Axial flow water turbine

The Axial flow turbines are used for Low head and relatively high flow rates in hydroelectric plants. Consequently they are suitable for tidal energy barrages or wave energy converters using overtopping. There are many types of axial flow turbines as Tubular, Rim, Bulb etc. The type depends to the arrangement of the electrical generator. The axial flow water turbines could be equipped with adjustable runner blades.

Absorbed [wave] power

The power which an oscillating system removes from the waves.

Absorption width

The same as capture width. A measure for a wave power device's ability to capture power from a wave. The ratio between absorbed power and the wave power level.


A device for storing energy for long or short periods and which can release the stored energy in the same form as it was supplied.

Added mass

The hydrodynamic forces due to waves on an immersed object can be expressed in terms of two complex components: one in phase with the acceleration and one in phase with the velocity of the device. The force in phase with the acceleration can be expressed in terms of an extra point mass fixed to the device - this is known as the added mass. The force in phase with the velocity of the device can be expressed in terms of a velocity force as an applied damping - this is known as the hydrodynamic damping.


The maximum extent or magnitude of a vibration or other oscillating phenomenon, measured from the equilibrium position or average value.

Angular frequency

Angular frequency ω (also referred to by the terms angular speed, radial frequency, and radian frequency) is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity.


An arrangement of similar devices. In ocean energy devices this means a number of similar devices arranged into a single group to provide a combined energy output. Also known as a "farm".

Asynchronous generator

Same as induction generator. A type of electric generator that produces alternating current that matches an existing power source, therefore adequate for use in RE applications. Below synchronism the machine acts as an induction motor (consuming electrical energy), at synchronism the machine acts as generator (driven by mechanical power).


A device which is aligned along with the predominant direction of wave incidence.

Availability Reliability Maintainability (ARM)

A formal analysis process conducted to determine the likely availability, reliability and maintainability of a system. This process recognises the connections between all these aspects. This allows a traceable analysis that can be used to predict the life costs of any system.


It is a type of axial flow turbine. A type of turbines for use in a tidal barrage. The bulb turbine is derived from Kaplan turbines with the generator contained in a waterproofed bulb submerged in the flow. The La Rance tidal plant near St Malo on the Brittany coast in France uses bulb turbines.


An anchored floating device. Traditionally these have served as navigation marks or for mooring but now can be incorporated to wave energy devices. They are typically small compared to the incoming wavelengths, thus are a common form of point absorber.

Capture width

The same as absorption width. A measure for a wave power device's ability to capture power from a wave. The ratio between absorbed power and the wave power level.

Closed-cycle OTEC System

Two basic OTEC system designs have been demonstrated to generate electricity: closed cycle and open cycle. In the closed-cycle OTEC system, warm seawater vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). The vapour expands at moderate pressures and turns a turbine coupled to a generator that produces electricity. The vapour is then condensed in another heat exchanger (condenser) using cold seawater pumped from the ocean's depths through a cold-water pipe. The condensed working fluid is pumped back to the evaporator to repeat the cycle. The working fluid remains in a closed system and circulates continuously.

Conversion efficiency

The conversion efficiency (η) of a device is the proportion of energy converted to a useful form (e.g. Electricity) compared to the total energy available to the device.


An effect whereby a mass moving in a rotating system experiences a force perpendicular to the direction of motion and to the axis of rotation.

Counrerrotating turbine

Air turbine consisting of two Wells Turbines placed close together with their blades rotating in opposite directions. Each behaves like a set of guide vanes for the other.


A mechanism for bringing about a reduction in the amplitude of a vibration or oscillating by extracting energy. In a power plant the damping is the part of the load that acts in phase with the velocity. Damping is usually expressed as the force per unit velocity [N/(m/s)].

Damping coefficient

See hydrodynamic damping.

Darrieus turbine

A cross-axis turbine type common in early wind turbine designs, which may have application in tidal stream energy, and possibly in wave energy. The Darrieus turbine has long, thin blades in the shape of loops connected to the top and bottom of the axle.

Deep water

Water sufficiently deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wave length is considered deepwater.

Deep water waves

A wave in water the depth of which is greater than one-half the wavelength.

Diffraction problem

A body, which is at least partly immersed in water, and which is able to perform oscillations, may interact with waves in various ways: In an incident wave the body may experience an oscillating force even if it is immobilized – this is known the diffraction problem or scattering problem. See also radiation problem.

Directional wave spectrum

A two-dimensional spectrum that shows how the wave energy is distributed between various directions of incidence, in addition to how it is distributed among various frequencies.


The part of a wave energy device that moves in response to the waves. Power is usually taken of from the relative motions of the reactor and displacer.


The retarding force exerted on a body moving relative to a fluid. Drag is usually an energy loss process. It can arise in water movements as friction on wetted surfaces or as vortex shedding from fluid flowing past solid object corners.


With particular application to tidal stream turbines; a duct is a cowling placed around a turbine to enhance the flow through the rotor. The term duct can also apply to the part of oscillating water columns where the air turbine is placed.

Environmental impact assessment (EIA)

The process of identifying, predicting, evaluating and mitigating the biophysical, social, and other relevant effects of development proposals prior to major decisions being taken and commitments made. Environmental assessment is a procedure that ensures that the environmental implications of decisions are taken into account before the decisions are made.

Exciting force

The force which an incident wave exerts on a body, when it is not moving.


The distance moved by a body relative to the instantaneous water surface level.

Far field

In a wave field a boundary distant from a certain point can be defined. The energy passing through this boundary must be in equilibrium. This boundary is known as the far field.

Fast tuning

Fast tuning requires changing characteristics of a device to adjust (or ideally to maximise) the energy capture. Fast tuning means adjustments for each wave or loosely over a period of around 1 second for real-sea waves. Also known as wave-by-wave tuning.

Free surface

In marine energy this typically means the surface of the water.

Guarantee of Origin

An electronic certificate representing 1 MWh of electricity production used for the purpose of proving to final customers the share or quantity of renewable energy that was supplied to them. (Directive 2009/28/ec)

Guide vanes

Guide vanes are used to enhance the performance of the Wells turbine.


Linear oscillatory motion (translation) of an immersed body or structure in the vertical direction.

Horizontal axis turbine

A tidal stream turbine mounted such that it rotates about a horizontal axis, typically running parallel with the flow direction.

Hybrid OTEC System

A hybrid cycle combines the features of both the closed cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine that produces electricity. The steam condenses within the heat exchanger and provides desalinated water.

Hydrodynamic damping

The hydrodynamic forces due to waves on an immersed object can be expressed in terms of two complex components: one in phase with the acceleration and one in phase with the velocity of the device. The force in phase with the acceleration can be expressed in terms of an extra point mass fixed to the device - this is known as the added mass. The force in phase with the velocity of the device can be expressed in terms of a velocity force as an applied damping - this is known as the hydrodynamic damping.

Impulse turbine

Most popular alternative to the Wells turbine for use in OWC plants. Its rotor is basically identical to the rotor of a conventional single-stage steam turbine of axial-flow impulse type. Since the turbine is required to be self-rectifying, instead of a single row of guide vanes there are two rows, placed symmetrically on both sides of the rotor. These two rows of guide vanes are like the mirror image of each other with respect to a plane through the rotor disc.


Latching is a method of control. The aim is to deliberately hold back or "latch" a device the term linear means that all oscillating variables are sinusoidal and proportional (in the case of waves) to wave height. Linear often implies relatively small motions or amplitudes.

Linear theory

Linear theory in wave energy often implies relatively small motions or amplitudes.

Marine current

See tidal stream. Can also relate to a flow of water induced by temperature gradients i.e. the Atlantic conveyor.

Mean wave power

Mean power is the average power in a real (polychromatic) sea. It is usually measured in kilowatts or megawatts.

Monochromatic wave

Wave with the same length and period.

Near field

The region near to a point where calculation of energy flux can be complex and difficult to determine. The near field contrasts with the far field, which is much simpler to describe mathematically.


(1) In beach terminology an indefinite zone extending seaward from the shoreline well beyond the breaker or surf zone. (2) The zone which extends from the SWASH zone to the position marking the start of the offshore zone, typically at water depths of the order of 20 m.

N-th order moment of wave spectrum

A power (n) of the frequency multiplied by the wave energy spectrum and integrated over all frequencies.

Ocean energy

Ocean energy covers a series of emerging technologies that use the power of waves, ocean currents, tides, ocean thermal energy gradient and salinity gradient to generate energy.

Ocean thermal energy conversion

Ocean thermal energy conversion or OTEC, is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient— the fact that the ocean's layers of water have different temperatures—to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power.

Open-cycle OTEC system

Two basic OTEC system designs have been demonstrated to generate electricity: closed cycle and open cycle. In an open-cycle OTEC system, warm seawater is the working fluid. The warm seawater is "flash"-evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kilopascals (kPa). The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam exiting the turbine is condensed by cold seawater pumped from the ocean's depths through a cold-water pipe. If a surface condenser is used in the system, the condensed steam remains separated from the cold seawater and provides a supply of desalinated water.

Oscillating Water Column (OWC)

A wave-power device consisting of an air chamber in which the front wall has an opening so as to let waves enter inside; the wave action makes the water level in the air chamber – known as pneumatic chamber - to oscillate and the air in the chamber is compressed and expanded generating an air flow through an air turbine.


Net movement of water across a selectively permeable membrane driven by a difference in osmotic pressure across the membrane. A selectively permeable membrane allows passage of water, but rejects solute molecules or ions. When freshwater and saltwater is separated by a proper membrane the freshwater will spontaneously migrate through the membrane and dilute the saltwater (the process known as osmosis).

Osmotic pressure

Pressure which, if applied to the more concentrated solution, would prevent transport of water across a semipermeable membrane.

Osmotic pressure difference

Between two bodies of water of different salt concentrations (salinity) there can exist a pressure difference. This is known as the osmotic pressure difference.


see ocean thermal energy conversion.


As used in marine energy: Overtopping is the method by which energy from the sea is extracted by allowing waves to impinge on a structure such that they force water up over that structure thus raising its potential energy (hydraulic head), kinetic energy or both. An overtopping device may or may not include a reservoir to contain the overtopped water. Often axial water turbine-generators are used to convert the hydraulic head to electricity.


See oscillating water column.


See also amplitude. Phase is a relative position of two parts of the same wave or between two waves. It is measured as an angle [either degrees or radians].

Pneumatic chamber

See Oscillating Water Column.

Point absorber

Wave-power device for which the horizontal extension is very small compared to predominant wavelenghts, and for which the ability to absorb (and/or radiate) wave energy is essentially independent of the direction of wave incidence.


Composed of more than one wavelength or frequency.

Pressure Retarded Osmosis (PRO)

It is a salinity gradient energy conversion technique that uses the osmotic pressure difference between seawater and fresh water to pressurize the saline stream, thereby converting the osmotic pressure of seawater into a hydrostatic pressure. Semipermeable membranes are used in this process. Other technique is the Reverse Electrodialysis (RED)

Radiation problem

A body, which is at least partly immersed in water, and which is able to perform oscillations, may interact with waves in various ways: If the body is forced into oscillation by external means, it may generate waves on otherwise still water – this is known the radiation problem. See also diffraction problem.

Regular wave

Wave which is periodic and has relatively long wave crests. The regular wave is closely sinosoidal and monochromatic if it is sufficiently low.

Relief valve

It is usual to equip an oscillating water column wave energy device with a relief valve (or by-pass valve) which allows dissipating excessive pneumatic energy in very energetic seas.

Reverse electrodialysis (RED)

Salinity gradient energy conversion technique in which ion selective membranes are used in alternate chambers with freshwater and seawater, where salt ions migrate by natural diffusion through the membranes and create a low voltage direct current. Other technique is the Pressure Retarded Osmosis (PRO)

Rim turbine

It is a type of axial flow turbine. In rim turbines the generator is mounted on the barrage at right angles to turbine blades (this turbine is used in Annapolis Royal in Nova Scotia).


Root mean square. Often used in marine energy to express characteristic values of pressure, water surface level or other physical entities of a given period of time.


Rotary oscillatory motion around a horizontal axis in the direction of longest extension of the immersed body, or alternatively, in the direction of wave incidence.

Salinity gradient

Energy can be extracted from the sea where large changes or salinity gradients exist. A semi-permeable membrane is placed between the two bodies of water. Slowly the less salty water moves into the salty water by osmosis.

Salinity Gradient Energy

Energy that can be captured by exploiting the pressure difference at the boundary between freshwater and saltwater.

Scatter diagram

Also known as a joint probability distribution. The scatter diagram is a table that shows the frequency of occurrence of different sea states specified by the significant wave height and energy period (often in parts per thousand).

Sea trial

Same as field trials. Last phase of testing of a concept, typically in prototype stage and scale 1:1 to 1:4. Focus lies not only on conversion efficiency or other limited issues, but on the seaworthiness in general, including mooring issues, survivability, maintenance and other practical aspects.

Selectively permeable membrane

see semipermeable membrane

Semipermeable membrane

Also termed a selectively permeable membrane, it is amembrane which retains the salt ions but allows water through. It is used to extract the power from Salinity Gradient with the Pressure Retarded Osmosis (PRO) process.

Shallow water

(1) Commonly, water of such a depth that surface waves are noticeably affected by bottom topography. It is customary to consider water of depths less than one-half the surface wavelength as shallow water. (2) More strictly, in hydrodynamics with regard to progressive gravity waves, water in which the depth is less than 1/25 the wavelength.


The influence of the seabed on wave behaviour. Manifested as a reduction in wave speed, a shortening in wave length and an increase in wave height.

Significant wave height

The average height of the one-third highest waves of a given wave group or sample. It is usually approximately equal to 4 times the square root of the zero order moment of wave energy spectrum (see spectral moment).


A measure of a device's ability to remain intact and operational in extreme environmental conditions.

Survival mode

An operation mode for a device that reduces the likelihood of damage being sustained during extreme/uncommon environmental conditions such as storms.


Wave that has propagated out from the region of wind generation.

Tank testing

Verification of hydrodynamic properties of a physical scale model of the device/structure under laboratory conditions. Typical sale factors range 10 to 100.


Line absorber which is aligned perpendicularly to the predominant direction of wave incidence.

Tidal barrage

Tidal barrage works in a similar way to that of a hydroelectric scheme, except that the dam is much bigger and spans a river estuary. A hard barrier is placed at a strategic point in an estuary with a high tidal range, thus creating an impoundment upstream of the barrage in conjunction with the banks of the estuary.

Tidal current

The rise and fall of the tides create horizontal movements of water. Usually these are of fairly low velocity, but local topography can greatly magnify them, for example in the straits between islands.

Tidal energy

The most notable ways to extract electrical energy from them tides are: a) tidal barrages and b) tidal stream turbines.

Tidal lagoon

Offshore tidal impoundment, or ‘tidal lagoon’ is a completely artificial impoundment that would be constructed in shallow water areas with a high tidal range. See also tidal barrage.

Tidal range

The vertical distance between the high and low tide.

Tidal range resource

The tidal range resource refers to the ‘gravitational potential energy’ that is created as a result of impounding a large volume of water on the high tide. This water is then passed through low-head turbines once a height difference is created on either side of the impoundment, generating electricity. There are two principal concepts for the design and placement of a tidal impoundment: Tidal barrage and tidal lagoon.

Tidal resource

There are two quite distinct categories of tidal resource: tidal stream and tidal range. The tidal stream resource is the kinetic energy contained in fast-flowing tidal currents, which are generally found in constrained channels. The tidal range resource refers to the gravitational potential energy that can be found in estuarine areas that exhibit a large difference in water height (their ‘tidal range’) between high and low tides. The technology used to exploit each of these resources is quite different. The two types of tidal resource are generally found in very different locations.

Tidal stream

The tides are generated by the rotation of the earth within the gravitational fields of the moon and sun. The relative motions of these bodies cause the surface of the oceans to be raised and lowered periodically, producing the bulk movement of water. Where these moving bodies of water meet land masses, channels or other underwater features they can be enhanced forming a tidal stream.

Tidal stream technologies

Tidal stream technologies work by extracting some of the kinetic energy from fast-flowing tidal currents and converting the kinetic energy to mechanical energy before being further converted to typically electricity. To do this they cannot completely block the path of the tidal currents, as otherwise there would be no energy to extract. Instead, they are designed to extract the maximum possible amount of energy whilst still allowing the sea to flow in a normal way – but with reduced energy


The periodic rising and falling of the water resulting from the gravitational attraction of the Moon and Sun and other astronomical bodies acting upon the rotating Earth. Although the accompanying horizontal movement of the water resulting from the same cause is also sometimes called the tide, it is preferable to designate the latter as TIDAL CURRENT, reserving the name TIDE for the range of vertical movement.

Trough of the wave

The lowest part of a waveform between successive crests. Also, that part of a wave below still-water level.

Tubular turbine

It is a type of axial flow turbine. In Tubular Turbine the runner is connected to a long shaft which drives the generator. Tubular turbines are proposed for the Severn tidal project in the United Kingdom.

Useful power

The useful power which is delivered by a wave-energy converter. The difference between absorbed wave power and power that is lost due to dissipative effects, such as friction and viscosity, etc.

Variable pitch turbine

Wells turbine use symmetrical profile blades with their chords in the plane of rotation. The possibility of the blade being able to change pitch so as to prevent the angle of incidence exceeding some maximum angle has been demonstrated numerically to be more productive than a fixed pitch turbine.

Vertical axis tidal stream turbine

A tidal stream turbine mounted such that it rotates about a vertical axis perpendicular to the flow of water.

Viscous drag

Drag caused by interaction with viscous fluid such as water.

Waterplane area

When a body pierces the surface of the water the area of the intersection between the body and the surface is the water-plane area.


Oceans waves are caused by winds blowing over the earth's surface. These winds transfer energy in shear to the water in the seas and oceans. This energy causes waves to form. It is from these waves that carry the energy with no net transfer of water in deep water. Energy can be extracted by marine energy technologies.

Wave crest

The wave crest is the peak of the wave. Since many water waves are wide compared with their height. The crests of successive waves proscribe parallel lines on the surface. Measurements of wave energy are usually related to a certain length of wave crest [kW/m].

Wave energy

Energy in or from waves. The total energy in a wave is the sum of potential energy, due to vertical displacement of the water surface, and kinetic energy, due to water in oscillatory motion.

wave energy converter

A technical device or system designed to convert wave energy to electrical energy or another kind of useful energy.

Wave energy spectrum

A mathematical or graphical description of how a wave state of irregular waves is distributed among the various frequencies.

Wave height

The vertical distance between a wave crest and the previous wave trough.

Wave load

The forces which waves exert on floating, submerged or bottom-standing structures. wave period The time for a wave crest to traverse a distance equal to one wavelength. The time for two successive wave crests to pass a fixed point.

Wave power

Mechanical power from waves, normally expressed in kilowatts per metre of wave crest length.

Wave tank

A test facility capable of producing (wide) wave of a known shape and type.

Wave trough

The lowest part of a waveform between successive crests. Also, that part of a wave below still-water level.

Wave-by-wave tuning

See fast tuning.


An envisaged plane which is perpendicular to the direction of wave propagation, and which moves with the propagation speed of the wave.


The horizontal distance between similar points on two successive waves measured perpendicular to the crest.

Wave-rider buoy

A device used to measure wave properties. The buoy rides the waves and estimates the wave positions and directions based on measurements of its own accelerations in different directions.

Wave-to-wire model

Mathematical model in the time domain that simulates the energy conversion chain.

Wells turbine

Air turbine using symmetrical profile blades with their chords in the plane of rotation. This turbine is selfrectifying, that is, its sense of rotation is the same for both of the two opposite air-flow directions. It is usual to equip the OWC wave energy device with such a turbine.

Working fluid

A fluid used to absorb and transfer heat energy.