It is found that marine energy can be used to generate enough power to meet the energy demands worldwide. To extract energy from the ocean, there are many techniques which include the energy from the waves, energy from currents, salinity gradients, and temperature gradients
Latest Developments and Advances in Marine Energy
Dr. Raj Shah, Blerim Gashi, Sharon Lin | Koehler Instrument Company
Introduction
As the energy demand continues to increase, the health of the environment continues to deteriorate due to the increase in the usage of fossil fuels. To aid the environment and prevent further damage, renewable energy research has become a hot topic aiming to identify and improve various renewable energy sources. These renewable energy sources include solar energy, wind energy, energy from biofuels and biomass, and geothermal energy. The most favorable renewable energy is marine energy. Marine energy refers to the energy found in waves, tides, and salinity gradients. Since approximately 71% of the earth is made up of water and contains approximately 332.5 million cubic miles of water [1], there is a high potential for energy in the ocean.
In addition to abundance, the energy is consistent and the amount of energy extracted can be predicted, unlike other renewable energy sources, making it a preferred energy source. It is found that marine energy can be used to generate enough power to meet the energy demands worldwide. To extract energy from the ocean, there are many techniques which include the energy from the waves, energy from currents, salinity gradients, and temperature gradients [2].
Wave Energy
As wind travels, it collides with the water, forming waves. This wave energy can be transformed into electricity [2]. There are three main types of power take-off methods that extract energy from waves.
The direct mechanical drive system uses an electric generator to convert the wave energy into electricity. Although it is found to have a 97% efficiency rate, the direct mechanical drive system is associated with high maintenance costs and is found to have a short life cycle. To make the direct mechanical drive system a reliable method to convert wave energy into electricity, there must be more research done on the electric generator [3]. The triboelectric nanogenerator is a recent invention that uses triboelectrification and electrostatic induction to convert wave energy into electricity. Unlike many generators that can only be used on low-frequency waves, the triboelectric nanogenerator can be used on waves with any frequency and it is not as costly. The triboelectric nanogenerator may be a great contender for converting wave energy into electricity, however, it is unknown what the conditions of the ocean may do to the nanogenerator [3].
Turbine transfer describes the use of liquid to power a turbine that is connected to a generator [4]. In the air turbine transfer system, a wave energy converter system is first used to convert the wave energy into pressurized air which travels through the turbine and into the generator which allows for the production of electricity. The air turbine transfer system is used when the waves are weak and slow. One benefit of air turbines is that they do not have to be located in the middle of the ocean like other devices, which means they will not be easily corroded and can be easily maintained. However, there is a high cost associated with the use of air turbines [3]. In the hydro turbine transfer system, the water travels through the turbine and drives the generator to produce electricity. It was discovered that the hydro turbine transfer has a 90% efficiency rate whereas the air turbine system has approximately 62.5% efficiency rate. Like other devices used to extract energy from the ocean, the hydro turbine can be easily damaged by the ocean water as it may harm the seals and the valves of the turbine which will decrease its efficiency [3].
Hydraulic systems use pressure to force the water through valves and into the actuator which converts the wave energy into electricity [4]. The hydraulic system consists of a buoy, ram, hydraulic motor, accumulator, and generator. The energy from the waves is absorbed from the ram and generates pressure which runs through the motor. The motor then allows the generator to convert the wave energy into electricity. The hydraulic system can produce a high yield of energy from low-frequency waves [3]. Although they are successful, it is difficult to contain the fluid in the system, the system also requires constant maintenance and it may be costly and difficult to store energy [4]. In addition to these challenges, the hydraulic system is found to have a lower efficiency when used in the real world when compared to the efficiency when used on the laboratory scale because when used on the laboratory scale, it doesn’t account for other factors like the effect of the ocean on the system. The fluid in the system may also lead to oil leakage which will harm marine life [3]. The hydraulic system may be a promising way to retrieve energy from waves, but there are many obstacles that need to be overcome before they can be applied on the commercial level.
Figure 1. Shows the hydraulic system [3]. The water travels through the ram and into the hydraulic cylinder before it goes through the accumulator and into the generator.
Tidal Energy
The rotational and gravitational forces of the Earth, Sun, and Moon pull the ocean waves into the low tide and high tide. To extract energy from these tides, hydrokinetic turbines are used. There are the vertical axis turbines, horizontal axis turbines, the venturi effect turbines, and the oscillating hydrofoil [2]. These tidal current turbines use the kinetic energy of the tides. In the vertical axis turbines, the turbine blades rotate at an angle perpendicular to the flow of water, whereas in the horizontal axis turbine, the turbine blades rotate at an angle parallel to the flow of water. In the tidal current turbines, the flow of the water causes the blades to move which turns the generator on through the gearbox [5]. Horizontal axis turbines are more efficient, easier to maintain as it is self-starting, and are cheaper than vertical axis turbines [6].
Another way to convert tidal energy into electricity is through tidal barrages, which is also known as a dam and deals with the tide’s potential energy. Single basin tidal barrages use three methods to convert tidal energy. One method is the ebb generation which traps the water in the basin during high tide and the water flows through low-head turbines which are then used to generate electricity. Another method is the flood generation method which uses a hydrostatic head that controls when the water can be allowed to flow through the turbine and into the basin. The last method is the two-way generation which combines the ebb generation and the flood generation methods. In a two-way generation, the water flows through the turbines when it is high tide or when the flood cycle is ending. Double basin tidal barrages use the ebb generation method whereby only the water travels through the first basin at high tides and is stored at the second basin. In 2009, there were four tidal plants that were reliable in generating energy [5].
The first tidal plant was built in 1967 in Brittany's Rance Estuary with a capacity of 240 megawatts. In 2011, the biggest tidal plant was built in South Korea and has a capacity of 254 megawatts. As technology advances, the production of energy from tidal waves will continue to increase. One of the main challenges due to these tidal plants is that the electromagnetic fields that are generated from these tidal plants negatively affect the growth of marine animals [7].
Ocean Currents vs Tidal Currents
Ocean currents are caused by the disturbances of winds on the ocean and the circulation of the ocean. Tidal currents are a result of the movements of the tides. The energy from the tidal currents is usually found between the seabed and the sea surface and can be extracted using tidal current energy converters. These converters include the horizontal axis twin-turbine, the vertical axis twin-turbine, and cross-flow devices. Ocean currents are found to be more consistent than tidal currents because ocean currents move in one direction whereas tidal currents can change directions because of the changes in flood and ebb cycles. Since the tidal currents change directions, it requires more energy which means tidal currents contain more energy than ocean currents. This might explain why ocean currents are generally slower than tidal currents [2]. It was discovered that when tidal currents flow in the same direction as the wave, the amplitude is decreased which decreases the tidal energy. Additionally, when the tidal current flows in the opposite direction of the wave, the amplitude of the wave increases which increases tidal energy [8]. Therefore, to increase the yield of electricity, the tidal energy from the tidal current that flows in the opposite direction of the waves should be used in the conversion of tidal energy into electricity.
Salinity Gradient
The salinity gradient describes the difference in salt levels between two bodies of water. This difference in salt levels produces the energy known as salinity gradient energy. Salinity gradient energy can be found in areas where the ocean and river meet, as there is a mixture of saltwater and freshwater. It is estimated that there are approximately 3.1 terawatts of salinity gradient energy throughout the globe [2].
Salinity gradient energy can be extracted through three different methods. One method is the pressure-retarded osmosis which uses semi-permeable membranes to transport water from the river to the sea which will cause an increase in static energy that is used to power the turbine. However, this method may not be reliable as it depends on the membrane used, therefore, more research is needed before pressure-retarded osmosis can be used on a commercialized scale [9]. Some types of membranes used in this process are cellulose acetate, polybenzimidazole, and poly(amide-imide) due to their increase in water flux, strength, and resistance against wear [10].
Another method of extraction is reverse electro-dialysis which pumps the saltwater and freshwater into membranes filled with anions and cations. This produces an electrochemical potential which produces a current. Like the pressure-retarded osmosis, reverse electro-dialysis also depends on the properties of the membranes. Recently, it was discovered that by improving the properties of the membrane, scientists were able to improve the efficiency of reverse electro-dialysis [9]. The last method is an electric double-layer capacitor which stores the charges in the saltwater and transports them into the freshwater and allows the charged ions to diffuse which creates electrostatic energy [9]. The introduction of ions from the saltwater into the freshwater may harm the marine life in freshwater as they are not used to the charged ions from the saltwater.
Ocean Thermal Energy Conversion
The upper layer of the ocean absorbs the energy from the sun and is used in the ocean thermal energy conversion cycles. The ocean thermal energy conversion cycles contain the warm seawater from the upper layers of the ocean and the cold seawater from the bottom layers of the ocean. The energy can be extracted from these cycles using open-cycle, closed-cycle, and hybrid-cycle techniques [2].
In an open-cycle extraction, a vacuum pump is used to ensure absolute zero pressure. The saltwater goes through the vacuum pump which leads to the formation of vapor, as shown in figure 2. This vapor is the freshwater and will go through a low-pressure turbine to enter the generator which is used to produce electricity. Since it is an expensive process and is found to have low efficiency, open-cycle extraction may not be widely used [11]. In a closed-cycle extraction, the saltwater goes through a refrigerant pump, a cooling agent, and vapor is formed, as shown in figure 3. The vapor travels through the turbine and into the electric generator, producing electricity. This method is cheaper than the open-cycle method, which makes it more favorable, however, it does not produce freshwater like the open-cycle method does [11]. The hybrid cycle is a mixture of the open-cycle and the closed-cycle techniques. In the hybrid-cycle extraction, the saltwater goes through the vacuum pump and the refrigerant pump and travels through the turbine and into the electric generator, as shown in figure 4 [11].
Figure 2. Show the open-cycle system which has a vacuum pump [11].
Figure 3. Show the closed-cycle system which has a refrigerant pump [11].
Figure 4. Show the hybrid-cycle system which has the vacuum pump from the open cycle and the refrigerant pump from the closed cycle.
Conclusion
Although marine energy is a promising renewable energy source, there are a lot of challenges that must be overcome before marine energy can be commercialized. The cost to extract marine energy is higher than the cost for other renewable energy sources. Additionally, there needs to be more research conducted on marine energy on the laboratory scale to commercialize the production of marine energy. Marine animals and plants may be harmed due to the extraction devices like turbines, cross-flow devices, and other energy-generating devices that are unknown to marine life. In addition to harming marine life, these extraction devices may also corrode easily and may not function properly due to the extreme weather conditions in the ocean [2].
Since there remains a lack of information and research regarding the extraction of energy from the ocean, the future of marine energy continues to remain unclear. The extraction of energy from the ocean may lead to erosion, deposition, and scour [12]. However, marine energy is promising as many countries have used tidal plants to extract tidal energy and have produced megawatts of electricity and there was no evidence leading to erosion or deposition. Additionally, marine energy is found to be the most reliable source of renewable energy, so there is a promising future in the commercialization of marine energy.
About Dr. Raj Shah .jpg)
Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 27 years. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s Long-Awaited Fuels and Lubricants Handbook 2nd Edition Now Available (https://bit.ly/3u2e6GY). He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honorific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology ) , Auburn Univ ( Tribology ) and Stony Brook University ( Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical engineering, Raj also has over 500 publications and has been active in the alternative energy industry for over 3 decades. More information on Raj can be found at https://bit.ly/3sayVgT
References
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