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Honors Theses Honors College (Sally McDonnell Barksdale Honors College)

Spring 5-1-2021

Evaluation and Sizing of Solar Powered Reverse Osmosis Water Evaluation and Sizing of Solar Powered Reverse Osmosis Water

Desalination System Desalination System

Sandip Subedi

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Recommended Citation Recommended Citation Subedi, Sandip, "Evaluation and Sizing of Solar Powered Reverse Osmosis Water Desalination System" (2021). Honors Theses. 1823. https://egrove.olemiss.edu/hon_thesis/1823

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EVALUATION AND SIZING OF SOLAR POWERED REVERSE OSMOSIS WATER

DESALINATION SYSTEM

By

Sandip Subedi

A thesis submitted to the faculty of The University of Mississippi in partial fulfillment of

the requirements of the Sally McDonnell Barksdale Honors College.

Oxford

May 2021

Approved by

---------------------------------------

Advisor: Dr. Tejas Pandya

---------------------------------------

Reader: Dr. Farhad Farzbod

---------------------------------------

Reader: Dr. Wen Wu

ii

© 2021

Sandip Subedi

All RIGHTS RESERVED

iii

ACKNOWLEDGEMENTS

I would like to thank Dr. Tejas Pandya for his continuous guidance, support, and

encouragement throughout the process of this work. I would also like to thank Dr. Wen

Wu and Dr. Farhad Farzbod for suggestions along the way, and also taking the time to be

on the committee. I am also thankful to all the researchers, scientists, engineers,

organizations, and committees whose references were vital in writing this report. Finally,

I would like to express my sincerest gratitude to my family for always supporting and

motivating me.

iv

ABSTRACT

SANDIP SUBEDI: EVALUATION AND SIZING OF SOLAR POWERED REVERSE OSMOSIS

WATER DESALINATION SYSTEM (Under the direction of Dr. Tejas Pandya)

About 71 % of the surface of Earth is water, out of which around 2.5 % is freshwater. The

majority of the surface of Earth is occupied by seawater which also contains dissolved

salt. The salt concentration in seawater makes it toxic to humans because the human

body is unable to get rid of the salt that comes from seawater. The demand for fresh

water is increasing due to the rise in global population. In this context, desalinated

seawater is considered as a viable resource to meet the growing demand for fresh water.

Desalination process is highly energy intensive due to the high salinity of the sea water.

Reverse Osmosis (RO) Desalination process has emerged as the leading and the cost-

effective membrane based desalination process that is currently dominating the

desalination market. Though several researches are being conducted in cost reduction of

desalination process, the use of renewable energy in sea water desalination is not being

widely studied. This paper presents the design of an RO desalination plant that can

produce 12 m3 of water in a day, and is powered by Photovoltaic system. The paper also

presents the water production cost analysis from an RO plant powered by a PV system,

and an RO plant powered by purchased electricity. Furthermore, the fossil fuel that can

be saved over the lifetime operation of the designed RO plant with a PV system is also

evaluated.

v

Table of Contents

LIST OF FIGURES ........................................................................................................ VI

LIST OF TABLES ......................................................................................................... VII

LIST OF ABBREVIATIONS .......................................................................................... VIII

1 INTRODUCTION ................................................................................................... 1

1.1 Seawater Desalination ........................................................................................................... 1

1.2 Reverse Osmosis .................................................................................................................... 3

1.3 Reverse Osmosis Desalination System .................................................................................... 4

1.4 Solar Energy .......................................................................................................................... 8

1.5 Photovoltaic System .............................................................................................................. 9

2 METHODOLOGY ................................................................................................ 12

2.1 Reverse Osmosis Plant Design .............................................................................................. 12

2.2 Photovoltaic System Design ................................................................................................. 14

3 RESULTS ............................................................................................................ 15

3.1 Reverse Osmosis Plant ......................................................................................................... 15

3.2 Photovoltaic System ............................................................................................................ 18

4 ANALYSIS .......................................................................................................... 19

4.1 Economic Analysis ............................................................................................................... 19

4.2 Fossil Fuel Savings ................................................................................................................ 21

5 CONCLUSION .................................................................................................... 22

6 REFERENCES ...................................................................................................... 23

vi

List of Figures

Figure 1: Major Desalination techniques and their contribution in World water

production 10

Figure 2: Reverse Osmosis Process 11

Figure 3: Single-stage RO Desalination system 12

Figure 4: Single-stage RO Desalination system with ERD 13

Figure 5: Two-stage RO Desalination system 14

Figure 6: Two-pass RO Desalination system 14

Figure 7: Solar Potential for the USA 15

Figure 8: Photovoltaic Cell Schematic 16

Figure 9: Photovoltaic System 17

Figure 10: Grid-tied PV system 18

Figure 11: Two pass RO plant with ERD 22

vii

List of Tables

Table 1: Composition of different water sources 20

Table 2: Specifications of LG Solar panel 21

Table 3: Water Flow, Pressure, TDS at different position of RO plant 24

Table 4: Product Water Concentration 25

viii

List of Abbreviations TDS Total Dissolved Solids

RO Reverse Osmosis

ERD Energy Recovery Device

USA United States of America

PV Photovoltaic

DC Direct Current

AC Alternate Current

IMS Integrated Membrane System

1

1 Introduction

1.1 Seawater Desalination The quantity of freshwater available on earth’s surface is only 2.5 % of total water

quantity, and only 0.008 % represents the accessible surface freshwater [1] . Water

shortages have affected many communities, and humans have long searched for a

solution to inadequate fresh water supplies. Desalination is one of the promising solutions

to overcome the shortage of fresh water. Desalination refers to the process of removing

the salts and minerals (contaminants) from either seawater or brackish water in order to

attain clean water suitable for human consumption and industrial and domestic usage [2].

Saline water can be classified depending on the Total Dissolved Solids (TDS) for brackish

water TDS is up to 10,000 ppm, and for seawater TDS is up to 45,000 ppm [3]. The

permissible limit of salinity in freshwater is in the range of 500 ppm to 1000 ppm [4]. Most

of the desalination systems require thermal and/or electrical input. Desalination is an

extensive energy process which requires about 10000 tons of fossil fuel per year to

produce 1000 m3 of water per day [5]. It is essential to replace the depleted fossil fuel by

renewable energy resources in order to decrease the carbon footprint and greenhouse

gases emission. Figure 1 shows the major desalination process and their contribution in

world water production. The RO Desalination is the leading desalination process in the

world. About 62 % of world water production comes from an RO desalination process [1].

2

This paper presents an overview of an RO desalination process, and a small-scale RO

desalination plant that runs on solar PV cells.

Figure 1: Major Desalination techniques and their contribution in World water production [1]

3

1.2 Reverse Osmosis Osmosis is a naturally occurring process where a semi-permeable barrier allows passage

of some molecules but not others. The direction of flow in the osmosis process is from

less concentrated solution such as freshwater to a more concentrated solution such as

seawater until the equilibrium is reached. Reverse Osmosis, as the name suggests, is the

opposite of the osmosis process. In the process of RO, by applying pressure impure water

forces to pass through a thin semi‐permeable membrane made of a thin film of cellulose

acetone affixed to a perforated plate or tube which catches the dissolved solids and

allows the water molecule to pass through it [6]. The hydrostatic pressure greater than

osmotic pressure of dissolved solid is applied on the concentrated side of the membrane.

Schematic view of the RO process is shown in Figure 2.

Figure 2: Reverse Osmosis Process [6]

4

1.3 Reverse Osmosis Desalination System RO desalination system works on the principle of RO process. It uses a high pressure pump

to increase the pressure on the seawater side and force the water across the

semipermeable membrane, leaving dissolved salts behind in the reject stream. The most

basic RO desalination system includes the pre-treatment module, high-pressure pumps,

the RO membrane module, and the post-treatment module, arranged as shown in Figure

3 [7]. In the pre-treatment module, the incoming feed water is pre-treated to remove

suspended solids, reduce potential fouling, and increase the lifespan of an RO membrane.

Post-treatment is required depending on the application of product water. It generally

involves remineralization and demineralization of the product water based on the

required purity. The high-pressure pump increases the pressure of the pre-treated water

and forces it to the membrane module. The membrane module blocks the dissolved salt

and allows the passage of water into the post-treatment module. The highly saline water

blocked by the membrane is rejected from the membrane module. The high-pressure

pump requires high energy to operate and is supplied in the form of electricity.

5

Figure 3: Single-stage RO Desalination system [7]

The energy can be extracted from rejected water and fed into the high-pressure pump

which reduces the energy required to operate the pump. The advancement of membrane

technology and energy recovery devices (ERDs) over the last 50 years has brought down

the energy intensity of an RO desalination systems [7]. Single-stage RO desalination

system with ERD is shown in Figure 4.

Figure 4: Single-stage RO Desalination system with ERD [7]

6

The performance of the RO desalination system can be improved by modifying the

configuration of the membrane module. The RO membrane module may contain one or

several modules with many membrane elements arranged in series or parallel in a

module. Figure 5 and Figure 6 represent two-stage and two-pass RO desalination systems

respectively. In a two-stage system, reject water from one membrane module is

pressurized through another membrane module, and the product water from both

membrane modules is collected together. In a two-pass system, the product water from

one membrane module is passed through another membrane module, and the final

product water is collected. The ultimate objectives of these different configurations of RO

desalination systems are to achieve a satisfied quality of water and long-term robustness

of the plant with increasing energy efficiency, water recovery, and membrane lifetime [8].

Figure 5: Two-stage RO Desalination system [7]

7

Figure 6: Two-pass RO Desalination system

8

1.4 Solar Energy

The alternatives of traditional fossil fuel as a source of electricity can be renewable energy

sources such as sun, wind, and water. It is essential to replace fossil fuel by renewable

energy resources in order to decrease the carbon footprint and greenhouse gases

emission. Solar energy can be a very efficient source to power an RO desalination plants

because of its availability near the coastal areas where seawater is abundant. The solar

energy potential for the USA is shown in Figure 7. It shows the average daily direct solar

energy incident on a perpendicular surface tracking the sun path. The coastal regions on

the west receives on average 4.0 – 7.0 kWh/m2/day, while the east, because of humidity

effect and cloud cover, receives on average about 4.0 kWh/m2/day.

Figure 7: Solar Potential for the USA [9]

9

1.5 Photovoltaic System The Photovoltaic System converts radiation from the sun into the usable electricity. This

conversion process is called the photovoltaic effect. When semiconductor materials are

exposed to light, the photons of the light ray excite the electrons in the semiconductor

material creating a significant number of free electrons. The flow of these electrons create

electricity. This is how Photovoltaic effect produces electricity. Photovoltaic cell is the

basic unit of the system where photovoltaic effect is utilized to produce electricity from

light. Figure 8 shows a schematic of a photovoltaic cell.

Figure 8: Photovoltaic Cell Schematic [21]

Silicon is the most widely used semiconductor material for constructing a photovoltaic

cell. The silicon atom has four valence electrons. In a solid crystal, each silicon atom shares

each of its four valence electrons with another nearest silicon atom hence creating the

covalent bond between them. Pure silicon is called an intrinsic semiconductor, but if a

small amount of impurities, usually called the dopant, is combined with the pure silicon,

10

an extrinsic semiconductor result. An n-type of semiconductor results if the dopant has

more electrons in the valence band than the base material. An n-type semiconductor

seems to have an excess of electrons even though the semiconductor is electrically

neutral. A p-type semiconductor appears to have a deficit of electrons, or an excess of

“holes'', although it is electrically neutral. The reason for doping a pure semiconductor

such as silicon is that a junction formed with n-type and p-type semiconductor material

enhances the flow of electricity and holes. The most fundamental components of the

photovoltaic panel are individual photovoltaic cells, which are assembled together to

form modules. That in combination with one another yields a whole photovoltaic system.

Figure 9 shows the photovoltaic system. This whole system generates electricity from

solar irradiance.

Figure 9: Photovoltaic System [11]

PV systems are generally of two different types, Off-Grid System and Grid-tied System.

The Off-Grid System doesn’t have an energy back-up system, whereas a Grid-tied system

11

has connection with the grid. In the Grid-tied system, the PV system converts solar energy

to DC electricity, which is further converted to AC electricity through an inverter and is

tied to the grid system [12]. This paper presents the design of a grid-tied PV system.

During the day the excess electricity is fed to the grid and the energy can be drawn back

at night from the grid. Figure 11 shows the Grid-tied PV system.

Figure 10: Grid-tied PV system [12]

12

2 Methodology

2.1 Reverse Osmosis Plant Design The impurities present in water vary in sizes. Pre-treatment is done initially to remove

large solute particles. Reverse Osmosis removes solute size ranging from 0.1 nanometer

to 1 nanometer which are ions present in water [13]. Table 1 shows the concentration in

different water sources.

Table 1: Composition of different water sources [13]

13

This paper presents the desalination of seawater. Hence, seawater composition from

Table 1 is taken to design an RO desalination plant. The concentration of Calcium,

Magnesium, Sodium, Bicarbonate, Sulphate, Chloride, Nitrate, Fluoride, and Boron are

400 mg/l, 1252 mg/l, 10561 mg/l, 140 mg/l, 2650 mg/l, 18980 mg/l, 1.5 mg/l, 1.4 mg/l,

and 4 mg/l respectively. These properties are used to design an RO desalination plant

using Integrated Membrane System (IMS) Design Software. IMS Design gives the user

complete control over input concentration of feed water and membrane selection based

on the required product water. The software also evaluates the energy consumption

based on the pressure supplied by the pressure pump.

14

2.2 Photovoltaic System Design The energy consumption obtained from the designed RO plant is used to design a PV

system. The PV system is designed in such a way that it supplies enough energy to operate

an RO plant. The RO plant is designed to operate in Biloxi, Mississippi. Solar irradiation is

taken into consideration to design a PV system. The average direct normal solar

irradiation of Biloxi, Mississippi is 5.0 kWh/m2/day [9]. This value is used to determine

annual solar energy available in the location, which is converted to electricity using a PV

system. Individual PV panels are available in the market supplied by companies like

Panasonic, Tesla, LG etc. Based on the energy required to power an RO plant, the required

number of PV panels are connected together which forms a PV system. This paper uses

specifications provided by LG Solar Panel to create a PV System. Table 2 shows

specifications of LG solar panel.

Table 2: Specifications of LG Solar panel [15]

15

3 Results

3.1 Reverse Osmosis Plant IMS Design software was used to design an RO plant. The concentration of impurities/ions

in seawater were used from Table 1. Different parameters were twisted such as number

of stages, number passes, number of membranes, ERD, and pressure pump to obtain

energy efficient and quality product water. Figure 10 shows the RO desalination plant

obtained from IMS design. The seawater is supplied from 1 which mixes with the

concentrate from the second membrane module. Some part of it goes to a pressure

exchanger which mixes with the high pressure concentrate from the first membrane

module, and some of it goes to a high pressure pump. The high pressure pump pressurizes

the seawater which eventually mixes with the high pressure concentrate coming from the

pressure exchanger, and passes through the first membrane module. The product water

from the first membrane module is further passed through another high pressure pump

and membrane module to obtain final product water. The water flow, pressure, and TDS

at each numbered hexagon is shown in Table 3. The product water is obtained at the rate

of 0.5 m3/h.

16

Figure 11: Two pass RO plant with ERD [Obtained from IMS Design Software]

Table 3: Water Flow, Pressure, TDS at different position of RO plant [Obtained from IMS Design Software]

The obtained product water concentration is shown in Table 4. The concentration of

Calcium, Magnesium, Sodium, Bicarbonate, Sulphate, Chloride, Nitrate, Fluoride, and

17

Boron are 0.001 mg/l, 0.004 mg/l, 4.174 mg/l, 0.085 mg/l, 0.022 mg/l, 6.326 mg/l, 0.019

mg/l, 0.024 mg/l, and 2.202 mg/l respectively. Based on the required quality of the

product water, it can be further mineralized or demineralized.

Table 4: Product Water Concentration [Obtained from IMS Design Software]

The energy is used in the RO plant to operate pressure pumps. Based on the pressure

required in the pumps, the energy is evaluated from the IMS Design software. The energy

to generate 1 m3 of desalinated water was found to be 3.85 kWh.

18

3.2 Photovoltaic System The energy to operate an RO plant is 3.85 kWh per m3 of product water. The product

water flow rate out of an RO plant is 0.5 m3/h. The specification of LG solar panel, average

direct solar irradiation of the location, inverter losses, and systems losses can be used to

evaluate the number of solar panels required to power the RO plant.

● Water generated in a day = 0.5 m3/h * 24 h = 12 m3

● Energy required = 3.85 kWh/m3 * 12 m3 = 46.2 kWh/day

● Average direct solar irradiation = 5.0 kWh/m2/day [14]

● Area of panel = 1.7272 m2 [15]

● Efficiency = 20.3 % [15]

● Energy produced by a panel = 5.0 kWh/m2/day * 1.7272 m2 * 0.203= 1.75 kWh/day

● Inverter losses = 4 % [11]

● System losses = 14 % [11]

● After accounting for these losses, Energy produced by a panel = 0.96* 0.86 *

1.75kWh/day

= 1.44 kWh/day

● Number of Panels = (46.2 kWh/day)/(1.44 kWh/day) = 33

The RO plant has the capacity of producing 12 m3 product water in a day which can be

operated by a PV system with 33 LG solar panels.

19

4 Analysis

4.1 Economic Analysis The use of an RO plant and PV system together reduces the cost of desalinated water.

The other option to power an RO plant would be to purchase energy from the grid. This

paper presents the water production cost differences in these two approaches, and

overall cost saved using one of these approaches.

The water production cost from an RO plant powered by PV system is $0.67/m3, whereas

the water production cost of RO plant powered by purchased electricity is $0.94/m3. Over

the lifespan of the RO plant, the amount that can be saved using an RO plant powered by

PV system is $23,588. The estimated lifetime of an RO plant is 20 years, whereas the

estimated lifetime of a PV system is 25 years. The energy generated from the PV system

for the remaining 5 years can be sold to Mississippi Power at a rate determined by the

Mississippi Public Service Commission [20].

• Capital Cost of an RO plant = $24,120

• Operation and Maintenance Cost of an RO plant= $1000/year (Estimated)

• Projected lifetime of an RO plant = 20 years

• Total cost of an RO plant (lifetime) = $24120 + $ 1000 * 20 = $ 44120

• Cost of 1 LG solar panel = $ 352

• Cost of a PV system (33 solar panels) = $ 352 * 33 = $11,616

20

• Operation and Maintenance Cost of PV system = $13.09/kW/year

• Power from 1 LG solar panel = 350 W = 0.35 kW

• Total power from 33 LG solar panel = 0.35 kW * 33 = 11.55 kW

• Total Operation and Maintenance Cost (Over lifetime of RO plant) = 13.09 * 11.55

* 20 = $ 3,024

• Total Cost of PV system = $ 11,616 + $ 3,024 = $ 14,640

• Cost of electricity in Mississippi = $ 0.1102/kWh

• Total Cost of electricity to power an RO plant = $ 0.1102 * 47.52 * 365 * 20 =

$38,228

• Total cost of RO and PV system = $ 44,120 + $ 14,640 = $ 58,760

• Total cost of RO system powered by electricity from grid = $44,120 + $38,228 =

$82,348

• Total water produced (lifetime) = 12 * 365 * 20 = 87,600 m3

• Cost of water production (RO powered by PV) = $ 58,760/87,600 m3 = $0.67/m3

• Cost of water production (RO powered by Conventional) = $ 82,348/87,600 m3 =

$0.94/m3

21

4.2 Fossil Fuel Savings

The use of solar energy as a source of electricity in an RO plant consequently saves fossil

fuel. It is essential to save fossil fuel by renewable energy resources in order to decrease

the carbon footprint and greenhouse gases emission.

● 8.3 kWh of energy is produced by 1 kg of coal. The efficiency of electricity

production from coal is 39 %.

● 1 kg of coal produces 0.39 * 8.3 = 3.237 kWh of electrical energy

● Total conventional energy substituted = 427, 680 kWh

● Mass of coal required = 427,680 kWh/3.327 kWh = 128,548 kg

● CO2 emission from 1 kg of Coal = 1.91 kg

● Total Carbon dioxide Emission = 1.91× 128,548 = 245,526 kg= 245.5 Metric ton

● Carbon dioxide Emission per annum = 245.5/25 ≅ 10 Metric ton

The RO plant powered by a PV system prevents 10 Metric ton of CO2 emission per annum.

22

5 Conclusion

The increasing demand of fresh water and limited availability of its sources have increased

the need of desalinated seawater in recent years. Among several desalination techniques,

RO desalination has emerged as a leading and cost-effective desalination process. The

desalination process is also an energy intensive process which requires a lot of energy to

operate. Solar energy can be used to power RO desalination plants. For this theoretical

study, the PV system was used to convert solar energy to electricity and supply it to the

RO plant. The water desalination cost from an RO plant powered by a PV system was

estimated to be around $0.67/m3, whereas the water desalination cost of an RO plant

powered by purchased electricity was estimated to be around $0.94/m3. The RO plant

otherwise operated by conventional fossil fuel energy sources could have serious

environmental impact. The solar powered RO desalination plant presented in this report

can produce 12 m3 of desalinated water every day. The desalinated plant is estimated to

save around 10 metric tons of CO2 per annum which aids to decrease greenhouse gases

emission. A detailed study and a scaled down prototype of a solar powered RO plant can

provide much needed insight into the technical and economic feasibility, and its impact

on the environment.

23

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<https://www.solarpowerworldonline.com/2015/10/what-are-some-common-types-of-solar-pv-and-storage-installations/> [Accessed 23 March 2021]. [13] Singh, R., n.d. Membrane technology and engineering for water purification. [14] Globalsolaratlas.info. 2021. Global Solar Atlas. [online] Available at: <https://globalsolaratlas.info/download/usa> [Accessed 23 March 2021]. [15] Solaris-shop.com. 2021. [online] Available at: <https://www.solaris-shop.com/content/LG350Q1C-A5%20Specs.pdf> [Accessed 23 March 2021]. [16] Wittholz, M., O'Neill, B., Colby, C. and Lewis, D., 2008. Estimating the cost of desalination plants using a cost database. Desalination, 229(1-3), pp.10-20. [17] Advisian.com. 2021. The Cost of Desalination - Advisian. [online] Available at: <https://www.advisian.com/en/global-perspectives/the-cost-of-desalination> [Accessed 23 March 2021]. [18] Energybot.com. 2021. Electricity Rates By State - EnergyBot. [online] Available at: <https://www.energybot.com/electricity-rates-by-state.html> [Accessed 23 March 2021]. [19] Nrel.gov, 2021. [Online]. Available: https://www.nrel.gov/docs/fy17osti/68023.pdf. [Accessed: 23- Mar- 2021]. [20] W. Save, R. Energy and S. Energy, "Net Metering & Your Bill", Mississippipower.com, 2021. [Online]. Available: https://www.mississippipower.com/residential/ways-to-save/renewable-energy/solar-energy/net-metering---your-bill.html. [Accessed: 23- Mar- 2021]. [21] PV Panels, 2021. [Online]. Available: http://www.apricus.com/solar-pv-systems-pv-panels-19.html#.YH_JzS2cZQI. [Accessed: 21 April 2021]