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In the thermoeconomic evaluation of energy systems both maximizing power output and economical profits should be considered as the main objective, replacing the usual increasing power to select the most suitable temperature. In this paper, an accurate procedure to evaluate the thermoeconomic and performance of installed INLET Air Cooling System in gas turbine was performed based on the measured data during the operation time of the system. First, describing processes evolving into the system and deriving relationships between flow rates, energy exchange, sizing design, etc. Second, the model with different design parameters such as the influence of the ambient temperature and the inlet air cooler performance have been simulated and analyzed. Analysis of the results shows that the output power of a gas turbine power plant without cooling system at August was 96.6MWh, but the output power for gas turbine power plant with cooling system at August was 120MWH. The life cycle cost of gas turbine power plant with cooling system is lower than the life cycle cost of gas turbine power plant without cooling by approximately 4% at inflation rate of 0% and 12% at inflation rate of 5%.
Keywards: Thermoeconomic, gas turbine, cooling system, life cycle cost.
In recent years, with the heightened awareness of the global warming issues and demand for cost effective power generation, the appraisal of power generation technology is inevitable. Therefore, a wide research activity is currently devoted about new technologies to boost the performance of gas turbine generators, inlet air cooler is expected to result in the boosting of power output of the gas turbine, and also create a noticeable improvement in efficiency. Since thermoeconomic analysis facilitates the determination of the optimum design parameters of gas turbine power plant systems for a set of operating conditions, thus the method becomes more important as the price of energy and investment costs increase. A thermoeconomic approach may be used to solve this problem; in fact, thermoeconomic is a technique, which combines thermodynamic analysis directly with economic aspects in order to optimize thermal systems like a gas turbine based cycles.
In an effort to boost the performance of gas turbine generators, Johnson (1989) suggested the use of evaporative coolers. De Lucia et.al., (1994) examined the operation of cogeneration gas turbine power plant with and without an air cooling system. Ondryas et al (1991) investigated various options for cooling the inlet air, including vapor compression chillers and aqua-ammonia absorption chillers. Szargut, (1999) studied the influence of ambient temperature on the operational indices of the gas turbine set. The author reported that lowering inlet air temperature leads to the increase of flow rate of combustion gases, which results in the increase of power output. Accadia and Vanoli (2004) used the structural method for the thermoeconomic optimization of the condenser of a vapour compression heat pump. Malewski and Holldorft, (1986) analyzed the performance of a gas turbine generator fitted with aqua-ammonia absorption chiller to cool the inlet air. In their system, the generator received the required heat from the exhaust gases via a direct contact heat exchanger Finally, McDonalds, (2003), studied the turbine performance with optional power booster including mechanical chillers with thermal storage system. Valdes et al. (2003) showed a possible way to achieve a thermoeconomic optimization of combined cycle gas turbine power plants. The optimization has been carried out using a genetic algorithm.
This paper focus on thermoeconomic evaluation of gas turbine power plant systems by adding inlet air cooler to a gas turbine power plant, and presents a new methodology for the analysis of inlet-air cooling technologies and ambient conditions. The detailed formulations for energy, energy, and economic analysis for the entire cogeneration plant are developed. Finally, the performance variations that the GT inlet-air cooling could produce in the rest of the bottoming elements, in order to obtain cash flows and other relevant economical variables that maximize the profit of the integrated system. In addition, it is observed that the thermoeconomic analysis of a system is able to provide suggestions about potential cost-effective improvements achievable by means of changes in the structure of the system.
In this study deals with single shaft gas turbine power plant type Siemens V94.2 similar to that one installed at the Baiji gas turbine power station. As shown in figure (1), single shaft gas turbines are configured in one continuous shaft and, therefore, all stages operate at the same speed (3000 RPM). These types of units are typically used for electricity generation.
Air Compressor model
Using the first law of thermodynamic and knowing the air inlet temperature to compressor, pressure ratio (rp) and isentropic efficiency for compressor, we can determine the compressor exit temperature:
So, the work of the compressor (Wc) when can be calculated from:
Where: The specific heat of air which can be fitted by the following equation for the range of 200K<T<800K (R): (Rahman, et al., 2010).
Where Ta in Kelvin.
The specific heat of flue gas is given by (Naradasu et al., 2007)
is the mechanical efficiency of the compressor.
Combustion chamber model
From energy balance in the combustion chamber (Nelson and Louay 2010):
Result the equation for the ratio ()
Where T3 = TIT = turbine inlet temperature.
The heat supplied is:
The shaft work of the turbine is given by:
The net work of the gas turbine (Wnet) is calculated from the equation:
And the gas turbine efficiency is:
Figure 1: Air cooling heat exchanger
Inlet air cooling of compressor
The first thermodynamic law is used for calculating cooling load of refrigeration cycle, the heat rejected from cooling air, Figure 2 is given by (Zhi-Gao, 2008):
Where: =mass flow rate of compressor inlet air at 45°C (summer condition).
=desired inlet air temperature, lets be 25°C.
=air inlet mean temperature at summer condition=45°C.
Qe =is the heat transferred through the evaporator.
Figure 2: The absorption refrigeration cycle. (Martínez-Lera and Ballester, 2010)
Simple gas turbine model
The life cycle cost as was pointed out previously is based on the actual power output that produced by the gas turbine throughout the year and can be written as follows (Zhang et al., 2005 ):
This equation can be written as:
LCCnc : life cycle cost without cooling
ICgt : initial cost of the gas turbine
EncAnu: annual energy cost without cooling
Fp : fuel price
Pncm: monthly power output without cooling
Mgt : maintenance cost of the gas turbine
Salgt : salvage value of the gas turbine
Pw : present worth value
Fw: future some of money
i : interest rate
n : period of investment
Ap :is the first payment in the series.
Gas turbine model with effect absorption cooling system
The single stage LiBr absorption system is used to boost the power output nearly to the design value by cooling the inlet air to 15ËšC which make the gas turbine operates at the design conditions ISO. Then, the life cycle cost of the combined system can be written as (Omer et al., 2009; Ozgur et al., 2010):
This equation can be written as:
LCCwc : life cycle cost with cooling
ICabs : initial cost of the absorption system
EwcAnu: annual energy cost with cooling
Padp : additional power output price
Pwcm : monthly power output with cooling
Padt : additional power produced due to the using of cooling system
Mgt : maintenance cost of the gas turbine
m : absorption life time.
Salabs : salvage value of the absorption system
Absorption cooling system with single stage LiBr-water used to reduce the inlet air temperature before entering to compressor of gas turbine power plant in order to increase the net power output to nearly design value. The absorption cooling system drive by recovered the heat energy from the exhaust gases that exit from the gas turbine power plant. Gas turbines are constant volume machines at a given shaft speed they always move the same volume of air. However the power output of a turbine depends on the flow of mass through it. This is precisely the reason why on hot days, when air is less dense, power output falls off. A rise of 10C temperature of inlet air decreases the power output by 1% and at the same time heat rate of the turbine also goes up which is a great concern of power producers. A simulation code developed by using matlab to investigate the effect of usine the inlet air cooling system on the thermal and economical performance of gas turbine power plant.
Figure 3: Gas turbine net power output with and without cooling system for January.
The gas turbine thermal cycle calculated by simulation program is shown in the figure 3 to 5. The daily net power output for gas turbine power plant with and without cooling for the January is found to be nearly constant and parallel to the design value of 2870 MWh as shown in figure (3), this is due to the low ambient temperature that remains during that month.
The detraction in power output was observed to occur during the hot months of May, June, July, August, September and October. Figure (4), shows the Effect of the daily time on the net daily power output for the gas turbine power plant with cooling and the gas turbine power plant without cooling for the month of June. The net power output when gas turbine power plant without cooling was found to be 2535.75MWh which represent an average reduction in daily power out of 11.9%, while the maximum reduction in net daily power output was obtained to be about 17.2%.
Figure (5) shows the Effect of the daily time on the net daily power output for the gas turbine power plant without cooling and the gas turbine power plant with cooling for August. The daily net power output was found to be 2507.4 MWh which indicates an average reduction in daily power out of 12.7% compared to the design value of 2871 MWh. The maximum reduction in daily power output was obtained to be approximately 19.5%.
Moreover, June, July and August are surely the hottest months in the year where the ambient temperature reaches over 45 ËšC. Therefore, the cooling system should operate at 100% capacity and 24 hours a day in order to boost the power production to near design value.
Figure 4: Gas turbine net power output with and without cooling system for June.
Figure 5: Gas turbine net power output with and without cooling system for August.
Figure 6: Life cycle costs for gas turbine power plant with and without cooling system for various interest rates at inflation rate of 0%.
Figure 7: Life cycle costs for gas turbine power plant with and without cooling system for various interest rates at inflation rate of 5%
In this study the lifetime of the gas turbine power plant as well as the single stage absorption system is considered to be 30 years. The values for interest rate were varied in the range of 0% to 10%, as the inflation rate is considered to be from 0% to 5%. Two main cases were considered for gas turbine, gas turbine without cooling and gas turbine without cooling. The life cycle cost is based on the annually net power output produced by the gas turbine power plant without using cooling system and then is calculated when the cooling system is used with gas turbine, subsequently, a comparison can be made between them.
Figure (6) shows the Effect of various interest rates at an inflation rate of 0% on the life cycle costs for the gas turbine power plant with and without cooling. The life cycle cost of gas turbine power plant with cooling system is lower than the life cycle cost of gas turbine power plant without cooling by approximately 4%. The life cycle saving also plotted for various value of interest rate at inflation rate of 0 %, which indicates that nearly 38 million dollars saving occurs for interest rate of 0.01%, then decreases as the interest rate increases as shown in this figure 6.
Figure (7) shows the Effect of various interest rates at an inflation rate of 5% on the life cycle costs for the gas turbine power plant with and without cooling. The life cycle cost of gas turbine power plant with cooling system is lower than the life cycle cost of gas turbine power plant without cooling by approximately 12%. However, it was observed that the life cycle costs increase as the inflation rate increase.
This study Focus on comparison of life cycle costs based on two technologies for both the gas turbine power plant without cooling system and gas turbine power plant with cooling system. The calculated costs included the capital investment costs, maintenance costs and energy costs, incurred throughout the lifespan of the gas turbine power plant. Finally, since the gas turbine power plant equipment cost information is proprietary and hard to acquire, it is difficult to measure sizably correct life cycle costs for both technologies. In addition, the summary of the major findings of this analysis following are:
A decrease in compressor inlet air temperature, keeping the other parameter unchanged decreases the LCC.
The life cycle cost for the gas turbine power with absorption refrigeration case is lower than that of using gas turbine power plant by nearly 4%.
Finally, economic analysis indicated that applying a single stage LiBr to augment the gas turbine power output in hot duration is a valuable technique and should be considered.
The authors would like to thank Universiti Malaysia Pahang for provides laboratory facilities and financial support under Doctoral Scholarship scheme.