Supercritical Power Research Articles

Effect of temperature and medium environment on corrosion fatigue behavior of Inconel 625

AbstractInconel 625 is a nickel‐based alloy with exceptional creep and corrosion resistance properties. It has emerged as a candidate material for operational and structural use in critical and supercritical power generation plants. Supercritical power plants work at high temperatures, pressure and in an erosive environment that makes metallic materials more vulnerable to corrosion fatigue. This study aimed to evaluate the fatigue crack growth rate (FCGR) at elevated temperatures (550–650°C) in a steam and supercritical water environment. All the experiments were performed at a water oxygen level of 8000 ppb. A cyclic stress of 1 Hz frequency and sine waveform was applied for the test. A 0.6 stress ratio was applied with a maximum stress intensity factor of 32 MPa√m. A direct current potential drop method was applied to detect the crack growth rate (CGR). For the given parameters, fractographic analysis validated the role of corrosion‐fatigue interaction as the primary source of variation in the FCGR. Results revealed that the CGR was enhanced with a rise in temperature under all test environments. The influencing mechanism of temperature on CGR was also discussed.

Study on supercritical CO2 power cycles for natural gas engine energy cascade utilization

An energy cascade utilization system (ECUS) composed of a natural gas engine (in the first stage) and supercritical CO2 power cycle (in the second stage) in series is established to improve thermal efficiency. From the perspective of energy and exergy, seven classical cycles and three engine coolant pre-heating cycles in the ECUS are investigated. Subsequently, the design criteria for supercritical CO2 power cycles are summarized. By introducing the concept of “Fuel–Product–Loss,” the thermodynamics and exergoeconomics of the ECUS are comprehensively investigated. Based on the foregoing, the multi-objective optimization of the ECUS is investigated using the Pareto front. Moreover, the effect of engine load on the ECUS is considered. Results show that the thermal efficiency of the ECUS increases from 42.4% to 48.94%, the exergy efficiency increases from 50.17% to 52.01%, and the unit product cost decreases from 14.98 to 13.19 ¢/kW∙h compared with the original engine (100% load).

Mass optimization of a recompression supercritical nitrous oxide and helium power system for space exploration

Due to high output power, long lifetime, high efficiency, and compact structure, the space nuclear power (SNP) system is considered an ideal choice for future large-scale and deep space missions. The system's mass, volume, and thermoelectric conversion efficiency directly affect and determine the system's performance compared with ground nuclear devices. The mass of the space nuclear power system is a vital parameter due to the limitation of cost, volume, and transportation. This study designed and optimized the recompression supercritical with nitrous oxide and helium (N2O–He) Brayton cycle with a small modular reactor for mass. Each Brayton cycle component model was developed to estimate the cycle performance systematically. The mass estimation model of the Brayton system includes a nuclear reactor, radiation shadow shield, Brayton cycle unit, recuperator, and radiator. A small modular reactor was designed to predict the minimum mass reactor suited to the given cycle conditions. The system mass optimization explored the trade-offs between the reactor, radiation shield, Brayton cycle unit, recuperator, and radiator to obtain the minimum mass of the space nuclear energy system to satisfy the operating requirements. Sensitivity parametric investigations are considered to evaluate the effects of crucial decision variables on the whole Brayton cycle mass. Furthermore, multi-objective optimization is performed to find optimum operating parameters to minimize the system mass. The results illustrate that the specific mass of the system is less than 20 kg/kW. The total mass of the whole system is 5605.93 kg using near-term materials, of which the Baryton Rotating Unit, radiator, and shadow shield mass dominate the total mass. The Brayton Rotating Unit, radiator, and radiation shield account for 44.64%, 32.53%, and 14.29%, respectively. It is concluded that after the dual-objective optimization analysis, the mass of the cycle decreased by 18.44%, which used the existing technical materials.

Thermodynamic optimization and performance study of supercritical CO2 thermodynamic power cycles with dry cooling using response surface method

This paper deals with thermodynamic optimization of supercritical CO2 recompression and partial cooling cycles operating at cycle maximum temperature of 680°C and maximum pressure of 250 bar. The primary goal to investigate the effects of variation in heat sink temperature (ambient air temperature), mass split fraction (X), and cycle minimum pressure (Pmin) on the thermal efficiency of the power cycles. Response surface method (RSM) is adopted to create a second-order polynomial equation in order to develop the relationship between cycle thermal efficiency and selected decision variables and to find global optimum cycle efficiency. In addition, classification of most influencing cycle parameter is carried out using ANOVA approach. In the case of a recompression cycle, the results demonstrate that heat sink temperature has the greatest impact on thermal efficiency, owing to low p-value and high F-value, followed by mass split fraction and minimum pressure. In a partial cooling cycle, the minimum pressure has the most significant impact on cycle thermal efficiency, followed by the mass split fraction and heat sink temperature. The global optimum combination for the recompression cycle is at heat sink temperature of 20°C, the mass split fraction of 0.3182, and a minimum pressure of 89 bar to obtain the highest thermal efficiency of 0.4963. In addition, the global optimum combination for partial cooling cycle is at heat sink temperature of 32.8 °C, mass split fraction of 0.34, and minimum pressure of 76 bar, which results in an optimum thermal efficiency of 0.4708.

Control strategies and dynamic experimental tests on the wide-range and rapid load regulation of a first pilot multi-megawatts fossil-fired supercritical CO2 power system

Supercritical CO2 (sCO2) power system is a research front in recent years due to its high efficiency and flexibility which is considered as a transformative power system in terms of consumption of the fluctuated and intermittent renewables. The merits of sCO2 power systems have been widely shown by small-scale test loops. However, it is still very lack of the investigations on the control strategies based on real-time dynamic operational data of large-scale multi-megawatts pilot power system, especially for wide-range and rapid load regulation to demonstrate the feasibility of large-scale utilization and commercialization. A worldwide first pilot multi-megawatts SMART (Supercritical CO2 Modular Advanced Research and Test) fossil-fired power system has been built and operated for more than 1000 h at TPRI (Thermal Power Research Institute), Xi’an, China with turbine inlet parameters of 20 MPa/600 °C/600 °C. The influence of key parameters on the dynamics of the system are experimental studied. In-depth tests of the wide-range load regulation and rapid load regulation of the SMART@TPRI power system are performed. The test results show that the key target parameters such as turbine inlet temperatures, compressor inlet temperature and pressure, sCO2 mass flow rate and exhaust flue gas temperature can be well regulated by a set of well-developed control modules to ensure an efficient and safe operation during load variations. The SMART@TPRI power system can run stably and flexibly within the whole range from 0 %Pe to 100 %Pe. The averaged load ramp and load reduction rate achieves 6.35 %Pe/min and −6.37 %Pe/min, respectively, which is almost 3 ∼ 4 times than that of steam power system, under the control strategy of sliding pressure with fixed turbine inlet temperature by a combination of several novel well-developed control modules.

Multi-Objective Optimization of a Solar Combined Power Generation and Multi-Cooling System Using CO2 as a Refrigerant

This paper proposes a new combined multi-cooling and power generation system (CMCP) driven by solar energy. Carbon dioxide is used as a refrigerant. A parabolic trough collector (PTC) is employed to collect solar radiation and convert it into thermal energy. The system includes a supercritical CO2 power system for power production and an ejector refrigeration system with two ejectors to provide cooling at two different evaporating temperatures. The CMCP system is simulated hourly with weather conditions for Tunisia. The PTC mathematical model is used to calculate the heat transfer fluid outlet temperature and the performance of the CMCP system on a specific day of the year. A 1D model of an ejector with a constant area is adopted to evaluate the ejector performance. The system’s performance is evaluated by an energetic and exergetic analysis. The importance of the system’s components is determined by an exergoeconomic analysis. The system is modeled using MATLAB software. A genetic algorithm is used for multi-objective optimization to determine the best values and solutions for the system’s design parameters. The optimal energy and exergy efficiencies were found to be 13.7 percent and 37.55 percent, respectively, and the total product unit cost was 31.15 USD/GJ.

A prototype recuperated supercritical co2 cycle: Part-load and dynamic assessment

High efficiency, flexibility and competitive capital costs make supercritical CO2 (sCO2) systems a promising technology for renewable power generation in a low carbon energy scenario. Recently, innovative supercritical systems have been studied in the literature and proposed by DOE-NETL (STEP project) and by a few projects in the EU Horizon 2020 (H2020) program aiming to demonstrate supercritical CO2 Brayton power plants, promising superior techno-economic features than steam cycles particularly at high temperatures.The H2020 SOLARSCO2OL project, which started in 2020, is building the first European MW-scale sCO2 demonstration plant and has been specifically tailored for Concentrating Solar Power (CSP) applications. After a detailed explanation of the modelling approach for steady and unsteady cycle simulations, this paper presents the off-design and dynamic analysis of such plant layout, which is based on a simply recuperated sCO2 cycle. The entire system model has been developed in TRANSEO environment. The part-load analysis ranged from 50% of nominal up to a 105% peak load, discussing the impact on compressor and turbine operating conditions. Full operational envelop has been determined considering cycle main constraints, such as maximum turbine inlet temperature and minimum pressure at compressor inlet.The off-design performance analysis highlights the most relevant relationships among the main part-load regulating parameters, namely molten salt mass flow rate, CO2 mass flow rate, total CO2 mass in the loop, and shaft line speed. The results show specific features of different control approaches, discussing the pros and cons of each solution, considering also its upscale towards commercial applications. In particular, the analysis shows that at 51% of load an efficiency decrease of 20% is expected. Finally, the dynamic characterization of the closed loop shows the relatively fast responsiveness of the plant to compressor speed variations, causing quick changes in CO2 mass flow rate, together with longer time scale phenomena related to the plant heat exchangers. In this respect, sCO2 plants demonstrate to have the potential to provide primary reserve for the electrical grid, as far as thermal stresses on main plant components are kept under acceptable limits.

Performance investigation of supercritical carbon dioxide power cycle with recuperative recompression technology

The supercritical carbon dioxide power cycle is an advanced thermoelectric conversion technology due to its high-power generation efficiency and compact structure. However, basic supercritical carbon dioxide power cycle performance is limited to turbine outlet temperature. To further improve thermoelectric conversion efficiency, recuperative recompression in the supercritical carbon dioxide power cycle is investigated for efficient energy utilization. Recuperative technology can reduce the amount of heat released to the environment, and recompression can reduce Compression power consumption. A thermodynamic model of recuperative recompression in a supercritical carbon power cycle system is built in this paper, and the system’s performance is investigated. The result reveals that the recuperative recompression in the supercritical carbon power cycle system has greater performance promotion potential than the basic supercritical carbon power cycle system.

Research on active disturbance rejection control of the main steam temperature of supercritical thermal power unit based on improved particle swarm optimization

The stability of the boiler’s main steam temperature is very important for the safe operation of the unit. At present, the mainstream PID control scheme is simple to use and convenient to set, but it cannot accomplish a acceptable ascendancy effect in the face of the main steam temperature control structure; with a long time delay, large inertia. In view of this situation, first of all, this paper introduces the linear active disturbance rejection control (LADRC) technology in a steam temperature control structure. The main loop adopts the linear active disturbance rejection controller and the subloop adopts the P-controller. Then, the particle swarm optimization algorithm is improved and used to modify the parameter of the LADRC, so that the LADRC system adjusted according to the theory can better complete the control task. Finally, a simulation to control the temperature is given, and the results show that the LADRC control system can track quickly and stably; in addition, as against with this traditional PID control strategy, it can achieve strong anti-interference performance.

Thermodynamic Optimization of Subcritical and Supercritical Organic Rankine Cycle Power Plants for Waste Heat Recovery in Marine Vessels

Abstract This study was aimed at comparing the optimal thermodynamic performance of subcritical and supercritical organic Rankine cycle (ORC) plants for waste heat recovery from ship engines. The technical impacts of adopting a supercritical ORC scheme relative to the usual subcritical one have not been explicitly reported in the literature for heat recovery in ship engines, hence this study. The fluids R245fa, R134a, and R600a were employed for analysis due to their versatility in real systems. The ORC plants were modeled and optimized in matlab using established zero-dimensional models to satisfy the first law mass and energy balances. Results showed that introducing a recuperator would increase ORC performance. For the R600a which exhibited the best performance among the three working fluids, a net power output of 488.3 kW was obtained for the subcritical ORC without a recuperator (SYS A) and 543.7 kW for the one with a recuperator (SYS B). Furthermore, a switch to a supercritical ORC configuration increased the net power by about 29% for R134a and 10% for R600a, and increased the thermal efficiency by about 2.2 percentage points for R134a and 0.5 percentage points for R600a, referencing the supercritical configuration without a recuperator (SYS C) and SYS A.

Recent Progress on High Temperature and High Pressure Heat Exchangers for Supercritical CO2 Power Generation and Conversion Systems

Heat exchangers for supercritical CO2 power generation and waste heat to power conversion systems have a significant impact on the overall cycle efficiency and system footprint. Key challenges for supercritical CO2 heat exchangers include ability to withstand high temperature and high pressure (typical temperature range of heat source 350 to 800 °C and typical required operating pressure range 150 to 300 bars), and large pressure differential between fluid streams. Other requirements are low pressure drop, high effectiveness and high reliability under thermal cycling. This paper presents recent developments in supercritical CO2 heat exchangers in terms of material selection, design, manufacture, and operation. Since heat exchangers represent a significant portion of the total system cost, another key challenge is to find a compromise between the heat exchanger type, cost, durability, and performance. This paper explores heat exchanger technologies, manufacturing techniques and materials for high temperature and high pressure heat exchangers for supercritical CO2 applications. It also identifies technology gaps and research needs to accelerate the development of effective designs to facilitate the commercialization of both supercritical CO2 heat exchanger technologies and power cycles.

Performance assessment of supercritical power plant based oil fuel under sliding-pressure operation

Energetic and exergetic analyses of a 626 MWe supercritical power plant-based mazout oil-fired are carried out to assess the system's performance. The energy losses and exergy destruction are evaluated for each system component using real actual data. The results show that the boiler is the primary source of irreversibility with 88.62% of the total exergy destruction. The intermediate pressure turbine maintains an exergy efficiency of 97% at 100%-load. Moreover, the maximum thermal efficiency is achieved at 100%-full load by 44.85% whereas, the maximum overall exergy efficiency of 40% is acquired at the maximum continuous rate condition of 105%-load.

Performance study of a novel supercritical CO2 solar-coal supplementary power generation system

• New supercritical CO 2 solar-coal supplementary power generation system is proposed. • The annual solar generating power accounts for 61.29% of the total power output. • Standard coal consumption rate is 161.11 g/kWh lower than pure coal-fired system. • The new system performances with the change of the load rate are revealed. • Effects of solar multiple and thermal energy storage capacity on new system are studied. To reduce the CO 2 emission and improve the solar energy utilization rate, a novel supercritical CO 2 solar-coal supplementary power generation system is proposed. Compared with the previous studies on supplementary systems, the novel system takes solar energy as the main heat source instead of coal, which greatly increases the proportion of solar energy input. The results show that, when the solar multiple (parameter representing the area of the heliostat field) is 3 and the thermal energy storage capacity is 16 h, on the summer and winter solstices, the proportions of solar generating power are 100% and 57.15%, respectively. The novel system can operate steadily during the whole year. The annual solar generating power is 161.07 GWh, accounting for 61.29%. The standard coal consumption rate of the novel system is 101.76 g/kWh, 161.11 g/kWh lower than that of the pure coal-fired system. The exergy efficiency for the novel system increases with an increasing operation load rate. Under 50%, 75%, 100%, and 125% load rates, the exergy efficiencies are 16.7%, 25.0%, 33.4%, and 38.0%, respectively. It is the first time to study the effects of the solar multiple and the thermal energy storage capacity on the energy, exergy, environmental, and economic performances of the supercritical CO 2 solar-coal supplementary power generation system. When the thermal energy storage capacity is 16 h, the solar multiple corresponding to the optimal economic performance is 2.5.

Dynamics and control implementation of a supercritical CO2 simple recuperated cycle

Interest in supercritical CO2 power cycles is constantly increasing, showing good efficiency, and promising competitive costs and enhanced flexibility with respect to competing systems. Some project within the EU Horizon 2020 program have studied these systems and aim to demonstrate them in large scale, following the example of the STEP project in US. This work is part of the effort of the SOLARSCO2OL project to build a sCO2-CSP power demo plant at MW scale. A dynamic model of a simple recuperated sCO2 cycle is developed in TRANSEO, using miniREFPROP to compute fluid properties, and table of properties are implemented, when possible, to enhance the performance of the code. Control logics is described and simple controllers implemented. Finally, controllers are tested showing the response of the main parameters of the plant to a ramp variation of the load. Stable compressor inlet pressure is achieved with an inventory control, while a stable turbine inlet temperature allows a high efficiency in part-load operation.

Comparative power density and exergy density analyses of a regenerative supercritical carbon dioxide power cycle

Comparative power density and exergy density analyses of a regenerative supercritical carbon dioxide power cycle

Performance assessment of supercritical power plant based oil fuel under sliding-pressure operation

Performance assessment of supercritical power plant based oil fuel under sliding-pressure operation

Experimental Study on a Supercritical CO2 Centrifugal Compressor Used in a MWe Scale Power Cycle

The centrifugal compressor is the core component of supercritical CO2 power cycle, and its performance and operation stability are research hotspots. However, there are few experimental studies, especially for compressors used in Mwe-scale power cycles. In this paper, based on a 1 MWe supercritical CO2 power cycle, a single-stage centrifugal supercritical CO2 compressor is designed with speed of 40,000 RPM, a pressure ratio of 2.5 and a mass flow of 16.3 kg/s. In order to carry out the compressor test, a general experimental platform for MWe sCO2 compressors is built. In the test, the mass flow range is 13.5~18 kg/s and the maximum experimental pressure ratio is close to 2.0. The performance curve of the compressor of 31,000 ± 1000 RPM is obtained, and the historical curve of the experiment is given. Then, the experimental curve is compared with the design curve using a dimensionless method. The isentropic head coefficient of the experimental curve is lower than the design value, and the experimental curves shift towards the boundary of small flow coefficient. Finally, the influence of compressor inlet condensation on compressor performance and the change of operating boundary is preliminarily explained.

Assessment of the Hot-Cracking Susceptibility of Welded Joints of the 7CrMoVTiB10-10 Bainitic Steel Used in Heat Exchangers

Bainitic steel containing approx. 2.25% Cr and 0.6 Mo with micro-additions of V, Ti, and B, designated as 7CrMoVTiB10-10 (T/P24), is one of the new construction materials used in new supercritical power units. The weldability of 7CrMoVTiB10-10 is defined as the hot-cracking susceptibility, it should be stated that the hot cracking in the welded joints of 7CrMoVTi10-10 is determined by phenomena occurring in the high-temperature brittleness range (HTBR). In this work, the HTBR is established for both the base material and welding conditions, taking into account the critical temperature-strain intensity (CST) and the critical strain speed (CSS). The HTBR for 7CrMoVTi10-10 is 122 °C wide and covers temperatures from 1394 °C to 1516 °C. Under imposed deformation conditions (typical during welding), the HTBR extends to a width of 293 °C towards lower temperatures, i.e., from 1516 °C to 1223 °C. The CSS = 0.83 1/s, the CST = 0.003 1/°C, and the Rf index = 0.12, which can be adopted as the criteria for the susceptibility of 7CrMoVTiB 10-10 to hot cracking under imposed deformation conditions. The hot cracking in 7CrMoVTiB10-10 occurs as a result of the loss of cohesion by the thin liquid layer crystallising between the growing weld crystals. Such cracks appear when the CSS or the CST is exceeded within the HTBR.

Offline reinforcement learning control for electricity and heat coordination in a supercritical CHP unit

With a high proportion of renewable energy generation connected to the power grid, the combined heat and power (CHP) units need to have flexible operation and control capabilities over a wide range of variable load conditions. In China, the proportion of supercritical combined heat and power (S-CHP) units gradually increases due to their high thermal efficiency. In this paper, we propose a data-driven environment modeling method and an offline reinforcement learning-based electricity–heat coordinated control approach for the wide and flexible load adjustment capabilities of the S-CHP unit. First of all, a modeling method based on the multiple multilayer perceptron (MLP) ensemble is proposed to address the possible over-fitting produced by a single MLP. Then, the state and reward are set considering the dynamic characteristics of the S-CHP unit. Moreover, policy training is implemented through a soft actor-critic algorithm with a maximum entropy model to ensure more robust search capabilities under variable load conditions and targets. The simulation results show that the generalization ability of the environment in the multiple MLP ensemble mode is more substantial than that in the single MLP mode. In addition, when the electric load command is between 267 MW and 325 MW, the offline reinforcement learning can significantly reduce the integral of absolute error matrices of the output parameters, demonstrating that the proposed strategy can achieve electricity–heat coordinated control under variable load conditions.

Investigation of the polygeneration system integrated with gas engine-driven heat pump system and CO2 Brayton cycle for waste heat recovery

A gas engine-driven heat pump system could provide cooling and heating, and fully recover the waste heat. The temperature of the waste heat recovered from the gas engine cylinder jacket and exhaust gas was more than 90℃ and 350℃, respectively. The supercritical CO2 Brayton power cycle is considered a promising primary mover of the combined cooling, heating, and power system to potentially provide higher efficiency. Therefore, a polygeneration system composed of gas engine-driven heat pump system and CO2 Brayton cycle was proposed for cooling, heating, and electricity. Experiments of gas engine-driven heat pump system were conducted and the performances were also investigated in the cooling mode. Meanwhile, the thermodynamic performances of the CO2 Brayton cycle were analyzed. The results indicated that the cooling capacity and waste heat increased with the increase of gas engine speed and decreased with the decrease of evaporator water inlet temperature, the primary energy ratio was 1.032, 0.963, and 0.893 at the gas engine speeds of 1260 r/min, 1510 r/min, and 1820 r/min, respectively, and the increasing range was over 43.5 %. In addition, the maximum net power output of CO2 Brayton cycle was 0.97, 1.68, and 2.14 kW, and the gas engine fuel economy was 12.05, 12.35, and 12.10 kg/kWh at the gas engine speeds 1260 r/min, 1510 r/min, and 1820 r/min, respectively. The decreasing range of gas engine fuel with CO2 Brayton cycle system was 8.6 %, 12.3 %, and 12.5 %, respectively.