International Journal of Exergy (18 papers in press)
Exergetic Performance Assessment of Solar Driven Combined CO2 Power and Refrigeration System
by Onder Kizilkan
Abstract: The scope of this study is to assess a solar driven combined power-refrigeration system working with carbon dioxide (CO2) through energy and exergy. The system consists of a supercritical Brayton cycle, a transcritical organic Rankine cycle, and a subcritical vapor compression refrigeration cycle. All three subsystems operate with carbon dioxide as the working fluid thanks to its zero-ozone depleting potential and negligible global warming potential. In addition, it is a sustainable working fluid. The combined systems heat energy demand is supplied by using parabolic trough solar collector system. Heat energy demand of organic Rankine cycle is supplied by the rejected heat of supercritical Brayton Cycle; while the refrigeration cycle is driven by the power generated from the organic Rankine cycle. The results show that the energy efficiencies of the Brayton and organic Rankine cycles are found to be 12.9% and 4.47%, respectively, while the performance coefficient of the refrigeration cycle is determined to be 3.35. The energy efficiency of the overall system is determined to be 20.89 %; while the exergy efficiency of the overall system is determined to be 12.95%. The exergy destruction rate of the whole system is calculated as 4891 kW.
Keywords: Solar energy; Combined power-refrigeration system; CO2; Parabolic trough solar collector.
COMPREHENSIVE ENERGY AND EXERGY ANALYSIS ON OPTIMAL DESIGN PARAMETERS OF RECUPERATIVE SUPERCRITICAL CO2 POWER CYCLE
by Veysi Bashan, Emrah Gumus
Abstract: This paper focuses mainly on the supercritical carbon dioxide (s-CO2) cycles performance as regards from the aspect of thermodynamic point of view. Detailed analyses have been performed by considering; the effects of compressor inlet temperature and pressure, turbine inlet temperature, compressor outlet pressure, recuperator pinch temperature and pressure drop. Turbine and compressor efficiencies on cyclic performance were investigated and the results have been given. Exergy efficiency and exergy destruction ratios have been provided with respect to compressor inlet temperature variation. Exergy efficiency decreased when compressor inlet temperature increased. Results show that an optimum input pressure value for high cycle efficiency should be a value between 75 bar and 90 bar. In addition, the pressure ratio should be between 2.5 and 3 in order to have high cycle efficiency and high output power. The effect of the increase in turbine efficiency is more important than compressor efficiency.
Keywords: Brayton cycle; Supercritical; s-CO2; Thermodynamic analysis; Exergy.
Exergy Analysis of a Target Drone Engine: An Experimental Study for TRS18
by Emre Aydin, Onder Turan, Ramazan Kose
Abstract: This article examines exergetic performance of an experimental TRS18 turbojet engine build in Anadolu University Faculty of Aeronautics and Astronautics Test-Cell Laboratory. In the analysis, engine performance parameters (pressure, temperature, fuel and airflow, thrust force) taken from the test-cell measurement devices. At the end of the work, total exergy efficiency of the target drone engine is obtained to be 42%. For TRS18 exergy destruction rate 0.872 MW in the combustor due to physical and chemical exergy destruction. For maximum exergy destruction rate is obtained to be 0.872 MW in the combustor due to physical and chemical exergy destruction, while it is 1.21 MW for the total for the engine. Improvement rates (IP) for combustion chamber, compressor and turbine are calculated to be 0.34 MW, 0.0025 MW and 0.007MW, respectively. As a conclusion, it is expected that results from this study can be useful to future design, research and experimental works related to small turbojets, auxiliary power units and target drone power systems.
Keywords: Turbojet; TRS18; exergy; missiles; propulsion; target drone; auxiliary power unit.
Performance Analysis of an Industrial Steam Power Plant with Varying Loads
by Mehmet Tontu, Mehmet Bilgili, Besir Sahin
Abstract: In this study, energy and exergy analyses of a coal-fired steam power plant with 660-MW capacity were conducted to define the system performance. Analyses were performed with three different rates of operating loads such as 100%, 70% and 40%. Influences of three different loads on the exergy destructions were investigated for all plant components, for example, boiler, turbines, heaters, condensers and pumps. In addition, the exergy efficiency of each component and the overall thermal efficiency of the steam power plant were computed. Heat addition in boilers, heat rejection in condensers, heat transfer in heaters, exergy destructions of all plant components (boiler, turbines, heaters, condensers and pumps), overall thermal and exergy efficiencies of the plant were evaluated in detail. The first-law efficiencies of thermal power plant were determined to be 41.5%, 39.7% and 36.4% % at three different loading capacities such as 100%, 70% and 40% respectively. The second-law efficiencies of thermal power plant were calculated to be 39.1%, 37.4% and 34.3% at loading capacities of 100%, 70% and 40% respectively. According to the obtained results, energy losses mainly happen in the condenser and exergy destructions mainly take place in the boiler. It is found that if the exergy destructions are reduced, the power plant efficiencies are positively affected.
Keywords: Coal-fired power plant; energy efficiency; energy and exergy analysis; exergy destruction; operating load.
Energy and exergy analyses of a combined cycle Kalina and Organic Rankine Cycles using waste heat
by Samad Jafarmadar, Mehdi Mansoury, Shahram Khalilarya
Abstract: In current research, wasted heat from a Reactivity Controlled Compression Ignition (RCCI) engine provides driving energy for Organic Rankine Cycle (ORC) and Kalina cycle. After developing a numerical model for this system, the effects of some decision variables on the energy and exergy efficiencies are studied. The obtained results show that, by adding ORC and Kalina cycles, energy and exergy efficiency values are increased while, brake specific values of Oxides of Nitrogen (NOx) and Carbon Monoxide (CO) emissions are decrease in selected three engine operation modes (4, 9 and 23 bar indicated mean effective pressure) without changing in fuels consumption.
Keywords: RCCI engine; Thermo/ Computational simulation; ORC cycle; Kalina cycle; Energy; Exergy; Emissions.
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.
The Effect of Plate-fin Types on the Thermal-hydraulic and Second Law Performances of a Vehicle Intercooler
by Ahmet Yasin SEDEF, Kemal BİLEN
Abstract: In this study, five fin types namely, plain fins, louvered fins, offset strip fins, wavy fins and perforated fins utilizable 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 intercoolers 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
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 of 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 center and amplitude of oscillation. To see in more detail the influence of oscillation, exergy budgets were analyzed. 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 minimizing the incident angles of solar irradiance around the noon.
Keywords: Attitude angle; Compound parabolic concentrator; Oscillation; Rotation; Exergy.
Controlled Variables Selection Based on Lost Work Minimization
by Ayoub Safari, Reza Eslamloueyan
Abstract: An important phase of control structure design is the selection of controlled variables (CVs), which has a large impact on controllability and resiliency of process units. The economic achievements and process profitability are also well investigated for identifying the effect of CVs selection upon them. But nowadays the tendency for promoting the energy efficiency and sustainable operation of process plants has caused the exergy to be emerged as another criterion for optimization of processes. Hence, the exergy efficiency and lost work are the aspects that the effect of CVs selection upon them are quantified in this study. The basic idea is to keep both economic and exergetic aspects by first optimizing the operating conditions economically, and then choosing the CVs by minimization of the lost work. The study also presents a local approach for simplifying the resulted mixed integer nonlinear programming (MINLP). The proposed method has been applied for selection of exergetically optimal CVs for two different case studies: a chemical reactor system and an ammonia synthesis plant. The best CVs for the CSTR causes 15 kW exergy saving. In the case of ammonia plant, the trade-off between exergy and other controllability criteria such as Niederlinski index or relative gain array (RGA) leads to the CVs that result in 1.2 kW exergy saving.
Keywords: control structure design; controlled variables; lost work minimization; exergy; self-optimizing 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 to ascertain the effects of the operating variables like; extraction pressure ratio, extracted mass rate, turbine inlet temperature, ambient relative humidity, and the mass of injected water on the first and second law efficiency of the gas turbine cycle utilizing Brayton refrigeration cycle for inlet air cooling and water injection for evaporative after cooling. Component-wise local irreversibility of the cycle was computed with the aim to locate the major thermodynamic losses. Combined application of inlet air cooling and evaporative after cooling enhanced cycles 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 as; compressor from 5% to 3%, combustion chamber from 19% to 21%, and exergy destruction in the reheater and turbines were reduced by a percent. 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. A nominal increase of more than 1% in cycle first and second law efficiency was observed when the ambient relative humidity rises from 60% to 80%. 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; Brayton refrigeration cycle; Inlet air cooling; Evaporative after cooling; First law; Second law.
Static and Dynamic Analyses for the Exergetic, Exergoeconomic, and Environmental Assessment of a High-Performance Building
by Raaid Al-Doury, M. Pinar Menguc
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. İt 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 14oC is recommended.
Keywords: Exergy; exergoeconomic; environmental; static analysis; dynamic analysis; low-energy buildings; high-performance buildings.
Entropy generation and irreversibility analysis of a compressible gas flow inside a two-dimensional uniform microchannel
by Khaleel Al Khasawneh
Abstract: The entropy generation for steady, compressible gas flow inside two-dimensional rectangular microchannel at slip flow regime with constant walls temperature has been investigated. The obtained solutions of the flow field are used to evaluate the entropy generation, Bejan number and irreversibility ratio inside the microchannel. Finally, the influence of Knudsen number, Brinkman number, aspect ratio, and pressure ratio on entropy generation rate and irreversibility were presented.
Keywords: rectangular microchannel; irreversibility; entropy generation; compressible gas flow; slip flow; Bejan number.
Exergy analysis and optimisation study of eliminating negative effects of electrical load variations in steam power plants
by Behnam Roshanzadeh, H. Ahmadi-Danesh-Ashtiani
Abstract: Energy consumption is increasing rapidly in the world, and sustainable development has been considered as one of the main goals for the countries. Existence of strong connection between permanent energy supply and sustainable development is undeniable, and power plants are known as the main source of energy supply in the world. In this paper, energetic and exergetic analyses of a steam power plant are performed, and the effect of electrical load variations on energy and exergy efficiencies is studied. Load may decrease due to environmental considerations or operation of the power plant during the night, but the results show that lowering the load has negative effects on energy and exergy efficiencies, specific fuel consumption and CO2 production per unit of power. Moreover, exergy analysis reveals that turbine's control valves play a considerable role in exergy destruction of the power plant. Based on these results, an optimisation is proposed which makes the efficiencies, specific fuel consumption and carbon dioxide production per unit of power almost independent from the load variations.
Keywords: energy; exergy; specific fuel consumption; CO2 production; fossil fuels; sustainable development; optimisation; power plant; control valves.
Investigation of the component exergy efficiencies of a two stage vapour compression cycle
by Sertac Samed Seyitoglu, Ali Kilicarslan
Abstract: The exergy efficiencies of the components (compressor I and II, condenser, evaporator and flash tank) of a two stage vapour compression refrigeration cycle were analysed with respect to evaporator and condenser temperatures for various refrigerants such a R1234ze, R1234yf, R236fa, R227ea and R134a. In the analysis, a computer program was developed by means of engineering equation solver (EES). The exergy efficiencies of components such as the evaporator, condenser, flash tank and compressor I increased while the exergy efficiency of compressor II decreased for the refrigerants used in the cycle for the evaporator temperatures ranging between -30°C to 0°C. The exergy efficiencies of the components such as compressor I and II increased while the exergy efficiency of condenser decreased, and the exergy efficiency of the evaporator stayed almost constant for the refrigerants used in the cycle for the condenser temperatures ranging between 40°C and 55°C. For all refrigerants, the highest exergy efficiencies (around 0.99) were obtained in the flash tank of the cycle while the lowest ones (around 0.15) were obtained in the condenser.
Keywords: vapour compression refrigeration; two-stage; exergy efficiency; R1234ze; R1234yf; R236fa; R227ea; R134a.
Exergy approach for advancing sustainability of a biomass boiler
by Marc Compton, Behnaz Rezaie, Marc A. Rosen
Abstract: An exergy analysis of the district energy plant at University of Idaho, Moscow, Idaho, USA is presented. Exergy flows through the components of the steam cycle through the biomass boiler are quantified to identify major sources of exergy destruction. A mathematical model is developed to determine sources of exergy destruction using measurements taken. The largest sources of exergy destruction are the boiler and furnace at 35% and 33% of the overall exergy losses, respectively, followed by the campus heating equipment at 5.7% and pressure reducing valve (PRV) at 3.5%. Parametric studies reveal that decreasing boiler steam pressure levels to reduce exergy destruction in the PRV results in increased exergy destruction rates in the boiler. Increasing boiler steam pressure levels instead reduces exergy destruction, but has negligible effects on the overall exergy efficiency of the complete cycle. This indicates that the PRV is limiting potential improvements in the boiler exergy efficiency.
Keywords: exergy accounting; wood chips; environmental impact; sustainability; district energy.
Radiative energy and exergy analyses of spectrally-selective surfaces for CSP systems
by Hayder Noori Mohammed, M. Pinar Mengüç
Abstract: This work presents a new methodology for calculations of spectral radiative energy and radiative exergy to evaluate the performances of concentrated solar power (CSP) systems. Spectral radiative properties and the operating temperature of selective surfaces, along with the temperature of the environment, are taken into account in analyses. The fundamental expressions needed for the spectral radiative energy and exergy analyses are introduced first. Then, the two approaches are used to assess the spectral performance of five selective coatings. The spectral analysis is performed in the wavelength range of 250 nm to 20,000 nm, while thermal analysis is carried out for the temperature range of 325 K to 800 K. NREL 6A coating was found to result in the highest radiative energy and radiative exergy for both efficiencies and gains, and for the best thermal stability compared to the other coatings.
Keywords: concentrating solar power systems; selective coating; spectral radiative energy analysis; spectral radiative exergy analysis.
An advanced exergy analysis based on the dysfunction and malfunction methodology for a combined cycle power plant: a Mexican case study
by R. Herrera, F. Méndez
Abstract: The present work studies the irreversibilities and inefficiencies of a combined power plant. The analysis predicts the malfunction and dysfunction distributions for a typical system located in Mexico. The study was conducted by first performing the energy balances of the system using the first law of thermodynamics. The concept of exergetic unit cost was then used to calculate the corresponding exergy destruction of the plant. And finally, structural theory and symbolic thermoeconomic analysis for constructing the malfunction and dysfunction conditions of the plant, showing clearly that for the combined cycle the irreversibilities are basically increased through the dysfunctions. The previous analysis shows the following results: the total increment of the irreversibility for the present cycle is 19879.33 kW, which represents about 3.5% of the fuel exergy required taking into account data of design; and the subsystems with the greatest dysfunctions correspond to both gas compressors.
Keywords: thermoeconomic diagnosis; exergy; cycle power; irreversibility.
Energy and exergy analyses of oxy-fuel combustion power plants with chemical looping air separation
by Shiyi Chen, Ahsanullah Soomro, Jun Hu, Wenguo Xiang
Abstract: This paper integrated chemical looping air separation (CLAS) with oxy-fuel combustion power plants. The results reveal that this type of oxy-fuel combustion technology displays higher energy efficiency than other systems such as post-combustion and oxy-fuel combustion with cryogenic air separation. Energy analysis reveals that the arrangement of a steam generation system based on a conventional pulverized coal power plant is more efficient for oxy-fuel combustion power plants with integrated CLAS technology. Thus, a power generation efficiency of 41.6% can be achieved. However, CO2 compression imposes an additional energy penalty of ~4%, resulting in a net power efficiency of 37.6%. On the other hand, exergy analysis reveals that the most significant exergy destruction portion of the system occurs in the combustion process. CO2 compression also imposes large exergy destruction. Finally, we concluded that the exergy destruction could be alleviated by reasonable arrangement of the heat exchangers.
Keywords: chemical looping air separation; oxy-fuel combustion; CO2 capture; exergy analysis; power plant.