Energy and the Environment-A Coastal Perspective

The production of energy from the creation of an ion gradient is a method which is far from new or original—this process is the foundation of energy generation in cellular respiration and photosynthesis. Humans, however, did not conceive the idea to generate electricity from the creation of an ion gradient until the 1970s and this technique was not pursued until recently. The production of electricity from salinity gradients is based on the natural tendency for freshwater to mix with saltwater. This technique relies on the use of a semi-permeable membrane and water’s osmotic properties. Currently, two methods of membrane technology are being pursued—pressure retarded osmosis (PRO) and reverse electrodialysis (RED).

Pressure retarded osmosis relies on the diffusion of freshwater across a semi-permeable membrane to a chamber containing seawater cause an increase in the water volume inside the seawater chamber and, consequently, pressurizing the water (IEA, 2009). The pressurized water then turns a turbine which generates electricity. The products of PRO are electricity and brackish water resulting from the mixing of freshwater and saltwater (IEA 2009).

Visual of a PRO system http://www.climatetechwiki.org/technology/jiqweb-ro

Reverse elctrodialysis is also a membrane-based electricity generation method but it relies on electrochemical reactions instead of osmotic pressure. This method utilizes a series of stacked membranes. Half of the membranes are permeable to sodium ion and half are permeable to chloride ions. Freshwater and saltwater flow alternately between each pair of membranes. The controlled diffusion of sodium and chloride ions in the water causes oxidation and reduction reactions at the iron ends of the device—otherwise known as the anode and cathode. The reverse elctrodialysis technique has been tested only at relatively minute scales with capacities around 100mW (IEA, 2009).

Visual of a RED System http://www.climatetechwiki.org/sites/climatetechwiki.org/files/images/extra/blue_power_02.jpg

One limiting factor of osmotic electricity production is that it is an entirely location-specific technology. Because this method relies on the mixing of freshwater and saltwater, osmotic power plants must be placed where a river meets saltwater. Other limiting factors of osmotic electricity production exist in the water properties. The salinity, volume, and turbidity of the water influence the effectiveness of electricity production. The higher the salinity gradient, the greater the pressure, and greater pressure leads to a higher rate of electricity generation. Similarly, a greater volume of water will yield more electricity. A major concern of salinity gradient device installments is the accumulation of biofouling. The accumulation of organisms on the water collection pipes would reduce the pressure of the collected water and hence reduce the amount of electricity produced. Therefore, the devices would need to be regularly cleaned and maintained in order to maintain optimal efficiency. Fortunately, the two major components necessary for osmotic power generators, the membrane and pressure exchanger, are fairly commercialized (Sandvik & Skihagen, 2008). Thus, this technology has the potential for widespread installation in the near future.

Norway PRO Installation http://images.nationalgeographic.com/wpf/media-live/photos/000/628/overrides/energy-osmotic-power-plant-electricity-from-seawater_62893_600x450.jpg

A PRO project exists in Norway and has a capacity of 4kW. This project was installed primarily as a test project and for experimental research on the PRO technology (OES-IA, 2009). One of the main objectives of this installation is to improve the efficiency of the PRO technology. As previously mentioned, RED technology has been tested at small scales. One of these small-scale installations exists in the Netherlands and its main purpose is to study the environmental impacts and improve the existing technology (OES-IA, 2009). Currently, salinity gradient electricity generation is expensive due to installation and operation costs. Moreover, the capacity is extremely low in comparison to other coastal and offshore energy devices and their installation prices. As investment in renewable energy increases, however, the price of salinity gradient energy technologies will likely decrease as the efficiency and capacity increase.

Citations:

• IEA, 2009. Ocean Energy: Global Technology Development Status. International Energy Agency Implementing Agreement on Ocean Energy Systems Annex I: Review, Exchange and Dissemination of Information on Ocean Energy Systems. IEA-OES document No.: T0104 available at: http://www.iea-oceans.org/
• OES-IA, 2009. International Energy Agency Implementing Agreement on Ocean Energy Systems Annual Report 2009. OES-IA document A09. Available at:  http://www.iea-oceans.org/
• Sanvik, S.O., Skihagen, S.E., (2008). Status of technologies for harnessing Salinity Power and the current Osmotic Power activities. Article to the 2008 annual report of the International Energy Agency Implementing Agreement on Ocean Energy Systems Annex I: Review, Exchange and Dissemination of Information on Ocean Energy Systems. Available at: http://www.iea-oceans.org/publications.asp?id=1