Page 118 - A Comprehensive Guide to Solar Energy Systems

116 A COMPrEHEnSIvE GUIdE TO SOlAr EnErGy SySTEMS

(shown in Fig. 6.5 [16]). In a direct system, the service water is directly circulated between
the water tank and the collector, while in an indirect system, a heat transfer fluid, usually
antifreeze, distilled water, or an organic fluid, is circulated through the solar collector. A
heat exchanger is employed to affect the heat transfer from the collector to the service wa-
ter in the tank. The heat exchanger could be inside or outside the hot water tank as shown
in Fig. 6.5A and B.
The indirect system, in most situations, performs better than the direct one. The in-
direct system is less climate-selective and more suitable for use in regions experiencing
cooler temperatures.

6.3.3 SWH Systems in Different Solar Collector Configurations

Solar collectors can have many variations according to their operating temperatures as
shown in Fig. 6.6 [17]. Unglazed panels and flat-plate water and air collectors are catego-
rized as low temperature solar collectors. Evacuated-tube, line-focus, and point-focus col-
lectors are classified as high temperature solar collectors.

6.3.3.1 Low Temperature Solar Collectors
Unglazed panels are most suitable for swimming pool heating as it is only necessary for
the water temperature to rise by a few degrees above ambient air temperature.
Flat-plate water collectors are the mainstay of domestic solar water heating (SWH)
worldwide. They are usually single glazing but may have an extra second glazing. The
absorber plate should have a high thermal conductivity to transfer the collected en-
ergy to the water with a minimum temperature drop. It usually has a black surface or
a selective coating that has both high optical absorption and low emission, to cut heat
losses.
Flat-plate air collectors are not as popular as water collectors and flat-plate air collec-
tors are usually used for space heating. One application of this type of collector involves
the linking up with a photovoltaic panel to produce both heat and electricity.

FIGURE 6.5 Schematic of the indirect SWH systems [16].