Exergy Diagram Research Articles

Exergo-economic analyses of two building integrated energy systems using an exergy diagram

This paper tackles the exergo-economic assessment of two integrated energy systems that allow the building heating demand to be achieved; a solar assisted heat pump using ice storage and an ambient air heat pump with photovoltaic panels. These systems are firstly described and their performance is discussed for various surfaces of solar collectors and photovoltaic panels. To this purpose, the total electrical energy consumption and the total cost are estimated while the total exergy destruction is divided according to renewable and non-renewable energy flows. This latter, which is really paid by the user, then serves as the exergy performance indicator. The ambient air heat pump with photovoltaic panels is found as the optimal solution from energy and economic perspectives while the solar assisted heat pump using ice storage provides the best exergy performance. To help analyze results, the system performance is illustrated by using an exergy diagram, where energy terms are shown on the abscise axis and energy quality factors are on the ordinate axis. This 2-D graphical representation, which can consider the results from dynamic analyses, helps understand the system operation and performance, and reveals the sources (components), the reasons (energy loss and/or energy potential factor degradation) and the nature (renewable, non-renewable flows) of irreversibility.

Development of an exergy diagram for integrated energy systems illustrating the irreversibility along energy paths

This paper tackles the development of an exergy diagram, adapted from the heat (Q)-Carnot factor (θ) 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 irreversibility sources (energy loss, 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 renewable and non-renewable sources. Thus, it permits the electricity use, which is really paid, to be emphasised, and the overall behaviour to be better understood. As well, the diagram is developed to illustrate the results of dynamic analyses by considering environment reference state variations. To this aim, the graphical construction has been applied to individual units and then, step by step, to a solar assisted heat pump using ice storage.

Development of an exergy diagram for integrated energy systems illustrating the irreversibility along energy paths

This paper tackles the development of an exergy diagram, adapted from the heat (Q)-Carnot factor (θ) 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 irreversibility sources (energy loss, 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 renewable and non-renewable sources. Thus, it permits the electricity use, which is really paid, to be emphasised, and the overall behaviour to be better understood. As well, the diagram is developed to illustrate the results of dynamic analyses by considering environment reference state variations. To this aim, the graphical construction has been applied to individual units and then, step by step, to a solar assisted heat pump using ice storage.

Exergy analysis of a MSF desalination plant in Yanbu, Saudi Arabia

This study focuses on determining exergy destruction in a MSF Desalination plant located in Yanbu Saudi Arabia. A non-iterative code in MATLAB is developed to perform mass, energy, and exergy balances around the plant at normal load of distillate production (~159kg/s) at 22.9°C in the winter and at peak load (~200kg/s) at 35.2°C in the summer. Exergy flowrates are calculated and exergy destruction around each of the equipment is estimated. An exergy diagram is prepared to indicate equipment with highest exergy destruction. As expected the major exergy destruction occurs at the Heat Recovery Section (74.9%). The simulation results indicate that as the number of stages increase from 25 to 31 the percent exergy destruction drops from 74.9% to 69.2%. The second law efficiency of the plant is 3.22% and 2.84% at normal and peak load respectively. This efficiency is quite low and several measures can be done to improve it by decreasing inlet exergy to the plant and controlling exergy losses during process operation. Two variations in MSF designs i.e., Once Through (MSF-OT) and Brine Mixing (MSF-M) are considered. It is found that the design of MSF-M is a viable option to minimize exergy losses.

Energy and Exergy Analysis of Kalina Cycle for the Utilization of Waste Heat in Brine Water for Indonesian Geothermal Field

The utilization of waste heat in a power plant syst em—which would otherwise be released back to the environment—in order to produce additional power increases the eff iciency of the system itself. The purpose of this s tudy is to present an energy and exergy analysis of Kalina Cycle System ( KCS) 11, which is proposed to be utilized to genera te additional electric power from the waste heat contained in geo thermal brine water available in the Lahendong Geot hermal power plant site in North Sulawesi, Indonesia. A modeling application on energy and exergy system is used to study the design of thermal system which uses KCS 11. To obtain the maximum power output and maximum efficiency, the system is optimized based on the mass fraction of working flu id (ammonia-water), as well as based on the turbine exhaust pressure. The result of the simulation is the optim um theoretical performance of KCS 11, which has the highest possible power output and efficiency. The energy flow diagra m and exergy diagram (Grassman diagram) was also presented for KCS 11 optimum system to give quantitative information regarding energy flow from the heat source to s ystem components and the proportion of the exergy input d issipated in the various system components.

Thermodynamic Analysis of Blast Furnace Slag Waste Heat-Recovery System Integrated with Coal Gasification

The blast furnace (BF) slag waste heat was recovered by an integrated system stage by stage, which combined a physical and chemical method. The water and coal gasification reactions were used to recover the heat in the system. Based on the first and second law of thermodynamics, the thermodynamic analysis of the system was carried out by the enthalpy–exergy diagram. The results showed that the concept of the “recovery-temperature countercurrent, energy cascade utilization” was realized by this system to recover and use the high-quality BF slag waste heat. In this system, the high-temperature waste heat was recovered by coal gasification and the relatively low-temperature waste heat was used to produce steam. The system’s exergy and thermal recycling efficiency were 52.6% and 75.4%, respectively. The exergy loss of the integrated system was only 620.0 MJ/tslag. Compared with the traditional physical recycling method producing steam, the exergy and thermal efficiencies of the integrated system were improved significantly. Meanwhile, approximately 182.0 m3/tslag syngas was produced by coal gasification. The BF slag waste heat will be used integrally and efficiently by the integrated system. The results provide the theoretical reference for recycling and using the BF slag waste heat.

Exergy analysis of micro-organic Rankine power cycles for a small scale solar driven reverse osmosis desalination system

Exergy analysis of micro-organic Rankine heat engines is performed to identify the most suitable engine for driving a small scale reverse osmosis desalination system. Three modified engines derived from simple Rankine engine using regeneration (incorporation of regenerator or feedliquid heaters) are analyzed through a novel approach, called exergy-topological method based on the combination of exergy flow graphs, exergy loss graphs, and thermoeconomic graphs. For the investigations, three working fluids are considered: R134a, R245fa and R600. The incorporated devices produce different results with different fluids. Exergy destruction throughout the systems operating with R134a was quantified and illustrated using exergy diagrams. The sites with greater exergy destruction include turbine, evaporator and feedliquid heaters. The most critical components include evaporator, turbine and mixing units. A regenerative heat exchanger has positive effects only when the engine operates with dry fluids; feedliquid heaters improve the degree of thermodynamic perfection of the system but lead to loss in exergetic efficiency. Although, different modifications produce better energy conversion and less exergy destroyed, the improvements are not significant enough and subsequent modifications of the simple Rankine engine cannot be considered as economically profitable for heat source temperature below 100 °C. As illustration, a regenerator increases the system’s energy efficiency by 7%, the degree of thermodynamic perfection by 3.5% while the exergetic efficiency is unchanged in comparison with the simple Rankine cycle, with R600 as working fluid. The impacts of heat source temperature and pinch point temperature difference on engine’s performance are also examined. Finally, results demonstrate that energy analysis combined with the mathematical graph theory is a powerful tool in performance assessments of Rankine based power systems and permits meaningful comparison of different regenerative effects based on their contribution to systems improvements.

Performance and parametric investigation of a binary geothermal power plant by exergy

Exergy analysis of a binary geothermal power plant is performed using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy diagram, and compared to the energy diagram. The sites with greater exergy destructions include brine reinjection, heat exchanger and condenser losses. Exergetic efficiencies of major plant components are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the plant are 4.5% and 21.7%, respectively, based on the energy and exergy of geothermal water at the heat exchanger inlet. The energy and exergy efficiencies are 10.2% and 33.5%, respectively, based on the heat input and exergy input to the binary Rankine cycle. The effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained.

Energy and exergy analyses in a thermal process of a production line for a cement factory and applications

The objective of this study is to determine the actual energy losses by performing energy and exergy analyses and to evaluate energy and exergy efficiency in each process for the cement factory. In these analyses, for each process energy and exergy diagrams have been constituted on the production line. Efficiencies (energy/exergetic) of the processes for the raw mill, the rotary kiln, the trass mill and the coal mill on the production line have been found as 84%/25%, 61%/49%, 74%/13%, 74%/18%, respectively. In addition, some suggestions concerning the reduction of energy losses have been proposed.

Understanding energy and exergy efficiencies for improved energy management in power plants

An extensive overview is provided of various energy- and exergy-based efficiencies used in the analysis of power cycles. Vapor and gas power cycles, cogeneration cycles and geothermal power cycles are examined, and consideration is given to different cycle designs. The many approaches that can be used to define efficiencies are provided and their implications discussed. Improvements of the management of energy in power plants that stem from understanding the efficiencies better are described. Examples are given to illustrate the efficiencies and their differences, with the results presented using combined energy and exergy diagrams. It is anticipated that the results will provide a convenient and practical tool for engineers and researchers dealing with the analysis, design, optimization and improvement of power cycles.

Energy and exergy analysis of a ground source (geothermal) heat pump system

Ground source heat pumps (GSHPs), often referred to as geothermal heat pumps (GHPs), offer an attractive option for heating and cooling residential and commercial buildings owing to their higher energy efficiency compared with conventional systems. GSHPs have been used for four years in the Turkish market, although they have been in use for more years in developed countries. The purpose of this study is to present an energy and exergy analysis of a GSHP system with a 50 m vertical 1.25 in. nominal diameter U-bend ground heat exchanger. This system was applied to a 65 m 2 room in the Solar Energy Institute, Ege University, Izmir, for the first time at the university level in Turkey. The Institute, built in 1986, has a livable floor area of 3000 m 2 and uses passive solar techniques. The heating and cooling loads of the room studied were 3.8 and 4.2 kW at design conditions, respectively. The system was commissioned in May 2000, and performance tests have been conducted since then. The exergy transports between the components and the consumptions in each of the components of the GSHP system were determined for the average measured parameters obtained from the experimental results in February 2001. The exergy diagram (the Grassmann diagram) was also presented for the GSHP system studied to give quantitative information regarding the proportion of the exergy input that is dissipated in the various system components.