Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 119

with high solar irradiation absorbance values ( ) for

long waves (selective surfaces). Selective surfaces consist in a thin layer with high
absorbance for the short waves of the solar irradiation, and relatively transparent to
the long waves of the thermal

ʱ ) and low emittance values ( ʵ

irradiation, deposited on a surface with high

re ectance and low emittance for small waves. Selective surfaces are important

when the temperature of the collector is much higher than the temperature of the
environment air. The absorber of commercial collectors is achieved through gal-
vanisation, anodisation, evaporation, and selective paint coating.
The tubes that the work

flows through must be an integral part of or joint to
the absorbent plate. A thermal welding between the tubes and the absorbent plate
must not imply excessive costs for the materials or work. The absorbent plates are
most often made up of copper or aluminum, because both metals are good heat
conductors. An absorbent plate must have a high thermal conductivity to transfer
the collected heat to the water with minimum losses. Extruded plates of plastic
materials resistant to ultraviolet raise may be used in collectors which function at
low temperatures, if the entire surface is in contact with the

uid

fluid which transfers the

heat.
In practice, most collectors are

fixed and that is why they must be so oriented as
to collect a maximum of solar irradiation during 1 day. This is the reason why xed

collectors are arranged in a leaning position, facing the Equator. Usually, for small
latitudes, the leaning angle of the collector is equal to the angle of the latitude, but it
increases with 10 . The leaning angle of the collector depends

on the latitude and the day of the year. If the leaning angle is equal to the latitude,
then the solar raise will be perpendicular to the surface of the collector in mid
March and September. To maximize the irradiation collected during summer, the
leaning angle of the collector must be smaller, and to maximize the irradiation
collected during winter, the leaning angle must be large.
In a

° at latitudes above 40 °

flat plate solar collector, the radiant energy incident on the surface cannot be
increased and that is why the surface needs to absorb as much of the incident
irradiation as possible, and the energetic losses of the collector must be as small as
possible.
The performances of the conventional

flat plate collector are much reduced
during cloudy weather, cold weather, or windy weather. Moreover, condensation
and humidity deteriorates the material inside the collector, having as a result a
decrease in performances and eventually the destruction of the system.
Solar collectors with evacuated tubes function differently from the

flat plate
collectors. The evacuated tube is made of borosilicate transparent glass, and the
absorber in the shape of a plate or a tube of selective cover copper.

These collectors were developed in many constructive variants namely heat pipe,
direct

—

flow, U-tube, etc. according to the method used to extract heat from the
evacuated tube, but all of them use vacuum to insulate the absorber. The vacuum
envelope reduces the heat losses through convection and conduction, so that higher
temperature may be reached than in the flat plate collector. These collector types are

formed from an array of evacuated tubes joined to a manifold through which the
heat transfer liquid (water or water/glycol)

ows.