Variable Pressure Ratio Research Articles

Thermodynamic of a novel advanced adiabatic compressed air energy storage system with variable pressure ratio coupled organic rankine cycle

In order to increase the cycle efficiency of compressed air energy storage, a novel advanced adiabatic compressed air energy storage system with variable pressure ratio based on organic Rankine cycle is presented. The thermodynamic model of the system is established and used to calculate the thermodynamic characteristics of system vs the number of expanders and their linked mode. Then, the system based on organic Rankine cycle subsystem was analysed. The results show that the system with variable pressure ratio reduces the compression process power consumption by 12.45% and increases the expander output power by 37.29% comparing with the advanced adiabatic compressed air energy storage, which make the cycle efficiency of the system increase from 40.16% up to 63.00%. Furtherly, the cycle efficiency of the system coupled organic Rankine cycle reached 70.53%.

Parametric Output of Penetration Length in De-Laval Nozzle using Computational Fluid Dynamics

A Contour shaped rocket nozzle, commonly known as Bell nozzle, is a more efficient form of a De-Laval nozzle useful for operation at different altitudes resulting in variable pressure ratio across the nozzle. Here the penetration length during the flow is calculated in a De-Laval nozzle using Computational fluid dynamics (CFD). The TOP nozzle] by G.V.R. Rao is modeled and simulated using CFD analysis by varying the divergent half angle of the bell nozzle. The geometry of different divergent angle of the bell nozzle is modeled using MATLAB code. The modeled geometry and mesh of the nozzle is simulated in ANSYS-FLUENT software. The simulation shows the variation in different flow parameter at different divergent half angles. The analysis so far for the same is done in a conical shaped rocket nozzle at the cost of varying the divergent length for changing the divergent half angle of the nozzle. This project simulates bell nozzle with constant divergent length. The simulation model is verified and validated using experimental and computational data at an NPR (Nozzle pressure ratio) of 1000. As the Rao Nozzle is used currently in rocket, missile, and satellite control systems worldwide. This research work to show how the penetration length effect the Mass-Weighted average Mach number in a particular designed bell nozzle with different divergent half angle. Based on the results obtained, the discussion is done about the parameters and the conclusion is given.

Design and off-design analysis of a Tesla Turbine utilizing CO2 as working fluid

The Tesla turbine is a bladeless expander; which principle of operation is based on the conversion of the viscous forces, developed by the flow while expanding through the rotor, in mechanical energy. It is especially suitable for small/micro size distributed energy systems (kW scale), mainly due to its very low cost, which results from the simple structure of the machine. The Tesla turbine works well at relatively moderate expansion ratios. Therefore, it is fit for CO2 power cycles applications that are characterised by small expansion ratio, despite the high pressure involved. In this work, the design and off-design analysis of a Tesla turbine for small/micro power application utilizing CO2 cycles is proposed. The optimized design was targeted for an inlet temperature of 150 °C and an inlet pressure of 220 bar. The final optimized geometry of the expander was defined, achieving a 23.4 W per channel power output with a 63% isentropic efficiency, when working with a 10.1 bar pressure drop at 2000 rpm. Furthermore, the turbine placement on the Baljè diagram was performed in order to understand the direct competitors of this machine. Finally, starting from the design configuration, the maps of efficiency at variable load and flow coefficients and that of reduced mass flowrate at variable pressure ratio were realized. Through the merging of these curves, the off-design maps of the Tesla turbine were obtained, highlighting a very limited sensitivity of the efficiency to variable working conditions, if rotational speed is adequately adjusted.

A compressed air energy storage system with variable pressure ratio and its operation control

The compressed air energy storage (CAES) system generally adopts compressors and turbines to operate under a constant pressure ratio. The system working parameters cannot adapt to load change, which causes the system efficiency to be limited. In order to improve CAES system efficiency, a novel variable pressure ratio CAES system is proposed to change the operation pressure ratio by controlling the opening or closing of valves between compressors and the opening or closing of valves between turbines. The thermodynamic analysis model of variable pressure ratio CAES system is established. The period of energy storage, the power consumption of compressor, the output work of turbine and the charging and discharging efficiency of system are calculated and analyzed for the 500 kW advanced adiabatic CAES system. The results show that compared with the corresponding constant pressure ratio CAES system, in the variable pressure ratio CAES system, the power consumption of compressor is reduced by 12%, the output work of turbine is increased by 17.9% and the charging and discharging efficiency of system is increased by 13.1%. Therefore, variable pressure ratio CAES system can not only significantly improve its energy storage efficiency, but also be easy to realize.

Transient Energy and Exergy Analyses of a Multistage Hydrogen Compression and Storage System

AbstractA new, practical, and efficient hydrogen compression and storage system is developed and analyzed thermodynamically through transient energy and exergy approaches. The first model of the multistage hydrogen compression system consists of multiple compressors with variable pressure ratios; all operating at the same variable pressure ratio. The second model has two fixed filling times, as a result of its operational mode. The second model integrates constant pressure ratio compressors with a controller that runs the required number of compressors based on the pressure in the vessel. The overall system and its components are simulated in Aspen Plus and Engineering Equation Solver, with which their performances are evaluated based on energy and exergy analyses.

Unsteady cavitation characteristics of liquid nitrogen flows through venturi tube

The dynamic cavitation characteristics of liquid nitrogen flow through a transparent venturi tube are experimentally investigated in a variable pressure ratio tunnel. The common results are presented at different pressure ratio, including the temporal and spatial changes of the temperature and pressure as well as the flow rate, together with the visual observation of the cavitation unsteady behaviors using the high-speed camera. The analyses on the measured dynamic pressure data and images reveal that the shedding frequency and length of cavity linearly increases, while the pressure amplitude exponentially increases, as the pressure ratio increases. Specially, a shock wave induced condensation front is observed propagating upstream within the attached cavity when the pressure ratio approximately exceeds 2.23. The wave generates from the collapse of the cavitation cloud and is considered to be the predominate mechanism of the cavitation shedding. A one dimensional theoretical equation in consideration of thermal effect is developed to estimate the speed of condensation front. And, the Strouhal number based on the condensation wave speed is found to be close to 0.5 for both water and liquid nitrogen cavitation, which can be utilized to estimate shedding frequencies for both cryogenic and room-temperature fluids. The initial cavitation development and characteristics are also investigated. The data from the present study can be used as a bench mark for CFD code validations.

Organic Fluids in a Supercritical Rankine Cycle for Low Temperature Power Generation

This paper presents a performance analysis of a supercritical organic Rankine cycle (SORC) with various working fluids with thermal energy provided from a geothermal energy source. In the present study, a number of pure fluids (R23, R32, R125, R143a, R134a, R218, and R170) are analyzed to identify the most suitable fluids for different operating conditions. The source temperature is varied between 125 °C and 200 °C, to study its effect on the efficiency of the cycle for fixed and variable pressure ratios. The energy and exergy efficiencies for each working fluid are obtained and the optimum fluid is selected. It is found that thermal efficiencies as high as 21% can be obtained with 200 °C source temperature and 10 °C cooling water temperature considered in this study. For medium source temperatures (125–150 °C), thermal efficiencies higher than 12% are obtained.

Thermodynamic Analysis of Integrated Molten Carbon Fuel Cell–Gas Turbine Cycles for Sub-MW and Multi-MW Scale Power Generation

This paper investigates the thermodynamic potential of the integration of molten carbon fuel cell (MCFC) technology with gas turbine systems for small-scale (sub-megawatt or sub-MW) as well as large-scale (multi-MW) hybrid cycles. Following the proposals of two important MCFC manufacturers, two plant layouts are discussed, the first based on a pressurized, externally reformed MCFC and a recuperated gas turbine cycle and the second based on an atmospheric MCFC, with internal reforming integrated within an externally fired gas turbine cycle. Different levels of components quality are considered, with an analysis of the effects of variable pressure ratios, different fuel mixture compositions (variable steam-to-carbon ratio) and turbine inlet temperature levels, together with potential advantages brought about by an intercooled compression process. The analysis shows interesting effects due to the peculiarity of the mutual interactions between gas turbine cycle and fuel cells, evidencing the importance of a careful thermodynamic optimization of such cycles. Results show the possibility to achieve a net electrical efficiency of about 57–58% for a small plant size (with a difference of 1.5–2 percentage points between the two layouts), with the potential to reach a 65% net electrical efficiency when integrated in advanced cycles featuring high-efficiency, large-scale equipment (multi-MW scale cycles).

Development of a Three-Dimensional Molten Carbonate Fuel Cell Model and Application to Hybrid Cycle Simulations

In this paper, a three-dimensional finite volume model of a molten carbonate fuel cell (MCFC) is presented. The model applies a detailed electrochemical and thermal analysis to a planar MCFC stack of given geometry and assigned input flows conditions, material properties and assigned external heat losses, calculating global energy balances and reactant utilization, as well as internal temperature and chemical composition profiles, pressure profiles, polarization losses, and current density and voltage distribution over the stack. The model is calibrated on the available data for an experimented MCFC stack manufactured by Ansaldo Fuel Cells. The comparison of 2D versus 3D simulations is first discussed, showing the importance of the 3D model for an accurate analysis of real stack operation under significant heat loss conditions, but also showing the mutual influence of heat transfer between adjoining cells and evidencing the variable electrical performances between different cells of the same stack. As a counterpart, application of the 2D model, which takes advantage by much shorter calculation time while keeping a reasonable accuracy in the overall energy balances, is anyway indicated for complex cycle analysis and cycle optimization, where a complete 3D stack analysis may be limited to selected cases. Then, the model is applied to the off-design analysis of hybrid cycles based on a recuperated gas turbine arrangement where variable pressure ratios and fuel supply conditions are assumed for the fuel cell. The effects of the operating pressure and of the adoption of different fuel compositions are considered, with a cycle layout that may include natural gas reforming or is directly fed by a syngas provided by an external source, simulating different fuel feedstocks (e.g., coal gasification). Complete results are presented, in terms of fuel cell and overall cycle performances.