The essential components of a solar water heater (Figure 11.15) 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. Over 10 million households worldwide have solar hot water systems.49
Figure 11.15 Design 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 countries such as Australia, Israel, Japan and the southern states of the USA (see box). In tropical countries, a solar cooking stove can provide an efficient alternative 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 provide for a greater degree of comfort and a more pleasant environment (see box).
Energy from the Sun: concentrating solar power
Solar energy can be converted into electricity either through its heat energy focused on to a boiler, for instance, to produce steam - known as concentrating solar power (CSP) - or by means of photovoltaic (PV) solar cells (see box). It is widely agreed that both possess the potential to be large contributors to global renewable energy. For instance, the current electricity needs of the United
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