Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 121

manufacturing techniques (Duffie and Beckman, 1991) to ensure good thermal contact
between the tubes and the plate.
4.1.3 Evacuated tube collectors
Evacuated-tube collectors are fabricated from either concentric glass tubes or a metal
tube end-sealed to and within a glass tube. An enclosed evacuated annular space and a
selective absorber surface provide a very low overall heat loss particularly when oper-
ated at higher temperatures. The evacuated space between the glazing and absorber
eliminates convective loss and long-wave thermal radiative heat loss is inhibited by
the deposition of the spectrally selective absorber coating on the absorber surface
(Morrison, 2001). Evacuated-tube solar collectors generally have low thermal mass.
The ability to heat-up rapidly (often from higher maintained overnight temperatures
than a flat plate collector) gives low utilizable insolation thresholds providing good
low insolation performance. Heat removal in evacuated tube collectors can be indirect
often using a volatile fluid in the absorber via a closed heat pipe (Tabassum et al.,
1988). More frequently water, as a heat transfer fluid, moving in a thermosyphon
through the collector is employed (Morrison, 2001, Budihardjo and Morrison, 2007).
Evacuated tube collectors can have copper absorbers or use a selectively coated glass
inner glass tube as the absorber. The latter all-glass type is now commonplace in China
with over 15 million square metres of collector area installed by 2010 (Norton, 2011).
A typical example of an evacuated tube collector is show in Figure 4.1.2.
Figure 4.1.2 An all-glass “wet type'' evacuated tube solar energy collector.