Net Cycle Efficiency Research Articles

Direct coupling of pressurized gas receiver to a brayton supercritical CO2 power cycle in solar thermal power plants

Three layouts of Brayton supercritical CO2 power cycles directly coupled to the receiver are proposed for Generation 3 solar power plants: conventional recompression, recompression with partial cooling, and recompression with intercooling. To achieve direct coupling, the solar heat is introduced downstream of the turbine, where CO2 pressure is lower. A higher temperature rise diminishes the receiver's dimensions, thus increasing its energy efficiency. It also lowers the average working temperature since the maximum temperature is fixed at 700 °C, thereby reducing losses. However, optical efficiency decreases as the receiver size diminishes. Both intercooling and partial cooling layouts further increase the cycle's net efficiency, which reduces the receiver's size, following similar trends observed with an increase in temperature rise. Considering all these effects, various factors push in opposite directions, affecting overall efficiency and costs. This competitive interplay results in overall efficiencies ranging from 30.26 % to 31.58 % and Levelized Costs of Electricity (LCOEs) between 162.47 €/MWh and 166.81 €/MWh. In conclusion, similar outcomes in terms of energy and economics are achieved with the three layouts, suggesting the simplest layout (recompression) as the most advisable. If thermal storage is incorporated, partial cooling becomes preferable due to its significant increase in the receiver's temperature rise.

Swirl Enhancement Effect on Turbine Rim Seal Performance

Abstract In gas turbines, sealing flow is extracted from the high-pressure compressor and supplied into the turbine wheel-space to suppress hot gas ingestion. Such hot gas ingestion is thought to be driven by the difference in either the static pressure or swirl between the mainstream and wheel-space flows. For the first time, experiments have been conducted in a low-speed single-stage axial turbine with a single-radial clearance rim seal and a “swirler” on the rotor disk. The objective is to evaluate the impact of enhanced wheel-space flow swirl on the rim seal performance. The swirler does not affect the mainstream flow. In the wheel-space, however, the swirler reduces static pressure; increases swirl; and improves rim seal performance (i.e., reduces the minimum sealing flowrate needed for hot gas ingestion prevention). Thus, the seal performance improvement occurs with increased difference in pressure and reduced difference in swirl between the mainstream and wheel-space flows. Therefore, it can be inferred that the rim seal performance depends more strongly on swirl than pressure. Preliminary gas turbine cycle performance studies indicate that net cycle efficiency benefits can be obtained.

Prospects for the Use of Hybrid Thermal Power Plants with Applying Solar Technologies in the People’s Republic of Bangladesh Energy Sector

There is an inextricable relationship and interdependence between the conditions of meeting energy consumption demands and environmental pollution. The interaction of these two factors and the development of production forces are gradually drawing attention to the problem of interaction between the thermal power engineering and environment. At the thermal power engineering early development stage, the main manifestation of this attention was the search in the environment for resources necessary to meet the energy consumption demands and stable energy supply to enterprises and residential buildings. In the subsequent, the problem boundaries were extended to the possibilities of a more complete use of natural resources through the search and improvement of processes and technology, extraction and enrichment, processing and combustion of fuel, as well as improvement of thermal power facilities. One of the ways to further develop thermal power plants and reduce their environmental impact is hybridization of solar technologies with conventional power plants. To overcome the limitations of simple cycle power plants, a method for hybridizing conventional power plants with solar technologies and its influence on performance are considered. The solar technologies used in the global energy sector are briefly overviewed. The efficiency of using hybrid power plants is studied with reference to the People's Republic of Bangladesh, which is one of the world’s sunniest and actively developing regions. The state of the republic’s energy sector, climatic features, and the possibility of using solar technologies for local conditions are analyzed. A solar thermal power plant model with a parabolic collector is considered; the efficiency of such power plants is analyzed, and technologies for using a combined-cycle power plant (CCPP) jointly with a solar steam generator for electricity generation are overviewed. The schemes of combined-cycle power plants with a solar steam generator were numerically analyzed. The analysis results have shown that during the summer state peak, the use of the integrated power unit makes it possible to additionally obtain approximately 6.5% of the unit total power output in comparison with a similar classical CCPP, but without using the field of solar collectors and a solar steam generator. The use of one modern GE 9371FB gas turbine unit produced by General Electric makes it possible to increase the net efficiency by 3.9% and bring the combined cycle net efficiency up to 59%. The improvements in the power unit are carried out with the minimal impact on the combined cycle operation due to a limited contribution from the solar energy based steam generation system. The integrated system makes it possible to cut the capital costs for the power plant construction. The combination of a CCPP with a solar installation will open the possibility to construct a highly maneuverable power unit with high efficiency indicators.

High-Efficiency Power Cycles for Particle-Based Concentrating Solar Power Plants: Thermodynamic Optimization and Critical Comparison

This paper investigates and compares several highly efficient thermodynamic cycles that are suitable for coupling with particle-in-tube fluidized-bed solar receiver technology. In such a receiver, high-temperature particles are used as both a heat transfer fluid and a storage medium. A dense particle suspension (DPS) is created through an upward bubbling fluidized-bed (UBFB) flow inside the receiver tubes, which constitutes the “particle-in-tube” solar receiver concept. Reaching higher temperatures is seen as a key factor for future cost reductions in the solar plant, as this leads to both higher power conversion efficiency and increased energy storage density. Three advanced thermodynamic cycles are analyzed in this work: the supercritical steam Rankine cycle (s-steam), supercritical carbon dioxide cycle (s-CO2) and integrated solar combined cycle (ISCC). For each one, 100% solar contribution, which is considered the total thermal input to the power cycle, can be satisfied by the solar particle receiver. The main findings show that the s-CO2 cycle is the most suitable thermodynamic cycle for the DPS solar plant, exhibiting a net cycle efficiency above 50% for a moderate temperature range (680–730 °C). For the other advanced power cycles, 45.35% net efficiency can be achieved for the s-steam case, while the efficiency of the ISCC configuration is limited to 45.23% for the solar-only operation mode.

Dynamic characteristics and control strategies of the supercritical CO2 Brayton cycle tailored for the new generation concentrating solar power

The supercritical CO2 (sCO2) Brayton cycle has the advantages of high efficiency, good flexibility and compact equipment, and is widely regarded as the ideal power cycle for the new generation concentrating solar power (CSP). The application scenario of the CSP determines that the unit's fast peak shaving capability must be considered. In this paper, a dynamic simulation model was developed for an indirect-heated 800 °C/550 °C sCO2 CSP test plant, containing a particle heat storage system. The effects of control modes of heating power, cooling power, mass flow rate (m), turbine rotational speed and sCO2 inventory on the system dynamic characteristics were researched. It was found that the control mode of changing particle/water mass flow rate had a significantly faster response speed than changing temperature. Using the turbine optimal rotational speed control mode could improve the net cycle efficiency from 15.30% to 16.12%, in a continuous linear load reduction process. The sCO2 inventory control module played a crucial role in limiting the fluctuation of compressor inlet pressure. The regulation mode of changing m and turbine inlet temperature (T4) synchronously and that of changing m with constant T4 were all proved to achieve the goal of fast peak shaving well, but the latter one was safer, which could greatly limit the temperature change rate of sCO2 and equipment in the fast peak shaving process. Our work not only offers an operation solution for the sCO2 CSP test plant, but also gives guidance for the efficient, flexible and safe design of the third generation CSP plants.

Parametric optimization of the semi-closed oxy-fuel combustion combined cycle

The report discloses thermodynamic studies of the semi-closed oxy-fuel combustion combined cycle. The computer simulation and parametric optimization approaches are described in details. The oxy-fuel cycle net efficiency in relationship to the carbon dioxide turbine exhaust pressure and the steam turbine inlet pressure is shown. The power production efficiency reduction is related to the turbine cooling losses is described. It is shown that the semi-closed oxy-fuel combustion cycle maximal net efficiency of 52.5% occurs at the initial temperature and pressure 1400°C and 60 bar at the carbon dioxide turbine exhaust pressure 0.5 bar and the steam turbine inlet pressure 90 bar. The cooling losses consideration leads to the net efficiency of 47.76% that is reached at the carbon dioxide turbine exhaust pressure 1 bar and the steam turbine inlet pressure 90 bar.

Thermodynamic analysis and optimization of Allam cycle with a reheating configuration

The Allam cycle is one of the most promising oxy-combustion systems and has attracted much attention from the energy industry. A novel layout of the Allam cycle, which combines a reheating configuration with the original Allam cycle, was proposed in the present study. The effects of key variables on the net cycle efficiency and net specific work of the new system were investigated. The Allam cycle with reheat was optimized to maximize the cycle efficiency and compared with other cycles. The results showed that the minimum cycle temperature and the outlet temperature of the second combustor have significant effects on the cycle performance, while the maximum cycle pressure has minimum effect on the cycle performance. The highest exergy destruction of the optimized cycle is contributed from the two combustors, which accounts for 25.42% of the total input exergy. The net efficiency of the optimized Allam cycle with reheat is up to 49.32%, a little higher than those of other oxy-combustion cycles and about 5 percentage points lower than that of the original Allam cycle. However, the net power output and net specific work are up to 904.6 MWe and 606.2 kJ/kg for this novel system, about 2.1 times and 2.2 times of those of the original Allam cycle, respectively.

Thermohydrodynamic analysis of the vertical gas wall and reheat gas wall in a 300 MW supercritical CO2 boiler

Supercritical CO2 (sCO2) Brayton cycle usually shows high efficiency and also has compact structure, which can be considered as a promising alternative in future coal-fired power plants. However, sCO2 boiler has different structures and configurations compared with the conventional steam boiler, especially for the arrangement of radiative heat transfer surfaces. Proper design of vertical gas wall and reheat gas wall is crucial to the success design of sCO2 boiler. In this study, a thermohydrodynamic model of radiative heat transfer surfaces was developed to predict pressure drops and wall temperature distributions. A sCO2 power cycle calculation model was employed to comprehensively analyze the influences of pressure drops on the thermal efficiency. A 300 MW sCO2 boiler was taken into account as the baseline case and thermohydrodynamic analyses of gas wall and reheat gas wall were carried out. The influences of flow directions and mass fluxes on wall temperature, pressure drops and thermal efficiency were obtained by the present models. Then, optimized structures of radiative heat transfer surfaces were proposed, i.e., vertical downward gas wall with tubes of φ34 × 4 mm and vertical upward reheat gas wall with tubes of φ57 × 6 mm. Accordingly, the net cycle efficiency achieved an optimized value of 50.07% under the conditions that the wall temperatures of radiative heat transfer surfaces did not exceed the allowable temperature.

Investigation on the use of a novel regenerative flow turbine in a micro-scale Organic Rankine Cycle unit

Reliable and low-cost expanders are fundamental for the competitiveness of small-scale Organic Rankine Cycle (ORC) plants using low-temperature heat sources. Regenerative flow turbines (RFTs) can be considered a low-cost and viable alternative expander, yet their performance needs to be fully investigated. Therefore, the use of an RFT in a micro-scale ORC test bench is investigated in this work through a modelling study. Specifically, three-dimensional CFD simulations are carried out to assess the performance of the considered expander with varying operating conditions and a numerical model of a non-regenerative, small-scale ORC system is developed to investigate its potential in waste heat recovery (WHR) applications.Using R245fa as the working fluid, the CFD analysis shows that the expander achieves a maximum total-to-static isentropic efficiency of about 44% in the investigated operating range. The small-scale ORC system has a net output power in the range 100–600 W and a net cycle efficiency of 1–2.3%. Moreover, a comparison with two scroll expanders having different built-in volume ratios shows that the RFT operates with higher isentropic efficiencies in low mass flow rates and pressure ratios thus highlighting its suitability for low-temperature WHR applications, especially when considerable fluctuations of the heat source are expected.

Parametric optimization of the thermodynamic cycle design for supercritical steam power plants

The paper presents an original methodology and model for the optimal design of power plants with supra-critical steam turbine cycle. By applying a thermodynamic analysis, the method finds the best configuration of the preheating system, the steam extractions from the turbine and the reheating steam pressure, for different main steam parameters and different high-pressure turbine configurations. A multistep optimization is made in the paper for a fixed fuel input. First, a parametric optimization of a given scheme, without restrictions was performed. Secondly, a technical parametric optimization with restrictions and the definition of the scheme is realized. Finally, a multi-objective optimization with the consideration of two criteria is made: a technical criterion (the thermal cycle net efficiency) and an economical one (the specific investment in the power plant’s equipment). The multi-objective optimum values for each studied case are obtained. The results from the Pareto frontier show that the technical optimum tends to the highest values of reheat pressure, while the economic one tends to the lowest values of reheat pressure. The results are in line with appropriate data from the literature.

HCFO-1224yd(Z) as HFC-245fa drop-in alternative in low temperature ORC systems: Experimental analysis in a waste heat recovery real facility

The organic Rankine cycle (ORC) has been growing in importance as a technology for producing electricity from low temperature waste and renewable heat sources. In small-scale applications, the most used working fluid has been HFC-245fa, although this is being substituted for low GWP alternatives. A new HCFO working fluid, HCFO-1224yd(Z), has appeared as a possible alternative for HFC-245fa. In this study, this alternative working fluid is analysed and, finally, the working fluid is experimentally tested as drop-in alternative in a commercial ORC system. The main purpose is to determine the feasibility of using HCFO-1224yd(Z) as a drop-in replacement for HFC-245fa in a real facility that was initially designed to operate with HFC-245fa. The results show how the use of HCFO-1224yd(Z) offers power output that ranges from 7.5% to 17.4% lower than that provided by HFC-245fa. Although HFC-245fa offers higher power output, the results show that HCFO-1224yd(Z) offers up to 7.7% higher cycle net efficiency; this is due to the higher input thermal rate required by HFC-245fa. Finally, HCFO-1224yd(Z) has been stated as a suitable alternative to HFC-245fa as a drop-in replacement in a small-scale, low-temperature ORC, which increases its attractiveness for heat source with higher temperatures.

Thermodynamic optimization and equipment development for a high efficient fossil fuel power plant with zero emissions

Abstract The transition to oxy-fuel combustion cycles is a perspective way to decrease harmful emissions into the atmosphere in the energy sector. The oxy-fuel, supercritical carbon dioxide Allam cycle (NET Power oxy-combustion cycle) is a promising technology with high efficiency. Emissions of carbon dioxide into the atmosphere could be decreased by 98.9% using this cycle for electricity production, which is 8–17% higher compare to gas turbine combined cycle with post-combustion carbon capture. However, there are several challenges on the way of its implementation, one of which is the development of a supercritical carbon dioxide gas turbine. The aim of the present study was to develop a construction of a high-power supercritical CO2 gas turbine with optimal thermodynamic parameters of the Allam cycle and to evaluate technical, environmental and economic characteristics of zero emission technology for the electricity production. To reach the goal, the calculation algorithm allowing to model the oxy-fuel combustion cycles was developed. The results of thermodynamic optimization showed that the highest value of cycle net efficiency equal to 56.5% is achieved for the turbine inlet temperature and pressure of 1083 °C and 30 MPa, the turbine outlet pressure of 3 MPa and the coolant temperature of 200 °C. The formed equipment requirements were used to create the design of a 335 MW supercritical carbon dioxide gas turbine, the main features of which were described in detail. The results of environmental characteristics analysis showed, that the specific amount of CO2 emitted from the cycle to the atmosphere is 0.0038 kg/kWh. The results of economic feasibility evaluation for the Allam cycle showed that the total specific investment costs including the price for carbon storage are equal to $1307.5/kW, which is quite competitive value.

New combined supercritical carbon dioxide cycles for coal-fired power plants

Supercritical CO2 (S-CO2) power cycle has been proven to be an efficient way to improve the efficiency of the coal-fired power plants. In this paper, new combined S-CO2 power cycles for coal-fired power plants were proposed to improve the performances of S-CO2 power cycle. A numerical model of S-CO2 cycle for coal-fired power plants was implemented. The effects of the key operating parameters on the performance of the new combined S-CO2 power cycles are numerically investigated and exergy destruction in each component is analysed. As a result, it can be found that the net cycle efficiency and specific power are improved by the new combined cycles, meanwhile the flue gas temperature is reduced. In contrast to the reference cycle, the net cycle efficiency and specific power of the combined Brayton cycle can be increased by 1.6% and 14.9 kJ/kg, respectively. Furthermore, the cycle exergy efficiency can be improved by 2.34%. As a result, a new insight is introduced for the development of S-CO2 Brayton cycles for coal-fired power plants.

An initial assessment of the value of Allam Cycle power plants with liquid oxygen storage in future GB electricity system

The Allam Cycle is a novel oxy-combustion gas turbine power cycle with a reported net cycle efficiency of 58–60% LHV and near-zero operating emissions. An Allam Cycle process model is developed displaying a net cycle thermal efficiency (LHV) of 58.0%, a higher value than previously reported in the literature, due to the inclusion of a bypass stream heat source. Novel modes of operation are added to improve plant operational flexibility, including a temporary increase in cycle efficiency to 66.1%, with the use of liquid oxygen storage to shift the energy penalty of oxygen production. This facilitates decoupling oxygen and electricity production and operates as a form of energy storage. For the first time, a purpose-built Unit Commitment and Economic Dispatch (UCED) model is used to investigate the impact of Allam Cycle plants and of liquid oxygen storage on system costs and grid CO2 intensities, taking the illustrative case of the GB electricity system. Over a representative winter week with high net demand, a fleet of 5–15 Allam Cycle plants operates with a capacity factor of, respectively 97%-90%, reducing system costs by 2.6%–6.7% and reducing electricity grid average CO2 intensity by 7.9%–19.0%. Adding oxygen storage to these plants allows surplus renewable energy generation to be stored, thus avoiding wind curtailment. Our initial findings indicate that oxygen storage can be valuable to both to plant operators and the system operators, but also that further work is required to evaluate non-energy revenue streams from the ancillary service market to determine whether the capital expenditure of liquid oxygen storage could be justified without financial incentives.

A comparative performance analysis of a solid oxide fuel cell and gas turbine combined cycles with carbon capture technologies

This study presents a performance prediction of triple combined cycles that use a solid oxide fuel cell (SOFC) and a gas turbine combined cycle (GTCC) with carbon capture technologies. Post- and oxy-combustion capture technologies were comparatively analyzed. The component design parameters of a commercial F-class gas turbine and SOFC were used. Minimizing the turbine inlet temperature (i.e., no extra fuel supplied to the combustor) resulted in higher net cycle efficiency. With post-combustion capture, the net cycle efficiency reached approximately 70 % when no fuel was supplied to the combustor, but the maximum CO2 capture rate was limited to 80 %. When a dual combined cycle was adopted, the CO2 capture rate increased to 91 %, while the net efficiency was approximately 69 %. With oxy-combustion capture, the optimum pressure ratio was higher than in the normal triple combined cycle, and the net cycle efficiency was lower than that of the post-combustion cycle. However, there was a critical advantage of a larger power output with nearly complete carbon capture. The impact of the location of the oxygen supply was examined in the oxy-combustion cycle with extra fuel supplied to the combustor, and supplying all the fuel to the SOFC improved the cycle performance.

Performance Enhancement of a Molten Carbonate Fuel Cell/Micro Gas Turbine Hybrid System With Carbon Capture by Off-Gas Recirculation

The demand for clean energy continues to increase as the human society becomes more aware of environmental challenges such as global warming. Various power systems based on high-temperature fuel cells have been proposed, especially hybrid systems combining a fuel cell with a gas turbine (GT), and research on carbon capture and storage (CCS) technology to prevent the emission of greenhouse gases is already underway. This study suggests a new method to innovatively enhance the efficiency of a molten carbonate fuel cell (MCFC)/micro GT hybrid system including carbon capture. The key technology adopted to improve the net cycle efficiency is off-gas recirculation. The hybrid system incorporating oxy-combustion capture was devised, and its performance was compared with that of a post-combustion system based on a hybrid system. A MCFC system based on a commercial unit was modeled. Externally supplied water for reforming was not needed as a result of the presence of the water vapor in the recirculated anode off-gas. The analyses confirmed that the thermal efficiencies of all the systems (MCFC stand-alone, hybrid, hybrid with oxy-combustion capture, hybrid with post-combustion capture) were significantly improved by introducing the off-gas recirculation. In particular, the largest efficiency improvement was observed for the oxy-combustion hybrid system. Its efficiency is over 57% and is even higher than that of the post-combustion hybrid system.

A novel cascade energy utilization to improve efficiency of double reheat cycle

Large mean terminal temperature difference of conventional regenerative air heater and high superheat degree of bleeding steams are the bottleneck of conventional steam rankine cycle, which limits the cycle for further efficiency improvement. In this study, a state-of-the-art double reheat power plant is selected as the reference cycle. A novel cycle covering steam, water and air process is invented based on the systematic combination of flue gas heat recovery and bleeding steam cascade energy utilization.Thermodynamic model is developed to analyze the performance improvement and exergy method is applied to validate and identify the cycle thermodynamic efficiency advantage. Simulation result presents novel cycle net heat rate is improved by 104.8 kJ/kWh and cycle net efficiency is increased by 0.61%. Feed water pump turbine inlet steam connection optimization accounts for 14 kJ/kWh net heat rate benefits for the novel cycle. Feasibility study proves it is possible to implement the novel cycle in a newly planned power plant. Based on techno-economic analysis, it is found that the total investment capital of novel cycle is 7.44 million US dollars and net annual revenue is 0.59 million US dollars compared with the reference cycle. Static capital investment paid back period is expected within 15 years in a newly planned power plant.

Evaluation of Bleed Flow Precooling Influence on the Efficiency of the E-MATIANT Cycle

Abstract This study aims to present a method for precooling bleed flow by water injection in the E-MATIANT cycle and to estimate its impact on the overall efficiency. The design parameters of the cycle are set up on the basis of the component technologies of today's state-of-the-art gas turbines with a turbine inlet temperature between 1100 and 1700°C. Several schemes of the E-MATIANT cycle are considered: with one, two and three combustion chambers. The optimal pressure ratio ranges for the considered turbine inlet temperatures are identified and a comparison with existing evaluations is made. For the optimal initial parameters, cycle net efficiency varies from 42.0 to 49.8%. A significant influence of turbine stage cooling model on optimal thermodynamic parameters and cycle efficiency is established. The maximum cycle efficiency is 44.0% considering cooling losses. The performance penalty due to the oxygen production and carbon dioxide capture is 20–22%.

Adapting the zero-emission Graz Cycle for hydrogen combustion and investigation of its part load behavior

A modern energy system based on renewable energy like wind and solar power inevitably needs a storage system to provide energy on demand. Hydrogen is a promising candidate for this task. For the re-conversion of the valuable fuel hydrogen to electricity a power plant of highest efficiency is needed.In this work the Graz Cycle, a zero-emission power plant based on the oxy-fuel technology, is proposed for this role. The Graz Cycle originally burns fossil fuels with pure oxygen and offers efficiencies up to 65% due to the recompression of about half of the working fluid. The Graz Cycle is now adapted for hydrogen combustion with pure oxygen so that a working fluid of nearly pure steam is available. The changes in the thermodynamic layout are presented and discussed. The results show that the cycle is able to reach a net cycle efficiency based on LHV of 68.43% if the oxygen is supplied “freely” from hydrogen generation by electrolysis.An additional parameter study shows the potential of the cycle for further improvements. The high efficiency of the Graz Cycle is also achieved by a close interaction of the components which makes part load operation more difficult. So in the second part of the paper strategies for part load operation are presented and investigated. The thermodynamic analysis predicts part load down to 30% of the base load at remarkably high efficiencies.

Influence of selected cycle components parameters on the supercritical CO2 power unit performance

The paper presents the influence of selected components parameters on the performance of supercritical carbon dioxide power unit. For this analysis mathematical model of supercritical recompression Brayton cycle was created. The analysis took into consideration changes in the net cycle power and efficiency for different compressor inlet temperatures. The results were obtained for a fixed minimum pressure of 7.4 MPa and fixed recompression split ratio. The studies conducted in this paper included also consideration of sensitivity of the cycle efficiency to a change in recuperators heat transfer area. In order to determine how each recuperator influences the cycle performance, an analysis of efficiency dependence on the recuperators area was made. Another parameters that were investigated are to a change in turbine and compressors isentropic efficiency and their influence on the cycle efficiency. In the reference cycle, isentropic efficiencies were set up as 88% for both the main and recompression compressor, and 90% for the turbine. Since isentropic efficiency is a sort of measure of broadly defined quality of a turbine or compressor, including airfoil shape, sealing, etc., it may be a significant cost factor that should be considered during cycle design. Therefore, a sensitivity analysis of cycle efficiency to both compressors and turbine isentropic efficiencies was conducted.