setiawan 2009 renewable-energy

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Design, economic analysis and environmental considerations of mini-grid hybrid

power system with reverse osmosis desalination plant for remote areas

Ahmad Agus Setiawan*, Yu Zhao, Chem. V. Nayar

Department of Electrical and Computer Engineering, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia

a r t i c l e i n f o

Article history:

Available online 21 July 2008

Keywords:

Hybrid system

Reverse osmosis

Economic analysis

Emissions

CO2NOxRemote areas

HOMER

Emergency relief

a b s t r a c t

This paper discusses the design process of a mini-grid hybrid power system with reverse osmosis de-salination plant for remote areas, together with an economic analysis and environmental considerations

for the project life cycle. It presents a design scenario for supplying electricity and fulfilling demand for

clean water in remote areas by utilising renewable energy sources and a diesel generator with a reverse

osmosis desalination plant as a deferrable load. The economic issues analysed are the initial capital cost

needed, the fuel consumption and annual cost, the total net present cost (NPC), the cost of electricity

(COE) generated by the system per kWh and the simple payback time (SPBT) for the project. The en-

vironmental considerations discussed are the amount of gas emissions, such as CO2 and NOx, as well as

particulate matter released into the atmosphere. Simulations based on an actual set of conditions in

a remote area in the Maldives were performed using HOMER for two conditions: before and after the

Tsunami of 26th December 2004. Experimental results on the prototype 5 kVA mini-grid inverter and

reverse osmosis desalination plant, rated at 5.5 kWh/day, are also presented here to verify the idea of

providing power and water supplies to remote areas.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

Providing energy for a community, in a sustainable manner,

nowadays has become a more and more important issue as we face

global warming and climate change realities. Power generation

engineers and designers have a responsibility to improve tech-

niques of energy conversion in order to reduce emissions of CO2and NOx, which are believed to be a source of environmental deg-

radation. Harnessing renewable energy sources which are abun-

dantly available in nature provides an opportunity to produce

energy in an environmentally friendly way.

There are many locations, especially in remote areas of de-

veloping countries, which have no access to a reliable power sup-

ply. This situation mainly is due to the geographical conditions ofthe areas, which make it uneconomic to build any connection to the

existing power grid lines. Most of these areas rely on diesel gen-

erators for their power supply. However, this conventional gener-

ation depends on the availability of fossil fuel that usually is quite

expensive. Beside that, the engines usually operate at lowefficiency

due to the typical loads in remote areas that vary considerably

during the day and night. Therefore, with an appropriate design,

combining a reliable diesel generator with a renewable energy

generator can solve these economic and environmental problems

to supply the energy demands for those particular areas in a sus-

tainable way.

In addition, there are plenty of opportunities for renewable

energy applications in emergency relief conditions, for example,

after the tsunami disaster that happened recently, as well as any

other natural disaster (such as earthquake, volcanic eruption, etc.),

which usually results in isolating affected areas, especially if it

happens in a remote location. Most of these areas usually suffer

from the destruction of their infrastructure such as power and

water supplies.

This paper presents a design process, economic analysis and

environmental considerations of a mini-grid hybrid power system

with reverse osmosis desalination plant for providing electricityand clean water supplies for remote areas. The design steps are

presented for supplying electricity and clean water in remote areas

by utilising renewable energy sources (wind and photovoltaic) and

a diesel generator with a reverse osmosis desalination plant as

a deferrable load. The economic analysis considersthe initial capital

cost needed, the fuel consumption and annual cost, the total net

present cost and the cost of electricity generated by the system per

kWh. Furthermore, the simple payback time (SPBT) calculation is

also presented in order to show the project feasibility from an

economic point of view. The environmental aspects analysed are

the amount of gas emissions such as CO2 and NOx as well as par-

ticulate matter released into the atmosphere.* Corresponding author. Tel.: 61 892 661784; fax: 61 892 662584.

E-mail address: [email protected] (A.A. Setiawan).

Contents lists available at ScienceDirect

Renewable Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / r e n e n e

0960-1481/$ see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.renene.2008.05.014

Renewable Energy 34 (2009) 374383
mailto:[email protected]://www.sciencedirect.com/science/journal/09601481http://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.sciencedirect.com/science/journal/09601481mailto:[email protected]


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Simulations based on the actual conditions in a remote area of

the Maldives were performed using HOMER, for two conditions:

before and after the Tsunami of 26th December 2004. Experimental

works on prototypes of a 5 kVA mini-grid inverter and a reverse

osmosis desalination plant, rated at 5.5 kWh/day, have been com-

pleted and the results are also presented here to verify the idea of

providing power and water supplies to remote areas.

2. Hybrid power system

There is a huge potential for utilising renewable energy sources,

for example solar energy, wind energy or micro hydropower, to

provide a quality power supply to remote areas. The abundant

energy available in nature can be harnessed and converted to

electricity in a sustainable way to supply the necessary power to

elevate the living standards of people without access to the elec-

tricity grid.

The advantages of using renewable energy sources for gener-

ating power in remote areas are obvious: the cost of transported

fuel is often prohibitive, and there is increasing concern about fossil

fuels and the issues of climate change and global warming. The

disadvantage of stand-alone power systems using renewable en-

ergy is that the availability of renewable energy sources has daily

and seasonal patterns which result in difficulties in regulating the

output power to cope with the load demand. Combining renewable

energy generation with conventional diesel power generation willenable the power generated from renewable energy sources to be

more reliable and affordable. This kind of electric power generation

system, which consists of renewable energy and fossil fuel gener-

ators together with an energy storage system and power condi-

tioning system, is known as a hybrid power system [5].

Fig. 1. Series hybrid power system configuration.

Fig. 2. Parallel hybrid power system configuration.

A.A. Setiawan et al. / Renewable Energy 34 (2009) 374383 375


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A hybrid power system has the ability to provide 24-h grid

quality electricity to the load. This system offers better efficiency,

flexibility of planning and environmental benefits compared to the

diesel generator stand-alone system. The usual maintenance costs

of a diesel generator can be decreased as a consequence of im-

proving the efficiency of operation and reducing the operational

time, which also means less fuel usage. Furthermore, the system

provides an opportunity for expanding its capacity in order to cope

with increasing demand in the future. This can be done by in-

creasing either the rated power of the diesel generator, renewable

generator or both.

The hybrid power systems which utilize renewable energy

generators can be classified into two basic configurations: series

hybrid system and parallel hybrid system.

2.1. Series hybrid power system

In a series hybrid system configuration, as shown in Fig. 1,

power generated by a diesel generator, wind generator and solar

PV array are used to charge a battery bank. The inverter will

convert the DC power stored in the battery bank to AC, at

a standard level of voltage and frequency, and then supply it to

the load.

Battery charging can be controlled either by controlling the

excitation of the alternator or by utilising wind/solar charger reg-

ulators that can prevent overcharging of the battery bank from the

wind/PV generators when the renewable power exceeds the load

demand and when the batteries are fully charged.

Fig. 3. AC coupling of parallel hybrid power system.

Fig. 5. Mini-grid hybrid power system with RO plant configuration.

0

2

4

6

8

10

12

14

16

18

20

Load(kW)

1 3 5 7 9 11 13 15 17 19 21 23

Hours

Fig. 4. Estimated daily load profile for a remote area in the Maldives after the tsunami.

A.A. Setiawan et al. / Renewable Energy 34 (2009) 374383376


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This configuration results in relatively simple implementation

but it has some drawbacks, such as: low overall efficiency, re-

quirement for a large battery bank to limit the depth-of-discharge,

and limited control of the diesel alternator since the system isbased on the level of charge in the battery rather than the site load.

2.2. Parallel hybrid power system

In a parallel hybrid system configuration, as shown in Fig. 2, the

diesel generator and renewable energy generator supply a portion

of the load demand directly. This system utilises a bi-directionalinverter, which is operated in parallel with the diesel generator and

can act as an inverter and rectifier/battery charger. The design

principles of this system are relatively complicated but it has su-

periority compared to the series configuration; for example, the

system load in this configuration can be met in the most optimal

way, the diesel efficiency can be maximized and there are possible

reductions in the capacities of diesel, battery and renewable energy

generators while meeting the load peaks.

The parallel hybrid power system, shown in Fig. 2, is a DC

coupling configuration where the renewable energy generators are

connected to a battery bank through the DC bus and supply ACload

through the bi-directional inverter. This configuration can further

be improved by connecting all of the renewable generators to the

AC bus to perform an AC coupling configuration, as shown in Fig. 3.Since this parallel topology has advantages regarding the control-

lability of the system, only this topology is considered for the de-

sign of a mini-grid hybrid power system in this paper [1].

3. Design procedures and simulation

There are a number of issues that should be considered in de-

signing a mini-grid hybrid power system: power demand topology,

cost of fuel in the particular area, availability of the renewable

energy resources, initial cost of the project including cost of each

component needed, project life time, interest rate, subsidization of

scheme, etc. Comparison analysis on the diesel-only system (which

is the common case foran autonomous power generation system in

remote areas), with the proposed hybrid system can be performedto observe the feasibility of the design project [7].

For this project, we selected one of the remote islands in the

Maldives which, after the tsunami disaster, now is populated by

approximately 300 people. Before tsunami, power supply was

maintained by operating two diesel generators with capacities of

40 kW and 32 kW. According to the Island Development Committee

(IDC),the influential authority in the island that makes decisionson

island development including electrification, the estimated load

distribution for the post-tsunami day is shown as in Fig. 4. This

primary load has an annual average of 298 kWh/day with an annual

peak of 37.4 kW [8].

In the simulation, in order to provide a consumable water

supply, a reverse osmosis desalination plant was chosen as a de-

ferrable load to the system. The RO plant should be capable ofsupplying 5 m3 of fresh water per day. This system consumes

Table 1

Economic analysis results from HOMER simulation

Parameter Before tsunami After tsunami

Diesel only PV-diesel Wind-diesel PV/wind-diesel

Ini ti al c apital c ost $ 57,600 $165,900 $185,900 $209, 900

Total net present

cost

$709,055 $646,359 $632,159 $635,386

Cost of electricity

($/kWh)

$0.510 $0.447 $0.437 $0.439

Annual fuel

consumed (L)

55,494 40,919 37,247 35,252

Annual fuel cost $35,516 $26,188 $23,838 $22,561

Table 2

Simple payback time calculations

Paramet er Di esel on ly PV- di esel Wi nd-die sel PV/wi nd-diesel

Initial cost $57,600 $165,900 $185,900 $209,900

Annual fuel cost $35,516 $26,188 $23,838 $22,561

SPBT in years 11.6 11 11.8

Table 3

Pollutants emitted from the power systems

Parameter Diesel only PV-diesel Wind-diesel PV/wind-diesel

CO2 (kg/yr) 146,135 107,753 107,753 98,829

CO (kg/yr) 361 266 266 229

UHC (kg/yr) 40 29.5 29.5 25.4

PM (kg/yr) 27.2 20.1 20.1 17.3

SO2 (kg/yr) 293 216 216 186

NOx (kg/yr) 3,219 2,373 2,373 2,045

Table 4Percentage of energy production

Paramet er Di esel on ly P V-die se l Win d-die se l PV/wi nd-diesel

Diesel 100% 86% 80% 76%

PV 0 14% 0 4%

Wind 0 0 20% 20%

Diesel efficiency 21.9% 30.5% 30.4% 30.4%

Water supply No Yes Yes Yes

Fig. 6. Equivalent circuit diagram of hybrid system.

Fig. 7. Energy management in hybrid system.



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2.5 kW of power and needs to be operated for 8 h per day. A fresh

water tank is designed to have storage capacity for two days de-mand. So, there will be a 20 kWh/day average deferrable load with

40 kWh storage capacities.

For both analytical and simulation purposes, we need to have

appropriate data on renewable energy availability for a certain

period of time. In this preliminary study case, they were obtained

from the Maldives government and the National Renewable Energy

Laboratory (NREL), USA [2,6]. With all necessary inputs collected,

we then carried out the simulation on this particular hybrid power

system modelling, using HOMER, to find the best configuration

[4,3]. The resultant system configuration for the mini-grid hybrid

system is shown in Fig. 5.

Simulations were performed for two conditions: before and

after the tsunami. Before the tsunami, the power generator consists

of two diesel generators, 40 kW and 32 kW. After the tsunami,

power demands including a reverse osmosis desalination plant are

supplied by using a mini-grid hybrid power system with several

topology options: PV/diesel, wind/diesel and PV/wind/diesel. The

economic analysis results obtained from the simulation for theproject period of 20 years are presented in Table 1.

We can calculate the simple payback time (SPBT), from Table 1

above, using the simple formulation below (Eq. (1)), and the results

are presented in Table 2 [9,10]. As can be seen, the simple payback

times for the hybrid systems are 11.6 years, 11 years, and 11.8 years

for PV-Diesel, Wind-Diesel, and PV/Wind-Diesel, respectively.

SPBT Excess Cost of hybrid system

Rate of saving(1)

From the simulation, we can also obtain the levels of emitted

pollutants to the environment. It shows the total amount of emis-

sions produced annually by the power systems. These emissions

are a by-product of the energy conversion processes, mainly oc-

curring in the diesel generator from fuel combustion. There are sixpollutants as simulation outputs: carbon dioxide (CO2), carbon

monoxide (CO), unburned hydrocarbons (UHC), particulate matter

(PM), sulfur dioxide (SO2) and nitrogen oxides (NOx); and they are

presented in Table 3.

In addition, the percentages of energy production from each

system are presented in Table 4, together with diesel operative

efficiency and supply of water capability.

From all of the simulation results above (Tables 14), we can

reveal that even though the diesel-only system has the least initial

capital cost, it results in the highest total net present cost for the

whole project, it emits more pollutant into the atmosphere, and the

diesel system operates at low efficiency (21.9%). The hybrid systems

utilising renewable energy generators offer a better economic

feasibility, lower emissions and the component diesel generator

Fig. 8. Prototype of 5 kVA mini-grid inverter.

Fig. 9. Experimental setup for mini-grid inverter.

Fig. 10. Efficiency of the 5 kVA mini-grid inverter.



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operates at a higher efficiency (above 30%) which ensures a longer

operation lifetime. Payback time for the hybrid systems is also quite

attractive with an average of 11.5 years in the 20-year period of the

project. In addition, the hybrid system topologies also provide

a fresh water supply as needed by people living in this particularremote area.

4. Prototype experimental results

Laboratory experiments on the prototype 5 kVA mini-grid in-

verter and reverse osmosis desalination plant rated at 5.51 kWh/

day were performed and the results are presented here to verify theidea of providing power and water supplies to remote areas.

Fig. 11. (a) Voltage and current waveforms, and (b) power output.

Fig. 12. THD for (a) voltage output, and (b) current output.

Fig. 13. Overload handling capability at 150% rated load.



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4.1. Prototype of mini-grid inverter

A power conditioning unit is the heart of the hybrid energy

system mentioned in the previous section. A bi-directional inverter

that can act both as inverter and charger is based on a voltage

source inverter (VSI). VSIs have DC input voltage as the source and

produce sinusoidal output voltage. During the inverting process the

inverter is operated to supply power with constant voltage, thus it

is operated in Voltage-Controlled VSI (VCVSI) mode. During the

charging process the inverter is operated in Current-Controlled VSI

(CCVSI) mode to ensure constant current to charge the battery

bank. Fig. 6 shows the equivalent circuit diagram of the hybrid

system where the inverter is operated in parallel with the diesel

generator to supply the load.

As shown in Fig. 7, during a low load period, power demand is

supplied by inverting energy from the battery bank through VCVSI

mode to provide a certain level of voltage and frequency output.

When the load increases and the inverter is not capable of pro-

viding the supply of power any longer, the diesel generator is op-

erated to cover the load demand, and the excess of the energy is

used to charge the battery in CCVSI mode.

Fig. 8 shows the prototype of a 5 kVA inverter which acts as

a power conditioner for the mini-grid hybrid system, and Fig. 9

shows the experimental setup in the control room.

The results of efficiency testing for various loading on the pro-

totype 5 kVA mini-grid inverter are presented in Fig. 10, which

shows the efficiency operation at 90% for a full load condition. The

inverter produces sinusoidal voltage and current output wave-

forms, as shown in Fig. 11, with small harmonics distortion

(1.1% THD) as shown in Fig. 12.

Additionally, overloading test results are presented in Fig. 13,

which shows the capability of the prototype inverter in handling

this situation for various changing loads. Fig. 14 shows the voltage

regulation by the inverter for a 0150% step load change.

4.2. Reverse osmosis desalination plant

The experimental works on the RO system were first performed

by conducting preliminary testing on the system prototype. Then,

based on these preliminary data, the system was modelled and the

mathematical formulation was derived. After that simulation was

performed using MATLAB-SIMULINK. Finally, the simulation results

were compared with the experimental results to verify accuracy of

the system modelling.

Using the MATLAB data fitting function, a mathematical ex-

pression was derived from the preliminary test (Fig. 15) that was

conducted on the RO unit and formulated as:

Yp0:0002X310:0037X

210:0824X10:000071X20:98 (2)

where, Yp

is the permeate flow rate, l/min; X1

is the motor fre-

quency, Hz; X2 is the feed water conductivity, ppm.

Fig. 14. Voltage regulation at 150% step load.

30004000

50006000

70008000

900010000

2025

3035

4042

4446

4850

5254

5658

60

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

FeedWat

erCondu

ctivity(p

pm)

SupplyPowerFrequency(Hertz)

InletFlo

wRateof

theProduc

tTank(L/m)

Fig. 15. Product water flows corresponding to the motor frequency and feed water conductivity.



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0 1 2 3 4 5 6 7 8

x 104

0.15

0.16

0.17

0.18

0.19

0.2

0.21

0.22

0.23

Time of a day (second)

PowerConsumption(kW)

Fig. 17. Transient power consumption over 24 h.

0 1 2 3 4 5 6 7 8 9

x 104

0.48

0.5

0.52

0.54

0.56

0.58

0.6

0.62

0.64

0.66

Time of a day (second)

Waterlevelintheproducttank(%)

Fig. 18. Water level in the product tank over 24 h.

0

Total Water Product

0

Total Energy Consumed

kWh

Water Capacity Motor Frequency

Toggle Control

power

To WorkspaceWater Demands

Subsystem

Scope1

16points

Conductivity of feed water

Inlet Flow Rate of the Product Tank

Reverse Osmosis Unit

Product flowTotal water amount

Product Water Totalization

Input Water Flow Rate

Water consumption Flow Rate Water Capacity

Initial Water Capacity

Product WaterTank

0

Power Consumption Display

Feed Water Conductivity

Motor Frequency

Power Consumption

Power Consumption Calculation

Water Consumption

Water Capacity Motor Frequency

Product Water Flow Rate

PID Controller

Mode Switch

650

Initial

Water Capacity

-K-

Gain

Motor Input Frequency 16points

Frequency Scaling

4000

Feed Water

Conductivity

Transient Power Total Energy

Energy Calculation

Fig. 16. SIMULINK block diagram model for RO system.


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More explicitly, Fig. 15 reveals the water productivity of the RO

system under different feed stream conditions, such as conductivity

and pressure. It can be observed that the product water flow is

higher when the motor frequency is higher and the feed water

salinity is lower. Furthermore, the output water flow does not

increase significantly as the motor augments beyond 40 Hz.

A SIMULINK model of the RO unit was created in accordance

with the membrane characteristic that was already obtained

(Eq. (2)). A PID controller was designed and modelled, as presented

in Fig. 16.

With an initialwater level settingat 50% of the product tank, the

simulations were carried out over both a 24-h and a 7-day period of

time. Simulation results on the RO system are presented in Figs. 17

20. It was found that the tanks water level stayed at above 60%

most of the time, as seen in Fig. 20. The RO plant has an additional

0 1 2 3 4 5 6 7

x105

0

0.05

0.1

0.15

0.2

0.25

Time of a week (second)

Powercon

sumption(kW)

Fig. 19. Transient power consumption over 7 days (one week).

0 1 2 3 4 5 6 7

x105

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

Time of a week (second)

Waterlevelintheproducttank(%)

Fig. 20. Water level in the product tank over 7 days (one week).

Table 5

Simulation results on RO system productivity and energy consumption

Feed water

conductivity (ppm)

Water productivity

over 24 h (l)

Energy consumption

over 24 h (kWh)

2000 1339 3.939

3000 1237 3.936

4000 1135 3.851

5000 1033 3.959

6000 930.4 4.078

7000 828.3 3.886

8000 726.1 3.824

9000 623.9 3.75

10000 521.7 3.818Fig. 21. Schematic diagram of the RO experimental unit.



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feature of energy saving with a proportional, integral and derivative

(PID) controller that monitors the demand and regulates the motor

speed accordingly, as shown in Fig. 19.

Simulation results on water productivity and energy consump-

tion for the reverse osmosis desalination plant are presented inTable 5. It is shown that both the system productivity and energy

needed by the system vary accordingly in respect to the feed water

conductivity. The most productive performance by the RO system

occurs at the lowest feed water conductivity.

A prototype reverse osmosis desalination plant rated at

5.51 kWh/day was built and tested. The schematic diagram for the

RO unit experimental system is shown in Fig. 21 and a photograph

of the measurement systems on the experimental setup is shown in

Fig. 22.

Experiments on the RO unit were conducted over 24 h for the

expected real situation feed water conductivity condition of

4000 ppm. Experimental data were collected and presented in

Table 6. It was noticed that the power consumption of the RO unit

was higher than the one obtained from the simulation result. Thismayhave been caused by the turbulence of the transient conditions

in the experiment, such as the temperature variations. However, in

general, the experimental result is still within the range of expected

power coverage by the mini-grid hybrid power system and thus the

system modelling can be justified.

5. Conclusions

This paper presents a design scenario to provide a reliable

supply of power and water to meet demands for remote areas and

emergency relief conditions. A simulation has been performed to

find the optimum configuration for a hybrid power system by uti-

lising HOMER software from NREL-USA. It is shown that a hybrid

power system can provide both power and water supplies withbetter performance, both economically and environmentally viable,

compared to the stand-alone diesel system. Laboratory experiment

results on the prototype 5 kVA mini-grid inverter and reverse os-

mosis desalination plant, rated at 5.51 kWh/day, were also pre-

sented in this paper to demonstrate the feasibility of providing

reliable power and water supplies to remote areas.

Acknowledgements

The authors would like to thank Curtin University of Technology

and industry partner Regen Power Pty Ltd, Australia, for providing

test facilities for this project. The first author would like to ac-

knowledge the Australian Development Scholarships for providing

support to pursue a Ph.D. degree in the Department of Electricaland Computer Engineering, Curtin University of Technology (Centre

for Renewable Energy and Sustainable Technologies Australia,

CRESTA).

References

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[3] Lambert T, Gilman P, et al. Micropower system modeling with HOMER. In:Farret FA, Simoes MG, editors. Integration of alternative sources of energy.

John Wiley & Sons; 2006.[4] Lilienthal P, Gilman P, et al. HOMER the micropower optimization model.

National Renewable Energy Laboratory; 2005.[5] Nayar CV, Phillips SJ, et al. Novel wind/diesel/battery hybrid energy system.

Solar Energy 1993;51(1):6578.[6] Renne D, George R, et al. Solar resource assessment for Sri Lanka and Maldives.

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[8] Setiawan AA, Zhao Y, et al. Design of hybrid power system with reverseosmosis desalination plant for Maldives, AUPEC 2006. Melbourne, Australia:Victoria University; 2006.

[9] Short W, Packey DJ, et al. A manual for the economic evaluation of energyefficiency and renewable energy technologies. Department of Energy, NationalRenewable Energy Laboratory; 1995.

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Fig. 22. Photograph of the measurement systems on the experimental beds.

Table 6

Experimental results on the RO system productivity and energy consumption

Feed water

conductivity (ppm)

System operation

time (h)

Total energy

consumption (kWh)

Water

consumption (l)

4000 24 5.51 1058

http://www.nrel.gov/homer/http://www.nrel.gov/homer/http://www.nrel.gov/homer/http://www.nrel.gov/homer/

setiawan 2009 renewable-energy

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