Environmental Engineering Reference
In-Depth Information
cases because the photovoltaic generator runs the pump only when the sun
shines. Chapter 4 describes photovoltaic systems in detail.
Solar thermal domestic water heating systems
The heating of domestic water involves much higher temperatures than for
swimming pool water. The simple absorbers used for swimming pool heating
are in most locations not suited to domestic hot water systems because the
absorber losses due to convection, rain and snow as well as heat radiation are
unacceptably high. Domestic water heating systems typically use collectors
that have much lower losses at higher water temperatures. These are either
flat-plate, evacuated flat-plate or evacuated tube collectors and are integrated
with collector storage systems. Collectors for domestic water heating are
described in the section on solar collectors, p85.
A complete system for domestic water heating consists not only of a closed
collector to heat the water. Further components such as a hot water storage
tank, pump and an intelligent control unit are needed to ensure a hot water
supply that is as comfortable as we expect from conventional systems.
A very simple system for solar water heating can be made of a black water-
filled tank that is exposed to sunlight in summer. If the tank is installed higher
than the tap, the warm water can be used without any further component. An
example for such an application is a solar shower that is sold as camping
equipment. In principle, it is a black sack hung on a high branch of a tree. If
this sack is exposed for some hours to solar radiation, a shower with solar
heated water can be taken.
However, this system does not meet the demand of daily routine. After the
sack is empty it must be refilled again by hand. To avoid this inconvenience,
sack and tap can be pressure-sealed and a hose can then be connected to
replace water automatically. As a further improvement a solar collector with a
high efficiency all year round can replace the sack. However, the collector
content is only sufficient for a very short shower and the water temperatures
will be very high. Therefore, a storage tank is needed. Two systems to integrate
hot water storage tanks into solar energy systems are described in the following
sections.
Thermosyphon systems
A thermosyphon system as shown in Figure 3.3 makes use of gravity. Cold
water has a higher specific density than warm water. It is therefore heavier and
sinks to the bottom. The collector is always mounted below the water storage
tank. Cold water from the bottom of the storage tank flows to the solar
collector through a descending water pipe. When the collector heats up the
water, the water rises again and flows back to the tank through an ascending
water pipe at the upper end of the collector. The cycle of tank, water pipes and
collector heats up the water until temperature equilibrium is reached. The
consumer can draw off hot water from the top of the tank. Used water is
replaced through a fresh supply of cold water through an inlet at the bottom
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