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Osmotic Power

Alex Myers

Holland College

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Abstract

This paper looks at the potential of osmotic power as a viable future renewable energy resource.

An in-depth investigation into the theory behind how osmotic pressure is used as a way of

producing power is presented. This paper examines the osmotic energy research which has been

carried out around the world. A summary of the research into finding the most effective

membrane for optimal power output is described. This paper looks at power potential of osmotic

energy and at the feasibility and potential for Hydro-Osmotic Power (HOP) plants in Canada.

Other areas of investigation include, what are the start-up costs, what is the payback period and

is this technology going to last far into the future. Finally, the environmental impacts of this

technology on the fresh and saltwater eco-systems are considered.

Keywords: osmosis, osmotic power, salt gradient, semi-permeable membrane, renewable

energy

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Table of Contents

Abstract ......................................................................................................................................................... 2

Introduction .................................................................................................................................................. 5

Background ................................................................................................................................................... 5

Osmosis in Nature ..................................................................................................................................... 5

History of Osmotic Energy ........................................................................................................................ 6

Methodology ................................................................................................................................................. 7

Current World Use of Osmotic Energy.......................................................................................................... 8

Europe ....................................................................................................................................................... 8

Japan ......................................................................................................................................................... 8

Canada ...................................................................................................................................................... 9

How Osmotic Energy Works ......................................................................................................................... 9

Membrane Technology ............................................................................................................................... 11

Power Potential........................................................................................................................................... 13

Future Potential for Improvement.......................................................................................................... 13

Osmotic Power in Canada ........................................................................................................................... 14

Feasibility and Potential .......................................................................................................................... 14

Possible Locations ................................................................................................................................... 15

Costs ............................................................................................................................................................ 16

Operating Costs ....................................................................................................................................... 17

Affordances ................................................................................................................................................. 18

Constraints .................................................................................................................................................. 18

Environmental Impacts ............................................................................................................................... 18

Conclusions ................................................................................................................................................. 20

References .................................................................................................................................................. 22

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Table of Figures

Figure 1. Simplified drawing of the osmotic power production process. Reprinted from Hydro Quebec.

(2015). A Renewable Energy Option Osmotic Power. Retrieved September 21, 2015, from

http://www.hydroquebec.com/sustainable-development/documentation-cente ................................... 10

Figure 2 Theoretical potential of osmotic power across continents. Reprinted from Stenzel, P., &

Wagner, H. (2010). Osmotic power plants: Potential analysis and site criteria. 3rd International

Conference on Ocean Energy, 5-5. ............................................................................................................. 15

Figure 3 Map of Canada illustrating possible locations and power outputs. Reprinted from Hydro

Quebec. (2015). A Renewable Energy Option Osmotic Power. Retrieved September 21, 2015, from

http://www.hydroquebec.com/sustainable-development/documentation-center/pdf ........................... 16

Figure 4 Cost comparison of renewable and non-renewable energy sources. Reprinted from Sharif, A. O.,

Merdaw, A. A., Aryafar, M., & Nicoll, P. (2014). Theoretical and experimental investigations of the

potential of osmotic energy for power production. Membranes, ............................................................. 17

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Introduction

In the search for clean renewable energy, one of the emerging technologies on the

horizon is osmotic energy. This form of energy production has been under development in

Europe by the Norwegian company Statkraft since the late 1990s (Skilhagen, 2008). One of the

benefits of developing osmotic power is that it can make use of existing hydro power and

desalination technology (Skilhagen, 2008). The intention is to have large scale energy production

from these hydro-osmotic power plants. These plants would most likely be placed where

freshwater naturally flows into the sea. The basis for this type of energy production is the

movement of freshwater into saltwater through a semi-permeable membrane which creates

pressure which then turns a turbine. The membrane accounts for 50% to 80% of the plant costs

and therefore is the subject of the majority of research into efficiency (Sharif, 2014). This

technology can also be scaled down to create electricity in small devices which could benefit

billions of people worldwide who have no electricity (Bruhn, 2014). Hydro Quebec is currently

looking at the possibility of osmotic power generation on 30 large rivers which empty into salt

water and hope to have this power on the market by 2020 (Hydro Quebec, 2015). This report

presents background information on osmotic energy production as well as current research that is

being carried out and assesses its feasibility and potential to be a viable energy source in the

future.

Background

Osmosis in Nature

Osmosis is a natural process which takes place all the time on a cellular level in plants

and animals. This is the process that allows trees to draw water from their roots all the way up to

the top of the tree (Urry et al., 2014). In general, osmosis occurs when water with low solute

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concentration moves through a semi-permeable membrane into water with a higher solute

concentration. A solute is a substance dissolved in a solution which in the case of osmosis in

plants could be sugar molecules (Urry et al., 2014). In a solution, the solute attracts water

molecules to itself and these are unable to pass freely through the selective membrane, only free

water molecules can pass through the membrane into the higher solute concentration (Urry et al.,

2014). This process will continue until the solute concentration on either side of the membrane is

almost equal (Urry et al., 2014). The movement of these free water molecules through the

membrane creates pressure. These same principles are how osmosis can be used to create

pressure which is then used to spin a turbine and create power.

History of Osmotic Energy

In the past, energy predominantly came from the burning of fossil fuels such as oil, coal

and natural gas. The carbon dioxide emissions from the burning of these fuels has reached

dangerous levels and has begun to have a negative effect on our climate. The shift away from

fossil fuels has created numerous unique ways of creating clean energy. In 1997, the Kyoto

Protocol put the pressure on countries around the world to reduce greenhouse gas emissions

which lead to solar power, wind power and other more obscure power generation methods such

as osmotic power. The natural process of osmosis has been known for hundreds of years and in

the 1970’s Prof. Sidney Loeb took this previous knowledge and came up with methods on how

to use osmotic pressure to create power with the use of membranes (Skilhagen et al., 2007).

Membranes are one of the most important factors in determining how much power an osmotic

power plant can produce. During the 1980’s and 1990’s semi-permeable membranes became

more efficient and membranes were being successfully introduced in industrial applications

(Skilhagen et al., 2007). Since 1997, Statkraft, a northern European power production company

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with its main focus in hydropower, has been taking part in research and development into

osmotic power (Skilhagen et al., 2007). Osmotic Energy gained a lot of attention in 2009 when

Statkraft opened the first porotype osmotic power plant in Hurum Norway (Stenzel & Wagner,

2010). Statkraft is currently the main osmotic technology developer in the world and have made

many advancements in membrane technology. Currently osmotic power plants are starting to be

built in other parts of the world as this technology becomes more of a viable option for power

creation.

Methodology

A literature search was carried out on various studies of osmotic energy. All the

information in this research paper came from trusted sources. The majority of the papers were

located through the Holland College online library services as well as internet searches on the

topic. These were credible technical reports and research papers on the subject of osmotic power.

These papers covered the period between 2007 and 2014, a time when osmotic power

went from prototype to operational. There was a preference for articles within this time period

because this is an emerging technology. The purpose of this research paper was to gather

knowledge on how osmotic power works and the feasibility of it being a reliable renewable

energy source in the future. The articles were read thoroughly and these references were

alphabetized and annotated. Then information was gathered to answer the questions that this

technical report set out to answer.

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Current World Use of Osmotic Energy

Europe

The Norwegian company Statkraft started working on osmotic power development in

1997. From 2009-2013, osmotic power was in the research stage during which time Statkraft

tested a prototype plant in a Fjord in Norway (Hydro Quebec, 2015). Fresh water from the Tofte

River and salt water from the Fjord were used to test the energy output of osmotic power but

were only able to produce less than 1W of power per square meter of membrane. One obstacle to

the success of osmotic power is the limited number of companies producing membranes but it is

expected that once osmotic power starts to grow, the number of companies producing high

efficiency membranes will increase as well (Halper, 2010).

The Dutch Research Centre in Holland has been studying osmotic power since 2007 and

a 50KW generating station using osmotic energy was set up in 2014 (Hydro Quebec, 2015). This

plant was built to help meet the countries goal of producing 14% of its energy needs from

renewable sources by 2020 (Kleiterp, 2012).

Japan

Research into osmotic power in Japan started in 2009. The Kyowakaiden Industry Co.

operates a prototype plant with Nagasaki University and Tokyo Institute of Technology in

Fukuoka (Halper, 2010). This plant was different from the plants in Europe that used the salt

gradient between river water and sea water. The Fukuoka plant made use of concentrated salt

water from a desalination plant and fresh water from a sewage treatment plant. This work was

carried out between 2010 and 2013 as part of the Mega-Ton Water System (Hydro Quebec,

2015).

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Canada

The first research into osmotic power in Canada took place from 2012 to 2013 with a

study into the feasibility of setting up power plants in the province of Quebec. Hydro Quebec

and the Norwegian company Statkraft set up a research project to look at a number of issues

about osmotic power such as water pretreatment, membrane performance, and sustainable

development. Hydro Quebec estimated that within the province, there were 30 large rivers that

flowed into salt water and had potential as possible locations for osmotic power plants (Hydro

Quebec, 2015). This energy potential was estimated to range from 0.889 TWh/yr to 10.545

TWh/yr (Jihye et al., 2015). The goal for Hydro Quebec is produce this power at a competitive

price by 2020 (Hydro Quebec, 2015).

How Osmotic Energy Works

For osmotic power plants to work they must be placed in locations with easy access to

both fresh water and sea water. These conditions exist where large rivers flow into the ocean.

Optimal conditions for this require that there is a limited amount of mixing of the fresh water and

salt water in the estuary, this maintains a high salt gradient. There are different ways of turning

osmosis into electrical energy but pressure retarded osmosis (PRO) is the most common form of

producing power. A PRO plant can be thought of as a desalination plant running in reverse.

Desalination plants utilize reverse osmosis (RO) which operates against the osmotic force. PRO

plants use the same osmotic force to produce energy (Gerstandt et al., 2007). Osmotic power is

produced by separating fresh water and salt water with a semi permeable membrane. The fresh

water and seawater must first go through filters to pretreat the water for better optimization of

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membrane performance. Natural osmosis causes fresh water to move through the membrane into

the saltwater side of the membrane (Hydro Quebec, 2015). Membranes must have high water

flow and salt retention capabilities, meaning the low salinity water can flow through easily but

the high salinity water cannot flow through the membrane. 80% to 90% of the water with low

salinity gradient is transferred into the pressurized salt water portion (Skilhagen et al., 2007).

This movement raises the pressure on the side with the high salinity. The pressure that is created

is referred to as osmotic pressure. This pressure and increase in volumetric water flow is then

used to spin a turbine which creates the electrical energy (Hydro Quebec, 2015). Figure 1 shows

a simplified diagram of an osmotic power plant.

Figure 1. Simplified drawing of the osmotic power production process. Reprinted from Hydro Quebec. (2015). A Renewable Energy Option Osmotic Power. Retrieved September 21, 2015, from http://www.hydroquebec.com/sustainable-development/documentation-cente

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For optimal operating conditions, the fresh water or feed water stays at ambient pressure

while the salt water or draw water pressure ranges from 11-15 bars (Skilhagen et al., 2007).

Osmotic flow causes water molecules to permeate through the membrane which increases the

hydraulic pressure difference across the membrane. Left on its own, the fresh water molecules

would eventually cause a decrease in the salinity of the salt water solution side causing

equilibrium, which stops the osmotic flow and energy production. Equilibrium can be avoided by

having a continuous inflow of both fresh and salt water and a discharge of the waste brackish

water, by creating a continuous flow, an osmotic power plant can produce power continuously

and consistently (Kleiterp, 2012). The flow of brackish water from the membrane module is

separated, 1/3 of the brackish water is sent to the turbine to create energy while the remaining 2/3

is recycled and used in a pressure exchanger to add pressure to the feed of the high salinity water

(Skilhagen et al., 2007).

Membrane Technology

A semipermeable membrane is an organic filter with extremely small holes. The

membrane will only allow small molecules like water to pass through (Adokar et al., 2013). In an

osmotic power plant the semipermeable membrane is set up with fresh water on one side and salt

water on the other. Through the process of osmosis, the fresh water moves through the

membrane into the salt water creating an increased pressure on the salt water side. This increased

pressure can be used to power a turbine which then creates electricity (Skilhagen et al., 2007).

There is a theoretical maximum pressure produced from this process of 26 bars, which is the

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equivalent of a 270 meter high water column. (Skilhagen et al., 2007). It is estimated that half of

the osmotic energy produced can be converted to electrical power (Skilhagen et al., 2007).

The semipermeable membrane in an osmotic power plant consists of one thin non-porous

layer, the diffusion skin and at least one layer of porous material which must be designed to

avoid salt build up. Water going into the membrane is filtered and with regular cleaning they can

last up to 7 years. These membranes are made up of spiral wound or hollow fibers and in an

osmotic power plant they are set up in parallel modules (Gerstandt et al., 2007).

A lot of the research carried out by osmotic energy companies has gone into finding an

efficient semipermeable membrane (Skilhagen et al., 2007). "Significant research into osmotic

energy took place from 1974-1985 but due to ineffective membranes, the key part of the osmotic

power plant, not much effort took place to establish this type of energy" (Gerstandt et al., 2007).

The efficiency and high cost of membranes was the main factor that has slowed down the

development of osmotic power plants (Kleiterp, 2012). The ideal membrane has high water

permeability and low salt permeability, low attraction to fouling substances and must be cost

effective. The membrane must also be able to withstand a high pressure differential between the

fresh and salt water (Adokar et al., 2013). Improvements in membrane technology have also

come from groups other than osmotic energy companies. NASA has an interest in developing

better membranes for treating water and producing electricity on spacecraft as well as

desalination plants that produce drinking water from seawater (Halper, 2010).

Pressure retarded osmosis (PRO) technology is the most common method used to

produce energy in osmotic power plants. The thin film composite (TFC) membrane has proven

to be the most efficient in reaching the theoretical goal of producing 5W/m2 (Gerstandt et al.,

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2007). Statkraft has teamed up with industrial companies to put more effort into improving the

quality and efficiency of membranes for osmotic energy production so that this form of

renewable energy can continue to develop (Skilhagen et al., 2007). "In order to exploit 10% of

the estimated European power potential, about 500 million m2 of membrane will be needed"

(Skilhagen et al., 2007).

Power Potential

The power potential of osmotic power depends very much on the location just like the

majority of renewable energies. The amount of usable water that can be extracted from the river

restricts how much power can be harnessed. How much water can be used is dependent on the

site and what the legal regulations are for ecological stability. Power potential is also affected by

changes due to seasonal water flow from the river (Stenzel et al., 2010). Statkraft's research

shows that in Norway the technical energy potential is around 12 TWh/year which could provide

Norway with 10% of its power needs. It is also estimated that in Europe there is potential for 170

TWh/year. Globally the total power potential is estimated to be approximately 1655 TWh

(Skilhagen et al., 2007). The potential for osmotic power is vast.

Future Potential for Improvement

Just like most forms of renewable energies, there is room for improvement to create more

power more efficiently. By increasing the velocity of water flow through the membrane the

energy produced is also increased. When the feed water and draw water are at a higher

temperature there is an increase in power (Touati et al., 2013). Temperature has an effect on the

water flux, water permeability, and the mass transfer coefficient. Because energy is proportional

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to the water flux, increasing the temperature increases that water flow which then increases the

amount of power recovered:

It can be seen that with the current first generation of PRO membranes, powers up to 150

kW could be obtained when both water feed temperatures are around 30°C. This value

decreases with lower temperatures (about 40 kW at 15°C) and higher feed salinities

(around 40 kWs when using 10 g/l draw solutions). Similarly, this power is expected to

significantly increase with temperature. Thus, PRO seems already a promising renewable

energy source in hot regions, and when there is adequate access to water at high

temperatures (industrial wastewaters, cooling towers, etc.) (Touati et al., 2013)

With all these improvement osmotic power output still needs to reach 5 W/m2 to become

a profitable source of energy. This will have to be achieved with the use of better membranes,

high salt gradients, and optimal temperature conditions.

Osmotic Power in Canada

Feasibility and Potential

The feasibility of osmotic power is for the most part dependent on membrane technology

becoming more efficient and reaching the desired output of 5 W/m2. It is also dependent on more

companies producing membranes because there is a shortage in the available supply. For

instance a 25MW power plant would require 5,000,000 m2 of semipermeable membrane

(Skilhagen et al., 2007). A financial feasibility study looked at whether a commercial osmotic

power plant is feasible if the capital costs of the infrastructure were included. He concluded that

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osmotic power plants are not feasible at the moment but improvement in plant design and

membrane technology could improve the possibility in the future (Kleiterp, 2012).

It is theorized that North America has a potential for 4.195 TWh/year (Stenzel et al.,

2010). There is great power potential all over the world particularly in Asia and South America.

Figure 2 shows the theoretical potential for osmotic power production in each continent.

Figure 2 Theoretical potential of osmotic power across continents. Reprinted from Stenzel, P., & Wagner, H. (2010). Osmotic power plants: Potential analysis and site criteria. 3rd International Conference on Ocean Energy, 5-5.

Possible Locations

Site selection for osmotic power plants is based on access to both fresh water and salt

water with minimal mixing of the two. In Canada, where there are many large rivers that empty

into bodies of salt water, there is a lot of potential for long term osmotic power production. "In

Quebec, studies by Hydro-Quebec's research institute (2011) estimated the exploitable osmotic

potential for the 30 large rivers emptying into salt water to be 1,860 MW. Fourteen of them

(totaling 1,060 MW) empty into the Gulf du Saint-Laurent (Gulf of St. Lawrence) and its

estuary. The challenge today is to generate osmotic power at a competitive cost by 2020" (Hydro

Quebec, 2015). Northwest Territories, Nunavut, British Colombia, Manitoba, Ontario, Quebec,

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and Newfoundland and Labrador all have potential for osmotic power (Hydro Quebec, 2015).

Figure 3 shows the areas in Canada with the highest osmotic power potential.

Figure 3 Map of Canada illustrating possible locations and power outputs. Reprinted from Hydro Quebec. (2015). A Renewable Energy Option Osmotic Power. Retrieved September 21, 2015, from http://www.hydroquebec.com/sustainable-development/documentation-center/pdf

Costs

Osmotic power is a relatively new method for producing energy and other than Statkraft's

prototype power plant there is not much information on costs associated with this technology. It

is estimated that the cost of osmotic energy will be comparable to other renewable energy

sources in the future and Statkraft expected that it would be similar to off shore wind power by

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2010-2015 but this has not yet been achieved (Skilhagen et al., 2007). Figure 4 shows a cost

comparison of renewable and non-renewable energy sources.

Figure 4 Cost comparison of renewable and non-renewable energy sources. Reprinted from Sharif, A. O., Merdaw, A. A., Aryafar, M., & Nicoll, P. (2014). Theoretical and experimental investigations of the potential of osmotic energy for power production. Membranes,

Operating Costs

Statkraft realized from the beginning that the development of membranes with high flow

rate and salt retention would be the main challenge to commercializing osmotic power

(Skilhagen et al., 2007). They estimated that once the technology was fully developed, the gross

generating cost would range from 7-14 cents/KWh with net costs around 60-75% based on

power plant efficiency (Hydro Quebec, 2015). Statkraft analyzed the cost of producing osmotic

energy by using their hydro power plants as the basis for comparison and concluded that in the

future the price of renewable energy would make osmotic power a commercially viable option

with expected pricing to range from 50-100Euro/MWh (Skilhagen et al., 2007).

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Affordances

There are a number of factors which make osmotic power a viable energy source. When

producing power there is no burning of fossil fuels which means no carbon emissions. Building

and running these power plants has minimal impact on the environment. This technology can run

24 hours a day 365 days a year, unlike wind or solar power. An osmotic power plant can also

operate using effluent from sewage treatment plants. The fuel for osmotic power is natural and

very abundant. There is also growing support from the public and politicians to implement more

renewable energies.

Constraints

Just like all energy sources osmotic power has its downsides. The membrane technology

is not yet at the level of cost effectiveness and efficiency to make this technology cost

competitive. The start-up costs to locate and build these power plants is quite high. Water

filtration and regular cleaning of membranes is needed to keep them from fouling. The brackish

wastewater must be disposed of properly so as to not affect the salt gradient near the power plant.

Environmental Impacts

There is not much known about the environmental impacts of osmotic power plants

because the technology is still fairly new, although this technology can be compared to water

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purification plants which are similar and well documented (Hydro Quebec, 2015). There are

numerous environmental effects to consider before beginning to build an osmotic power plant:

sediment displacement, shore line erosion, water quality from chemical and waste water spills,

local aquatic animal and plant species, changes in flow rate, and current direction (Hydro

Quebec, 2015). Habitats and vegetation will be modified with possible effects on aquatic life due

to changes in salinity from the large amount of discharged brackish water (Hydro Quebec, 2015).

There is potential for effects from the use of chemicals used to clean membranes and pretreat

water along with waste sludge and used membranes. Conflict with shipping vessels and fishing

may also arise (Hydro Quebec, 2015). The ambient water temperature would rise from the

production of energy although this is only estimated to be a 1/2°C increase (Hydro Quebec,

2015). In the grand scheme of things the environmental impacts of an osmotic power plant are

fairly minor. There are no studies so far that have looked at the life cycle assessment of an

osmotic power plant but it is estimated to be the same as other renewable energies (Hydro

Quebec, 2015). Osmotic power plants do not produce any greenhouse gasses or airborne

contaminants during the production of power, the only emissions produced are during the

building stages. The visual impact of these plants can be very minimal as they can be built

mostly underground (Hydro Quebec, 2015). Most rivers run into the ocean in cities or industrial

areas which means osmotic power plants could be built in these areas without affecting unspoiled

land. Careful selection of where the inlet and outlet of the plant are located can help lessen the

effect on the marine ecosystem (Adokar et al., 2013). Skilhagen states that:

Statkraft has assessed the environmental optimization and pre-environmental impact of

an osmotic power plant located at a river outlet and not found any serious obstacles. A

combination of river flow regulatory compliance and careful engineering of the intake

OSMOTIC POWER 20

and outlet of brackish water would reduce the impact on the river environment to a

minimum. (Skilhagen et al., 2008).

Conclusions

The need to reduce greenhouse gas emissions has led to research into alternative energy

sources such as osmotic power. Renewable energy produced by osmotic power is available 24

hours a day all year long, unlike wind and solar which has problems with interruptions in power

production. There are some major challenges before osmotic energy can become cost

competitive with other renewable energies. One of the biggest challenges is the development of

efficient and cost effective membranes. However as more osmotic power plants are built the

need for more membrane manufacturers will increase which will further the development of

membrane technology. Improving membrane efficiency is essential to achieving 5 W/m2 which

is the point where osmotic power becomes cost effective. Most renewable energies such as wind

and solar power started out as being inefficient and not very feasible, but through continued

development they were able to become some of the top renewable energies used today. Another

challenge that faces osmotic power production is finding the most suitable location for optimal

energy output with the highest salinity gradient. In addition, the infrastructure cost of the power

plant will be high.

Osmotic power production has a low impact on the environment because it does not

produce greenhouse gasses. The impact on the aquatic environment around the plant location is

also low, but there are a few potential problems such as production of brackish waste water

which may affect plant and animal life near the outlet of the plant. As well, during the

construction phase of the power plant, there may be disruption and erosion of the shoreline and

OSMOTIC POWER 21

riverbed which could be avoided through careful construction practices. There is also concern

that the chemicals used to clean the membranes and filters will affect the aquatic environment.

There are a number of rivers across Canada which show potential for osmotic power production.

The rivers with the highest potential are located in Quebec. With continued improvement in

membrane technology, osmotic power could be a possible future renewable energy source for

Canada and the rest of the world.

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