Page 119 - A Comprehensive Guide to Solar Energy Systems

Chapter 6 • Solar Water Heaters 117

FIGURE 6.6 Classification of solar collectors [17].

6.3.3.2 High Temperature Solar Collectors
An evacuated-tube collector consists of a set of modular tubes, where convective heat
losses are minimized by virtue of the vacuum in the tubes. The absorber plate is a metal
strip in the center of each tube, and a heat pipe is used to carry the collected energy to the
water, which circulates along a header at the top of the pipe array.
line-focus collectors can concentrate sunlight onto a pipe running down the center of
a trough that could be pivoted to track the sun up and down or east to west. It is mainly
used for generating steam for electricity plants. A line-focus collector can be orientated
with its axis in either a horizontal or a vertical plane.
Point-focus collectors are also used for steam generation or driving a Stirling engine,
but needs to track the sun in two dimensions.

6.3.3.3 Comparison of the Evacuated-Tube and Flat-Plate Collectors
Flat-plate SWH systems have been used worldwide owing to their structural simplicity and
low cost. However, evacuated-tube collectors with heat pipe arrays are growing in popu-
larity as they have many advantages over the flat-plate collectors. The main features of the
evacuated-tube and flat-plate collectors are compared in Table 6.1 [13].