SALINITY GRADIENT POWER Energy can be captured by exploiting the pressure difference at the boundary between freshwater and saltwater. This is called Osmotic Energy. The difference of potential between freshwater and salt water is called the Salinity Gradient. The potential for osmotic energy exists where ever a stream or river enters the ocean. Most people are familiar with reverse osmosis where freshwater is obtained from saltwater. Reverse osmosis consumes energy and produces freshwater from seawater. Osmosis consumes freshwater in the presence of seawater and produces energy (the freshwater becomes saltwater). Salinity Gradient is a technology that takes advantage of the osmotic pressure differences between salt and fresh water or water of different salinity. Salinity gradient power is completely renewable and sustainable. It is the highest energy concentration (i.e., energy density) of all marine renewable energy sources, an ultra-dense energy resource. Despite its potentiality, salinity gradient energy is often-overlooked source of marine renewable energy. Various concepts on how to make use of salinity gradient power were proposed more than twenty years ago. One such concept is the Pressure-Retarded Osmosis (PRO). In PRO, seawater is pumped into a pressure chamber where the pressure is less than the osmotic pressure difference between fresh water (or low salinity water) and seawater (or higher salinity water). Freshwater flows through a semi permeable membrane and increases the volume (or pressure) within the chamber; a turbine is spun as the pressure is compensated. Early technical advances were not considered promising, mainly because they relied on expensive membranes. Membrane technologies have advanced, but to date, they remain the technical barrier to economical energy production. Efforts are underway to address those issues and alternatively develop designs that eliminate membrane. The alternative technologies include vapour compression and the patented hydrostatic generator . The principle of salinity gradient energy is the exploitation of the entropy of mixing freshwater with saltwater. The potential energy is large, corresponding to 2.6 MW m3/sec freshwater when mixed with seawater. This energy source is not easy to understand, as it is not directly sensed in nature in the form of heat, waterfalls, wind, waves, or radiation. Several methods have been proposed to extract this power. Among them are the difference in vapor pressure above
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SALINITY GRADIENT POWEREnergy can be captured by exploiting the pressure difference at theboundary between freshwater and saltwater. This is called OsmoticEnergy. The difference of potential between freshwater and salt wateris called the Salinity Gradient. The potential for osmotic energyexists where ever a stream or river enters the ocean.
Most people are familiar with reverse osmosis where freshwater isobtained from saltwater. Reverse osmosis consumes energy and producesfreshwater from seawater. Osmosis consumes freshwater in the presenceof seawater and produces energy (the freshwater becomes saltwater).
Salinity Gradient is a technology that takes advantage of theosmotic pressure differences between salt and fresh water or waterof different salinity. Salinity gradient power is completelyrenewable andsustainable. It is the highest energy concentration (i.e., energydensity) of all marine renewable energy sources, an ultra-denseenergy resource. Despite its potentiality, salinity gradient energyis often-overlooked source of marine renewable energy.
Various concepts on how to make use of salinity gradient power wereproposed more than twenty years ago. One such concept is thePressure-Retarded Osmosis (PRO). In PRO, seawater is pumped into apressure chamber where the pressure is less than the osmoticpressure difference between fresh water (or low salinity water) andseawater (or higher salinity water). Freshwater flows through a semipermeable membrane and increases the volume (or pressure) within thechamber; a turbine is spun as the pressure is compensated.
Early technical advances were not considered promising, mainlybecause they relied on expensive membranes. Membrane technologieshave advanced, but to date, they remain the technical barrier toeconomical energy production. Efforts are underway to address thoseissues and alternatively develop designs that eliminate membrane. Thealternative technologies include vapour compression and the patentedhydrostatic generator .
The principle of salinity gradient energy is the exploitation of theentropy of mixing freshwater with saltwater. The potential energy islarge, corresponding to 2.6 MW m3/sec freshwater when mixed withseawater. This energy source is not easy to understand, as it is notdirectly sensed in nature in the form of heat, waterfalls, wind,waves, or radiation. Several methods have been proposed to extractthis power. Among them are the difference in vapor pressure above
freshwater and saline water and the difference in swelling betweenfresh and saline waters by organic polymers. However, the mostpromising method is the use of semipermeable membranes. The energycan then be extracted as pressurized brackish water by pressureretarded osmosis (PRO) or direct electrical current by reverseelectrodialysis (RED).
WHAT IS OSMOTIC PRESSURE?
"Sucking" means creating a partial vacuum that will allow atmosphericpressure to push a liquid toward the one who may have thought he waspulling it, the sucker.
From which it follows that water cannot possibly be sucked to aheight greater than a column whose weight (per area) balances thepressure of the atmosphere. That height is around 33 feet (the lengthof a barometer can be considerably shortened by using mercury ratherthan water for the liquid column).
So how do trees 100 ft tall get their water to the top branches?
By osmotic pressure. Osmosis is the diffusion of a solution with alower concentration (i.e., a more dilute solution) through asemipermeable membrane into a solution with a higher concentration soas to equalize the concentrations. Osmotic pressure is what forcesthe liquid through the membrane, and this pressure (in the case ofsalty sea water on one side of the membrane and fresh water on theother) will hold up a column of not 33, but 775 feet!
TECHNOLOGIES?
TWO TYPES OF SYSTEM TO HARNESS ENERGY :
1. THERMAL
Pressure Retarded Osmosis (PRO)
Reverse ElectroDialysis (RED).
2. MEMBRANE
Vapor Compression
Hydrocratic Generator
Withthe REDmethod, ion selective membranes are used in alternate chambers withfreshwater and seawater, where salt ions migrate by natural diffusionthrough the membranes and create a low voltage direct current. Anarray of alternating anion and cation exchange membranes is calledRED. With the PRO method, another type of membrane, similar to reverseosmosis membranes used for sea water desalination, is used. These PROmethod membranes are much more permeable to water than to salt. Iffresh and saltwater are separated by such membranes, natural osmosiswill force the freshwater through the membrane to the saltwater sidewhere hydrostatic pressure up to 26 bars can be created. The twomethods are quite different in their working principles, but it is thesame potential energy that is exploited.
Significant research took place from 1975 to 1985 and gave variousresults regarding the economy of PRO and RED plants. The total costvaried from 0.02 to far above 1.3 USD per kWh [1-6]. In a recentreview on renewable energy [7] the cost of salinity power is said tobe prohibitive. However, a closer look at the sources reveal thatmany negative conclusions were based on the performance of existingequipment and not new purpose-built equipment. Measurements withmembranes for reverseosmosis and not equipmentadapted to osmosis. Arecent prestudy [8]concluded that a total powercost with PRO could be 0.035- 0.07 USD per kWh. Thisprestudy resulted in furtherdevelopment work by Norway'slargest producer ofhydroelectric power,Statkraft SF, incollaboration with Europeanmembrane expertise andbacking from the EU commission. It is important to note that small-scale investigations into salinity power production take place inother countries like Japan, Israel, and the United States.
Hydrocratic Generator• Free Energy of Mixing• No membranes• Renewable• Plentiful
Methods of Implementation
Why Salinity Gradient Power?• Renewable• Sustainable• High energy density• Untapped• No fuel costs• No CO2 emission
Advantages
The advantageous properties of salinity power can be summarized as follows:
No CO2 or other significant effluents or any global environmentaleffects;
Completely renewable;
Non-periodic (unlike wind or wave power);
Suitable for small or large scale plants (modular layout).
Drawbacks
The primary drawbacks of salinity power are:
Some plant equipment has yet to be developed with the necessary efficiency;
High capital costs for plant construction -mostly buildings and machinery;
Energy cost is very sensitive to membrane cost and efficiency;
Membranes used for plants are vulnerable to fouling.
Conclusions
Salinity power is one of the largest sources of renewable energy thatis still not exploited. The exploitable potential world-wide is estimated to be 2000 TWh annually. One of the reasons that this renewable source has not drawn more attention is that it is not readily evident to most people. Another reason is that considerable technological development is necessary to fully utilize this resource. Along with the the lack of efficient and suitable plant components, some pessimistic cost forecasts have been issued. The potential cost of energy from this source is higher than most traditional hydropower, but is comparable to other forms of renewableenergy that are already produced in full-scale plants.
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