than that of a state-of-the-art ultra-supercritical steam power plant with same paremeters (about 46%–47%). Finally, the effects of the second split flow ratio on net efficiency were analyzed, and the equations to calculate the optimal range of second split flow ratio was derived.
•Code of supercritical CO2 cycle tailored for coal-fired power plants is developed.•Three improved cycles are proposed by using a low temperature economizer.•sCO2 cycle with second split flow is an effective layout for coal-fired power plants.•Net efficiency rises from 45.96% to 50.71% at 31MPa/600 °C/620 °C by this improvement.•Calculation equations of the maximum/minimum second split flow ratio are derived.
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•SOFC-GT hybrids were designed for various levels of SOFC fuel utilization, Uf.•System efficiency was high, >70% LHV, between 65 and 90% Uf using syngas fuel.•Lower Uf increased the ...Nernst potential and GT inlet T, while SOFC size dropped.•Peak efficiency was found with SOFC only providing 65% of the power share.•Hybrid designs with 40–60% output power from SOFC had comparable economics.
A computational analysis was conducted to optimize the design of a solid oxide fuel cell - gas turbine hybrid power generator, focusing on the impact that fuel utilization within the fuel cell has on system efficiency and installed costs. This is the first ever design-study considering the effect of fuel utilization on performance, as well as on the optimum power split. This hybrid system attained high electric generation efficiencies (>70%) over a wide range of operating conditions (60% < fuel utilization < 90%) while the fuel cell stack size decreased in proportion to decreasing the fuel utilization. A one-dimensional fuel cell model was used to simulate the fuel cell while GateCycle® was used to simulate the performance of the associated recuperated turbine and various subsystems necessary for thermal management. For each test case, the size of the solid oxide fuel cell, gas turbine, and recuperator, as well as the fuel and air flow rates, hot-air bypass set point, and heat exchange effectiveness in the solid oxide fuel cell manifold were varied to obtain 550 MWe output. In addition, anode recycle, turbomachinery efficiency, and various thermal management options were tested. The maximum system efficiency (75.6%) was attained for the single-pass solid oxide fuel cell with highly efficient turbomachinery when the solid oxide fuel cell used 80% of the incoming fuel. Efficiency was essentially flat from 75% fuel utilization through 85% fuel utilization. Employing anode recycle starting at 65% resulted in roughly 1 percentage point efficiency decrease for each percent increase in fuel utilization. For minimized solid oxide fuel cell degradation, a near 50:50 power split case was studied resulting in 68.6% efficiency and the solid oxide fuel cell using 55% of the incoming fuel. Because of shifting half of the power generation to the gas turbine, the size of the fuel cell stack was reduced by 25% as compared to that at maximum efficiency (80% fuel utilization).
•The partial cooling cycle has a relatively large temperature range of heat input.•Larger ranges decrease the cost of two-tank thermal energy storage.•Larger ranges decrease tower mass flow rate and ...associated parasitics.•Colder average receiver temperatures decrease receiver thermal losses.•Partial-cooling cycle achieves lower levelized cost of energy vs recompression.
This analysis investigates the design, cost, and performance of the simple, recompression, and partial-cooling configurations of the supercritical carbon dioxide power cycle integrated with a molten salt power tower concentrating solar power system. This paper uses a steady-state model to design each cycle with varying amounts of recuperator conductance to understand performance and cost trade-offs. The recompression cycle can achieve a higher thermal efficiency than the partial-cooling cycle, and the partial-cooling cycle achieves a higher thermal efficiency than the simple cycle. The partial-cooling cycle is the most expensive cycle because it requires more total turbomachinery capacity. However, the partial-cooling cycle has the largest temperature range of heat input. This feature leads to cheaper two-tank thermal energy storage, higher receiver efficiencies, and lower mass flow rates in the power tower. Crucially, the lower mass flow rates significantly reduce pump electricity consumption relative to the recompression-cycle system. Consequently, this study finds that the power tower system integrated with the partial-cooling cycle is both cheaper and generates more net electricity than systems integrated with the other two cycles. Finally, this paper presents a parametric study on the air-cooler approach temperature and shows that small approach temperatures can improve cycle efficiency and increase the temperature range of heat input, which can lead to smaller optimal approach temperatures than may be expected.
•A novel dual ORC system is designed for engine waste heat and LNG cold.•Exhaust gas and jacket cooling water are considered as heat sources.•LNG and boil-off gas are considered as heat sinks.•ORC ...loops are optimized to produce the maximum net work output.
The marine sector produces a large portion of total air pollution, so the emissions of the engines used must be improved. This can be achieved using a new eco-friendly engine and waste-heat recovery system. A dual-fuel (DF) engine has been introduced for LNG carriers that is eco-friendly and has high thermal efficiency since it uses natural gas as fuel. The thermal efficiency could be further improved with the organic Rankine cycle (ORC). A novel dual-loop ORC system was designed for DF engines. The upper ORC loop recovers waste heat from the exhaust gas, and the bottom ORC loop recovers waste heat from the jacket cooling water and LNG cold. Both ORC loops were optimized to produce the maximum net work output. The optimum simple dual-loop ORC with n-pentane and R125 as working fluids produces an additional power output of 729.1 kW, which is 4.15% of the original engine output. Further system improvement studies were conducted using a recuperator and preheater, and the feasibility of using boil-off gas as a heat sink was analyzed. Optimization of the system configuration revealed that the preheater and recuperator with n-pentane and R125 as working fluids increase the maximum net work output by 906.4 kW, which is 5.17% of the original engine output.
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