Optimum Heat Power Cycles for Specified Boundary Conditions

1991 ◽  
Vol 113 (4) ◽  
pp. 514-521 ◽  
Author(s):  
O. M. Ibrahim ◽  
S. A. Klein ◽  
J. W. Mitchell
Keyword(s):  
Heat Exchanger ◽  
Boundary Conditions ◽  
Power Output ◽  
Maximum Power ◽  
Working Fluid ◽  
Carnot Cycle ◽  
Heat Power ◽  
Power Cycle ◽  
Thermal Capacitance ◽  
Optimum Heat

Optimization of the power output of Carnot and closed Brayton cycles is considered for both finite and infinite thermal capacitance rates of the external fluid streams. The method of Lagrange multipliers is used to solve for working fluid temperatures that yield maximum power. Analytical expressions for the maximum power and the cycle efficiency at maximum power are obtained. A comparison of the maximum power from the two cycles for the same boundary conditions, i.e., the same heat source/sink inlet temperatures, thermal capacitance rates, and heat exchanger conductances, shows that the Brayton cycle can produce more power than the Carnot cycle. This comparison illustrates that cycles exist that can produce more power than the Carnot cycle. The optimum heat power cycle, which will provide the upper limit of power obtained from any thermodynamic cycle for specified boundary conditions and heat exchanger conductances is considered. The optimum heat power cycle is identified by optimizing the sum of the power output from a sequence of Carnot cycles. The shape of the optimum heat power cycle, the power output, and corresponding efficiency are presented. The efficiency at maximum power of all cycles investigated in this study is found to be equal to (or well approximated by) η=1−TL,in/φTH,in where φ is a factor relating the entropy changes during heat rejection and heat addition.

scholarly journals OTEC Maximum Net Power Output Using Carnot Cycle and Application to Simplify Heat Exchanger Selection

Entropy ◽  
2019 ◽  
Vol 21 (12) ◽  
pp. 1143 ◽  
Author(s):  
Kevin Fontaine ◽  
Takeshi Yasunaga ◽  
Yasuyuki Ikegami
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Pressure Drop ◽  
Power Output ◽  
Heat Exchangers ◽  
Power Plants ◽  
Seawater Temperature ◽  
Carnot Cycle ◽  
Ocean Thermal Energy ◽  
Net Power Output

Ocean thermal energy conversion (OTEC) uses the natural thermal gradient in the sea. It has been investigated to make it competitive with conventional power plants, as it has huge potential and can produce energy steadily throughout the year. This has been done mostly by focusing on improving cycle performances or central elements of OTEC, such as heat exchangers. It is difficult to choose a suitable heat exchanger for OTEC with the separate evaluations of the heat transfer coefficient and pressure drop that are usually found in the literature. Accordingly, this paper presents a method to evaluate heat exchangers for OTEC. On the basis of finite-time thermodynamics, the maximum net power output for different heat exchangers using both heat transfer performance and pressure drop was assessed and compared. This method was successfully applied to three heat exchangers. The most suitable heat exchanger was found to lead to a maximum net power output 158% higher than the output of the least suitable heat exchanger. For a difference of 3.7% in the net power output, a difference of 22% in the Reynolds numbers was found. Therefore, those numbers also play a significant role in the choice of heat exchangers as they affect the pumping power required for seawater flowing. A sensitivity analysis showed that seawater temperature does not affect the choice of heat exchangers, even though the net power output was found to decrease by up to 10% with every temperature difference drop of 1 °C.

Effects of Several Major Irreversibilities on the Thermodynamic Performance of a Regenerative MHD Power Cycle

2005 ◽  
Vol 127 (2) ◽  
pp. 103-118 ◽  
Author(s):  
Jincan Chen ◽  
S. K. Tyagi ◽  
S. C. Kaushik ◽  
V. Tiwari ◽  
Chih Wu
Keyword(s):  
Power Output ◽  
Working Fluid ◽  
Temperature Ratio ◽  
Cold Side ◽  
Power Cycle ◽  
Thermodynamic Performance ◽  
Compressor Efficiency ◽  
Increasing Function ◽  
Optimum Relation ◽  
Cycle Temperature

This communication presents the thermodynamic analysis along with a detailed parametric study of an irreversible regenerative MHD power cycle. The power output is adopted as the objective function and optimized with respect to the cycle temperature ratio for a typical set of operating parameters. The power output is found to be an increasing function of the effectiveness and the heat capacitance rates on the hot- and cold-side reservoirs, the regenerative effectiveness, and the compressor and generator efficiencies, while it is found to be a decreasing function of the working fluid heat capacitance rates and the Mach number. The effects of the cold-side effectiveness and heat capacitance rate are found to be more than those of the other side effectiveness and heat capacitance rates on the performance of the cycle. The effect of the compressor efficiency is found to be more than that of the generator efficiency on the power output while it is reverse in the case of thermal efficiency. It is also found that there is an optimum relation among the various heat capacitance rates at which the cycle attains the maximum performance.

Sintering of Solid Particulates Under Elevated Temperature and Pressure in Large Storage Bins for Thermal Energy Storage

2014 ◽  
Author(s):  
R. C. Knott ◽  
D. L. Sadowski ◽  
S. M. Jeter ◽  
S. I. Abdel-Khalik ◽  
H. A. Al-Ansary ◽  
...  
Keyword(s):  
Heat Exchanger ◽  
Size Distribution ◽  
Thermal Energy ◽  
Solar Power ◽  
Physical Characteristics ◽  
Working Fluid ◽  
Concentrated Solar Power ◽  
Power Cycle ◽  
Solid Particulates ◽  
Storage Bins

This research is a part of the DOE-funded SunShot project on “High Temperature Falling Particle Receiver.” Storing thermal energy using solid particulates is a way to mitigate the time of day dependency of concentrated solar power. Small particles may be stored easily, and can be used as a heat transfer medium to transfer heat to the power cycle working fluid through a heat exchanger. This study examines the physical characteristics of solid particulates of different materials kept inside large storage containers. Particle behavior at the expected high temperatures of the concentrated solar power cycle combined with the elevated pressure experienced within the storage container must be evaluated to assess the impact on their physical properties and ensure that the particles would not sinter thereby impacting flow through the system components particularly the receiver and heat exchanger. Sintering is a process of fusing two or more particles together to form a larger agglomerate. In the proposed concentrated solar power tower design, particles will experience temperatures from 600°C to 1000°C. The increase in temperature changes the physical characteristics of the particle, along with any impurities that could form particle to particle bonds. In addition, the hydrostatic pressure exerted on particles stored inside a storage unit increases the probability of sintering. Thus, it is important to examine the characteristics of particles under elevated temperatures and pressures. The experimental procedure involves heating particulates of a known mass and size distribution to temperatures between 600°C and 1000°C inside a crucible. As the temperature is held constant, the particulate sample is pressed upon by a piston pushing into the crucible with a known constant pressure. This process is repeated for different temperatures and pressures for varying lengths of time. The resulting particulates are cooled, and their size distribution is measured to determine the extent of sintering, if any, during the experiment. The particulates tested include various types of sand, along with alumina particles. The data from this experiment will allow designers of storage bins for the solid particulates to determine when significant sintering is expected to occur.

Exploration of Phase Change Within a Capillary Flow Driven Square Micro-Channel Using Lattice Boltzmann Method and Experimental Boundary Conditions

2015 ◽  
Author(s):  
E. Borquist ◽  
G. Smith ◽  
L. Weiss
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Boundary Conditions ◽  
Phase Change ◽  
Lattice Boltzmann ◽  
Capillary Flow ◽  
Working Fluid ◽  
Micro Channel ◽  
External Work ◽  
Boltzmann Method

Previously published research examined the overall efficiency of heat transfer through a copper plated micro-channel heat exchanger. However, since the device is sealed and composed entirely of copper, understanding the phase change, temperature field, and density field of the working fluid is difficult empirically. Given that the efficiency was shown to be greatly increased by the working fluid phase change, this understanding within the device is important to designing devices of greater efficiency and different working fluids. One method of determining device and component performance is numerical modeling of the system. Fluids that undergo phase change have long frustrated those attempting to successfully numerically model systems with acceptable stability. Over the past twenty years, the lattice Boltzmann method (LBM) has transformed the simulation of multicomponent and multiphase flows. Particularly with multiphase flows, the LBM “naturally” morphs the phase change interface throughout the model without excessive computational complexity. The relative ease with which LBM has been applied to some multicomponent/multiphase systems inspired the use of LBM to track phase change within the previously recorded experimental boundary conditions for the copper plated heat exchanger. In this paper, the LBM was used to simulate the evaporation and condensation of HFE-7200 within a capillary flow driven square micro-channel heat exchanger (MHE). All initial and boundary conditions for the simulation are exactly those conditions at which the empirical data was measured. These include temperature and heat flux measurements entering and leaving the MHE. Working fluid parameters and characteristics were given by the manufacturer or measured during experimental work. Once the lattice size, initial conditions, and boundary conditions were input into MATLAB®, the simulations indicated that the working fluid was successfully evaporating and condensing which, coupled with the capillary driven flow, allowed the system to provide excellent heat transfer characteristics without the use of any external work mechanism. Results indicated successive instances of stratified flow along the channel length. Micro-channel flow occurring due to capillary action instead of external work mechanisms made differences in flow patterns negligible. Coupled with the experimentally measured thermal characteristics, this allowed simulations to develop a regular pattern of phase interface tracking. The agreement of multiple simulations with previously recorded experimental data has yielded a system where transport properties are understood and recognized as the primary reasons for such excellent energy transport in the device.

Design and Optimization of Waste Heat Recovery Unit Using Carbon Dioxide as Cooling Fluid

2014 ◽  
Author(s):  
Geir Skaugen ◽  
Harald T. Walnum ◽  
Brede A. L. Hagen ◽  
Daniel P. Clos ◽  
Marit J. Mazzetti ◽  
...  
Keyword(s):  
Heat Exchanger ◽  
Heat Recovery ◽  
Waste Heat Recovery ◽  
Waste Heat ◽  
Working Fluid ◽  
Design And Optimization ◽  
Power Cycle ◽  
Turbine Efficiency ◽  
Recovery Unit ◽  
Process Simulations

This paper describes design and optimization of a Waste Heat Recovery Unit (WHRU) for a power cycle which uses CO2 as a working fluid. This system is designed for offshore installation to increase gas turbine efficiency by recovering waste heat from the exhaust for production of additional power. Due to severe constraints on weight and space in an offshore setting, it is essential to reduce size and weight of the equipment to a minimum. Process simulations are performed to optimize the geometry of the WHRU using different objective functions and thermal-hydraulic models. The underlying heat exchanger model used in the simulations is an in-house model that includes the calculation of weight and volume for frame and structure for the casing in addition to the thermal-hydraulic performance of the heat exchanger core. The results show that the for a set of given process constraints, optimization with respect to minimum total weight or minimum core weight shown similar results for the total installed weight, although the design of heat exchanger differs. The applied method also shows how the WHRU geometry can be optimized for different material combinations.

scholarly journals Thermal performance analysis of organic flash cycle using R600A/R601A mixtures with internal heat exchanger

2020 ◽  
pp. 296-296
Author(s):  
Guidong Huang ◽  
Songyuan Zhang ◽  
Zhong Ge ◽  
Zhiyong Xie ◽  
Zhipeng Yuan ◽  
...  
Keyword(s):  
Heat Exchanger ◽  
Power Output ◽  
Thermal Efficiency ◽  
Electricity Generation ◽  
Internal Heat ◽  
Working Fluid ◽  
Cycle System ◽  
Internal Heat Exchanger ◽  
Generation Costs ◽  
Net Power Output

In this study, the thermal performance of an internal heat exchanger-organic flash cycle system driven by geothermal water was investigated.R600a/R601a mixtures were selected as the working fluid. The effects of the mole fraction of mixtures on the heat absorption capacity of the heater, the temperature rise of cold working fluid in the internal heat exchanger, net power output, thermal efficiency, and electricity generation costs were analyzed. The net power outputs, electricity generation costs, and thermal efficiency of the internal heat exchanger-organic flash cycle and simple organic flash cycle systems were compared. Results showed that the system using theR600a/R601a mixtures (0.7/0.3mole fraction) has the largest net power output, which increased the net power output by 3.68% and 42.23% over the R601a and R600a systems, respectively. WhentheR600a mole fraction was 0.4, the electricity generation costs reduction of the internal heat exchanger-organic flash cycle system was the largest (1.77% compared with the simple organic flash cycle system).The internal heat exchanger can increase the thermal efficiency of organic flash cycle, but the net power output does not necessarily increase.

Techno-Economic Evaluation of the Effect of Impurities on the Performance of Supercritical CO2 Cycles

2019 ◽  
Author(s):  
Ladislav Vesely ◽  
Vaclav Dostal ◽  
Jayanta Kapat ◽  
Subith Vasu ◽  
Scott Martin
Keyword(s):  
Economic Evaluation ◽  
Heat Exchanger ◽  
High Efficiency ◽  
Operating Conditions ◽  
Rate Of Return ◽  
Working Fluid ◽  
Levelized Cost Of Electricity ◽  
Tube Heat Exchanger ◽  
Power Cycle ◽  
Effect Of Impurities

Abstract The development of new power generation technologies are necessary to meet growing energy demands and emission requirements. The supercritical carbon dioxide (S-CO2) cycle is one such technology; it has relatively high efficiency, potential to enable 100% carbon capture, and compact components. The S-CO2 cycle is adaptable to almost all of the existing power producing methods including fossil, solar, and nuclear technologies. However, it is known that the best combination of the operating conditions, equipment, working fluid and cycle layout determine the maximum achievable efficiency of a cycle. Impurities in the cycle have some effect on the S-CO2 power cycle as presented in our previous work. The effect of impurities is positive or negative and affects all components. The effect of mixture compositions on the techno-economic evaluation is important information for the global understanding of the effect of mixtures on the S-CO2 power cycle. This paper focuses on the techno-economic evaluation of a hypothetical power plant with the S-CO2 power cycle. Two cases are considered for techno-economic evaluation. The difference between these cases is in the heat source and the associated heat exchanger (PCHE and shell and tube heat exchanger). Cost estimation was performed for three indicators (the levelized cost of electricity, the internal rate of return, and the net present value), which are important for economic viability and the rate of return of the project.

CFD Simulation of an Alfa-Stirling Engine to Study the Geometrical Parameters on the Engine Performance

2019 ◽  
Author(s):  
Ana C. Ferreira ◽  
Senhorinha F. C. F. Teixeira ◽  
Ricardo F. Oliveira ◽  
José C. Teixeira
Keyword(s):  
Heat Exchanger ◽  
Power Output ◽  
Cfd Simulation ◽  
Engine Performance ◽  
Stirling Engine ◽  
Operating Conditions ◽  
Design Parameters ◽  
Working Fluid ◽  
Cold Temperatures ◽  
Temperature Heat

Abstract An alpha-Stirling configuration was modelled using a Computational Fluid Dynamic (CFD), using ANSYS® software. A Stirling engine is an externally heated engine which has the advantage of working with several heat sources with high efficiencies. The working gas flows between compression and expansion spaces by alternate crossing of, a low-temperature heat exchanger (cooler), a regenerator and a high-temperature heat exchanger (heater). Two pistons positioned at a phase angle of 90 degrees were designed and the heater and cooler were placed on the top of the pistons. The motion of the boundary conditions with displacement was defined through a User Defined Function (UDF) routine, providing the motion for the expansion and compression piston, respectively. In order to define the temperature differential between the engine hot and the cold sources, the walls of the heater and cooler were defined as constant temperatures, whereas the remaining are adiabatic. The objective is to study the thermal behavior of the working fluid considering the piston motion between the hot and cold sources and investigate the effect of operating conditions on engine performance. The influence of regenerator matrix porosity, hot and cold temperatures on the engine performance was investigated through predicting the PV diagram of the engine. The CFD simulation of the thermal engine’s performance provided a Stirling engine with 760W of power output. It was verified that the Stirling engine can be optimized when the best design parameters combination are applied, mostly the regenerator porosity and cylinders volume, which variation directly affect the power output.

Direct Method to Maximize Net Power Output of Rankine Cycle in Low-Grade Thermal Energy Conversion

2010 ◽  
Vol 2 (2) ◽  
Author(s):  
Faming Sun ◽  
Yasuyuki Ikegami
Keyword(s):  
Energy Conversion ◽  
Thermal Energy ◽  
Power Output ◽  
Direct Method ◽  
Rankine Cycle ◽  
Working Fluid ◽  
Carnot Cycle ◽  
Low Grade ◽  
Thermal Energy Conversion ◽  
Net Power Output

Using ammonia as working fluid, enthalpy equations corresponding to every point in Rankine cycle for low-grade thermal energy conversion (LTEC) are presented by employing curve-fitting method. Analytical equations of Rankine cycle analysis are thus set up. In terms of temperatures of the evaporator and condenser, the equation related to Rankine cycle net power output is then achieved. Furthermore, by using theoretical optimization method, the results of the maximum net power output of a Rankine cycle in LTEC are also reported. This study extends the recent flurry of publications about Rankine cycle power optimization in LTEC, which modified the ideal Rankine cycle to a Carnot cycle by using an average entropic temperature to achieve the theoretical formulas. The proposed method can better reflect the performance of Rankine cycle in LTEC since the current work is mainly based on the direct simulations of every enthalpy points in Rankine cycle. Moreover, the proposed method in this paper is equally applicable for other working mediums, such as water and R134a.

High-Power Multi-Stage Rankine Cycles

1995 ◽  
Vol 117 (3) ◽  
pp. 192-196 ◽  
Author(s):  
O. M. Ibrahim ◽  
S. A. Klein
Keyword(s):  
Power Output ◽  
Waste Heat ◽  
Thermodynamic Cycle ◽  
Rankine Cycle ◽  
Maximum Power ◽  
Heat Sources ◽  
Power Cycles ◽  
Power Cycle ◽  
Multi Stage ◽  
Limit Power

This paper presents an analysis of the multi-stage Rankine cycle aiming at optimizing the power output from low-temperature heat sources such as geothermal or waste heat. A design methodology based on finite-time thermodynamics and the maximum power concept is used in which the shape and the power output of the maximum power cycle are identified and utilized to compare and evaluate different Rankine cycle configurations. The maximum power cycle provides the upper-limit power obtained from any thermodynamic cycle for specified boundary conditions and heat exchanger characteristics. It also provides a useful tool for studying power cycles and forms the basis for making design improvements.

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