Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 129

solar savings fraction is to be provided. (Yazdanshenas and Furbo, 2007; Letz
et al., 2009; Yazdanshanas et al., 2008). The annual variations of space and water
heating loads can be quite different.

• integral collector storage (Smyth et al., 2006; 1998; 1999; 2001a, 2001b; 2003;

2004) in which the heat water store is also the solar energy collector.

Combi + ; these are solar assisted heat pumps (Morrison, 1984; Troi et al., 2008).

•

• large scale interseasonal energy storage systems (Schmidt et al., 2004).

• swimming pool heating (Ruiz and Martinez, 2010); employing flat plate collectors

often unglazed using a low-cost plastic pipe absorber.

• photovoltaic solar water heaters (Fanney and Daugherty, 1997; Fanney et al.,

1997) where a electrical heating element immersed the water to be treated is
powered by a photovoltaic array.

In addition to evacuated tube and flat plate collectors, solar water heating can also
be accompanied using photovoltaic/thermal collectors. Such latter collectors use the
85-95% of the incident on a photovoltaic array not connected to electricity (Ji et al.,
2006; Chow et al., 2008) as heat.

4.4.2 Applicability of particular collector types to specific
outlet temperatures and diffuse fractions
Solar energy availability can be diurnally or annually out-of-phase with a heat load.
When the heating needs are at their peak, the supply of solar energy can be at its lowest.
However, domestic and industrial hot water needs often do not vary over the year and
solar collectors can be used for producing hot water during the summer.
Solar energy water heaters can be categorised as either active or passive. An active
system requires a pump to drive the heated fluid through the system, whereas a passive
system requires no external power. Distributed systems (Prapas et al., 1993) comprise
a solar collector, hot water store and connecting pipework; they may be either active
or passive. In the former, a pump actuated by temperature sensors via a control circuit
temperature difference between collector inlet and outlet is actuated by the required
to convey the fluid from the collector to the store (Prud'homme and Gillet, 2009;
Wuestling et al., 1985). The pump may be powered by a photovoltaic module giving
automony from mains power (Al-Ibrahim et al., 1988). In a passive thermosyphon
solar water heater; fluid flow is due to buoyancy forces occurring in a closed circuit
comprising a collector, hot water store and the connecting pipework produced by
the difference in densities of the water in the collector and that of the cooler water
in the store.
Concentrating solar energy collectors require high direct components of insola-
tion to be effective. Extensive use of solar power generation is likely to be most
viable where the annual direct fraction is above 60%. This delineates their geo-
graphical applicability in Figure 4.4.1. Similarly solar water system productivity is a
function of insolation and ambient temperature. For identical load profiles and temper-
atures solar savings fractions and continuous autonomous provision can be determined
(Yohanis et al., 2006a; 2006b). A geographical distribution of the latter is shown in
Figure 4.4.2.