Utilization of Solar Spectral Regions Spectral Response of Materials

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Why this extended discussion of the solar spectral distribution? The primary reason is provided by the example discussing the sensitivity of our eyes, in the previous

Human Eye Spectral Response

Fig. 7. Temperature and color relationships for various natural and artificial sources of optical radiation.

Fig. 7. Temperature and color relationships for various natural and artificial sources of optical radiation.

Artificial Optical Radiation
Fig. 8. Decreasing direct beam spectral irradiance as aerosol optical depth at 500 nm increases from 0.05 (top curve) to 0.04 (bottom curve).
Daytime Solar Spectrum
Fig. 9. Human eye daytime (photopic) relative spectral response (right axis) and solar spectrum (jagged curve).

Section. Figure 9 shows the wavelength region where the human eye responds, the spectral response of the eye, overlaying an Air Mass 1.5 global solar spectrum. The peak of this response is at 555 nm, corresponding to the color green.

Many of the materials used in renewable, and specifically, solar energy system applications have a significant response to the solar spectrum over a limited spectral interval.

The green color of most plant leaves is the result of absorption of blue and red light by chlorophyll, which reflects almost all of the light in the region around 550 nm, as shown in Fig. 10.

Our skin contains compounds that absorb ultraviolet light with wavelengths shorter than 400 nm. These compounds react with the UV photons to produce sun-tans, sunburns, or even cancer.

Similarly, many semiconductors, such as silicon, germanium, etc. produce a flow of electrons (the photovoltaic effect) when photons of a certain wavelength interact with the materials. Figure 11 shows Air Mass 1.5 direct normal, diffuse sky, and total global solar spectral distributions, with indications of the spectral regions where our vision, plants, and various photovoltaic materials interact with the solar spectral distribution.

Absorption spectra of Chlorophyll a and b b

Absorption spectra of Chlorophyll a and b b

Generation Various Spectral
400 500 600 700

Wavelength / (nm)

Fig. 10. Blue and red light absorbed by chlorophyll (types a and b shown) used in photosynthesis. Green (around 550 nm) is reflected and not used.

Spectral Regions
Fig. 11. Solar spectral distributions and the various regions of spectral sensitivity for vision, plant photosynthesis, and photovoltaic conversion technologies.

Figure 12 shows that various combinations of photovoltaic materials can be constructed to respond over different spectral ranges, utilizing more or less of the solar spectral distribution.8

The different response regions shown in Fig. 12 are the result of the band gap between bound electrons in the material and the conduction band for electrons (and holes) in terms of energy (in electron volts, eV). The energy, E, of a photon is related to the wavelength as E = (h c) / X. In semiconductors suitable for Photovoltaic applications, the band gaps are relatively close together, so photons with relatively low energy (0.5 to 1.5 eV) can stimulate electrons to enter the conduction band. We can convert the power versus wavelength plot to one of number of photons versus energy, as on the left of Fig. 13. On the right of Fig. 13 we show the relationship between the available solar energy and projected conversion efficiencies of some PV materials can approach 50%, if concentrated (focused by lens or mirrors) solar radiation is used in conjunction with future generation materials.

Cadmium Selenide
Fig. 12. Spectral response regions for various photovoltaic technologies. CIG stands for cadmium indium gallium selenide.
Photovoltaic Efficiency Wave Length
Fig. 13. Correlation of available solar spectral photon energies with band gap of present and future generation photovoltaic materials. New represents some new combination of materials optimized or tailored for a specific band gap.

As Fig. 13 shows, future generation materials with "designer" band gaps can produce higher efficiency devices to generate more electricity with the same solar spectrum. By stacking the available materials, additional components of the solar spectrum contribute to the overall production of conduction electrons, or electric current.

Optimization of the performance of solar energy systems, as well as building thermal performance (heating and cooling loads), and daylighting (window performance) all require knowledge of the terrestrial solar spectral distribution. Optical properties of materials such as transmittance, reflectance, and absorption are always dependent on the wavelength of the incident radiation. For example, Fig. 14 shows the properties of several components of a window for building applications. The top left panel shows the properties of a single pane of glass which permits thermal infrared radiation into a room; high inside reflectance keeps the thermal energy trapped, offsetting the need for more heating energy in a cold climate.

In the lower panels of the figure, properties of each pane of a double pane structure are shown. Low IR transmittance (lower left) of the outer pane keeps thermal infrared solar radiation from entering the building. The broad transmittance band of the inner layer and low inside reflectance of the inner pane allows thermal infrared energy to escape. This structure reduces the cooling load in a sunny environment.

Spectral Response Reflectance

Fig. 14. Examples of optical properties of materials (reflectance, transmittance) for window structures. When used in conjunction with solar spectral distributions, energy savings can be computed.

Fig. 14. Examples of optical properties of materials (reflectance, transmittance) for window structures. When used in conjunction with solar spectral distributions, energy savings can be computed.

Spectral Response Pyranometer

Fig. 15. Spectral response of thermopile pyranometer measuring total solar radiation is shown with thick black line. Spectral radiance (brightness) of the sky dome (blue line). The cut-off at 3000 nm means the radiometer will not respond to the infrared sky radiation that peaks at 7000

nm (7 micrometers).

Fig. 15. Spectral response of thermopile pyranometer measuring total solar radiation is shown with thick black line. Spectral radiance (brightness) of the sky dome (blue line). The cut-off at 3000 nm means the radiometer will not respond to the infrared sky radiation that peaks at 7000

nm (7 micrometers).

Similar principles can be used for designing selective absorbers, where the goal is absorb as much of the solar spectrum as possible and convert that absorbed energy into heat, or thermal energy. Conversely, reflective materials can be designed to select only visible (cold mirror) or infrared (hot mirrors) to isolate and direct selected portions of the solar spectrum for various applications.

Knowledge of the optical properties of materials in relation to the solar spectrum is also important in measuring broadband solar radiation. For instance, a pyranome-ter used to monitor total solar radiation for a renewable energy system has a spectral response (due to the special glass dome protecting the detector) that does not respond to the thermal infrared radiation of the sky beyond 3000 nm, as shown in Fig. 15. However, there will be thermal infrared radiation exchanged between the radiometer and the sky dome, which will influence the measurement performance of the pyra-nometer.9

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