Creating Clean Water and Energy – at the Same Time!
Predictions of worldwide record droughts in the years ahead, along with news of hitting peak oil prices, have led scientists to seek ways to solve both issues. A Cambridge, Massachusetts-based desalination start-up called Oasys has come up with a way to supply both clean water and inexhaustible energy.
Yale researchers Rob McGinnis and Dr. Menachem Elichalem have created a desalination system called Engineered Osmosis. This process relies on the design on of a membrane-based water separation and power generating system. Current membrane systems rely on normal hydraulic pressure to push the water through the membranes, thus using a lot of energy and still being limited by the osmotic pressure.
Using natural process
Engineered Osmosis uses the natural water flux as the driver for transport between membranes; hydraulic pressure is used only to create intentional resistive force. One such use of Engineered Osmosis is to create potable water from a process known as “forward osmosis” (FO). This process draws “pure” water from its contaminants to a solution of concentrated salts via this semi-permeable membrane in what is known as an osmotic pressure gradient.
Forward osmosis does not require mutiple stages, large heat transfer areas and large pumping volumes, to create clean water like current desalination processes. Instead, FO uses a draw solution, dissolved solutions in water that are easily rejected by semipermeable membranes, in the form of concentrated solution of ammonium (created by the dissolution of ammonia and carbon dioxide in water). Both the draw solution and salt are rejected by the membrane and the solution is directed towards a distillation column where low temperature heat strips the ammonia and carbon-dioxide gases for reuse. This then produces freshwater containing less than 1ppm of ammonia.
Good for water AND energy
Osmosis can not only create fresh water, it also can create sustainable energy. Oasys and another company called Statkraft have harnessed the basic elements of osmosis to create energy. Statkraft is a renewable energy company in Norway created the first osmotic power plant in Tofte on November 24, 2009. The plant generates power by exploiting what happens when fresh and salt water are mixed. River water is pushed on one side of the membrane and salt water on the other, which draws the fresh water towards the salt and raises the pressure to the salty side of the membrane. With help of pressurized exchangers, the water will help spin a turbine.
The membrane can produce around 3 watts of power per square meter – 5 watts at most. This means a 5 million square meter factory could only produce 25 KW of energy, not very practical for normal applications. Oasys, on the other hand, believes that it can produce 200 watts of power due to the high pressure ammonia-carbon dioxide salt used during the forward osmosis process. Instead of using fresh water supply, the power plants will contain two tanks, one for the salt water and the other for the newly desalinated water. The waste heat from the power plant would desalinate the water and if the plant needs more power during peak hours, the salt and fresh water could be combined to create more power. While the efficiency is still only at 50-80 percent, much of the energy input is waste heat. Oasys recently opened up a small plant in South Boston and secured a $10 million Series A round of financing [PDF].
Water shortages and power issues are not something in the distant future. Companies like Oasys are turning to natural processes, something as simple as osmosis, in ways that it can not only hydrate the world but also power it.