Electricity generation and utilization of fuel resources have long been topics of significant interest to researchers in scientific institutions, energy corporations, and governments. The rapidly increasing demand for electricity generation is related to an optimistic view of economic growth [1
], but can also adversely impact the environment and human health [2
]. Most large-scale power plants worldwide are fossil-fuel power plants [3
], with the cost of generating electricity fluctuating wildly due to resource shortages [4
]. Thus, with the continuing demand for further increases in the standard of living, energy analyses of fossil fuel power plants are becoming more urgent and electrical power generation has been the topic of many studies which emphasize enhancing power generation efficiency.
Fossil-fuel power plants can be classified into gas cycle, steam cycles, and combined gas-steam cycles. The market for electrical power generation tends to prefer gas-steam cycles in a combined heat and power (CHP) configuration since these can reach around 60% efficiency [5
]. However, the performance of the gas-steam cycle relies very much on the gas turbine, comprising a compressor, combustion chamber and the turbine, the performance of which can be affected by factors which include fuel type, fuel temperature and mass flow rate, ambient inlet air temperature and humidity, site elevation, inlet and exhaust losses, air extraction, performance degradation, and steam and water injection for power augmentation [6
]. Understanding the impact of these parameters on gas cycle performance and power generation is critical. Theoretically, the evaluation of energy systems, including gas-cycles, can typically be performed by evaluating energetic performance based on the first law of thermodynamics [7
]. Another evaluation method is to investigate exergetic performance and evaluate according to the second law of thermodynamics [8
]. Using energetic and exergetic analysis together provides a significant understanding of the system being evaluated.
Several studies have confirmed that ambient temperature significantly affects the cycle’s performance [11
]. The impact of varying ambient temperature has been shown to cause drastic fluctuations in electric-power output and efficiency, especially at high ambient temperatures [14
]. Most of the research in this area has indicated that exergy destruction is lowest in the compressor, followed by the turbine, and is at maximum in the combustion chamber [15
The wider use of exergy analysis as a tool for assessing various thermal energy applications is a topic of interest for many researchers [20
]. However, the assessment of thermal energy systems should consider the economic factors that are necessary to improve system efficiency and reduce operational costs. In this regard, there are two common ways to perform an economic assessment of thermal energy systems. The first way is by applying Life Cycle Costing (LCC), and the second way is to apply an exergoeconomic assessment. LCC is a method to assess all costs associated with the life cycle and has been widely used for economic evaluation of thermal energy systems [23
]. It is considered to be a tool that can help decision makers estimate the total costs of ownership over the lifetime of the thermal energy system being evaluated. Taner and Sivrioglu [27
] indicated that calculating the LCC of fossil fuel power plant alternatives requires calculating the costs in present value including investment cost (capital cost), non-fuel operating and maintenance (O&M) costs, fuel costs, and salvage value.
However, for a better understanding of the alternatives, an exergoeconomic assessment provides a more detailed evaluation of the economic performance of each state in thermal energy systems [28
]. This approach is applied to identify costs that result from exergy destruction in each component in the thermal energy system. It does this by combining the second law of thermodynamics with economic principles. Several studies available in the literature have considered utilizing exergy analysis to perform an exergo-economic assessment of a GT cycle. Seyyedi, et al. [29
] performed detailed thermodynamic, economic, and environmental analyses to optimize a GT cycle. Their study included determining the effect of air preheaters on the thermodynamic cycle and consequential environmental impact.
Boyaqchi and Molaei [30
] conducted exergy, exergo-economic, and exergo-environmental assessment of a gas turbine cycle. Their work was based on the multi-objective particle swarm optimization (MOPSO) algorithm, and the objective functions were the total cost rate, exergy efficiency, and CO2
emissions. Igbong and Fakorede [31
] conducted an exergo-economic analysis of a GT plant, studying the effects of turbine inlet temperature and compressor pressure ratio. Arora [32
] conducted exergoeconomic research on GT power plant components: the combustion chamber, compressor, and exhaust. Mousafarash and Ameri [33
] conducted exergy and exergo-economic analyses of a GT power plant considering its performance at different ambient temperatures and partial loads. Avval et al. [34
] conducted an exergy-exergo-economic and exergo-environmental multi-objective optimization analysis of a GT power plant. They studied the influence of different design parameters on turbine exergy efficiency, total cost rate of the system, and CO2
emission, investigating how high ambient temperatures led to a low density of air intake to the compressor, enhancing the performance of the gas cycle while minimizing the losses through highly efficient components (i.e., gas turbine, air compressor, combustion chamber) [35
A study conducted by Almutairi, et al. [36
] raised the question of whether the type and configuration of the air compressor can impact on the overall engine performance by interacting with the operating conditions. They investigated several gas turbines with different compressor configurations using the first and second laws of thermodynamics and a detailed exergy analysis. The analysis considered the performance of each configuration as a function of compressor isentropic efficiency, compressor pressure ratio, ambient temperature, and load variation. They found that compressor configuration did affect the performance of the gas turbines relative to the manufacturer’s data, as predicted by the exergetic and energetic analysis. No studies were found in the literature which used economic and exerge-oconomic investigations to test the feasibility of using gas turbines with different compressor configurations. Because of the importance of gas-turbine engines in power applications especially for electricity generation, the present work attempts to fill the research gap by investigating different GT engines with different compressor configurations according to economic, exergo-economic, and exergeo-environment assesments.
For the economic assessment, the study presents a LCC for each GT engine studied, considering the total costs incurred throughout the lifecycle period. For the exerge-oecnomic assessment, the study combines the exergy analysis with economic influences and includes all related costs resulting from inefficiencies in the thermodynamic processes in the major components in the given GT engines. For the exergeo-environment assessment, the study presents three environmental indicators by which to assess the most environmentally and ecologically favorable gas-turbine engine. The study in this regard summarizes its aims as:
Producing a LCC assessment for a lifespan of 20 years for the given GT engines with respect to ISO conditions and variation in ambient temperature and loading conditions.
Calculating the payback period of each GT engine studied where the delivered power is assumed to be electricity sold at tariff prices.
Estimation of exergy costing for all equipment in the given GT engines.
Estimation of CO2 emissions at ISO conditions that result from each GT engine considering three environmental indicators, environmental destruction coefficient, environmental destruction index, and environmental benign index.
3. Results and Discussion
For the GT engine configurations considered, the economic, exergo-economic, and exergeo-environment analyses have been performed and the results are presented in detail in this section. Four different models of GT engine configurations have been considered based on both energetic and exergetic analyses. The compressor configurations of the GT engines were the primary consideration of this work.
The analysis was of the four GT engine configurations under different operational scenarios. The first scenario was to run the given engines under ISO conditions and carry out energetic and exergetic analyses at production loads of 60%, 70%, 80%, 90%, and 100%. For the second scenario, the four engines were run under different ambient temperatures at 100% production load.
3.1. Economic Analysis
The analysis estimated the economic feasibility of each gas engine by calculating the LCCs over 20 years, but did not consider revenues that might be acquired through the sale of electricity. It was assumed that expected costs must be paid over the period of the study plan (20 years). Thus, the work focused on the investment required at the beginning, to be paid for construction and installation, the annual cash needed to operate and maintain the gas engine, the annual cash needed for fuel supply, and finally the cash return from selling the engine after 20 years. Figure 2
shows (a) lifecycle costs at varying operating loads, and (b) lifecycle costs at varying ambient temperatures.
The four configurations studied were of different sizes, so the lifecycle costs were presented in terms of the cash required to be paid during the life cycle relative to the net power of the GT engine. From Figure 2
a, as would be expected, the cost of the net power supplied by the GT engines decline with the increase in production load. In this regard, the centrifugal-centrifugal configuration was the most economically feasible, ranging from 10,000 USD/kW at 60% production load to about 8500 USD/kW at full load. At all operational loads, the order of the other engine configurations was consistent; axial centrifugal, axial, and two-stage centrifugal. However, the axial-centrifugal and the two-stage centrifugal both have the same economic performance at full load, as shown in Figure 2
The cost of the net output power of the engine increased with increase in ambient temperature can be seen in Figure 2
b. The figure shows that the two-stage centrifugal configuration and the axial-centrifugal configuration have the same LCC at 288 K; this is compatible with the findings presented in Figure 2
a. However, the LCC for the two-stage centrifugal increases more rapidly then for the axial-centrifugal as the environmental temperature increases. The centrifugal-centrifugal configuration over the environmental temperature range, considered consistently, has the lowest LCC value. The axial configuration has the lowest rate of increase of LCC, with environmental temperature intersecting with the centrifugal-centrifugal configuration at 328 K.
shows the payback periods of each configuration if the output net power is assumed to be electricity sold at tariff price. The feasibility of the four configuration increased with increase in operation load and decrease in ambient temperature, as shown in Figure 3
a,b. The gas engine with centrifugal-centrifugal configuration consistently gave the shortest payback period. All configurations perform exactly as expected with respect to the LCC. Figure 3
a shows that running the two-stage centrifugal engine is not feasible at 60% load if it is intended to produce electricity over a working life time of 20 years.
From Figure 3
b, all configurations have shorter payback periods at lower ambient temperatures, with the centrifugal-centrifugal again giving the shortest payback period at all environmental temperatures, save at 328 K, where it is beaten by the axial configuration.
In conclusion, it can be said that, generally, the centrifugal-centrifugal configuration outperformed all the other configurations in economic feasibility.
3.2. Exergy Economic Analysis
shows the values of both exergoeconomic factor, fk
, and relative difference in cost, rk
, as percentages for the gas turbine engine with axial compressor. The greatest value for rk
occurs in the combustion chamber (CC); this means that it is the most important component from an exergoeconomic viewpoint. Next is the axial compressor, then the LPT, HPT, and GEN, respectively. The CC, relative to the other components, has a high rate of exergy destruction and a correspondingly low value for fk
The value of fk for all components is below 50%; this means that a cost saving for the entire system could be attained by enhancing the efficiency of the components, even if this incurs an increase in the cost of capital invested. The cost of inefficiencies in each component can be addressed based on operating conditions and design criteria, with the component with the highest relative cost difference having priority.
The values of fk
as a percentage of GT engine with two-stage centrifugal compressor are shown in Figure 5
. The CC is again the greatest cost source relative to all other components. A study of the literature confirms that for gas turbines, the CC is accepted as the primary source of exergy destruction, greater than for any of the rotating components. The gas turbine engine with a two-stage centrifugal compressor has a relatively slight effect on rk
when compared to the axial compressor (reference case), due to its relatively low efficiency and capital cost. However, the impact on LPC and HPC is apparent and there is a need for greater efficiency even if it enhances capital cost because the exergy destruction makes a greater contribution to total cost. From an exergoeconomic perspective, after the CC, the LPC is the next most significant component, followed by the HPC. The gas turbine component’s economic efficiency is reduced slightly relative to turbines in the reference case due to the compressor configuration, which affects operating conditions and the quantity of extracting power.
shows the exergoeconomic variables (rk
) for the GT engine with axial-centrifugal compressor. The main findings extracted from this figure can be summarized in four points: first, the axial compressor (LPC) shows greater economic efficiency than the centrifugal compressor due to its lower level of irreversibilities, despite its higher investment cost. Second, after the CC, the cold section components have more effect on enhancing the entire system’s cost-effectiveness. Third, the potential for improvement is higher in the centrifugal compressor (HPC). Finally, the irreversibility contributed to the axial compressor is lower than for the reference case. which suggests the target of a relative reduction in investment cost as well.
shows the exergoeconomic variables (rk
) for the GT engine with centrifugal-centrifugal compressor. From this figure, it is concluded that: first, despite low exergy destruction, the LPT is most important, followed by the combustion chamber, due to varying compressor configurations. The LPC always shows better economic performance than the HPC because of cost due to inefficiencies. Therefore, modifying the design of the compressors by increasing component efficiency or adjusting the operating condition to reduce exergy destruction within the component is necessary. The values of fk
for the intermediate pressure turbine (IPT) and electric generator are 63% and 56%, respectively. These values suggest that capital cost should be reduced at the expense of component efficiency. Moreover, the GT engine with centrifugal-centrifugal compressor configuration enhances the exergoeconomic efficiency for all rotating components compared to the GT with two-stage centrifugal compressor. All turbine components registered higher values of fk
than for any of the previous cases investigated.
Ambient temperature is deemed a useful statistic by which to assess the local climatic conditions. Figure 8
shows the sum of component costs and exergy destruction for every component considered in the GT with an axial compressor under ISO and Kuwait climate conditions. The CC has the greatest value of
, followed by AC, the HPT, the LPT, and the GEN, respectively. Increasing ambient temperature adversely affects gas turbine engine efficiency and production because of increased compressor load. In the AC and HPT, an increase in ambient temperature increases both exergy destruction costs and total cost. However, for the CC, an increase in ambient temperature will increase the temperature of the air entering the CC; this causes a reduction in fuel consumption and exergy destruction. A further decrease in exergy destruction costs in the CC is possible by fine-tuning the excess air, or if the reactants are preheated using waste heat from the exhaust gases, decreasing heat lost from the CC. Pre-heating the reactants has the added benefit of decreasing the rate of fuel combusted.
For GT engines producing electricity, grid imbalances or other limitations can lead to low demand, with the GT operating at part-load; an unfavorable condition that reduces engine efficiency. The effect of change in load, from full to 65% part load on exergy destruction and component costs (
) for the GT engine with axial-centrifugal compressor is shown in Figure 9
. The value of
for each component increases when the power setting is reduced to part-load due to the increase in specific fuel consumption. The axial compressor (LPC) shows less change with load variation than the centrifugal compressor due to its higher efficiency and lower fuel exergy.
3.3. Exergyenvironmental Analysis
The environmental impact when converting energy resources into different forms of energy can be assessed quantitatively using an exergo-environmental methodology. Environmental impact has been investigated using different indicators (see Table 7
). All three environmental indicators show that the GT with axial-centrifugal compressor is the most environmentally favorable and most ecologically efficient, producing the least ecological damage.
also shows the CO2
emissions for all four proposed GT engines. CO2
emission is strongly related to engine efficiency, as it is a measure of the fuel consumed in the combustion process. The CO2
value increases as the air-fuel ratio increases until the maximum value is achieved at the stoichiometric air-to-fuel ratio (when oxygen from the air and fuel are in perfect balance for combustion) and then decreases in the presence of excess air, e.g., the air-to-fuel ratio increases further. This leads to an important point: for maximum combustion efficiency, the proportion of carbon dioxide in the flue gases should be just less than its peak value. The gas turbine engine with an axial-centrifugal compressor achieved the lowest value of CO2
emissions per kWh when compared with others.
The current study illuminated some significant outcomes of small-scale gas turbine engines from an economical and environmental perspective. The end-user requirement is essential in selecting the appropriate gas turbine among the proposed engines. Lifecycle costing introduces valuable information about the expenses, while exergoeconomic factors help to enhance the system’s cost-effectiveness. It is interesting to note that with small-scale gas turbine engines, the engine size and maintenance cost significantly impact feasibility and economic efficiency; unlike in large-scale capacity, the maintenance cost impact is considered minor. The environmental impact of all gas turbine engines is only associated with operational efficiency and better utilization of resources.