Exergoeconomic analysis of a combined water and power plant 4

Figure 5.7 Variation of Overall heat transfer coefficient with heating medium flow rate Feed flow: 0.25 m3/hr, Feed temp.. ~300C, HM temp.: 550C and Feed concentration: 30,000 ppm The in

CHAPTER-5 RESULTS AND DISCUSSION

In this research project, both experimental and numerical investigations were

conducted to determine the performance characteristics of the system under different

operating conditions In addition, projections of the systems performance in

multi-effect mode and parametric studies were referred after validation of the simulation

model This chapter mainly contains the results and discussion of the important

findings The chapter has been divided in the following sections:

• Validation of simulation results of the combined cycle power plant with

Thermoflow® and other published literature

• Experimental parametric investigations of the MED and RO systems

• Thermal and hydraulic performance comparisons of different tube profiles

• Comparison of experimental and simulation results

Description of each of the sections is included in the following section

5.1 Combined cycle power plant simulation results

The major part of the research project is to utilize the waste heat from the combined

cycle power plant at full load Moreover, to analyze the effect of bleeding steam out

from the power plant at part load condition was considered In order to do so, a model

combined power plant (Siemens V94.2 Gas turbine series) has been taken whose

operating parameters are given in Table 5.1 A series of simulation runs were carried

out on the system to determine the performance characteristics under different

operating conditions Commercially available simulation software Thermoflow® was

used for the verification of the simulation results The simulation results follow the

following sequence:

a Power plant efficiency, steam cycle efficiency based on First law of

thermodynamics comparison with Franco et al (2002)

b Gross power output and other important power plant parameters

verification with Thermoflow®

c Comparison with Rensonnet et al (2007) for power plant efficiency,

exergetic efficiency and water cost comparison

Table 5.1 Different input parameters for combined cycle power plant for Siemens

V94.2

Gas Turbine Work Output, W GT(full-load) 250 MW

Steam Turbine Work Output W ST(full-load) 115MW

Compressor Inlet temperature, T 1 25 o C Compressor Inlet Pressure, P 1 101.3 kPa Pressure ratio, P 2 /P 1 11.65 Gas Turbine inlet temperature 1076 °C Gas Turbine exit temperature, T 4 586 °C Efficiency of gas turbine at design load , ηGT 0.89

Efficiency of compressor at design load, ηC 0.89

Efficiency of steam turbine, ηST 0.9 Mass flow rate to the compressor, m 1 513 kg/s

Temperature of flue gas leaving HRSG II, T 6 85 o C

HRSG I condensate inlet pressure, P d 60 bars

Temperature of water after exit of HRSG I , T d 500 o C

Pressure of bled steam, P e 3 bar Pressure at condenser, P b 0.07 bar

5.1.1 Power plant simulation results

The developed simulation program for the power plant was executed with the data

from Table 5.1 and checked with that of the Franco et al (2002), as shown in Table

5.2 The simulation program can predict within ±8% of the published results From

the simulation results, it can be concluded that the program developed can predict the

performance of the power plant fairly well under different conditions

Table 5.2 First law Power plant efficiencies and comparison with Franco et al (2002)

5.1.2 Power plant simulation validation with Thermoflow ®

The reliability test of the developed power plant simulation program can be ensured

with the checking of gross power output from gas turbine (GT), steam turbine (ST)

and their efficiencies, respectively

So, several load conditions were tested with Thermoflow® taking the input parameters

same as that of Table 5.1 Table 5.3 compares the gross output power of GT, ST, and

total gross power output of the combined power plant Lastly thermal efficiency of the

power plant from full load / rated load to 60% of the rated load condition was

checked From Table 5.3, it shows that if the load is decreased by 20% (from 100% to

80%) the power output reduces by almost 30% Moreover, the thermal efficiency

decreases drastically if the load is reduced

Load (%) Steam cycle efficiency, η, η

ST Combined cycle efficiency, η, ηcc

Simulation Franco et al (2002) Simulation Franco et al (2002)

Table 5.3 Comparison of results with thermoflow for combined cycle power plant

5.2 Combined water and power plant simulation results

In the previous section, the thermal efficiency of the combined cycle power plant was

studied Now, the combined cycle is coupled to the MED plant (CC+MED), and

followed by the RO plant coupled to the combined cycle with the MED plant

(CC+MED+RO) In case of CC+MED+RO plant, then unused electrical load is

imposed onto the power plant to run the RO plant Now, the imposed electrical load

will depend on the water demand which is met by RO This electricity consumed by

RO is simulated to be 5%, 10% and 15% of the designed operating load of the

combined power plant The trend of water production with efficiency for different

configurations of the combined water and power plant is also studied

Thermoflow Simulation Thermoflow Simulation Thermoflow Simulation

5.2.1 Effect of Combined cycle load on Thermal Efficiency

The trend of efficiency with load is shown in Figure 5.1 It shows that, the combined

cycle power plant has the lowest efficiency among the 3 modes of configurations

Thermal efficiency of the combined water and power plant increases by about 5%

when the MED plant is coupled to the power plant In addition, when the RO plant

operates, there is a further increase of about 10%

Figure 5.1 Variation of thermal efficiency with Combined cycle load

Therefore, the efficiencies of the combined water and power plant will increase with

increasing capacity of the RO plant The CWPP graphs show more sensitiveness

across the cases rather than on the load itself These results show tremendous prospect

in terms of savings in the primary energy consumption and output of the CWPP

In short, the efficiency of the combined water and power plant increase with

increasing capacity of the RO plant Even when it is just the MED plant that is

coupled to it making use of the thermal energy inherent in the flue gas that is going

out of the heat recovery steam generator of the plant, it has a higher efficiency than

that of a combined cycle power plant alone

5.2.2 Combined Water and Power Plant (CWPP) result comparison with

Rensonnet et al (2007)

Water production cost relies heavily on electricity cost, steam cost and above all the

exergetic efficiency of the Combined Water and Power Plant (CWPP) Rensonnet et

al (2007) studied the different combinations of desalination plants combined with

power plant In that study, when the plant runs at CC+MED mode, steam is bled from

steam turbine deliberately for producing water, thereby, reducing the electricity

output It in turn increases the electricity cost When the plant runs at CC+RO no

steam is bled from the steam turbine So, the electricity output also increses The

effect is clearly shown on the exergetic efficiecny The maximum benefit of this type

of combined water and power plant is gained when the plant runs at CC+MED+RO

mode

Table 5.4 Comparison of power plant parameters with Rensonnet et al (2007)

Rensonnet et al (2007) Simulation Result

CC+MED CC+RO

CC+ME

D +RO

CC+ME

D +RO Power of

Cost of

distillate

(US$/m 3 )

5.2.3 Effect of Combined cycle load on Water Production

The water production of 1) the combined cycle power plant with the MED plant, and

2) the combined cycle and power plant with the MED and RO (using 5% of the design

capacity of the power plant) are studied

It is observed from the figure that the water production decreases with increasing

electrical load This can be explained by the changes in the amount of steam available

when the load changes When the electrical load decreases, the amount of steam

available either to produce water through MED or produce electricity is increased as

the amount of steam that is bled will also increase Therefore, more steam can be sent

to the MED plant for the production of distillate Hence water production will

decrease with electrical load

Figure 5.2 Variation of Water Production with Combined cycle load

Naturally, with RO being added on to the combined power and water plant, the water

production will increase Through the simulation, it is found that the RO plant will

produce 4.54 MIGD of water Hence, the curve for CC+MED+RO (5%) is shifted by

4.54 MIGD upwards relative to that of CC+MED And for another 5% increase of

load to power the RO, an additional 4.54 MIGD of water can be produced

5.2.4 Electricity cost variation with Oil Price

The electricity cost is directly proportional to the cost of oil per barrel for the power

plant From Figure 5.3, when the cost of the barrel increases, the electricity cost also

increases The cost of the fuel influences the cost of running the combined water and

power plant directly Since electricity is the product of the plant, according to the cost

balancing equation, a higher fuel cost will, therefore, equate to a higher electricity

Figure 5.4 Exergetic efficiencyof power plants for different electrical loads

From Figure 5.4, as the load increases, the exergetic efficiency of the combined cycle

power plant also increases in a parabolic curve During part load condition, the

efficiency of the turbine and the compressor decreases As a result, lesser amount of

electricity is produced during the process, therefore, the lower exergetic efficiency of

the power plant Another interesting point is as the load increases the rate of

improving exergetic efficiency also decreases

5.3 Experimental Results of MED

A series of experiments were carried out on the system to determine the performance

characteristics under different operating conditions The variables considered in this

study are as follows:

• Heating medium flow rate

• Heating medium temperature

• Feed flow rate

• Feed temperature and flashing

Top brine temperature

0.39 0.40 0.41 0.42 0.43 0.44 0.45 0.46 0.47

The impacts of these operating variables have been analyzed in this section in terms

of the following parameters

Detail results for the different tube materials are shown in tabular form in appendix E

5.3.1 Effect of Heating Medium Flow Rate

Hot water was used as the heating medium (HM) in the shell side of the evaporator

The thermal performance of the desalination system was investigated with hot water

flow rate ranging from 1.5 m3/hr to 3.5 m3/hr The feed concentration was in average

30,000 ppm From Figure 5.5, it is clearly evident that the increase of hot water flow

rate enhances the thermal performance of the desalination system with the increase in

performance ratio and decrease in the specific heat transfer area for each of the

tube-bundle For corrugated Cu-Ni (90-10) tube bundle, the performance ratio increases

from 0.75 to 0.914 due to increased vapour production rate which is most significant

compared to other tube-bundles In MED or MSF, the increase of heating steam flow

rate increase the production but it leads to decrease in performance ratio as energy

input increases, El-Dessouky (1998) Specific heat transfer area found to decrease by

40%, 60%, 37% and 35% for Single-fluted Al, Smooth Cu-Ni, Corrugated Cu-Ni and

PTFE-Coated tube, respectively, when the HM flow rate was changed from 1.5 to 3.5

m3/hr The fresh water production rate for different tube-bundles is shown in Figure

5.6 Increase of heating medium flow rate increases the shell-side heat transfer

co-efficient and eventually the available heat flux for the incoming feed which in turns

improves system’s thermal performance

Figure 5.5 Variation of performance ratio and specific heat transfer area with

heating medium flow rate (Feed flow: 0.25 m3/hr, Feed temp ~300C, HM

temp.: 550C, Feed concentration: 30,000 ppm)

Performance ratio

Specific heat transfer area

Figure 5.6 Variation of fresh water production rate (kg/hr) with heating

medium flow rate (Feed flow: 0.25 m3/hr, Feed temp ~300C, HM temp.: 550C

and Feed concentration: 30,000 ppm)

The specific heat transfer area for each of the tube-bundles decreases with the

increase in hot water flow rate, as shown in Figure 5.5, which improves the economy

of the desalination system with a fixed heat input The specific heat transfer area is the

maximum for PTFE-coated smooth Aluminum tube-bundle due to less production of

vapour with the same heat transfer area as compared to other tube-profiles The result

resembles with the study made by Dessouky et al (1999), where performance of

plastic heat exchanger made of PTFE was evaluated An increase of 2 to 4 times more

heat transfer area than metal exchanger was found there whereas in the present study

it is found to be 1.2 times as only coating is applied to the metal surface instead of

plastic heat exchanger So, if the coating durability proves to be efficient through

proper investigations after a long period of operation, PTFE coated metal tube may be

a good option for reducing the scale formation

Figure 5.7 shows the improvement of overall heat transfer co-efficient due to heating

medium flow rate variation

Figure 5.7 Variation of Overall heat transfer coefficient with heating medium flow

rate (Feed flow: 0.25 m3/hr, Feed temp ~300C, HM temp.: 550C and Feed

concentration: 30,000 ppm)

The increased heating medium flow rate increases Reynolds number and,

consequently, shell-side heat transfer co-efficient leading to an improvement of the

overall heat transfer coefficient The average salt concentration in product water was

24-32 ppm, which is fairly below the standard set by WHO (500 ppm) for potable

water Average uncertainty in fresh water production was found to be about 1.9%,

while that for experimental overall heat transfer coefficient was 12%

5.3.2 Effect of Heating Medium Temperature

The temperatures of the hot water were varied between 47 and 650C with a heating

medium flow rate of 3.5 m3/hr From Figure 5.8, it is evident that with the increase of

∆TH, i.e the difference between the temperature of hot water and saturation

temperature of the evaporator, the vapour production increased almost linearly

Figure 5.8 Variation of fresh water production rate (kg/hr) with heating

medium temperature (Feed flow: 0.25 m3/hr, Feed temp ~300C, HM flow

rate: 3.5 m3/hr and Feed concentration: 30,000 ppm)

The increase was found to be more prominent for each of the tube-bundle with

different tube-profile, when ∆TH was more than 150C As the heating medium

temperature was increased, the heat flux to the feed water increased as well The

vapour production starts at an earlier in the tube because of the higher heat flux Tube

side heat transfer co-efficient increases because of the increased quality and heat flux

For the corrugated tube, an increase in hot water temperature from 470C to 650C,

increased the production from 7.312 to 55.56 kg/hr Corrugated Cu-Ni (90-10) tube

exhibits higher performance compared to other tube profiles

Figure 5.9 shows the concentration factor, Xb/Xf, increases with the increase of

heating medium temperature This can be attributed to the higher vapour production

rate, which increases the concentration of the blow down brine solution The

concentration factor increases from 1.03 to 1.29 with the increase of ∆TH from 5.50C

to 23.50C for corrugated Cu-Ni tube-bundle due to increased production In MED or

MSF, blow down concentration ratio normally lies between 1.7-1.8 reported by Wu

(2003)

11.05

Figure 5.9 Variation of concentration factor with heating medium temperature

(Feed flow: 0.25 m3/hr, Feed temp ~300C, HM flow rate: 3.5 m3/hr and Feed

concentration: 30,000 ppm)

The concentration factor is minimum for PTFE-Coated Al tube due to its slightly

lower production compared to the other tube profiles

Figure 5.10 shows the increase in fraction of equilibrium (β) with the increase of

heating medium temperature which improves the thermal performance of the system

With the increase of β, the feed temperature quickly approaches the saturation

temperature corresponding to the evaporator pressure

Figure 5.10 Variation of fraction of equilibrium with the heating medium

temperature (Feed flow: 0.25 m3/hr, Feed temp ~300C, HM flow rate: 3.5

m3/hr and Feed concentration: 30,000 ppm)

5.3.3 Effect of Feed Flow Rate

The effect of feed water flow rate on the performance of the system was investigated

for flow rates ranging from 0.21 m3/hr to 0.75 m3/hr The concentration of the feed

brine solution was 30,000 in average Figure 5.11 shows that with the increase of feed

flow rate, the performance ratio decreases

The uncertainty in fresh water production is within 2.5-3.1% and for fraction of

equilibrium, it is 5-8.2%, as calculated from the standard deviation of the measured

temperature

Figure 5.11 Variation of performance ratio with feed flow rate (HM flow: 2.5

m3/hr, Feed temp ~300C, HM temp.: 550C and Feed concentration: 30,000

ppm)

With the increase of feed flow rate, the fluid velocity increases which reduces the

residence time Consequently, it allows less time for the fluid to attain thermal

equilibrium As seen from Figure 5.12, the fraction of equilibrium decreases with the

increase of feed flow rate As a result, the vapour production decreases which

decreases the performance ratio as shown in Figure 5.11

In the study made by Darwish et al (1976), it was also found that an increase of the

feed flow rate in a flashing chamber decreases the efficiency of the flash chamber

With the increase of feed flow rate for each of the tube-bundle, the concentration

factor decreases while the specific heat transfer area increases, as seen from Figure

5.13 The implication of this result is that increasing feed flow rate to get higher yield

of fresh water production needs higher heat transfer area For the present study, an

increase in feed flow rate from 0.21 to 0.75 m3/hr, increases the required heat transfer

area for single-fluted Aluminum tube profile by 35% to get the production at

0.21m3/hr with the same heat input

Figure 5.12 Variation of fraction of equilibrium with feed flow rate (HM

flow: 2.5 m3/hr, Feed temp ~300C, HM temp.: 550C and Feed concentration:

Figure 5.13 Variation of specific heat transfer area and concentration factor

with feed flow rate (HM flow: 2.5 m3/hr, Feed temp ~300C, HM temp.: 550C

and Feed concentration: 30,000 ppm)

Concentration factor

Specific heat transfer area

5.3.4 Effect of feed temperature and flashing

In any thermal desalination process, preheating of the feed increases the systems’

thermal performance substantially Preheating allows the feed to the evaporator to

reach thermal equilibrium quickly and initiates flashing which increases the

production rate Figure 5.10 shows the gradual decrease of the performance ratio with

the increase of subcooled temperature range Alternatively, as the difference between

the feed and saturation temperature decreases, the performance ratio increases

gradually The reason may be because as the feed temperature rises, it takes a shorter

residence time to reach near the thermal equilibrium and nucleation starts earlier in

the tube However, the change was not found to be too significant when preheating

below the saturation temperature This fact is evident from Figure 5.15 with the

gradual increase of the overall heat transfer coefficient

Figure 5.14 Variation of performance ratio with feed temperature (HM flow:

2.5 m3/hr, Feed flow rate: 0.25 m3/hr, HM temp.: 550C and Feed

concentration: 30,000 ppm)

Figure 5.15 Variation of Overall heat transfer coefficient with feed

temperature (HM flow: 2.5 m3/hr, Feed flow rate: 0.25 m3/hr, HM temp.: 550C

and Feed concentration: 30,000 ppm)

Flashing effect

In a thermal desalination plant, the flashing effect contributes significantly to the

improvement of system performance Flashing effect can be efficiently utilized when

feed temperature is more than the saturation temperature inside the evaporator

Figure 5.16 shows the increase of performance ratio with the increase of degree of

superheat of the feed salt water With the increase of degree of superheat, the flashing

effect becomes more prominent than boiling As flashing occurs within the bulk of the

liquid, vapour production improves significantly which increases the performance

ratio For the case of corrugated tubes, 44% more vapour production was observed

when the degree of superheat increased from 0 to 100C The performance ratio

improves from 1.05 to 1.41

Figure 5.16 Variation of Performance ratio with feed temperature (HM flow:

2.5 m3/hr, Feed flow rate: 0.25 m3/hr, HM temp.: 550C and Feed

concentration: 30,000 ppm)

The increase of vapour production for the case of Single-fluted Aluminum

tube-bundle, smooth Cu-Ni tube-bundle and PTFE-coated Aluminum tube bundle found to

be 30%, 20% and 35%, respectively, for the increase of degree of superheat from 0 to

100C

Flashing efficiency can be defined as the ratio of actual amount of vapour generated

by flashing to the theoretical amount of vapour that would be generated under

adiabatic irreversible process Figure 5.17 shows the increase of flashing efficiency

with the increase of liquid superheat from 0 to 100C As the liquid superheat

increases, it leads to a higher evaporation rate The bubble formation increases in the

liquid bulk and creates turbulence This turbulence promotes the improved flashing

efficiency For the case of corrugated tube-bundle, an improvement of flashing

efficiency from 75% to 95% had been observed when the feed salt water was

superheated in the range of 0 to 100C

Figure 5.17 Variation of flashing efficiency with the feed temperature (HM

flow: 2.5 m3/hr, Feed flow rate: 0.25 m3/hr, HM temp.: 550C and Feed

concentration: 30,000 ppm)

In the study made by Khalil et al (1981), it was found that with 40C of superheating,

the flashing efficiency increases from 42.5% to 50 % at a flow rate of 3.2 m3/hr of the

feed water to a flash desalination unit In a preliminary optimization analysis on

design parameters of the VTE-MED process coupled with nuclear heating reactor, Wu

and Du (2003) found that preheating of feed with higher superheat can increase the

systems performance considerably Similar responses have been obtained in the

present system

5.3.5 Effect of top-brine temperature

Top brine temperature (TBT) is an important design consideration for any thermal

desalination plant It is the maximum allowable operating temperature of any effect in

Multi-effect desalination (MED) or Multi-stage flash desalination (MSF) plant The

effect of top-brine temperature had been tested in the present system The range of

temperature tested was 41.50C to 500C The feed flow rate was 0.25 m3/hr with

temperature of 300C and average feed concentration of 30,000 ppm The heating

medium temperature and flow rate were maintained 550C and 3.5 m3/hr, respectively

Figure 5.18 shows that, as the top brine temperature increases, the vapor production

increases as well However the response was found to be insignificant

Figure 5.18 Variation of fresh water production with top brine temperature

(HM flow rate: 3.5 m3/hr, Feed flow rate: 0.25 m3/hr, Feed Temp.: 300C; HM

temp.: 550C and Feed concentration: 30,000 ppm)

The effect of top brine temperature on the performance ratio of the system is shown in

Figure 5.19 As the top brine temperature increases, the difference between the feed

temperature and saturation temperature in the evaporator increases as well So, more

energy input is needed to raise the feed temperature to reach saturation temperature in

the evaporator As a result, the systems performance ratio decreases with the increase

of top brine temperature (TBT) This fact is evident from Figures 5.18 and 5.19

Figure 5.19 Variation of performance ratio with top brine temperature (HM

flow rate: 3.5 m3/hr, Feed flow rate: 0.25 m3/hr, Feed Temp.: 300C; HM temp.:

550C and Feed concentration: 30,000 ppm)

5.3.6 Effect of variable concentration

In the experimental investigations, almost all of the experimental runs were

conducted with saltwater of average concentration of 30,000 ppm However, in order

to determine the effects of variable concentrations of the feed water on the

performance of the system, some experiments were conducted with 15,000, 25,000

and 35,000 ppm as well Identical conditions of other operating parameters had been

maintained during those experiments Figure 5.20 shows the effect of variable

concentration on the production rate It is evident from Figure 5.20 that the variable

concentrations do not influence greatly the production rate of the system, except slight

decrease with increasing concentration

Figure 5.20 Variation of production rate on variable concentration (HM flow

rate: 3.5 m3/hr; HM temperature 550C; Feed temperature: 300C; Feed flow rate

0.25 m3/hr and Chamber pressure: 80 mbar)

The slight decrease of the production rate can be explained by Figure 5.21 As seen

from Figure 5.21, with the increase of the feed water concentration; the boiling point

elevation (BPE) also increases, as BPE is function of salinity and top brine

temperature As BPE is one of the non-equilibrium components; the increase of BPE

reduces the fraction of equilibrium in the evaporator which is displayed in Figure

5.21

The product water concentration was also observed to be nearly unaffected with the

increasing feed concentration This fact is evident from Figure 5.22 The average TDS

in product water obtained from different experimental investigations lies between 20

and 30 ppm This range is well below the range set by WHO (World Health

Organization) which is less than 500 ppm, as stated by Malek et al (1992)

Figure 5.21 Variation of BPE and fraction of equilibrium with variable

concentrations of feed (HM flow rate: 3.5 m3/hr; HM temperature 550C; Feed

temperature: 300C; Feed flow rate 0.25 m3/hr and Chamber pressure: 80 mbar)

Figure 5.22 Variation of product water concentration with variable

concentrations of feed (HM flow rate: 3.5 m3/hr; HM temperature 550C; Feed

temperature: 300C; Feed flow rate 0.25 m3/hr and Chamber pressure: 80 mbar)

5.4 Thermal hydraulic performance comparisons for different tube profiles

Thermal hydraulic performance consideration is very important for choosing a

certain tube material or geometry for thermal desalination process Effective design,

selection of tube profile and material may increase energy efficiency of a desalination

plant For this research project, four different kinds of tube profiles were selected for

the design of the evaporator The aim was to select tube material and tube profile for

better performance in desalination application using waste heat in the form of hot

water Comparisons of both experimental and simulation performance have been

made in this section

5.4.1 Feed temperature distribution inside the tube

The feed water temperature distribution inside the tube has been found from the

simulation Simulation results are shown in Fig 5.23 for a heating medium flow rate

of 3.5 m3/hr, temperature of 650C, and feed water flow rate and temperature of 0.25

m3/hr and 300C, respectively The saturation temperature inside the evaporator has

been considered as 41.50C at a pressure of 80 mbar As shown in Figures 5.23 and

5.24, the temperature reaches the saturation condition at a particular distance and

then remains constant, as boiling starts From Figure 5.23, the response is quicker for

corrugated tube compared to the smooth profile For the case of corrugated tube, the

additional surface area promotes better heat transfer As the fluid passes through the

corrugated tube, the turbulence level increases because of the continuous breakdown

of the boundary layer development in the groove area compared to smooth profile,

where the formation of boundary layer impedes better heat transfer As obtained from

the simulation, the corrugated tube attains saturation temperature at a distance of 0.16

m whereas it is 0.26 m for the smooth profile tube

Figure 5.23 Variation of the feed water temperature along the length of the tube

(heating medium temperature: 650C; heating medium flow rate: 3.5 m3/hr; feed flow

rate: 0.25 m3/hr; temperature: 300C; saturation temperature: 41.50C)

Figure 5.24 shows the temperature distribution of feed water for single-fluted Al tube

profile and PTFE-Coated smooth Al tube profile For the single-fluted aluminum tube

profile, the feed salt-water reaches saturation condition a bit earlier compared to

PTFE- coated smooth Al tube profile As seen from the Figure, the difference with

regards to distance for both of the tube profile cases is marginal The reason may be

for the case of coated tube, the coating thickness is too thin (75 micron) to provide

substantial thermal resistance As reported in the literature, the porous coating of Al,

in fact, provide additional nucleation sites compared to uncoated smooth profile This

implies the potential usage of coated tube for the desalination application for the

prevention of scaling

Figure 5.24 Variation of the feed water temperature along the length of the tube

(heating medium temperature: 650C; heating medium flow rate: 3.5 m3/hr; feed flow

rate: 0.25 m3/hr; temperature: 300C; saturation temperature: 41.50C)

5.4.2 Variation of vapour quality along the length of the tube

Figure 5.25 shows simulation results of the increase of vapor quality along the

vertical length of the tube for different tube profiles As shown in the figure, the

quality is zero at the entrance of the tube when single-phase feed water enters the

tube Once the fluid reaches thermal equilibrium at evaporator pressure, vapor quality

increases rapidly As seen from the figure, for the same operating condition vapour

quality for corrugated tube types start at earlier distance and at increased rate

compared to other tube

Figure 5.25 Variation of quality along the length of the tube (heating medium

temperature: 650C; heating medium flow rate: 3.5 m3/hr; feed flow rate: 0.25 m3/hr;

temperature: 300C; saturation temperature: 41.50C)

For the corrugated profile, this can be attributed, to the grooved area which comes in

less contact with the flowing fluid As a result the tube wall temperature is higher in

the grooved area This may produce more nucleation sites in the corrugated profile

compared to other profiles

5.4.3 Shell side heat transfer coefficient

Figure 5.26 shows the variation of experimental results of shell side heat transfer

coefficient with heating medium flow rate As shown from the figure, the shell-side

heat transfer coefficient increases for each of the tube profile with the increase of flow

rate The increase of flow rate increases the Reynolds number, which increases

outside heat transfer coefficient It is evident from the figure that the fluted and

corrugated exterior of the Cu-Ni and Al exhibits higher heat transfer coefficient than

smooth exterior With the extended surface, it creates more turbulence in the shell

side fluid However, the extended surface is more efficient when steam is condensed

on it For corrugated and fluted profile, this is very significant

Figure 5.26 Variation of the shell side HTC with HM flow rate (heating medium

temperature: 650C; feed flow rate: 0.25 m3/hr; temperature: 300C; saturation

temperature: 41.50C)

5.4.4 Tube side heat transfer coefficient

Figure 5.27 shows the variation of heat transfer coefficient (HTC) at the inner surface

of the tube with heating medium flow rate As heating medium flow rate is increased,

the heat flux in the shell side increased as well This increased heat flux in the shell

side promotes better heat transfer to the feed water inside the tube and vapour

generation starts at an earlier distance As the vapour starts to generate in the tube,

the two-phase heat transfer coefficient dominates the inside heat transfer coefficient,

as it is a function of quality As seen from the figure, both of the Cu-Ni tube profiles

provide significant improvement in inside HTC than the Aluminum

Figure 5.27 Variation of the tube-side heat transfer coefficient with heating medium

flow rate (heating medium temperature: 650C; heating medium flow rate: 3.5 m3/hr;

feed flow rate: 0.25 m3/hr; temperature: 300C; saturation temperature: 41.50C)

For the Cu-Ni, the tube wall thickness is 1.65 mm, whereas, it is 3 mm for Aluminum

The lower thickness permit less thermal resistance to heat flow which causes better

tube wall heat transfer distribution for the case of Cu-Ni (90-10) Inside heat transfer

coefficients have been obtained as 1.79, 2.103, 2.69 and 1.47 kW/m2K for

Single-fluted Aluminum, smooth Cu-Ni (90-10), corrugated Cu-Ni (90-10) and PTFE-coated

Aluminum tube profiles, respectively, at a heating medium flow rate of 3.5 m3/hr

The advantage of the Cu-Ni (90-10) profile over aluminum is that it can be

maintained thin during the evaporator design However, for the case of Aluminum it

is not possible as it affects the structural rigidity of the evaporator But Cu-Ni (90-10)

is more expensive than Aluminum and that’s why it’s still popular However, an

economic justification can also be made for the optimum performance of the

desalination plant

5.5 MED parametric evaluation

For any performance evaluation study, it is very important to do a parametric analysis

of the system This parametric study helps to uncover the better operating region,

locates major areas of improvement and above all, improves the plant performance

For this reason, the next section is dedicated to MED sub system parametric

evaluation The variables considered are as mainly top brine temperature (TBT) and

Number of effects

A thorough parametric analysis is performed on the MED plant to understand the cost

formation of the distillate The change in exergetic efficiency and destruction,

distillate production and cost of product with respect to the change in the input

parameter is investigated The significance in the changes of its installation such as

varying number of effects of the MED plant is also discussed The simulation model

developed for MED uses the property functions in Appendix F

5.5.1 Distillate production with varying number of effect

In Figure 5.28, the dependence of distillate production on the number of effects in a

MED system is shown For the same TBT of 65oC, the production increases for less

number of effects The change in distillate production increases by about 20% for

increasing the number of effects from 9 effects to 12 effects and 18% for 12 effects to

15 effects From Effect 1, the distillate production decreases gradually This is due to

the decrease in temperature of the subsequent steam input as well as the decrease in

the amount of vapor from one effect to another

Figure 5.28 Distillate productions for different No of effects at constant TBT

5.5.2 Effect of Top Brine Temperature on Distillate production

Figure 5.29 Distillate production variations with number of effects

From Figure 5.29, the top brine temperature of the MED plant has a positive effect on

distillate production When the TBT is increased from 55oC to 65oC, it increases by

about 12% while increasing from 55oC to 75oC increases by about 21% This is due to

the higher exergy input of the MED plant which causes more vapors to form The

amount of distillate in the first effect is influenced by the mass and temperature of the

steam For the same number of effects, when TBT is raised, the temperature

difference across the effect also increases and accounts for the positive shift in

production

5.5.3 Effect of Top Brine Temperature on exergy efficiency

As seen from Figure 5.30, with higher top brine temperature, there is lower exergetic

efficiency of the desalination process Since the temperature of the last effect is kept

constant, a higher top brine temperature will indicate a greater temperature loss along

the MED effect Therefore when the temperature of the steam is much higher than the

top brine temperature, the exergetic efficiency will become lower

Effect Number Figure 5.30 Graph of exergy efficiency with varying top brine temperature

5.5.4 Exergy efficiency for varying number of effects

Figure 5.31 Graph of exergy efficiency with varying number of effect

The general exergy efficiency of the desalination process increases as the number of

effects increases This is shown in Figure 5.31 This is due to the increase in

temperature difference of the brine between effects as the number of effects

decreases The large drop in exergetic efficiency in the last effect is due to the leftover

brine that is released from the MED plant This causes large amount of exergy to be

lost to the environment There is also greater drop in exergetic efficiency in MED

plant with lower number of effects This is due to the higher temperature drop from

effect to effect This results in higher exergy loss and thus leads to increase in rate of

the decrease of exergetic efficiency of the MED plant

5.5.5 Cost per unit exergy for varying number of effects

Reducing the number of effects can cause the cost per unit exergy of the product to

increase as exhibited in Figure 5.32 This can be attributed to the higher temperature

loss between each effect which results in higher exergetic destruction Since more

exergy is destroyed in the process, a higher portion of the cost of the fuel will be

transferred to the product where the overall cost of the product is equal to the cost of

Cost Per Unit Exergy of Distllate With Number of

Effects Along MED Plant

graph follows a quadratic equation with different minimum points for MED of

different effects As the number of effect increases, the minimum point of the curve

becomes smoother and more effects are able to generate distillate with approximate

equal cost per unit exergy

5.5.6 Overall heat transfer Co-efficient for varying number of effects

Overall heat transfer coefficient of the MED plant depends on the heat transfer area as

well as the log mean temperature difference (LMTD) The heat transfer coefficient is

very high for the first effect as shown in Figure 5.33 because of the highest

temperature difference between the incoming preheated seawater and primary steam

source introduced here In the 2nd effect, there is a sharp drop in heat transfer

coefficient as a result of the drop in temperature difference between the steam

produced in first effect and the brine temperature in the 2nd effect It may also because

of the introduction of the pre heaters that are introduced at 2nd effect onwards For all

Figure 5.33 Graph of transfer heat transfer coefficient along MED plant

top brine temperatures of 55, 65 and 750C as temperature difference increases, the

transfer heat coefficient follows As the TBT increases, for fixed blow down

temperature, the temperature difference across each effect also increase As a result,

the heat transfer coefficient increases

5.6 Experimental Results for single tube vertical heat exchanger

So far, simulation results from the developed mathematical model as well as some

experimental results have been discussed in the previous sections From these results

it’s quite obvious that the TBT, number of effects, overall heat transfer coefficients

play immense role in design of the MED system It also occurs that, in order to find

more efficient MED system more detail experimental results are required to find out

the heat transfer characteristics of the effects Experiments were conducted for all

tube configurations for the variables illustrated in both Table 4.5 and Table 4.6 A

more detailed diagram of the variables considered in these experimental runs are

shown in Figure 4.24

5.6.1 Effect of Heating Medium Temperature

The heating medium temperature has a significant effect on the overall heat transfer

coefficient of the evaporator tubes It can be observed from Figure 5.34 that, for all

materials under investigation with insertions, there is an average increase of

60%-100% in overall heat transfer coefficient for an increase in heating medium

temperature of 65oC to 85oC The same trend was also experienced for the corrugated

tube which is illustrated in Figure G1 The results of the smooth tubes are available in

the Figure G2 Trends observed are similar to that of Prasanta et al (2004) As seen

from figure below, at any constant heating medium temperature, Stainless Steel has

the lowest overall heat transfer coefficient while Copper is the most effective with

47% increase in overall heat transfer coefficient

Figure 5.34 Variation of overall heat transfer coefficient with heat medium

temperature for smooth tubes with insertions

This phenomenon is attributed to the increase in heat flux to the feed water as a result

of the increase in heat medium temperature There is a rapid initial increase in the

overall heat transfer coefficient however the rate of increase decreases due to sensible

heat loss to the surroundings

5.6.2 Effect of insertions on water production

Another performance measure on the effectiveness of the evaporator tubes is the

water production rate Referring to Figure 5.35, water production rate increases for

both smooth tubes and smooth tubes with insertions for increasing heat medium

temperature This general trend can also be observed in the corrugated tubes featured

in Figure G3 The increase in water production rate is a direct consequence of

increased overall heat transfer coefficient as mentioned earlier Vapour production

occurs at an earlier distance in the tube because of the higher heat flux thus

consequently producing more vapour and distillate For both, with and without inserts,

copper shows the best water production rate compared to the other materials Copper

with inserts shows about 100% increase in water production

Figure 5.35 Variation of water production rate with heat medium temperature for

smooth tubes and smooth tubes with insertions