116 practice makes perfect Advanced English Reading and Comprehension
maximize the absorption of infrared radiation, and mirrors or iber optics to enhance natural
lighting.
6 he most recognizable solar technology, photovoltaic (PV) cells, convert direct sunlight into
electricity. Developed in 1941, the modern silicon PV cell, or solar cell, is a wafer made of two lay-
ers of crystalline silicon, a semiconducting material derived from sand or quartz. When sunlight
strikes a PV cell’s surface, electrons are knocked out of place, creating an electrical current. Solar
cells are soldered together to form modules, which are combined into panels sandwiched between
sheets of glass within an aluminum frame. PV panels, which work best in direct sunlight, can be
mounted on tracking devices to follow the sun’s movement. Solar panels are assembled into solar
arrays; the larger the array, the greater the energy output.
7 In the late 1950s, solar cells were adopted by NASA to power satellites. he space industry’s
pioneering of solar technology for use in spacecrat has contributed to the advancement, reliabil-
ity, and cost efectiveness of solar technology. PV panels are most commonly seen on rootops and
walls, but nowadays, anything from clocks, watches, calculators, road signs, and bus shelters to
houses and commercial buildings can run on power generated by PV systems. Areas of PV arrays
known as solar farms produce electricity on a scale large enough to be distributed through a
power grid to thousands of homes. he world’s largest solar farm, Waldpolenz Solarpark near
Leipzig, Germany, covers 110 hectares^2 and uses state-of-the-art thin-ilm photovoltaic technol-
ogy to produce 40,000 megawatts (MW) of electricity.
8 Concentrating solar power (CSP) uses large mirrors or receiving collectors to concentrate
the sun’s radiation on pipes, tubes, or cylinders illed with luid or gas. An intense heat of 160° to
540°C (300° to 1,000°F) creates steam or expands gas to drive a turbine or electric generator.
High-temperature collectors, such as giant parabolic dishes covered with mirrors, generate large
amounts of electricity that is fed into a power grid. In the 1980s, nine thermal solar power plants
with a combined capacity of 354 MW were built in the Mojave Desert. he Ivanpah Solar Power
Facility, which is still under construction in the southwestern United States, will have a combined
capacity of 392 MW.
9 Another form of collector technology is solar thermal energy, which heats swimming pools
and household water. he irst solar water heater was patented in the United States in 1891, and by
1897, nearly a third of the homes in Pasadena, California, had installed a solar water heating sys-
tem. Interest in these systems revived in the late 1970s, but in the mid-1980s, their popularity
declined with energy prices and the expiration of a federal tax credit.
10 Solar power has been the subject of intense debate. he principal issues are the following:
◆ Fuel supply Sunlight is free, renewable, and available to everyone, albeit more plentiful in
areas closest to the equator, such as the Sahara Desert, which receives in excess of 4,000 hours
of sunlight per year. Of course, the sun shines only during the day and its intensity depends on
its position in a cloudless sky.
◆ Environmental impactEssentially, solar power releases zero emissions, and no or very few
polluting chemicals or gases into the environment. he manufacture of solar equipment, how-
ever, occurs in factories that burn fossil fuels and emit CO 2. Solar cells are also made of chemi-
cals that can end up as toxic waste.
◆ Costs Processing silicon for PV cells is expensive and consumes time and energy. Although
silicon is abundant, prices are subject to demand and are expected to rise in the future. Never-
theless, solar cell manufacturing has become more eicient and cheaper, and prices have been
falling steadily since the 1970s. Engineers continue to explore nonsilicon technology and to
develop generations of thinner, cheaper, more lexible, more eicient, and more cost-efective
solar cells for diverse applications. Microphotovoltaic substances can be printed on surfaces or(^2) One hectare is the equivalent of 2.47 acres.