14 June 2002
water and space heating. These collectors usually consist of four basic components:
casing, back insulation, absorber plate assembly, and a transparent cover. Figure 3-4
shows the typical components of a flat plate collector. The absorber panel is a flat
surface that is coated with a material that readily absorbs solar radiation in the thermal
spectrum. Some coatings, known as "selective surfaces", have the further advantage
of radiating very little of the absorbed energy back to the environment. Channels
located along the surface or within the absorber plate allow the working fluid to
circulate. Energy absorbed by the panel is carried to the load or to storage by the fluid.
The absorber panel is encased in a box frame equipped with insulation on the back
and sides and one or two transparent covers (glazing) on the front side. The glazing
allows solar radiation into the collector while reducing convective energy losses from
the hot absorber plate to the environment. Similarly, back insulation is used to reduce
conductive energy loss from the absorber plate through the back of the collector.
(b) Unglazed. Unglazed collectors are the least complex collector type
and consist of an absorber plate through which water circulates. This plate has no
glazing or back insulation. These collectors are often made of extruded plastic because
they are designed to operate at relatively low temperatures. Since they are not
thermally protected, these collectors should be operated only in warm environments
where lower thermal losses will occur. Swimming pool heating is the most common use
of unglazed collectors.
(c) Evacuated Tube. Evacuated tube collectors are best suited for
higher temperature applications, such as those required by space cooling equipment or
for higher temperature industrial process water heating. Convective losses to the
environment are decreased in this type of collector by encapsulating the absorber and
fluid path within a glass tube that is kept at a vacuum. Tracking mechanisms and/or
parabolic solar concentrating devices (simple or compound) are often used, resulting in
somewhat higher equipment costs.
Collector Efficiency and Performance.
(a) Definitions. Collector efficiency is defined as the fraction of solar
energy incident upon the face of the collector that is removed by the fluid circulating
through the collector. Several parameters are defined as follows:
= heat transfer fluid inlet temperature.
Ta = ambient air temperature.
= solar irradiance on the collector
Ac = solar collector surface area.
FR = collector heat removal factor, a dimensionless parameter describing the ratio
of actual energy gained by the collector to that which would be gained, in the
limit, as the absorber plate temperature approaches the fluid inlet
temperature. This value is similar to a conventional heat exchanger's