1. Introduction

The exhaust gases of gas turbine power plant carry a significant amount of thermal energy that is usually expelled to the atmosphere; this causes a reduction in net work and efficiency of gas turbine. On the other hand, the generated power and efficiency of gas turbine plants depend largely on the temperature of the inlet air, so that they both increase as the inlet air temperature decreases. Decreasing the temperature of air inlet to the compressor could be achieved by installing an absorption refrigeration cycle (ARC) at gas turbine inlet, powered by the thermal energy of exhaust.

The feasibility of waste heat utilization in a process is dictated by the temperature, quantity, and availability of the waste heat source, the hot exhaust gases in the case of gas turbine power plant. The use of waste heat of the exhaust gases lead to many benefits in the operation of gas turbine power plants including the reduction in the energy input of the plant, increasing energy efficiency, reducing carbon dioxide and other emissions, and reducing the operating costs. Thus waste heat utilization is an attractive improvement when applied.

Many researches have been done on the different methods of inlet air cooling including the evaporative cooling, refrigeration cooling, and hybrid cooling[1, 10]. But, these different methods show a mall increase in efficiency and power outputs of the plant because an additional energy is required to operate these cooling systems. However, the use of absorption cooling system powered by the hot exhaust gases leads to a higher efficiency and power output since no external power is required and the process of air cooling will be a heat recovery process of waste heat[2]. This research aims to study the effect of inlet air cooling by absorption refrigeration system on gas turbine performance.

2. Gas Turbine Specifications

A specific gas turbine model was selected to determine the temperature and quantity of waste heat available, The GE MS5001 model used in Rehab Power Station in the northern of Jordan was selected with the following specifications.

Table 1. GE MS5001 Gas turbine specifications Specification Value Unit Rated Power 24.6 MW Air Flow Rate 118 kg/s Exhaust Temperature 484 °C Efficiency 32.98 % Pressure Ratio 10.5

The basic components of a gas turbine shown in Figure 1 below are compressor, a combustion chamber (air heater), a turbine, and a regenerator. The compressor model has ambient air as an inlet; the mass flow rate at the compressor inlet was selected to be equal to the manufacturer given value of 118 kg/s for the gas turbine inlet and was kept constant. The outlet, which goes to the combustion chamber, is determined by assuming zero pressure drops at the compressor inlet, a compressor ratio of 10.5, and an isentropic efficiency of 83%[3].

The combustion chamber has two inlets, the compressor outlet and the fuel, the fuel was assumed to be pure methane at 15°C and 1300 kPa. At full load its mass flow rate is set to match the manufacturer-specified firing temperature. Once the full load fuel rate is set in this case 6230 kg/hr, part loading is simulated by adjusting the fuel mass flow rate. The chamber is assumed to have zero pressure drops.

The reactor products exit the combustion chamber and enter the turbine. The turbine’s isentropic efficiency is assumed to be 85%, the pressure ratio is taken to be 10.5 and the pressure drop at the gas turbine exit is assumed to be zero. The turbine exhaust is synonymous with the exhaust of the gas turbine.

The gas turbine exhaust goes to a heat exchanger block, which simulates the removal of waste heat. Zero pressure drops was assumed, and the exit temperature of the block was selected to be 200°C. This is because condensation issues might arise at temperatures lower than 200°C, thus, 200°C is a conservative approximation[4, 5].

Figure 1. Regenerative Gas Turbine (RGT) Modeling and Integration using EES

3. Absorption Chiller

An absorption chiller is a closed loop cycle that uses waste heat to provide cooling or refrigeration. Absorption chillers use has been limited by their relatively poor efficiency at delivering cooling compared to vapor compressions cycles. For comparison, an absorption chiller typically has a coefficient of performance (COP) between 0.4 and 1.5, based on heat input.

However, absorption chillers continue to be viable in some applications because they are able to utilize low temperature (<200°C) heat to provide cooling. In this sense, a COP for an absorption chiller cannot be compared to the COP of other cooling cycles because the input energy for an absorption chiller can be essentially free, for example when it is waste heat, as it would otherwise have been unused in most processes.

Thus, in processes where low temperature waste heat is available and cooling is desired, it often makes sense to implement an absorption chiller to increase the overall energy efficiency of the process absorption chillers use a refrigerant-absorbent pair as a working fluid. The two most common combinations of working fluids used are ammonia/water and water/Lithium Bromide[6, 7].

4. Modeling of Ammonia/Water Absorption Cycle

A single-effect ammonia (NH₃)-water absorption cycle shown in Figure 3 is developed in this study and coupled with the gas turbine to cool the ail inlet to the compressor of gas turbine. The state points and assumptions of the cycle operation are summarized in Table 2.

Table 2. State points and assumptions

5. Thermodynamic Analysis

A. desorber

Applying the energy equation for the desorber of the absorption system yields:

(1)

Where h₁₄ is the enthalpy of gas exiting the desorber are respectively the mass flow rates of gas entering the desorber, the weak solution exiting the desorber, the refrigerant exiting the desorber and the strong solution entering the desorber h₉ ,h₁₀ , h₁:and are the corresponding enthalpies[8,9,10].

Applying the mass balance equation yields:

(2)

Using the concentration–mass flow rate relation gives:

(3)

Where x_ss and x_ws are the concentrations of the strong and weak solutions respectively. Both concentrations are known and equal to 0.4 and 0.3. The high pressure of the cycle, P_high, is equal to the condenser pressure, and the low pressure of the cycle, P_low, is equal to the evaporator pressure. The temperature of the outlet high-concentration mixture, T₁₀ can be determined as a function of its concentration and pressure using a program called Engineering Equation Solver (EES) as follows:

(4)

The corresponding enthalpy of the high concentration mixture, h₁₀ can be determined as a function of its concentration and temperature using EES so that.

(5)

An equation can be written for the temperature of gas at the desorber outlet in relation to the temperature of the high concentration mixture at point 10 of the cycle (Figure 2).

Figure 2. Schematic diagram of the recuperated cycle with absorption inlet cooling

(6)

The gas pressure at the outlet of the desorber can be determined using assumption. The enthalpy of the outlet gas can be determined as a function of its pressure and temperature using EES as:

(7)

An equation can be written for the temperature of the outlet steam in relation to the outlet high-concentration mixture at point 10 using the following assumption:

(8)

The enthalpy of the outlet steam can be determined as a function of its pressure and temperature using EES as:

(9)

B. Heat exchanger

The heat exchanger (HE2) effectiveness is defined as:

(10)

Where T₁₁ and T₈ are the temperatures of the outlet weak solution and inlet strong solution respectively. The temperatures T₈ can be determined as a function of pressure and concentration using EES as:

(11)

Applying the energy balance equation yields:

(12)

Where h₁₁ and h₈ are the enthalpies of the outlet strong solution and inlet weak solution respectively. The enthalpies h₁₁ and h₈ can be determined as a function of the temperatures and concentrations of the solutions using EES as:

(13)

(14)

Equations (1 to 14) are solved simultaneously for the unknowns h₁, h₁₁ , h₈ , h₁₀ , h₉, h₁₄ , T₁ , T₁₁ , T₈, T₁₀ and T₁₄ using EES. The results were presented in Figures 3 to 6 shown below.

6. Results and Discussion

From the previous analysis of the designed absorption cooling cycle. The outside air is cooled before entering the compressor and as a result the performance of the power plant will be better. The analysis were carried out for various exhaust gas turbine temperatures (153-200)°C and the cooling capacity has been getting cooler air up to 7°C for (T_amb=35°C ) The results of this study are shown in Figure 3 and 4 below.

Figure 3. cooling capacities of chiller designs at various exhaust temperatures (T13)

Figure 4. Inlet air cooled °C at various exhaust temperatures (T13)

Figure 5. Inlet cooling improves the work net of the gas turbine cycle

Figure 6. inlet cooling improves the work net of the gas turbine cycle

The inlet air cooling improves both the efficiency and the work net of the gas turbine cycle as shown in the Figures (5) and (6) below. As a result of decreasing inlet air temperature, the work net of the power plant improved by 5.9 percent and the efficiency of the power plant improved by about 0.49 percent.

7. Conclusions

The exhaust gases of gas turbine power plant carry a significant amount of thermal energy that is usually expelled to the atmosphere; this causes reduction in net work and efficiency of gas turbine. On the other hand, the generated power and efficiency of gas turbine plants depend largely on the temperature of the inlet air, So that they both increase as the inlet air temperature decreases. The mentioned two problems can be solved by installing an absorption refrigeration cycle (ARC) at gas turbine inlet, working with thermal energy of exhaust gases. In this research, effect of inlet air cooling on gas turbine performance is studied. The work shows that:

• The compressor air inlet temperature decreases as exhaust temperatures (T₁₃) increase. As a result the net work of the gas turbine will increase.

• Absorption cooling could improve efficiency by about 0.49 percent.

• Absorption cooling could improve the work net cycle by about 5.9 percent.

Nomenclature

ARC: Absorption Refrigeration Cycle.

COP: Coefficient of Performance.

e: heat exchanger effectiveness

EES: Engineering Equation Solver.

h: enthalpy (kJ/kg).

: mass flow rate (kg/s).

P: pressure.

RGT: Regenerative Gas Turbine.

T: temperature (^oC).

x: concentration (mass fraction).

Subscripts

amb : ambient.

g : generator (desorber).

NH₃ : ammonia.

r : refrigerant.

ss : strong solution.

ws : weak solution.