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Figure 2-8. Evacuated tube solar heat collector.
2.1.10 Other types of solar collectors.  The three most common types of
solar collectors are flat plate collectors, evacuated tube collectors, and
concentrating  collectors.  Due to certain cost and performance advantages,
flat plate collectors have been used extensively for residential DHW and space
heating applications.  Evacuated tube and concentrating collectors are used
mostly in solar applications requiring very high temperatures.  Some applica-
tions requiring large solar arrays are using evacuated and concentrating
collectors.  A brief description follows. Evacuated-tube collectors.  Figure 2-8 shows an evacuated-tube
collector.  This type of collector uses a vacuum between the absorber and the
glass outer tube to significantly reduce convection and conduction heat losses.
Evacuated-tube collectors operate essentially the same as flat-plate
collectors.  Solar radiation passes through the outer glass tube and is
absorbed by the coated absorber.  Heat energy is transferred to fluid flowing
through the absorber.
Most evacuated-tube designs collect both direct and diffuse radiation
efficiently, but certain types are specifically designed for more efficient
collection of direct radiation.  Although evacuated-tube collectors are
considerably more expensive than typical flat-plate collectors, they are much
more efficient and more cost effective when high collection temperatures are
needed for operating absorption chillers or for industrial process heating.
They may not be as efficient as flat-plate collectors at low-temperature
applications such as domestic water heating and space heating.  For these
applications, evacuated tube collectors should be judged on a cost per Btu
basis to determine their effectiveness.  Maintenance costs may be higher and
heat dissipation coils may be required.  Comparisons can be made using data
similar to that in Figure 2-7 and Table 2-6, although evacuated tube
collectors normally show a higher "all-day" performance than that indicated by
the instantaneous values of Table 2-6.

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