Supercritical Carbon Dioxide Power Cycle Research Articles

A Study of Supercritical Carbon Dioxide Power Cycle for Concentrating Solar Power Applications Using an Isothermal Compressor

For the concentrating solar power (CSP) applications, the supercritical carbon dioxide (s-CO2) power cycle is beneficial in many aspects, including high cycle efficiencies, reduced component sizing, and potential for the dry cooling option. More research is involved in improving this technology to realize the s-CO2 cycle as a candidate to replace the conventional power conversion systems for CSP applications. In this study, an isothermal compressor, a turbomachine which undergoes the compression process at constant temperature to minimize compression work, is applied to the s-CO2 power cycle layout. To investigate the cycle performance changes of adopting the novel technology, a framework for defining the efficiency of the isothermal compressor is revised and suggested. This study demonstrates how the compression work for the isothermal compressor is reduced, up to 50%, compared to that of the conventional compressor under varying compressor inlet conditions. Furthermore, the simple recuperated and recompression Brayton cycle layouts using s-CO2 as a working fluid are evaluated for the CSP applications. Results show that for compressor inlet temperatures (CIT) near the critical point, the recompression Brayton cycle using an isothermal compressor has 0.2–1.0% point higher cycle thermal efficiency compared to its reference cycle. For higher CIT values, the recompression cycle using an isothermal compressor can perform above 50% in thermal efficiency for a wider range of CIT than the reference cycle. Adopting an isothermal compressor in the s-CO2 layout can imply larger heat exchange area for the compressor which requires further development.

Thermodynamic analysis of a novel dual expansion coal-fueled direct-fired supercritical carbon dioxide power cycle

The direct-fired supercritical CO2 power cycle not only has the potential of reaching high efficiency but also has inherent ability to capture almost all of the combustion derived CO2. A novel direct-fired supercritical CO2 power cycle layout is proposed in this paper, using the syngas produced by coal gasification as the fuel. The proposed cycle layout is specially designed to facilitate heat integration between the power cycle, the fuel conversion process and other auxiliary subsystems. Heat from the air compressor intercooler and the low temperature syngas is introduced to the regenerator to correct its imbalanced heat exchange, a typical problem of the supercritical CO2 power cycle that is caused by the abrupt physical property variation. Design considerations of the proposed cycle layout are discussed in detail. The result shows that the net efficiency is 42.1%, with near-zero CO2 emissions. The proposed cycle layout is then further modified by integrating more heat from the oxygen compressors and the syngas compressor, which reduces the hot end temperature difference of the regenerator to less than 10 °C and increases the net efficiency to 43.7%. Heat integration through novel cycle layout has been proved essential to guarantee the high efficiency of the supercritical CO2 power cycle.

Energy and exergy analyses of a parabolic trough solar power plant using carbon dioxide power cycle

This paper examines both energy and exergy performances of parabolic trough collectors (PTCs), as part of a solar power plant, under different design and operating conditions. The proposed solar power plant utilizes an innovative supercritical carbon dioxide (S-CO2) power cycle to convert the heat produced by the PTCs to power. In addition, the present system integrates a thermal energy storage (TES) to overcome the intermittent nature of solar energy and extend the hours of operation. Therefore, detailed thermodynamic and heat transfer analyses are conducted to assess heat losses, exergy destructions, and energy and exergy efficiencies. The state-of-the-art PTCs technologies are considered to set the design parameters used in the modeling of the solar field. Furthermore, the effects of varying some operating conditions on the energy and exergy performance of the PTCs and the S-CO2 power cycle are investigated. These parameters include beam radiation intensity, beam incidence angle, and receiver emittance. Subsequently, the resultant impacts of changing these parameters on the overall solar power plant energy and exergy efficiencies are examined. The energy and exergy efficiencies of the PTC are found to be 66.35% and 38.51%, respectively.

Characteristics and optimization of supercritical CO2 recompression power cycle and the influence of pinch point temperature difference of recuperators

Supercritical carbon dioxide (S-CO2) power cycle has been under the spotlight for years as one of the promising solutions to resolve the energy and environmental problem. The most of previous studies focused on cycle performances and applicable configuration. In this study, detailed and extensive analysis on the recompression S-CO2 power cycle was carried out to reflect more practical and desirable conditions. This study focused on the optimization of the S-CO2 cycle by evaluation of the irreversibility of the recuperators using the effectiveness analysis and pinch point temperature difference (PPTD) analysis. Split ratio from the main stream to the recompressor and compression ratio were considered as independent parameters. From the simulation, the characteristics and performance of the recompression cycle were discussed. The optimum operating conditions were determined in combinations of PPTDs of the two recuperators. With the performance curves, control strategy of the two independent parameters is ranged for desirable operation of S-CO2 power cycle.

Parametric study of a direct-fired supercritical carbon dioxide power cycle coupled to coal gasification process

The direct-fired supercritical carbon dioxide power cycle not only has the potential of reaching high efficiency, but also achieves near 100% carbon capture in a more inherent and natural manner. A conceptual flow sheet of the direct-fired supercritical carbon dioxide power cycle that is coupled to the coal gasification process is built in this study. A detailed turbine cooling model is added to better assess the performance of the power plant. To investigate the effect of key cycle variables on the cycle performance and determine potential chances for efficiency improvement, a parametric study is conducted in this paper. The result shows that the turbine inlet temperature and turbine outlet pressure have significant effect on the cycle efficiency while the effect of the turbine inlet pressure is minor. The cycle efficiency is susceptible to the inlet condition of the carbon dioxide pump, which is the weakness of the supercritical carbon dioxide power cycle. The flow sheet and some of the cycle parameters are modified based on the conclusions drawn from the parametric study. The net efficiency of the power plant is calculated to be 38.87% (on lower heating value basis) at a turbine inlet temperature of 1200 °C, while capturing most of the carbon dioxide derived from combustion.

Heat integration and optimization of direct-fired supercritical CO2 power cycle coupled to coal gasification process

Supercritical carbon dioxide (sCO2) power cycle is a promising power cycle that has drawn much attention in recent years. Among various layouts, the direct-fired sCO2 power cycle achieves not only high efficiency but also near zero emission. Coal gasification and air separation process are indispensable processes when coal is used as fuel. To explore the best cycle performance, the above two processes have to be tightly integrated with the sCO2 power cycle in both mass and energy. Key parameters that govern the heat integration should also be optimized to achieve better performance. A two-stage method is applied for simultaneous optimization and heat integration of a direct-fired sCO2 power cycle coupled to coal gasification process. First, boundary parameters of the heat integration process are optimized. Then the heat exchanger network, which reveals viable heat integration scheme, is designed to fulfill the optimized target. The result shows that the net efficiency reaches 39.29% when heat integration of ASU is not considered. If heat integration of ASU is considered, the net efficiency increases to 40.97%, showing an efficiency increment of 1.68%. When the CO2 turbine pressure ratio is also optimized, the net efficiency increases further to 41.41%.

Preliminary experimental study of a supercritical CO2 power cycle test loop with a high-speed turbo-generator using R134a under similarity conditions

Research on applying a supercritical carbon dioxide power cycle (S-CO2) to concentrating solar power (CSP) instead of a steam Rankine cycle or an air Brayton cycle has been recently conducted. An S-CO2 system is suitable for CSP owing to its compactness, higher efficiency, and dry-cooling capability. At the Korea Institute of Energy Research (KIER), to implement an S-CO2 system, a 10 kWe class test loop with a turbinealternator-compressor (TAC) using gas foil bearings was developed. A basic sub-kWe class test loop with a highspeed radial type turbo-generator and a test loop with a capability of tens of kWe with an axial type turbogenerator were then developed. To solve the technical bottleneck of S-CO2 turbomachinery, a partial admission nozzle and oil-lubrication bearings were used in the turbogenerators. To experience the closed-power cycle and develop an operational strategy of S-CO2 operated at high pressure, an organic Rankine cycle (ORC) operating test using a refrigerant as the working fluid was conducted owing to its operational capability at relatively lowpressure conditions of approximately 30 to 40 bar. By operating the sub-kWe class test loop using R134a as the working fluid instead of CO2, an average turbine power of 400 W was obtained.

Current Status of Research on the Application of Supercritical Carbon Dioxide Power Cycle in Fossil Fired Power Generation

Supercritical carbon dioxide power cycle (sCO2 cycle) is characterized of its simple system, compact structure and high efficiency, and can be coupled with fossil fired heat source to form advanced fossil fired power generation system. The sCO2 cycle has two types, i. e. indirectly heated and direct-fired cycle, and the former can use coal fired furnace as heat source, and the latter can use gas combustor as heat source. Based on the current status of research and development of sCO2 cycle throughout the world, it is concluded that sCO2 cycle can be applied to fossil fired power generation, and it is expected that large-scale clean and high-efficiency power generation system can be developed in the near future so as to overcome the development bottleneck of conventional fossil fired power generation technology.

Cost comparison of printed circuit heat exchanger to low cost periodic flow regenerator for use as recuperator in a s-CO2 Brayton cycle

Supercritical Carbon Dioxide (sCO2) power cycles have the potential to deliver high efficiency at low cost. However, in order for an sCO2 cycle to reach high efficiency, highly effective recuperators are needed. These recuperative heat exchangers must transfer heat at a rate that is substantially larger than the heat transfer to the cycle itself and can therefore represent a significant portion of the power block costs. Regenerators are proposed asa cost saving alternative to high cost printed circuit recuperators for this application. A regenerator is an indirect heat exchanger which periodically stores and releases heat to the working fluid. The simple design of a regenerator can be made more inexpensively compared to current options.The objective of this paper is a detailed evaluation of regenerators asa competing technology for recuperators within an sCO2 Brayton cycle. The level of the analysis presented here is sufficient to identify issues with the regenerator system in order to direct future work and also to clarify the potential advantage of pursuing this technology. A reduced order model of a regenerator is implemented into acycle model of an sCO2 Brayton cycle. An economic analysis investigates the cost savings that is possible by switching from recuperative heat exchangers to switched-bed regenerators. The cost of the regenerators was estimated using the amount of material required if the pressure vessel is sized using ASME Boiler Pressure Vessel Code (BPVC) requirements. The cost of the associated valves is found to be substantial for the regenerator system and is estimated in collaboration with an industrial valve supplier. The result of this analysis suggests that a 21.2% reduction in the contribution to the Levelized Cost of Electricity (LCoE) from the power block can be realized by switching to a regenerator-based system.

Evaluation of Property Methods for Modeling Direct-Supercritical CO2 Power Cycles

Direct supercritical carbon dioxide (sCO2) power cycles are an efficient and potentially cost-effective method of capturing CO2 from fossil-fueled power plants. These cycles combust natural gas or syngas with oxygen in a high pressure (200–300 bar), heavily diluted sCO2 environment. The cycle thermal efficiency is significantly impacted by the proximity of the operating conditions to the CO2 critical point (31 °C, 73.7 bar) as well as to the level of working fluid dilution by minor components, thus it is crucial to correctly model the appropriate thermophysical properties of these sCO2 mixtures. These properties are also important for determining how water is removed from the cycle and for accurate modeling of the heat exchange within the recuperator. This paper presents a quantitative evaluation of ten different property methods that can be used for modeling direct sCO2 cycles in Aspen Plus®. Reference fluid thermodynamic and transport properties (REFPROP) is used as the de facto standard for analyzing high-purity indirect sCO2 systems, however, the addition of impurities due to the open nature of the direct sCO2 cycle introduces uncertainty to the REFPROP predictions as well as species that REFPROP cannot model. Consequently, a series of comparative analyses were performed to identify the best physical property method for use in Aspen Plus® for direct-fired sCO2 cycles. These property methods are assessed against several mixture property measurements and offer a relative comparison to the accuracy obtained with REFPROP. The Lee–Kessler–Plocker equation of state (EOS) is recommended if REFPROP cannot be used.

Techno-economic assessment of technological improvements in thermal energy storage of concentrated solar power

The technological and economic impact of design changes in thermal energy storage of concentrated solar power (CSP) systems is assessed. It is shown that the system costs change with the types of storage tanks and also that the operation temperature is limited by the thermal properties of the thermal storage medium. In addition, the cost of energy can be substantially reduced by replacing the conventional power cycle with more advanced power cycles, such asa supercritical carbon dioxide power cycle. Using two types of thermal storage tanks and two thermal storage media, cases are generated incorporating combinations of the design options. A sensitivity analysis is used to investigate the impacts of each technological improvement. The results of this work will contribute to predicting the impact of research and improving the economics of the CSP system.

Fundamental Thermo-Economic Approach to Selecting sCO2 Power Cycles for CSP 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 levels, 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. A thorough sensitivity analysis based on turbine inlet temperature and pressure ratio is done for twelve sCO2 cycles, considering their effects on thermal efficiency and specific work, along with solar share and temperature rise across the solar receiver. 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 always increase the receiver temperature rise 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 strongly affect 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. 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. The results obtained for the two most representative sCO2 cycles show somewhat unexpected results.

Experimental research and theoretical analysis of flow instability in supercritical carbon dioxide natural circulation loop

In the last decade, supercritical carbon dioxide power cycle has attracted worldwide attention. The characteristics of flow instability are critical for the design and safe operation of supercritical carbon dioxide power cycle. In the present paper, theoretical and experimental study was carried out to investigate the characteristics of flow instabilities in supercritical carbon dioxide natural circulation loop. A new explanation of the mechanism of flow oscillation in supercritical carbon dioxide natural circulation has been put forward. It found that the pressure fluctuation, which resulted from the variation of heat transfer mode, might be amplified under the condition of appropriate non-dimensional pressure drop in supercritical fluids natural circulation loop. Effects of typical thermal parameters on flow instabilities of supercritical carbon dioxide natural circulation were discussed in detail. Results showed that, for supercritical fluids natural circulation system, an increase in the system pressure and the local resistance coefficient in the cold section, and a decrease in the local resistance coefficient in the hot section could enhance the system stability.

Corrosion Behavior of Steels in Supercritical CO2 for Power Cycle Applications

In order to understand issues with corrosion of heat exchanger materials in direct supercritical carbon dioxide (sCO2) power cycles, a series of autoclave exposure experiments and electrochemical experiments have been conducted. Corrosion behaviors of 347H stainless steel and P91 martensitic-ferrtic steel in sCO2 environment have been compared. In autoclave exposure tests performed at 50°C- 245°C and 80 bar. Mass change measurements, surface characterization, and corrosion product analysis have been conducted to understand the corrosion behavior of steels in sCO2 containing H2O and O2. Electrochemical tests performed at room temperature and 50°C, a simulation environment of water condensation phase with dissolved CO2 was prepared to evaluate the corrosion resistance of materials. From both types of experiments, generally 347H showed higher corrosion resistance than P91.

Low-Leakage Shaft-End Seals for Utility-Scale Supercritical CO2 Turboexpanders

Supercritical carbon dioxide (sCO2) power cycles could be a more efficient alternative to steam Rankine cycles for power generation from coal. Using existing labyrinth seal technology, shaft-end-seal leakage can result in a 0.55–0.65% points efficiency loss for a nominally 500 MWe sCO2 power cycle plant. Low-leakage hydrodynamic face seals are capable of reducing this leakage loss and are considered a key enabling component technology for achieving 50–52% thermodynamic cycle efficiencies with indirect coal-fired sCO2 power cycles. In this paper, a hydrodynamic face seal concept is presented for utility-scale sCO2 turbines. A 3D computational fluid dynamics (CFD) model with real gas CO2 properties is developed for studying the thin-film physics. These CFD results are also compared with the predictions of a Reynolds-equation-based solver. The 3D CFD model results show large viscous shear and the associated windage heating challenge in sCO2 face seals. Following the CFD model, an axisymmetric finite-element analysis (FEA) model is developed for parametric optimization of the face seal cross section with the goal of minimizing the coning of the stationary ring. A preliminary thermal analysis of the seal is also presented. The fluid, structural, and thermal results show that large-diameter (about 24 in.) face seals with small coning (of the order of 0.0005 in.) are possible. The fluid, structural, and thermal results are used to highlight the design challenges in developing face seals for utility-scale sCO2 turbines.

Development of the turbomachinery for the supercritical carbon dioxide power cycle

A supercritical carbon dioxide power cycle has been focused as a promising power cycle because of its compactness, a high efficiency, and a wide applicability. A 10-kWe-class simple unrecuperated supercritical carbon dioxide Brayton experimental loop including a turbo-alternator-compressor unit using a centrifugal compressor and a radial turbine was developed. A shrouded type of a compressor impeller and a turbine wheel with labyrinth seals were designed to overcome thrust balancing problems of the high-pressure fluid turbomachinery. In addition, this type has no issues on the thermal growth collision failure and clearance loss between a shroud and a wheel. Preliminary operation at 30,000 rpm, turbine inlet temperature of 83 °C, and pressure of 8500 kPa was successful. It is founded that all states of the cycle existed in the supercritical region. Copyright © 2015 John Wiley & Sons, Ltd.

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Performance improvement of dry cooled advanced concentrating solar power plants using daytime radiative cooling

In this study, utilization of daytime radiative cooling to enhance the performance of air-cooled concentrating solar thermal power plants is investigated. Water scarcity and environmental concerns are the driving forces for solar thermal power plants to use dry cooling systems. In order to overcome the energy conversion efficiency penalties associated with using air cooled technologies various supplemental cooling techniques have been proposed. Recent advancements in manufacturing structures with selective radiative properties have made the daytime radiative cooling to the cold outer space practical. In this work, the efficiency improvement of the air-cooled advanced supercritical carbon dioxide power cycles coupled with a radiative cooler is explored.It is shown that for the simple supercritical carbon dioxide cycle operating at hot source temperature equal 550°C by employing 14.02m2/kWe radiative cooler, it is possible to overcome the efficiency losses due to air cooling and the net output of the cycle improves by 5.0%. At hot source temperature equal 800°C, the required radiative cooler area is 4.38m2/kWe and respective performance improvement is equal 3.1%. For the recompression supercritical carbon dioxide cycle operating at hot source temperature equal 550°C by employing 18.26m2/kWe radiative cooler, it is possible to overcome the efficiency losses due to air cooling and the net output of the cycle improves by 7.5%. At hot source temperature equal 800°C, the required radiative cooler area is 10.46m2/kWe and respective performance improvement is equal 4.9%.

Development of a Solar Receiver Based on Compact Heat Exchanger Technology for Supercritical Carbon Dioxide Power Cycles

Supercritical carbon dioxide (s-CO2) can be used both as a heat transfer and working fluid in solar power tower plants. The main concern in the design of a direct s-CO2 receiver is the high operating pressures, i.e., close to 20 MPa. At such high pressures, conventional receivers do not exhibit the necessary mechanical strength or thermal performance. In this paper, a receiver based on compact heat exchanger technology is developed. The receiver consists of a group of plates with square-shaped channels which are diffusion bonded together to tolerate the high operating pressure. A computational model is developed and validated against data in the literature. Inconel 625 is used as the base material because of its superior resistance against corrosion in the presence of s-CO2. The receiver heats s-CO2 with mass flow rate of 1 kg/s from 530 °C to 700 °C under a solar flux density of 500 kW/m2. The influence of different parameters on the performance of the receiver is evaluated by a parametric analysis. Subsequently, a multi-objective optimization is performed to determine the optimal geometry of the heat exchanger considering the tradeoff between objective functions, such as unit thermal resistance and pressure drop. The design variables are hydraulic diameter, number of layers, and distance between the channels. The mechanical strength of the system is the constraint to the problem, which is evaluated using an ASME code for the pressure vessels. Finally, the temperature profiles inside the channels and the surface of the receiver are presented. It is shown that the fluid reaches the desired temperature while the maximum temperature of the surface remains well below the material limit.

Integration between direct steam generation in linear solar collectors and supercritical carbon dioxide Brayton power cycles

Direct Steam Generation in Parabolic Troughs or Linear Fresnel solar collectors is a technology under development since beginning of nineties (1990's) for replacing thermal oils and molten salts as heat transfer fluids in concentrated solar power plants, avoiding environmental impacts. In parallel to the direct steam generation technology development, supercritical Carbon Dioxide Brayton power cycles are maturing as an alternative to traditional Rankine cycles for increasing net plant efficiency and reducing balance of plant equipments dimensions and cots. For gaining synergies between these two innovative technologies, in this paper, Direct Steam Generation and Brayton power cycles are integrated in line-focusing solar power plants. Four configurations are studied: Configuration 1 consists on installing a condenser between solar field and power cycle; condensing the heat transfer fluid (steam water) with the balance of plant working fluid (carbon dioxide). The condenser would be a shell & tubes type. Along tubes carbon dioxide flows, and steam water condensates at shell-side. Main advantage of the condenser equipment is the high heat transfer coefficient at water condensing-side, reducing condenser dimension and weight. The main disadvantage of this configuration is the high operating pressure required in solar field for condensing steam into liquid water. This pressure should be between 150 bar and 175 bar for obtaining 400 °C at turbine inlet. In the Configuration 2, the superheated steam delivered by solar collectors transfers the heat energy in a primary heat exchanger to the balance of plant working fluid. In this configuration the steam not condensate into liquid water, and only reduces the temperature from 550 °C–560 °C to 420 °C. The steam pressure drops in solar field along receivers, headers and heat exchangers are compensated by means of steam compressors. This second solution is compatible with higher turbine inlet temperatures, up to 550 °C. The keystones of this second configuration are the steam conditions at compressor inlet, pressure ∼175 bars and temperature ∼420 °C, for minimizing steam compressor electrical consumption. The third design solution (Configuration 3) includes a solar field with direct steam generation in solar collectors with boiling recirculation mode, but the balance of plant is integrated by two Brayton power cycles in cascade. The first power cycle operating at 550 °C turbine inlet, and the second cycle at 410 °C turbine inlet. Main advantage is the integration between a validated direct steam generation technology (recirculation boiling mode) with the Brayton power cycles avoiding steam compressors, a technology not yet commercially available, and main drawback of this design is the increasing number of balance of plant equipments. The Configuration 4 is very similar to the Configuration 2, with the same direct steam generation solar field with superheated steam without condensing, and a single reheating stage solar field with molten salt as heat transfer fluid.The Configuration 1 provides similar efficiency and net power output, for similar solar field effective aperture area, as obtained with molten salt solar collectors with supercritical carbon dioxide power cycle (recompression with main compression intercooling cycle provides 36.6% net efficiency, for a maximum 400 °C turbine inlet). The second design solution (Configuration 2) net efficiency is not very much impacted for steam compressor electrical consumption recompression cycle net efficiency is 43.6% with steam solar field, versus 45.16% with molten salt solar field, in both cases with 550 °C turbine inlet. The Configuration 3 performance is ∼39.7% with two cascade Brayton power cycles with recompression and main compression intercooling. Finally, the Configuration 4 optimum plant performance is obtained for the recompression cycle with a net efficiency ∼45.77%, and is constrained by the molten salt drawbacks (material corrosion, material cost, environmental impact, etc).