**9. Spectral selectivity surface applications**

We now discuss a new specialized area of solar energy application, based on the spectral selectivity property of a surface. It is a new and special innovative concept in solar energy application.

It was discovered that optical properties of materials can be modified to select wavelengths of the solar spectrum to transmit, or absorb or reflect. On these principles the following applications are possibe:


Spectral selectivity of a surface is achieved by applying special coatings on substrates, which may be transparent or opaque, with the intention of modifying the optical properties of the surface, such that the surface selects wavelengths of the solar spectrum to transmit or absorb or reflect. These properties are: transmittance, absorbance, reflectance, emittance, absorption coefficient (**α**) extinction coefficient (**k**) refractive index (**n**) to mention a few, and upon which relevant applications are based. Surfaces of different material coatings will produce different values of these optical properties at different wavelengths of the solar spectrum.

Solar radiation is transverse oscillating electric and magnetic fields. The electromagnetic fields interact with the electric charges of the material of the surface on which solar radiation is incident. The interaction results in the modifications of the solar radiation at different parts of its spectrum. As a result, some parts of the radiation are absorbed, some are transmitted, and some are reflected back to space (Granquist, 1985; Lovern, et al, 1976). Thus, by spectral selectivity of a surface, it is meant surfaces whose values of absorptance, emittance, tramittance and reflectance of radiation and other related optical properties vary with wavelengths over the spectral region, 0.3≤ λ ≤ 3µm (Loven, et al. 1976; Maniel and Maniel, 1976).

For example, a spectral selective surface having high absorptance in the wavelength range 0.3 µm ≤ λ ≤ 3µm, and high reflectance at 3 µm ≤ λ≤ 100 µm will appear black with regards to the short wavelengths range, 0.3 µm ≤ λ ≤ 3µm and at the same time appear an excellent mirror in the thermal region, i.e. 3 µm ≤ λ≤ 100 µm. A device with these properties is called a "heat mirror".

We shall discuss briefly, for example, the principle of the following spectral selectivity applications of solar energy.

i. Heat mirror

14 Solar Radiation

cost backing such as glass or plastics. Electrical contacts, anti-reflective coatings, and protective layers are also applied directly to the backing materials. The films conform to the shape of the backing, a feature that allows them to be used in such innovation product

This is a new solar energy electric conversion technology in which solar cell is currently being developed from various organic matters (dyes). They are sort of thin films discussed above. The crystallized silicon solar cells have being a standard technology in solar conversion devices for over fifty years. However they are still expensive, and relatively inefficient (they have achieved only 50% efficiency so far). Right now, various types of organic solar cells from dye materials are being studied and may soon replace the silicon solar cells, because they (organic solar cells) will be fabricated with greater efficiency, low cost processes, and they will be more versatile than silicon solar cells. Further still, they have added advantages of being thinner, lighter and more colourful

We now discuss a new specialized area of solar energy application, based on the spectral selectivity property of a surface. It is a new and special innovative concept in solar energy

It was discovered that optical properties of materials can be modified to select wavelengths of the solar spectrum to transmit, or absorb or reflect. On these principles the following

Spectral selectivity of a surface is achieved by applying special coatings on substrates, which may be transparent or opaque, with the intention of modifying the optical properties of the surface, such that the surface selects wavelengths of the solar spectrum to transmit or absorb or reflect. These properties are: transmittance, absorbance, reflectance, emittance, absorption coefficient (**α**) extinction coefficient (**k**) refractive index (**n**) to mention a few, and upon which relevant applications are based. Surfaces of different material coatings will produce different values of these optical properties at

as flexible solar electric roofing shingles.

**9. Spectral selectivity surface applications** 

**8.4 Organic solar cells** 

than silicon solar cells.

applications are possibe: Selective absorbers, Heat mirrors, Reflective materials, Anti-reflective,

 Fluorescent concentrator, Holographic films, Cold mirrors, Radiative cooling, Optical switching,

Solar control window.

Transparent insulating materials,

different wavelengths of the solar spectrum.

application.


#### **9.1 Heat mirror**

A solar collector with a highly selective absorber in the short wavelengths range of solar radiation, that is, at 0.2 ≤ λ ≤ 3μm, will reflect very highly the thermal radiation (IR) component of solar radiation. This implies that the device is black to this short wavelengths range because it absorbs them, and forms an excellent mirror in the thermal region because it reflects them. The device is called a "Heat mirror". Thus heat mirror is essentially a device that transmits or absorbs the short wavelengths radiation (UV – VIS) and reflects long thermal wavelengths (IR) of solar radiation. That is, it is a window to the short wavelengths and a mirror to the long wavelengths. Such a surface is therefore suitable for architectural windows in buildings, where low temperature or cooling effects is desired. This device therefore may be adaptable for passive cooling in a tropical climate region.

The heat mirror device is obtained by using a semiconductor–Metal Tandem. Thus, it can be called absorber-reflector Tandem. The semiconductor components are arranged to reflect the thermal radiation (IR), while the metal components absorb or transmit the UV – VIS radiation. A heat mirror device is also called a transmitting selective surface.

In the arrangement of the components, the reflective layer surface is arranged to cover the non-selective absorber base. In this way, the selective reflector reflects the thermal infrared radiation ( λ > 3 µm) and transmits the short wavelength range ( λ < 3 µm). The short wavelength radiation transmitted by the reflector is absorbed by the black absorber base. Some highly doped semi conductors such as InO2, SnO2 or the mixture of the two, Indium-Tin-Oxide (ITO), have been used successfully to produce the reflector component of the device (Seraphin, 1979). A heat mirror may therefore be used to separate heat radiation (IR) and light radiation (VIS) of the solar spectrum. The IR energy separated could be used for thermal purposes such as the thermo-photovoltaic.
