Environmental Engineering Reference
In-Depth Information
the 1920s as cheap diesel and natural gas became available. Being an astute businessman,
Bailey made a second fortune with his Day and Night Gas Water Heater.
Modern Systems
The vast majority of solar thermal units are used to heat water to temperatures of between
20° and 80° Celsius for domestic use. In the simplest case, the unit produces warm water
for sanitary use (swimming pools, showers, etc.). More complex systems are required to
generate hot water for central heating. Some industrial applications also exist, in which
higher temperatures are achieved. These, however, invariably involve concentrating solar
radiationusingparabolasandmirrors,andthereforefallwithinthecategoryofconcentrated
solar power (see Section 4.6 ).
All modern solar water heating systems share two principal characteristics: the use of
a collector to absorb solar radiation and a separate tank to store the hot water. The heat is
transferred from the collector to the tank either using convection, the natural flow of all
liquids and gases from cold to hot, or pumps that circulate the heat-absorbing fluid (see
Figure 4.16 ) .
Figure 4.16. Model of a simple solar heating system. Cold water from the mains (A)
flows into the storage tank, (B) a fluid passes through the solar collector, (C) is heated by
solar radiation, and (D) releases its heat into the water in the tank while passing through a
coil (E). The hot water rises to the top of the tank and is released through an exit valve
(F).
By far the most common active solar heating systems are flat-plate collectors (covering
90percent ofthemarket). These resemble solarphotovoltaic (PV)panels andaregenerally
mounted on roofs. They consist of four elements: a transparent cover that allows the sun's
rays to pass through but reduces heat loss; a black sheet of metal attached to a grid of
narrow tubes (the absorber); a heat-transporting fluid (usually water mixed with antifreeze)
that carries the heat from the tubes to the storage tank; and an insulating backing. The fluid
circulates through a grid of tubes embedded in the collector, then flows through a coil in
the storage tank, releasing its heat into a tank of water that is replenished from the main
water supply (see Figure 4.17 ) .
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