Solar Radiation Spectrum

The sun supplies us its energy in the form of a specturm of electromagnetic radiation that spans a large range of wavelengths from 200 nm to more that 50 000 nm with the peak around 500 nm. Approximately 47% of the incident extraterrestrial solar radiation is in the visible wavelengths from 380 nm to 780 nm. The infrared portion of the spectrum with wavelengths greater than 780 nm accounts for another 46% of the incident energy and the ultraviolet portion of the spectrum with wavelengths below 380 nm accounts for 7% of the extraterrestrial solar radiation.

Watt Engineering Ltd. On the Nature and Distribution of Solar Radiation US Government Printing Office Stock No. 016-000-00044-5, March 1978

As the sunlight passes through the atmosphere, a large portion of the UV radiation is absorbed and scattered. Air molecules scatter the shorter wavelengths more strongly than the longer wavelengths. This scatters more blue light and is the reason the sky appears blue. Water Vapor and atmospheric dust further reduce the amount of direct sunlight passing through the atmosphere. On a clear day approximately 75% of the extraterrestrial direct normal irradiance passes through the atmosphere without being scattered or absorbed.

The "incident solar spectrum" is the portion of energy, which is transported by the different ranges of wavelengths to the Earth's surface. The total radiation power which arrives on the Earth's surface is approximately 1000 W/m².

Selective Absorbers

The basic task of a solar absorber is to absorb as much solar radiation as possible. Black surfaces are particularly suitable for this task because they absorb nearly the entire solar radiation and convert it into heat.

The disadvantage is that the absorber surface becomes hot and starts to emit energy in the form of heat radiation (infrared radiation). Thus, about 50% of the absorbed energy is lost!

In order to reduce the loss by heat radiation, we take advantage of the fact that 99% of the solar radiation energy is irradiated in a wavelength range below 2.5µm, whereas 99% of the heat radiation of a 100°C warm surface (maximum work temperature of a warm water collector) is emitted above this characteristic wavelength (λ_cut-off = 2.5µm).

Thus, the ideal solar energy absorber must fulfill the following conditions:

• maximum possible absorption of solar radiation within the range below 2.5µm, and
• minimum emission of heat radiation above 2,5µm.

The layered structure of TiNOX "selective" absorbers exploits this fact, so that compared to a black surface, the losses by heat radiation are decreased significantly by selective absorbers.