Solar radiation is a mixture of light of varying wavelengths, some of which are visible to the naked eye and some of which are not. If solar radiation intensity is represented in a graph as a function of wavelength, the solar radiation spectrum is obtained (see Figure 2.42). The radiation for this spectrum at the edge of the Earth’s atmosphere (AM0) differs somewhat from the mean radiation incident on the Earth’s surface in
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Absorption by water vapor in the atmosphere
E = hv
Figure 2.42 Spectrum of solar radiation: (i) Intensity as a function of wavelength and photon energy, (ii) Ultraviolet (UV) spectrum: 100nm < l < 380 nm, (iii) Visible spectrum: 380nm < l < 780nm, (iv) Infrared (IR) spectrum: 780 nm < l < 1 mm, (v) AM0: extraterrestrial radiation spectrum, (vi) AM1.5: spectrum of radiation on the Earth’s surface following penetration of 1.5 times the density of the atmosphere
Central Europe (AM1.5), by virtue of the fact that a portion of this radiation is lost through atmospheric reflection, absorption and scatter.
As is well known, light consists of both waves and particles. In order to understand how solar cells work, it is essential to be aware of the fact that light is composed of a large number of individual light quanta (light particles) or photons, each of which exhibits a very specific energy E, which in turn has a very specific relationship with wavelength and frequency. The following equation applies to photon energy:
E — h ■ v — h
E — photon energy (often indicated as electron volts (eV) rather than joules, where 1eV= 1602 ■ 10~19J) n — frequency (Hz) l — wavelength (m)
h — Planck’s constant — 6.626 ■ 10~34Ws2 c — speed of light — 299 800km/s — 2.998 ■ 108m/s
A side-by-side comparison of the AM1.5 and AM0 spectra in the solar spectrum (Figure 2.42) clearly shows that certain wavelength ranges are absorbed in whole or in part by specific elements of the atmosphere. According to [Sta87], the AM1.5 spectrum (see Figure 2.42) equates to an irradiance of 835 W/m2, which is the irradiance that occurs at sea level after solar radiation has passed through 1.5 times
the density of the atmosphere. On the other hand, solar cell output Pmax and efficiency Zpv are usually determined using the AM1.5 spectrum, increased by the factor 1.198 — 1/0.835, which equates to an irradiance of 1 kW/m2 (see the bolded black curve in Figure 3.19 and the tabular values in [Gre95]).
Hence the spectral intensity distribution of solar radiation is a key factor for PV systems since a photon can only release electrical energy in a solar cell if the photon carries at least one specific energy (band gap energy Eg) whose nature varies according to the material involved. Lower-energy photons do not promote energy conversion. With higher-energy photons, only a portion of the energy – namely, the band gap energy – can be used (see Chapter 3). The band gap energy of monocrystalline silicon is around 1.1 eV. On overcast days, chiefly diffuse radiation is available in the visible spectrum, i. e. the percentage of visible light in the spectrum is somewhat greater (see Figure 2.42). Hence, under such conditions reference cells register somewhat higher irradiance relative to pyranometer readings (see Figure 2.47).