2.4 “Lux” as a Unit of Light Measurement
2.6 Irradiance and Irradiation
‘Irradiance’ is an instantaneous quantity describing the flux of solar radiation incident on a surface with the unit W×m−2. Thus, irradiance is solar ‘power density’. The irradiance value changes very much, in a given location, depending on the moment of the day. The typical variation of solar irradiance in tropical regions on a clear sunny day is a bell curve, as shown in Fig.2.10.
By referring to ‘spectral irradiance’ one refers to the ‘spectral power density’.
Thus, the unit of spectral irradiance or spectral power density is W×m−2×nm−1. Irradiation7is the integration of irradiance over a specified period of time (MJ× m−2per hour, day, week, month, year as the case may be). Thus, it is simply the ‘time integral of irradiance’ or ‘energy’ received at a given location on earth. Irradiation measures the solar radiation energy density usually in Wh×m−2.
7Alternative terms used for irradiation are a) solar insolation b) radiant exposure. Although the International Electrotechnical Vocabulary (IEV [11]) defines “irradiation” as “exposure to ionizing radiation”, the term is routinely used in the PV community instead of “radiant exposure” (i.e.
the radiant energy incident on an element of the surface). An alternative term commonly used is
“insolation”.
30 A. Funde and A. Shah
Fig. 2.10 aIrradiance variation through a typical clear-sky day of the month of April at Pune, India (measured data) andbThe monthly-average of daily global solar radiant exposure throughout a year at Pune, India. The data of (b) is based on the Pvsyst®software simulated data for the location of Pune, India, derived from the MeteoNorm®synthetic data
The daily and yearly global irradiance give us, thus, a precise indication of the energy of incident sunlight to a specified area and during a specified period of time.
One needs to know these quantities, at a given location to accurately predict the future performance of a solar power plant (see also Chap.10especially Sect. 10.2.5).
For better clarity of the terms used, following terms are drawn from the IEC Glossary. Web address:http://std.iec.ch/glossary.
[Following elaboration is referring to Publication: IEC TS 61836, ed. 2.0 (2007–
12): Solar photovoltaic energy systems—Terms, definitions and symbols].
Irradiance:
(Symbol:G) (Unit: W×m−2)
electromagnetic radiated power per unit of area.
(a) direct irradiance: irradiance from the sun’s disk and from the circumsolar region of the sky within a subtended angle of 8.7×10−2rad (5°)
(b) diffuse irradiance: irradiance excluding that portion which contributes to direct irradiance
(c) global irradiance: sum of the direct and diffuse irradiance (or simply ‘irradi-ance’)
(d) global horizontal irradiance (GHI) global irradiance on a horizontal surface NOTE Global horizontal irradiance, GHI usually takes into consideration the albedo, although the albedo is often negligible in GHI.
(e) direct normal irradiance (DNI): irradiance received from a small solid angle centered on the sun’s disc on a plane perpendicular to the sun’s rays
(f) in-plane irradiance, or plane of array irradiance (POAI): irradiance received from the sun as a combination of direct normal irradiance and all forms of diffuse light in the plane of the PV array
(g) integrated irradiance: continuously integrated spectral irradiance over a speci-fied range of wavelengths
2 Solar Spectra 31 NOTE: If the spectral range is limited, the range is to be stated. If not, then the irradiance is integrated over the total or almost total range of wavelengths.
Integrated irradiance is measured by a pyranometer (see Chap.10).
(h) spectral irradiance: (Symbol:El) (Unit: W×m−2×nm−1): irradiance per unit wavelength
Irradiation:
(Symbol:H) (Unit: J×m−2)
irradiance integrated over a specified time interval
(a) diffuse irradiation: diffuse irradiance integrated over a specified time interval (b) direct irradiation: direct irradiance integrated over a specified time interval (c) global irradiation: global irradiance integrated over a specified time interval (d) total irradiation: (Symbol:HT): total irradiance integrated over a specified time
interval.
References
1. Standard Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables,ASTM E490-00a.
West Conshohocken, PA: ASTM International; (2014)
2. Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data. Int. Electrotechm. Comm.IEC 60904-3: Edition 4.0-Part 3 (2019)
3. Terrestrial photovoltaic (PV) modules—design qualification and type approval—Part 1–3: spe-cial requirements for testing of thin-film amorphous silicon based photovoltaic (PV) modules.
Int. Electrotechm. Comm.IEC 61215-1-3(2016)
4. D. Yogi Goswami, Solar Energy Resources, inEnergy Conversion, ed. by D. Goswami, F.
Kreith (CRC Press, Boca Raton, 2017).https://doi.org/10.1201/9781315374192
5. N. Jenkins, J. Ekanayake, The Solar Energy Resource. Renewable Energy Engineer-ing(Cambridge University Press, Cambridge, 2017), pp. 120–137.https://doi.org/10.1017/
9781139236256.005
6. D.L. King, J.A. Kratochvil, W.E. Boyson, Measuring Solar Spectral and Angle-of-Incidence Effects on PV Modules and Solar Irradiance Sensors.26th IEEE PV Specialists Conference, pp. 1113–1116 (1997).https://doi.org/10.1109/pvsc.1997.654283
7. Data provided by Kristijan Brecl and Marko Topic, University of Ljubljana, Slovenia in personal communication (October 2019)
8. K. Mertens,Photovoltaics—Fundamentals, Technology and Practice(pp. 21–42), 1st Edn.
(Wiley Ltd., 2014)
9. J. Quill, G. Fedor, P. Brennan, E. Everett, Quantifying the Indoor Light Environment,Q-Lab Corporation, (2007). https://www.q-lab.com/resources/technical-bulletins.aspx LX-5026—
Quantifying Indoor Light. Accessed 25 Nov 2019
10. S. Kim, M. Jahandar, J.H. Jeong, D.C. Lim, Recent Progress in Solar Cell Technology for Low-Light Indoor Applications. Curr. Altern. Energy3, 3 (2019).https://doi.org/10.2174/
1570180816666190112141857
11. International Electrotechnical Vocabulary.www.electropedia.org. Accessed on 25 Dec 2019
32 A. Funde and A. Shah
Adinath Funde is Assistant Professor at School of Energy Studies, Savitribai Phule Pune University (formerly University of Pune), Pune, India from the year 2011. He is coordinator for Solar Photovoltaic and Energy Storage initiatives of the depart-ment, School of Energy Studies of his University. His broader research interests are Renewable Energy Conversion and Energy Storage. His present specific research interests include low cost alternative materials for solar photovoltaics, battery electrode materials for sodium ion chemistry, and iron flow batteries. He has worked as Visiting Post-doctoral Fellow at Aalto Univer-sity, Finland, where he worked on hybrid structure solar cells.
He is member of several professional organizations. Adinath obtained his Ph.D. in Physics from University of Pune in 2011 on research in thin-film solar cells.
Arvind Shahis the Founder of the Photovoltaics Research Lab-oratory (PV-Lab), at the Institute of Microtechnology (IMT), in Neuchâtel, Switzerland.
PV laboratory, Neuchâtel, has done pioneering work in the establishment of low-cost production methods for solar cells based on silicon: It introduced a novel plasma-assisted depo-sition method called “VHF depodepo-sition” permitting a signifi-cant increase in the deposition rate for thin-film silicon layers.
It also introduced microcrystalline silicon, deposited by VHF plasma, and with very low oxygen content, as novel absorber layer, within thin-film solar cells.
From 1979 to 2005, Arvind was a professor at the University of Neuchâtel. From 1987 to 2005, he was additionally a part-time professor at the EPFL Lausanne.
In 1975, he founded and co-directed the Centre for Elec-tronics Design and Technology (CEDT) at the Indian Institute of Science in Bangalore. CEDT is now one of India’s leading University Centres in the field of electronics.
Since 2006, he has been active as a scientific consultant to the PV laboratory and to various Industries, in Europe, India and the USA.
He received the Swiss Solar Prize, together with Johannes Meier in 2005. He received the Becquerel Award in 2007.