Design and Operation of Small-Scale Photovoltaic-Driven Reverse Osmosis (PV-RO) Desalination Plant for Water Supply in Rural Areas

doi:10.4236/cweee.2012.13004

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Computational Water, Energy, and Environmental Engineering, 2012, 1, 31-36

http://dx.doi.org/10.4236/cweee.2012.13004 Published Online October 2012 (http://www.SciRP.org/journal/cweee)

Design and Operation of Small-Scale Photovoltaic-Driven

Reverse Osmosis (PV-RO) Desalination Plant for Water

Supply in Rural Areas

Fawzi Banat1, Hazim Qiblawey2, Qais Al-Nasser3

1Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates

2Department of Chemical Engineering, Qatar University, Doha, Qatar

3Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan

Email: [email protected]

Received August 30, 2012; revised September 3, 2012; accepted October 10, 2012

ABSTRACT

The alarming water and energy crisis in many regions of the world can be eased by combining renewable energy with

desalination technologies. The ADIRA project funded by the EU looked for demonstrating the feasibility of water de-

salination in areas around the Mediterranean by installing a number of autonomous desalination systems (ADS) which

are able to convert brackish or seawater into potable water for the needs of small communities. W ithin the activities of

the ADIRA project a reverse osmosis unit powered by photov oltaic electricity was installed in a village in the northern

part of Jordan with a capacity of 0.5 m3/day. The system was composed of a softener, reverse osmosis unit, PV panels

(432 Wp) and storage batteries. Residential type “OSMONICS” membrane (TFM-100) was utilized in the RO unit.

Field tests were performed on brackish water (1700 mg/L total dissolved solids (TDS)). This paper sheds the light on

the process flow diagram, sizing of the system main components and presents some of the results obtained.

Keywords: Reverse Osmosis; Water Filters; Ion Exchange System; Membranes; Carbon Filters

1. Introduction

Small capacity desalination units utilizing the reverse

osmosis (RO) technology and powered by photovoltaic

(PV) cells, is a potential solution for providing freshwa-

ter to small commin utes in isolated arid areas that have 1)

saline water problems; 2) no access to the electricity grid;

and 3) plenty solar resources. PVRO has minimal envi-

ronmental impact, can be easily designed and assembled

for different demand profiles using modular components

[1], and can be easily maintained and repaired.

Options of PV-RO configurations are available to over-

come the intermittent nature of solar power, th ese are: 1)

Use of fossil fuel to make up the gaps (grid-connected

systems), 2) store the solar energy, 3) run the desalina-

tion plant intermittently. Systems without a grid-connec-

tion are generally described as standalone or autonomous

systems.

Numerous renewable energy-powered RO plants, pri-

marily PV-battery systems of small to medium capacity

(0.5 to 50 m3/day), have been built in different locations

of the world. For example, Herold and Neskakis [2] pre-

sented a small PV-driven reverse osmosis desalination

plant on th e island o f Gran Cana ria with an ave rage da ily

drinking water production of 0.8 - 3 m3/d. The plant was

supplied by a stand-alone 4.8 kWp photovoltaic (PV)

system with additional battery storage of 60 kWh. The

nominal production was 1 m3/day. The specific energy

consumption of th is system was considered high with 16

$/m3 production cost.

The Energy Research Institute o f King Abdulaziz City

for Science and Technology (KACST) conducted exten-

sive research on a PV-battery-inverter RO system in Sa-

dous, Saudi Arabia. The RO system produced on average

5.7 m3/day, converting brackish water from 5700 ppm

TDS to 170 ppm TD S w ith an av era ge 30 % r ecov er y rate

[3].

In 2001 Solar Energy Systems (SES) in Australia work-

ed on commercializing a PV-RO unit, developed at Mur-

doch University, that is capable of producing 100 gallons

per day of water from feed water containing up to 5000

ppm TDS [4]. They installed approximately 20 systems,

primarily in the desert area of Australia. The system was

designed for 15 to 20 percent water recovery. Part of the

reasoning for the low water recovery was to reduce pro-

blems with scaling.

Carvalho et al. [5] presented the cost of PV-RO de-

salination plant with batteries in stalled in the community

of Ceara, of Brazil. The specific energy consumption of

F. BANAT ET AL.

produced water was around 3.03 kWh/m3 with cost of

12.76 $/m3.

Riffel and Carvalho [6] presented a small-scale battery-

less PV-RO plant for stand-alone applications that oper-

ates at variable flow/pressure conditions in equatorial ar-

eas to desalinate brackish water.

Mohamed et al. [7] presented the experimental results

of a small seawater RO system, installed at the Univer-

sity of Athens, equipped with an ERD of the Clark pump

type.

Qiblawey et al. [8] presented experimental results of a

PV powered household RO unit installed in Jordan and

operated with tap water having 350 mg/L and 720 mg/L

total dissolved solids. The unit was operated with and

without storage batteries. The specific energy consump-

tion of the battery system ranged from 1.1 kWh/m3 to 4.3

kWh/m3 and ranged from 1.1 kWh/m3 to 1.5 kWh/m3 for

the battery-less system.

In the context of ADIRA project a PV-RO unit with

production capacity of 0.5 m3/day was designed and in-

stalled in a village in the northern part of Jordan. The

ADIRA project is one of the MEDA projects financially

supported by the European Union (EU) for the develop-

ment of the water sector in the Middle Eastern and North

African (MENA) countries. This paper presents sizing of

the unit components alon g with some results.

2. Method

2.1. Process flow diagram

The process flow diagram of the PV-RO system is shown

in Figure 1. The system has three major components, a

PV array, a spiral wound membrane module, and a sof-

tener. The softener treats raw water from mineral ions

that cause scaling problems. The pretreatment step con-

sists of 4 stages: Softener, 5 Micron sediment filter,

granular activated carbon filter (GAC) and 1 Micron

sidemen filter. The system was fed with untreated brack-

ish water with a salinity of 1700 mg/L. In these experi-

ments, four residential membrane modules “OSMON-

ICS” type (TFM-100) were utilized. Electricity needed

by the system was partially supplied by the PV array

which consists from 8 PV modules each 54 Wp. Since

the RO unit needs a stable power supply, two batteries

(12 V, 230 Ah) were connected in series to increase the

voltage up to 24 V. The energy produced by the PV is

transferred through the solar charge regulator to battery

storage capable of storing enough energy for extra opera-

tion hours after sunset. The stored energy is transferred

back to regulator unit for powering the loads.

Solar charge controller was used to connect PV panels

to storage batteries. Charge controllers block reverse

current and prevent battery overcharge. Also prevent

battery over discharge, protect from electrical overload,

and display battery status. Its purpose is to keep batteries

and loads properly fed and safe for the long term. A se-

ries of temperature sensors (Pico Technology, UK) were

installed throughout the system in order to measure the

temperature of ambient, feed water, and the PV panels.

Two flow meters (FLR1000, USA) were installed to

measure the volumetric flow rate of raw water feed and

permeate. A pressure sensor (Omega PX309, USA) was

installed in the feed stream to control the pressure of the

feed pump. TDS probes (HMDigital, USA) were in-

stalled in the feed stream and in the permeate stream in

order to measure the quality of fed and treated water. A

pyranometer (PYR-PA2.5, USA) was installed to meas-

ure the global irradiation during the operation time. Two

clamps meter (Pico Technology, UK) were installed to

measure the available current from PV and to measure

the charger current of batteries.

The rechargeable batteries used aimed mainly for stor-

ing energy during the day to make it available through

nights to ensure continuous operation. Figure 2 shows

illustrative block diagram of the PV-RO system. The

system was tested for about 10 months; the aim of the

system testing was to investigate the water production

quantity and quality as well as the specific energy con-

sumption of the unit under different operating condi-

tions.

2.2. Sizing of the System

2.2.1. Daily En e rgy Requirement

The total daily energy requirements for the RO unit, sof-

tener unit and the auxiliaries (sensors, data acquisition

system etc.) have been determined as follows:

2.2.2. RO Lo a d

Two high pressure pumps (HPP): Volts = 24 VDC,

Maximum current = 1.2 A

Power of one HPP = 24 * 1.2 = 28.8 W

Total Power = 28.8 * 2 = 57.6 W

Hours of o pe ration (av erage p e r da y ) = 8 h

Total RO energy required/day = 57.6 * 8 = 460.8 Wh

2.2.3. Intake Pump

Intake pump: Volts = 220 VAC, 50 Hz, Current = 0.54 A,

Output p ower = 60 W

Load including inverter losses (assuming the inverter

losses (ηinv) to be about 10% = 60/0.9 = 66.7 W

Hours of o pe ration = 8 h

Total energy required/day = 66.7 * 8 = 533.3 Wh

2.2.4. Softener Feed Pump

Softener pump: Volts = 230 VAC, 50 Hz, Current = 6.2

A, Output pow er = 1.0 h p

Power of Soften er pump = 1 hp = 746 W

F. BANAT ET AL.

Salt Water

Int ake

Softener

Fresh Water

Tank

Control

Electronics

PV ArraySoften er

Pump

Feed

Pump

Pre-Filter

AssemblyHigh P ress ur e

Pump RO

Modules

Reject Water

Tank

Figure 1. Process flow diagram.

Sola r Re gu la tor

12 V/24 V, 20 A

Solar Energy

PV Panel

(34.8 V, 433 Wp)

ect wate r

RO Unit

(57.6 W , 24 VDC)

Fresh

water

Intak e Pump

(60 W, 220 VAC)

Soft en er Unit

(746 W, 220 V AC)

Feed

Water

Battery set

(230 Ah, 24 V)

Po wer L i n e

Figure 2. Block diagram of the system.

Load including inverter losses = 746/0.9 = 828.9 W

Hours of o per ation = 8 h

Total energy requ ired/day = 828.9 * 8 = 6631.2 Wh

2.2.5. Auxiliaries Load

Data acquisition system including sensors: Volts = 10

VDC, Current = 100 - 500 mA.

Maximum Power = 0.5 A * 1 0 V = 5 W

Hours of o pe ration = 24 h

Total Auxiliaries energy required/day = 5 * 24 = 120

Solar charge regulator: Volts = 24 VDC, Current = 15.8

Maximum Power = 0.0 1 58 A * 24 V = 0.38 W

Hours of operati on = 2 4 h

Total energy required/day = 0.38 * 24 = 9.1 Wh

The daily energy required including the losses for in-

verters = 460.8 + 533.3 + 6631.2 + 120 + 9.1 = 7754.4

Wh/day.

2.2.6. PV Pane ls

The theoretical daily energy requirement for the system

is about 7.754 kWh/day (EL), including the inverter

losses.

The battery losses (ηb) is about 15% [9] and the PV

thermal losses (ηth) is about 15% [9] also. The average

peak sunshine hour (PSSH) in Jordan is about 7 h. The

peak power of the PV module can be determined as fol-

lows:



Lth

power of the PVE(PSSH)

7.75470.85 0.85

1.533KWp

Peak b







The size of PV module must be such as to produce

1.533 kW with operating voltage more than 24 V, in or-

der to charge the batteries. Commercially available PV

modules of polycrystalline silicon type of 54 Wp as peak

power were selected (ISC = 3.31 A, V

OC = 21.7 V, Im =

3.11 A, Vm = 17.4 V at STC). The system needs ap-

proximately 30 (1533/54) PV modules in order to cover

the daily energy requirements. Considering that the DC

side operating voltage of 24 V, then 2 PV modules in

series are required (2 * 17.4 = 34.8 V). These (2 PV

F. BANAT ET AL.

modules in series) have to be organized in sub-arrays of

15 parallel strings (15 * 3.11 A = 46.65 A), each string

connected to the charge controller. The total array peak

power is (3 .11 A * 17.4 V) * (2 * 15) = 1.6 23 kWp.

In the initial design of the PV modules; only the en-

ergy required for high pressure pumps as well as the aux-

iliaries were considered with 2 4 hou rs of o peration ({(2 *

24 V * 1.2 A) + (5 W + 0.38 W)} * 24 = 1511.52

Wh/day). Intake pump and the softener pump were in-

stalled later to improve the operational efficiency of the

RO unit, and they were powered directly from the elec-

tricity grid, not from the PV array. The desired peak

power of the PV module based on the new consideration

is about 350 Wp (1511.52/(6 * 0.85 * 0.85)), the aver-

age peak sunshine of 6 h is used to be on the safe side.

The PV array of the system consists of 4 parallel

strings each of 2 series PV modules, the total array peak

power of the system is 432.9 Wp ((3.11 * 17.4) * (2 * 4))

which covers the amount of energy required.

2.2.7. Battery Storage

The battery of the PV-RO system was designed to act as

energy storage to run the system whenever insufficient

solar irradiation is available (cloudy days and nights). If

80% [9] maximum depth of discharge (DOD) is consid-

ered, the required maximum battery capacity per day is

calculated as follows:



Battery capacity (Wh)EDOD

1511.52 0.8 0.85

2223 Wh







Two batteries of 2760 Wh (230 Ah * 12 V) connected

in series were selected, producing 5520 Wh (230 Ah * 24

V) in total. The battery storage was able to operate the

RO unit for more than 59 h continua lly.

 







DOAdayBattery capacityWhDODEWhd

5520 0.8 0.85 1511.522.48days59.6h





 

2.2.8. Charge Reg ulator

Charge regulators (controllers) are rated based on the

amount of amperage they can process from a solar array.

If a controller is rated at 20 amps it means that you can

connect up to 20 amps of solar panel output current to

this controller. A suitable charge regulator was used in

the PV-RO system with the specification of 12 V/24 V,

20 A.

3. Results and Discussion

3.1. Metrological Data

Average values of solar irradiation (W/m2), insolation

(kWh/m2/d), and ambient temperature (˚C) were recorded

for different months during the year 2007 and presented in

Table 1. As shown, the month of September was the hottest

(26˚C) with the highest solar insolation (7.5 kWh/m2/d).

The RO unit was designed to operate 24 h/day. The

energy required was supplied by a PV array and battery

storage was used to power the unit during the low solar

intensity periods. The generated energy form the PV array

varied in accordance with variations in the environmental

conditions (insolation and ambient temperature). These

variations affected the amount of daily produced water.

The average daily generated energy was calculated

during different months depending on the peak power of

the PV array (Ppeak), average daily peak sunshine hours

(PSSH), battery efficiency factor (ηb), and the PV array

thermal factor (ηth), and can be determined as follows:

generated peak

EPPSSH th b







Using the above equation, a comparison was made

between the average daily generated energy during

months and that required by the load to operate the sys-

tem for 24 h daily as shown in Figure 3.

As shown in Figure 3 the energy generated during

summer months exceeds that required by the s ystem (ex-

cluding the softener and feed pump) by about 40%. The

energy generated varies from month to month depending

mainly on the sky if sunny or cloudy.

3.2. System Performance

The solar irradiation and the ambient temperature had a

significant influence on the performance of the PV sys-

tem. As irradiation increases, the PV current increases

significantly due to high energy absorbed by the PV

modules. It increased by 46% when the irradiation in-

creased by 52% as illustrated in Table 2. The power

output follows the behavior of the modules current and

increases as irradiation increases. Slight variation in the

PV voltage was observed when the irradiation increased.

Table 1. Monthly average values for insolation, irradiation,

and ambient temperature.

Month Insolation

(kWh/m2/d) Irrad.

(W/m2) Ambient Temperature (˚C )

Av. Max. Min.

Sep, 077.5 595.4 26 40 18

Oct, 076.65 564.3 24 39 14

Nov, 075.3 485.4 17 33 6

Dec, 074.8 457.3 11.5 23 1.7

Jan, 084.7 442.2 7 19.3 0

Feb, 085.5 480 10.6 27.6 1.4

Mar, 086.6 531 18.6 37.7 5.1

F. BANAT ET AL. 35

Daily re quir ed

energ y by RO

Average daily

generated energy

0.5

1.5

2.5

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Months

Energy (kWh/da y )

Figure 3. The average daily generated energy versus the

required by the RO unit at different months.

Table 2. The effect of irradiation on the power output.

Irradiation

(W/m2) Module Ampere

(A) Array Ampere

(A) Array Power (W)

0 0 0 0.0

200 0.75 3 104.4

400 1.25 5 174.0

600 2 8 278.4

800 2.6 10.4 361.9

1000 3.11 12.44 432.9

3.2.1. S pe cific Energy Consu m ption

The theoretical specific energy consumption (SEC) for

the PV-RO system was determined as follows:

SEC (kWh/m3) = Input Power * Opr. hours/Daily

produced flow

Input powersoftener pump powerintake pum

power

+high pressure pump power

=746 + 60 +57.6 =863.6 W

SEC=13.82 kWhm





 



With the sof tener the SEC was 13.82 kWh/m3 but was

1.9 kWh/m3 without it.

3.2.2. S ystem Res ults

At an operating pressure of 4.5 bar, the average feed flow

was 99 L/h and the permeate flow was around 34 L/h

(34% permeate recovery). Operating the system for three

hours produced 11 9 L of pe r meate as shown in Figure 4.

The effect of feed temperature on both of recovery and

salt rejection are illustrated in Figures 5 and 6, respective-

ly. The percentage of recovery increased from 30% to

38% when feed water temperature increased from 12.4˚C

to 21.2 ˚C, and the rejection of salts decreased from 98%

to 97.4% when feed water temperature increased from

13.8˚C to 21.4˚C.

120

150

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Operating hours (h)

Daily production (L/d)

Figure 4. Daily production as a function of operating hours

(operating pressure = 4.5 bar).

25%

29%

33%

37%

41%

45%

10. 0

12. 0

14.0

16. 0

18.0

20.0

22. 0

Temperature

%Recovery

Figure 5. Percentage recovery as a function of feed tem-

perature (operating pressure = 4.5 bar).

97.2%

97.4%

97.6%

97.8%

98.0%

98.2%

10.0

11.6

13.2

14.8

16.4

18.0

19.6

21.2

22.8

Temperature

%Salt Rejection

Figure 6. Percentage of salt rejection as a function of feed

temperature (operating pressure = 4.5 bar).

At an operating pressure of 4.5 bar, the average feed

flow was 99 L/h and the permeate flow was around 34

L/h (34% permeate recovery). Operating the system for

three hours produced 119 L of permeate as shown in

Figure 4.

The effect of feed temperature on both of recovery and

salt rejection are illustrated in Figures 5 and 6, respec-

tively. The percentage of recovery increased from 30% to

F. BANAT ET AL.

5. Acknowledgements

38% when feed water temperature increased from 12.4˚C

to 21.2˚C, and the rejection of salts decreased from This work was supported by funds from the European

Union through the ADIRA project. Financial support

from the Petroleum Institute in Abu Dhabi to publish the

work is gratefully acknowledged.

98% to 97.4% when feed water temperature increased

from 13.8˚C to 21.4˚C.

The recovery percentage affected salt passage through

the membrane. In fact, the increase in recovery was asso-

ciated with a decrease in salt rejection. For example, as

the recovery percentage increased from 30% to 40%, the

salt rejection decreased from 98% to 97.5%. REFERENCES

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The average specific energy consumption (SEC) was

about 26 kWh/m3 at an operating pressure of 4.5. The

SEC reached 19.4 kWh/m3 for a recovery percentage of

40.6% and reached 32 kWh/m3 for 30% of recovery as

shown in Figure 7.

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Figure 7. The relati on between recovery perc entage and the

specific energy consumption (operating pressure = 4.5 bar).