Marine energy or marine power (also sometimes referred to as ocean energy, ocean power, or marine and hydrokinetic energy) refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in the world's oceans creates a vast store of kinetic energy, or energy in motion. Some of this energy can be harnessed to generate electricity to power homes, transport and industries.
The term marine energy encompasses both wave power i.e. power from surface waves, and tidal power i.e. obtained from the kinetic energy of large bodies of moving water. Offshore wind power is not a form of marine energy, as wind power is derived from the wind, even if the wind turbines are placed over water.
The oceans
have a tremendous amount of energy and are close to many if not most
concentrated populations. Ocean energy has the potential of providing a
substantial amount of new renewable energy around the world.
Global potential
There is the potential to develop 20,000–80,000 terawatt-hours
per year (TWh/y) of electricity generated by changes in ocean
temperatures, salt content, movements of tides, currents, waves and
swells
Indonesia,
as an archipelagic country that is three quarters ocean, has 49 GW
recognized potential ocean energy and has 727 GW theoretical potential
ocean energy.
Forms of ocean energy
The oceans represent a vast and largely untapped source of energy in the form of surface waves, fluid flow, salinity gradients, and thermal differences.
Marine and Hydrokinetic (MHK) or marine energy development in
U.S. and international waters includes projects using the following
devices:
Wave power converters in open coastal areas with significant waves;
Strong ocean currents are generated from a combination of temperature, wind, salinity, bathymetry,
and the rotation of the Earth. The Sun acts as the primary driving
force, causing winds and temperature differences. Because there are only
small fluctuations in current speed and stream location with no changes
in direction, ocean currents may be suitable locations for deploying
energy extraction devices such as turbines.
Ocean currents are instrumental in determining the climate
in many regions around the world. While little is known about the
effects of removing ocean current energy, the impacts of removing
current energy on the farfield environment
may be a significant environmental concern. The typical turbine issues
with blade strike, entanglement of marine organisms, and acoustic
effects still exists; however, these may be magnified due to the
presence of more diverse populations of marine organisms using ocean
currents for migration purposes. Locations can be further offshore and
therefore require longer power cables that could affect the marine
environment with electromagnetic output.
At the mouth of rivers where fresh water mixes with salt water,
energy associated with the salinity gradient can be harnessed using
pressure-retarded reverse osmosis process and associated conversion
technologies. Another system is based on using freshwater upwelling
through a turbine immersed in seawater, and one involving
electrochemical reactions is also in development.
Significant research took place from 1975 to 1985 and gave
various results regarding the economy of PRO and RED plants. It is
important to note that small-scale investigations into salinity power
production take place in other countries like Japan, Israel, and the
United States. In Europe the research is concentrated in Norway and the
Netherlands, in both places small pilots are tested. Salinity gradient
energy is the energy available from the difference in salt concentration
between freshwater with saltwater. This energy source is not easy to
understand, as it is not directly occurring in nature in the form of
heat, waterfalls, wind, waves, or radiation.
Water typically varies in temperature from the surface warmed by
direct sunlight to greater depths where sunlight cannot penetrate. This
differential is greatest in tropical
waters, making this technology most applicable in water locations. A
fluid is often vaporized to drive a turbine that may generate
electricity or produce desalinized water. Systems may be either open-cycle, closed-cycle, or hybrid.
Solar energy from the Sun creates temperature differentials that
result in wind. The interaction between wind and the surface of water
creates waves, which are larger when there is a greater distance for
them to build up. Wave energy potential is greatest between 30° and 60°
latitude in both hemispheres on the west coast because of the global
direction of wind. When evaluating wave energy as a technology type, it
is important to distinguish between the four most common approaches: point absorber buoys, surface attenuators, oscillating water columns, and overtopping devices.
The wave energy sector is reaching a significant milestone in the
development of the industry, with positive steps towards commercial
viability being taken. The more advanced device developers are now
progressing beyond single unit demonstration devices and are proceeding
to array development and multi-megawatt projects.
The backing of major utility companies is now manifesting itself
through partnerships within the development process, unlocking further
investment and, in some cases, international co-operation.
At a simplified level, wave energy technology can be located
near-shore and offshore. Wave energy converters can also be designed for
operation in specific water depth conditions: deep water, intermediate
water or shallow water. The fundamental device design will be dependent
on the location of the device and the intended resource characteristics.
Environmental effects
Common environmental concerns associated with marine energy developments include:
the risk of marine mammals and fish being struck by tidal turbine blades
the effects of EMF and underwater noise emitted from operating marine energy devices
the physical presence of marine energy projects and their potential to alter the behavior of marine mammals, fish, and seabirds with attraction or avoidance
the potential effect on nearfield and farfield marine environment and processes such as sediment transport and water quality
The Tethys database provides access to scientific literature and general information on the potential environmental effects of marine energy.
Waves are generated primarily by wind passing over the sea's
surface and also by tidal forces, temperature variations, and other
factors. As long as the waves propagate slower than the wind speed just
above, energy is transferred from the wind to the waves. Air pressure
differences between the windward and leeward sides of a wave crest and surface friction from the wind cause shear stress and wave growth.
Wave power as a descriptive term is different than tidal power,
which seeks to primarily capture the energy of the current caused by
the gravitational pull of the Sun and Moon. However, wave power and
tidal power are not fundamentally distinct and have significant
cross-over in technology and implementation. Other forces can create currents, including breaking waves, wind, the Coriolis effect, cabbeling, and temperature and salinity differences.
As of 2022, wave power is not widely employed for commercial
applications, after a long series of trial projects. Attempts to use
this energy began in 1890 or earlier, mainly due to its high power density.
Just below the ocean's water surface the wave energy flow, in
time-average, is typically five times denser than the wind energy flow
20 m above the sea surface, and 10 to 30 times denser than the solar
energy flow.
In 2000 the world's first commercial wave power device, the Islay LIMPET was installed on the coast of Islay in Scotland and connected to the UK national grid. In 2008, the first experimental multi-generator wave farm was opened in Portugal at the Aguçadoura wave park. Both projects have since ended.
Wave energy converters can be classified based on their working principle as either:
oscillating water columns (with air turbine)
oscillating bodies (with hydroelectric motor, hydraulic turbine, linear electrical generator)
The first known patent to extract energy from ocean waves was in 1799, filed in Paris by Pierre-Simon Girard and his son. An early device was constructed around 1910 by Bochaux-Praceique to power his house in Royan, France. It appears that this was the first oscillating water-column type of wave-energy device. From 1855 to 1973 there were 340 patents filed in the UK alone.
Modern pursuit of wave energy was pioneered by Yoshio Masuda's 1940s experiments.
He tested various concepts, constructing hundreds of units used to
power navigation lights. Among these was the concept of extracting power
from the angular motion at the joints of an articulated raft, which
Masuda proposed in the 1950s.
Salter's 1974 invention became known as Salter's duck or nodding duck, officially the Edinburgh Duck. In small-scale tests, the Duck's curved cam-like body can stop 90% of wave motion and can convert 90% of that to electricity, giving 81% efficiency. In the 1980s, several other first-generation prototypes were tested, but as oil prices ebbed, wave-energy funding shrank. Climate change later reenergized the field.
The world's first wave energy test facility was established in Orkney, Scotland in 2003 to kick-start the development of a wave and tidal energy industry. The European Marine Energy Centre(EMEC) has supported the deployment of more wave and tidal energy devices than any other single site.
Subsequent to its establishment test facilities occurred also in many
other countries around the world, providing services and infrastructure
for device testing.
The £10 million Saltire prize challenge was to be awarded to the
first to be able to generate 100 GWh from wave power over a continuous
two-year period by 2017 (about 5.7 MW average). The prize was never awarded. A 2017 study by Strathclyde University and Imperial College
focused on the failure to develop "market ready" wave energy devices –
despite a UK government investment of over £200 million over 15 years.
Public bodies have continued and in many countries stepped up the
research and development funding for wave energy during the 2010s.
This includes both EU, US and UK where the annual allocation has
typically been in the range 5-50 million USD. Combined with private funding, this has led to a large number of ongoing wave energy projects.
Like most fluid motion, the interaction between ocean waves and
energy converters is a high-order nonlinear phenomenon. It is described
using the incompressible Navier-Stokes equations
where is the fluid velocity, is the pressure, the density, the viscosity, and the net external force on each fluid particle (typically gravity). Under typical conditions, however, the movement of waves is described by Airy wave theory, which posits that
In an ideal flow, the viscosity is negligible and the only external force acting on the fluid is the earth gravity . In those circumstances, the Navier-Stokes equations reduces to
When considering small amplitude waves and motions, the quadratic term can be neglected, giving the linear Bernoulli equation,
and third Airy assumptions then imply
These constraints entirely determine sinusoidal wave solutions of the form
where determines the wavenumber of the solution and and are determined by the boundary constraints (and ). Specifically,
The surface elevation can then be simply derived as
a plane wave progressing along the x-axis direction.
Consequences
Oscillatory motion is highest at the surface and diminishes exponentially with depth. However, for standing waves (clapotis) near a reflecting coast, wave energy is also present as pressure oscillations at great depth, producing microseisms. Pressure fluctuations at greater depth are too small to be interesting for wave power conversion.
The behavior of Airy waves offers two interesting regimes: water
deeper than half the wavelength, as is common in the sea and ocean, and
shallow water, with wavelengths larger than about twenty times the water
depth. Deep waves are dispersionful:
Waves of long wavelengths propagate faster and tend to outpace those
with shorter wavelengths. Deep-water group velocity is half the phase velocity. Shallow water waves are dispersionless: group velocity is equal to phase velocity, and wavetrains propagate undisturbed.
The following table summarizes the behavior of waves in the various regimes:
Airy gravity waves on the surface of deep water, shallow water, or intermediate depth
Wave power formula
In deep water where the water depth is larger than half the wavelength, the wave energy flux is
with P the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, Te the wave energy period, ρ the water density and g the acceleration by gravity. The above formula states that wave power is proportional to the wave energy period and to the square
of the wave height. When the significant wave height is given in
metres, and the wave period in seconds, the result is the wave power in
kilowatts (kW) per metre of wavefront length.
For example, consider moderate ocean swells, in deep water, a few
km off a coastline, with a wave height of 3 m and a wave energy period
of 8 s. Solving for power produces
or 36 kilowatts of power potential per meter of wave crest.
In major storms, the largest offshore sea states have significant
wave height of about 15 meters and energy period of about 15 seconds.
According to the above formula, such waves carry about 1.7 MW of power
across each meter of wavefront.
An effective wave power device captures a significant portion of
the wave energy flux. As a result, wave heights diminish in the region
behind the device.
Energy and energy flux
In a sea state, the meanenergy density per unit area of gravity waves on the water surface is proportional to the wave height squared, according to linear wave theory:
where E is the mean wave energy density per unit horizontal area (J/m2), the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy, both contributing half to the wave energy density E, as can be expected from the equipartition theorem.
The waves propagate on the surface, where crests travel with the
phase velocity while the energy is transported horizontally with the group velocity. The mean transport rate of the wave energy through a vertical plane of unit width, parallel to a wave crest, is the energy flux (or wave power, not to be confused with the output produced by a device), and is equal to:
with cg the group velocity (m/s).
Due to the dispersion relation for waves under gravity, the group velocity depends on the wavelength λ, or equivalently, on the wave periodT.
Wave height
is determined by wind speed, the length of time the wind has been
blowing, fetch (the distance over which the wind excites the waves) and
by the bathymetry
(which can focus or disperse the energy of the waves). A given wind
speed has a matching practical limit over which time or distance do not
increase wave size. At this limit the waves are said to be "fully
developed". In general, larger waves are more powerful but wave power is
also determined by wavelength, water density, water depth and acceleration of gravity.
This
device floats on the surface, held in place by cables connected to the
seabed. The point-absorber has a device width much smaller than the
incoming wavelength λ. Energy is absorbed by radiating a wave with
destructive interference to the incoming waves. Buoys use the swells'
rise and fall to generate electricity directly via linear generators, generators driven by mechanical linear-to-rotary converters, or hydraulic pumps. Energy extracted from waves may affect the shoreline, implying that sites should remain well offshore.
Surface attenuator
These
devices use multiple floating segments connected to one another. They
are oriented perpendicular to incoming waves. A flexing motion is
created by swells, and that motion drives hydraulic pumps to generate
electricity.
Oscillating wave surge converter
These devices typically have one end fixed to a structure or the seabed while the other end is free to move. Energy
is collected from the relative motion of the body compared to the fixed
point. Converters often come in the form of floats, flaps, or
membranes. Some designs incorporate parabolic reflectors to focus energy at the point of capture. These systems capture energy from the rise and fall of waves.
Oscillating water column
Oscillating water column
devices can be located onshore or offshore. Swells compress air in an
internal chamber, forcing air through a turbine to create electricity. Significant noise is produced as air flows through the turbines, potentially affecting nearby birds and marine organisms. Marine life could possibly become trapped or entangled within the air chamber. It draws energy from the entire water column.
Overtopping device
Overtopping
devices are long structures that use wave velocity to fill a reservoir
to a greater water level than the surrounding ocean. The potential
energy in the reservoir height is captured with low-head turbines.
Devices can be on- or offshore.
Submerged pressure differential
Submerged pressure differential based converters
use flexible (typically reinforced rubber) membranes to extract wave
energy. These converters use the difference in pressure at different
locations below a wave to produce a pressure difference within a closed
power take-off hydraulic system. This pressure difference is usually
used to produce flow, which drives a turbine and electrical generator.
Submerged pressure differential converters typically use flexible
membranes as the working surface between the water and the power
take-off. Membranes are pliant and low mass, which can strengthen
coupling with the wave's energy. Their pliancy allows large changes in
the geometry of the working surface, which can be used to tune the
converter for specific wave conditions and to protect it from excessive
loads in extreme conditions.
A submerged converter may be positioned either on the seafloor or
in midwater. In both cases, the converter is protected from water
impact loads which can occur at the free surface. Wave loads also diminish in non-linear
proportion to the distance below the free surface. This means that by
optimizing depth, protection from extreme loads and access to wave
energy can be balanced.
Floating in-air converters
Floating in-air converters potentially offer increased reliability
because the device is located above the water, which also eases
inspection and maintenance. Examples of different concepts of floating
in-air converters include:
roll damping energy extraction systems with turbines in compartments containing sloshing water
Artificial reef accumulation near fixed installations
Potential disuption to roosting sites
Potential
Wave energy's worldwide theoretical potential has been estimated to be greater than 2 TW.
Locations with the most potential for wave power include the western
seaboard of Europe, the northern coast of the UK, and the Pacific
coastlines of North and South America, Southern Africa, Australia, and
New Zealand. The north and south temperate zones have the best sites for capturing wave power. The prevailing westerlies in these zones blow strongest in winter.
The National Renewable Energy Laboratory
(NREL) estimated the theoretical wave energy potential for various
countries. It estimated that the US' potential was equivalent to 1170
TWh per year or almost 1/3 of the country's electricity consumption. The Alaska coastline accounted for ~50% of the total.
Note that the technical and economical potential will be lower than the given values for the theoretical potential.
Environmental impacts must be addressed.
Socio-economic challenges include the displacement of commercial and
recreational fishermen, and may present navigation hazards. Supporting infrastructure, such as grid connections, must be provided.
Commercial WECs have not always been successful. In 2019, for example,
Seabased Industries AB in Sweden was liquidated due to "extensive
challenges in recent years, both practical and financial".
Current wave power generation technology is subject to many technical limitations.
These limitations stem from the complex and dynamic nature of ocean
waves, which require robust and efficient technology to capture the
energy. Challenges include designing and building wave energy devices
that can withstand the corrosive effects of saltwater, harsh weather
conditions, and extreme wave forces.
Additionally, optimizing the performance and efficiency of wave energy
converters, such as oscillating water column (OWC) devices, point
absorbers, and overtopping devices, requires overcoming engineering
complexities related to the dynamic and variable nature of waves.
Furthermore, developing effective mooring and anchoring systems to keep
wave energy devices in place in the harsh ocean environment, and
developing reliable and efficient power take-off mechanisms to convert
the captured wave energy into electricity, are also technical challenges
in wave power generation.
As the wave energy dissipation by a submerged flexible mound breakwater
is greater than that of a rigid submerged structure, greater wave
energy dissipation is expected due to highly deformed shape of the
structure.
Wave farms
A
wave farm (wave power farm or wave energy park) is a group of colocated
wave energy devices. The devices interact hydrodynamically and
electrically, according to the number of machines, spacing and layout,
wave climate, coastal and benthic geometry, and control strategies. The
design process is a multi-optimization problem seeking high power production, low costs and limited power fluctuations.
U.S. patent 1,930,958 — 1932 Wave Motor
- Parsons Ocean Power Plant - Herring Cove Nova Scotia - March 1925.
The world's first commercial plant to convert ocean wave energy into
electrical power. Designer - Osborne Havelock Parsons - born in 1873
Petitcodiac, New Brunswick.
A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass. By contrast, a land breeze or offshore breeze is any wind that blows from a landmass toward or onto a large body of water. The term offshore wind may refer to any wind over open water. Sea breezes and land breezes are both important factors in coastal regions' prevailing winds.
Sea breeze and land breeze develop due to differences in air pressure created by the differing heat capacities of water and dry land. As such, Sea breezes and land breezes are more localised than prevailing winds. Since land heats up much faster than water under solar radiation, a sea breeze is a common occurrence along coasts after sunrise.
On the other hand, dry land also cools faster than water without solar
radiation, so the wind instead flows from the land towards the sea when
the sea breeze dissipates after sunset.
The land breeze at nighttime is usually shallower than the sea
breeze in daytime. Unlike the daytime sea breeze, which is driven by
convection, the nighttime land breeze is driven by convergence.
Cause
Sea breeze
The sea has a greater heat capacity than land, so the surface of the sea warms up more slowly than the surface of the land. As the temperature of the surface of the land rises, the land heats the air above it by convection. The hypsometric equation
states that the hydrostatic pressure depends on the temperature. Thus,
the hydrostatic pressure over the land decreases less at higher
altitude. As the air above the coast has a relatively higher pressure,
it starts moving towards the sea at high altitude. This creates an
inverse airflow near the ground.
The strength of the sea breeze is directly proportional to the
temperature difference between the land and the sea. If a strong
offshore wind is present (that is, a wind greater than 8 knots
(15 km/h)) and opposing the direction of a possible sea breeze, the sea
breeze is not likely to develop.
Land breeze
At
night, the land cools off faster than the ocean due to differences in
their heat capacity, which forces the dying of the daytime sea breeze as
the temperature of the land approaches that of the ocean. If the land
becomes cooler than the adjacent sea surface temperature,
the air pressure over the water will be lower than that of the land,
setting up a land breeze blowing from the land to the sea, as long as
the environmental surface wind pattern is not strong enough to oppose
it.
Effects
Sea breeze
A sea-breeze front is a weather front created by a sea breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, the front can sometimes trigger thunderstorms.
If the flow aloft is aligned with the direction of the sea breeze,
places experiencing the sea breeze frontal passage will have benign, or
fair, weather for the remainder of the day. At the front warm air
continues to flow upward and cold air continually moves in to replace it
and so the front moves progressively inland. Its speed depends on
whether it is assisted or hampered by the prevailing wind, and the
strength of the thermal contrast between land and sea. At night, the
sea breeze usually changes to a land breeze, due to a reversal of the
same mechanisms.
Thunderstorms caused by powerful sea breeze fronts frequently occur in Florida, a peninsula bounded on the east and west by the Atlantic Ocean and Gulf of Mexico,
respectively. During the wet season, which typically lasts from June
through September/October, any direction that the winds are blowing
would always be off the water, thus making Florida the place most often
struck by lightning in the United States, and one of the most on Earth. These storms can also produce significant hail due to the tremendous updraft it causes in the atmosphere especially during times when the upper atmosphere is cooler such as during the spring or fall.
On calm summer afternoons with little prevailing wind, sea
breezes from both coasts may collide in the middle, creating especially
severe storms down the center of the state. These thunderstorms can
drift towards either the west or east coast depending on the relative
strengths of the sea breezes, and sometimes survive to move out over the
water at night, creating spectacular cloud-to-cloud lightning shows for
hours after sunset. Due to its large size Lake Okeechobee may also contribute to this activity by creating its own lake breeze which collides with the east and west coast sea breezes.
In Cuba similar sea breeze collisions with the northern and southern coasts sometimes lead to storms.
Land breeze
Land
breeze, which consists of cool air coming from the land, pushes the
warmer air upwards over the sea. If there is sufficient moisture and
instability available, the land breeze can cause showers, or even
thunderstorms, over the water. Overnight thunderstorm development
offshore due to the land breeze can be a good predictor for the activity
on land the following day, as long as there are no expected changes to
the weather pattern over the following 12–24 hours. This is mainly
because the strength of the land breeze is weaker than the sea breeze. The land breeze will die once the land warms up again the next morning.
Utilisation
Wind farms are often situated near a coast to take advantage of the normal daily fluctuations of wind speed resulting from sea or land breezes. While many onshore wind farms and offshore wind farms do not rely on these winds, a nearshore wind farm
is a type of offshore wind farm located on shallow coastal waters to
take advantage of both sea and land breezes. For practical reasons,
other offshore wind farms are situated further out to sea and rely on
prevailing winds rather than sea breezes.