Net Cycle Research Articles

Investigation of the part load behavior for the R-Graz cycle with pure oxygen and hydrogen fuel

ABSTRACT In order to address the future demands for large-scale and efficient hydrogen storage and utilization, while considering the need of power dispatch and peak shaving, in this article, based on the cycle part load models and the conceptual design of the main components, the R-Graz cycle design parameters have been reoptimized, and subsequently, an analysis of the part load behavior and performance characteristics was conducted. The results show that within the load range discussed in this paper, i.e., 50% to 100%, the components of the R-Graz cycle are well matched, enabling high-efficiency operation. The net cycle efficiencies are 64.4% and 71.0% for 50% and 100% load respectively. Within this wide load range, the performance of the R-Graz cycle surpasses that of the Graz cycle consistently. When the load decreases, key parameters of the R-Graz cycle’s main components, such as rotational speed, pressure, temperature, mass flow rate and split ratio also decrease. Performance analysis under varying load conditions reveals that the maximum temperature in the cycle is a critical parameter affecting the efficiency of the R-Graz cycle. The research findings of this article lay the foundation for the practical application of the R-Graz cycle.

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.

Large deviations and fluctuation theorems for cycle currents defined in the loop-erased and spanning tree manners: A comparative study

The cycle current is a crucial quantity in stochastic thermodynamics. The absolute and net cycle currents of a Markovian system can be defined in the loop-erased (LE) or spanning tree (ST) manner. Here we make a comparative study between the large deviations and fluctuation theorems for LE and ST currents, i.e., cycle currents defined in the LE and ST manners. First, we derive the exact joint distribution and large deviation rate function for the LE currents of a system with a cyclic topology and also obtain the exact rate function for the ST currents of a general system. The relationship between the rate functions for LE and ST currents is clarified and the analytical results are applied to examine the fluctuations in the product rate of a three-step reversible enzyme reaction. Furthermore, we examine various types of fluctuation theorems satisfied by LE and ST currents and clarify their ranges of applicability. We show that both the absolute and net LE currents satisfy the strong form of all types of fluctuation theorems. In contrast, the absolute ST currents do not satisfy fluctuation theorems, while the net ST currents only satisfy the weak form of fluctuation theorems under the periodic boundary condition. Finally, the fluctuation theorems for cycle currents are applied to study the fluctuations in entropy production along a single stochastic trajectory.

Tight uncertainty relations for cycle currents.

Several recent inequalities bound the precision of a current, i.e., a counter of the net number of transitions in a system, by a thermodynamic measure of dissipation. However, while currents may be defined locally, dissipation is a global property. Inspired by the fact that, ever since Carnot, cycles are the unit elements of thermodynamic processes, we prove similar bounds tailored to cycle currents, counting net cycle completions, in terms of their conjugate affinities. We show that these inequalities are stricter than previous ones, even far from equilibrium, and that they allow us to tighten those on transition currents. We illustrate our results with a simple model and discuss some technical and conceptual issues related to shifting attention from transition to cycle observables.

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.

Performance Analysis of Heat Exchangers and Integrated Supercritical CO2 Brayton Cycle for Varying Heat Carrier, Cooling and Working Fluid Flow Rates

Supercritical CO2 power systems offer the potential of reduced system footprint and improved thermal efficiency, through the development and adoption of compact heat exchangers. Among these heat exchangers, the microtube, printed circuit, and plate heat exchangers are emerging as the most promising technologies for heat addition to the cycle, heat recuperation and heat rejection, respectively. To investigate the performance of supercritical CO2 recuperated Brayton cycle for heat to power conversion, simulation models of the heater, recuperator and cooler were developed using the distributed modeling approach and the ε-NTU method and then integrated with turbomachinery models to form the cycle model. The influences of flow rates of the heat carrier, cooling and working fluids on the heat exchanger performance and the integrated system were investigated. For the studied power system and under the off-design operating conditions, the net thermal efficiency of the cycle varies between 14.1% and 16.8%. Results show that increasing in the working fluid flow rate remains the net power output of the cycle but decreases the net cycle thermal efficiency, while increasing in the heat carrier fluid increases both, and the increase of cooling fluid increases the net power output but maintains the net thermal efficiency.

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.

Effects Of Nitrogen Concentrations On The Biomass, Lipid And Biodiesel Production Potentials Of Spirogyra Specie

The international dependence on non-renewable power supply including fossil fuels has led the sector toward power insecurity. This has necessitated the worldwide attempt to reduce the dependence on fossil fuels and broaden an economically feasible and scalable alternative fuel supply so that it will additionally lessen huge emissions of CO2 within side the surroundings which in flip will result in discount environmental pollution. Biofuels have the potentials to offer a feasible opportunity to fossil fuels and can lessen overall CO2 emissions. Algae appear like a rising supply of biomass for biodiesel that has the capacity to absolutely displace fossil fuel. Biodiesel has gain sufficient interest in latest years because of its green nature, non-poisonous characteristics, biodegradability and decrease net carbon cycle as compared to standard diesel fuel. In this research, Spirogyra sp. was collected from Tubu river, a tributary of River Kaduna, as a source of biomass feedstock for biodiesel production. Spirogyra sp. turned into cultured in various nitrogen (2.94 x 103 (100%), 2.20 x 103 (75%), 1.47 x 103 (50%), 0.73 x 104 (25%), and 0.000 (0%)) concentrations in Bold’s Basal Media (BBM) in a laboratory test for 21 days at 25-28°C ambient temperature. Oil from Spirogyra sp. become extracted in soxhlet extractor using n-hexane as solvent and transformed into biodiesel through transesterification reaction. The highest biomass growth, oil, lipid content and biodiesel production were observed in treatment with 75% (2.20 x 103) Spirogyra sp. has a big capability for biodiesel production.

Does Firms’ Working Capital Efficiency Matter in Dividend Payout: Empirical Evidence from India

The objective of this study is to assess the impact of working capital efficiency (WCE) (proxied by net trade cycle ( NTC)) on the dividend payout (proxied by dividend payout ratio ( DPR)) of the listed non-financial Indian firms. This study addresses the research gap in the literature about the influence of WCE on firms’ earnings distributed to equity shareholders as dividends in the Indian market. We have used secondary data of 150 firms that were listed on the Bombay Stock Exchange for the period 2012–2018 and analysed the data of the sample firms using the Tobit regression model. Our results indicate a negative and significant relationship between the NTC and the DPR of the sample firms. However, the beta coefficient of NTC was found to be very small, suggesting that the lower NTC may not inevitably generate a higher DPR of firms in the Indian context.

Estimating net ideal cycle time for body-in-white production lines

In the automotive industry, a Body in White (BIW) refers to the first step, the basic structure, in the production of a vehicle. Once a BIW production line has been built, the (maximum) capacity is fixed and throughput is therefore limited by the equipment specified during the design phase. The main metric used to inform the production line design is the Net Ideal Cycle Time (NICT). Unfortunately, the state of practice to estimate the NICT is a basic heuristic that does not account for production variation. In this paper, we challenge the current estimation approach by proposing an alternative that assumes actual production to follow a Weibull distribution. The proposed model is derived and estimated from empirical data. The results suggest that BIW production lines have traditionally been designed with too low a capacity, resulting in planned throughput rarely being achieved. On the other hand, increasing the design capacity implies a higher initial investment. In this paper it is demonstrated that the higher investment required is offset by reduced losses, resulting in more reliable planning and returns.

Thermodynamic performance analysis of state of the art gas turbine cycles with inter-stage turbine reheat and steam injection

Inter-stage turbine reheat is an effective gas turbine retrofit which can easily be used with simple and steam injected (SI) gas turbines as well. Although reheat provides higher inlet temperatures for HRSG in SI cycles and also increases net work output significantly, reheat combustor increases fuel consumption and thermal efficiency may still decrease. Therefore effects of reheat and steam injection in terms of thermodynamic performance require a detailed thermodynamic investigation. In this regard, simple, reheat, steam injected (STIG) and reheat steam injected (RHSTIG) gas turbine cycles are compared using the state of the art cycle parameters. Optimal performance parameters are determined using a new comprehensive cycle model which simulates combustion process regarding 14 exhaust species. It has been found that reheat provides a significant improvement on the cycle net work but it is not suitable for cycles having low pressure ratios if the only concern is maximum thermal efficiency. Results show that a good compromise between the maximum net work and maximum thermal efficiency is observed when reheat pressure is equal to the 0.4th power to the maximum cycle pressure. At this case, reheat provided 35.5% improvement in net cycle work with an efficiency penalty of only 5%.

Dual effects of long-chain alcohols in phase change capture of SO2: Increasing mutual solubility and decreasing product viscosity

To realize the goal of energy-saving SO2 capture process, a series of novel SO2 phase change absorbents (SPCAs) composed of 2-[2-(Dimethylamino)ethoxy]ethanol (DMEE)/long-chain alcohols/hexadecane were investigated in this work. The long-chain alcohols had dual effects in SPCAs, i.e., increasing mutual solubility of DMEE and hexadecane, and decreasing viscosity of the absorption products. Three alcohols, including butyl, hexyl and decyl alcohol, could turn the immiscible mixture of DMEE and hexadecane into homogeneous solution to form SPCAs. The decyl alcohol in absorption product could efficiently decrease the viscosity from 2338.1 mPa·s to 362.02 mPa·s, which is beneficial to gas-liquid mass transfer and fluid transport process. Meanwhile, the NMR and FTIR results indicated that DMEE chemically absorbed SO2, forming a zwitterionic alkyl sulfite compound. The long-chain alcohol formed hydrogen bonds with the alkyl sulfite and it didn't change the reaction mechanism. Noteworthy, the SPCA can absorb 0.823 mol SO2/mol DMEE (0.396 g SO2/g DMEE) at 293.15 K and 0.002 atm, which was exceptionally attractive. Furthermore, the absorbed SO2 could be partly released under vacuum at 110 °C and the net cycle capacity can reach 73% ~ 74% of the maximum capacity, which indicated the SPCAs have a promising application prospect.

Steady-state off-design modeling of the supercritical carbon dioxide recompression cycle for concentrating solar power applications with two-tank sensible-heat storage

Concentrating solar power researchers are evaluating the potential of the supercritical carbon dioxide recompression cycle to improve the thermal efficiency and decrease the capital costs of next-generation systems. This analysis investigates the steady-state off-design performance of a recompression cycle integrated with two-tank sensible-heat thermal energy storage as the ambient temperature and heat-transfer fluid (HTF) inlet conditions to the cycle change. This paper presents off-design component models and then cycle convergence and control models to maximize net cycle power output while constraining the high-side pressure and air-cooler fan power to their respective design values and fixing the cycle HTF outlet temperature to its design value. Results show that inventory control and air-cooler fan power are important control parameters that can be optimized to maximize off-design power output as the ambient temperature and HTF mass flow rate diverge from design. The high-side pressure and fan power constraints cause the cycle net power to degrade when the ambient temperature is warmer than the design value, and this study calculates a maximum mass flow rate for hot days above which the cycle cannot achieve the design HTF outlet temperature. Finally, the analysis shows that optimizing compressor shaft speeds can improve cold-day performance by around 1.5 percentage points and part-load performance by up to 2.5 percentage points versus a baseline case with no active compressor control.

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.

Reference Design and Simulation Framework of a Multi-Megawatt Airborne Wind Energy System

In this paper, we present the design and computational model of a representative multi-megawatt airborne wind energy (AWE) system, together with a simulation framework that accounts for the flight dynamics of the fixed-wing aircraft and the sagging of the tether, combining this with flight control and optimisation strategies to derive the power curve of the system. The computational model is based on a point mass approximation of the aircraft, a discretisation of the tether by five elastic segments and a rotational degree of freedom of the winch. The aircraft has a wing surface area of 150 m2 and is operated in pumping cycles, alternating between crosswind flight manoeuvres during reel out of the tether, and rapid decent towards the ground station during reel in. To maximise the net cycle power, we keep the design parameters of the aircraft constant, while tuning the operational and controller parameters for different wind speeds and given contraints. We find that the presented design can generate a net cycle power of up to 3.8 megawatts.

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.

Do Firms Approach the Target Working Capital Requirement? A Case of Pakistan

Abstract The study investigates the dynamism of working capital requirement (WCR) in non-financial firms listed on the Pakistan Stock Exchange from the period of 2007 to 2013. The purpose of this research is to analyze whether firms follow the target WCR, to estimate the speed with which firms adjust towards its target WCR and to investigate the firm-specific and macroeconomic determinants of WCR. Difference GMM technique is used to analyze the speed and determinants of WCR to avoid the problems of endogeneity and unobservable heterogeneity. The study gives evidence that there is an existence of target WCR in firms of Pakistan and firm require 1.6 years to completely adjust back to target WCR. The factors which are statistically significant in the determination of WCR are the level of economic activity in the country, operating cash flow, profitability, leverage, financial distress, and financing cost. The WCR is measured by net trade cycle of a firm.

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.