the blackened surface in a solar collector that absorbs incoming solar radiation, converts it to thermal energy, and feeds it into the heat transfer fluid.

air change

a measure of the air exchange in a building where one air change is an exchange of a volume of air equal to the interior volume of a building in question.

amorphous silicon

a type of silicon made up of atoms with no fixed order, as in glass, so it is not crystalline. Also known as thin-film silicon.


an assembly of PV modules connected in series and/or parallel.


the least massive, yet the most important part of the Earth for life. Through the atmosphere pass nearly all the elements that form living organisms. The atmosphere protects life from the rigours of space and establishes the climate.

auxiliary heating

the conventional (i.e. non-solar) contribution to the total load (e.g. gas boiler, etc.).

black water

wastewater generated by a household, including toilet wastes, that is entirely non-useable. See grey water.

calorific value

describes the energy content of a unit mass or volume of a fuel (kWh kg-1, Jkg-1, kWh m-3, J m-3).

carbon dioxide, CO2

a gas used by plants and produced by respiration and burning. It has a faintly pungent smell and is present in the air at 280 parts per million but in our breath at 4%. Carbon dioxide in the air helps, through the greenhouse effect, to keep the earth warm, but too much can lead overheating.

chimney or stack effect

the tendency of air or gas in a vertical passage, such as a duct, to rise when heated due to its lower density in comparison with that of the surrounding air or gas. In buildings, there is a tendency towards displacement caused by the difference in temperature between the internal heated air and external un-heated air, and therefore the difference in density of both air masses.

collector, flat plate

an assembly containing a panel of metal or other suitable material, usually a flat black colour on its sun side, that absorbs sunlight and converts it into heat (see absorber). It is usually in an insulated box, covered with glass or plastic on the sun side to take advantage of the greenhouse effect. In the collector, this heat transfers to a circulating fluid, such as air, water, oil or antifreeze.


the transfer of heat from a warmer region to a colder region within the same substance without mass transfer. The rate of transfer will depend on the thermal conductivity of the material.


the transfer of heat within gases and liquids, due to the free flow of particles. A convection current is where rising hot air is replaced by cold air die to its higher density.

crystalline silicon

a type of silicon in which the atoms have a regular diamond-like structure. Also know as single crystal or polycrystalline.

degree of efficiency

the ratio of useful energy (heat) to the energy used (solar radiation) - relating to a solar collector. The rate of energy lost will depend on the degree of efficiency. If a solar collector has a degree of efficiency of 0.6 - 60% of the radiation received is converted into useful heat, and 40% is lost in the form of optical and thermal losses.

diffuse solar radiation

the solar radiation as received on a surface with the exception of the solid angle subtended by the sun's disk (direct solar radiation). Solar radiation reaches the Earth indirectly through scattering in the atmosphere.


pollutants that are toxic to humans.


there are several different forms of energy, including, thermal energy (heat), mechanical energy or electromagnetic energy (radiation). Energy is given in different units, for example as watt-hours (Wh), kilowatt-hours (kWh) or joules (J). One joule is one watt-second (Ws). 1kWh= 1000Wh = 3,600,000 J (=3.6 MJ).

global solar radiation

the full radiation impacting on a horizontal surface. Consists of direct radiation and diffuse radiation.

diffuse radiation

the energy from the sun which is scattered by molecules in the atmosphere and changes direction before reaching the earth's surface - thereby reducing direct radiation. As an annual average in northern Europe, diffuse radiation is between 50% and 60% of all global solar radiation. Diffuse radiation also heats up solar collectors.

direct radiation

the energy from the sun which reaches the earth without being absorbed or scattered by molecules, dust and clouds. With a clear sky, global solar irradiance consists only of direct radiation.

grey water

wastewater generated by a household from the sink, bath, shower and laundry but not by the toilet. Grey water can be treated and recycled for use in the toilet flush or washing machine instead of being returned to the mains. See black water.

heat exchanger

a device through which heat is transferred from one liquid to another. It usually consists of a coiled arrangement of metal tubing.

heat load

the total energy required for space heating.

heat loss

heat flow through building envelope components (walls, windows, roof, etc.).

heat pump

a thermodynamic device that transfers heat from one medium to another. The first medium (the source) cools as the second (the heat sink) warms up.

heating season

the period of the year during which heating a building is necessary to maintain comfortable conditions. UK standard - approx 33 weeks a year.

hybrid solar heating system

solar heating system combining active and passive techniques.

I-V curve

plot of current versus voltage for a PV cell, module or array as a load is increased from short circuit to open circuit under fixed conditions of irradiance and cell temperature.

infrared radiation

electromagnetic radiation at the wavelengths emitted by a red-hot body, longer than the wave-lengths of visible light.

incident angle

the angle between the sun's rays and a line perpendicular (normal) to the irradiated surface.


a contraction of 'incoming solar radiation' meaning the amount of solar energy incident on a given area over a certain period of time; a common unit of insolation is kW m-2 day-1, often referred to as peak hours per day.

internal gains

the energy dissipated inside the heated space by people (body heat) and appliance (lighting, cooker, etc.). A proportion of this energy contributes to the space heating requirements (kWh).


the upper layer of the air where some of the gas molecules have been ionized by high energy radiation or atomic particles from the Sun. Older forms of radio communication depended on its presence for transmission beyond the horizon.

kilowatt hour

energy unit equivalent to 1000 W used for 1 hour; also referred to as a 'unit' of electricity: 1 kWh=3.6 MJ.

open circuit voltage

the maximum voltage between the terminals of a PV cell or module when only connected by a voltmeter, so no current is flowing. Symbol is Voc.


the orientation of a surface is in degrees of variation away from solar south, towards either east or west. Solar or true south should not be confused with magnetic south, which can vary owing to magnetic declination.

ozone layer

a layer of air 15-50 km above the Earth's surface, where most atmospheric ozone is found. It absorbs some of the ultraviolet radiation of the Sun, and in doing so grows warmer and serves to lessen the ultraviolet received at the earth's surface. It is being depleted to some extent by a complex sequence of reactions involving man-made emissions from the Earth's surface of methane, chlorine and nitrogen compounds.

peak hours per day

an alternative unit to kWh m-2 day-1 for daily insolation.

peak power

the maximum amount of power a PV module can generate under standard test conditions of irradiance and cell temperature; units of Wp.


made from dry, unrefined scrap wood (sawdust and wood shavings). They ideally have a diameter of 6 mm and are 10-13 mm long. One kilogram of wooden pellets has a calorific value of about 5 kWh. They require only half the storage volume of bulk wood. Wooden pellet burners can be used as combination systems in connection with a solar thermal system for domestic water heating as well as for room heating support.


the process by which plants use the energy from sunlight to synthesize organic compounds for their growth from carbon dioxide in the air and water. The process also produces oxygen.

photovoltaic (PV)

the process of direct conversion of light into electricity within a material.


the spontaneous release of energy in the form of high energy particles or radiation from atomic nuclei. Some elements essential for life, such as potassium, carbon, and hydrogen, have naturally occurring isotopes that are radioactive.

radio-frequency radiation

electro-magnetic radiation at low frequencies and long wavelengths compared with light or heat radiation. Radio-frequency radiation is generated naturally by lightning and stars, but on the Earth overwhelmingly more by humans for communication and cooking.


the ability of a surface to reflect radiation incident upon it.

solar radiation

the energy-carrying electromagnetic radiation emitted by the sun. This radiation comprises many frequencies, each relating to a particular class of radiation:

  • high-frequency/short-wavelength ultraviolet;
  • medium-frequency/medium-wavelength visible light;
  • low-frequency/high-wavelength infrared

Solar radiation is relatively unimpeded until it reaches the Earth's atmosphere. Here some of it is reflected back out of the atmosphere and some of it is absorbed. That which reaches the Earth's surface unimpeded is referred to as 'direct' solar radiation. That which is scattered by the atmosphere is referred to as 'diffuse' solar radiation. The combination of direct and diffuse is called 'global'.


the substance dissolved in a solvent. Together the solute and the solvent form a solution.


the layer of the earth's atmosphere immediately above the troposphere where the air is stratified. It is warmer than the upper troposphere and contains the ozone layer.

thermal transmittance

the thermal transmission through 1m2 area of a given structure (e.g. a wall consisting of bricks, thermal insulation, cavities, etc.) divided by the difference between the environmental temperature on either side of the structure. Usually called 'U-value' (W m-2 K).

trace gases

gases such as methane, nitrous oxide, ozone, carbon monoxide, dimethyl sulphide, and methyl chloride that exist at low concentrations in theatmosphere. Their low concentration belies their importance


the process of altering the tilt of a module through the day in order to face the sun and thus maximize the power output.


the ratio of radiant energy transmitted through a substance to the total radiant energy incident on its surface. In solar technology, transmittance is always affected by the thickness and composition of the glass cover plates on the collector or window, and to a major extent by the angle of incidence between the sun's rays and a line normal to the surface.

trombe wall

a black-painted masonry (earth, brick, block, stone or mass concrete) wall with glazing on the southerly side. The wall acts as absorber, heat store and emitter. Felix Trombe was the French designer who designed the Trombe Wall.


the layer of the Earth's atmosphere about 10km from the surface. The air in the troposphere is well mixed and it is where the clouds are mostly to be found.

ultraviolet radiation

electro-magnetic radiation in the range of frequencies immediately above visible light and below x-rays. The ultraviolet radiation reaching the Earth's surface has both harmful and beneficial effects.


waste disposal

material sent to landfill or incineration.

water extraction

mains, surface and groundwater consumption.


an area of land that drains to a common outlet, such as the outflow of a lake, the mouth of a river or any point along a stream channel.


unit of power which is the rate of flow of energy, whether electrical, light or heat; definition is 1 W = 1 J s-1; for electrical, also equals 1VA.

watt hour

convenient unit of energy corresponding to use of 1 W for 1 hour; 1Wh = 3600 J, 860 calories or 3.41 Btu.

wind speed

the speed of the air measured in accordance with the recommendations of the World Meteorological Organization, normally measured at 10 m above ground level.