EVACUATED TUBE COLLECTORS:
Another type of active solar collector consists of several
glass tubes, each of which has concentric inner and outer
walls. The annular space between these glass tubes has
been evacuated of air and thus acts like a Thermos®
bottle. Convective heat transfer between the inner and
outer glass tubes is essentially eliminated. A coated
copper absorber strip with attached tubing is located
within the inner glass tube, as shown in figure 12.
Most current-generation evacuated tubes have a
specialized fluid sealed within the internal copper tubing.
When heated, this fluid changes from liquid to vapor
and rises toward the top of the tube. It then passes into
a small copper capsule that fits tightly into a manifold
assembly at the top of the collector. Heat conducts
though this copper capsule into fluid circulating along
the manifold. The fluid sealed within the evacuated
tubes never contacts the fluid in the manifold. As heat
is released from the fluid within the evacuated tube, it
condenses back to a liquid and flows back to the bottom
of the tube ready to repeat the cycle.
FLAT PLATE COLLECTORS:
The principal component in this type of collector is the
absorber plate, which is usually an assembly of copper
sheet and copper tubing. The top surface of the absorber
plate is coated with dark colored paint or electroplated
“selective surface” coating that absorbs the vast majority
of solar radiation striking it. The instant solar radiation
strikes this surface it is converted to thermal energy
(e.g., heat). The copper sheet acts as a wick to conduct
this heat toward the copper tubing that is welded or
otherwise bonded to the sheet. Heat moves across the
copper sheet toward the tubes because the fluid flowing
through the tubes is cooler than the absorber sheet. This
fluid absorbs the heat and carries it out of the collector.
To minimize heat loss, the absorber plate is usually
housed in an enclosure made of aluminum and capable of
withstanding many years of exterior exposure. The sides
and back of this enclosure are insulated with materials
capable of withstanding temperatures in excess of 350ºF,
which might occur if the collector is exposed to intense
sunlight without fluid flow through its absorber plate.
The upper surface of the enclosure is usually tempered
glass with a low iron oxide content. Tempered glass can
withstand high thermal stress as well as potential impact
from hailstones or other objects. Low iron oxide content
glass minimizes absorption of solar radiation as it passes
through on its way to the absorber plate.
