The power of osmosis13 April 2010
With more than a hundred years of experience in developing and operating hydro power, Norwegian utility Statkraft is now on course to strengthen its position in the quest for new forms of renewable energy generation, including the development of osmotic power. Stein Erik Skilhagen reveals all
In the context of climate and environmental challenges, R&D has a key role to play in finding new solutions. From a company’s perspective, R&D is also about safeguarding business outlook and shaping growth ambitions. This means that we need to improve existing technologies as well as work on building new renewable energy solutions.
Statkraft has been engaged in developing new renewable energy technologies since the early 1990s. Based on the company’s history as a major Norwegian power generator, our focus has been on harvesting the energy that is available along the far-reaching Norwegian coastline. For more than a decade we have been working internationally, in close collaboration with R&D parties, as well as with universities in order to find ways to produce renewable energy from the natural forces of the ocean.
It has been known for centuries that mixing freshwater and seawater releases energy. For example, a river flowing into the salty ocean is releasing large amounts of energy. The challenge is to utilise this energy, since the energy which is released from the mixing of salt and freshwater leads only to a very small increase of the local water temperature. During the last few decades at least two concepts for converting this energy into electricity instead of heat have been identified.
One of these is pressure retarded osmosis (PRO). Thanks to this technology it may be possible to utilise the enormous potential of a new, renewable energy source. This potential represents a worldwide electricity production of more than 1600TWh/yr – equivalent to half the annual power generation in the European Union.
For pressure retarded osmosis, also known as osmotic power, the released chemical energy is transferred into pressure instead of heat. This was first pointed out by Professor Sidney Loeb in the early 1970s, when he designed the world’s first semi-permeable membrane for desalination of saline water for production of drinking water based on reverse osmosis. Osmotic power is based on naturally occurring osmosis, triggered by nature’s drive to establish equilibrium between different concentrations in liquids. Osmosis is a process by which solvent molecules pass through a semi-permeable membrane from a dilute solution into a more concentrated solution (Figure 1).
The difference in concentration of salt between seawater and freshwater creates a strong force towards mixing. The effects of this strong force to mix can be intensified through a special membrane which separates salt and freshwater in a finite space and which only lets the water pass through the membrane, while the salt ions are rejected. In this way, an osmotic pressure can be achieved by the amount of freshwater moving to the seawater side. This pressure can be in the range of 24 to 26 bars depending on the salt concentration of seawater.
More precisely, in a PRO system, filtered freshwater and seawater are led into a closed system as illustrated in Figure 2. Before entering the membrane modules, the seawater is pressurised to about half the osmotic pressure, approximately 12-14 bars. In the module freshwater migrates through the membrane into the pressurised seawater. This results in an excess of diluted and pressurised seawater which is then split into two streams. One third of this pressurised seawater is used for power generation in a hydro power turbine, and the remaining part passes through a pressure exchanger in order to pressurise the incoming seawater. The outlet from such a plant will mainly be diluted seawater (brackish water) that will be led either back to the river mouth or into the sea.
Consequently, the higher the salinity gradient between fresh and saltwater, the more pressure will build up in the system. Similarly, the more water that enters the system, the more power can be produced. At the same time, it is important that the freshwater and seawater is as clean as possible. Substances in the water may get captured within the membrane’s support structure or on the membrane surfaces, reducing the flow through the membrane and causing a reduction in power output and overall system efficiency. This phenomenon, commonly known as fouling, is linked to the design of the system, to the characteristics of the membrane, to the membrane module, and to the pre-treatment of the freshwater and the seawater.
The majority of an osmotic power plant will be designed of existing ‘off-the-shelf’ technology. The key components are the membranes, the membrane modules, and the pressure exchangers. The lion’s share of efforts to commercialise osmotic power is dedicated to improving and scaling up these components.
Statkraft’s focus on osmotic power began in 1997 when the Norwegian research organisation SINTEF was engaged to perform feasibility studies on behalf of the company. The result of the study was that osmotic power could have significant global potential. However, it was also revealed that one component would require improvement, and this was the semi-permeable membrane.
Based on the early findings, the work since then has mainly been focused on the design and production of a semi-permeable membrane optimised for osmotic power. From economical calculations and estimations of the development in the energy market, a target for the efficiency of the membranes has been set at 5W/m2 for producing osmotic power on a commercial basis. During these years the power density of the membrane developed by Statkraft has increased from less than 0.1W/m2 up to today’s membranes producing close to 3W/m2. The development has been aimed at testing commercial membranes, as well as developing new membranes designed for osmotic power.
In addition to the development of the membrane, there are also significant activities on the design and development of the membrane modules. The standard spiral wound module design has limitations both in the internal flow pattern and pressure losses, and there are also limitations with regards to the common design for scaling up to larger units. Since an osmotic power plant will require several million square metres of membrane, the modules should contain several hundreds or even thousands of square metres. In this respect, the following design criteria have been suggested:
• The elements must be able to have flow on both the freshwater and the seawater side of the membrane.
• The elements must contain a large membrane area.
• Fouling must be minimised.
• The design must be cost-effective.
The development of osmotic power is managed by Statkraft, and is executed mainly by research groups in Germany, Norway and the Netherlands, as well as in the US. There are, however, several other groups working on elaborative topics both in North America and in Asia.
Based on the development of the membranes, along with an improved understanding of the system technology and potential, Statkraft decided to build a complete system for testing the osmotic power concept in 2007. It has designed a prototype plant where pressure retarded osmosis is used to drive a turbine, based on feed of seawater and freshwater from natural sources.
The osmotic power prototype
The world’s first osmosis driven prototype for power generation was put into operation on 24 November 2009 during a grand ceremony. The first electricity produced was used for preparing a cup of tea for the Crown Princess of Norway Mette-Marit.
In the southern part of Norway a complete prototype of an osmotic power plant has been built. Several considerations had to be made related to the choice of location. Firstly, fresh water and seawater with satisfactory quality must be available. Secondly, the location had to be readily available for researchers, suppliers and governmental representatives, hence it could not be too far from a large city. Based on these requirements, the small community of Tofte, one hour drive from Oslo, was chosen.
The prototype represents a major milestone towards the commercialisation and creates a unique test site for future technology development. The plant did generate the first small kWh of electricity from osmosis in November 2009, and the first proof of the concept of producing power by osmosis has been recorded.
The prototype is designed to be used as a laboratory for the ongoing development of the technology. In this respect, it will contribute to technology enhancements in order to reach the objective of producing power at a competitive cost, and creating the basis for upsizing the various components to commercial scale.
In addition to the on-going research, with the main focus being on the membranes and the membrane modules, the prototype will serve as a catalyst for developing partnerships and building relationships with interested parties. The prototype facilitates the creation of partnership for development of osmotic power outside Statkraft’s core geographic area, and it increases awareness among governments and manufacturers. Furthermore, the prototype will be a starting point to test and measure environmental challenges such as measuring potential algae bloom related to the discharge of brackish water.
Design of the prototype
The prototype is designed with all necessary systems and components for continuous PRO operation. Based on the assumption that a membrane with an efficiency of 5 W/m2 will be developed during the lifetime of the plant, 10kW installed power capacity was set as the overall design criteria. This gave the lead for water supply for both water qualities, as well as sizing of the individual components.
The seawater feed to the plant is supplied through water pipes from approximately 30m below sea level, just outside the harbour. The water is filtered through a mesh before it enters the plant.
The freshwater at the Tofte location flows from a small lake up in the hillside. A major focus activity will be to identify the minimal pre-treatment necessary to operate the plant, and to design the appropriate system optimised to fulfil the requirements for operating the membrane system in a continuous mode through the entire lifetime of the membranes.
For this plant 2000m2 of membrane has been installed based on a modified spiral wound 8” module. This is a convenient and standardised design where membranes can be replaced easily and also is a standard that other suppliers can relate to. After some time in operation we also expect to test alternative module design optimised for PRO operation. The first membranes installed are based on conventional cellulose acetate membranes, redesigned for PRO operation. The membranes will be replaced when new and improved membranes are designed and produced in sufficient amounts.
Besides the membrane system, the plant is equipped with two specially designed energy recovery devices. Although this technology is well proven in desalination systems, the installation in this plant is unique due to the low operating pressure. It will be very important to learn the operations of these units in PRO, and also to test the efficiency and leakages experienced in a low pressure system.
A turbine with a generator is installed to generate electricity from the pressurised water. With continuous flow of water at approximately 12 bar, a Pelton turbine was chosen. To be able to generate as much electricity as possible from the membranes installed, the turbine must be optimised for the correct flow of water at the given pressure. In a full-scale installation, the combined efficiency of the turbine and generator is expected to exceed 85%.
The overall objectives of the prototype are two-fold. Firstly, confirming that the designed system can produce power on a reliable 24-hour/day production. Secondly, the plant will be used for further testing of technology achieved from parallel research activities to substantially increase the efficiency. The performance and efficiency of the individual component, as well as the system efficiency as a whole, will be directed towards the targets for commercial production of osmotic power. These activities will mainly be focused on membranes, membrane modules, pre-treatment of water, pressure exchanger equipment, and power generation (turbine and generator).
The road towards commercialisation
What is necessary for establishing osmotic power as a major contributor to renewable energy generation in the future?
Statkraft has spent significant time and effort on the development of osmotic power, and will continue to do so. The solution is very attractive due to the environmentally friendly solution it offers, but to really make this a new and attractive solution in the renewable energy market it will depend on three major factors.
• 1) Supplier industry: It is well known that osmotic power was founded in the field for desalination. It is crucial that the future suppliers for osmotic power, such as membrane manufacturers, are willing to spend time and resources on bringing the technology from where it is today and improve and scale it up to an industrial size.
• 2) Energy utilities: Statkraft’s strategic long-term interest in osmotic power is to include it in its renewable energy portfolio. With the increasing focus on the environment, this will also be the case for several other utilities around the world. Other utilities must show their belief in osmotic power – Statkraft alone will not be able to establish a global osmotic power market necessary to realise its potential.
• 3) Governments and framework conditions: During the last few decades one has seen the growth of new solutions for harvesting renewable energy in Europe. These days, several European countries give substantial economic support to the establishment and growth of new, renewable solutions. Such framework conditions will also be critical for osmotic power. With a predictable support scheme and incentives for competitiveness also in the early maturing phase, both the supplier industry and the utilities will be ready to participate. Statkraft has already encouraged the European Union to include osmotic power as a recognised part of the marine energy sector, and will continue to do so also for individual countries.
Stein Erik Skilhagen, head of osmotic power, Statkraft AS, Lilleakerveien 6, 0216 OSLO, Norway. Email: [email protected]
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