scholarly journals Closed Irreversible Cycles Analysis Based on Finite Physical Dimensions Thermodynamics

Proceedings ◽  
2020 ◽  
Vol 58 (1) ◽  
pp. 37
Author(s):  
Gheorghe Dumitrascu ◽  
Michel Feidt ◽  
Stefan Grigorean
Keyword(s):  
Energy Efficiency ◽  
Mathematical Models ◽  
Heat Input ◽  
Thermal Conductance ◽  
External Heat ◽  
Working Fluid ◽  
Heat Output ◽  
Internal Irreversibility ◽  
Operational Constraints ◽  
Physical Dimensions

The paper develops generalizing entropic approaches of irreversible closed cycles. The mathematical models of the irreversible engines (basic, with internal regeneration of the heat, cogeneration units) and of the refrigeration cycles were applied to four possible operating irreversible trigeneration cycles. The models involve the reference entropy, the number of internal irreversibility, the thermal conductance inventory, the proper temperatures of external heat reservoirs unifying the first law of thermodynamics and the linear heat transfer law, the mean log temperature differences, and four possible operational constraints, i.e., constant heat input, constant power, constant energy efficiency and constant reference entropy. The reference entropy is always the entropy variation rate of the working fluid during the reversible heat input process. The amount of internal irreversibility allows the evaluation of the heat output via the ratio of overall internal irreversible entropy generation and the reference entropy. The operational constraints allow the replacement of the reference entropy function of the finite physical dimension parameters, i.e., mean log temperature differences, thermal conductance inventory, and the proper external heat reservoir temperatures. The paper presents initially the number of internal irreversibility and the energy efficiency equations for engine and refrigeration cycles. At the limit, i.e., endoreversibility, we can re-obtain the endoreversible energy efficiency equation. The second part develops the influences between the imposed operational constraint and the finite physical dimensions parameters for the basic irreversible cycle. The third part is applying the mathematical models to four possible standalone trigeneration cycles. It was assumed that there are the required consumers of the all useful heat delivered by the trigeneration system. The design of trigeneration system must know the ratio of refrigeration rate to power, e.g., engine shaft power or useful power delivered directly to power consumers. The final discussions and conclusions emphasize the novelties and the complexity of interconnected irreversible trigeneration systems design/optimization.

scholarly journals Finite Physical Dimensions Thermodynamics Analysis and Design of Closed Irreversible Cycles

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3416
Author(s):  
Gheorghe Dumitrașcu ◽  
Michel Feidt ◽  
Ştefan Grigorean
Keyword(s):  
Thermal Conductance ◽  
Heat Output ◽  
Operational Parameters ◽  
Analysis And Design ◽  
The Mean ◽  
Internal Irreversibility ◽  
Heat Transfers ◽  
Operational Constraints ◽  
Log Temperatures ◽  
Physical Dimensions

This paper develops simplifying entropic models of irreversible closed cycles. The entropic models involve the irreversible connections between external and internal main operational parameters with finite physical dimensions. The external parameters are the mean temperatures of external heat reservoirs, the heat transfers thermal conductance, and the heat transfer mean log temperatures differences. The internal involved parameters are the reference entropy of the cycle and the internal irreversibility number. The cycle’s design might use four possible operational constraints in order to find out the reference entropy. The internal irreversibility number allows the evaluation of the reversible heat output function of the reversible heat input. Thus the cycle entropy balance equation to design the trigeneration cycles only through external operational parameters might be involved. In designing trigeneration systems, they must know the requirements of all consumers of the useful energies delivered by the trigeneration system. The conclusions emphasize the complexity in designing and/or optimizing the irreversible trigeneration systems.

Influence of Working Fluid Properties on Internal Irreversibility of Power Systems

2010 ◽  
Author(s):  
Gheorghe Dumitrascu ◽  
Aristotel Popescu
Keyword(s):  
Heat Transfer ◽  
Power Systems ◽  
Direct Method ◽  
External Heat ◽  
Working Fluid ◽  
Second Law ◽  
Fluid Properties ◽  
Set Up ◽  
Internal Irreversibility ◽  
Heat Exchanges

Thermodynamic techniques used to analyze and optimize irreversible cycles involve mainly both the exergy model, i.e. second law efficiency, and the entropy generation minimization [1]. This paper presents an appraising comprehensive direct method based on two overall numbers of irreversibility, external and internal ones. These two numbers link the first and second laws of Thermodynamics inside the first law efficiency, called in this paper the irreversible first law efficiency. The number of external irreversibility is the ratio of second law effectiveness of the cycle external heat exchanges, which were set up by relating the real heat exchange of the working fluid with external heat reservoirs, to the ideal completely reversible one, assuming both a heat transfer area tending to infinity and a heat transfer temperature difference tending to zero. The number of internal irreversibility manages the irreversible entropy production along the cycle. This paper includes selected numerical results regarding the internal irreversibility connected to the nature of the working fluid in a Joule-Brayton engine cycle.

Efficiency Optimizations of an Irreversible Brayton Heat Engine

1998 ◽  
Vol 120 (2) ◽  
pp. 143-148 ◽  
Author(s):  
C.-Y. Cheng ◽  
C.-K. Chen
Keyword(s):  
Heat Transfer ◽  
Thermal Efficiency ◽  
Heat Exchangers ◽  
Heat Engine ◽  
Thermal Conductance ◽  
Working Fluid ◽  
Reservoir Temperature ◽  
Temperature Ratio ◽  
Conductance Ratio ◽  
Internal Irreversibility

A steady-flow approach for finite-time thermodynamics is used to calculate the maximum thermal efficiency, its corresponding power output, adiabatic temperature ratio, and thermal-conductance ratio of heat transfer equipment of a closed Brayton heat engine. The physical model considers three types of irreversibilities: finite thermal conductance between the working fluid and the reservoirs, heat leaks between the reservoirs, and internal irreversibility inside the closed Brayton heat engine. The effects of heat leaks, hot-cold reservoir temperature ratios, turbine and compressor isentropic efficiencies, and total conductances of heat exchangers on the maximum thermal efficiency and its corresponding parameters are studied. The optimum conductance ratio could be found to effectively use the heat transfer equipment, and this ratio is increased as the component efficiencies and total conductances of heat exchangers are increased, and always less than or equal to 0.5.

Experimental and Numerical Simulation for Thermal Investigation of Oscillating Heat Pipe Using VOF Model

2020 ◽  
Vol 38 (1A) ◽  
pp. 88-104
Author(s):  
Anwar S. Barrak ◽  
Ahmed A. M. Saleh ◽  
Zainab H. Naji
Keyword(s):  
Thermal Resistance ◽  
Heat Pipe ◽  
Heat Input ◽  
Cfd Simulation ◽  
Working Fluid ◽  
Maximum Deviation ◽  
Inlet Temperature ◽  
Maximum Heat ◽  
Oscillating Heat Pipe ◽  
Vof Model

This study is investigated the thermal performance of seven turns of the oscillating heat pipe (OHP) by an experimental investigation and CFD simulation. The OHP is designed and made from a copper tube with an inner diameter 3.5 mm and thickness 0.6 mm and the condenser, evaporator, and adiabatic lengths are 300, 300, and 210 mm respectively.  Water is used as a working fluid with a filling ratio of 50% of the total volume. The evaporator part is heated by hot air (35, 40, 45, and 50) oC with various face velocity (0.5, 1, and 1.5) m/s. The condenser section is cold by air at temperature 15 oC. The CFD simulation is done by using the volume of fluid (VOF) method to model two-phase flow by conjugating a user-defined function code (UDF) to the FLUENT code. Results showed that the maximum heat input is 107.75 W while the minimum heat is 13.75 W at air inlet temperature 35 oC with air velocity 0.5m/s. The thermal resistance decreased with increasing of heat input. The results were recorded minimum thermal resistance 0.2312 oC/W at 107.75 W and maximum thermal resistance 1.036 oC/W at 13.75W. In addition, the effective thermal conductivity increased due to increasing heat input.  The numerical results showed a good agreement with experimental results with a maximum deviation of 15%.

Microcontroller Based Single Axis Intelligent Control Sun Tracker for Parabolic Trough Collectors

2012 ◽  
Vol 562-564 ◽  
pp. 1772-1775
Author(s):  
Shakeel Akram ◽  
Farhan Hameed Malik ◽  
Rui Jin Liao ◽  
Bin Liu ◽  
Tariq Nazir
Keyword(s):  
Energy Efficiency ◽  
Embedded System ◽  
Alternative Energy ◽  
Power Plants ◽  
Tracking System ◽  
Appropriate Technology ◽  
Working Fluid ◽  
The Sun ◽  
Solar Collectors ◽  
Parabolic Trough

Due to the complex design and high costs of production, solar thermal systems have fallen behind in the world of alternative energy systems. Different mechanisms are applied to increase the efficiency of the solar collectors and to reduce the cost. Solar tracking system is the most appropriate technology to increase the efficiency of solar collectors as well as solar power plants by tracking the sun timely. In order to maximize the efficiency of collectors, one needs to keep the reflecting surface of parabolic trough collectors perpendicular to the sun rays. For this purpose microcontroller based real time sun tracker is designed which is controlled by an intelligent algorithm using shadow technique. The aim of the research project is to test the solar-to-thermal energy efficiency by tracking parabolic trough collector (PTC). The energy efficiency is determined by measuring the temperature rise of working fluid as it flows through the receiver of the collector when it is properly focused. The design tracker is also simulated to check its accuracy. The main purpose to design this embedded system is to increase the efficiency and reliability of solar plants by reducing size, complexity and cost of product.

scholarly journals Field Test on Performance of an Air Source Heat Pump System Using Novel Gravity-Driven Radiators as Indoor Heating Terminal

2021 ◽  
Vol 9 ◽  
Author(s):  
Jie Jia ◽  
Xuan Zhou ◽  
Wei Feng ◽  
Yuanda Cheng ◽  
Qi Tian ◽  
...  
Keyword(s):  
Energy Efficiency ◽  
Heat Pump ◽  
Field Test ◽  
Thermal Response ◽  
Heat Output ◽  
Pump System ◽  
Heat Pump System ◽  
Air Source Heat Pump ◽  
Gravity Driven ◽  
Indoor Comfort

The simultaneous need for energy efficiency and indoor comfort may not be met by existing air source heat pump (ASHP) technology. The novelty of this study lies in the use of a new gravity-driven radiator as the indoor heating terminal of ASHPs, aiming to provide an acceptable indoor comfort with improved energy efficiency. To confirm and quantify the performance improvement due to the proposed system retrofit, a field test was conducted to examine the system performance under real conditions. In the tests, measurements were made on the refrigerant- and air-side of the system to characterize its operational characteristics. Results showed that the proposed radiator has a rapid thermal response, which ensures a fast heat output from the system. The proposed system can create a stable and uniform indoor environment with a measured air diffusion performance index of 80%. The energy efficiency of the proposed system was also assessed based on the test data. It was found that the system’s first law efficiency is 42.5% higher than the hydraulic-based ASHP system. In terms of the second law efficiency, the compressor contributes the most to the overall system exergy loss. The exergy efficiency of the proposed system increases with the outdoor temperature and varies between 35.02 and 38.93% in the test period. The research results and the analysis methodology reported in this study will be useful for promoting the technology in search of energy efficiency improvement in residential and commercial buildings.

scholarly journals INCREASING THE EFFICIENCY OF ELECTRIC DRIVES WITH PERIODICAL LOADING BY USING COMPREHENSIVE MATHEMATICAL MODELING MEANS

2021 ◽  
Author(s):  
Olena Bibik ◽  
◽  
Oleksandr Popovich ◽  
Keyword(s):  
Mathematical Modeling ◽  
Energy Efficiency ◽  
Steady State ◽  
Optimal Design ◽  
Mathematical Models ◽  
Quasi Steady State ◽  
Periodic Load ◽  
Electromechanical Systems ◽  
Periodic Loading ◽  
Asynchronous Motors

The mode of operation of induction motors (IMs) affects their performance. In most cases, motors are optimally designed for steady state operation. When operating in other modes, additional attention is required to the problems of energy efficiency. Induction motors are the most common type of electromechanical energy converters, and a significant part of them operate under conditions of periodic changes in the load torque. The work is devoted to solving the problem of increasing the energy efficiency of asynchronous motors of electromechanical systems with a periodic load, including pumping and compressor equipment. The traditional solution to this problem for compressor equipment is the optimal design of an IM under static conditions, as well as the use of flywheels, the use of an IM with an increased slip value and controlled IM with a squirrel-cage rotor and with frequency converters. In this work, the modes of operation of asynchronous motors with periodic loading are investigated. For this, complex mathematical models are developed in the simulation system. Such models are effective in modeling taking into account periodic load changes: repetitive transient processes, their possible asymmetry and non-sinusoidality, increased influence of nonlinearity of electromagnetic parameters. In complex mathematical modeling, the mutual influence of the constituent parts of the electromechanical system is taken into account. Simulation allowed quantifying the deterioration in energy efficiency under intermittent loading, in comparison with static modes. Criteria for evaluating quasi-static modes have been developed and areas of critical decrease in efficiency have been determined. The paper proposes and demonstrates a methodology for solving this problem. For this purpose, tools have been created for the optimal design of asynchronous motors as part of electromechanical systems with periodic loading. These tools include: complex mathematical models of electromechanical systems with asynchronous motors with periodic load, mathematical tools for determining the parameters of quasi-steady-state modes, the methodology of optimal design based on the criterion of the maximum efficiency of processes under quasi-steady-state modes of operation. The possibilities, advantages and prospects of using the developed mathemati-cal apparatus for solving a number of problems to improve the efficiency of electric drives of compressor and pumping equipment are demonstrated. It is shown that by taking into account quasi-static processes, the use of complex mathematical models for the optimal design of asynchronous motors with a periodic load provides an in-crease in efficiency up to 8 ... 10%, relative to the indicators of motors that are de-signed without taking into account the quasi-static modes. The areas of intense quasi-steady-state modes are determined using the devel-oped criterion. In these areas, there is a critical decrease in efficiency compared to continuous load operation. A decrease in efficiency is associated with a decrease in the amount of kinetic energy of the rotating parts compared to the amount of electromagnetic energy. In connection with the development of a frequency-controlled asynchronous drive of mechanisms with a periodic load, the relevance of design taking into account the peculiarities of quasi-static has increased significantly. For example, a variable frequency drive of a refrigerator compressor or a heat pump can increase energy efficiency up to 40%, but at low speeds, due to a decrease in kinetic energy, the efficiency can decrease to 10 ... 15%, unless a special design methodology is applied. This problem can be solved by using the complex mathematical modeling tools developed in the article.

Performance of Oil Separator According to the Depth of the Outlet Tube

2015 ◽  
Author(s):  
Seongil Jang ◽  
Joon Ahn ◽  
Si Hyung Lim
Keyword(s):  
Energy Efficiency ◽  
Energy Consumption ◽  
Pressure Drop ◽  
Separation Efficiency ◽  
Lubricating Oil ◽  
Tube Length ◽  
Working Fluid ◽  
Energy Losses ◽  
Refrigeration System ◽  
Outlet Tube

Recent years have witnessed a growing concern over saving energy because of global warming issues and energy price hikes caused by increased oil prices. The need to improve energy efficiency to reduce energy consumption has been raised. Refrigeration systems are also expected to have their energy efficiency improved. A refrigeration system’s the compressor uses lubricating oil. Lubricating oil, along with refrigerant, circulates in a refrigeration system. During this process, the pressure drop increases, and the heat transfer coefficient decreases. Moreover, insufficient lubricant may incur a decrease in performance and damage to a compressor. Therefore, an oil separator is used to separate the lubricant and return it to the compressor. Since an oil separator causes an additional pressure drop, energy consumption should be decreased by increasing the oil separator’s separation efficiency and decreasing the pressure drop. The recent increase in development of large-scale buildings such as skyscrapers and large supermarkets has also increased the demand for large refrigeration machines. At the same time, refrigeration piping is becoming longer, and refrigerant must circulate up to the highest points. A high-pressure head and long piping configuration inevitably increase the quantity of lubricant left on the pipe wall, which in turn increases the loss of lubricants. The increased length and fall height for lubricants to circulate with refrigerant increase the related energy loss. In order to use a compressor in a high-head long-piping refrigeration system, the separation efficiency of the oil separator must be improved. Doing so will also reduce energy losses. Even with an improved separation efficiency, however, an increased pressure drop means additional energy losses. Thus, an oil separator with high separation efficiency and low pressure drop should be designed. So using the Numerical analysis, designed a new oil separator. A series of numerical simulation has been carried out to study peformance of a cyclone type oil separator, which is designed for the compressor of a refrigeration system. Working fluid is R22, which is a typical refrigerant, and mineral oil droplet is supplied. Depending on the outlet tube length, separation efficiency varies from 98.74 to 99.25%. Considering both of the separation efficiency, outlet tube length of the separator has been designed as 158 mm and oil separator length is 310mm.

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