Adiabatic Losses in Stirling Refrigerators

1996 ◽  
Vol 118 (2) ◽  
pp. 120-127 ◽  
Author(s):  
L. Bauwens
Keyword(s):  
Volume Ratio ◽  
Poor Performance ◽  
Absolute Temperature ◽  
Coefficient Of Performance ◽  
Dead Volume ◽  
Temperature Ratio ◽  
Small Temperature ◽  
Stirling Cycle ◽  
Engine Model ◽  
Cylinder Flow

The Stirling cycle has been used very effectively in cryocoolers; but efficiencies relative to the Carnot limit are typically observed to peak for absolute temperature rations of about two, which makes it less suitable for low-lift refrigeration. The adiabatic loss appears to be responsible for poor performance at small temperature differences. In this paper, adiabatic losses are evaluated, for a temperature ratio of 2/3, taking into account the effect of phase angle between pistons, of volume ratio, of the distribution of the dead volume necessary of reduce the volume ration, and of the distribution of displacement between expansion and compression spaces. The study is carried out numerically, using an adiabatic stirling engine model in which cylinder flow is assumed to be stratified. Results show that the best location for the cylinder dead volume is on the compression side. Otherwise, all strategies used to trade off refrigeration for coefficient of performance are found to be roughly equivalent.

Design and Evaluation of a MEMS-Based Stirling Microcooler

2013 ◽  
Vol 135 (11) ◽  
Author(s):  
Dongzhi Guo ◽  
Jinsheng Gao ◽  
Alan J. H. McGaughey ◽  
Gary K. Fedder ◽  
Matthew Moran ◽  
...  
Keyword(s):  
Volume Ratio ◽  
Operating Conditions ◽  
Coefficient Of Performance ◽  
Dead Volume ◽  
Working Fluid ◽  
Cooling Power ◽  
Phase Lag ◽  
Power Losses ◽  
Cold Side ◽  
Swept Volume

A new Stirling microrefrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated. The cooling elements are to be fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Air at a pressure of 2 bar is the working fluid and is sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. Parametric study of the design shows the effects of phase lag between the hot space and cold space, swept volume ratio between the hot space and cold space, and dead volume ratio on the cooling power. Losses due to regenerator nonidealities are estimated and the effects of the operating frequency and the regenerator porosity on the cooler performance are explored. The optimal porosity for the best system coefficient of performance (COP) is identified.

Design and Evaluation of MEMS-Based Stirling Cycle Micro-Refrigeration System

2011 ◽  
Author(s):  
Dongzhi Guo ◽  
Jingsheng Gao ◽  
Alan J. H. McGaughey ◽  
Matthew Moran ◽  
Suresh Santhanam ◽  
...  
Keyword(s):  
Volume Ratio ◽  
Cooling Capacity ◽  
Operating Conditions ◽  
Heat Extraction ◽  
Dead Volume ◽  
Working Fluid ◽  
Phase Lag ◽  
Refrigeration System ◽  
Cold Side ◽  
Stirling Cycle

A Stirling cycle micro-refrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated thermodynamically. The cooling elements are each 5 mm-long, 2.25 mm-wide, have a thickness of 300 μm, and are fabricated in a stacked array on a silicon wafer. A 0.5 mm-long regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms. The diaphragms are 2.25 mm circles driven electrostatically. Helium is the working fluid, pressurized at 2 bar and sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally 90° out of phase such that heat is extracted to the expansion space and released from the compression space. The bulk silicon substrate on which the device is grown is etched with “zipping” shaped chambers under the diaphragms. The silicon enables efficient heat transfer between the gas and heat source/sink as well as reduces the dead volume of the system, thus enhancing the cooling capacity. In addition, the “zipping” shaped substrates reduce the voltage required to actuate the diaphragms. An array of vertical silicon pillars in the regenerator serves as a thermal capacitor transferring heat to and from the working gas during a cycle. In operation, the push-pull motion of the diaphragm makes a 300 μm stroke and actuates at a frequency of 2 kHz. Parametric study of the design shows the effects of phase lag, swept volume ratio between the hot space and cold space, and dead volume ratio on cooling capacity. At TH = 313.15 K and TC = 288.15 K and assuming a perfect regenerator, the thermodynamic optimization analysis gives a heat extraction rate of 0.22 W per element and cooling capacity of 30 W/cm2 for the stacked system. Evaluation of the stacked system shows that the COP will reach 6.3 if the expansion work from the cold side is recovered electrostatically and used to drive the hot side diaphragm.

Three-Dimensional Analysis on the Performance of the Thermoelectric Micro-Cooler

2005 ◽  
Author(s):  
Kong Hoon Lee ◽  
Ook Joong Kim
Keyword(s):  
Temperature Difference ◽  
Three Dimensional ◽  
Coefficient Of Performance ◽  
Thermoelectric Cooler ◽  
Small Temperature ◽  
Small Current ◽  
Thermoelectric Element ◽  
Maximum Value ◽  
Column Type ◽  
P Type

Three-dimensional numerical analysis has been carried out using the FEMLAB software package to figure out the performance of the thermoelectric micro-cooler. A small-size and column-type thermoelectric cooler is considered and Bi2Te3 and Sb2Te3 are selected as the n- and p-type thermoelectric materials, respectively. The thickness of the thermoelectric element considered is 5 to 20 μm and the thickness affects the performance of the cooler. The effect of parameters such as the temperature difference, the current, and the thickness of the thermoelectric element on the performance of the cooler has also been investigated. The coefficient of performance (COP) is the primary factor to evaluate the performance of the cooler and the COP varies with the parameters. The COP has the maximum value at a certain current and the value decreases with the temperature difference or the thickness. The predicted results also show that the performance can be improved for thick thermoelectric element at the small temperature difference and small current.

scholarly journals ZnO Thin Film Transistor: Effect of Traps and Grain Boundaries

2018 ◽  
Vol 17 (1) ◽  
pp. 41-43
Author(s):  
A.A. Razak ◽  
W.H. Khoo ◽  
Suhana Mohamed Sultan
Keyword(s):  
Grain Boundary ◽  
Semiconductor Industry ◽  
High Surface ◽  
Thin Film Transistor ◽  
Volume Ratio ◽  
Poor Performance ◽  
High Resistivity ◽  
Sensing Applications ◽  
Two Factors ◽  
Temperature Deposition

Recently ZnO has drawn a lot of attention in semiconductor industry due to its interesting features. High exciton binding energy, high resistivity against radiation, high breakdown voltage, low temperature deposition are some of the interesting features of this material. Zinc oxide TFT device gains an increasing interest for its potential in sensing applications due to its biocompability, chemical stability and  simple fabrication process with various methods and high surface-to-volume ratio. However, ZnO TFT devices from previous work exhibited poor ION and field effect mobility. This work investigates the cause of its poor performance by focusing only two factors: traps and defects in the channel and grain boundary. The work was performed in Silvaco TCAD 2D simulator. It was found that a single grain boundary in the channel causes a reduction of the ION by 95%. The effect in the ION is less severe when traps and defects were introduced in the ZnO channel. The results can assist in optimizing the TFT device performance for sensing applications.

scholarly journals Холодильный коэффициент составной охлаждающей термоэлектрической ветви

2019 ◽  
Vol 53 (5) ◽  
pp. 685
Author(s):  
И.А. Драбкин
Keyword(s):  
Temperature Difference ◽  
Coefficient Of Performance ◽  
Large Temperature ◽  
Small Temperature ◽  
Energy Characteristics

AbstractOne of the main energy characteristics of a cooling leg is the maximum available temperature difference (Δ T _max). Its increase indicates an increase in the coefficient of performance (COP) at all temperatures. The use of a segmented leg makes it possible to increase the Δ T _max value; however, the maximum available COP increases only at large temperature differences, whereas at small temperature differences the maximum available COP is even smaller than in a simple leg.

scholarly journals Performance Optimization of an Air-Standard Irreversible Dual-Atkinson Cycle Engine Based on the Ecological Coefficient of Performance Criterion

2014 ◽  
Vol 2014 ◽  
pp. 1-10 ◽  
Author(s):  
Guven Gonca ◽  
Bahri Sahin
Keyword(s):  
Performance Optimization ◽  
Pressure Ratio ◽  
Performance Criterion ◽  
Coefficient Of Performance ◽  
Cycle Performance ◽  
Temperature Ratio ◽  
Finite Rate ◽  
Ecological Performance ◽  
Atkinson Cycle ◽  
Internal Irreversibility

This paper presents an ecological performance analysis and optimization for an air-standard irreversible Dual-Atkinson cycle (DAC) based on the ecological coefficient of performance (ECOP) criterion which includes internal irreversibilities, heat leak, and finite-rate of heat transfer. A comprehensive numerical analysis has been realized so as to investigate the global and optimal performances of the cycle. The results obtained based on the ECOP criterion are compared with a different ecological function which is named as the ecologic objective-function and with the maximum power output conditions. The results have been attained introducing the compression ratio, cut-off ratio, pressure ratio, Atkinson cycle ratio, source temperature ratio, and internal irreversibility parameter. The change of cycle performance with respect to these parameters is investigated and graphically presented.

scholarly journals IN-CYLINDER MASS FLOW ESTIMATION AND MANIFOLD PRESSURE DYNAMICS FOR STATE PREDICTION IN SI ENGINES

2014 ◽  
Vol 54 (3) ◽  
pp. 240-247 ◽  
Author(s):  
Wojnar Sławomir ◽  
Boris Rohal-Ilkiv ◽  
Peter Šimončic ◽  
Marek Honek ◽  
Csambál Jozef
Keyword(s):  
Control Strategies ◽  
Estimation Method ◽  
Mean Value ◽  
Intake Manifold ◽  
Flow Estimation ◽  
Engine Model ◽  
Pressure Dynamics ◽  
Estimation And Control ◽  
Cylinder Flow ◽  
And Control

The aim of this paper is to present a simple model of the intake manifold dynamics of a spark ignition (SI) engine and its possible application for estimation and control purposes. We focus on pressure dynamics, which may be regarded as the foundation for estimating future states and for designing model predictive control strategies suitable for maintaining the desired air fuel ratio (AFR). The flow rate measured at the inlet of the intake manifold and the in-cylinder flow estimation are considered as parts of the proposed model. In-cylinder flow estimation is crucial for engine control, where an accurate amount of aspired air forms the basis for computing the manipulated variables. The solutions presented here are based on the mean value engine model (MVEM) approach, using the speed-density method. The proposed in-cylinder flow estimation method is compared to measured values in an experimental setting, while one-step-ahead prediction is illustrated using simulation results.

Optimization of Cooled Shields in Insulations

1984 ◽  
Vol 106 (4) ◽  
pp. 871-875 ◽  
Author(s):  
J. C. Chato ◽  
J. M. Khodadadi
Keyword(s):  
Dissipation Rate ◽  
Heat Dissipation ◽  
Absolute Temperature ◽  
Entropy Production Rate ◽  
Insulation Layer ◽  
Temperature Ratio ◽  
Insulation System ◽  
Simple Method ◽  
Practical Applications ◽  
Cooling Loads

A relatively simple method has been developed to optimize the location, temperature, and heat dissipation rate of each cooled shield inside an insulation layer. The method is based on the minimization of the entropy production rate, which is proportional to the heat leak across the insulation. The results show that the maximum number of shields to be used in most practical applications is three. However, cooled shields are useful only at low values of the overall, cold wall to hot wall absolute temperature ratio. The performance of the insulation system is relatively insensitive to deviations from the optimum values of temperature and location of the cooling shields. Design curves are presented for rapid estimates of the locations and temperatures of cooling shields in various types of insulations, and an equation is given for calculating the cooling loads for the shields.

First and second law analysis of a Stirling engine with imperfect regeneration and dead volume

2009 ◽  
Vol 223 (11) ◽  
pp. 2595-2607 ◽  
Author(s):  
J Harrod ◽  
P J Mago ◽  
K Srinivasan ◽  
L M Chamra
Keyword(s):  
Entropy Generation ◽  
Thermal Efficiency ◽  
Engine Performance ◽  
Stirling Engine ◽  
Dead Volume ◽  
Second Law ◽  
Work Output ◽  
Volume Percentage ◽  
Stirling Cycle ◽  
Second Law Analysis

This article discusses the thermodynamic performance of an ideal Stirling cycle engine. This investigation uses the first law of thermodynamics to obtain trends of total heat addition, net work output, and thermal efficiency with varying dead volume percentage and regenerator effectiveness. Second law analysis is used to obtain trends for the total entropy generation of the cycle. In addition, the entropy generation of each component contributing to the Stirling cycle processes is considered. In particular, parametric studies of dead volume effects and regenerator effectiveness on Stirling engine performance are investigated. Finally, the thermodynamic availability of the system is assessed to determine theoretical second law efficiencies based on the useful exergy output of the cycle. Results indicate that a Stirling engine has high net work output and thermal efficiency for low dead volume percentages and high regenerator effectiveness. For example, compared to an engine with zero dead volume and perfect regeneration, an engine with 40 per cent dead volume and a regenerator effectiveness of 0.8 is shown to have ∼60 per cent less net work output and a 70 per cent smaller thermal efficiency. Additionally, this engine results in approximately nine times greater overall entropy generation and 55 per cent smaller second law efficiency.

Numerical Simulation Study of the Influence of Structural Parameters of Combustion Chamber on In-Cylinder Flow Field of Diesel Engine

2013 ◽  
Vol 860-863 ◽  
pp. 1729-1732
Author(s):  
Guo Cheng Li ◽  
Ping Sun ◽  
Peng Hu
Keyword(s):  
Diesel Engine ◽  
Flow Field ◽  
Combustion Chamber ◽  
Structural Parameters ◽  
Volume Ratio ◽  
Initial Boundary ◽  
Swirl Flow ◽  
Swirl Ratio ◽  
Depth Ratio ◽  
Cylinder Flow

Based on the entity model of the type 4B26 diesel engine, calculated by CFD FIRE and combined with the software BOOST for the initial boundary conditions, the influence of combustion chamber structural parameters, such as boss height, surface-volume ratio and diameter-depth ratio of combustion chamber, on in-cylinder flow field of diesel engine was investigated. The results show that the influence of the boss height on flow field in the cylinder and the transient swirl ratio is obvious, and increasing the boss height is beneficial to urge the formation of mixture rapidly. Reducing the surface-volume ratio is beneficial for improving the maximum transient swirl ratio, and the air strength maintains well also, but has little influence to the retentivity of the swirl intensity. Meanwhile, reducing the diameter-depth ratio does not only improves the air flow movement strengthen in the combustion chamber, but also enhances the maximum transient swirl ratio, and the retentivity of swirl flow movement is satisfying.

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