Performance indices of a power plant using exergy-based analyses

ASSESSMENT OF PERFORMANCE INDICES OF A POWER

PLANT USING EXERGY-BASED ANALYSES

ABSTRACT

The objective of this applied industrial research was to conduct an exergy-based analysis for an Open Cycle Gas Turbine (OCGT) in Abu Dhabi (UAE) in order to evaluate its performance under design conditions and during summer weather conditions. The first explanation for this investigation is that CO2 emissions from power generation plants in the UAE are responsible for about 33% of the 200 Million tons of the total CO2 emitted in 2013 in the country [1]. The second reason for this industrial project is that the standard conditions used for the design of gas turbines are 288K, sea level atmospheric pressure and 60% relative humidity [2]. However, the average summer weather conditions in Abu Dhabi are T=316K and a relative humidity of 50%. As a consequence, the effects of summer weather conditions on different performance indices of the power plant were also studied. Aspen Hysys V8.6 with the Soave-Redlich-Kwong (SRK) equation were used as tools to simulate the power plant under standard conditions

For the first performance index under investigation YD (exergy destruction ratio), results of the exergy analysis showed that the combustor is the main contributor to the exergy destruction of the power plant. Summer conditions increased its exergy destruction ratio by 21.2 % and decreased its exergetic efficiency by 2.6%. However, due to the positive effects of higher temperatures, the summer weather conditions decreased the exergy destruction ratio of the turbine by 55% and increased its exergetic efficiency by 4.3%.

The contribution of the cost of the exergy destruction in each equipment of the plant to the total cost of the final product (electricity) of the power plant was investigated using an exergoeconomic analysis of the plant and the second performance index rk (relative cost difference). In accordance with the first investigation, the combustor had also the highest contribution to the cost of the final product (r=15.4%). The summer weather conditions had increased the cost rate of its exergy destruction by 19.8%. and its relative cost difference by 18.2 %. On the other hand, the summer weather conditions have decreased the cost of exergy destruction of the turbine by 14.3 %. and its contribution to the cost of the final product (r) by 31.6%.

The effects of summer weather conditions on the environmental impact of the power plant were investigated using two different performance indices. The relative difference of exergy-related environmental impacts (rb) was utilized for each equipment of the power plant and the environmental impact of a kWh of electricity (EIE) was used for the power plant. In agreement to the exergetic analysis, the results of the exergoenviromental analysis indicated that the combustor presents the highest environmental impact of exergy destruction. The summer weather conditions further increased this impact by 21.6%. In addition, the combustor also had the highest contribution to the total environmental impact of the final product (rb =14.4%), while summer weather conditions increased this contribution by 20.8%. The expander had the second highest environmental impact of exergy destruction and summer weather conditions decreased this impact by 53.7%. The expander had the lowest contribution to the total environmental impact of the final product (rb =8.4%), while summer weather conditions decreased this contribution by 58.3%.

The environmental impact of a kWh of electricity of the power plant under standard conditions was 37.8 mPts/kWh (exergy destruction only), and 54.7 mPts/kWh (both exergy destruction and exergy loss). The summer weather conditions increased these impacts by 6.6% and 10.7 % respectively. It could be concluded that the negative effects of summer weather conditions on the performance of the combustion chamber and the compressor were partly compensated by their positive effects on the performance of the turbine.

RESEARCH TEAM

EXERGY ANALYSIS

Omar Mohamed Alhosani1, Abdulla Ali Alhosani1

EXERGOECONOMIC ANALYSIS

Ahmed Nabil Al Ansi1, Mubarak Salem Ballaith1, Hassan Ali Al Kaabi1

EXERGOENVIRONMENTAL ANALYSIS

Mohamed Mohamed Alhammadi 1, Mubarak Haji Alblooshi¹, Fontina Petrakopoulou ²

PRINCIPAL INVESTIGATOR (PI):

Zin Eddine Dadach1

NOMENCLATURE

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INTRODUCTION

The inefficiencies linked to irreversibilities in energy-conversion systems are invisible in a typical energy balance (1st Law of Thermodynamics) but can be evaluated using an exergy analysis (2nd Law of Thermodynamics). According to the theory, the exergy destroyed in each component of a power generation plant represents the loss of performance related to irreversibilities. To enhance the performance of the plant, efforts will be mainly focused on the equipment that presents the highest exergy destruction since it will offer the largest improvement of the exergy efficiency of the plant. The first performance index under investigation is the exergy destruction ratio YD. In order to investigate the summer weather conditions on this performance index, an exergy analysis of the plant will be conducted during a typical summer day and compared to the results under standard design conditions The final objective of increasing exergy efficiency of power plants is to reduce the consumption of fuel and minimize their environmental impact. In exergoeconomic and exergoenvironmental analyses, exergy destruction costs and environmental impacts are linked to irreversibilities [3-4]. The combination of exergy with costs was first used by Keenan [5] and the concept of exergoeconomics was introduced by Tsatsaronis [6].The exergoeconomic accounting method is used in the design and operation of gas turbines to calculate the costs of final products as well as the costs of the exergy destroyed within each equipment. It can be considered as an exergy-aided cost reduction approach that uses the exergy costing principle [7]. Following the exergy analysis of the power plant, a detailed exergoeconomic analysis of the plant based on Specific Exergy Costing (SPECO) method is presented in this investigation. One of the objectives of the analysis is to compare the values of the performance index rk (contribution to the cost of the final product of the power plant) calculated during summer atmospheric conditions to the values obtained under design conditions.

In order to ensure sustainable operation, the environmental impact of the plant must also be investigated. To achieve this goal, the environmental impact of each plant’s component must be compared to its corresponding impact under standard conditions. The final step of this investigation is to study the environmental impact of the plant by comparing the values of the performance index rb (environmental impact difference) of each component of the plant during a typical summer day and under standard conditions. The effects of summer weather conditions on the power plant will be investigated by comparing the values of the performance index EIE (environmental impact of electricity) during summer weather conditions and under standard conditions.

BACKGROUND

Concept of Exergy

Exergy is commonly defined as the maximum theoretical work that can be extracted from a “combined system” consisting of a “system” under study and its “environment” as the system passes from an initial state to a state of equilibrium with the environment [8]. When a system is in equilibrium with the environment, the state of the system is called ‘dead state’ and its exergetic value is zero. According to Bejan et al.[7], total exergy (ET) of a stream is constituted by four main components:

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The physical exergy (Eph) is often described as the maximum theoretical useful work obtainable as the system passes from its initial state (P, T) to the “restricted dead state” (P0, T0). On the other hand, the chemical exergy (Ech) is the maximum useful work obtainable as the system passes from the “restricted dead state”, where only the conditions of mechanical and thermal equilibrium are satisfied, to the “dead state” where it is in complete equilibrium with the environment [9]. The kinetic (Ek) and potential (Ep) exergies are associated to the system velocity and height, respectively measured relative to a given reference point. When a system is at rest relatively to the environment (Ek=Ep=0), the total mass specific exergy (eT) of a stream is defined as:

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Standard Chemical Exergy of a gas mixture

The chemical exergy per mole of gas (k) is given by the following equation [9]:

For a mixture of gases, the chemical exergy per mole of the mixture could be estimated using [9]:

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The exergy of fuel is equivalent to the calculated reversible work. The values of exergy of hydrocarbons and other components are listed in the literature [3] and the chemical exergy of a fuel could be estimated using equation (4). It should be noted that the value of the specific chemical exergy of a fuel at dead-state conditions is between the lower (LHV) and higher (HHV) heating values of the fuel [9].

Exergy Balance in Open Systems

Unlike energy, exergy is not conserved in any real process. As a consequence, an exergy balance must contain a “destruction” term, which vanishes only for a reversible process. The general form of exergy balance for a control volume could be written as [9]:

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For a steady state system, equation (5) could be rewritten as:

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In equation (6), the total specific exergy transfer at the inlets and outlets could be written as:

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h and s are properties at the inlet and the outlet of the system. h0 and s0 are respectively the specific enthalpy and the specific entropy of the restricted dead state.

Exergy analysis of an Open Cycle Gas Turbine

As shown in Figure 1, the power plant under investigation is an Open Cycle Gas Turbine (OCGT). Exergy destruction (ED) within a component of a power generation plant is the measure of irreversibility that is the source of performance loss. An exergy analysis will determine the magnitude and the source of thermodynamic inefficiencies in a power plant.

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Figure 1: Schematic Diagram of an Open Cycle Gas Turbine [7]

Based on Figure 1, the exergy destruction (ED) and exergy efficiency (EE) for the three main component of an open cycle gas turbine (OCGT) are defined using the equations [7]:

Compressor

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Combustor

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Turbine

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For a power generation plant producing electricity using the combustion of fuel, the rate of product exergy of the kth equipment ( is the exergy of the desired output resulting from the operation of the component, while the rate of fuel exergy of the same component is the expense in exergetic resources for the generation of the desired output. The rate of fuel exergy and product exergy of the three main components are defined in Table1.

Table 1: Rate of fuel and product exergy for each component [7]

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The rate of exergy destruction within the kth component, (ED)k, is calculated as the difference between its rate of fuel and product exergy [7]:

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And the exergy destruction ratio in each equipment could be written as:

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Economic analysis

The economic model takes into account the cost of the components, including amortization and maintenance, and the cost of fuel consumption. The first stage of an economic analysis of a power plant is to estimate the purchased equipment cost (PEC). The capital needed to purchase and install equipment is called the fixed capital investment (FCI). A number of methods have been published to estimate the purchase cost of equipment based on design parameters [7, 10-11]. The levelized cost method of Moran [12] is considered in this investigation. The amortization cost (AC) for a component (k) of the power plant depends on the initial cost (IC) and the actual cost factor (ACF):

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¹Department of Chemical and Petroleum Engineering, Higher Colleges of Technology, Abu Dhabi, UAE

² Department of Thermal and Fluid Engineering, University Carlos III of Madrid, 28911 Madrid, Spain