Environmental Engineering Reference
In-Depth Information
Aperture Cover
Vertical
Reflector
θ
a
Absorber, with or
without selective
surface
θ
a
∅
Horizontal
θ
a
= Acceptance half-angle
∅
= Angle of inclination
Envelope might or
might not be present
Modified reflector to
prevent thermal
short circuit
Figure 4.3.1
A compound parabolic concentrator.
When using a dielectric medium with a refractive index of
>
1, as the concentrator,
light will be refracted to a smaller angle of incidence rendering it possible to accept
insolation across a larger acceptance angle. A concentrator filled with a high refractive
index low-iron dielectric would increase the concentration ratio by 52%.
4.4 SOLARWATER HEATING
4.4.1 Overview
The diverse different forms of solar energy water heaters that are now available
(Norton, 2011) include:
•
thermosyphon solar water heaters (Norton and Probert, 1986) operate by natural
convection, can be direct or indirect and can use a variety of flat-plate or evacuated
tube collectors.
•
pumped-circulation solar water heaters (Reddy, 1987; Duffie and Beckman, 1991)
usually indirect and have larger collector area than thermosyphon systems, they
also can use either flat plate or evacuated tube collectors. The pump is usually
activated by a set collector outlet-inlet temperature difference though photovoltaic
modules have also been used to power pumps (Parker, 1976; Al-Ibrahim et al.,
1998). Most pumped systems are indirect (Frazer et al., 1995; Parent and Van der
Meer, 1990; Yazdanshenas et al., 2008; Too et al., 2009).
•
combisystems (Weiss, 2003) (a contraction of “combined systems'') these provide
both space heating and hot water. These, invariably indirect, systems require care-
ful design of heat exchanger arrangement and central strategies of the optimal
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