Radiation Optics

Optical Energy Capture – The rate of optical (short wavelength) radiation incident on absorber/receiver will be the solar irradiance resource for that type of collector and its tracking (global (total) solar irradiance for a flat-plate collector and direct (beam) solar irradiance for a concentrating collector). Since the capture area of the collector may not be aimed directly at the sun, this resource must be reduced to account for the angle of incidence. The area of the collector on which the solar irradiance falls is called the aperture (opening) area of the collector. The incident solar resource then is

where:

• Ia – solar irradiance entering the collector aperture (global (total) or direct (beam))(W/m2)
• Aa – aperture area of the collector (m2)

This solar resource is reduced by a number of losses as it passes from the aperture of the collector to the absorber. These processes depend on the type and design of the specific collector, but here we include the important optical loss mechanisms, and will drop the unimportant terms in future chapters as we discuss specific types of collectors. The rate of optical (short wavelength) energy reaching the absorber or receiver is the product of the incoming solar resource multiplied by a number of factors, all less than 1.0 describing this reduction

where

• – capture fraction (fraction of reflected energy entering or impinging on receiver)
• – reflectance of any intermediate reflecting surfaces
• – transmittance of any glass or plastic cover sheets or windows
• – absorptance of absorber or receiver surface

The first two terms above apply only to concentrating collectors. The capture fraction is a measure of both the quality of the shape of the reflecting surface, and the size of the receiver. Often this is described in terms of ‘spillage’ i.e. the fraction of reflected energy not impinging on or entering the receiver. A poorly shaped concentrator, or a receiver too small will make this number considerably less than 1.0.

Transmissivity of The Cover System

The transmittance is the fraction of solar radiation passing through all transparent cover material that sunlight passes through on its way to the absorber. Cover sheets of glass or plastic are used on flat-plate collectors, above the absorber to reduce convective heat loss.

For parabolic trough collectors a glass tube surrounds the absorber tube for the same reason. High-temperature cavity receivers may incorporate a quartz glass cover to keep the gas in the receiver separate from outside air or to permit pressurization of the gas within the cavity. In all cases, the use of a cover sheet reduces the solar radiation passing to the receiver/absorber. Their benefit for reducing heat losses from the absorber must at least balance this reduction.

Transmittance of the cover also depends on the wavelength of light passing through it. Glass for example transmits most radiation in the visible spectrum, but does not transmit much in the infrared region. Therefore, an absorber covered with glass will receive most of the incoming, short wavelength radiation, but will not transmit much of the long wavelength radiation loss coming from the absorber. This characteristic of glass is the reason that glass greenhouses loose very little energy at nighttime. Carbon dioxide buildup gives our atmosphere a similar property and therefore the name ‘greenhouse effect’.

On the other hand, plastic covers have high transmittance values at very long wavelengths. Solar collectors using plastic covers can be used for nighttime cooling since radiation loss to the nighttime sky can be significant. This is also why greenhouses in warm climates use plastic rather than glass as a cover. Without nighttime long-wavelength radiation loss, the average temperatures would be too high for optimal plant growth.