Environmental Engineering Reference
In-Depth Information
Solar equipment for heating water is usually passive. In this process, solar light is
absorbed in a panel at the top of a building reservoir, through which water circu-
lates. The warmed water is then stored and distributed. The collectors are panels
covered with glass, under which water circulates in metallic tubes. Frequently there
is an auxiliary electric or gas system to heat the water in the reservoir when it falls
below 50°C (122°F), such as during periods when the Sun is not shining or covered
by clouds or in the winter.
Solar energy has been used for many years to heat water. Countries such as Is-
rael and cities such as Barcelona in Spain have enacted mandatory laws and in-
centive programs for the installation of panels, which efficiently replaces the use
of fossil fuels and fuel wood for heating. In hot climates, solar heating may sat-
isfy approximately 75% of the demand for water heating. In the cold climate of
Europe, this proportion falls to about 20%, or even less. China managed to popular-
ize a low-cost solar water heating system, with prices cheaper than in the Western
countries: US$120-US$150/m
2
versus US$700-US$800/m
2
in Europe. In China,
at least 30 million users had thermal heaters in 2006.
In this technology sunlight is focused on a collector to heat a fluid to a few hundred
degrees Celsius, producing steam for electricity generation. The most popular tech-
nology to do this uses parabolic mirrors, which are large mirrors shaped in such
a way that they concentrate the solar rays incident on them on a pipe placed at
the center of the curved surfaces, where a circulating fluid is heated. The exist-
ing projects are still marginally competitive, and research and development are still
needed in this area. Large-scale power plants using parabolic mirrors operate in
California (at a power of 350 MW). Spain intends to start operating two units soon,
totaling 100 MW, and has more than 1,000 MW in final design phase.
What is wave energy?
The oscillation of the surface of the sea, caused by winds, can be used to drive
mechanical devices that generate electricity. The energy potential that can be ob-
tained is very large (2.5 million MW), but the technology needed to do this is in
its infancy. The main reason is the low density of energy: if the typical length of
ocean waves is 60 m and their maximum height is 3 m, the maximum power one
