Photovoltaics - Terms Origin

The term photovoltaics derives from the Greek word phōs meaning light and the word volt, named by italian physicist Alessandro Volta (1745-1827). Photovoltaics is a science, which examines light-electricity conversion, respectively, photon energy-electric current conversion. In other words it stands for light-current conversion. First known use of word photovoltaic in the literature was in the book Elements of Electro-biology, Or the Voltaic Mechanism of Man of Electro-pathology, Especially of the Nervous System and of Electro-therapeutics written by Alfred Smee (1818-1877) in 1849. Smee was surgeon, but he also significantly contributed to progress of electro-metallurgy and electrical physiology. In his book published in 1849, following text can be found on page 15:

Upon exposing the apparatus to intense light, the galvanometer was instantly deflected, shewing that the light had set in motion a voltaic current, which I propose to call a photo-voltaic circuit.

Smee has also used term photo-voltaic battery on page 14. As many other technical (and common) words even significant amount of solar energy enginering terminology based on ancient greek language. Some words were derived directly from greek language other indirectly from latin language. Few words originate also in arabic language, they were overtaken from arabic language in the early middle century.

What is Photovoltaics?

The light to current conversion takes place within solar cells, which can be amorphous, polycrystalline or monocrystalline, according to their structure. In most cases they are made of silicon. Solar-module consists of many solar cells, which are electrically connected and placed between glass and tedlar plate, and framed by an (usualy) aluminium frame. A number of solar-modules and other components (batteries, charge regulators, inverters...) can form large photovoltaic systems. The first PV system applications developed were applied as an energy source for satellites. Though it sounds unusual at first, it is a fact that solar cells represent a very important part of information technology revolution in the last few decades. Without solar cells there wouldn't be so many communication satellites, which found the information technology revolution. The consequences are fast development in the field of information technologies and computer sciences, Internet etc. Data and voice transfer possibilities, which are offered to you by modern communications techniques, were made possible by a small piece of silicon in the form of a solar cell. Nowadays, photovoltaic systems are applied as an energy source in many cases. Photovoltaic systems are an excellent solution to electricity production regardless of your whereabouts - even at high latitudes of Himalayas or in Antarctica photovoltaic systems have been put into service. Intensive research in the field of solar energy has taken place more than a century ago. The solar energy use does not require fossil or other-more or less poisonous-fuels and it doesn't require much maintenance. No toxic emissions are produced. Supported by suitable financial aid solar energy is fully compatible with other energy sources. Therefore, solar energy is - along with other renewable energy sources (wind, hydrogen, biomass etc.) the most promising energy source for the next decades. I can but agree with the solar pioneer Frank Shuman who, at the beginning of the 20th century, said: "... it is the most rational source of power ..."

Basic Units and Measures

Units and measures used in solar energy engineering are, with some exceptions, used also in physics, mechanical engineering and some other branches of enginnering. Basically units and measures can be devided into three major groups: terms related to solar radiation, terms related to light transmission and terms related to sun-earth geometry.

Global irradiance[1] G W/m2
Beam irradiance Gb W/m2
Diffuse irradiance Gd W/m2
Spectral irradiance Eλ W/m2μm-1
Global irradiation[2] H J/m2
Beam irradiation Hb J/m2
Diffuse irradiation Hd J/m2

TABLE 1: Most common units and measures related to solar radiation

Altitude (0 to ±90°)[3] α degrees (°)
Surface tilt angle
(0 do ±90° tilt toward equator positive)
β degrees (°)
Azimuth of surface[4]
(-180° to + 180° toward W negative)
γ degrees (°)
Declination (0 to ±23.45°)[5] δ degrees (°)
Solar azimuth angle γs degrees (°)
Incidence angle (0 to +90°) i degrees (°)
Zenith angle (0 to +90°) θ degrees (°)
(0 to ±90°, N from equator positive)
Ψ degrees (°)
Hour angle[6]
(-180° to +180°, noon 0°, afternoon positive)
ω degrees (°)

TABLE 2: Most common units and measures related to Sun-Earth geometry[7]

Specific heat c J/kgK
Thermal conductivity k W/mK
Extinction coefficient Κ -
Refraction index k -
Absorptance α -
Emmitance ε -
Reflectance ρ -
Density ρ kg/m3
Transmittance τ -

TABLE 3: Most common units and measures related to radiation (light) transmission


[1] Common term meaning solar irradiance is also solar power.
[2] The unit Wh/m2 (kWh/m2) is often used in technical practise in Europe. Sometimes also term insolation (incident solar radiation) is used. Please look for further information and additional symbols in suggested literature.
[3] In computer programmes all angles are usually expressed in radians. One radian is equal to 180/π degrees. One degree = radian⋅(180/π)
[4] Azimuth angle can also be expressed in range from 0° to +360°, clockwise from North positive. Such values are more common in architecture for example in solar energy enginering azimuth toward south is usually 0° as presented in the Table 3.
[5] Declination value calculated by Cooper's formulae, more precise methods are also available. For details see site analysis section.
[6] Hour angle is calculated by simple equation h = ± 15°⋅number of hours from/to solar noon.
[7] For description of Sun-Earth geometrical relations term solar geometry is also sometimes used.

Terminology Origin

ebook Smee, Alfred. 1849. Elements of Electro-biology, Or the Voltaic Mechanism of Man of Electro-pathology, Especially of the Nervous System and of Electro-therapeutics. London: Longman, Broen, Green & Longmans.

Origin of Technical Terms - Books & Dictionaries

Green, T. M. (2007), The Greek & Latin Roots of English, Rowman & Littlefield Publishers, Inc.; 4th Edition, ISBN 978-0742547803.
Papaioannou, M., Papaioannou, P. (2003), English and Greek Words of Hellenic Origin, Ellin, ISBN 978-9602867921.
Konstantinidis, A. (2003), Greek words in english language (greek language), Self published, ISBN 960-90338-2-2. Ellin, ISBN 978-9602867921.
Glare, P. W. G. (1983), Oxford Latin Dictionary, Oxford University Press, ISBN 978-0198642244.
Smith, W., Hall, T. D. (2000), Smith's English-Latin Dictionary, Bolchazy-Carducci Publishers, ISBN 978-0865164918.
Smith, W., Lockwood, P. (1994), Chambers Murray Latin-English Dictionary, Chambers; Rei Sub edition, ISBN 978-0550190031.
Liddell, H.G., Scott, R. (1945), An Intermediate Greek-English Lexicon: Founded upon the Seventh Edition of Liddell and Scott's Greek-English Lexicon, Oxford University Press, ISBN 978-0199102068.

Units and Measures - Papers & Books

Units and Symbols in Solar Energy; Solar Energy, vol. 71, no. 1, 2001.
Beckman, W. A et al. (1978), Units and symbols in solar energy; Solar Energy, vol. 21, no. 1, p. 65-68.
Rösemann, R. (2011), A Guide to Solar Radiation Measurement, From Sensor to Application; Gengenbach Messtechnik, 2nd Edition, ISBN 978-90-817397-0-2.
Cardarelli, F. (2003), Encyclopaedia of Scientific Units, Weights and Measures: Their SI Equivalences and Origins, Springer, ISBN 978-1852336820.
Gupta, S. V. (2009), Units of Measurement: Past, Present and Future. International System of Units (Springer Series in Materials Science), Springer, ISBN 978-3642007378.
Lindeburg, M. R. (1998), Engineering Unit Conversions, Professional Publications, Inc., ISBN 978-1591260998.


www - Here you will find interactive calculators for many measurement systems both commonly used like metric and U.S. Avoirdupois and quite exotic like Ancient Greek and Roman.
Electropedia - Electropedia: The World's Online Electrotechnical Vocabulary.
The NIST Reference on Constants Units and Uncertainty.
English weigths and measures.