Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 122

4.1.4 Collector components
4.1.4.1 Absorber plate
The plate and tubes of both a flat-plate and metal-in-glass evacuated tube solar energy
collector absorber are usually made of a metal with high thermal conductivity such
as copper or aluminum. Good heat transport is thus provided through the plate to
the heat transfer fluid. An ideal absorber plate has a high solar absorbance surface
to selectively absorb as much as possible of the incident insolation, together with low
emittance to long-wave thermal radiation so that long-wave radiative heat losses are
low (Norton, 1992). Such solar selective absorbers often consist of two layers with
appropriately different optical properties; often a thin upper layer that exhibits high
absortance to solar radiation whilst being relatively transparent to thermal radiation is
deposited on an underlying surface whose high reflectance provides low emittance to
thermal long-wave radiation. Alternatively a heat selective mirror that has high solar
transmittance with high infrared reflectance can be placed on top of a non-selective
high absorbtance material. An example of this latter of selective surface combination is
“black chrome'' that consists of microscopic chromium particles deposited on a metal
substrate; the chromium particles reflect long-wave thermal radiation, but shorter
wavelength insolation passes between the chromium particles.
In very low cost low temperature solar energy collectors for applications, such as
swimming pool heating (Ruiz and Martinez, 1992), all these guidelines for absorber
materials and fabrication are inappropriate as the minimum initial cost usually domi-
nates design choices. Black paint which has a high absorptivity but not being selective
is equally high emittance is used on absorbers or is the dark pigment in unglazed often
ground-mounted plastic coils through which water is solar heated.
4.1.4.2 Aperture cover
Most glasses are almost completely transparent to the shortwave radiation the asso-
ciated with insolation, but nearly opaque to long-wave thermal radiation. When
employed as an aperture cover, glass inhibits successfully radiative heat loss from an
absorber plate to ambient or to the lower temperature radiative heat sink that the sky
often constitutes. Glass is thus the aperture cover material employed most commonly
for solar energy collectors (Norton, 1992). It is desirable that a large part of the incom-
ing direct, diffuse and ground reflected solar radiation is transmitted through the cover
and used efficiently to heat the transfer fluid. This means that the transmittance of an
aperture cover must be high which requires both low reflectance and absorptance.
The reflectance of a material depends on its refractive index and the angle of insola-
tion incidence. It can vary for different wavelengths of insolation transmittance and
decreases with increasing angle of insolation incidence (Duffie and Beckman, 1991).
An aperture cover glazing with a smaller refractive index exhibits a lower reflectance
and higher transmittance. The overall solar transmittance of an aperture cover glazing
is the normalized sum of each quantized wavelength of the solar radiation spectrum
transmitted by the aperture cover. Since iron absorbs light in the visible part of the
spectrum, the transmittance of glass for the solar spectrum decreases with increasing
iron concentrations. Thus low-iron glass typically with an iron content below 0.06
percent is preferred for flat plate collector aperature cover glazing.