Optimum Pressure Ratio Research Articles

06/02744 Optimal heat conductance distribution and optimal intercooling pressure ratio for power optimisation of irreversible closed intercooled regenerated Brayton cycleWang, W. H. et al. Journal of the Energy Institute, 2006, 79, (2), 116–119

This paper presents a method for the dimensioning of the low-energy District Heating (DH) piping networks operating with a control philosophy of supplying heat in low-temperature such as 55 °C in supply and 25 °C in return regularly while the supply temperature levels are being boosted in cold winter periods. The performance of the existing radiators that were formerly sized with over-dimensions was analyzed, its results being used as input data for the performance evaluation of the piping network of the low-energy DH system operating with the control philosophy in question. The optimization method was performed under different mass flow limitations that were formed with various temperature configurations. The results showed that reduction in the mass flow rate requirement of a district is possible by increasing the supply temperature in cold periods with significant reduction in heat loss from the DH network. Sensitivity analysis was carried out in order to evaluate the area of applicability of the proposed method. Hence varied values of the original capacity and the current capacity of the existing radiators were evaluated with the design temperature values that were defined by two former radiator sizing standards.

An Investigation of Energy versus Exergy Based Optimization for Power Cycles

A comparison of optimization studies based on energy and exergy methods and for the ideal and actual working models of common power cycles are performed. The cycles considered include the simple gas-turbine cycle, the regenerative gas-turbine cycle, the regenerative gas turbine cycle with reheating and intercooling, the simple steam cycle, the reheat steam cycle, and the combined gas-vapor cycle. The optimization is performed to determine the optimum pressure ratios in gas-turbine cycles and the optimum boiler pressures in steam cycles that maximize the thermal efficiency of the cycle in energy method and the exergy efficiency in exergy method. The optimum points are also searched for maximizing the net work of the cycle in both energy and exergy methods. The results show that the optimum boiler pressures that maximize the network are identical based on both energy and exergy approaches and for both ideal and actual operating models in simple steam and reheat steam cycles. The optimum boiler pressures that maximize the cycle efficiency are about the same based on both energy and exergy approaches when ideal operations are considered in simple and reheat steam cycles. The optimum points differ when actual operations are considered for these cycles. The optimum pressure ratios that maximize the network are identical based on both energy and exergy approaches but different depending on the selection of an ideal or actual model in simple and regenerative gas-turbine cycles, and in combined cycle. No agreement with respect to the optimum pressure ratios that maximize the cycle efficiency is observed for all types of gas-turbine cycles including the combined cycle.

Optimal heat conductance distribution and optimal intercooling pressure ratio for power optimisation of irreversible closed intercooled regenerated Brayton cycle

In the present paper, power is optimised for an irreversible closed intercooled regenerated Brayton cycle coupled to constant temperature heat reservoirs in the viewpoint of finite time thermodynamics. The power optimisation is performed by searching the optimal heat conductance distributions among the four heat exchangers, namely the hot and cold side heat exchangers, the intercooler and the regenerator for fixed total heat exchanger inventory and by searching the optimal intercooling pressure ratio. When the optimisation is performed with respect to the total pressure ratio of the cycle, the maximum power is maximised twice and a double maximum power is obtained. During the analysis, the heat resistance losses in the four heat exchangers, the irreversible compression and expansion losses in the compressors and the turbine are taken into account.

On the simultaneous optimization of pressure and layout for gas permeation membrane systems

We present a methodology for the simultaneous optimization of pressure and network configurations for gas separation membrane permeators. The methodology targets and refines pressure clusters for efficient operation of membrane networks and follows a three-stage strategy. The first stage produces a pressure target curve (PTC) that allows the identification of Pareto optimal pressure cluster combinations. This is followed by a second stage, where the different optimal pressure ratios are used in an optimal search for process structures to identify the performance of the individual clusters. The third stage processes the information generated in the first two stages in a generalized process superstructure model. Throughout the methodology, a modified process synthesis model for membrane network optimization and design is employed which can be optimized robustly using the simulated annealing algorithm. Three illustrative examples are presented to demonstrate the proposed methodology for simultaneous pressure and layout optimization.

Power density optimisation of an endoreversible closed variable-temperature heat reservoir intercooled regenerated Brayton cycle

SYNOPSIS Taking power density, defined as the ratio of power output to the maximum specific volume in the cycle, as the objective function, this paper applies the theory of finite time thermodynamics to find the optimal distribution of heat conductance of the hot- and cold-side heat exchangers, the optimal intercooling pressure ratio, the optimal total pressure ratio and the optimal heat capacity ratio between working fluid and heat reservoir of an endoreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs with heat resistance losses in the hot- and cold-side heat exchangers, the intercooler and the regenerator, by using detailed numerical calculation. The maximum power density, the double-maximum power density and the triple-maximum power density are obtained by optimisation. The effects of some design parameters, including the cycle inlet heat reservoir temperature ratio, the inlet temperature ratio of cooling fluid in the intercooler and the cold-side heat reservoir, and the total heat exchanger inventory, on the maximum power density and the corresponding efficiency, optimum intercooling pressure ratio, and optimum heat conductance distributions between the hot- and cold-side heat exchangers, the double-maximum power density and the corresponding efficiency and optimum total pressure ratio, as well as the triple-maximum power density, are analysed.

Design and Off-Design Characteristics of the Alternative Recuperated Gas Turbine Cycle With Divided Turbine Expansion

In order to fully address the characteristics of the alternative recuperated cycle with divided turbine expansion, both design and off-design analyses have been performed. Two types of mechanical design are assumed: two shaft and single shaft. In particular, optimal pressure ratio division between the high- and low-pressure turbines is evaluated for the single-shaft configuration. It is predicted that the alternative recuperated cycle hardly exhibits sensible design efficiency advantage over the conventional recuperated cycle for moderate turbine inlet conditions and with usual component performances. An advantage of the alternative cycle with single-shaft design is that thermal efficiency is less sensitive to compressor pressure ratio compared to other configurations, and we can also have flexibility in the turbine division without much efficiency loss. The part load analyses have been carried out with the aid of realistic component maps and models for off-design operation. In addition to the general fuel only control, a variable speed control is assumed as the part load operating strategy of the single-shaft configuration. Obvious advantage with the alternative cycle is observed in the variable speed operation of the single-shaft design. With this strategy, the part load efficiency of the alternative cycle is far superior to the conventional cycle. Almost constant efficiency is predicted for a wide power range.

Efficiency optimization of an irreversible closed intercooled regenerated gas-turbine cycle

Thermal efficiency is optimized for an irreversible closed intercooled regenerated gas-turbine cycle coupled to variable-temperature heat reservoirs in the viewpoint of the theory of thermodynamic optimization (or finite-time thermodynamics). It is first performed by searching the optimum intercooling pressure ratio and the optimum heat conductance distributions among the four heat exchangers (the hot- and cold-side heat exchangers, the intercooler, and the regenerator) for fixed total heat exchanger inventory. Secondly, the optimization is performed further with respect to the total pressure ratio of the cycle, the maximum efficiency is maximized twice and the double-maximum efficiency is obtained. Thirdly, the optimization is performed additionally with respect to the thermal capacitance rate matching between the working fluid and the heat reservoir, the double-maximum efficiency is maximized again and a thrice-maximum efficiency is obtained. In the optimization, the following effects are taken into account: the irreversibility of heat transfer in the four heat exchangers, the irreversible compression and expansion losses in the compressors and the turbine, the pressure drop loss in the piping, and the finite thermal capacity rates of the three heat reservoirs. The optimization method is applied to the conceptual design of a closed-cycle intercooled regenerated helium turbine power plant for high-speed warship propulsion. The numerical example shows that the method herein is valid and effective.

Membrane processes for post-combustion carbon dioxide capture: A parametric study

Much of the research in the area of carbon dioxide recovery and storage focuses on minimizing the energy required for CO 2 capture, as this step corresponds to the major cost contribution of the overall process (capture, transportation, injection). Out of the three traditional methods of CO 2 capture (absorption, adsorption and membrane processes), absorption is considered to be the best available technology for post-combustion application. However, amine absorption requires 4–6 GJ/tonne of recovered CO 2, in a large part due to significant energy consumption associated with the regeneration step. In this paper, we perform a systematic analysis of the separation performances and associated energy cost of a single-stage membrane module. First, the operational limits are identified in terms of permeate composition and CO 2 recovery ratio via a systematic parametric study for CO 2/N 2 mixtures. The energy consumption of the capture step is then evaluated and compared with the performance of amine absorption. Next, the search for an optimal strategy in terms of compression energy for a combination of membrane capture and CO 2 injection has been addressed. The results allow the identification of an optimal membrane pressure ratio for a given set of conditions.

Performance analysis on various system layouts for the combination of an ambient pressure molten carbonate fuel cell and a gas turbine

This study analyses hybrid systems combining a molten carbonate fuel cell (MCFC) operating at ambient pressure and a gas turbine. Various possible system layouts, with the major difference among these layouts being the heating method of the turbine inlet gas, are proposed and their design performances are simulated and comparatively analyzed. Power of the MCFC in the hybrid system is explained in terms of the cathode inlet air temperature. Power of the gas turbine differs among various layouts because of large difference in the turbine inlet temperature. The direct firing in front of the turbine allows far higher turbine inlet temperature, and thus greater gas turbine power than the indirect heating of the inlet gas. The optimum pressure ratio of the directly fired system is higher than that of the indirectly fired system. The directly fired system allows not only much larger system power and higher optimum efficiency but also greater flexibility in the selection of the design pressure ratio of the gas turbine. In addition, the directly fired system can better accommodate the specifications of both current micro gas turbines and advanced gas turbines than the indirectly fired system.

Photo reduction of CO 2 to methanol using optical-fiber photoreactor

Greenhouse gases such as CO 2 are the primary cause of global warming. One of the best routes to remedy CO 2 is to transform it to hydrocarbons using photo reduction. CO 2 was photocatalytically reduced to produce methanol using a Hg lamp with wavelength 365 nm in a steady-state optical-fiber photoreactor. The optical-fiber photoreactor, comprised of nearly 120 Cu/TiO 2-coated fibers, was designed and assembled to transmit and spread light uniformly inside the reactor. TiO 2 film was coated on optical fiber using a dip-coating method. Cu-loaded titania solutions were prepared by a thermal hydrolysis method. The thickness of Cu/TiO 2 film was 53 nm. The coating film consisted of very fine spherical particles with diameters of near 14 nm. The XRD spectra indicated the anatase phase for all TiO 2 and Cu/TiO 2 films. The wavelength of absorption edge on Cu/TiO 2 was near 367 nm, equivalent to a bandgap of 3.3 eV. The most active Cu species on TiO 2 surface were Cu 2O clusters, and they played an important role for the formation of methanol. The methanol yield increased with UV irradiative intensity. Maximum methanol rate was 0.45 μmole/g cat h using 1.2 wt.%-Cu/TiO 2 catalyst at 1.29 bar of CO 2, 0.026 bar of H 2O, and 5000 s mean residence time under 16 W/cm 2 UV irradiation. Higher than 1.2 wt.% Cu loading gave a lower rate of methanol yield because of the masking effect of Cu 2O clusters on the TiO 2 surface. The Langmuir–Hinshelwood model was established by correlating experimental data to describe the kinetic behavior. An optimum pressure ratio of H 2O/CO 2 was found in the photo reduction of CO 2 for maximum methanol yield.

Performance optimisation for an irreversible variable-temperature heat reservoir air refrigerator

SYNOPSIS The performance analysis and optimisation of an irreversible air refrigerator with variable-temperature heat reservoirs is carried out by taking the cooling load density, i.e., the ratio of cooling load to the maximum specific volume in the cycle, as the optimisation objective using finite-time thermodynamics (FTT) or entropy generation minimisation (EGM) in this paper. The analytical formulae for the relationships between cooling load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, and the irreversible compression and expansion losses in the compressor and expander. The influences of the effectiveness of the heat exchangers, the inlet temperature ratio of the reservoirs, and the efficiencies of the compressor and expander on the cooling load density versus and pressure ratio are provided by numerical examples. The cooling load density optimisation is performed by finding the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for a fixed total heat exchanger inventory, and the optimum heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the effectiveness of the heat exchangers between the working fluid and heat reservoirs, the efficiencies of compressor and expander, the inlet temperature ratio of heat reservoirs, the heat conductance distribution and the heat capacity rate matching between the working fluid and the heat reservoirs on the maximum cooling load density are provided by numerical examples. Optimisation of refrigeration plant design leads to a reduction in size of the compressor, expander, and the hot- and cold-side heat exchangers.

Implementation of 3-Port Condensing Wave Rotors in R718 Cycles

The use of a novel 3-port condensing wave rotor is suggested to enhance the turbocompression in a refrigeration cycle that works only with water (R718) as a refrigerant. Although the implementation of such a wave rotor essentially reduces the size and cost of R718 units, their efficiency may also be increased. The condensing wave rotor employs pressurized water to pressurize, desuperheat, and condense the refrigerant vapor, all in one dynamic process. The underlying phenomena of flash evaporation, shock wave compression, desuperheating, and condensation inside the wave rotor channels are described in a wave and phase-change diagram. The thermodynamic process is shown in pressure-enthalpy and temperature-entropy diagrams. Based on the described thermodynamic model, a computer program was generated to evaluate the performance of R718 baseline and wave-rotor-enhanced cycles. The effect of some key parameters on the performance enhancement is demonstrated as an aid for optimization. A performance map summarizes the findings. It shows optimum wave rotor pressure ratio and maximum relative performance improvement of R718 cycles by using the 3-port condensing wave rotor.

Preliminary Study of a Novel R718 Compression Refrigeration Cycle Using a Three-Port Condensing Wave Rotor

Using a novel 3-port condensing wave rotor enhancing the turbocompression in a R718 refrigeration cycle, which uses only water as a refrigerant, has been introduced. The wave-rotor implementation can increase efficiency and reduce size and cost of R718 units. The condensing wave rotor employs pressurized water to pressurize, desuperheat, and condense the refrigerant vapor—all in one dynamic process. The underlying phenomena of flash evaporation, shock wave compression, desuperheating, and condensation inside the wave rotor channels are described in a wave and phase-change diagram. The thermodynamic process is shown in pressure–enthalpy and temperature–entropy diagrams. A computer program based on a thermodynamic model was generated to evaluate the performance of R718 baseline and wave-rotor-enhanced cycles. The effect of some key parameters on the performance enhancement is demonstrated as an aid for optimization. A performance map summarizes the findings. It shows optimum wave rotor pressure ratio and maximum relative performance improvement of R718 cycles by using the 3-port condensing wave rotor.

Energy analysis of a cogeneration plant using coal gasification and solid oxide fuel cell

This paper presents a conceptualized combined heat and power (CHP) scheme based on coal gasification and with a high temperature, pressurized solid oxide fuel cell (SOFC) in the topping cycle and a bottoming steam cogeneration cycle. An energy analysis is done for this CHP plant. The study reveals that such a plant offers a substantial saving in fuel with respect to separate plants for the same power and utility heat. The total power of the plant optimizes at a pressure ratio for a given cell operating temperature. However, this optimum pressure ratio increases with higher cell operating temperatures. On the other hand, the utility heat rate decreases with the increasing pressure ratio for any cell operating temperature. In a combined effect of these utility outputs (i.e. power and utility heat), the overall performance of the plant, expressed by fuel energy savings ratio (FESR), is found to optimize at a particular pressure ratio for a given cell operating temperature. For example, the maximum FESR is found to be 30% for an optimum pressure ratio of about 18 for a cell operating temperature of 1273 K.

Power optimization of an irreversible closed intercooled regenerated brayton cycle coupled to variable-temperature heat reservoirs

In this paper, power is optimized for an irreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs in the viewpoint of the theory of thermodynamic optimization (or finite-time thermodynamics (FTT), or endoreversible thermodynamics, or entropy generation minimization (EGM)) by searching the optimum intercooling pressure ratio and the optimum heat conductance distributions among the four heat exchangers (the hot-and cold-side heat exchangers, the intercooler and the regenerator) for fixed total heat exchanger inventory. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum power is maximized twice and the double-maximum power is obtained. Further, as the optimization is performed with respect to the thermal capacitance rate matching between the working fluid and the heat reservoir, the double-maximum power is maximized again and a thrice-maximum power is obtained. In the analysis, the heat resistance losses in the four heat exchangers, the irreversible compression and expansion losses in the compressors and the turbine, the pressure drop loss in the piping, and the effects of finite thermal capacity rate of the three heat reservoirs are taken into account. The effects of the heat reservoir inlet temperature ratio, the total heat exchanger inventory and some other cycle parameters on the cycle optimum performance are analyzed by a numerical example. The optimum results are compared with those reported in recent reference for the conceptual design of a closed-cycle intercooled regenerated gas turbine nuclear power plant for marine ship propulsion. The numerical example shows that the method herein is valid and effective.

Power optimization of an endoreversible closed intercooled regenerated Brayton cycle

In this paper, power is optimized for an endoreversible closed intercooled regenerated Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization (EGM). The effects of some design parameters, including the cycle heat reservoir temperature ratio and total heat exchanger inventory, on the maximum power and the corresponding efficiency are analyzed by numerical examples. The analysis shows that the cycle dimensionless power can be optimized by searching the optimum heat conductance distributions among the hot- and cold-side heat exchangers, the regenerator and the intercooler for fixed total heat exchanger inventory, and by searching the optimum intercooling pressure ratio. When the optimization is performed with respect to the total pressure ratio of the cycle, the maximum dimensionless power can be maximized again.

Design Of Flat Pad Aerostatic Bearings With TheAid Of Finite Element Analysis

A need has been recognised for an aerostatic, flat pad bearing design procedure that has the simplicity of use of design charts, whilst retaining the accuracy offered by finite difference or finite element models. To meet ths requirement a finite element code has been combined with the automatic mesh generation procedure described here. The results provided by this code for a range of bearing designs are also presented. It is shown that the identification of an optimum pressure ratio is more complex than with journal bearings, due to the lack of a natural, concentric position and the variation in pressure ratio over a number of orifices. It is concluded that the multi-criteria nature of the bearing design problem should be taken into account at an early stage. A code capable of determining a Pareto optimal but criteria constrained set of bearing designs is proposed.

Power density analysis and optimisation for an irreversible closed variable-temperature heat reservoir simple Brayton cycle

SYNOPSIS In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as the objective for performance analysis of an irreversible closed Brayton cycle coupled to variable-temperature heat reservoirs with reference to finite time thermodynamics (FTT) or entropy generation minimisation (EGM). The analytical formulae describing the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, the irreversible compression and expansion losses in the compressor and turbine, and the effect of the finite thermal capacity rate of the heat reservoirs. When the heat transfers between the working fluid and the heat reservoirs are carried out ideally and the thermal capacity rates of the heat reservoirs are infinite, this paper confirms those results obtained in recent literature. The obtained results are also compared with those results obtained by using the maximum power criterion. The influences of some design parameters, including the temperature ratio of the heat reservoirs, the effectivenesses of the heat exchangers between the working fluid and the heat reservoirs and the efficiencies of the compressor and the turbine, on the maximum power density are provided by numerical examples, and the advantages and disadvantages of maximum power density design are analysed. The maximum power density optimisation is performed in two aspects. The first is to identify the optimum heat conductance distribution corresponding to the optimum power density of the hot- and cold- side heat exchangers for the fixed heat exchanger inventory. The second is to identify the optimum thermal capacitance rate corresponding to the optimum power density between the working fluid and the high-temperature heat source for a fixed ratio of the thermal capacitance rates of two heat reservoirs. The influence of some design parameters, including the temperature ratio of the heat reservoirs, the heat exchanger inventory between the working fluid and the heat reservoirs and the efficiencies of the compressor and the turbine, on the optimum heat conductance distribution, the optimum thermal capacitance rate corresponding, the maximum power density, and the corresponding optimum pressure ratio, are provided. The power plant design with optimisation leads to a higher efficiency and smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers.

Gas-phase ion chemistry in silane/propyne mixtures

The gas-phase ion chemistry of the silane/propyne mixture was studied by ion trap mass spectrometry with the aim of finding the best experimental conditions leading to the formation of Si- and C-containing ion clusters of increasing size. To this purpose, the optimal pressure ratio among the two gases was about 1:1. Ion/molecule reactions were investigated and rate constants determined for propyne alone and for the silane/propyne systems. In these systems, several mixed ions were formed; in particular, the Si 2CH 5 + ion species is obtained and reaches 20% abundance after 1 s reaction time under the experimental conditions used here.

Cooling Load Density Optimization of an Irreversible Simple Brayton Refrigerator

The performance optimization of an irreversible simple Brayton refrigerator coupled to constant-temperature heat reservoirs is carried out by taking the cooling load density, i.e., the ratio of cooling load to the maximum specific volume in the cycle, as the optimization objective using finite-time thermodynamics (FTT) or entropy generation minimization (EGM) in this paper. The analytical formulae about the relations between cooling load density and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, and the irreversible compression and expansion losses in the compressor and expander. The influences of the effectiveness of the heat exchangers, the temperature ratio of the reservoirs, and the efficiencies of the compressor and expander on the cooling load density versus COP are provided by numerical examples. The cooling load density optimization is performed by searching the optimum pressure ratio of the compressor, and searching the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for the fixed total heat exchanger inventory. The influences of some design parameters, including the effectiveness of the heat exchangers between the working fluid and heat reservoirs, the efficiencies of compressor and expander, the temperature ratio of heat reservoirs, on the maximum cooling load density, the optimum heat conductance distribution and the optimum pressure ratio are provided by numerical examples. The refrigeration plant design with optimization leads to a smaller size including the compressor, expander, and the hot- and cold-side heat exchangers.