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.


In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes). (US Dept. of Energy)

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 dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron. (US Dept. of Energy)


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.

Acetic acid

The acid most commonly associated with vinegar, it is the most commercially important organic acid and is used to manufacture a wide range of chemical products, such as plastics and acetobacter but, except for making vinegar, is usually made through synthetic processes. Derivatives of acetic acid which may be formed by substitution reactions. Mono- and di-substituted, as well as, halogenated compounds have been synthesised. Experimentally, alpha- and n2- substituted acetic acids have been examined for their anti-inflammatory activity and effect on the central nervous system respectively. Additionally, limited exposure data has been collected on dibromo and dichloroacetic acids to determine whether they pose health effects. (

Acid detergent fiber (ADF)

Organic matter that is not solubilized after one hour of refluxing in an acid detergent of cetyltrimethylammonium bromide in 1N sulfuric acid. ADF includes cellulose and lignin. This analytical method is commonly used in the feed and fiber industries. (Source: Milne, T.A.; Brennan, A.H.; Glenn, B.H. Sourcebook of Methods of Analysis for Biomass Conversion and Biomass Conversion Processes. SERI/SP-220-3548. Golden, CO: Solar Energy Research Institute, February 1990.)(NREL Biomass Glossary)

Acid hydrolysis

The treatment of cellulosic, starch, or hemicellulosic materials using acid solutions (usually mineral acids) to break down the polysaccharides to simple sugars.(NREL Biomass Glossary)

Acid insoluble lignin

Lignin is mostly insoluble in mineral acids, and therefore can be analyzed gravimetrically after hydrolyzing the cellulose and hemicellulose fractions of the biomass with sulfuric acid. ASTM E-1721-95 describes the standard method for determining acid insoluble lignin in biomass.(NREL Biomass Glossary)

Acid soluble lignin

A small fraction of the lignin in a biomass sample is solubilized during the hydrolysis process of the acid insoluble lignin method. This lignin fraction is referred to as acid soluble lignin and may be quantified by ultraviolet spectroscopy. (Source Ehrman, T. Determination of Acid-Soluble Lignin in Biomass. NREL-LAP-004. Golden, CO: National Renewable Energy Laboratory, September 9, 1996.)(NREL Biomass Glossary)

Activated shelf life

The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level. (US Dept. of Energy)

Activation voltage(s)

The voltage(s) at which a charge controller will take action to protect the batteries. (US Dept. of Energy)

Actual or total catchment area (km²)

The "actual or total catchment area" of a water-course at a give point is equal to the algebraic sum of the natural catchment area at that point and the diverted catchment areas which modify its inflows. By extension, the actual catchment are of a head installation is defined as the sum of the actual catchment areas of each of its intakes. (UNIPEDE)

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.

Adjustable set point

A feature allowing the user to adjust the voltage levels at which a charge controller will become active. (US Dept. of Energy)


An emissions control device that removes VOCs from a gas stream as a result of the gas attaching (adsorbing) onto a solid matrix such as activated carbon. (CEPA)

Aerobic fermentation

Fermentation processes that require the presence of oxygen.(NREL Biomass Glossary)

Agricultural residue

Agricultural crop residues are the plant parts, primarily stalks and leaves, not removed from the fields with the primary food or fiber product. Examples include corn stover (stalks, leaves, husks, and cobs); wheat straw; and rice straw. (NREL Biomass Glossary)


See amperage interrupt capability. (US Dept. of Energy)

Air mass (sometimes called air mass ratio)

Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness). (US Dept. of Energy)


The shape of the blade cross-section, which for most modern horizontal axis wind turbines is designed to enhance the lift and improve turbine performance. (US Dept. of Energy)


An alcohol is an organic compound with a carbon bound to a hydroxyl group. Examples are methanol (CH3OH) and ethanol (CH3CH2OH).(NREL Biomass Glossary)


Any of a class of highly reactive organic chemical compounds characterized by the common group CHO and used in the manufacture of resins, dyes, and organic acids.(NREL Biomass Glossary)


A group of aquatic, photosynthetic, eukaryotic organisms ranging from unicellular to multicellular forms, and generally possess chlorophyll but lack true roots, stems and leaves characteristic of terrestrial plants.


A substance having highly basic properties; a strong base.

Alkali lignin

Lignin obtained by acidification of an alkaline extract of wood.(NREL Biomass Glossary)

Alkaline hydrolysis

The use of solutions of sodium hydroxide (or other alkali) in the treatment of cellulosic material (wood) to break down cellulose to simple sugars.(NREL Biomass Glossary)

Ambient temperature

The temperature of the surrounding area. (US Dept. of Energy)

Amorphous semiconductor

A non-crystalline semiconductor material that has no long-range order. (US Dept. of Energy)

Amorphous silicon

A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon a polycrystalline silicon. (US Dept. of Energy)

Amperage interrupt capability (AIC)

Direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current. (US Dept. of Energy)


Unit for the quantity of electricity obtained by integrating current flow in amperes over the time in hours for its flow; used as a measure of battery capacity. (US Dept. of Energy)


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


Amylase is present in organisms such as molds, bacteria, yeasts and plants. In plants, the amylase can be found in seeds to break down starch into sugar to be used by the embryo to induce growth.


Occurring in the absence of oxygen or not requiring oxygen to live.

Anaerobic digestion

Degradation of organic matter by microbes in the absence of oxygen to produce methane and carbon dioxide.(NREL Biomass Glossary)


A device to measure the wind speed. (US Dept. of Energy)

Angle of incidence

The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.(US Dept. of Energy)

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.

Anhydrous (adj.)

Containing no water.


The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode. (US Dept. of Energy)

Antireflection coating

A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission. (US Dept. of Energy)

Aquatic plants

Any plant adapted to grow in water or aqueous habitats


A polysaccharide that is mostly a polymer of arabinose.


A sugar of the pentose class that is a constituent of many plant gums.


See photovoltaic (PV) array. (US Dept. of Energy)


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".

Array current

The electrical current produced by a photovoltaic array when it is exposed to sunlight. (US Dept. of Energy)

Array operating voltage

The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load. (US Dept. of Energy)

Ash fusion temperature

The temperature at which a special test cone made from particles of ash obtained from the coal will (1) begin to deform, i.e., soften, or (2) completely deform or fuse into a blob. (

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.


A tool resembling a large corkscrew, for boring holes in wood. (Oxford Dictionary)

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.

Average wind speed

The mean wind speed over a specified period of time. (US Dept. of Energy)

Azimuth angle

The angle between true south and the point on the horizon directly below the sun.


100 percent biodiesel.


Fuel mix of 20% biodiesel and 80 diesel.


Sugar cane waste (

Barrier energy

The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier. (US Dept. of Energy)

Batch distillation

A process in which the liquid feed is placed in a single container and the entire volume is heated, in contrast to continuous distillation in which the liquid is fed continuously through the still.

Batch fermentation

A process in which cells or micro-organisms are grown for a limited time. At the beginning of the fermentation, an inoculum is introduced into fresh medium, with no addition or removal of medium for the duration of the process.


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.


Biobutanol is a 4-carbon alcohol (butyl alcohol) produced from the same feedstocks as ethanol including corn, sugar beets, and other biomass feedstocks. Butanol is generally used as an industrial solvent in products such as lacquers and enamels, but it also can be blended with other fuels for use in conventional gasoline vehicles (US Department of Energy).


A general term describing the use of biological systems to transform one compound into another. Examples are digestion of organic wastes or sewage by microorganisms to produce methane and the synthesis of organic compounds from carbon dioxide and water by plants. (US Biomass board)


An alternative fuel that can be made from any fat or vegetable oil; It can be used in any diesel engine with few or no modifications. Although biodiesel does not contain petroleum, it can be blended with diesel at any level or used in its pure form. (CWET)


The production, conversion, and use of material directly or indirectly produced by photosynthesis (including organic waste) to manufacture fuels and substitutes for petrochemical and other energy-intensive products.(NREL Biomass Glossary)


Biomass converted to liquid or gaseous fuels such as ethanol, methanol, methane, and hydrogen.(NREL Biomass Glossary)


A gaseous mixture of carbon dioxide and methane produced by the anaerobic digestion of organic matter. (NREL Biomass Glossary)

Biogenic Source

Biological sources such as plants and animals that emit air pollutants such as volatile organic compounds. Examples of biogenic sources include animal management operations and oak and pine tree forests. (See also natural sources.) For more information, visit our Ecosystems Impact website. (CEPA)


"Bioliquids" means liquid fuel for energy purposes other than for transport, including electricity and heating and cooling, produced from biomass;(2009/28/EC)


Any plant-derived organic matter. Biomass available for energy on a sustainable basis includes herbaceous and woody energy crops, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, aquatic plants, and other waste materials including some municipal wastes. Biomass is a very heterogeneous and chemically complex renewable resource.(NREL Biomass Glossary)

Biomass / Biogas

Biomass conversion

The process by which organic materials, such as wood waste or garbage, are burned for direct energy or electrical generation, or by which these materials are converted to synthetic natural gas (Platts)

Biomass processing residues

Byproducts from processing all forms of biomass that have significant energy potential. For example, making solid wood products and pulp from logs produces bark, shavings and sawdust, and spent pulping liquors. Because these residues are already collected at the point of processing, they can be convenient and relatively inexpensive sources of biomass for energy. (NREL Biomass Glossary)


A landfill where the waste actively decomposes rather being simply buried in a "dry tomb (CWET)


See building integrated photovoltaics. (US Dept. of Energy)


The aerodynamic surface that catches the wind. (US Dept. of Energy)

Blocking diode

A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards. (US Dept. of Energy)

Boron (B)

The chemical element commonly used as the dopant in photovoltaic device or cell material. (US Dept. of Energy)


A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth. (US Dept. of Energy)

Boundary layer

That part of the atmosphere that is adjacent to the Earth’s surface and which is affected by the properties of that surface. (CarbonTrust)


Various systems used to stop the rotor from turning. (US Dept. of Energy)


Undergrowth, twigs, and small branches, typically used for firewood or kindling.

Building integrated photovoltaics

A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems. (US Dept. of Energy)


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.

Bypass diode

A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light. (US Dept. of Energy)

Cadmium (Cd)

A chemical element used in making certain types of solar cells and batteries. (US Dept. of Energy)

Cadmium telluride (CdTe)

A polycrystalline thin-film photovoltaic material. (US Dept. of Energy)


The layer of reproducing cells between the inner bark (phloem) and the wood (xylem) of a tree that repeatedly subdivides to form new wood and bark cells.(NREL Biomass Glossary)

Canopy layer or sublayer

The part of the atmospheric boundary layer occupied by the roughness elements (buildings in the urban case). (CarbonTrust)

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.


Organic compounds made up of carbon, hydrogen, and oxygen. It includes cellulosics, starches, and sugars. (NREL Biomass Glossary)


Noise or vibration causing damage to the turbine blades as a results of bubbles that form in the water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure.

Cell (battery)

A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). See also photovoltaic (PV) cell. (US Dept. of Energy)

Cell barrier

A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge. (US Dept. of Energy)

Cell junction

The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone. (US Dept. of Energy)

Characteristic generation (kWh)

This concept of "characteristic generation", which is specified to the hydro field, represents what could have been generated by a head installation during a given period. It is obtained by adding the energy losses to the net generation. All energy losses except those when an actual flow exceeds the maximum usable flow of a head installation. By extension, a mean characteristic generation of a head installation can be calculated over a long period. This concept is different from energy capability because it does not take into account the variability of storage in its reservoirs. It should be noted that the generation in question is a net generation. The latter is generally calculated on the basis of the generation at the machine terminals less the transformer losses, as well as the consumption of all the electrical auxiliaries (auxiliary hydro sets supplied by another head installation, auxiliary services supplied by the distribution network, etc…). (UNIPEDE)

Charge controller

A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status. (US Dept. of Energy)

Charge factor

A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate. (US Dept. of Energy)

Chemical vapor deposition (CVD)

A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate. (US Dept. of Energy)

Classification of hydro-electric head installations

Hydro-electric head installations are classified according to the use that can be made of the cumulative flow that they receive, depending on whether the cumulative flow must be used within a short period of time or whether it can be retained for a certain period. This criteria is based on the reservoir filling period "D" calculated using the annual characteristic mean flow. Run-of-river head installations: These head installations normally operate on base load and use the cumulative flow continuously or receive (or use) environmental flow releases. Storage head installations: Hydro-electric head installations storing their cumulative flows wholly or partly in their retaining works in order to generate during hours of higher demand. According to the filling period of a reservoir it can be defined as follows : Pondage 2 hours < D < 400 hours Reservoir D = 400 hours These head installations are normally operated in such a way as to allow load following. By extension, when the operation of a head installation is directly related to a that of a reservoir upstream and the intermediate inflows are negligible, these head installations must be considered to belong to the same category as the one which governs them. Different types of head installations with pumping: Pumped storage head installations or head installations with pumping are those in which water can be raised by means of pumps and stored, to be used later for the generation of electrical energy. The basic difference between the various types of pumped storage head installation arise from the way in which the turbines and pumps are arranged in the hydraulic circuits. In all types of pumped storage stations, the pumps and turbines are connected to one or several interconnected upper reservoir(s). When the pumps and turbines are connected to the same lower reservoir or to reservoirs which are themselves connected, the pumping cycle can be repeated many times. A distinction must be made according to whether the upper reservoir is fed by significant natural flows. If, on the other hand, the pumps and turbines are connected to physically separate lower reservoirs without a hydraulic connection between them except by way of an upper reservoir, there can be no pump/turbine cycle as such and the pumps have only the role of pumping into the upper reservoir the contribution of water acquired at their level. A station having this latter configuration is known by the term "station with contributory pumping". These head installations are classified in the categories referred to in definitions above, according to the filling period of the reservoir "D". Pure pumped storage head installation: A "pure pumped storage head installation" is one without significant natural cumulative flows into the upper reservoir. Note : The natural cumulative flow into the upper reservoir, in an average year, permits a utilisation period of the maximum electrical capacity in the turbine mode less than or equal to 250 hours (average value in France and Italy). Mixed pumped storage head installation: A "mixed pumped storage head installation" is one with significant natural cumulative flow into the upper reservoir.

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.

Cloud enhancement

The increase in solar intensity caused by reflected irradiance from nearby clouds. (US Dept. of Energy)


The use of a mixture of two fuels within the same combustion chamber.(NREL Biomass Glossary)

Concentrating photovoltaics (CPV)

A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells. (US Dept. of Energy)

Concentrating solar power (CSP)

A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator. (US Dept. of Energy)


A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times. (US Dept. of Energy)

Conduction band (or conduction level)

An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge. (US Dept. of Energy)


Tree, usually evergreen, with cones and needle-shaped or scalelike leaves, producing wood known commercially as softwood. .(NREL Biomass Glossary)

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.


See Inverter. (US Dept. of Energy)


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.

Crystalline silicon

A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon. (US Dept. of Energy)

Cut-in wind speed

The wind speed at which a wind turbine begins to generate electricity. (US Dept. of Energy)

Cutoff voltage

The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery. (US Dept. of Energy)

Cut-out wind speed

The wind speed at which a wind turbine ceases to generate electricity. (US Dept. of Energy)


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.

Dangling bonds

A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface. (US Dept. of Energy)

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 Thermal

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.


The removal of a substantial portion of the water from any substance. (NREL Biomass Glossary)


The removal of hydrogen from a chemical compound.

Dendritic web technique

A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools. (US Dept. of Energy)


Mass per unit of volume. (US Dept. of Energy)

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.

Diffuse insolation

Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation. (US Dept. of Energy)

Diffuse radiation

Radiation received from the sun after reflection and scattering by the atmosphere and ground. (US Dept. of Energy)

Diffusion furnace

Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material. (US Dept. of Energy)

Direct insolation

Sunlight falling directly upon a collector. Opposite of diffuse insolation. (US Dept. of Energy)

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.

Distributed generation

Localized or on-site power generation. (US Dept. of Energy)

Distributed power

Generic term for any power supply located near the point where the power is used. Opposite of central power. See also stand-alone systems. (US Dept. of Energy)

Distributed systems

Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system. (US Dept. of Energy)

Diverted catchment area (km²)

The "diverted catchment area" is a catchment area, the inflows to which are artificially diverted (by gravity or pumping) from their natural course to another water course for energy purposes or some other purpose. The characteristics of the deviation works may result in a limitation of the deviated flows, and so not all the inflows to that catchment area may be diverted. A diverted catchment area may itself be natural or actual. (UNIPEDE)


On the opposite side from the direction from which the wind blows. (US Dept. of Energy)


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.


A mixture of 10% ethanol and 90% gasoline based on volume.


The liquid or gas discharged after processing activities, usually containing residues from such use. Also discharge from a chemical reactor.


A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell. (US Dept. of Energy)

Electron hole pair

The result of light of sufficient energy dislodging an electron from its bond in a crystal, which creates a hole. The free electron (negative charge) and the hole (positive charge) are a pair. These pairs are the constituents of electricity. (US Dept. of Energy)

Elemental analysis

The determination of carbon, hydrogen, nitrogen, oxygen, sulfur, chlorine, and ash in a sample. See ultimate analysis. (NREL Biomass Glossary)

Energy crop

A crop grown specifically for its fuel value. These include food crops such as corn and sugar cane, and nonfood crops such as poplar trees and switchgrass. (NREL Biomass Glossary)

Energy head height (m)

The "energy head height" of a head installation for a given flow in the intake works, is the difference in altitude between the cross section of the charging point of the intake works and, depending on the case, either the tail-race cross section, or the Pelton reference level. Two characteristic values are defined: Maximum energy head height (m) - The "maximum energy head height" is defined as the difference between the maximum normal operating level and the maximum tail race level or the Pelton reference level. It is measured at the maximum usable flow of all the generator sets of the head installation. Normally it corresponds to the maximum electrical capacity of the head installation. Mean energy head height (m) - The "mean energy head height" is the difference, for the average usable flow in operation, between the mean operating level and the mean tailrace level in operation or the Pelton reference level.

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.

Epitaxial growth

The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal. (US Dept. of Energy)


The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox). (US Dept. of Energy)


A compound formed from the reaction between an acid and an alcohol. In esters of carboxylic acids, the -COOH group of the acid and the -OH group of the alcohol lose water and become a -COO- linkage. (NREL Biomass Glossary)

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 pyrolysis

Pyrolysis in which reaction times are short, resulting in higher yields of certain fuel products, which may range from primary oils to olefins and aromatics depending on the severity of conditions. (NREL Biomass Glossary)

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.


Any material used as a fuel directly or converted to another form of fuel or energy product. (NREL Biomass Glossary)


The area upwind of a site, over which the air has travelled. (CarbonTrust)

Fill factor

The ratio of a photovoltaic cell's actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance. (US Dept. of Energy)

Fischer-Tropsch process

A collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a synthetic lubrication oil and synthetic fuel, typically from coal, natural gas, or biomass. (US Energy information administration)

Fixed bed

A bed of closely spaced particles through which gases move up or down for purposes of gasification or combustion. (NREL)

Fixed carbon

The carbon remaining after heating in a prescribed manner to decompose thermally unstable components and to distill volatiles. Part of the proximate analysis group. (NREL)

Fixed tilt array

A photovoltaic array set in at a fixed angle with respect to horizontal. (US Dept. of Energy)

Flash point

The temperature at which a combustible liquid will ignite when a flame is held over the liquid; anhydrous ethanol will flash at 51 degrees Fahrenheit.

Flat-plate array

A photovoltaic (PV) array that consists of non-concentrating PV modules. (US Dept. of Energy)

Flat-plate module

An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight. (US Dept. of Energy)

Flat-plate photovoltaics (PV)

A PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array. (US Dept. of Energy)

Flexible Fuel Vehicle

Automobile capable of running on gasoline and high-ethanol blends interchangeably (

Flow separation

The process by which an eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides. (US Dept. of Energy)

Fluidized bed

A gasifier or combustor design in which feedstock particles are kept in suspension by a bed of solids kept in motion by a rising column of gas. The fluidized bed produces approximately isothermal conditions with high heat transfer between the particles and gases. (NREL)


Rate of transport. (CarbonTrust)

Forestry residues

Includes tops, limbs, and other woody material not removed in forest harvesting operations in commercial hardwood and softwood stands, as well as woody material resulting from forest management operations such as pre-commercial thinnings and removal of dead and dying trees.(NREL Biomass Glossary)

Free surface

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

Fresnel lens

An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point. (US Dept. of Energy)


Wood used for conversion to some form of energy, primarily in residential use. (NREL)

Full sun

The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter). (US Dept. of Energy)


Plant-like organisms with cells with distinct nuclei surrounded by nuclear membranes, incapable of photosynthesis. Fungi are decomposers of waste organisms and exist as yeast, mold, or mildew. (NREL)


An aldehyde derivative of certain biomass conversion processes; used as a solvent. (NREL)


A passive protection for the turbine in which the rotor folds either up or around the tail vane. (US Dept. of Energy)


Any chemical or heat process used to convert a feedstock to a gaseous fuel.


A gasoline extender made from a mixture of gasoline (90%) and ethanol ethanol or wood alcohol (3%).

Glowing combustion

A reaction between oxygen or an oxidizer and the surface of a solid fuel, allowing emission of heat and light without a flame - also known as surface burning.

Gross maximum head height (m)

The "gross maximum head height" of a head installation is the difference in level between the maximum normal operating level in the retaining and the minimum tail-race level works. When the tail-race level is affected by the head height of another installation, the tail-race level corresponds to the maximum normal operating level of the downstream reservoir. It is defined assuming the following conditions: maximum normal operating level, zero flow through turbines,zero guaranteed flow.

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)

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)

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)

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)

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)

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.


One of the botanical groups of dicotyledonous trees that have broad leaves in contrast to the conifers or softwoods. The term has no reference to the actual hardness of the wood. The botanical name for hardwoods is angiosperms. Short-rotation, fast growing hardwood trees are being developed as future energy crops.


Horizontal axis wind turbine. (US Dept. of Energy)

Heat rate

The amount of fuel energy required by a power plant to produce one kilowatt-hour of electrical output. A measure of generating station thermal efficiency, generally expressed in Btu per net kWh. It is computed by dividing the total Btu content of fuel burned for electric generation by the resulting net kWh generation.

Heat transfer efficiency

Useful heat output released / actual heat produced in the firebox.

Heating value

Higher heating value (HHV) is the potential combustion energy when water vapor from combustion is condensed to recover the latent heat of vaporization. Lower heating value (LHV) is the potential combustion energy when water vapor from combustion is not condensed.


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


Any of various simple sugars that have six carbon atoms per molecule

Higher heating value (HHV)

The heat produced by combustion of one unit of substance at constant volume in an oxygen bomb calorimeter under specified conditions. The conditions are: initial oxygen pressure of 2.0-4.0 MPa (20-40 atm), final temperature of 20°-35°C, products in the form of ash, liquid water, gaseous CO2 and N2, and dilute aqueous HCl and H2SO4. It is assumed that if significant quantities of metallic elements are combusted, they are converted to their oxides. In the case of materials such as coal, wood, or refuse, if small or trace amounts of metallic elements are present, they are unchanged during combustion and are part of the ash. Also known as gross heat of combustion.


The total carbohydrate fraction of wood; cellulose plus hemicellulose.

Horizontal axis turbine

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


The central part of the wind turbine, which supports the turbine blades on the outside and connects to the low-speed rotor shaft inside the nacelle. (EERC)


The offspring of genetically different parents The term is applied as well to the progeny from matings within species and to those between species. Hybrids combine the characteristics of the parents or exhibit new ones.

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.



An organic compound that contains only hydrogen and carbon. In vehicle emissions, these are usually vapors created from incomplete combustion or from vaporization of liquid gasoline. Emissions of hydrocarbons contribute to ground level ozone.


A process in which hydrogen is added to organic molecules at high pressures and moderate temperatures; usually used as an adjunct to catalytic cracking.

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.


Treatment of substances with hydrogen and suitable catalysts at high temperature and pressure to saturate double bonds.


The conversion, by reaction with water, of a complex substance into two or more smaller units, such as the conversion of cellulose into glucose sugar units.

Hydrostatic equilibrium

The state of balance between the force of gravity and the vertical component of the pressure gradient force. It is a state of the atmosphere in which there is no vertical acceleration of the air. (CarbonTrust)

III-V cell

A high-efficiency solar cell made from materials including Group III and Group V elements from the periodic table. (US Dept. of Energy)

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.

Incident light

Light that shines onto the face of a solar cell or module. (US Dept. of Energy)

Incremental energy costs

The cost of producing and/or transporting the next available unit of electrical energy above a previously determined base cost. NREL

Inertial sublayer

The part of the atmospheric boundary layer that is much lower than the boundary layer depth but much higher than the surface roughness elements. (CarbonTrust)

Infrared radiation

Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light. (US Dept. of Energy)


A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell. (US Dept. of Energy)


The solar power density incident on a surface of stated area and orientation.


A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells. (US Dept. of Energy)

Intermediate catchment area (km²)

The "intermediate catchment area" of a water-course at a given point is equal to the difference between its own actual catchment area and those of the works situated immediately upstream to it. In the case of a head installation the inflow from the intermediate catchment area represents the theoretical "inevitable run of the river" when all the upstream head installations are shut down, without overflow, and when all independent storage works are closed. (UNIPEDE)

Intrinsic layer

A layer of semiconductor material, used in a photovoltaic device, whose properties are essentially those of the pure, undoped, material. (US Dept. of Energy)

Inverted metamorphic multijunction (IMM) cell

A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency. (US Dept. of Energy)


A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid. (US Dept. of Energy)


A device that converts direct current (DC) to alternating current (AC). (US Dept. of Energy)


An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged. (US Dept. of Energy)


Tropical plant emerging as a potential new feedstock for biodiesel due to its resistance to drought and pests, and its seeds which contain up to 40% oil. When crushed and processed, the seeds’ oil can be used in a standard diesel engine and its residue can be processed into biomass. (Chartis)

Junction box

A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary. (US Dept. of Energy)

Junction diode

A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes. (US Dept. of Energy)


The width of a cut used to create wafers from silicon ingots, often resulting in the loss of semiconductor material. (US Dept. of Energy)

Klason lignin

Lignin obtained from wood after the non-lignin components of the wood have been removed with a prescribed sulfuric acid treatment. A specific type of acid-insoluble lignin analysis. NREL


Engine sound that results from ignition of the compressed fuel-air mixture prior to the optimal moment. EFC

Kraft process

Chemical pulping process in which lignin is dissolved by a solution of sodium hydroxide and sodium sulfide. NREL

Landfill gas

Biogas produced from the natural degradation of organic material in landfills. NREL

Langley (L)

Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2. (US Dept. of Energy)


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.


The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material. (US Dept. of Energy)


The period during which a system is capable of operating above a specified performance level. (US Dept. of Energy)

Life-cycle cost

The estimated cost of owning and operating a photovoltaic system for the period of its useful life. (US Dept. of Energy)

Light trapping

The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers. (US Dept. of Energy)

Light-induced defects

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light. (US Dept. of Energy)


Energy-rich material contained in biomass that can be used for boiler fuel. EFC

Lignin pseudo-molecule for modeling

The lignin ratio of methoxy groups to phenylpropanoid groups (MeO:C9) is used to calculate an ultimate analysis for the lignin pseudo-molecule. This ultimate analysis is used to estimate other properties of the molecule, such as its higher and lower heating values. NREL


Refers to plant materials made up primarily of lignin, cellulose, and hemicellulose. logging residues: The unused portions of growing-stock and non-growing-stock trees cut or killed by logging and left in the woods. US biomass board

Linear theory

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

Lower heating value (LLV)

The heat produced by combusting one unit of a substance, at atmospheric pressure under conditions such that all water in the products remains in the form of vapor. The net heat of combustion is calculated from the gross heat of combustion at 20°C by subtracting 572 cal/g (1030 Btu/lb) of water derived from one unit mass of sample, including both the water originally present as moisture and that formed by combustion. This subtracted amount is not equal to the latent heat of vaporization of water because the calculation also reduces the data from the gross value at constant volume to the net value at constant pressure. The appropriate factor for this reduction is 572 cal/g.


The polymer of mannose with a repeating unit of C6H10O5. Can be hydrolyzed to mannose.


(C6H1206) A six-carbon sugar. A product of hydrolysis of mannan found in the hemicellulose fraction of biomass.


Marine current

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


Mixture of grain and water that is converted through the fermentation process into alcohol.

Mass closure (%)

The percent by weight of the total samples extracted from the biomass sample compared to the weight of the original sample. It is a sum of the weight percent of moisture, extractives, ash, protein, total lignin, acetic acid, uronic acids, arabinan, xylan, mannan, galactan, glucan, and starch. This is a good indicator of the accuracy of a complete biomass compositional analysis.

Maximum usable flow or plant capacity flow (m³/s)

The "maximum usable flow" of a head installation is the maximum flow corresponding, under continuous operating conditions, to the complete opening of the turbines at maximum normal operating level of the reservoir and the minimum flow of the receiving water course or possibly the normal reservoir level of the downstream reservoir. It is not necessarily the same as the flow which corresponds to the maximum electrical capacity output.

Mean wave power

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


The sum of the physical and chemical processes involved in the maintenance of life and by which energy is made available to the organism.


(CH4) The major component of natural gas. It can be formed by anaerobic digestion of biomass or gasification of coal or biomass.

Methanol (wood alcohol)

(CH3OH) An alcohol formed by catalytically combining carbon monoxide with hydrogen in a 1:2 ratio under high temperature and pressure.


A small groove scribed into the surface of a solar cell, which is filled with metal for contacts. (US Dept. of Energy)


Any microscopic organism such as yeast, bacteria, fungi, etc.

Mill Residue

Wood and bark residues produced in processing logs into lumber, plywood, and paper.


A common method of pricing electricity in the U.S. Tenths of a U.S. cent per kilowatt hour.


The use of multiple inverters connected in parallel to service different loads. (US Dept. of Energy)

Module derate factor

A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module. (US Dept. of Energy)


The amount of water and other components present in the biomass sample that are volatilized at 105oC.

Moisture free basis

Biomass composition and chemical analysis data is typically reported on a moisture free or dry weight basis. Moisture (and some volatile matter) is removed prior to analytical testing by heating the sample at 105oC to constant weight. By definition, samples dried in this manner are considered moisture free.

Monochromatic wave

Wave with the same length and period.


Solar silicon solidified into a uniform, cylinder-shaped crystal as a result of conditions during the crystallization process. (Deutsche Solar)


The cultivation of a single species crop.


Fabricated as a single structure. (US Dept. of Energy)


A simple sugar such as a five-carbon sugar (xylose, arabinose) or six-carbon sugar (glucose, fructose). Sucrose, on the other hand is a disaccharide, composed of a combination of two simple sugar units, glucose and fructose.


The simplest form of silane (silicon analogues of alkane hydrocarbons). It is used in solar silicon production. (Deutsche Solar)


Based on the form of the surface i.e. based on the dimensions and distribution of roughness elements. (CarbonTrust)


A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline. (US Dept. of Energy)

Municipal solid waste

Any organic matter, including sewage, industrial, and commercial wastes, from municipal waste collection systems. Municipal waste does not include agricultural and wood wastes or residues.


The body of a propeller-type wind turbine, containing the gearbox, generator, blade hub, and other parts. (US Dept. of Energy)

Native lignin

The lignin as it exists in the lignocellulosic complex before separation.

Natural catchment area (km²)

The "natural catchment area" of a water course at a given point consists of all the areas from which rainfall would flow by surface run-off into the bed of the watercourse at the point in question in the absence of any upstream division. By extension, the natural catchment area of a head installation is defined as the sum of all the natural catchment areas of its intakes. (UNIPEDE)

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.

Net head height (m)

The "net head height of a head installation", under specified conditions of inflow and operation, is the head height actually used by its turbines, i.e. the difference between the level corresponding to the manometric height at the turbine inlet, taking into account the equivalent velocity head at this point, and, o in the case of reaction turbines, the tail-race level increased by the equivalent velocity head at that point o in the case of impulse turbines, the average level of injection (Pelton). In practice, the net head height is the gross maximum head height less the sum of the pressure losses in the intake works for the given flow.

Net heat of combustion

see lower heating value

Neutral detergent fiber (NDF)

Organic matter that is not solubilized after one hour of refluxing in a neutral detergent consisting of sodium lauryl sulfate and EDTA at pH 7. NDF includes hemicellulose, cellulose, and lignin.

Nitrogen fixation

The transformation of atmospheric nitrogen into nitrogen compounds that can be used by growing plants.

Nitrogen oxides (NOx)

A product of photochemical reactions of nitric oxide in ambient air, and the major component of photochemical smog.

Non-condensing, controlled extraction turbine

A turbine that bleeds part of the main steam flow at one (single extraction) or two (double extraction) points.

Non-forest land

Land that has never supported forests and lands formerly forested where use of timber management is precluded by development for other uses.

N-th order moment of wave spectrum

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


Negative semiconductor material in which there are more electrons than holes; current is carried through it by the flow of electrons. (US Dept. of Energy)

N-type semiconductor

A semiconductor produced by doping an intrinsic semiconductor with an electron-donor impurity (e.g., phosphorus in silicon).

N-type silicon

Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon. (US Dept. of Energy)

O&M costs

Operation and maintenance costs. (US Dept. of Energy)

Obukhov length

A quantity that characterizes the relative importance of mechanically and thermally produced turbulence. (CarbonTrust)

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.


Measure of a fuel’s resistance to self-ignition (see ‘Knock’)

One-axis tracking

A system capable of rotating about one axis. (US Dept. of Energy)


Solar power systems which are connected to a regional electricity grid. When a large amount of electricity is produced due to optimum levels of sunlight, electricity is fed into the grid; electricity can be drawn from the grid if necessary (when it is dark). (Deutsche Solar)

Open-circuit voltage (Voc)

The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing. (US Dept. of Energy)

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.

Operating point

The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array. (US Dept. of Energy)

Organic compound

An organic compound contains carbon chemically bound to hydrogen. Organic compounds often contain other elements (particularly O, N, halogens, or S).

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.

Oven dry ton

An amount of wood that weighs 2000 lb at 0% moisture content.


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.


An oxygenate is a compound which contains oxygen in its molecular structure. Ethanol and biodiesel act as oxygenates when they are blended with conventional fuels. Oxygenated fuel improves combustion efficiency and reduces tailpipe emissions of CO.

Parallel connection

A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage. (US Dept. of Energy)


A fine liquid or solid particle such as dust, smoke, mist, fumes, or smog, found in air or emissions.

Peak sun hours

The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2. (US Dept. of Energy)

Peak watt

A unit used to rate the performance of solar cells, modules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified. (US Dept. of Energy)


A short cylindrical piece meant for burning, produced through compressing pulverized dry biomass, such as corn stalk, cassava stalk, waste wood, sawdust, pinchip, etc


Plant that doesn’t have to be planted every year like traditional row crops. EFC


Any petroleum-based substance composed of a complex blend of hydrocarbons derived from crude oil, including motor fuel, jet oil, lubricants, petroleum solvents, and used oil.


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].


In plants, the inner bark; the principal tissue in a tree concerned with the transport of sugars and other nutrients from the leaves.


Conversion of light into other forms of energy by chemical, biological, or physical processes.


An electric current induced by radiant energy. (US Dept. of Energy)

Photoelectric cell

A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers. (US Dept. of Energy)

Photoelectrochemical cell

A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use. (US Dept. of Energy)

Photovoltaic (PV)

Pertaining to the direct conversion of light into electricity. (US Dept. of Energy)

Photovoltaic (PV) array

An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module. (US Dept. of Energy)

Photovoltaic (PV) cell

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell. (US Dept. of Energy)

Photovoltaic (PV) conversion efficiency

The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device. (US Dept. of Energy)

Photovoltaic (PV) device

A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms. (US Dept. of Energy)

Photovoltaic (PV) effect

The phenomenon that occurs when photons, the "particles" in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available. (US Dept. of Energy)

Photovoltaic (PV) generator

The total of all PV strings of a PV power supply system, which are electrically interconnected. (US Dept. of Energy)

Photovoltaic (PV) module

The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). (US Dept. of Energy)

Photovoltaic (PV) panel

Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current). (US Dept. of Energy)

Photovoltaic (PV) system

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components. (US Dept. of Energy)

Photovoltaic-thermal (PV/T) system

A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system or solar thermal system. (US Dept. of Energy)

Physical vapor deposition

A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate. (US Dept. of Energy)


A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices. (US Dept. of Energy)

Pitch control

Pitch control allows the wings of a wind turbine to be adjusted to suit the prevailing conditions.

Plug-and-play PV system

A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation. (US Dept. of Energy)

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.

Point-contact cell

A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection. (US Dept. of Energy)


Composed of more than one wavelength or frequency.


See multicrystalline. (US Dept. of Energy)

Polycrystalline silicon

A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon. (US Dept. of Energy)

Polycrystalline thin film

A thin film made of multicrystalline material. (US Dept. of Energy)


A large molecule made by linking smaller molecules (monomers) together.


A long-chain carbohydrate containing at least three molecules of simple anhydrous sugars linked together. Examples include cellulose and starch.

Power coefficient

The ratio of the power extracted by a wind turbine to the power available in the wind stream. (US Dept. of Energy)

Power curve

A chart showing a wind turbine’s power output across a range of wind speeds. (US Dept. of Energy)

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)

Process development unit

An experimental facility that establishes proof of concept, preliminary process economics, and engineering feasibility for a pilot or demonstration plant.

Process heat

Energy, usually in the form of hot air or steam, needed in the manufacturing operations of an industrial plant.


The ethanol content of a liquid at 60 degrees Fahrenheit stated as twice the percent by volume of the ethyl alcohol.


A protein molecule is a chain of up to several hundred amino acids and is folded into a more or less compact structure. Because about 20 different amino acids are used by living matter in making proteins, the variety of protein types is enormous. In their biologically active states, proteins function as catalysts in metabolism and to some extent as structural elements of cells and tissues.

Proximate analysis

The determination, by prescribed methods, of moisture, volatile matter, fixed carbon (by difference), and ash. The term proximate analysis does not include determinations of chemical elements or determinations other than those named.

P-type semiconductor

A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon). (US Dept. of Energy)


See photovoltaic(s).


An instrument used for measuring global solar irradiance. (US Dept. of Energy)


An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc. (US Dept. of Energy)


The breaking apart of complex molecules by heating in the absence of oxygen, producing solid, liquid, and gaseous fuels.

Quantum efficiency (QE)

The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight. (US Dept. of Energy)

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.

Rankine cycle

A thermodynamic cycle used in steam turbines to convert heat energy into work. Concentrating solar power plants often rely on the Rankine cycle. In CSP systems, mirrors focus sunlight on a heat-transfer fluid. This is used to creates steam, which spins a turbine to generate electricity. (US Dept. of Energy)

Rated output capacity

The output power of a wind machine operating at the rated wind speed. (US Dept. of Energy)

Rated wind speed

The lowest wind speed at which the rated output power of a wind turbine is produced. (US Dept. of Energy)


A chemical reaction is a dissociation, recombination, or rearrangement of atoms.

Recombinant DNA

DNA that has been artificially introduced into a cell, resulting in alteration of the genotype and phenotype of the cell, and is replicated along with natural DNA. Used in industrial micro-organisms to produce more productive strains.


A device that converts alternating current to direct current. See also inverter. (US Dept. of Energy)

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.

Remote systems

See stand-alone systems. (US Dept. of Energy)

Renewable diesel

Biofuel technology whereby fuel is created from biomass, oils, and fats without a chemical process. Advantages of renewable diesel include improved ignition, higher renewable content, greater fuel stability, a broader selection of feedstock, and lower greenhouse gas emissions. Chartis

Residues in biomass

Byproducts from processing all forms of biomass that have significant energy potential such as, making solid wood products and pulp from logs produces bark, shavings and sawdust, and spent pulping liquors. Because these residues are already collected at the point of processing, they can be convenient and relatively inexpensive sources of biomass for energy.

Resistive voltage drop

The voltage developed across a cell by the current flow through the resistance of the cell. (US Dept. of Energy)

Reverse current protection

Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode. (US Dept. of Energy)

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)

Ribbon (photovoltaic) cells

A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material. (US Dept. of Energy)

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.


The rotating part of a wind turbine, including either the blades and blade assembly or the rotating portion of a generator. (US Dept. of Energy)

Rotor diameter

The diameter of the circle swept by the rotor. (US Dept. of Energy)

Rotor speed

The revolutions per minute of the wind turbine rotor. (US Dept. of Energy)

Roughness layer or sublayer

The part of the atmospheric boundary layer that is not much higher than the surface roughness elements. (CarbonTrust)


A simple sugar or a more complex compound that can be hydrolyzed to simple sugar units.


A conversion process using acids, bases, or enzymes in which long-chain carbohydrates are broken down into their component fermentable sugars.

Sacrificial anode

A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure. (US Dept. of Energy)

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).

Schottky barrier

A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal. (US Dept. of Energy)


An air pollution control device that uses a liquid or solid to remove pollutants from a gas stream by adsorption or chemical reaction.

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.

Secondary silicon

Recycled silicon. (Deutsche Solar)

Selectively permeable membrane

see semipermeable membrane


Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process. (US Dept. of Energy)


See multicrystalline. (US Dept. of Energy)

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.

Series connection

A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage. (US Dept. of Energy)

Series controller

A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery. (US Dept. of Energy)

Series regulator

Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array. (US Dept. of Energy)

Series resistance

Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections. (US Dept. of Energy)


Reduction of wind speed through obstacles (trees, houses, other wind turbines etc). This creates turbulence, which means that systems cannot deliver full power. Shadowing is taken into account when preparing earnings forecasts. (EcoVane)

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.

Short Rotation Intensive Culture (SRIC)

The growing of tree crops for bioenergy or fiber, characterized by detailed site preparation, usually less than 10 years between harvests, usually fast-growing hybrid trees and intensive management (some fertilization, weed and pest control, and possibly irrigation).

Shunt controller

A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less. (US Dept. of Energy)

Siemens process

A commercial method of making purified silicon. (US Dept. of Energy)

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).

Silicon (Si)

A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide). (US Dept. of Energy)

Sine wave

A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle. (US Dept. of Energy)

Sine wave inverter

An inverter that produces utility-quality, sine wave power forms. (US Dept. of Energy)

Single-crystal material

A material that is composed of a single crystal or a few large crystals. (US Dept. of Energy)

Single-crystal silicon

Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon. (US Dept. of Energy)

Slow pyrolysis

Thermal conversion of biomass to fuel by slow heating to less that 842 degrees Fahrenheit (450 degrees Celsius) in the absence of oxygen.

Soft costs

Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection. (US Dept. of Energy)


Generally, one of the botanical groups of trees that in most cases have needle-like or scale-like leaves; the conifers; also the wood produced by such trees. The term has no reference to the actual hardness of the wood. The botanical name for softwoods is gymnosperms.


Solar cell

See photovoltaic (PV) cell.

Solar constant

The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 watts per square meter or 492 Btu per square foot. (US Dept. of Energy)

Solar cooling

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners. (US Dept. of Energy)

Solar energy

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners. (US Dept. of Energy)

Solar noon

The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky. (US Dept. of Energy)

Solar resource

The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours. (US Dept. of Energy)

Solar spectrum

The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation. (US Dept. of Energy)

Solar thermal electric systems

Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough. (US Dept. of Energy)

Solar-grade silicon

Silicon crystals which have a sufficiently high grade of purity to be used in photovoltaic applications. The chemical element silicon is a semi-conductor and forms crystals with a resistant diamond structure. It is the second most common element in the earth's crust after oxygen. For processing in the solar sector, raw silicon is purified to achieve solar-grade silicon and then cast into blocks for further processing into wafers. (Deutsche Solar)

Split-spectrum cell

A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See also mulitjunction device. (US Dept. of Energy)


A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate. (US Dept. of Energy)

Square wave

A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics. (US Dept. of Energy)

Square wave inverter

A type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency. (US Dept. of Energy)

Staebler-Wronski effect

The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light. (US Dept. of Energy)

Stall control

In contrast to pitch control, stall control is mainly used on smaller wind turbines. Here, the blades are locked in place and do not adjust during operation - instead, they are designed and shaped to increasingly ‘stall’ the blade’s angle both to maximize power output and to protect the turbine from excessive wind speeds. (EcoVane)

Stand-alone system

An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage. (US Dept. of Energy)

Standard reporting conditions (SRC)

A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions. (US Dept. of Energy)

Standard test conditions (STC)

Conditions under which a module is typically tested in a laboratory. (US Dept. of Energy)

Standby current

This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low. (US Dept. of Energy)

Stand-off mounting

Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle. (US Dept. of Energy)


A molecule composed of long chains of a-glucose molecules linked together (repeating unit C12H16O5). These linkages occur in chains of a-1,4 linkages with branches formed as a result of a-1,6 linkages (see below). This polysaccharide is widely distributed in the vegetable kingdom and is stored in all grains and tubers.

Start-up wind speed

The wind speed at which a wind turbine rotor will begin to spin. See also Cut-in wind speed. (US Dept. of Energy)


The dried stalks and leaves of a crop remaining after the grain has been harvested.


A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load. (US Dept. of Energy)

Structural chemical analysis

The composition of biomass reported by the proportions of the major structural components; cellulose, hemicellulose, and lignin.


The base on which an organism lives or a substance acts upon (as by an enzyme).


The physical material upon which a photovoltaic cell is applied. (US Dept. of Energy)


Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load). (US Dept. of Energy)


The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero. (US Dept. of Energy)


The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum. (US Dept. of Energy)

Surface layer

For the large scale meteorological community, this is synonymous with inertial sublayer. However the urban meteorological community often uses the term to mean the inertial sublayer and roughness sublayer combined. (CarbonTrust)


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.


A component of lignin, normally only found in hardwood lignins. It has a six-carbon aromatic ring with two methoxyl groups attached.

Tail-race level (m)

The "tail-race level" is the level of the water which has passed through the turbines, measured at the end of the turbine discharge works. It is a function of the flow which passes through the turbine and of either: the level of a still at the end of the discharge works, or the flow in the turbine outlet reception bay, or the level of the downstream reservoir, if it has an influence. (UNIPEDE)

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.


A liquid product of thermal processing of carbonaceous materials.

Temperature factors

It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures. (US Dept. of Energy)


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

Thermochemical conversion

The use of heat to change substances chemically to produce energy products.

Thermophotovoltaic cell (TPV)

A device where sunlight concentrated onto an absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation. (US Dept. of Energy)

Thick-crystalline materials

Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons. (US Dept. of Energy)

Thin film

A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells. (US Dept. of Energy)

Thin film photovoltaic module

A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See also amorphous silicon. (US Dept. of Energy)

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.

Tilt angle

The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection. (US Dept. of Energy)

Tin oxide

A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass. (US Dept. of Energy)

Tip speed ratio

The speed at the tip of the rotor blade as it moves through the air divided by the wind velocity. This is typically a design requirement for the turbine. (US Dept. of Energy)

Total internal reflection

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor. (US Dept. of Energy)

Total lignin

The sum of the acid soluble lignin and acid insoluble lignin fractions.

Total solids

The amount of solids remaining after all volatile matter has been removed from a biomass sample by heating at 105°C to constant weight.


The base structure that supports and elevates a wind turbine rotor and nacelle. Modern turbines are typically constructed using a tubular steel tower. Older wind turbines and windmills used the latticetype tower, which consists of a crisscrossed network of steel or wood members. (EERC)

Tracking array

A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy. (US Dept. of Energy)


A chemical process which reacts an alcohol with the triglycerides contained in vegetable oils and animal fats to produce biodiesel and glycerin.

Transparent conducting oxide (TCO)

A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials. (US Dept. of Energy)


A triglyceride is an ester of glycerol and three fatty acids. Most animal fats are composed primarily of triglycerides.

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.


The changes in wind speed and direction, frequently caused by obstacles. (US Dept. of Energy)

Two-axis tracking

A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal). (US Dept. of Energy)

Ultimate analysis

The determination of the elemental composition of the organic portion of carbonaceous materials, as well as the total ash and moisture. See elemental analysis


Electromagnetic radiation in the wavelength range of 4 to 400 nanometers. (US Dept. of Energy)

Underground feeder (UF)

May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures. (US Dept. of Energy)


On the same side as the direction from which the wind is blowing—windward. (US Dept. of Energy)

Uronic acid

A simple sugar whose terminal -CH2OH group has been oxidized to an acid, COOH group.

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.

Vacuum distillation

The separation of two or more liquids under reduced vapor pressure; reduces the boiling points of the liquids being separated.

Vacuum evaporation

The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum. (US Dept. of Energy)

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.


A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground. (US Dept. of Energy)


Vertical axis wind turbine. (US Dept. of Energy)

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.


A solid or liquid material that easily vaporizes.

Volatile matter

Those products, exclusive of moisture, given off by a material as a gas or vapor, determined by definite prescribed methods that may vary according to the nature of the material.


A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot. (US Dept. of Energy)

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.

Weibull distribution

In order to calculate the mean power delivered by a wind turbine from its power curve, it is necessary to know the probability density distribution of the wind speed. The Weibull distribution method is used for this purpose, simply showing the distribution of the proportion of time spent by the wind within narrow bands of wind speed. (EcoVane)

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.

Wet scrubber

An air pollution control device used to remove pollutants by bringing a gas stream into contact with a liquid.

Whole tree chips

Wood chips produced by chipping whole trees, usually in the forest. Thus the chips contain both bark and wood. They are frequently produced from the low-quality trees or from tops, limbs, and other logging residues.

Willstatter lignin

Lignin obtained from the lignocellulosic complex after it has been extracted with hydrochloric acid.


Wind Assessments

The wind speed at a site is calculated by experts and compiled in a report, which takes data from site and local weather stations. The assessment of a specific site surrounding obstacles, hills, mountains, trees are also considered. At the end of an opinion is the average wind speed of a location in meters per second at hub height, and the Weibull distribution. (EcoVane)

Wind farm

A group of wind turbines often owned and maintained by one company. Also known as a wind power plant. (US Dept. of Energy)

Wind power class

A system designed to rate the quality of the wind resource in an area, based on the average annual wind speed. The scale ranges from 1 to 7, with 1 being the poorest wind energy resources and 7 representing exceptional wind energy resources. (EERC)

Wind power density

A way to define the amount of wind power contained in a given area for use by a wind turbine, measured in watts per square meter. (EERC)

Wind vane

A device used to measure wind direction. (EERC)


A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath. (US Dept. of Energy)


A solid lignocellulosic material naturally produced in trees and some shrubs, made of up to 40%-50% cellulose, 20%-30% hemicellulose, and 20% -30% lignin.

Working fluid

A fluid used to absorb and transfer heat energy.


The liquid remaining from a brewing mash preparation following the filtration of fermentable beer.


A polymer of xylose with a repeating unit of C5H804, found in the hemicellulose fraction of biomass. Can be hydrolyzed to xylose. Gross heat of combustion: Qv(gross)=17751.9 Jg-1.


(C5H10O5) A five-carbon sugar. A product of hydrolysis of xylan found in the hemicellulose fraction of biomass.


The initial movement of logs from the point of felling to a central loading area or landing.


The movement of the tower top turbine that allows the turbine to stay into the wind (US Dept. of Energy)


Any of various single-cell fungi capable of fermenting carbohydrates.

Zenith angle

The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead). (US Dept. of Energy)