EXERGOECONOMIC ANALYSIS OF A POWER PLANT IN ABU DHABI. Ahmed Nabil Al Ansi, Mubarak Salem Ballaith, Hassan Ali Al Kaabi, Advisor: Zin Eddine Dadach

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1 EXERGOECONOMIC ANALYSIS OF A POWER PLANT IN ABU DHABI Ahmed Nabil Al Ansi, Mubarak Salem Ballaith, Hassan Ali Al Kaabi, Advisor: Zin Eddine Dadach

2 INTRODUCTION Following a previous exergy analysis of a power plant in Abu Dhabi (UAE), a detailed exergoeconomic analysis of the plant based on Specific Exergy Costing (SPECO) method is presented in this investigation. The objective of this applied research is to compare the values of the different exergoeconomic variables of the Open Cycle Gas Turbine (OCGT) calculated during summer atmospheric conditions to the values obtained from the simulation of the plant using design conditions.

3 An exergy analysis of a power generation plant will determine the magnitude and the source of thermodynamic inefficiencies

4 Exergy destruction (ED) within a component the plant is the measure of irreversibility that is the source of exergy efficiency (EE) loss Compressor (ED) K = W K m air (ex T2 ex T1 ) (1) Combustor Turbine (EE) K = 1 (ED) K W K (2) (ED) cc = m air x ex T2 + m fuel x ex fuel m fg. ex T3 (3) (EE) cc = 1 (ED) cc m air x ex T2 +m fuel x ex fuel (4) (ED) T = m fg x ex T3 ex T4 W T (5) (EE) T = 1 (ED) T m fg x ex T3 ex T4 (6)

5 Rate of fuel and product exergy for each equipment For a power plant, the rate of product exergy of the k th equipment ( E P,k ) is the exergy of the desired output resulting from the operation of the component, while the rate of fuel exergy of the same component ( E F,k ) is the expense in exergetic resources for the generation of the desired output. Equipment E F,k (MW) E P,k (MW) Compressor W K m air (ex T2 ex T1 ) Combustor m air x ex T2 m fg. ex T3 + m fuel x ex fuel Turbine m fg x ex T3 ex T4 W P + W K

6 Rate of exergy destruction and destruction ratio The rate of exergy destruction within the k th component, (ED) k, is calculated as the difference between its rate of fuel and product exergy [5]: (ED) k = E F,k E P,k (7) And the exergy destruction ratio in each equipment could be written as: Y D,k = (ED) k E F,T (8)

7 Previous results of the exergy analysis during a typical summer day [P= kpa, T= 316 K and absolute humidity = 0.03 kg.m-3] Equipment Exergy destruction (kw) ED (%) Y D (%) EE Compressor , Combustor Turbine Power plant

8 Economic analysis 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). The purchase cost of equipment based on design parameters: The levelized cost method of Moran is considered in this investigation Knowing the amortization cost (AC) for a component (k) of the power plant and The capital recovery factor (CRF), the annualized cost of an equipment (k) could be estimated as: ( C) k $. year 1 = AC k x CRF i, n k (9)

9 Cost function The capital cost rate for the equipment (k) of the plant [5]: Z K = C kx φ k 3600x N φ k and N are respectively the maintenance factor and the annual number of operating hours. Typical values of the maintenance factor (φ k =1.06) and the annual number of operating hours (N=8000). The cost function of the three main equipment of an open cycle gas turbine could be written as: Compressor Z K = 71.1x m 1 x Combustor 1 (0.9 η K ) (10) xp r xln P r x4.84 x 10 9 (11) Z cc = 46.08xm 1 x 1 + e (0.018xT ) x(0.995 (P 3 /P 2 ) 1 x 4.84 x 10 9 (12) Turbine Z T = xm fg x 0.92 η st 1 xln P 3 P 4 x 1 + e (0.036xT x4.84 x 10 9 (13)

10 Exergoeconomic analysis Exergy costing of a power generation plant involves cost balance for each component separately. As shown in the Table, in a cost balance around the k-th component, the total cost of all exiting streams (e) is equal to the total cost of the entering streams (i) plus the appropriate charges due to capital investment and the expenses for operations and maintenance ( Z k ). Equipment Cost balance Auxiliary equations Compressor C 2 = C 1 + C w,k + Z K (14) C 1 =0 (15) Combustor C 3 = C 2 + C F + Z CC (16) Turbine C 4 + C w,k + C P = C 3 + Z T (17) C w,k W K = C W,T W T (18) and C 4 E 4 = C 3 E 3 (19)

11 Cost of exergy destruction Invisible in cost balance equations, the exergy destruction cost for each equipment of the power plant could be estimated by combining the equations related respectively to the exergy destruction (ED) and the cost rate balance. The product exergy rate ( E P.k ) could be assumed fixed and the unit cost of fuel of the k component (c F,k ) is independent of the exergy destruction. In this situation, the cost of exergy destruction represents the cost rate of additional fuel that must be supplied to the component (k) of the power plant to compensate the exergy destruction within the equipment and is defined as : C D,k = c F,k x (ED) k (20)

12 The average cost of fuel (c F,k ), the average unit cost of product (c P,k ) c F,k = C F,k E F,k (21) c P,k = C P,k E P,k (22)

13 Cost rate of fuel and product for the main components of the power plant Equipment Cost rate of fuel C F ($/hr.) Cost rate of product C P ($/hr.) Compressor C w,k C 2 C 1 Combustor C 2 + C F C 3 Turbine C 3 C 4 C W,T

14 Relative cost difference (r k ) & exergoeconomic factor (f k ) r k = c P,k c F,k c F,k = 1 EE k EE k + Z k c F,k x E P,k (23) f k = Z k c F,k x[ ED) k + Z k (24)

15 Results: Values of the exergy of fuel and product for each component Equipment E F,k (MW) E P,k (MW) Compressor W K = m air (ex T2 ex T1 )= Combustor m air x ex T2 + m fuel x ex fuel = m fg. ex T3 = Turbine m fg x ex T3 ex T4 = W P + W K = 380

16 Values of the exergy of fuel and product for each component (design conditions) In the second stage of the investigation, exergy analysis and Aspen Hysys V8.6 with the Soave-Redlich-Kwong (SRK) equation of state were utilized in order to simulate the process under design conditions (T=288K, absolute humidity of kg.m -3 ). Validation of the simulation results: The net output of the power plant under design conditions (T=288K, RH=60%) is 160 MW (Source: Company) and our simulation results indicate that the net power produced at design conditions is 163 MW. The relative shift of 1.9% presents a validation of the simulation. The corresponding values of the exergy of fuel and product for each component during design conditions are shown in Table : Equipment E F,k (MW) E P,k (MW) Compressor W K = m air (ex T2 ex T1 )= Combustor m air x ex T2 + m fuel x ex fuel = m fg. ex T3 = Turbine m fg x ex T3 ex T4 = W P + W K = 362.8

17 RESULTS OF THE THERMOECONOMIC ANALYSIS OF THE POWER PLANT For a selected cost of natural gas of 2.6$/GJ [12], an exergy of natural gas of 48.6 MJ/kg and the flow rate of 9.6 kg/s, the total cost of the fuel is estimated at 4232 $/hr. Cost rate of the different equipment and streams of the plant is shown in Table. Stream C ($/hr.) c ($/GJ) W K W T Fuel

18 Exergoeconomic parameters for each equipment respectively for design conditions and summer conditions Equipment EE (%) Compressor Combustor Turbine Equipment EE (%) Compressor Combustor Turbine ED Y D Z K c F,k c P,k C D,k C D,k + Z k r k f k (MW) (%) $/hr. $/GJ $/GJ ($/hr.) ($/hr.) (%) (%) ED Y D Z K c F,k c P,k C D,k C D,k + Z k r k f k (MW) (%) $/hr. $/GJ $/GJ ($/hr.) ($/hr.) (%) (%)

19 ANALYSIS OF RESULTS According to the literature, the components having the highest value of the sum ( C D,k + Z k ) are the most important components from the exergoeconomic viewpoint. The value of the relative cost difference (r) for a component shows the degree to which it contributes to increasing the final cost of the product based on the input of the component. The component having a low value of the exergoeconomic factor (f) means that the greater part of the product cost is due to exergy destruction and loss.

20 Combustor For the summer atmospheric conditions, the combustor has the highest value of the exergy destruction of MW and the highest value of the sum ( C D,k + Z k = $/hr.). The combustor has also the highest contribution to the cost of the final product (r=18.2) and the lowest contribution of the capital investment (f k = 2.2). The analysis of these variables indicate that the high cost of the product is mainly due to the cost of exergy destruction within the combustor. This situation becomes more important during summer conditions. This could be explained by the fact that the fuel supply may not meet the gas turbine combustion requirements. Moreover, it could be cost effective to invest in a process control system based a continuous measurement of both O 2 and CO leaving the combustor. This could provide the needed information for effective combustion for significant energy savings by minimizing excess air.

21 Turbine For the summer atmospheric conditions, the turbine has the lowest exergy destruction of MW and the second highest value of the sum ( C D,k + Z k = $/hr.). The turbine has also the lowest contribution to the cost of the final product (r=10.4) and the highest contribution of the capital investment (f k = 51.4). Since the turbine has the highest capital cost rate, the large decrease of its exergy destruction (55%) during summer conditions did not have an important impact on the value of the exergoeconomic factor. However, compared to design conditions, the power plant lost 4.66 % of its net power output. This problem could be partially solved by reducing the exergy loss due to the high temperature of the exhaust gas (700 K). It is then recommended to invest in a heat recovery steam generator (HRSG) system to increase the net power output of the plant.

22 Compressor For the summer atmospheric conditions, the compressor has the second highest exergy destruction of MW and the lowest value of the sum ( C D,k + Z k = $/hr. ). The compressor has also the second highest contribution to the cost of the final product (r=16.9) and the second highest contribution of the capital investment (fk= 45.3). The findings are in concordance with the increase of the exergy destruction of 16.2% during summer conditions. Previous results show that temperature has more negative effects than the absolute humidity on the power plant. A thermo-economic optimization study is needed in order to select between the different cooling systems (ex: fogging cooling) in order to decrease the negative effects of the high temperatures on the performance of the compressor.

23 Conclusion The effects of summer conditions on the exergy destruction within the three main components of the open cycle gas turbine and their exergy efficiencies were estimated in a previous investigation and summarized in this paper. Summer atmospheric conditions have increased the total exergy destruction of the power plant and its cost rate respectively by 1.95% and 10%. The increase of the cost of exergy destruction within the combustion chamber and the compressor (respectively 19.8% and 10.5%) are compensated by a decrease of 14.3% of the cost of exergy destruction within the turbine

24 The study showed that the effects of increasing ambient temperature on the combustion chamber attracts the maximum cost in terms of exergy destruction and, thus, constitutes the prime target for capital investment. The effects of irreversibilities within the combustion chamber could be minimized by selecting the adequate fuel to meet the requirements of the combustion and investing in a process control system based on the measurements of the concentration of oxygen and carbon monoxide in the exhaust gas. The cost of exergy destruction in the compressor could be lowered by investing in a system for cooling ambient air before compression. The exergy loss due to the high temperature of the exhaust gas could be minimized by adding a heat recovery steam generator (HRSG) system at the hot stream leaving the turbine.

25 Reference This applied research is based on a methodology described in: Bejan A; Tsatsaronis G., Moran M: Thermal Design and Optimization; J. Wiley & Sons Edition. (1996) Ahmed Nabil Al Ansi, Mubarak Salem Ballaith, Hassan Ali Al Kaabi, Zin Eddine Dadach: Exergoeconomic Analysis of a Power Plant in Abu Dhabi (UAE), International Journal of Energy Engineering Dec. 2015, Vol. 5 Iss. 6, PP