Page 27 - Applied Photovoltaics

thermoelectric effect (voltage generated by heat differences across junctions of
dissimilar material) forms the basis of the more accurate equipment (pyrometers,
pyrheliometers) since this effect is less sensitive to the wavelength of light.
Obtaining accurate solar insolation data in an appropriate form is obviously important
for designing photovoltaic systems but it is sometimes a difficult task. One of the
most widely available data forms is the average daily, monthly, quarterly or annual
total global (direct and diffuse) radiation falling on a horizontal or tilted surface.
Examples are shown in Fig. 1.12, which gives the quarterly-average global isoflux
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contours for each quarter in MJ/m per day. Similar global plots are available from
Sandia National Laboratories (1991). Where possible, more exact data for each
particular location should be sought, preferably in the form of direct and diffuse
components rather than global insolation levels. Some sources of insolation data are
listed in Appendix D. Direct and diffuse components have been measured and are
available for some locations. Data for several Australian sites have been processed
into a range of forms useful to solar energy engineers and architects (Lee et al.,
2003).

Peak sun hours data
Average daily insolation values for each month are sometimes presented in the form
of ‘peak sun hours’. Conceptually, the energy received throughout the day, increasing
from low intensity in the morning, peaking at solar noon and declining during the
afternoon, is compressed into a reduced duration of noon intensity sunlight (Sandia
National Laboratories, 1991). If the intensity of noon insolation (peak sun) is
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approximated to 1.0 kW/m , the number of peak sun hours coincides with the total
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daily insolation measured in kWh/m .
Sunshine hours data
A form in which solar insolation data are commonly available is as ‘sunshine hours’
(SSH) (Twidell & Weir, 2006). This term indicates the number of daily hours of
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sunlight above a certain intensity, approximately 210 W/m , for a given period
(usually a month), but gives no indication of absolute values and applies only to the
direct component of sunlight. The measurements of ‘sunshine hours’ are made on a
Campbell-Stokes sunshine hours instrument by concentrating parallel rays of light
onto a small area of moving tape, which burns if the sun is shining brightly. Diffuse
light cannot be concentrated in the same way and is not recorded by the instrument.
The resulting data are not very high quality and are not recommended for use except
where they can be reliably correlated with irradiation (Standards Australia, 2002), but
are available for many locations where irradiation has not been recorded.
For PV system design, the difficulty is in converting SSH data to a useable form.
Here we consider techniques for estimating, from SSH data, the monthly average of
daily global radiation incident on a horizontal surface (Iqbal 1983)

R R (a b n / N ) (1.12)
0 d
where R is defined by Eqn. (1.11), n is the recorded monthly average of bright
o
sunshine hours per day, as measured by a Campbell-Stokes instrument, a and b are

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