While our recent look at residential solar may lead you to believe harnessing that power is a newer initiative, humans have been exploiting solar energy for thousands
of years to heat their homes, cook, and produce hot water. Some of the
earliest written references to technology consciously designed to
capture the Sun’s rays come from ancient Greece. Socrates himself said,
“in houses that look toward the south, the sun penetrates the portico
in winter, while in summer the path of the sun is right over our heads
and above the roof, so that there is shade.” He is describing how Greek
architecture exploited the different paths of the Sun through the sky at
different times of the year.
By the fifth century BCE, the Greeks were struggling with an energy crisis.
Their predominant fuel, charcoal from trees, was scarce since they had
stripped their forests in order to cook and heat their houses. Wood and
charcoal were rationed, and olive groves needed protection from the
citizenry. The Greeks addressed their energy shortage by carefully
planning the layout of their cities to ensure that each house could take
advantage of the sunshine in the way Socrates described. The
combination of technology and enlightened government policy worked, and a
crisis was avoided.
Technologies for harnessing the thermal energy in sunlight have only
continued to grow over time. Colonists in New England borrowed the
ancient Greek homebuilding techniques to keep warm in the harsh winters.
Simple passive solar water heaters, little more than a black-painted
barrel, were sold commercially in the United States in the late 19th century. And more elaborate solar heating systems
were developed to pipe water through absorbing and/or focusing panels.
The hot water is stored in an insulated tank until needed. In climates
subject to freezing, a two-fluid system is used, where the Sun heats a
water/antifreeze mixture that passes through coils embedded in the
storage tank, which does double-duty as a heat exchanger.
These days, a variety
of sophisticated commercial systems are available for water and space
heating in the home. Solar thermal systems are deployed throughout the
world, with the largest installed base per capita found in Austria, Cyprus, and Israel.
A solar water heating system on a roof in Washington, DC.
But modern solar truly starts in 1954 with the discovery of a practical way to make electricity from light: Bell Labs uncovered
the fact that silicon could make a photovoltaic material. This finding
created the foundation for today's solar cells (essentially the devices
converting light energy into electricity) and ushered in a new era of
solar power. Aided by intense research ever since, it's an era that
continues today as solar appears poised to become the dominant source of
power in the future.
What is a solar cell?
The most common type of solar cell is a semiconductor device made
from silicon—a cousin of the solid-state diode. The familiar solar
panels are made from a number of solar cells wired together to create
the desired output voltage and current. Those cells are surrounded by a
protective package and topped with a glass window.
Solar cells generate electrical power using the photovoltaic effect, a fact that didn't come from Bell Labs. Instead, this was first discovered
in 1839 by French physicist Alexandre-Edmond Becquerel (son of
physicist Antoine Cesar Becquerel and father of physics Nobelist Henri
Becquerel, the discoverer of radioactivity). A little more than a
century later, Bell Labs had its solar cell breakthrough, providing the
foundation of the most common solar cells.
In the language of solid state physics, a solar cell is formed from a p-n junction
in a silicon crystal. The junction is made by “doping” different areas
of the crystal with small amounts of different impurities; the interface
between these regions is the junction. The n side is a conductor with electrons as the carriers of current, and the p
side has “holes,” or areas with missing electrons that act as current
carriers within the crystal. In the region near the interface, the diffusion of charges
creates a local “built-in voltage” across the interface. When a photon
enters the crystal, if it has enough energy, it may dislodge an electron
from an atom, creating a new electron-hole pair.
The p-n junction of a standard solar cell.
By Bhpaak / CC BY-SA 4.0 via Wikimedia Commons
The newly freed electrons are attracted to the holes on the other side
of the junction, but they are prevented from crossing it due to the
built-in voltage. However, if a pathway is provided through an external
circuit, the electrons can travel through it and light our homes along
the way. When they reach the other side, they recombine with the holes.
This process can continue as long as the Sun continues to shine.
The energy required to transform a bound electron into a free one is
called the “band gap.” It’s the key to understanding why photovoltaic
(PV) cells have an intrinsic limit on efficiency. The band gap is a
fixed property of the crystal material and its dopants. Those dopants
are adjusted so that solar cells have a band gap close to the energy of a
photon in the visible region of the spectrum. This is a practical
choice, because visible light isn’t absorbed by the atmosphere (phrased
differently, we humans have evolved to see in the most common
wavelengths).
Photons come in fixed amounts of energy, which means their energy is
quantized. That also means a photon with energy less than the band gap
(say, one in the infrared part of the spectrum) won’t create a charge
carrier. It will simply heat the panel. Two infrared photons together
will do no better, even if their combined energy would be enough to
bridge the gap. A photon with excess energy (an ultraviolet photon, for
example) will knock an electron loose, but the excess energy will also
be wasted.
Since efficiency is defined as the ratio of light energy striking the
panel divided by electrical energy extracted—and since much of this
light energy will necessarily be wasted—the efficiency can not be 100
percent.
The band gap of a silicon PV solar cell is 1.1 electron volts (eV).
As can be seen from the diagram of the electromagnetic spectrum
reproduced here, the visible spectrum lies just above this, so visible
light of any color will produce electrical power. But this also means
that for each photon absorbed, excess energy is wasted and converted
into heat.
The upshot is that even if the PV panel is flawlessly manufactured and conditions are ideal, the theoretical maximum efficiency is about 33 percent. Commercially available solar panels typically achieve about 20 percent efficiency.
Perovskites
Most of the solar panels commercially deployed are made from
the silicon cells described above. But research into other materials
and strategies is underway in laboratories around the world.
Some of the most promising recent research for silicon alternatives has involved materials called perovskites. The mineral perovskite (CaTiO3) was named
in 1839 in honor of Count Lev Aleksevich Perovski (1792-1856), a
Russian mineralogist. It can be found on every continent and in the
clouds of at least one exoplanet.
The word “perovskite” is also used for synthetic compounds that have
the same orthorhombic crystal structure as the naturally occurring
mineral (or a closely related one) and share a structurally similar
chemical formula.
Crystal structure of natural perovskite.
Solid state | CC BY-SA 3.0
Depending on which elements are used, perovskites can display a wide variety of useful properties, such as superconductivity, giant magnetoresistance, and photovoltaic activity. Their use in PV cells has generated a great deal of optimism, as they have shown an unprecedented increase in efficiency from 3.8 percent to 20.1 percent in the past seven years of laboratory research. This rapid rate of progress inspires confidence that further gains are likely, especially as the factors limiting efficiency are becoming clearer.
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