Giuma Fellah

In this work, the total exergy destruction in a component of a system is split into endogenous and exogenous parts to assess the thermodynamic performance of a gas turbine unit, which is located in the SARIR power station, in Libya. The power station consists of three units, Siemens gas turbines, type SGT5-PAC 4000F, each unit with 285 MW rated capacity with the three ordinary components (air compressor, combustion chamber and turbine) is considered.
The total exergy destruction is not only due the deficiency of that component, but also occurs by the deficiencies of the remaining components. The endogenous exergy destruction takes place when the other components of the system work perfectly without any exergy destruction, and the considered component works with its normal condition. Splitting the total exergy destruction into endogenous and exogenous parts must be considered when decision is made to enhance the thermodynamic performance of a system. The exergy destruction can be split into an avoidable and unavoidable exergy destruction, each of them can be split into an endogenous and exogenous exergy destruction. Only part of the exergy destruction can be avoided, the remaining cannot be avoided due to economic issues and technological limit.
The results show that for the compressor, 82% of the exergy destruction is endogenous and 18% is exogenous exergy destruction, for the gas turbine, 96.6% of the exergy destruction is endogenous and 3.4% is exogenous exergy destruction and for the combustion chamber, 69.60% of the exergy destruction is endogenous and 30.40% is exogenous exergy destruction. That is, 75.82 MW of the exergy destruction in the combustion chamber is exogenous, hence the performance of the other two components (compressor and turbine) must be improved or replaced to elevate the thermodynamic performance of the combustion chamber. The later finding cannot be recognized without splitting the total exergy destruction into endogenous and exogenous exergy destruction.

The purpose of this work is to contribute to the issue of recovering cold exergy from cold states. Material at temperatures less than atmospheric temperature such as liquefied natural gas (LNG), may possess an abundant amount of so called cold exergy, which is usually not recognized nor recovered during the regasification process. Derna power plant is selected to explore this issue, by utilizing the cold exergy which is possessed by the Liquefied Natural Gas (LNG), here, the gasification process takes place by utilizing the heat energy which is rejected by the condenser of the selected steam power station. Natural gas is compressed and liquefied to lower the specific volume for shipping at extremely low temperatures, around 110K. The latent heat (heat of evaporation) of methane (the major component of the natural gas) at 0.1MPa is 512 kJ/kg. Cold exergy could be recovered by transferring heat from the atmosphere (from seawater for instance) to a heat engine, and rejecting heat to a sink, for instance to the LNG during the evaporation process. The results show that 40.84% of the input physical exergy of the LNG is destroyed during the conventional regasification process. The net power output of the selected steam power plant could be raised by 12.1 % when connected with the regasification cycle

It is well known that carbon dioxide is one of the major pollutant agents to our environment, and its emission must be limited. Power plants mostly burn fossil fuel to generate electricity and hence produce a tremendous amount of pollutants such as CO2. In this work, a simple gas-turbine cycle is connected with a steam cycle unit to form a combined gas-steam power unit and hence, improves its thermal performance. Since testing at large scale is so costly, it is likely to use process simulation software to evaluate such processes. Post-combustion removal is one possible way to lower CO2 emissions from industrial plants, including power stations. Instead of discharging CO2 which is contented in the exhaust gas to the atmosphere, the carbon dioxide can be seized, transported and stored securely in a number of places, including geological formations, and depleted oil and gas reservoirs. The most favorable method for CO2 removal is by absorption in an amine-based solvent followed by desorption. The simplest and most used amine for CO2 removal is MEA (Mono-Ethanol Amine). The net power output of the selected simple-gas turbine unit for the analysis is 287.4 MW with thermal efficiency of 28.98%. Modeling and simulating this power unit indicates 40.7851 kg/s of CO2 is
emitted to the atmosphere. The aim of this work is to reduce this amount and contribute to the clean environment issues. According to this study, the rate of the emitted CO2 is reduced to 0.3888 kg/s, and to 0.4085 kg/s, when CO2 removal cycle is connected to the simple gas turbine cycle and to the combined gas-steam cycle, respectively.

Exergy analysis is the traditional approach to evaluate the performance of thermal systems from thermodynamic point of view, where exergy destruction (irreversibility) is determined to assess the deficiency of the plant's components. However, the costs of those deficiencies can be determined only through thermoeconomic analysis. The aim of this work is to simulate the performance of multi-stage desalination unit from thermoeconomic point of view. The Specific Exergy Costing (SPECO) method is adopted for simulation. The results show that the minimum unit cost of the product water is obtained for the three stages unit, the obtained result is in good agreement with that given in the literature. The minimum unit cost is found at compressor efficiency of 91% for the singlestage and at 82% for the multi-stages. It is found that, by reducing the pressure ratio of the compressor in the three-stage system from 2.15 to 1.6, the specific power consumption can be reduced by 40.5%., and the unit product cost by 2.75%.

The objective of the current work is to provide a deeper awareness about the thermodynamic performance and cost effectiveness of thermal systems. To seek the potential for improvement, the avoidable part of the exergy destruction and the avoidable investment cost must be identified. The exergy destruction and the associated investment cost are split into avoidable and unavoidable parts. Modified exergatic efficiency (effectiveness) and a modified exergoeconomic factor are introduced and
compared with the corresponding conventional effectiveness and exergoeconomic factor. A simple gas turbine cycle is taken as an example to explore the advantages of such approach to give a rational judgment of the performance of thermal systems. The results show that, for the whole plant the conventional exergoeconomic factor is calculated as 16.15% and the conventional effectiveness as 24.67%, while the modified exergoeconomic factor is calculated as 36.85%, and the modified effectiveness as 40.14%

Exergoeconomic (thermoeconomic) analysis is performed on Alkhoms steam power plant. The nominal power of the plant is 120 MW. The analysis is based on real-time data and performed for three different loads. The main factor of load variation is the variation of the steam mass flow rate. These loads are 120 MW (full load), 60 MW (part load), and 100 MW (real-time operation). It is worth to mention that high-pressure heaters are out of service these days. A systematic and general methodology for defining and calculating exergetic efficiencies, exergy destruction, and exergy related to costs in thermal systems is presented. The methodology is based on the Specific Exergy Costing (SPECO) method. Results of the exergy analysis showed the exergetic efficiency (effectiveness) increases from 34.74% at the real-time operation to 40.96% at full operating load, and hence the ratio of the total exergy destruction to fuel input exergy decreases from 64.46% at a real-time operation to 59.6 at part...

Zuara desalination plant is installed and operated by General Electricity Company of Libya (GECOL), It is located 10 km east of Zuara city, and 150 km west of Tripoli, the Libyan capital. The plant is based on Multi-Effect evaporation technology. It consists of three units with total capacity of 40000 m 3 /day. The plant was supplied and constructed by French company SIDEM and started producing distillate water in March 2006. In this paper, the plant performance from thermodynamics point of view is performed. The analysis is based on design data, performance test data, and real time data. Both thermo-mechanical (physical) and chemical exergies, are considered. Using performance test data, the results show that the de-superheating process contributes to about 44% of the total exergy destruction, while the nine effective cells destroy about 26%, the distillate condenser destroys 22%, and the seawater pre-heaters destroy 8% of the total exergy destruction in the unit. The analysis show...

Exergoeconomic (thermoeconomic) analysis is performed for the unit GT14 of South Tripoli (Libya) gas turbine power plant. The designed electrical power of the unit is 100MW (based on ISO conditions). The full operating load (electrical) is 85MW. The analysis is based on real time data and performed for three different loads; those are 85% (full operating load) 60%, and 40% of design load. A systematic and general methodology for defining and calculating exergetic efficiencies, exergy destruction and exergy related costs in thermal systems is presented. The methodology is based on the Specific Exergy Costing approach. Results of exergy analysis show the exergetic efficiency increases from 20.54% at 40% design load to 29.12% at full operating load, and hence, the ratio of the total exergy destruction to fuel input exergy decreases from 61.03% at 40% design load to 48.63% at full operating load load, and the ratio of exergy loss with the exhaust gases to the input fuel exergy slightly ...

The aim of this work is to demonstrate the power of excel software in simulating the performance of thermal systems, a steam power plant is taken as an example for the demonstration. The idea is to show how one can analysis and simulate the performance of thermal systems within Excel environment. The thermodynamic properties are obtained by the excel tools developed for thermodynamics. The tools developed by a University of Alabama research team. The platform of these tools is the Microsoft Excel. The obtained properties using these tools are tested and found in good agreements with other sources. Energy balance is performed on each heater to find the bleeding mass flow rates. The obtained equations are solved simultaneously by using excel software. The functions MMULT and MINVERSE are used to multiply the matrices and to find the inverse and hence the mass flow rates. The solver facility can be used to optimize the thermal efficiency and/or the plant irreversibility.

In the hot climate zones where the ambient temperature may reach 50oC, the net power output of the gas turbine cycles is extremely deteriorated. In order to reduce the effect of the high ambient temperature, specifically in the summer session, the inlet air could be cooled to the standard temperature of 15oC.The goal of this article is to determine from the thermodynamic point of view, the feasibility of utilizing an absorption refrigeration cycle to enhance the thermodynamic performance of the combined cycle when the ambient temperatures vary between 15oC and 50oC.This article proposes utilizing the exhaust gasses of the gas turbine cycle to power both a steam cycle and ammonia-water absorption refrigeration cycle in a combined power-refrigeration cycle. The task of the absorption refrigeration cycle is to keep the inlet air temperature at 15oC.It is found that the net power output and the thermal efficiency of the power-refrigeration cycle could be increased by 30.16% and 3.12% re...

Exergoeconomic (thermoeconomic) analysis is performed for the unit GT14 of South Tripoli (Libya) gas turbine power plant. The designed electrical power of the unit is 100MW (based on ISO conditions). The full operating load (electrical) is 85MW. The analysis is based on real time data and performed for three different loads; those are 85% (full operating load) 60%, and 40% of design load. A systematic and general methodology for defining and calculating exergetic efficiencies, exergy destruction and exergy related costs in thermal systems is presented. The methodology is based on the Specific Exergy Costing approach. Results of exergy analysis show the exergetic efficiency increases from 20.54% at 40% design load to 29.12% at full operating load, and hence, the ratio of the total exergy destruction to fuel input exergy decreases from 61.03% at 40% design load to 48.63% at full operating load load, and the ratio of exergy loss with the exhaust gases to the input fuel exergy slightly ...

Numerical simulation is implemented to evaluate the effect of sand mold thicknesses on the solidification time for pure iron. The finite volume technique; explicit time scheme; enthalpy method is adopted for the simulation. The thermo-physical properties of the mold and the metal are assumed to be temperature independent, however, for the metal, the properties can be different in the solid and liquid phases. The outer surfaces of the mold transfer heat by convection to surroundings. It is found that the solidification time can be controlled by the mold thickness only when the ratio of the sand size to metal size is smaller than or equal to a definite value. This ratio can be considered as the critical one, beyond which the mold thickness has no effect on the solidification time.

This paper explores numerically the internal entropy generation and hence the internal irreversibility for an annular sensible heat thermal energy storage unit. Finite volume technique is adopted for the analysis with fully implicit numerical technique. A storage unit composed of two horizontal concentric cylinders with different diameters is considered for the analysis. The space between the two cylinders is filled with the storage material. Two different materials with different thermal diffusivities are considered for the analysis; those are cast iron and brick. The outside surface of the storage unit is insulated, while the inside surface exchanges heat by convection with air. The only mode of heat transfer inside the storage material is the heat transfer by conduction. The problem is a two-dimensional one. It is found that during the transient period, both the storage capacity and the internal entropy generation in cast iron are substantially higher than those in brick. However...

The aim of this paper is devoted to assess the thermodynamics performance of unit one of Zawia city desalination plant. The plant was supplied by French company SIDEM and constructed by Turkish ENKA TEKNIK Company started producing distillate water in July 2010. The plant is based on multiple-effect distillation with thermal vapor compression technology and consists of four units with total capacity of 80,000 m 3 /day. Design and real time data in summer and winter sessions are adopted for the analysis. The analysis reveals that the thermal vapor compression and the effects are the main source of exergy destruction. The major exergy destruction takes place during the thermal vapor compression processes, which contribute to an average value of 45.76%% of the consumed exergy. Losses contribute to about 22.25% of the consumed exergy. Effects, pre-heaters and final condenser contribute on average to 16.13%, 7.84%, and 8.02% of the consumed exergy, respectively.

Exergy analysis is performed for unit three of Zawia combined cycle power plant. The plant is one of the largest power plants in Libya; located near Zawia city, 50 km West of Tripoli. The nominated power is 1,440MW of electrical power. Unit three consists of two units of gas turbines, two heat recovery steam generators and one unit of the steam turbine. It has a total capacity of 450MW. The objective of the simulation is to predict the contribution of unit's components to exergy destruction and the overall performance from the thermodynamic point of view. Design data and real-time data are adopted for the analysis. The results show that there is a lessening in the thermodynamics performance when operating the plant on off design conditions. It is found that the exegetic effectiveness by using design and real-time data of unit three are 54.88%, and 43.75% respectively. The total exergy destructed in the unit is 356.34MW (contributes to 42.57% of input exergy) and 556.28MW (contri...

Combined cycle power plants are now becoming widely accepted for generating electricity in Libya. Some of these plants are being installed, and others are planned to be built in the near future. The thermodynamic performance of these plants depends on the ambient temperature which varies considerably from one season to another in Libya. The object of the current study is to investigate the effects of these variations on the performance of combined cycle power plants. In the study, the hot effluent of two selected gas turbine power units is taken to be utilized to generate steam in a heat recovery steam generator and operate a proposed dual pressure steam power cycle. The influence of ambient temperature variations on gas, steam, and combined cycles performance is simulated using HYSYS software. A real time data including ambient temperature, air mass flow rate, pressure ratio and fuel flow rate, for the selected gas unit are obtained at full load operation. These data are extrapolat...

The paper presents the development, state-of-the-art, and applications of exergy analysis in the evaluation of the performance of thermal energy storage units. Formulas dealing with different sources of irreversibilities for complete charging-discharging cycles are introduced. The contributions of the most popular authors to the subject are focused on. Comparison of the present authors' results related to sensible heat storage units, utilizing a relatively simple iteration technique with that using the well-known optimization code „GRG2", shows an excellent agreement.

This paper presents a numerical model of a two-dimensional axi-symmetrical transient controlled heat conduction of Phase Change Material (PCM). The system consists of a copper tube surrounded by a coaxial cylinder which forms an annular gap around the tube. The annular is filled with PCM while the two-phase fluid undergoes heat exchange through a tube wall. The finite volume approach for numerical modeling of phase change problem is employed. The essential feature of this approach is that the evolution of latent heat is accounted for by the definition of total enthalpy (H), while the two-phase fluid flows is modeled via a single energy equation with empirical correlation for the heat transfer coefficient. A parametric study is carried out to find out influence of different kinds of refrigerants, refrigerant saturation temperature, discharging time, and refrigerant mass flow on the temperature distribution inside the PCM, and the amount of heat released by PCM. Three refrigerants wit...

There are several successful methods to improve the performance gas turbine power plants, among which are the refrigerated inlet air-cooling gas turbine cycle where cold air at inlet of the compressor is introduced The objective of the present work is to enhance the performance of unit six of gas turbine power plant of the city of Misurata. The inlet air will be cooled to lower temperatures by an aqua-ammonia absorption chiller which is driven by the tail-end heat recovered from the engine exhaust gases. A computer program has been developed to find the effect of ambient and inlet temperatures on the performance of the gas turbine power plant A real time data for the gas turbine units, computer model and theoretical study are used to check the validity of the proposed model. From the results attained it can be said that the combined system (cooling/power) achieves gains in concluded power, exergy effectiveness, and specific fuel consumption, of about 80.7, 57.04 and 16.47% respectiv...

The scope of the present paper is to perform a thermodynamic analysis for a reverse osmosis desalination plant, in order for physical and chemical exergy to be calculated at all plant states. The analysis shows that the amount of the chemical exergy is very small compared with the physical exergy. It is found that 74.07% (122.77 kW) of the plant incoming exergy is destroyed in the membrane, the two throttling valves and all other components destroy 12.73% (21.10 kW) and 8.92% (14.78 kW) of the incoming exergy, respectively. The total amount of exergy which is destroyed at all plant's component is 158.65 kW of the 165.75 kW incoming exergy, and hence the plant effectiveness is found to be 4.2821%. The plant effectiveness could be raised to 4.8642% and an amount of 173,773 kW. hr per year of the electrical exergy could be saved, if a hydraulic turbine were implemented to save 94% of the destroyed exergy at the two throttling valves.

The objective of this paper is to evaluate the thermodynamic performance of a refrigeration unit in which the simple vapor compression refrigeration cycle is replaced by an ejector where the refrigerant is compressed thermally to the desired condenser pressure. Different refrigerants operating under different source and evaporator temperatures are simulated to predict the unit performance. Low quality waste heat is used to power the refrigeration unit. There is a great deal of waste heats being released into environment, such as exhaust gas from turbines and engines, and waste heat from industrial plants, which cause thermal environmental pollution. In addition, there are also abundant geothermal resources and solar energy available in the world.

Zuara desalination plant is installed and operated by General Electricity Company of Libya (GECOL), It is located 10 km east of Zuara city, and 150 km west of Tripoli, the Libyan capital. The plant is based on Multi-Effect evaporation technology. It consists of three units with total capacity of 40000 m3/day. The plant was supplied and constructed by French company SIDEM and started producing distillate water in March 2006. In this paper, the plant performance from thermodynamics point of view is performed. The analysis is based on design data, performance test data, and real time data. Both thermo-mechanical (physical) and chemical exergies, are considered. Using performance test data, the results show that the de-superheating process contributes to about 44% of the total exergy destruction, while the nine effective cells destroy about 26%, the distillate condenser destroys 22%, and the seawater pre-heaters destroy 8% of the total exergy destruction in the unit. The analysis shows that the unit effectiveness is about 17% during the performance test (full load); however, for half load performance the effectiveness decreases severely to 4.670%.

ABSTRACT
The aim of this paper is devoted to assess the thermodynamics performance of
unit one of Zawia city desalination plant. The plant was supplied by French company
SIDEM and constructed by Turkish ENKA TEKNIK Company started producing
distillate water in July 2010. The plant is based on multiple-effect distillation with
thermal vapor compression technology and consists of four units with total capacity of
80,000 m3/day. Design and real time data in summer and winter sessions are adopted for
the analysis. The analysis reveals that the thermal vapor compression and the effects are
the main source of exergy destruction. The major exergy destruction takes place during
the thermal vapor compression processes, which contribute to an average value of
45.76%% of the consumed exergy. Losses contribute to about 22.25% of the consumed
exergy. Effects, pre-heaters and final condenser contribute on average to 16.13%,
7.84%, and 8.02% of the consumed exergy, respectively.

This paper explores numerically the internal entropy generation and hence the
internal irreversibility for an annular sensible heat thermal energy storage unit. Finite
volume technique is adopted for the analysis with fully implicit numerical technique. A
storage unit composed of two horizontal concentric cylinders with different diameters is
considered for the analysis. The space between the two cylinders is filled with the
storage material. Two different materials with different thermal diffusivities are
considered for the analysis; those are cast iron and brick. The outside surface of the
storage unit is insulated, while the inside surface exchanges heat by convection with air.
The only mode of heat transfer inside the storage material is the heat transfer by
conduction. The problem is a two-dimensional one. It is found that during the transient
period, both the storage capacity and the internal entropy generation in cast iron are
substantially higher than those in brick. However, as the steady state condition is
approached with the increase in charging time, the discrepancies in the thermodynamic
performance between the two storage materials are reduced.

This paper presents a numerical model of a two–dimensional axi-symmetrical transient controlled heat conduction of Phase Change Material (PCM). The system consists of a copper tube surrounded by a coaxial cylinder which forms an annular gap around the tube. The annular is filled with PCM while the two-phase fluid undergoes heat exchange through a tube wall.
The finite volume approach for numerical modeling of phase change problem is employed. The essential feature of this approach is that the evolution of latent heat is accounted for by the definition of total enthalpy (H), while the two-phase fluid flows is modeled via a single energy equation with empirical correlation for the heat transfer coefficient. 
A parametric study is carried out to find out influence of different kinds of refrigerants, refrigerant saturation temperature, discharging time, and refrigerant mass flow on the temperature distribution inside the PCM, and the amount of heat released by PCM.
Three refrigerants with different properties are selected for the present study; those are, carbon dioxide (R744), Freon 134a (R134a) and Freon 407C (R407C). The results show that the refrigerant R744 is characterized with the best heat transfer characteristics, and the lowest solidification time of PCM.

The basic gas turbine cycle has low thermal efficiency, which decreases in the hard climatic conditions of operation. There are several successful methods to enhance their performances, among which is the refrigerated inlet air-cooling gas turbine cycle where cold air at the inlet of the compressor is introduced.
A computer program has been developed to find out the effect of the ambient and inlet temperatures on the performances of unit six of gas turbine power plant of Misurata city by utilizing an ammonia- water absorption cycle.
From the results attained it can be said that the combined system (cooling/power) achieves gains in concluded power, exergy effectiveness, and specific fuel consumption, of about 80.7, 57.04 and 16.47% respectively.