Global Warming

300 Energy andtransport for thefuture

Solar water heating

The essential components of a solar water heater (Figure 11.11) are a set

of tubes in which the water flows embedded in a black plate insulated

from behind and covered with a glass plate on the side facing the Sun. A

storage tank for the hot water is also required. A more efficient (though

more expensive) design is to surround the black tubes with a vacuum

to provide more complete insulation. Ten million households worldwide

have solar hot water systems.^48

Figure 11.11Design of a solar water heater: a solar collector

connected to a storage tank through a circulating pump. Alternatively,

if the storage is above the collector, the hot water will collect through

gravity flow.

In tropical countries, a solar cooking stove can provide an efficient al-

ternative to stoves burning wood and other traditional fuels. Thermal

energy from the Sun can also be employed effectively in buildings

(it is called passive solar design), in order to provide a modest boost

towards heating the building in winter and, more importantly, to pro-

vide for a greater degree of comfort and a more pleasant environment

(see box).

Solar heat can also be employed to provide heating to produce steam

for the generation of electricity. To produce significant quantities of

steam, the solar energy has to be concentrated by using mirrors. One

arrangement employs trough-shaped mirrors aligned east-west which

focus the Sun on to an insulated black absorbing tube running the length

of the mirror. A number of such installations have been built, particularly

in the USA, where solar thermal installations provide over 350 MW

of commercial electricity. The high capital cost of such installations,