International Journal of Exergy (12 papers in press)
ENERGETIC AND EXERGETIC ANALYSIS OF A MULTI-STAGE TURBINE, COAL-FIRED 173 MW POWER PLANT
by Joshua Clay, James Mathias
Abstract: This paper reports the results of an energy and exergy audit on a 173 MW pulverized coal fired, subcritical steam power plant unit in the Midwestern United States. Energy and exergy analysis determined the thermal efficiency and identified the greatest sources of irreversibilities and also mapped the flow of energy and exergy throughout the cycle. The overall cycle energetic and exergetic efficiencies were found to be 32.8% and 33.7%, respectively. The largest source of exergy destruction in the cycle was found to be within the steam generator. Options explored to increase the exergetic efficiency were to improve the steam turbines to operate with higher temperature steam, to bring into the condenser water from the lake that is closer to the ambient temperature, and to replace a valve after the boiler feedwater pump with a hydro-turbine to produce some power.
Keywords: Exergy; power generation; heat exchangers; turbines.
Modeling of Energy and Exergy Efficiencies of a Horizontal Axis Wind Turbine Based on the Blade Element Momentum Theory at Different Yaw Angles
by Ali Khanjari, Esmail Mahmoodi, Ali Sarreshtehdari, Mahmood Chahartaghi
Abstract: In this study, energy and exergy analyses of a horizontal axis wind turbine are presented based on blade element momentum theory (BEM) under different yaw angles. Velocity, temperature, pressure and specific humidity are considered as metrological parameters. The wind flow is assumed steady, and exergy destruction are taken to account for wind blades. The computations are validated by wind tunnel data from the MEXICO wind turbine experiment. The results show that the BEM can predict the axial and tangential forces in different spans. It is interesting to note that by increase of yaw angle the exergy destruction and exergy flow decrease inversely due to rise of entropy generation in rotor plan. Also, by increasing of yaw angle from 0ᴼ to 15ᴼ at wind speeds from 10 to 24 m/s, the energy and exergy efficiencies see a slight decrease in comparison with yaw angle of 0ᴼ. At wind speeds from 18 to 24 m/s, under yaw angle of 30ᴼ, by rising the wind speed, energy and exergy efficiencies increase due to reduction of the axial force on the wind turbine rotor. At wind speed 24 m/s under the yaw angle 30ᴼ, energy and exergy efficiencies increase 14.84% and 14.89%, respectively in comparison with the yaw angle of 0ᴼ.
Keywords: Yaw angle; Energy and exergy efficiencies; MEXICO wind turbine; BEM modeling.
Entropy Generation and Irreversibility Analysis for Two-Dimensional Microhartamann Gas Flows
by Khaleel Al Khasawneh, Duaa Kharouf
Abstract: In this study, the entropy generation for steady, compressible gas flow inside two-dimensional rectangular microchannel at slip flow regime with constant walls temperature under the influence of magnetic field was analytically investigated. The obtained velocity and temperature distributions were used in this study. The influences of changing several parameters; such as, Knudsen number, Brinkman number, Hartmann number, aspect ratio, electric field to magnetic field ratio and pressure ratio were investigated. A finite volume method was used for numerical validation. The obtained results showed that the total entropy generation decreases in the direction toward the wall of the microchannel and increases along the flow direction. The total entropy generation increases as Knudsen number, aspect ratio, electric to magnetic field ratio, pressure ratio and Brinkman number increases. Entropy generation due to the magnetic field effect had the maximum value except the region near the walls of the microchannel. Entropy generation due to the friction effect was low along the centreline and increase toward the walls, while entropy generation due to the heat transfer effect was very low as compared to the friction effect and magnetic effect.
Keywords: microchannel; entropy generation; irreversibility; Bejan number; slip flow; Knudsen number; magnetic field; Hartmann number.
Development of an exergy diagram for integrated energy systems illustrating the irreversibility along energy paths
by Etienne Saloux, Mikhail V. Sorin, Alberto Teyssedou
Abstract: This paper deals with the development of an exergy diagram, adapted from the heat (Q) - Carnot factor (theta) one, where energy quality factors are given in the ordinate axis while energy terms constitute the abscise axis. Energy and exergy balances are then shown and the nature of the sources of irreversibility (energy loss or/and energy potential degradation) is revealed, contrary to traditional Grassman diagrams. In this paper, the graphical construction is extended to integrated energy systems where energy paths are differentiated according to their renewable (solar, etc.) and non-renewable (electricity, etc.) sources. Thus, it permits the use of electricity, which is really paid, to be emphasized, and the overall behavior to be better understood. As well, the diagram is developed to illustrate the results of dynamic analyses; in particular, it takes into account the variations of the environment reference state. To this aim, the graphical construction has firstly been applied to individual units and then, step by step, to a solar assisted heat pump using ice storage. Finally, the calculation of mean values for considering the results of dynamic analyses is discussed.
Keywords: Exergy; exergy diagram; irreversibility; integrated energy systems; exergy reference environment; exergy efficiency; solar collectors; heat pump; thermal energy storage.
Techno-environmental analysis of a parabolic dish assisted recompression with and without reheat s-CO2 Brayton cycle
by Muhammad Abid, Muhamamd Khan, Tahir Ratlamwala
Abstract: This comparative research concentrates on the integration of a parabolic dish system with two types of supercritical CO2 Brayton cycles, (a recompression with reheat and without reheat) solar assisted Brayton cycles. The performance of both systems has been numerically analyzed by comparing their net power output, overall energetic and exergetic efficiencies, exergo-environmental analysis and individual components exergy destruction rates by varying several input parameters (direct normal irradiance (DNI), ambient and inlet temperature of receiver, turbine and main compressor inlet temperatures, pressure ratio). The engineering equation solver (EES) is employed for numerical modeling. Super critical CO2 has been circulated into the Brayton cycle loop, whereas, fresh water is used as a heat transfer fluid in solar collector. The simulations has been performed for Pakistan (Southern Punjab, latitude for the location is 29o 25 / 5.0448 N while longitude is 71o 40 / 14.4660 E), where, DNI is higher and almost equal to1000 W/m2. The simulation results show that the overall energy and exergy efficiencies of recompression with reheat system are considerably higher (almost 11.50%) than the recompression without reheat system. The thermal efficiency of reheat system is 47.75% whereas, 45.01% efficiency has been found for the later system. The overall first and second law efficiencies are 30.37% and 32.70% for reheat system, whereas, they are found to be 27.26% and 29.6% for the system without reheat. Moreover, receiver collector has a major role in exergy destruction rate, nearly 40 % of the whole system and slightly decreases with an increase in minimum cycle temperature (compressor inlet temperature). The increase in minimum cycle temperature and pressure ratio increases the overall efficiency of the reheat system but for without reheat system it reduces due to the increased main compressor work. The exergo-environmental impact index and impact factor are reduced from 2.217 to 1.876 and 0.689 to 0.652, respectively.
Keywords: Parabolic dish system; s-CO2; Brayton cycle; Energy and Exergy efficiency; exergo-environmental.
Energy and exergy analyses of a solar powered multi-effect cooling cycle
by Abdul Khaliq, Esmail M. A. Mokheimer, Rajesh Kumar
Abstract: High demand of air-conditioing and refrigeration along with deep-freezing is persistently increasing across the globe, mainly due to climate change and improvement in living standard. Solar thermal energy is one of the viable options for cooling production because of the synchronization of peak cooling demand and maximum available solae energy in the summer afternoons. In this context, this study aims to assess the thermodynamic performance of a novel solar powered multi-effect cooling cycle through the cascaded utilization of energy and exergy.The effects of parameters such as; direct normal irradiation (DNI), turbine inlet temperature, turbine back pressure, and evaporator temperature of ERC were ascertained on the energetic and exergetic performance of the cycle. Exergy destruction occurs throughout the plant components is quantified and illustrated using an exergy flow diagram, and compared to the corresponding energy flow diagram. The exergy efficiency of the cycle was significantly less than its corresponding energy efficiency. Computational analysis further revealed that the maximum exergy losses of more than 34% occur in the solar field followed by 7.25% and 6.75% in the components of ARC and CRC, respectively. Percentage of these exergy losses indicates the sites where the efforts should be made to improve the real performance of proposed cooling cycle.
Keywords: solar; refrigeration; efficiency; ejector; absorption.
EXTREMELY SMALL ENERGY REQUIREMENT BY POLIOVIRUS TO PROLIFERATE ITSELF IS THE KEY TO AN OUTBREAK OF AN EPIDEMIC
by Mustafa Ozilgen, Sevgi Eylul Ferahcan, Ayse Selcen Semercioz
Abstract: Poliovirus is an RNA virus which proliferate itself in the intestines. Thermodynamic assessment of the proliferation process has been assessed after calculating the energy and exergy of the RNA with the group contribution method. Mass, energy and exergy balances are performed to calculate the energy and exergy exploited from the host cell as 4.65x10-19 and 3.35x10-17 kJ, respectively. In a typical epidemic in 1988, 350,000 cases were reported, implying that the total of 1.627x10-9 kJ of energy 1.174x10-7 kJ of exergy were exploited from them during the epidemic. Although the energy and exergy efficiency of virus generation in a host cell were very small, e.g., 1.376 x 10-6 and 9.78 x 10-5, respectively; energy and exergy employed in this process was very small implying that extremely small energy and exergy requirement by poliovirus to proliferate itself is the key to an outbreak of an epidemic.
Keywords: Virus; epidemic; energy; exergy; devastation of the host cells.
The effect of plate-fin types on the thermal-hydraulic and second law performances of a vehicle intercooler
by Ahmet Yasin Sedef, Kemal Bilen
Abstract: In this study, five fin types namely, plain fins, louvered fins, offset strip fins, wavy fins and perforated fins utilisable in the design of vehicle intercoolers are theoretically examined. The aim is to see their effect on the thermal-hydraulic and second law performances of the intercooler. In the analysis, Ford Cargo TCI engine specifications are used to establish intercooler's operating conditions. Totally, 15 different fin geometries of five types are analysed. The results show that offset strip 1/9-24.12 fin provide highest thermal performance, with 99.8% effectiveness and 33.23 kW heat transfer while causing 12.57% pressure loss in the hot fluid. Conversely, plain 9.03 fin maintains the least pressure drop, 0.53%, but with effectiveness of 43.3%. For rational efficiency, plain fins possess highest values ranging 0.37 to 0.405. Moreover, as the hot fluid flow rate increases flow imbalance diminishes and thus rational efficiency increases up to a certain point.
Keywords: intercoolers; plate-fin heat exchangers; heat transfer; effectiveness; pressure drop; entropy generation; exergy analysis.
Influence of rotational oscillation on exergy efficiency of compound parabolic concentrator
by Yoshiki Nishi, Yusuke Tokui
Abstract: This study examined the influence of the oscillatory time variation in the attitude angle of compound parabolic concentrator (CPC) on the thermal performance of CPC. Two experimental CPC models were constructed, one of which was fixed and the other one was provided with two rotational motions (pitch and roll) under sunlight irradiance. By comparing the temperatures of thermal collectors in the two CPCs, the influence by the oscillation was evaluated. Experimental results demonstrated that the oscillation can decrease or increase the temperatures depending on the centre and amplitude of oscillation. To see in more detail the influence of oscillation, exergy budgets were analysed. It is found that the rotational motions of CPC tend to decrease the exergy efficiency by a few percentages, whereas it can improve the efficiency by a few percentages by minimising the incident angles of solar irradiance around the noon.
Keywords: attitude angle; compound parabolic concentrator; CPC; oscillation; rotation; exergy.
Controlled variables selection based on lost work minimisation
by Ayoub Safari, Reza Eslamloueyan
Abstract: An important phase of control structure design is the selection of controlled variables (CVs). The economic criteria and process profitability are extensively used for identifying the CVs selection. Nowadays, the exergy is another criterion for process optimisation. In this study, a novel technique is developed for incorporating the exergy efficiency and lost work in CVs selection during plant-wide control structure design. The basic concept behind the proposed method is to select the set of CVs that leads to the least process lost work when a process under feedback control encounters disturbances. In this way, at first the optimum operating conditions are determined by an economic objective function, and then CVs are specified by minimisation of the plant lost work. The study also presents a local approach for simplifying the solution of the obtained mixed integer nonlinear programming (MINLP).
Keywords: control structure design; controlled variables; CVs; lost work minimisation; exergy; self-optimising control.
A theoretical investigation on exergy analysis of a gas turbine cycle subjected to inlet air cooling and evaporative after cooling
by Abdul Khaliq, Suleman Haroon, M.A. Habib
Abstract: A thermodynamic investigation based on energy and exergy analyses was carried out for the gas turbine cycle utilising Brayton refrigeration cycle for inlet air cooling and water injection for evaporative after cooling. Combined application of inlet air cooling and evaporative after cooling enhanced cycle's exergetic output from 29% to 33% and reduces the exergy loss from 24% to 3%. This arrangement in gas turbine also changed the percentage of local irreversibility of the cycle components considerably. At a given ambient relative humidity of 60%, proposed modification in basic gas turbines boost the cycle first and second law efficiency by 3.7% and 4.2%, respectively. The results obtained reveal that application of reverse Brayton refrigeration cycle for inlet air cooling along with evaporative after cooling could be a most promising option for performance enhancement of gas turbines operated in hot and humid climatic regions.
Keywords: gas turbine; inlet air cooling; evaporative after cooling; first law; second law; Brayton refrigeration cycle.
Static and dynamic analyses for the exergetic, exergoeconomic and environmental assessment of a high-performance building
by Raaid Rashad Jassem Al Doury, M. Pinar Mengüç
Abstract: This paper presents an exergetic, exergoeconomic and environmental assessment of an existing high-performance building using both static and dynamic analyses. The IEA ECBCS framework is adopted for exergy analysis, whereas the SPECO method is used to implement the exergoeconomic analysis. The environmental impact is considered in a quantitative fashion. It is noted that a dynamic analysis using average hourly temperatures is preferred over a static analysis. However, if a simpler static analysis is to be used, an annual average temperature needs to be identified for specific climate zone and building type. For Istanbul, an average temperature of 14°C is recommended.
Keywords: exergy; exergoeconomic; environmental; static analysis; dynamic analysis; low-energy buildings; high-performance buildings.