Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 118

Fig. 15 Flat plate solar collector [ 17 ]

with liquid or with air. In a collector with liquid, the solar energy heats the liquid

flowing through the tubes attached to the absorbent plate. The tubes may be
arranged in parallel using entrance and exit collectors or in coils. The coils ensure a
uniform

flow, but raise problems when emptied, since they cannot be totally
emptied, in order to avoid the freezing of the water.
The cover has the role of reducing losses through convection from the absorbent
plate through the air stratum in between the two surfaces, as well as losses through
irradiation from the collector, the cover being transparent for the short waves of the
irradiation received from the Sun and almost opaque for the long waves of the
thermal irradiation emitted by the absorbent plate. The cover may be made of
transparent or translucent glass or of plastic materials. Translucent glass with a low
content of iron is the most widely used material because it transmits a high per-
centage of the solar irradiation (approximately 0.85 0.90 of the normal incident

irradiation on the glass surface) and is almost opaque for the thermal irradiation
emitted by the absorber.
The sheets or plates made of plastic have a good transmittance for the short
waves of the irradiation received from the Sun, but a high transmittance (of
approximately 0.4) for the long waves of the thermal irradiation. The use of plastic
materials is limited by the temperature they are resistant at without deteriorating or
modifying dimensions. Few plastic materials resist for a long time to the action of
the ultraviolet rays of the Sun. The advantages of the plastic covers consist in

-

flexibility, reduced mass, and the fact that they do not brake when hail falls or
stones hit. Antire exive covers and textured surfaces improve transmittance.

The absorbance of the collector surface for the short waves of the solar irradi-
ation depends on the nature and color of the cover and on the angle of incidence.
Since almost all black paints re

ect approximately 10 % of the incident irradiation,
the absorbent plates undergo electrolytic or chemical treatments to obtain surfaces