1. Exergo-economic analysis of a seawater reverse osmosis desalination
plant with various retrofit options
Muhammad Ahmad Jamil, Bilal Ahmed Qureshi, Syed M. Zubair ⁎
Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
H I G H L I G H T S
• Seawater reverse osmosis desalination plant with four different retrofit options is studied.
• First- and second-law analyses are carried out to estimate energy requirements and second-law efficiency.
• The product cost is compared by performing exergo-economic analysis using reliable seawater properties.
• Analysis revealed that with a pressure exchanger, energy consumption can be reduced by 24%.
• It is also shown that post-treatment and distribution sections increase the product cost by about 20%.
a b s t r a c ta r t i c l e i n f o
Article history:
Received 5 May 2016
Received in revised form 22 September 2016
Accepted 28 September 2016
Available online 8 October 2016
The current study is focused on carrying out exergo-economic analysis of a seawater reverse osmosis (SWRO) de-
salination plant. The main objective is to compare the performance as well as the product cost of an existing
SWRO plant, including post-treatment and distribution sections, for four different retrofit options made by cou-
pling high-efficiency pressure exchangers (PXs) in place of conventional energy recovery turbines. For this pur-
pose, first- and second-law analysis is carried out to estimate the energy requirements and second-law efficiency
for each retrofit option. Finally, the product cost is compared by performing an exergo-economic analysis using
appropriate seawater properties for the calculations. The analysis revealed that, by introducing a PX, the specific
energy consumption (SEC) can be reduced by about 24%; thus, increasing the second-law efficiency. Besides this,
it is also demonstrated that the addition of post-treatment and distribution sections enhances the product cost by
almost 20%. Furthermore, the study suggested that using a booster pump with a PX (as used in retrofit # 3) is best
suited for enhancing the plant capacity compared to retrofit # 4 in which a PX is used in place of the pump. It has
the least product water cost among all the options discussed.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Reverse osmosis
Seawater
Retrofit
Pressure exchanger
Exergo-economic analysis
1. Introduction
Reverse osmosis (RO), a membrane-based desalination system is
one of the most frequently used techniques for treatment of seawater.
From 1970 till today, this technology has been widely used, studied
and improved over the time [1]. It has lower start-up time, decreased
environmental impacts (in terms of emissions) and easier operation
and maintenance. Energy analysis of RO systems operating under differ-
ent capacities with and without energy recovery devices (ERDs) reveals
that their energy consumption can be greatly reduced by coupling ERDs
[2–6]. Coupling of Pelton turbines (as energy recovery turbines “ERTs”)
with RO systems is one of the oldest energy recovery methods [7,8]. Iso-
baric pressure exchangers (PXs) are relatively modern and better
devices in this regard [9]. A detailed discussion of the working and selec-
tion of ERDs is carried out by various investigators [10–15].
Besides this, exergy analysis has been used as one of the most impor-
tant tools by researchers [16,17] frequently to identify the components
with the greatest exergy destruction. Cerci [18] and Aljundi [19] ana-
lyzed two different RO plants using actual plant data and reported the
throttling valves and membrane modules to be the primary locations
for exergy destruction. Romero et al. [20] carried out a similar study
for a complete plant including pre-treatment, post-treatment and distri-
bution sections. The above studies proposed that the second-law effi-
ciency of the plants can be improved by installing pump-motors
equipped with variable frequency drives and replacing throttle valves
on the brine stream with a PX.
Another useful way of analyzing the desalting systems is to combine
the exergy and cost analysis known as exergo-economic analysis.
Lozano and Valero [21] presented the theory of exergetic costs which
is considered to be one of the major approaches in this field. Based on
Desalination 401 (2017) 88–98
⁎ Corresponding author.
E-mail address: smzubair@kfupm.edu.sa (S.M. Zubair).
http://dx.doi.org/10.1016/j.desal.2016.09.032
0011-9164/© 2016 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal

2. this theory, Romero et al. [22] carried out an exergo-economic analysis
of an RO plant and reported the product cost to be 0.70 €/m3
. El-Emam
and Dincer [23] performed a similar analysis for different seawater sa-
linities and estimated the product cost to be 2.45 $/m3
for a salinity of
35 g/kg. Spiegler and El-Sayed [24,25] contributed significantly to the
field of thermo-economics by developing the correlations for the rate
of fixed cost of various components of desalination systems. They sug-
gested that the main focus should be on the exergy destruction which
constitutes mainly the operating resources of any desalination system
rather than the making resources (fixed cost). Some studies [26–28]
were focused on analyzing solar-powered desalination systems. The
unit product cost for a small-scale solar-powered membrane distillation
unit was reported to be 15 $/m3
by [26] which is much higher than con-
ventional systems. However, the unit product cost for a large scale PV/
RO system was estimated as 1.3 $/m3
by [27], which is slightly higher
than conventional systems (0.75 $/m3
) due to higher electricity cost.
Penate and Rodriguez [29] proposed and analyzed four different retrofit
options to provide upgradation opportunities for existing SWRO plants
working with conventional ERTs. The results of energy, exergy and ther-
mo-economic analysis for all the retrofit options were compared to
identify the best one with a minimum product cost.
1.1. On exergy calculation model
Fitzsimons et al. [30] examined and compared six different exergy
calculation models and showed that these models affect the final results
significantly. The study suggested that, among these, the electrolyte
model approach is not suitable because seawater is not an ideal mixture.
Similarly, the approaches used by Cerci [18] and Drioli et al. [31–33] are
not suitable for desalination system analysis because of ideal mixture
assumptions and specific separation assumptions, respectively.
Sharqawy et al. [34] functions and Pitzer et al. [35,36] equations can
be used to calculate thermodynamic properties of seawater and other
electrolytes, respectively. A similar issue regarding the definition of sec-
ond-law efficiency is highlighted by various authors [17,37–39] in their
studies. Some of them [14,40] define it as the ratio of the total exergy
leaving to the total exergy entering the system, while others [16,17] as
the ratio of product to fuel exergies. Qureshi and Zubair [41] discussed
the applicability of these definitions and suggested that the second
one is more appropriate for desalination systems.
Based on the above discussion, the current study is focused on
reassessing and improving the work of Penate and Rodriguez [29] by
considering the following: (a) post-treatment and distribution sections
in the current analysis, (b) use of reliable and updated seawater proper-
ties recently compiled by Sharqawy et al. [34], (c) an appropriate defini-
tion of the second-law efficiency suggested by Qureshi and Zubair [41],
and (d) plant performance as a function of important input parameters
such as unit electricity cost, feed salinity and high-pressure pump (HPP)
efficiency.
2. System description and modeling
The system under consideration consists of a 10,000 m3
/d seawater
reverse osmosis (SWRO) plant equipped with two membrane modules
of the same capacity coupled with two identical ERTs. The schematic for
this configuration is shown in Fig. 1, in which one HPP per train is used
to raise the feed pressure. The system is analyzed under four different
possible retrofit options, calculations for each retrofit option are per-
formed and the results are compared with the standard configuration.
In the first two retrofit options, the plant capacity remains the same
while the focus is to minimize the energy consumption by replacing
ERTs with a PX. The last two options are proposed to increase the
plant capacity as well as minimize the energy consumption. The data
used for analysis of the plant is listed in Table 1. The analysis presented
in the paper is based on the following assumptions that are also consid-
ered by [29,41]: (a) the dead state is taken as the conditions of the feed,
i.e., P0 = 101.325 kPa, T0 = 20 °C, S0 = 35 g/kg and operating temper-
ature is considered constant throughout the system, (b) an overall pres-
sure drop in RO modules, pipes and valves is considered to be 160 kPa,
(c) feed water pressure at HPP inlet is taken as 351.325 kPa and the re-
covery ratio is 45%, (d) effect of permeate back pressure, reverse salt dif-
fusion, concentration polarization and system leakages are considered
negligible, (e) thermo-physical properties of seawater are based on
the correlations provided by Sharqawy et al. [34], and (f) efficiencies
of the various components are, ηHPP =78%, ηBP =77%, ηFP =78%,
ηDP =78%, ηMotor =92%, ηPX =90% [29].
For numerical simulation, engineering equation solver (EES) soft-
ware is used with updated seawater properties compiled by Sharqawy
et al. [34].
2.1. First-law analysis
To carry out the first-law analysis, the mass balance (Eq. (1)) and the
solution balance (Eq. (2)) are applied. For a steady-state system, these
can be expressed as,
X
in
_m ¼
X
out
_m ð1Þ
X
in
_mS ¼
X
out
_mS ð2Þ
Pump and turbine work is calculated using Eq. (3) and Eq. (4), re-
spectively as,
ẆPump ¼
Q
̇
ΔP
ηPump
ð3Þ
ẆTB ¼ ηTB Q̇ΔP ð4Þ
The PX efficiency is described as [41,42]:
ηPX ¼
Q
̇
B;oPB;o þ Q
̇
F;oPF;o
Q
̇
B;iPB;i þ Q
̇
F;iPF;i
ð5Þ
Specific energy consumption (SEC) is one of the important parame-
ters for comparing plants working under different capacities because it
compares the energy requirement for a unit product. It can be expressed
as [41]:
SEC ¼
W
̇
in
3600
X
out
Q
̇
p
ð6Þ
2.2. Second-law analysis
This analysis measures the extent of irreversibility in terms of exergy
destruction, which is calculated by applying exergy-balance on each
component, separately:
X
fuel
_X−
X
products
_X ¼ _XD þ _XL ð7Þ
The second law efficiency is calculated, as described in [17,41]:
ηII ¼
_Wl; min
_Win
ð8Þ
89M.A. Jamil et al. / Desalination 401 (2017) 88–98

3. The concept of least work of separation and the minimum of least
work of separation is well explained by Mistry et al. [17]. The latter
can be written as:
_Wl; min ¼
X
products
_X−
X
fuel
_X ð9Þ
3. Retrofit options
The retrofit options proposed in this study are divided into two
categories:
3.1. Retrofit options for constant capacity
These options are proposed with an aim to reduce energy consump-
tion when the total plant capacity is to be maintained same as in the
standard configuration. In this category, the standard plant is retrofitted
with high-efficiency ERDs as explained below.
3.1.1. Retrofit option # 1
It consists of one HPP instead of two (used in the standard configu-
ration) and the two ERTs are replaced by a high-efficiency isobaric ener-
gy recovery device. One HPP is sufficient for both the trains so this
configuration requires replacement of the HPP motor with a slightly
higher capacity and, thus, requires minor affordable changes in the elec-
tric wiring and other protection systems. The feed pump does not re-
quire any alteration because the same amount of feed water is to be
pumped at the same pressure. Neither the recovery ratio nor the flow
rates are modified. However, this configuration requires installation of
a booster pump (BP) for which new channeling of the feed water and
brine is required. Civil work is not required that much because the
ERD can be installed in the space left by ERTs. However, piping needs
to be modified because the flows are distributed over the plant in differ-
ent ways as shown in Fig. 2. The feed-water is pumped to both the HPP
Fig. 1. Schematic of the standard SWRO plant.
Table 1
Operational data for standard configuration SWRO plant [29].
Parameter Flow (m3
/h) Pressure (kPa)
Total feed 926 101.325
Feed per train 463 5961.325
HPP inlet 463 351.325
Permeate stream per train 208 101.325
Total permeate 416 101.325
Brine stream/Turbine inlet 255 5801.325
Brine discharge/Turbine Outlet 255 101.325
Fig. 2. Schematic of retrofit option # 1.
90 M.A. Jamil et al. / Desalination 401 (2017) 88–98

4. and ERD inlet and its pressure is raised as it passes through these de-
vices. Water from both circuits combine at state 8 and then distributed
evenly to each train. Permeate from both the trains is obtained as shown
in the figure. The high-pressure brine stream from each train is directed
to the ERD where it loses its pressure energy to raise the pressure of in-
coming feed water and then rejected back to the sea. The operational
data for this configuration is given in Table 2.
3.1.2. Retrofit option # 2
It consists of an isobaric pressure exchanger with a BP to raise the
pressure of the feed water and is proposed to avoid replacement of
the existing motors coupled to the HPP. One BP is also installed prior
to the HPP to maintain the required pressure, which avoids substantial
electrical modifications by keeping the flow rates and the plant-capacity
constant. Flow arrangements remain same as discussed in the previous
option and schematic is shown in Fig. 3. Table 3 summarizes the opera-
tional data for this retrofit option.
3.2. Configurations to increase the plant capacity
These options are proposed with an aim to increase the plant capac-
ity by introducing high capacity RO trains with a higher number of pres-
sure vessels. The plants are also retrofitted with high-efficiency ERDs to
minimize the energy consumption. In the current study, only one train
is shown for the sake of analysis because the two trains are identical.
These types of retrofits are recommended for the cases where new in-
vestments can be made to upgrade the plants and sufficient space is
available to accommodate high capacity RO trains.
3.2.1. Retrofit option # 3
In this case, each HPP is retrofitted to allow 35% reduction in the feed
flow rate because an isobaric ERD and a BP is used to manage the pres-
sure of the remaining feed. The schematic for this option is shown in
Fig. 4. The new train consists of 23 pressure vessels and works with
the same recovery rate and the train capacity is increased from 5000
m3
/day to 7200 m3
/day. Pipe diameters are to be modified slightly
and more parallel pipes are to be installed. A new pipe is to be installed
for the low- and high-pressure feed flow to the ERD and BP. No doubt,
this configuration consumes more net energy but provides 40% more
permeate which reduces the SEC. The operational parameters used for
defining the new capacity are given in Table 4.
3.2.2. Retrofit option # 4
This retrofit option is a modified form of the previous one (see Fig. 5)
in which an isobaric ERD is used as a second stage HPP. A portion of the
feedwater is pressurized by the HPP while the rest of it gets pressurized
by the ERD that uses the brine (from the first stage) as the working fluid.
This retrofit produces about 50% more water than the standard configu-
ration. It allows for the existing HPP and RO train to be used without any
new installation of energy consuming devices. The operational data for
this retrofit is given in Table 5.
A sample calculation for the first- and second-law analyses (of the
2nd retrofit option) is given in Appendix A.
4. Exergo-economic analysis
For exergo-economic analysis, the plant is divided into three subsys-
tems for each retrofit option. Subsystem 1 consists of a feed pump and a
pre-treatment section and subsystem 2 consists of HPPs, BP (if needed)
and ERDs. Subsystem 3 includes the post-treatment and distribution fa-
cility. The schematic of subsystems for this analysis is shown in Fig. 6.
The first step here is to calculate the exergy of each stream which is
the sum of physical and chemical exergies [34]. The other calculations
depend on the model used. For instance, the model used in the current
paper, which is based on the approach used by Romero et al. [22], re-
quires the calculation of exergy destruction in each component for the
cost analysis. The only difference is that, in [22], the analysis for each
component is carried out separately but, in our case, the components
are combined in the form of subsystems. The calculations are given in
the following sections.
Table 2
Operational data for SWRO plant retrofit option # 1 [29].
Parameter Flow (m3
/h) Pressure (kPa)
Total feed 926 101.325
Feed per train 463 5961.325
HPP inlet 416 351.325
ERD feed 510 351.325
Permeate stream per train 208 101.325
Total permeate 416 101.325
Brine stream/ERD brine inlet 510 5761.325
Brine discharge/ERD brine outlet 510 221.325
Fig. 3. Schematic of retrofit option # 2.
Table 3
Operational data for SWRO plant retrofit option # 2 [29].
Parameter Flow (m3
/h) Pressure (kPa)
Total feed 926 101.325
Feed per train 463 5961.325
Low pressure BP inlet 416 351.325
HPP inlet 463 351.325
ERD feed inlet 510 351.325
Permeate stream per train 208 101.325
Total permeate 416 101.325
Brine stream/ERD brine inlet 510 5801.325
Brine discharge/ERD brine outlet 510 101.325
91M.A. Jamil et al. / Desalination 401 (2017) 88–98

5. 4.1. Exergy costs of flow streams
The exergetic cost of any stream is based on the exergy required to
produce it. In the present study, the exergy cost calculations are based
on three assumptions [20]: (a) exergy cost of any input stream (from
the environment) is equal to its exergy value, (b) exergy cost of any use-
less flow (such as blowdown) is considered as zero, and (c) inlet and
outlet components of any fuel have the same unitary exergy cost.
The unitary exergy cost (a dimensionless parameter) of a stream
represents the ratio of its exergetic cost to power input. It tells us
about the exergy power required to produce that exergy stream. For dif-
ferent streams, the value of the exergy rate and unit exergetic cost are
different and can be calculated, as explained below.
The intake stream (0) is taken as the dead state so its specific exergy,
exergy rate and unitary exergy cost will be zero.
Cx0 ¼ Ẋ0 ¼ 0 ð11Þ
The stream leaving subsystem 1 will have certain pressure with ref-
erence to the dead state, so it will have certain exergy rate. Its unitary
exergy cost can be expressed as,
Cx1 ¼
W
̇
Subsystem1
Ẋ1
ð12Þ
where ẆSubsystem1 represents the work supplied to the feed pump and
pre-treatment unit.
Based on the assumption stated above, the exergy cost of the blow-
down is taken as 0 because it has no further utility.
Cx2 ¼ 0 ð13Þ
The product stream has certain exergy rate with reference to the
dead state. Its unitary exergy cost can be calculated as
Cx3 ¼
Ẇ
Subsystem1 þ Ẇ
Subsystem2 þ Ẇ
Subsystem3
Ẋ
3
¼
Ẇ
Subsystem;tot
Ẋ
3
ð14Þ
where ẆSubsystem;tot represents the total work supplied to produce this
product stream. This includes work input to the feed pump, HPP and
BP (if required). The stream exergy (fuel, product, and losses) values
of the subsystems (see Fig. 6) are given in Table 7.
4.2. Exergo-economic costs of the flow streams
The next step in this analysis is to calculate the exergo-economic
costs of the flow streams. Tables 8–10 show the values used in the
exergo-economic analysis taken from the literature [29]. The unit
exergo-economic cost or the unit cost, in c$/MJ, of a stream can be
Fig. 4. Schematic of retrofit option # 3.
Table 4
Operational data for SWRO plant retrofit option # 3 [29].
Parameter Flow
(m3
/h)
Pressure
(kPa)
Total feed 1334 101.325
Feed per train 667 5961.325
HPP inlet 300 351.325
ERD feed inlet 367 351.325
Permeate stream per train 300 101.325
Total permeate 600 101.325
Brine stream/ERD brine inlet 367 5761.325
Brine discharge/ERD brine outlet 367 101.325
Fig. 5. Schematic of retrofit option # 4.
92 M.A. Jamil et al. / Desalination 401 (2017) 88–98

6. calculated by applying the general formula which states that the unit
exergo-economic cost of any product stream is equal to sum of the
costs of fuel streams and fixed cost of the components producing it.
This is expressed as [22]:
Cp ¼ ∑
f
Cf
X
˙
f
X
˙
p
þ
Z
˙
X
˙
p
ð15Þ
The rate of exergo-economic cost Ċp(in c$/s) is calculated as [22]:
Ċp ¼ Ċi þ Celectricity Ẋ
þ Ż ð16Þ
The cost of fresh (desalted) water, in c$/m3
, for each retrofit option is
calculated as:
γp ¼
C
̇
p
Q̇p
ð17Þ
A sample calculation for the exergo-economic analysis is given in
Appendix B.
5. Results and discussion
5.1. First and second law analysis
This section compares the power requirements, specific energy con-
sumptions and second law efficiencies of the systems that are discussed
in the previous section.
5.1.1. Standard configuration plant
Referring to Table 6, it is obvious that, for the standard plant, the net
energy requirement reduces to 1573 kW from 2251 kW after installa-
tion of ERTs. SEC for the standard configuration is 3.78 kWh/m3
. This
is expected to be reduced after installation of high-efficiency ERDs. It
is important to emphasize that selection of any retrofit option requires
the following parameters to be analyzed critically:
(a) overall plant efficiency,
(b) new operational data and recovery rate,
(c) civil works and hydraulic network,
(d) capital investment associated with retrofitting, and
(e) management of the new system in terms of operation and main-
tenance, etc.
Keeping the above facts in mind, four different possible retrofit op-
tions are analyzed for comparison purpose.
5.1.2. Retrofit option # 1
Table 6 illustrates that, for the same plant capacity, retrofit option #
1 shows a considerable reduction in energy requirements compared to
the original plant. The table shows there is no change in the feed
pump work because this section is same for all the retrofit options hav-
ing same plant capacity. However, coupling of an isobaric PX reduces
overall energy consumption by 20.63% compared to the original plant.
Additionally, second-law efficiency is also increased from 19.47% to
24.53% for this option. Therefore, one can say that this retrofit is more
efficient from second-law viewpoint compared to the standard plant
because of better energy recovery.
Table 5
Operational data for SWRO plant retrofit # 4 [29].
Parameter Flow
(m3
/h)
Pressure
(kPa)
Total feed 1436 101.325
Feed per train 1 463 5961.325
Feed per train 2 255 5711
HPP inlet 463 351.325
ERD feed inlet 255 351.325
Permeate stream per train 1 208 101.325
Permeate stream per train 2 112 101.325
Total permeate 640 101.325
Brine train 1/ERD brine inlet 255 5761.325
Brine train 1/ERD brine outlet 255 101.325
Brine train 2 143 101.325
Fig. 6. Schematic for exergo-economic analysis.
93M.A. Jamil et al. / Desalination 401 (2017) 88–98

7. 5.1.3. Retrofit option # 2
This retrofit is obtained by slightly modifying the previous one with
an aim to avoid major replacements. The input power is distributed dif-
ferently in this retrofit because of an additional BP. From Table 6, it can
be seen that SEC for this option is reduced by 19.31% compared to the
standard plant. The second-law efficiency, for this case, is 24.15%,
which is slightly lower than the previous retrofit but higher than the
standard one. So, we may say that the energy consumption for both of
these retrofit options lies in the same range because of the same plant
capacity. Hence, overall cost for modification is the only deciding factor
among these two.
5.1.4. Retrofit option # 3
The energy requirements for components like feed-pump, HPP and
distribution pump are higher for this retrofit option because it has
higher plant capacity compared to the standard configuration. When
we compare the results (refer to Table 6), this retrofit has the lowest
SEC value of 2.98 kWh/m3
and highest second-law efficiency of 24.7%.
It gives energy saving of 21.15% compared to all other retrofits. There-
fore, this retrofit is recommended where higher capacity RO trains can
be accommodated.
5.1.5. Retrofit option # 4
It is another retrofit option that can also be used to enhance the plant
capacity. However, the analysis shows, (refer to Table 6) that this option
is not as efficient energetically as the previous ones. It can be seen from
this table that SEC for this retrofit is 3.86 kWh/m3
, which is the highest
among all, including the standard configuration. It shows an increase of
about 2% in the energy requirement compared to the base system. In ad-
dition, second-law efficiency has the least value of about 19%. So, we can
say that this retrofit is not suitable both from first- and second-law
considerations.
5.2. Exergo-economic analysis
Table 11 summarizes both exergy values and unitary exergy costs of
flow streams. The exergy rates of streams leaving subsystem 1 and 2 are
same for the first two retrofit options because of their same capacity,
while these are different for the last two because of different capacities.
It is important to emphasize that unitary cost of stream 1 is same for all
the retrofit options since subsystem 1 consists of the same equipment in
all cases. The difference, however, in work supplied due to different
mass flow rates is compensated by the product stream exergy and
their ratio remains same. However, the unitary exergy cost of product
stream is different for all retrofit options since different types of equip-
ment are attached to each system with dissimilar capacities. Table 12
summarizes the final product cost for all retrofit options. It can be
seen that retrofit option # 3 has the least product cost of 70.34 c$/m3
followed by option # 2 with a value of 72.24 c$/m3
. Among the first
two retrofits, the second one has lower cost compared to the first one.
This is because no change in HPP or any other component is required
and only a BP is introduced to meet the demand. We note that option
# 4 has the highest product cost of 83.07 c$/m3
. This is primarily due
to the fact that it needs larger modifications and has the highest energy
consumption and irreversible losses in the system components,
resulting in the lowest second-law efficiency.
5.3. Comparison with literature
To assess the importance and effectiveness of the modifications in-
vestigated in the current work, it is necessary to compare the present re-
sults with the one published in literature [29]. Figs. 7 to 9 compare the
SEC, second-law efficiency and product cost for all the plant configura-
tions with and without, post-treatment and distribution sections. The
Table 6
First law analysis results.
Parameter Standard plant Retrofit option #1 Retrofit option #2 Retrofit option #3 Retrofit option #4
Feed pump work (kW) 82.44 82.44 82.44 118.8 127.8
HPP work (kW) 925 816.1 594.1 1201 1850
BP work (kW) n/a 57.03 297.13 47.66 n/a
Product pump (kW) 209.4 209.4 209.4 302.1 322.2
Pelton turbine work (kW) −339.2⁎ n/a n/a n/a n/a
Total energy requirement (kW) 1573 1248 1268 1788 2472
SEC (kWh/m3
) 3.78 3.00 3.05 2.98 3.86
Energy saving (%) n/a 20.63 19.31 21.15 −2.18⁎⁎
ηII (%) 19.51 24.53 24.15 24.70 19.06
n/a stands for not applicable; * represents energy produced by the system; and ** represents increase in SEC compared to the base configuration.
Table 7
Stream exergies of the subsystems.
Stream Subsystem 1 Subsystem 2
Fuel (f) _X0 þ _X4
_X1 þ _X5
Product (p) _X1
_X3
Losses (L) 0 Ẋ2
Table 8
Data used in economic analysis [29].
Parameter Value
Taxes 0.35
Amortization period 8 years
Lifetime 15 years
Loan Interest rate 0.06
Mean inflation rate 0.02
Annual increasing of capital goods
above or below inflation rate
0.00
Years in which the devices should be
replaced
Intake and pumping once during lifetime,
while membrane every five years
Electricity cost 0.1344 ($/kWh)
Annual increasing of O M costs
above or below inflation rate
0.00
Annual increasing of product cost
above or below inflation rate
0.00
Annual availability 0.95
Table 9
Input data costs for each retrofit options analyzed.
Parameter Retrofit
option #1
Retrofit
option #2
Retrofit
option #3
Retrofit
option #4
Subsystem 1 cost 0.2274 M$ 0.2274 M$ 0.4558 M$ 0.4872 M$
Subsystem 2 cost 1.1268 M$ 1.167 M$ 1.467 M$ 1.596 M$
Specific O M Cost
(insurance, labor,
overheads,
breakdowns, fuel
excluded)
0.1512 $/m3
0.1512 $/m3
0.1456 $/m3
0.1568 $/m3
94 M.A. Jamil et al. / Desalination 401 (2017) 88–98

8. major outcomes of the comparisons can be summarized in the following
paragraphs.
Fig. 7 shows that SEC results for the current analysis (without addi-
tional sections) are in excellent agreement with the work of Penate and
Rodriguez [29] and confirms the model validity. It can be seen that after
introducing post-treatment and distribution sections, SEC values show
an increase of 15 to 20% due to additional energy consuming compo-
nents. It is, however, important to note that second-law efficiency
(refer to Fig. 8) values, calculated by using updated seawater properties
and an appropriate definition of the efficiency, show a 50 to 60% de-
crease compared to the values reported in the literature for all the
cases invesitgated. In addition, second-law efficiency values decrease
by 13 to 16% after incorporation of the post-treatment and distribution
sections.
Fig. 9 shows that the unit product cost (in c$/m3
) calculated by the
current approach is higher than the one reported in [29]. It is, however,
close to the one reported in [22,43]. The possible reasons for this differ-
ence include the use of updated seawater properties and additional
post-treatment and distribution sections that are considered in the
present investigations. It should be noted from Fig. 9 that coupling of
the post-treatment and distribution sections increases the final product
cost by 25 to 35% for all the cases.
5.4. Parametric investigation
In this section, the effect of input parameters like feed salinity,
pump efficiency and input energy cost on the plant performance
are discussed. All the operating conditions that are used in this
study are mentioned in the corresponding figures.
Fig. 10 shows that, by introducing high-efficiency HPPs, SEC of the
plant can be greatly reduced. The base system shows an abrupt re-
duction in SEC because two HPPs are involved and have the highest
energy consumption. While in the retrofit options, due to the pres-
ence of PXs, the energy consumption of HPPs is already lower than
the base system. Thus, the increase in their efficiency does not re-
duce the SEC as abruptly as in the base system. Similarly, Fig. 11
shows a significant rise in second-law efficiency for the base system
with an increase in HPP efficiency compared to other retrofit options
for the same reason. Though it is obvious that by introducing high-ef-
ficiency components, lower SEC, and higher second-law efficiency
can be obtained, the current analysis is carried out to give an idea
that how and how much the performance of a plant changes with
these parameters.
Table 10
Effective rate of fixed costs for different subsystems [29].
Retrofit option #1
Subsystem (in c$/s)
Retrofit option #2
Subsystem (in c$/s)
Retrofit option #3
Subsystem (in c$/s)
Retrofit option #4
Subsystem (in c$/s)
(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)
0.3523 1.932 1.364 0.375 1.818 1.364 0.772 2.386 1.790 0.829 2.614 1.960
Table 11
Stream exergy values and unitary exergy costs of streams.
Parameter
_X0
(kW)
_X1
(kW)
_X2
(kW)
_X3
(kW)
Cx0
(--)
Cx1
(--)
Cx2
(--)
Cx3
(--)
Retrofit option # 1 0 64.31 0 306.2 0 1.282 0 3.369
Retrofit option # 2 0 64.31 0 306.2 0 1.282 0 3.433
Retrofit option # 3 0 92.4 0 441.6 0 1.282 0 3.342
Retrofit option # 4 0 99.72 0 473.3 0 1.282 0 4.54
Table 12
Product costs for the retrofit options.
Parameter Retrofit option # 1 Retrofit option # 2 Retrofit option # 3 Retrofit option # 4
Unit exergo-economic cost, C (c$/MJ) 35.85 35.90 37.31 42.07
Rate of exergo-economic cost, _Cp (c$/s) 8.37 8.34 11.73 14.78
Fresh water cost, γp (c$/m3
) 72.48 72.24 70.34 83.07
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Base System Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4
SEC(kWh/m3)
Plant Type
Literature Current analysis Current analysis with post-treatment distribution
Fig. 7. Specific energy consumption for different plant configurations.
0
10
20
30
40
50
60
Base System Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4
ηII(%)
Plant Type
Literature Current analysis Current analysis with post-treatment distribution
Fig. 8. Comparison of second-law efficiency for various plant configurations.
95M.A. Jamil et al. / Desalination 401 (2017) 88–98

9. Feed salinity is another parameter that affects the second-law ef-
ficiency of the plants as shown in Fig. 12. When the feed salinity is in-
creased, the recovery ratio decreases which is inversely proportion
to the input energy. The second-law efficiency is obtained by divid-
ing the minimum of least work of separation by input energy so it in-
creases with the feed salinity.
It is important to understand the variation of the product cost
against the input energy cost because the price of electricity is different
in every locality. It can be seen from Fig. 13 that the final product cost
increases linearly with the electricity cost. However, the first and second
retrofit options show an interesting shift at higher electricity cost. For
example, at an electricity cost ≤0.22 $/kWh, the second retrofit option
has lower product cost while, for higher values of unit electricity cost,
the first option gives better results from an economic standpoint.
6. Concluding remarks
A seawater reverse osmosis plant discussed by Penate and
Rodriguez [29] is re-evaluated by using updated seawater properties
and an appropriate definition of the second law efficiency for four
different retrofit options along with the base system. Furthermore,
the study is also updated by adding the post-treatment and distribu-
tion sections that were not considered in the previous investigation.
The study provides reliable information about improving the
existing SWRO plant in terms of energy consumption by introducing
high-efficiency PXs as well as upgrading the plant capacity. The
major findings of the present study can be summarized as:
• Compared to conventional energy recovery turbines, modern PXs
are more efficient and their installation resulted in 18 to 23%
reduction in SEC of the plant.
• About 50 to 60% reduction in second-law efficiency was observed
by using the updated and reliable seawater properties as well as
by using an appropriate relation for second-law efficiency.
• The installation of post-treatment and distribution sections in-
creases the energy consumption as well as the product cost and de-
creases the second-law efficiency of the plant. About 15 to 20%
increase in SEC, 25 to 30% increase in product cost and 13 to 16% re-
duction in second-law efficiency is observed by introducing these
sections. This fact suggests that these should not be neglected for
a reliable design and analysis of desalination plants.
• Among all the retrofit options discussed in the paper, options 2 and
3 present better results compared to options 1 and 4 in terms of the
product cost.
0
10
20
30
40
50
60
70
80
90
Retrofit # 1 Retrofit # 2 Retrofit # 3 Retrofit # 4
Specificproductcost,γγp(c$/m3)
Plant Type
Literature Current analysis Current analysis with post-treatment distribution
Fig. 9. Comparison of specific product cost for various plant configurations.
Fig. 10. Specific energy consumption vs high-pressure pump efficiency.
ηmotor = 92%, SF = 35g/kg, TF B = 65g/kg
Fig. 11. Second-law efficiency vs high-pressure pump efficiency.
Fig. 12. Second-law efficiency vs feed salinity.
Fig. 13. Variation of the specific product cost vs electricity cost.
96 M.A. Jamil et al. / Desalination 401 (2017) 88–98

10. • For a constant plant capacity, option 2 is recommended for an elec-
tricity cost of ≤0.22 ($/kWh) since it has slightly lower product cost
compared to the first option. While at higher electricity costs, the
first option is more favorable.
• Retrofit option 3 is the best possible choice because of the lower
SEC and product cost. However, it requires higher capacity RO
trains which may not be affordable in every case.
• Retrofit option 4 shows the worst performance. It has the highest
product cost among all the retrofit options because of higher SEC
and lower second-law efficiency values.
The present study clearly shows that reliable seawater properties,
method of calculation and plant layout must carefully be selected
while analyzing any desalination system as they can affect the final
results significantly.
Nomenclature
C unit exergo-economic cost (c$/MJ)
Cx unitary exergy cost
Ċp rate of exergo-economic cost (c$/s)
ESaving energy saving (%)
_m mass flow rate (kg/s)
P pressure (kPa)
Q̇ volume flow rate (m3
/s)
S salinity (g/kg)
T temperature (°C)
Ẇ power requirement (kW)
Ẋ exergy rate (kW)
Ż rate of fixed costs (c$/s)
Greek letters
Δ change in quantity
η efficiency
γ specific cost (c$/m3
)
Subscripts
0 dead state
B brine
D destroyed
F feed
i inlet
II second law
in input
l least
L loss
min minimum
o outlet
p product
Rf# retrofit option
TB turbine
Tot total
Abbreviations
BP booster pump
DP distribution pump
EES engineering equation solver
ERT energy recovery turbine
ERD energy recovery device
HPP high-pressure pump
LP low-pressure pump
PVs pressure vessels
PX pressure exchanger
RO reverse osmosis
SWRO seawater reverse osmosis
SEC specific energy consumption, (kWh/m3
)
Acknowledgement
The authors acknowledge the support provided by King Fahd Uni-
versity of Petroleum Minerals through the project IN151001.
Appendix A. First- and second-law analysis calculations
For sample calculations, retrofit option # 2 is presented here because
it includes almost all the components that are discussed in this paper.
The pump work is given by,
ẆPump ¼
Q
˙
ΔP
ηPump
ðA À 1Þ
Now, we will calculate the individual pump works.
Feed pump work:
_WFP ¼
0:2572 Â 351:325−101:325ð Þ
0:78
¼ 82:44 kW
Low-pressure BP work:
_WLP;BP ¼
0:1155 Â 1951:325−101:325ð Þ
0:77
¼ 240 kW
BP work:
_WBP ¼
0:14167 Â 5961:325−5651:325ð Þ
0:77
¼ 57:03 kW
HPP work:
_WHPP ¼
0:1156 Â 5961:325−1951:325ð Þ
0:78
¼ 594:3 kW
Distribution pump work:
_WDP ¼
0:1155 Â 1402−101:325ð Þ
0:78
¼ 192:599 kW
With a motor efficiency of 0.92, the total pump work comes out to
be,
_Wtot ¼
82:435 þ 240 þ 57:03 þ 594:3 þ 192:599
0:92
¼ 1267:8 kW
The SEC is given by,
SEC ¼
_W˙
in
3600 Â
X
out
_
Q
˙
p
ðA À 2Þ
For retrofit option # 2, it is found to be,
SEC ¼
1267:8
416
¼ 3:05 kWh=m
3
The energy saving is then calculated as:
ESaving ¼
3:78−3:05
3:78
 100 ¼ 19:31%
The second-law efficiency is given as,
ηII ¼
W
˙
l; min
W
˙
in
ðA À 3Þ
97M.A. Jamil et al. / Desalination 401 (2017) 88–98

11. For retrofit option # 2, it is found to be,
ηII ¼
306:2
1267:4
 100 ¼ 24:15%
Appendix B. Sample exergo-economics calculation
The sample calculations for exergo-economic analysis are given in
this appendix. In this regard, retrofit option # 2 is presented here ac-
cording to the procedure given in Section 4.
The unitary exergy cost of streams is expressed as,
Cx# ¼
W
˙
Subsystem#
X
˙
#
ðB À 1Þ
For stream 1,
Cx1 ¼
89:603
64:31
¼ 1:393
For stream 3,
Cx3 ¼
1267:8
306:8
¼ 4:13
Now, the unit exergo-economic cost, in c$/MJ, is described as
Cp ¼ ∑
f
Cf
X
˙
f
X
˙
p
þ
Z
˙
X
˙
p
ðB À 2Þ
Cp ¼
3:7931 Â 10−5
 100  89:603
64:31
þ
0:3636
64:31
þ
3:7931 Â 10−5
 100  968:51
306:2
þ
1:818
306:2
þ
3:7931 Â 10−5
 100  209:34
306:2
þ
1:3636
306:2
¼ 35:92 c$=MJ
Now, the rate of product cost, in c$/s, is described as,
Ċp ¼ Ċi þ Celectricity Ẋ
þ Ż ðB À 3Þ
where Ċi represents the cost of inlet stream to that subsystem. This
gives,
Ċ p ¼ 8:35 c$=s:
The fresh water cost, in c$/m3
, is given by,
γRf #2;3
¼
C
˙
p
Q
˙
p
ðB À 4Þ
Therefore,
γRf #2;p
¼ γRf#2;3
¼
8:35 Â 3600
416
¼ 72:25 c$=m3
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