Flat-plate and evacuated tube collectors operate by the "greenhouse effect." As
seen in Figure 2-17, the heat energy from the sun is in short waves, which make
their way easily through the molecules of the glazing material. Once inside they
Fluid flowing through the
strike, and are absorbed by, the black absorber plate.
absorber passageways carries the heat away.
Heat energy attempting to re-radiate from the absorber does so in much longer
waves than the original incoming energy, and does not move back out through the
glazing. In addition, most absorbers are coated with a "selective" surface which
absorbs large percentages of available heat, and re-radiates small amounts.
A Typical Flat-plate
Evacuated tube collectors further reduce heat loss by surrounding the hot absorber
with a vacuum. This eliminates heat loss by convection (heat movement involving
the warming and lifting of a gas or liquid). The vacuum serves as the insulation.
The glass tube functions as the frame and the glazing.
2.7 COMPONENT OPERATION