Efficiency improvement and exergy destruction

Journal of Engg. Research Vol. 1 - (1) June 2013

pp. 289-315, 2013

Eciency improvement and exergy destruction reduction by combining a power and a multi-eect boiling desalination plant A. A. ALSAIRAFI*, I. H. AL-SHEHAIMA* AND M. DARWISH** *Department of Mechanical Engineering, College of Engineering and Petroleum, Kuwait University - P.O. Box 5969, Safat 13060, Kuwait ** Qatar Environment and Energy Research Institute, Qatar Foundation - P.O. Box 5825, Doha, Qatar

ABSTRACT Electric power and desalted seawater demand is increasing in Kuwait mainly due to residential and industrial growth, especially in summer season. In the past six years, Kuwait citizens have been facing the problem of automated power and water disconnection because of the electricity and water production is lower than the consumption. A common idea for resolving such a problem is to build new power plants but this solution is not practical due to environmental issues. Another choice but more engineer challenging approach for resolving this problem is to improve the eciency and performance of the already existing power plants. Currently, there are six power plants in Kuwait; four of them have both stand-alone gas-turbine and steam-turbine power plants, one is steam power plant and one is gas turbine power plant. Combined power and desalination plant are more attractive in Kuwait since they have higher thermal eciency than traditional ones and both electric power and process heat (e.g., desalting) can be produced simultaneously. The relatively low temperature multi-eect desalination (MED) process (around 75 o C saturated temperature as the heat source) is thermodynamically the most ecient of all thermal distillation processes (source, and consumes about 2 kWh/m3 pumping energy). In this study, factors aecting the performance of a combined power and MED-desalination plant have been studied. This includes the atmospheric humidity, compressor inlet air temperature, top brine temperature, desalination unit capacity, cooling water temperature, and the number of evaporation stages of the MED unit. A ®rst- and second-law analysis of the proposed system was carried out under several operating conditions. As an example, a 125 MW Siemens V94.2 gas turbine of Al-Zour gas turbine power plant in Kuwait has been selected. It is found that the overall thermal eciency of the proposed system increases signi®cantly as the desalination unit capacity increases and this increase can reach 25% (from th;cc = 55.1% to 69.9% at T1 = 30oC and = 0.30) as the capacity increases from 1 to 5 MIGD. The total work generated decreases insigni®cantly, i.e. the low pressure steam turbine power decreases from 27 MW to 23 MW while the total power output decreases from 208 MW to 204 MW. In addition, as the desalination unit gain ratio increases, the total exergy destruction is reduced, i.e. the desalination unit exergy destruction increases from 12 MW to 15 MW but the total exergy destruction of the

290

A. A. Alsaira®, I. H. Al-Shehaima and M. Darwish

cycle decreases from 352 MW to 349 MW. It can be concluded that the low-temperature MED process oers a potential ecient solution to the current Al-Zour power generation plant. Keywords: Combined cycles; energetic analysis; exergy analysis; heat recovery

steam generator - HRSG; multi-eect distillation - MED. NOMENCLATURE

Acronyms

BPE EES GT HRSG MED MSF NF RO ST TBT TIT TVC UF

Boiling point elevation Engineering Equation Solver Gas turbine Heat recovery steam generation Multi-Eect distillation Multi-Stage Flash Nano®ltration Reverse Osmosis Steam turbine Top brine temperature Turbine inlet temperature Thermal Vapor Compression Ultra®ltration

Notation

Cavg D Di

1T E_ f GR h

Average speci®c heat Total distillate water Distillate water for each eect Temperature dierence between the evaporation stages Rate of energy input by fuel Gain ratio Speci®c enthalpy

Eciency Improvement and Exergy Destruction Reduction by Combining a Power ...

TD

End condenser eectiveness Average latent heat for the desalination unit mass ¯ow-rate Desalination unit number of evaporation stages Pressure at each state Rate of entropy Speci®c entropy Desalination unit average temperature Cooling water temperature State temperature Temperature after feed water heater Temperature at each evaporation stage (eect) of desalination unit

Tn

Last evaporation stage (eect) temperature

k Lavg

_

m Neects P

_

S s Tavg tc T tb

Tv Uc Ue UFH V

_ X_

WE

Xavg Xb Xf

Vapor temperature at each evaporation stage (eect) Condenser overall heat transfer coecients Desalination eect overall heat transfer coecient Feed water heaters overall heat transfer coecient Velocity Gas turbine net power output Rate of exergy destruction Average water salinity Brine salinity Feed salinity

Greek Letters I II

9 water

First-law Eciency Second-law Eciency Relative humidity Speci®c exergy Water density

291

292


Subscripts b e i in isen gen out

0 1, 2...

Boundary Exit of the proposed control volume Inlet to the proposed control volume Inlet to the proposed control volume Isentropic Generation Exit of the proposed control volume Dead-state conditions State points in the Fig. 1 INTRODUCTION

Kuwait depends on oil products for producing both electrical energy and fresh drinking water. The annual electricity consumption is 45.2 billion kWh (about 13142 kWh per capita), whilst the consumption of fresh water averages 129780 MIG (about 103 IG/day per capita) as given by Statistical year book on electricity, 2009. Kuwait is ranked the 6th nation of the world in terms of electrical consumption per capita due to massive utilization of air conditioning systems, use of fossil fuel to operate desalination facilities for producing fresh water, and poor electricity rationalization. The desalinated water represents 73.5% of the total national water resources, and 93.0% of fresh water (Darwish & AIÂ-Najem 2005). Kuwait is approaching a stage of suering from shortage of natural fresh water which is needed for day to day drinking, washing, agriculture, and industrial utilization. The natural water that comes from the Arabian Gulf has a high concentration of salt. In addition, water drawn from underground aquifers in Kuwait is most often very brackish and completely undrinkable. Apparently, the lack of fresh water is threatening the country development. Increasing the demand for fresh water will be a necessity in the near future due to the expected rise in population rates, enhanced living standards, and the growing industrial and agricultural developments. Today, Kuwait depends on desalination technologies for the production of fresh water that has less than 500 mg/1 dissolved solids (Alawadhi 2002). A number of seawater desalination techniques have been developed worldwide during the last several decades to increase the supply of fresh water in dry areas of the world. Among them, there are two major desalination techniques that are commercially used in dierent parts of the world: thermal and membrane


293

methods. Thermal desalination comprises several technologies such as MultiStage Flash (MSF) (Darwish et al., 1995), Multi-eect distillation (MED) (ElNashar & Qamhiyeh 1995; Al-Shammiri & Safar 1999), and Thermal Vapor Compression (TVC) (Darwish 1988; Aly 1995; Hamed et al., 1996), while desalination by the membrane method comprises Reverse Osmosis (RO) (Van der Bruggen 2003; BoÂ dalo-Santoyo et al., 2003), Nano®ltration (NF) (Van der Bruggen et al., 2003), Ultra®ltration (UF) (Cherkasov et al., 1995), Electrodialysis and Electrodialysis Reversal (Grebenyuk et al., 1998). Comparison between the thermal and membrane desalination techniques can be found in several references (Darwish et al., 1989; Afgan et al., 1999; Darwish 2007). Increasing the thermal eciency of the existing power plants in Kuwait would reduce the amount of fuel required to produce the current level of electric power generation from these plants and likely reduce other environmental emissions. Seawater desalination can be integrated with a power plant by the utilization of extracted steam as the source of heat for thermal desalination. This results in a signi®cant lowering of the energy cost of seawater desalination (Darwish et al., 2006; Darwish et al., 2007). On the other hand, combined power and water systems in which high grade energy is used to produce power in a power plant and low-grade power plant output heat to run a thermal desalination unit to produce fresh water from saline water is generally more attractive since it is a more energy, economy and environment-pro®table way when compared with separate power-only and wateronly systems (Afgan et al., 1999). The electricity and water consumption in hot, dry countries depends on the time of year with maxima for both water and electricity in the summer months and markedly reduced electricity consumption in winter, although water consumption does not fall to the same extent. The development of electric power and desalination plants in recent years is increasing the interest in understanding the plants currently in use since the cost of electric power and desalinated water are still high for many people, so there is a need to learn how to reduce costs as well as understand how the techniques work before trying to develop new technologies. This study proposes a system which can be applied to an existing gas turbine thermal power plant by extracting steam from a low-pressure steam turbine as energy source input to run a multi-eect desalination (MED) unit. The application of such a method would provide a potential route to examine MED-thermal power plant technique that could be utilized to produce more clean and ecient system in the future. A simple thermodynamic analysis of the system is performed using mass, energy and exergy balance. In addition to the system eciency enhancement, a relationship between the total exergy destruction of the proposed system and the gained ratio of the desalination unit was obtained to evaluate and improve system eciency at a given state in Kuwait environment and to identify suitable operating conditions to minimize exergy destruction.

294


MULTI-EFFECT DISTILLATION TECHNOLOGY

A seawater desalination process separates saline seawater into two streams: a fresh water stream containing a low concentration of dissolved salts and a concentrated brine stream. Previously, multi-eect distillation (MED) was used to evaporate saline water in order to get drinking water and salt as commodities, see Figure 1. Earlier con®gurations suered from scaling problems due to the high temperatures involved in the process. Nonetheless, newer developed con®gurations are able to operate at lower temperatures which minimize scaling and other problems. The operating temperatures run around 70oC, and the plant size depend on the number of eects used (usually from 8 to 16 evaporation stages ``or eects'').

Fig. 1: Schematic diagram of the proposed combined cogenerative power plant

with a multi-eect distillation process


295

METHOD Thermodynamic analysis for the combined cycle

To determine the performance of the proposed system, the steady-state component models are used. Every component is modeled in consideration of mass, energy, and second law of thermodynamics balances. In this study, the hot products of combustion is idealized as air with variable speci®c heats which is produced in the gas turbine cycle and is selected as a typical waste heat to the heat recovery steam generation for the proposed system analysis. Main parameters of the proposed combined cycle for the calculations are listed in Table 1. The values within the parentheses represent the variable range for parametric analysis while given by manufacturer or assumed in this work. The thermodynamic properties of the working ¯uid are calculated by the engineering equation solver EES, which has high accuracy thermodynamic and transport property database (Klein 2008). Table 1. Main input parameters to the combined cycle program.

Compressor inlet temperature [oC] Compressor pressure ratio Turbine inlet temperature [oC] Fuel Lower Heating Value [kJ/kg fuel] Fuel Higher Heating Value [kJ/kg fuel] Humidity ratio [%] Low pressure in HRSG [kPa] High pressure in HRSG [kPa] Condenser pressure [kPa] Pinch point temperature [oC] Deaerator pressure [kPa]

15 [0-55] 11.65 1076 43100 46200 15 [0-100] 300 10000 10 15 200

The thermal power plant components' equations for the proposed system analysis are based on the following assumptions: 1. The ¯ow is steady and the state of the working ¯uid at each speci®c location within the system does not change with the time. 2. The open loop GT cycle is simple (no regeneration or intercooling) with air as a working ¯uid. 3. Pressure drop and heat loss in pipe lines are assumed negligible.

296


4. To avoid moisture erosion at the steam turbine outlet, the quality of the saturated water vapor turbine exhaust is maintained higher than 88.0%. 5. The reactants and the exhaust gases of the combustion chamber are considered as ideal gases. 6. The compressor and the gas turbine have an isentropic eciency of 88.3%. 7. The steam turbine and the pumps have an isentropic eciency of 80.0% and the combustion eciency is 99.0%. 8. The HRSG unit is a dual-pressure superheater and economizer counter current heat exchanger. 9. Steam leaving the steam cycle condenser and deaerator is assumed saturated liquid. The kinetic and potential energy changes of the steam are usually small in comparison to the work and heat transfer terms and are therefore usually neglected. The governing equations include three main equations of mass, energy and entropy balances as well as gas-vapor mixture equations and are taken from reference (Cengel & Boles 2008). Thus, the steady-¯ow mass and energy equations reduce to:

X _ ÿX _ 0 X X _ ÿ _ _ ÿ _ mi

Q_ in ÿ Q_ out

Win

1

me

Wout

mi hi

me he

0

2

where m_ is the mass ¯ow rate of the system, Q_ is the heat rate, W_ is the power, and h is the enthalpy of the system. The relative humidity, , is the ratio of the amount of moisture that the air holds (mv) to the maximum amount of moisture the air can hold at the same temperature (mg), which can be written as: '

mv mg

PPv

g

3

where Pv is the partial pressure of water vapor, and Pg is the partial pressure of dry air. Pg is the saturation pressure at a given temperature. After ®nding Pv, and given the total mixture pressure P, the humidity ratio ! can be determined as (Cengel & Boles 2008): 0:622'Pg 4 ! P ÿ 'Pg


297

The objective product of the overall cycle is the generated electrical energy,

_ . The thermodynamic performance is based on the ®rst law eciency, and is

WE

de®ned as:

I

_

W_ E

5

Ef

where E_ f is the rate of fuel energy input to the GT cycle. W_ E For simple cycle, is equal to the gas turbine net power output, i.e. W_ GT , and for the overall combined cycle, W_ E is equal to the total work output, i.e. W_ GT W_ ST . The waste heat recovery steam generation eciency is de®ned as: HRSG

m_ 7 h8 ÿ h7Q m_ 16 h17 ÿ h16

6

HRSG;in

The thermal eciency of the actual cycle is reduced during operation because it involves irreversibilities. The sources of irreversibilities could be external (i.e. heat loss by radiation and convection) or internal (i.e. friction, chemical reactions). A second-law analysis of these cycles reveals where the largest irreversibilities occur and what their magnitudes are. In recent decades, exergetic performance analysis has found a useful method to identify the location, magnitude and sources of irreversibilities of thermal power plants (Verkhivker & Kosoy 2001; Rosen & Dincer 2004). In addition, it also provides a more meaningful assessment of plant individual components' eciency. The exergy of a ¯uid stream 9 at any state can be determined from: 2

h ÿ h0 ÿ T0s ÿ s0 V2 gz

7

The exergy destruction for a steady-¯ow system can be expressed, in the rate form, as: X_ dest T0 S_ gen T0

X _

_

Qsys;out ÿ me se Tb;out

X_

_

Qsys;in mi si ÿ Tb;in

8

where Tb,in and Tb,out are the temperatures of the system boundary in which heat is transferred into and out of the system, respectively. The de®nition for the second-law eciency applies to devices that are not intended to produce or consume work. The second-law eciency of a system may be de®ned during a process as:

298


II

recovered destroyed 1 ÿ Exergy Exergy Exergy supplied Exergy supplied

9

The consumption of exergy or work potential during a process was calculated to determine the second-law eciency. The exergy can be supplied or recovered at various amounts in various forms such as heat, work, internal energy, and enthalpy. The exergy recovered and the exergy destroyed (the irreversibility) must add up to the exergy supplied; hence, the system was de®ned precisely in order to identify correctly any interactions between the system and its surroundings. Thermodynamic analysis for the desalination unit

A mathematical model for utilization an extracted steam as energy input to feed a MED desalination unit has been developed based on the Al-Zour gas turbine power plant which consists of Figure 1 a combined power plant with a backpressure steam turbine. The MED desalination plant is composed of one desalination unit with variable unit producing capacity. The desalination unit main parameters are given in Table 2. The required MED capacity in MIGD can be converted to kg/s as follows: D Capacity 2 0:004546 2 1000000 2 water 2 24 2 3600

10

To simplify the analysis there are some assumptions used: 1. The speci®c heat (Cavg) of distillate, feed water and brine are assumed to be constant and evaluated at the average boiling temperature in the ®rst and last eect. 2. The latent heat (Lavg) is assumed to be constant for steam as well as the vapor generated in each eect it is evaluated at the same above average temperature. 3. Equal temperature drops (1T) between the MED evaporation stages and between the steam and top brine temperature in the ®rst MED eect. 11 1T TND;1 ÿ TÿD1;n effects 4. Boiling point elevation (BPE) is constant through the eect and it is evaluated at Tavg and Xavg, where Xavg is calculated as follows: Xf Xb 12 Xavg 2


299

5. Overall heat transfer coecient of the eects (Ue) is assumed to be constant. 6. Overall heat transfer coecient of the feed water heaters (HFH) is assumed to be constant 7. Condenser parameters (Ue) overall heat transfer coecient of the eects, (k) end condenser eectiveness and (tc) cooling water temperature are constant. 8. Generated vapor temperature (Tv;n) in each eect can be calculated from: Tv;n

TD;n ÿ BPE

13

Table 2. Main input parameters to the desalination unit program

Desalination unit capacity [MIGD] 5 [1-5] Last eect temperature, Tn [oC] 38 Brine salinity [ppm] 72000 Feed salinity [ppm] 42000 Desalination eect overall heat transfer coecient, 3 Ue [W/m2.K] Feed water heaters overall heat transfer coecient, 2.4 UFH [W/m2.K] Condenser overall heat transfer coecients, Uc [W/ 2.4 m2.K] End condenser eectiveness, k 0.7 Desalination unit number of eects, Neects 10 [6-22] o 28 [5-30] Cooling water temperature, tc [ C] o Top brine temperature, TBT [ C] 70 [65-75] Average latent heat for the desalination unit, Lavg 2348 [kJ/kg] Boiling point elevation, BPE 0.7 Average speci®c heat for seawater, cavg [kJ/kg.K] 3.9

RESULTS AND DISCUSSIONS

The MED desalination unit is coupled with a conventional gas/steam combined cycle. The eect of its coupling on the gas/steam cycle is studied used the Engineering Equation Solver (EES) software to evaluate thermodynamic properties. The gas turbine component models are initially

300


validated by comparison with the computed results from (Cengel & Boles 2008) using a range of pressure ratio and dierent isentropic eciencies for the compressor and the gas turbine. Then, the EES code results are compared with the actual data from the Siemens gas turbine manufacturer's data to demonstrate the models' accuracy. The simulation results from this work are in good agreement with the Siemens data, and the maximum deviation is insigni®cant (less than 5.0%) for the main operating parameters such as T, P, power output, and thermal eciency. After that, the EES code results match the results are compared with the actual data from (Darwish & Al Najem 2004) for combined cycle with back pressure steam turbine driving MED system. Also, the parametric study has the same results as the one given in (Al-Fahed et al., 2009) for the eect of inlet air temperature and relative humidity on the gas cycle performance. Figure 2 shows the eect of top brine temperature on combined cycle thermal eciency at dierent inlet air temperature (15 oC, 30 oC, 50oC) and relative humidity (0.3 and 0.6). The thermal eciency increases by 0.1% when the TBT increases from 65 oC to 75oC at a ®xed ambient temperature and relative humidity. It is a very small dierence because the MED works on low steam grade. On the other hand, a minimal eect of 0.1% increase is noticed in the cycle eciency when the relative humidity is increased at the same inlet air temperature. If the inlet air temperature is increased at the same relative humidity, the hybrid cycle eciency increases by 2.0%. The main reason is the reduction in the fuel mass. 56.2

η th,CC [%]

56.0 55.8

@ T[1]=15oC, φ [1]=0.6

55.6

@ T[1]=15oC, φ [1]=0.3

55.4

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

55.2 55.0 54.8 54.6 54.4 54.2 54.0 64

66

68

70

TBT [oC]

72

74

76

Fig. 2: The eect of top brine temperature on the hybrid combined GT-ST-MED

cycle eciency at dierent inlet air temperature and relative humidity


301

72.5 @ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

68.5

η th,CC [%]

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

64.5

60.5

56.5

52.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Capacity [MIGD]

Fig. 3: The eect of desalination unit capacity on the hybrid combined GT-ST-MED

cycle eciency at dierent inlet temperature and relative humidity

Figure 3 shows the eect of the desalination unit capacity on the hybrid cycle thermal eciency at dierent inlet temperature and relative humidity. The hybrid cycle thermal eciency increases as the capacity is increased. The thermal eciency increases by 25.0% when the capacity is increased at constant relative humidity. The main cause of this increase is using low grade steam to produce large amount of distillate water. The heat recovered by the desalination unit is increased signi®cantly. If the capacity and relative humidity are ®xed, an increase of 6.0% in hybrid thermal eciency is noticed when the ambient temperature increases because the fuel mass ¯ow rate is decreased. At the same capacity and ambient temperature, the relative humidity has no eect on the thermal eciency. The number of evaporation stages is very important and studying its eects is essential. Figure 4 shows the number of evaporation stages' eects on the combined cycle eciency at dierent inlet temperature and relative humidity. The thermal eciency decreases by 7.0% as the number of eects is increased from 6 to 22 at 15oC inlet air temperature and = 0.6. The thermal eciency increases by 1.0% when the inlet air temperature is increased at same relative humidity. This is due to reduction in the mass of steam extracted which leads to a signi®cant reduction in the heat consumed by desalination unit.

302

A. A. Alsaira®, I. H. Al-Shehaima and M. Darwish 59.0 58.0

@ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

η th,CC [%]

57.0

@ T[1]=30oC, φ 1]=0.3 @ T[1]=50oC, φ [1]=0.3

56.0 55.0 54.0 53.0 52.0

5

9

13

Neffects

17

21

25

Fig. 4: The eect of the number of desalting stages on the combined cycle eciency

at dierent inlet temperature and relative humidity

Figure 5 presents the eect of cooling water temperature on hybrid cycle at dierent inlet temperatures and relative humidity. At constant relative humidity, the increase in the cooling water temperature results in a decrease in the thermal eciency by 2.0%. This decrease is caused by the reduction in extracted steam mass ¯ow rate which reduces the heat consumed by the desalination unit. The thermal eciency increases by 1.0% as inlet air temperature is increased at constant cooling water temperature. A minimal eect of 0.7% increase in the thermal eciency is predicted as the relative humidity is increased from 0.3 to 0.6 at ®xed cooling water temperature because the fuel mass ¯ow rate is reduced. 57.4

η th,CC [%]

57.1 56.7

@ T[1]=15oC, φ [1]=0.6

56.4

@ T[1]=15oC, φ [1]=0.3 @ T[1]=30oC, φ 1]=0.3

56.0

@ T[1]=50oC, φ [1]=0.3

55.7 55.3 55.0 54.6 54.3 53.9 0

5

10

15

tc [°C]

20

25

30

35

Fig. 5: The eect of cooling water temperature on the combined cycle eciency


Figure 6 shows the eect of top brine temperature on the net power produced


303

by steam turbine at dierent inlet temperature and relative humidity. An insigni®cant increase by 0.6% in steam turbine power output when top brine temperature was increased from 65oC to 75oC at = 0.6 and 15oC inlet air temperature. Top brine temperature increases resulting in an increase in the extracted steam mass ¯ow rate which reduces the steam cycle power output. A reduction of 11% in steam power output when the inlet air temperature is increased at = 0.3 and 75oC top brine temperature. This 11% reduction occurs because the exit temperature from the gas turbine is reduced; therefore, less energy enters the heat recovery steam generation. There is no eect of relative humidity on the steam cycle power output at constant relative humidity and inlet air temperature. 69 68 @ T[1]=15oC, φ [1]=0.6

67

@ T[1]=15oC, φ [1]=0.3 @ T[1]=30oC, φ [1]=0.3

WST,net [MW]

66

@ T[1]=50oC, φ [1]=0.3

65 64 63 62 61 60 65

67

69

TBT [°C]

71

73

75

Fig. 6: The eect of top brine temperature on the net power produced by steam turbine


The eect of MED desalination unit capacity on the steam turbine power output at dierent inlet temperature and relative humidity is shown in Figure 7. The net power decreases by 6% when the MED capacity is increased at = 0.6 and 15oC inlet air temperature. This 6% reduction is due to large amount of extracted steam. It is noticed that as the air inlet temperature increases, the steam cycle net power output decreases by 12% at constant relative humidity and capacity. The cause of this 12% reduction is the decrease in exit temperature and energy entering the heat recovery steam generation. There is no eect of humidity at steam cycle power output at constant inlet air temperature. Figure 8 shows the number of evaporation stages eect on the steam turbine net power at dierent inlet temperatures and relative humidity. If the number of evaporation stages is increased, the steam power output increases by 2% at

304


15oC inlet air temperature and = 0.3. The main reason is the reduction in the extracted steam mass ¯ow-rate. The steam power output decreases by 11%, when the inlet air temperature is increased at constant number of evaporation stages and relative humidity. 70 68

@ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

WST,net [MW]

66

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

64 62 60 58 56

1

1.5

2

2.5

3

3.5

4

4.5

5

Capacity [MIGD]

Fig. 7: The eect of desalination unit capacity on the net power produced by steam turbine


69

67

@ T[1]=15oC, φ [1]=0.6

WST,net [MW]

@ T[1]=15oC, φ [1]=0.3 @ T[1]=30oC, φ [1]=0.3

65

@ T[1]=50oC, φ [1]=0.3

63

61

59 5

9

13

Neffects

17

21

25

Fig. 8: The eect of desalination unit evaporation stages on the net power produced

by steam turbine at dierent inlet temperature and relative humidity

The eect of desalination unit capacity on the combined cycle net power output at dierent inlet temperature and relative humidity is presented in Figure 9. The result shows that as the capacity is increased, the net total power


305

decreases linearly by 2% at = 0.6 and 15oC inlet air temperature. Because of the increment in the capacity, the amount of extracted steam increases and the steam cycle power decreases which aect the total power. Also it shows that as the inlet air temperature increased, the net total power decreases by 18% at constant capacity and relative humidity. A 0.3% minimal eect on the net total power output, when the relative humidity increases. 230

@ T[1]=15oC, φ [1]=0.6

220

@ T[1]=15oC, φ [1]=0.3

Wtotal,net [MW]

@ T[1]=30oC, φ [1]=0.3

210

@ T[1]=50oC, φ [1]=0.3

200

190

180 1

1.5

2

2.5

3

3.5

4

4.5

5

Capacity [MIGD]

Fig. 9: The eect of desalination unit capacity on the net power produced by combined cycle


Figure 10 shows the eect of top brine temperature on the steam turbine exergy destruction at dierent inlet temperature and relative humidity. Steam cycle exergy destruction increases by a small amount of 0.6% when the top brine temperature is increased at 15oC inlet air temperature and = 0.3. The cause of the 0.6% increment is due to the extracted steam mass ¯ow rate increase which resulting high exergy value. At constant top brine temperature and relative humidity, the steam cycle exergy destruction decreases by 9% when the inlet air temperature increased. The main reason being that the energy of the exhaust gases is reduced as the inlet air temperature is increased, and the steam cycle exergy is reduced.

306

A. A. Alsaira®, I. H. Al-Shehaima and M. Darwish 74 72 @ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

Xdest,ST,cycle [MW]

70

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

68 66 64 62 60 65

67

69

TBT [°C]

71

73

75

Fig. 10: The eect of top brine temperature on the exergy destruction by steam turbine cycle


Figure 11 shows the eect of the capacity on the steam cycle exergy destruction at a dierent inlet temperature and relative humidity. The steam cycle exergy destruction increases by 14% as the capacity is increased at 15oC inlet air temperature and = 0.3. This is due to an increase in extracted mass ¯ow rate which has high exergy. A reduction of 16% in steam cycle exergy destruction is recorded when the inlet air temperature is increased and the capacity and relative humidity are ®xed. 84

Xdest,ST,cycle [MW]

80

@ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

76

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

72 68 64 60

1

1.5

2

2.5

3

3.5

4

4.5

5

Capacity [MIGD]

Fig. 11: The eect of desalination unit capacity on the exergy destruction by steam turbine cycle



307

The eect of evaporation stages number for the desalination's unit on the steam cycle exergy destruction at dierent inlet temperature and relative humidity is shown in Figure 12. A reduction of 4% in the steam cycle exergy destruction occurs when the number of evaporation stages is increased and the inlet air temperature and humidity is decreased. The 4% reduction in steam cycle exergy is caused by the decreased of steam extracted from the steam turbine which has high exergy value. The steam cycle exergy destruction decreases by 8% as the inlet air temperature is increased and the relative humidity and number of evaporation stages were ®xed. Figure 13 shows the eect of top brine temperature on the desalination exergy destruction at a dierent inlet temperature and relative humidity. The results show that as the top brine temperature is increased, the desalination exergy increases by 22% at 15oC inlet air temperature and 0.3. Because of the extracted steam ¯ow rate is increased, the desalination exergy increases by 22%. Extracted steam ¯ow rate has a high exergy value. It is also shown that as the inlet temperature is increased, the exergy destruction decreases by 55%. A decrease in the energy leaving the gas turbine will result in a reduction of 55% in exergy which leads to a decrease in the steam cycle exergy. 74 72

@ T[1]=15oC, φ [1]=0.6 @ T[1]=15oC, φ [1]=0.3

Xdest,ST,cycle [MW]

70

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

68 66 64 62 60 5

9

13

Neffects

17

21

25

Fig. 12: The eect of desalination unit evaporation stages on the exergy destruction

by steam turbine cycle at dierent inlet temperature and relative humidity

308

A. A. Alsaira®, I. H. Al-Shehaima and M. Darwish 3.0

@ T[1]=15oC, φ [1]=0.6

2.6

@ T[1]=15oC, φ [1]=0.3

Xdest,des [MW]

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

2.1

1.7

1.2

0.8 65

67

69

71

73

75

TBT [°C]

Fig. 13: The eect of top brine temperature on the exergy destruction by desalination unit


Figure 14 shows the eect of the capacity on the desalination exergy destruction at dierent inlet temperature and relative humidity. A reduction of 60.5% in desalination exergy destruction is recorded when the inlet air temperature is increased at 5 MIGD capacity and = 0.3. The main reason is a reduction in the energy leaving the gas turbine which leads to a reduction in the exergy in steam turbine cycle. The exergy destruction increases four times as the capacity is increased from 1 to 5 MIGD at constant inlet air temperature and relative humidity. This is due to an increase in steam extracted from steam turbine. 14 @ T[1]=15oC, φ [1]=0.6

12

@ T[1]=15oC, φ [1]=0.3 @ T[1]=30oC, φ [1]=0.3

Xdest,des [MW]

10

@ T[1]=50oC, φ [1]=0.3

8 6 4 2 0 1

1.5

2

2.5

3

3.5

4

4.5

5

Capacity [MIGD]

Fig. 14: The eect of desalination unit capacity on the exergy destruction by desalination unit



309

Figure 15 shows the number of evaporation stages eect on the desalination exergy destruction at dierent inlet temperatures and relative humidity. It is noticed that as the number of eects increased, the desalination exergy destruction decreases. At a number of evaporation stages less than 10, the change in exergy destruction is greater than the number of evaporation stages more than 10 stages. A reduction of 70% in exergy destruction occurs if the number of evaporation stages is increased from 6 to 22 at a constant inlet air temperature and humidity. This is due to the reduction in the steam mass extracted from the steam turbine. Also a 63% decrease in exergy destruction is noted when the inlet air temperature is increased from 15oC to 30oC at a constant relative humidity and a number of eects. The exergy destruction decreases by 63% because the energy leaving the gas turbine is decreased which reduces the steam cycle exergy. 5 @ T[1]=15oC, φ [1]=0.6

4

@ T[1]=15oC, φ [1]=0.3

Xdest,des [MW]

@ T[1]=30oC, φ [1]=0.3 @ T[1]=50oC, φ [1]=0.3

3

2

1

0 5

7

9

11

13

Neffects

15

17

19

21

23

Fig. 15: The eect of desalination unit evaporation stages on the exergy destruction by desalination

unit at dierent inlet temperature and relative humidity

The eect of cooling water temperature on the desalination exergy destruction at dierent inlet temperature and relative humidity is presented in Figure 16. MED exergy destruction decreases by 19% when cooling water temperature is increased at constant inlet air temperature and relative humidity. The cause is the reduction in steam mass ¯ow rate extracted to the MED desalination unit. A 60% decrease in MED exergy destruction occurs as the inlet air temperature increased at constant cooling water temperature and relative humidity.

310

A. A. Alsaira®, I. H. Al-Shehaima and M. Darwish 3.5 @ T[1]=15oC, φ [1]=0.6

3.0

@ T[1]=15oC, φ [1]=0.3 @ T[1]=30oC, φ [1]=0.3

Xdest,des [MW]

2.5

@ T[1]=50oC, φ [1]=0.3

2.0 1.5 1.0 0.5 0.0 0

5

10

15

tc [°C]

20

25

30

35

Fig. 16: The eect of cooling water temperature on the exergy destruction by desalination unit


Figure 17 shows the exergy destruction of desalination unit as a function of the gain ratio at dierent desalination unit capacity. For a 1 MIGD capacity, it is noticed that the exergy destruction of the desalination cycle decreases linearly with the gain ratio; this decrease is 3.78% as the gain ratio increases from 6.85 to 7.1. On the other hand, the exergy destruction is decreased linearly by 18.23% when the gain ratio increased from 6.885 to 7.1 at constant capacity of 5 MIGD. At constant gain ratio and top brine temperature, the exergy destruction is increased by 100% when the capacity increased from 1 to 2 MIGD. The most important conclusion from this ®gure is that high gain ratio and lower exergy destruction can be obtained for the relatively low top brine temperature. A multi-eect desalting system consumes both thermal and mechanical energy. The thermal energy is extracted from steam turbine where the speci®c consumed heat is equal to 23.78 kJ/kg of distillate water. The mechanical energy consumed by the unit's pumps to move the streams is in the range of 2 kWh/m3; and for MSF is 4 kWh/m3 (Darwish et al., 2010). The mechanical energy equivalent to the MED consumed thermal energy has a value of 6.605 kWh/m3 while for MSF system is equal to 14 kWh/m3. This makes the consumed equivalent mechanical energy by the MED system as 8.605 kWh/m3 and 18 kWh/m3 for MSF. The results show that the consumed equivalent mechanical energy will be reduced by 38.5% by using MED system rather than using the widely used MSF in Kuwait. The MED system is an attractive desalting system in Kuwait since the cost of water production will be reduced by 38.5%.


311

16 Capacity = 1 M IGD

14

Capacity = 2 M IGD Capacity = 3 M IGD

Xdest,des [MW]

12

Capacity = 4 M IGD Capacity = 5 M IGD

10 8 6 4 2 0 6.85

6.90

6.95

7.00

7.05

7.10

7.15

GR

Fig. 17: Exergy destruction of desalination unit as a function the gain ratio

and dierent desalination unit capacity

CONCLUSIONS

The study carried out in this paper represents an eort to simulate and analyze a modi®ed combined power and MED desalination plant of Al-Zour so as to achieve the best possible thermal eciency with minimum exergy destruction. The required heat energy for the MED desalination unit can be supplied by the back pressure steam turbine of the power plant. A computer program has been developed to simulate the thermodynamic properties of dierent components using the Engineering Equation Solver EES for the proposed system. The major ®ndings from this work are the following: . The overall plant thermal eciency increases signi®cantly as the desalination unit capacity is increased. This increase can reach 25% (from 54.71% low end) as the capacity increases from 1 to 5 MIGD at the condition = 0.6, T1 = 15oC and TBT = 65oC. . The total power output decreases slightly as the desalination unit capacity increases. This decrease can be as small as 2% (from 227.6 MW to 224.3 MW) as the capacity increases from 1 to 5 MIGD at the condition = 0.6, T1 = 15oC and TBT = 65oC. . The top brine temperature, the desalination unit capacity, the desalination unit number of eects, and the cooling water temperature have an insigni®cant eect on the total exergy destruction for coupled MED with the combined cycle.

312

. .


The multi-eect desalting system reduces the consumed equivalent mechanical energy by 38.5% comparing with the widely used MSF. For a same desalination unit capacity, the exergy destruction of the proposed system decreases linearly with the gain ratio. At constant gain ratio and top brine temperature, the exergy destruction of the desalination unit is increased signi®cantly when the capacity is increased but the total exergy destruction of the cycle remains almost unchanged. The desalination unit exergy destruction increases from 12 MW to 15 MW while the total exergy destruction of the cycle decreases from 352 MW to 349 MW when the capacity is 5 MIGD. Therefore, it can be concluded that the low-temperature MED process oers a potential ecient solution to the current Al-Zour power generation plant. REFERENCES

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Submitted : 19/4/2012 Revised : 30/9/2012 Accepted : 1/10/12

314


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