In this paper, the different types of Marine Energy will be discussed and examined to gain a better understanding of how it can be used as a renewable resource and the technology available currently for capturing such energy. Marine Energy is defined as any energy carried by ocean waves, tides, currents, salinity, and ocean temperatures. This paper will examine Wave Power, Tidal Power, Ocean Current, Ocean Thermal and Osmotic Power as means for collecting Marine Energy. Each technology will have its own section with its own evaluation, covering different aspects such as; how the technology works, different types, locations and much more. The scope of this paper is not to compare or promote Marine Energy to other renewable resources such as Solar or Wind, but to only inform and provide a technical evaluation of Marine Energy and its technologies as a whole.
In this section, ‘Wave Energy’ or more currently referred to as, ‘Marine and Hydrokinetic Energy’, will be examined.
Waves are created by wind, which is an indirect effect of uneven solar radiation from the sun, and by the motion of the Earth. As the name suggests, Wave Energy is harvested by a Wave Energy Converter (WEC) that converts a waves Kinetic and gravitational potential energy into electricity. There are four main groups of Marine and Hydrokinetic Devices that will be examined which are: 1) point absorbers, 2) attenuators, 3) oscillating water columns, and 4) overtopping devices.
Point absorbers are devices that float on top of or near the surface of the water, connected to the ocean floor by a cable or stationary base. These devices follow the vertical movements of the waves in any direction and absorb the wave energy by reacting some motion to a fixed resistance or more specifically, a reaction point.
The figure above shows two examples of point absorbers on the left-hand side of the picture. The point absorber on the left is attached to the ocean floor by a cable or mooring line. This allows free motion of the buoy, where the heavy submerged ballast plate reacts back from the oscillating motions transferred from the waves to the buoy. This reaction between the buoy and ballast plate causes the hydraulic pump in between them to rotate a generator making electricity. The middle absorber produces electricity in the same way except instead of having a submerged reacting plate, the floating buoy reacts with a fixed point attached to the ocean floor. Although both point absorbers fall under the ‘Nearshore’ category, typically, the middle design would be used in shallower water and the left design would be used in a deeper water environment.
Attenuators are long, offshore surface floating devices that are aligned parallel to the wave direction. The attenuator is attached by a cable to the ocean floor allowing it to freely move about the waves fluctuations. As a wave passes through the long body of the attenuator; segments either rise or dip into the peaks and troughs of the waves. In between each segment of the attenuator are joints that contain hydraulic rams which pump hydraulic fluid into a motor connected to a generator. With each rise and dip of the segments, a lot of force is generated between the joints which pump the fluid. The fluid pumped in runs the motor thus turning the generator creating electricity. A representation of a attentutor can be seen on the right side in the picture above or here below.
Oscillating Water Columns
An oscillating water column is an onshore structure located near rocks or cliffs with a deep sea bottom. The structure has a partially submerged chamber that allows waves to push up and down within, causing bidirectional air fluctuations. These air fluctuations turn a turbine connected to a generator that converts the wave energy into electricity.
As the waves on the outside of the chamber wall fluctuated, this causes the water enclosed in the chamber to oscillate up and down. As the water oscillates, this compresses and decompressed the air within the chamber like a piston, back and forth. The air passes through a special turbine called a ‘Wells Turbine’, which rotates no matter the direction of the air passing through it. This is the main design criteria of an oscillating water column because the air flow switches directions with each push and pull of the waves. Without a Wells turbine, the turbine would only rotate on the push of the air, leaving the pull portion unused thus wasting potential energy generation. This process repeats continuously as waves propagate back and forth outside the chamber.
An overtopping device is an onshore or floating device that contains a reservoir above sea level. The reservoir fills with water by allowing the waves to ‘overtop’ the walls of the structure. The water collects in the reservoir and gravity pushes the water down through a low head turbine back into the ocean, collecting its potential energy and making electricity in the process.
The onshore overtopping devices consist of a funnel-like channel, which allows the waves to work its way up via the funneling effect, to an above sea level basin. As the basin is filled with water collecting potential energy, channels return the water back to the ocean, first running through a low head turbine, producing electricity.
Wave energy is a truly abundant source of energy around the world. There have been many studies of the potential amount of energy that waves alone can generate around the world, however, each study uses its own way of determination and includes biases by the researchers. These studies do however all agree that the amount of energy in its totality is extensive. Using studies outline in World Energy Resources – Marine Energy, 2016 and Alternative Energy Systems by B.K. Hodge, the potential wave energy can be described here.
A study conducted in 2010 by the International Panel on Climate Change, Mork et al.(2010) estimates the total theoretical wave energy potential to be 32 petawatt-hours per year (PWh/yr) which is twice the global electricity supply in 2008, 17 PWh/yr. Below is Table 1, that illustrates the theoretical potential of wave energy by region in 2010 by Mork et al.
It can be seen that Asia and Australasia have the highest potential of wave energy with North and South America also receiving a generous amount. It is noted that these estimates do not account for geographical, technical or economic constraints and that the amount of energy harnessed from these amounts would be much less. Additional studies are mentioned where a Pelc and Fujita (2002) estimates 5.5 PWh/yr while a Thorpe (1999) and Cornett (2008) approximate 2 PWh/yr. Although these numbers are vastly different, it still paints a picture of just how much wave energy is possibly available as a green source of energy. As technology becomes more available and cost-efficient, the amount of energy harnessing ability will also increase.
As an addition, Alternative Energy Systems by B.K. Hodge, shows a map of the United States representing the Estimated US tidal current and wave energy resource by Thresher (2014).
The figure shows the power available in megawatts (MW), and the energy harvested in terawatt-hours per year (TWh/yr).
Before analyzing current and future wave energy projects, the short history of wave energy will first have to be outlined to gain an understanding of where it all started and where the technology is going.
The first known patent for wave energy dates back to 1799 by a Frenchman named Pierre-Simon Girard. He and his son designed a machine that would harness the oceans wave energy and could power pumps, sawmills and the like. Research and inventions for wave energy then took a major halt with the rise of fossil fuels and the industrializing world. Then in 1910, Bochaux-Praceique constructed a device to light and power his home via wave energy. This would seem to be the first oscillating water column.
The father of modern wave energy converts came around 1940, by the name of Yoshio Masuda in Japan. He developed a navigation buoy that was powered by wave energy by use of an air turbine. This device was later recognized as a floating oscillating water column.
Then in 1973, the oil crisis pushed further research into developing ocean wave energy devices. As stated in Interesting Facts about the History of Wave Energy, ‘Researchers from the University of Edinburgh, Norwegian Institute of Technology, US Naval Academy, Bristol University, University of Lancaster, and MIT joined forces and developed the Edinburgh Duck, a device that could harness the power of ocean waves and convert it into energy.’. Allegedly in control tests, the Duck could stop 90% of wave motion and convert 90% of that to electricity giving 81% efficiency.
The progression of wave power was then halted once more when the oil crisis ended in the 1980’s where the UK’s and the world’s first major wave program slowed its research. Norway however, in 1985 established the world’s first wave power station which incorporated two full-sized shoreline oscillating water columns rated at 350 and 500 kW. The UK finally got back up to speed in 1991, installing its first official wave energy plant in Scotland, using a 75 kW oscillating water column.
The rest of the 1990’s had more developments mostly in Europe following support from the government with 2 million ECUs committed to ocean energy development.
The History of Wave Technology will be continued under current developments starting in the 2000’s.
When the 2000’s began, big players in the Ocean Energy Market entered the field. Ocean Power Technologies in the United States and UK’s Pelamis.
Pelamis was the first company to establish a wave energy system connected to the grid off the coast of Portugal in Aguacadora in 2004. They were the first to install a wave energy array of Attenuators (described above), installing 3 Pelamis devices with a total of 2.25 MW in 2008. Unfortunately, this project was decommissioned shortly after due to maintenance issues and bankruptcy of the company from the global economic crisis.
In Sweden, the world’s largest commercial wave energy array is being developed. Starting in 2015 in Sotenas, the array currently delivers 1.05 MW of capacity with 32 point absorber buoys and with a plan to max out at 10 MW being discussed in 2017-18.
The United States currently is a bit behind on the wave energy scale. A commercial wave park in Reedsport, Oregon was stopped in 2013 due to legal and technical problems. The park was supposed to have ten, 1.5MW PowerBuoys installed in Spring 2013. The States does have a device being tested in Kaneohe Bay Oahu, Hawaii called ‘Azura’. The Azura is the first wave energy device in the United States to be supplying energy to the grid. The Azura is 45-tons located at a depth of 30 meters and can generate 20 kilowatts of power.
There are a few other small projects around the globe located in Australia, UK, Ghana, Spain, etc. This does, however, paint a picture of how behind wave energy technology is from being commercially implemented around the globe. Even with all of the potential energy generation the waves poses, wave energy converters are not being utilized. The next section will cover why this may be.
The advantages of Wave energy are straightforward and easily recognizable. There is an extremely large supply of ocean wave energy every day, it is a clean source of energy with little to no CO2 emissions, has the highest energy density compared to other renewable resources being 800 times more dense than air, it has negligible land use, there are a variety of ways to harness wave energy, and waves are easily predictable and consistent. However, the reason than wave energy has not been utilized around the globe is because currently, the disadvantages of the technology outweigh the advantages. It can be stated first that the technology has many technological and socio-economic hurdles to overcome. These challenges will be addressed in this section.
The first issue is the need for advanced materials for wave energy converters. First and foremost, wave energy devices are exposed to very hard environmental conditions from the salinity of the water, storms, strong tides etc. The need for advanced materials to counteract this problem is prominent. Stated in World Energy Resources – Marine Energy, 2016 ‘ deployment and utilization of materials other than steel for the structure and prime mover, such as steel reinforced concrete, rubber or fiber reinforced polymer to provide advantages such as weight savings. Innovative device coatings will also help protect materials from corrosion, water absorption, cavitation, etc. in the marine environment.’. This in itself provides an issue which is, the more advanced the materials, the higher the cost. It will take research and development to find a balance between cost of material and life optimization.
The next issue to be addressed is the high installation, maintenance and operation costs. For any of the offshore or even nearshore wave energy converters, the cost of installing the device especially with a seafloor foundation is huge and time costly. The speed of installation needs to be increased with advanced foundation installation techniques. Similar techniques need to be found to reduce the cost and time of retrieval for operation and maintenance.
One of the largest issues with wave energy converters is the need for higher power outputs and efficiencies. Different structural configurations need to be tested to maximize the amount of energy that can be transferred by the wave. This will take time and is a key factor as to why many (WEC) have not been deployed where power output of the devices is more important than scaling of current technology.
Another major issue that would need to be addressed is that wave technology can become a hindrance to private and commercial vessels. Wave energy converters are located near the coasts where a large portion of the population lives. Installation of these devices would have to take into account the extensive ports that have cargo ships, cruise ships, recreational vehicles and beachgoers.
The issues listed above are the largest hurdles that wave energy must face. The remaining problems include: only suitable for certain locations, the effect on the marine ecosystem, the need for a reduced cost of electrical infrastructure such as cable installation and operation, and mostly just innovations in all associated technology.
The road to getting wave energy or ocean technology in general, caught up with other forms of renewable energy resources will be speculated below.
Future of Wave Energy
Wave Energy and Marine Energy specifically, is behind in progression of utilization compared to other forms of renewable energy technologies. As stated in World Energy Resources – Marine Energy, 2016 ‘ The relative immaturity of wave energy technology can be illustrated by the lack of convergence around one single device design, with R&D funding split between several different device types’. A graph utilized in the World Energy Resource document can be seen below to show the distribution of R&D by Wave Energy Type.
With so many different technologies, not one device can be put ahead of the others and focused on for development; thus slowing down progression.
The budget for ocean energy technologies is also a causing factor for its lacking development. Between 1974 and 2013 the global public budget for ocean energy RD&D was $1.6BN (World Energy Resources 2016). This was significantly less than most other renewable technologies: Solar (US$23.3BN), biofuels (US$14.1BN), wind (US$6.8BN), geothermal (US$6.2BN), etc. (World Energy Resources 2016). Wave Energy and its counter technologies need more government and private investment to push the technology further into development. It seems that the lack of investments or maybe more specifically, the focus of investments in Solar, Wind, biofuels, etc. has halted the progression of wave energy technology.
The International Energy Agency mentions that Ocean Energy Systems established ‘ global maps of ocean energy resources: wave, tidal, thermal, and salinity gradients. These maps, together with technological developments, cost reductions, synergies with other industries, markets, policies, and challenges for ocean energy have been synthesized in An International Vision for Ocean Energy.’ By 2050, OES has a goal of installing 337 GW of capacity, creating 1.2 million jobs and reducing 1 billion tonnes of CO2 emissions. For this to be feasible, more money will need to be invested in the ocean energy sector.
Osmotic Power Generation
Commonly referred to as ‘Blue Energy,’ this energy generation type uses a process called Pressure-retarded osmosis to generate hydraulic power which is then converted into electricity.
As one of the most commonly overlooked types of marine power generation, Osmotic power typically falls into the ‘other’ category as it does not need to be placed on or near a large body of water. Defined as ‘A spontaneous movement of solvent molecules through a semipermeable membrane from the region of lower solute concentration into a region of higher solute concentration, in an effort to equalize concentrations on both ends of the membrane’ (Marketwatch). The energy generation process, however is still relatively simple. By harnessing this natural flow of water and creating a higher pressure differential, energy is created.
Reverse osmosis, the more common of the two, is most familiar in the field of salt water desalination. By pushing saline water through the correct filter or membrane, the user is able to trap the salt water and produce purified water. On the opposite end of the spectrum, regular osmosis is still a relatively simple process. When used to produce electricity or the energy to do so, two ‘fuels’ are needed. Saline water and purified fresh water. These fluids are then placed in a two-part tank, or a long stretch of connected piping, that is separated by a thin film. This semipermeable film acts as the gatekeeper between the two fluids and will only allow fresh water to pass. Relying upon water’s natural tendency to dilute itself, the freshwater side will flow into and mix with the saline water of the other side. Energy is created from this movement because of the higher resultant pressure on the saltwater side. Shown below is an image detailing the rise in pressure after the pure water dilutes to the salt water in the center of the tank.
Capturing this water movement through the pressure change, the high-pressure salt water will flow at an increased rate into a turbine generator to then produce electricity (PowerMag). Another way to visualize the osmosis process is to think of the effect salt water has on the body. When a person drinks a glass of salt water, any fresh water within the body will naturally flow to the stomach (through osmotic diffusion) to help dilute the solution. The apparent lack of dehydration explains the sickly feelings a person typically gets when it is consumed.
Statkraft Osmotic Power Plant
Currently, there is only one functional osmotic power plant in the globe. Located in Tofte, Norway, the Statkraft Osmotic Power Plant is the first of its kind. Built by the Statkraft Group (Europe’s largest renewable energy company with a revenue of nearly 53 billion NOK), the facility was designed with research in mind. The capacity of the facility is extremely small, with the maximum power potential topping out at a mere 10 kW. Opening in November of 2009, the facility is fully funded by Statkraft with the hope that the technology can progress enough to build a larger, more functional plant in its place. The plant’s water circulation layout, as shown below, was designed with flexibility in mind in order to accommodate the ever-adapting needs of the researchers.
In order for the system to be efficient, let alone produce electricity, the total energy usage of both the freshwater pumps and the saltwater pumps needed to consume less than the turbine generated. Achieving this, the most current plant layout was able to produce about 4 kW of electricity, much lower than their original estimations (PowerMag). The plant consumes salt water at a rate of 20 liters per second and fresh water at a rate of 10 liters per second. When at the membrane, the salt water is subject to 60% to 85% of the total osmotic pressure. Overall, the project was designed to ‘boil the kettle’ (New Atlas) to help other companies get a foot in the door with this technology.
Globally, all countries with access to both fresh and saltwater are able to produce electricity using pressure retarded osmosis at the same rate. Therefore, the efficiency lies in the technology. With today’s technology, these systems can achieve efficiencies as high as 56% (Environmental Science & Technology). Spread across the globe, it is estimated that 2,000 TWh per year can be harvested from this type of generation (ScienceDirect).
Although most research and implementation of osmotic power falls in the 21st century, the earliest mention of its potential to generate electricity dates back to the year 1954 when R.E. Pattle wrote a dissertation suggesting that the conversion of fresh and saltwater produces a profound energy that has yet to be harnessed. He later goes on to say that ‘When a volume V of a pure solvent mixes irreversibly with a much larger volume of a solution, the osmotic pressure of which is P, the free energy lost is equal to PV. The osmotic pressure of seawater is about 20 atmospheres, so that when a river mixes with the seas, free energy equal to the obtainable from a waterfall 680 ft. high is lost,’ (Pattle). Although these words are not entirely accurate, he does lay the foundation for a technology that will take over fifty years to come into fruition. Later on, in the 1970’s, an Israeli professor by the name of Sidney Loeb finally came up with an idea how to harness this energy. He began to design the first semipermeable membrane used to generate electricity through PRO.
Statkraft, after working for four years on the plant in Norway, announced in 2013 that they would no longer pursue it due to stale returns. In addition, the future plant that was designed to produce 1-2 mW was also canceled (Ridden, 2009). From their research, it was found that the materials needed to produce the membrane were neither cheap enough or efficient enough to continue with a large-scale facility. When calculated, their current membrane had an energy density of about 1W/m^2 while an economically viable membrane would require nearly 5W/m^2 of output. With that being said, their initial goal was still complete. Other companies saw the work they put forth and are now able to continue the work that they started in hopes of finding a viable solution to Statkraft’s problems. If created properly, Osmotic power generation or Blue Energy could have the ability to become a leading competitor in renewable energy.
Ocean Current Energy
Ocean current energy is the energy captured from Earth’s natural currents in the ocean. Energy is harnessed through a generator and sent back to shore through underwater cables to produce electricity. Currents in the ocean are affected by wind, water salinity, temperature, the topography of the ocean floor, and the earth’s rotation. These complex currents will flow relatively constant in one direction, unlike tides. Currents have a large advantage over tides because they are more predictable and are always flowing. This means there is a great potential to capture this constant flow to produce nonstop energy. These currents can turn a turbine or move across an oscillating hydrofoil to capture the energy. It is estimated that if 1/1,000th of the potential energy from the Gulf was captured, it could supply the entire state of Florida with 35% of its energy needs. Ocean current energy is one of the least developed and unknown types of marine power because there has not been enough research or money that has been put into the development of current energy. Due to the lack of research and funds, there are currently no commercial systems used today to generate power.
Considering marine power is a fairly new technology, there is not much history behind them. Marine current energy was first brought up in the mid-1970s after the oil crisis. Conceptual designs were being developed by the UK, but there was never anything implemented. Later in the 80’s, countries such as Canada and Japan were getting involved as well by doing some small research projects. The first step was to find locations where current energy would even be possible. Locations off the coast of the UK showed that the potential of current energy could generate up to 58 TWh/year, which is 19% of the UK’s demand. The largest hurdle is that the technology is still just too far behind. Funding is limited so research is lacking. Today, there is just not enough evidence in the technology to prove that these will be feasible or economical. Currently, there are still not any commercial current energy farms, but there are some prototypes being researched.
Ocean Current Turbines
A current turbine prototype has been built by a team of researchers at Okinawa Institute of Science and Technology Graduate University (OIST). They have placed these turbines in the Kuroshio Current, which is located along Japan’s coast. These turbines are very similar to wind turbines that are seen today. Since water is 800 times denser than air; when water hits these turbines around 12 mph, that is comparable to 110 mph winds. This means that current energy has the potential to be ten times more effective than wind energy. The main goal of these turbines was to keep it simple with as little parts as possible. This will reduce the amount of maintenance that needs to be done. Since these are placed far off the coast in the ocean, it can be difficult to conduct maintenance on these machines. The design consists of a float, generator, counterweight, and three blades. Below is a picture of the prototype built by OIST in 2016.
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