Osmotic Power: A Review Hablillah bin Mohd Hazim.

Master of Electrical Engineering (power)

subzeerow@yahoo.com

INTRO TO OSMOTIC POWER The growth of population, increasing energy demand and clean water as well as the increasingly polluted environment constraints have forced the researchers and scientists to explore alternative sources and new technologies which more environmentally friendly, clean and profitable to various parties. One of the discoveries in the realization of this objective is to use the Osmotic Power. Osmotic power is an alternative energy under the category of renewable energy source. It is like hydro power, which both use hydro turbines to generate electricity, but there are two major differences in the types of water resources and energy conversion means for both these systems. Hydroelectric power plants use water from the dam, while the osmotic power plant, the energy generated from the reaction of river water and sea water via membrane and no dam is required.

Figure 1: Osmotic power system

Two basic of osmotically membrane processes commonly practiced in engineering systems are Forward Osmosis (FO) and Pressure Retarded Osmosis (PRO). It is estimated that the world's energy production through PRO achieve 2000TWh/year while world energy production through renewable energy sources reached 10, 000 TWh/year in the future.

OSMOTIC POWER SISTEM BASIC

Figure 2: Membrane consept

Osmosis is the process of transferring the solvent (usually water) through a semi-permeable membrane from a high concentration solvent that flow into a low concentrations solvent or vice versa. Osmotic pressure is defined as the hydrostatic pressure required for stopping the spread of a solvent through a membrane. The relationship between osmotic pressure and salinity levels showed a direct proportion where the higher the salinity level difference between the two sources of water (fresh water and sea water) in the chamber, the higher the osmotic pressure would result.

Figure 3: FO, PRO and RO Meanwhile the PRO is an intermediate osmosis process that occurs between FO and RO, where the hydrostatic

pressure of the draw solution is lower than the osmotic pressure difference through the membrane. This allows the water seeping from the fresh water into the salt water. A direction in the process of osmotic water flux is shown in Figure 4 below.

Figure 4: The relation among water flux, pressure and energy in osmotic power system

PRO performance is influenced by the concentration polarization. As FO, PRO experience internal concentration polarization (ICP) in a porous membrane substrate, resulting in severe loss of energy and reduce water flux. This can be either due to the dilution of draw solution in the active layer facing feed solution orientation (AL-FS) or the concentration of feed solution in the active layer facing draw solution orientation (AL-DS).

Figure 5: AL-DS orientation for PRO

Furthermore, external concentration polarization (ECP) can also affect the performance of PRO particularly for low hydrodynamic shear force or high water absorption rate. Membrane type, feed solution/draw solution concentration and operating temperature are also an important parameter governing the performance of the PRO.

OSMOTIC POWER PLANT

The flow of fresh water from the river to the sea enables the release of energy. This energy can be harvested in a natural process called osmosis. Fresh water and salt water will be pump into the chamber and separated between them by using a semi-permeable membrane. Fresh water will flow through the membrane which has a different level of salinity compared to sea water. This will increase the pressure of the sea water, which is then fed to a turbine to produce power.

Figure 6: Osmotic power plant Statkraft, Norway

Osmotic power plant can be built in the industrial area and it is also suitable built near the mouth of the river which is a cross between fresh water and salt water. The membrane system is the heart of the osmotic power plant equipped with a number of many membrane and water pipe line to rotate the turbines that produce amounts of electricity.

Figure 7: Membrane system

At present, there have been a prototype osmotic power plant developed by the Norwegian company, Statkraft and the technology is being studied specifically by the company. Almost 25 TWh per year has been generated from this process in Norway and on 24 November 2009, the first osmotic power plant was opened by Princess Mette-Marit in Norway that has managed to produce 4 kW beginnings in 2015 and is targeted to generate the output of 25 MW which equal to a farm small wind power plant. By using asymmetric membrane, ICP occurs in a porous membrane substrate, which reduces the osmotic driving force across the functional layer, thus reducing the water flux. In PRO, the AL-DS is considered to be mechanically stable, as well as an external hydraulic pressure applied to the inside of salt water. In this case, the concentrative ICP occurs in porous membrane substrate, and water flux is given by:

where: Jw water fluks via membrane ; A and B water and solute permeability of the

active layer; D and F osmotic pressure draw solution and the

feed water, P hydraulic pressure difference across the

membrane; Km mass transfer coefficient in the

membrane substrate,which is the ratio of solute diffusivity in water(D) over the structure parameter(S) of the membrane support layer.

S product of membrane support layer thickness (l) and tortuosity() over its porosity():

SYSTEM BENEFITS AND WEAKNESSES Osmotic power does not require a large area to develop compared to normal hydro system. It is suitable as a private power generation for industries. It is clean, minimal environmental impact where its use renewable energy sources and does not emit carbon dioxide. On the other hand, it is quite expensive, especially for the membrane degradation rate and is still in the research stage. Besides, its may disrupt marine life in the vicinity. It also included the complex engineering design.

SUMMARY Nowadays, osmotic power is seen still relatively new. As far as this review is written based on the previous published papers, it appears that there is only one power plant using this technology in Norway and the study of membrane efficiency and other essential parameters are quite less. In my opinion, greater efforts should be made to develop this technology as it being more clean and the potential for future energy sustainability. Viewed from the Malaysia country, it may be said to be very suitable as this country surrounded by the sea and Malaysia has many estuaries for development of osmotic power plant as well as being well-suited developed within the industry and do not need a big space. REFERENCE 1. Minmin Zhang, Dianxun Hou, Qianhong She,

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