Solar power

Solar power has experienced significant growth in the twenty-first century, becoming a major energy resource in the United States and abroad. Various technologies have been developed to harness solar power, and these technologies—as well as their installation—have become increasingly more affordable throughout the 2010s and 2020s. By 2024, approximately 210 gigawatts of solar capacity was installed in homes and businesses across the country, and experts predicted this number would continue to increase in subsequent decades as demand for clean energy continued to grow amid rising concerns over climate change. In 2024, solar comprised an estimated 6 percent of the electricity supply in the US, with leaders hoping to expand that figure closer to 50 percent by 2050.

Insolation

The sun’s hydrogen fusion reaction (the same as humankind’s hydrogen bomb) produces a relatively steady and uninterrupted 380 billion kilowatts of energy. This energy is released into space at the solar surface at a rate of over 56 million watts per square meter. Spreading out radially as it travels outward, this power is greatly reduced by distance. By the time it reaches Earth, a single square meter of the surface receives about 1.1 kilowatts. Nonetheless, the total of the sun’s energy received on the planet is about 85 trillion kilowatts. The portion that falls on the United States amounts to five hundred times more energy than the nation’s total need. If the United States could tap into this energy with only 10 percent efficiency, it could meet all of its energy requirements from what falls on merely 2 percent of its surface. The opportunity is similarly available for most countries.

Although humans are only beginning to learn how to harness this power, other life-forms have been using it for at least 2 billion years. The solar energy consumed by plants through photosynthesis is enormous. Within the United States, the amount of solar energy used by vegetation equals at least 20 million barrels of oil per day. Solar energy is the power that drives Earth’s weather and stirs ocean currents; therefore, any utilization of wind power, ocean thermal power, or the energy stored in plants (whether burning firewood or fuels distilled from biomass) is indirectly an application of solar power. For that matter, fossil fuels represent solar energy stored in plants and animals that lived hundreds of millions of years ago, but such broad interpretations of solar power have little operational meaning. In the context of this discussion, solar power will mean insolation—the radiant energy of the sun as it falls on Earth’s surface.

Challenges to Utilization

The practical application of solar power to modern civilization’s energy requirements presents some difficult challenges. One problem is that solar energy, although abundant and widely distributed, is not concentrated, whereas humankind’s energy demands tend to require a considerable amount of power at very confined locations. Virtually all of North America south of the 48th parallel receives more than 12,000 kilojoules of solar energy per square meter of its surface every day, with large areas of the West and South enjoying more than 16,000 kilojoules. Yet it takes about 55,000 kilojoules to heat the hot water required by a family of four; this power application alone theoretically requires 3 to 4 square meters of collection device to satisfy one small family’s demand for water for cooking, washing, and bathing. In actuality, it requires closer to 10 square meters because the collection devices operate at far below 100 percent efficiency.

Another serious problem in utilizing solar power is the pattern of its availability. Solar power at a given site peaks every day as the sun passes overhead and disappears entirely during the hours of darkness. Seasonal variations in the location of the sun affect the number of hours of energy received, and as distance from the tropics increases, the incoming sunlight strikes at ever more acute angles, further diminishing the power available per square meter.

Finally, cloud cover seriously interferes with solar energy reception, making overcast days almost as unproductive as nighttime. The average number of cloudy days per year at a given location is a factor as significant as latitude. For example, sunny, equatorial Kenya enjoys almost twice the insolation as cloudy Nigeria, only 6 degrees of latitude away. Relatively sunny Washington, DC, at 38 degrees north latitude, has more insolation to work with than Nigeria.

These problems notwithstanding, it has been demonstrated that solar power can be successfully harnessed over a wide area on Earth’s surface. Various schemes to collect and concentrate the energy and manage its utilization and storage to match availability to demand have already been developed. Moreover, public opinion is strongly favorable toward solar power as an alternative to the continued use of fossil fuels or the large-scale development of nuclear power. Experts continue to predict that widespread utilization of solar power will continue into the future.

Thermodynamic and Photovoltaic Potentials

The essence of the energy problem facing the world is not an energy shortage but the cost of deriving and distributing “high-quality” energy from abundant, renewable, but “low-quality” sources. Quality, as traditionally applied to energy, means temperature first and foremost. The value of a unit of energy depends greatly on the temperature at which it can be delivered because, under the laws of thermodynamics, a unit at high temperature can do more useful work than a low-temperature unit. As it falls upon Earth’s surface, solar energy is considered a low-quality thermal resource: It is fine for making “sun tea” and heating swimming pools, but it will neither spin the wheels of industry nor light a modern, industrialized society. The fact that it is pure, clean energy creating no harmful by-products for the environment is of little consequence if it cannot do the work required of it.

Fortunately for the future, solar power can be collected and concentrated to raise its thermal quality. In fact, solar energy’s thermodynamic potential (the highest heat at which it can be realistically supplied) is about 5,000 degrees Celsius, which is 3,000 degrees higher than the thermodynamic potential of conventional nuclear power and equal to the theoretical maximum heat available from the complete combustion of pure carbon in pure oxygen. Moreover, solar radiation can generate energy in other forms than heat. The most important of these to modern civilization is its photovoltaic potential—that is, its ability to mobilize electrons in various semiconductor materials, thereby creating an electrical current. Sunlight is also capable of splitting water molecules into hydrogen and oxygen, a fact of enormous significance to more advanced power technologies.

Thermal Systems

Solar power technologies already available are quite varied but can be grouped generally as thermal, photovoltaic, and photolytic. Thermal schemes are those best known and understood by the average consumer because they apply solar power to energy needs in traditional ways.

Solar thermal systems that require no mechanisms are termed passive designs. They rely on the choice of materials and the size and careful placement of building elements to control the flow of heat within the living space. Typically, large, south-facing glass walls admit solar heat in winter but are shadowed by deep roof overhangs in summer. Interior design facilitates the natural convective flow of warm and cold air and the storage of energy. In favorable locations, good passive thermal architecture can provide as much heating and cooling as an efficient building needs.

“Flat plate” collectors used to heat water are an excellent example of active solar thermal technology. Water is pumped through piping inside a collector designed to gather and hold the solar heat that strikes it. As the water circulates through the collector, its temperature may be raised to as much as 100 degrees Celsius. Such systems most commonly supply domestic hot water but may also be used for space heating and cooling of residences and larger buildings. (Refrigerated cooling with solar-heated water involves a process different from mechanical refrigeration but is not a new idea. Absorption refrigeration, as it is called, was used in early refrigerators.) Other active solar thermal systems for space heating and cooling use the flow of water over building surfaces to add or subtract heat as needed.

Another application of active solar thermal power is in so-called heat engines, which use the expansion, contraction, or evaporation of a fluid or gas to obtain mechanical motion. A number of designs exist, dating back to the early eighteenth century, but all have suffered from low efficiency until now, when they are being interfaced with advanced thermal collectors whose much higher temperatures provide the efficiency needed. These designs promise to be increasingly important in both terrestrial and space applications.

Also requiring much higher temperatures (from 250 to more than 500 degrees Celsius) are schemes that use the sun to create superheated steam to operate turbine-powered generators. Solar farms use parabolically shaped troughs lined with mirror strips to produce temperatures of 375 degrees Celsius. A working fluid is pumped through the collectors and then to a central heat exchanger to create steam for a generating plant. Another approach, first introduced in Italy, involves reflecting the insolation captured by an array of mirrors to a central “power tower,” within which the sum of all the reflected insolation produces operating temperatures in excess of 1,000 degrees Celsius. Solar farms such as a those developed in the Mojave Desert—where massive solar arrays such as the Riverside East Solar Energy Zone and the Ivanpah Solar Electric Generating System stretch across hundreds of thousands of acres—use heliostats that follow the sun's trajectory and condensers that convert turbine steam back into water, along with generators and solar-field-integration software.

Photovoltaics

Photovoltaics—the direct generation of electricity from sunlight—was pioneered in the 1950s as an offshoot of semiconductor research. As the space age dawned, devices called solar cells became familiar as the power sources for many satellites. Their very high cost and low efficiency were of little consequence in these applications, as alternatives were either even more expensive or nonexistent. Their high cost stemmed partly from the fact that the silicon disks had to be obtained from ultrathin slices of pure silicon crystal, laboriously grown in laboratories to avoid contamination. Even the slicing operation added significantly to the cost because half of every crystal produced was consumed by the saw cuts. Low efficiencies compounded the problem, as a great many of the expensive cells were needed to provide even minimal current. Yet persistent research into new materials and techniques has been changing this gloomy picture. In fact, photovoltaic power generation has dropped so much in cost that major utility companies have put photovoltaic generating stations online to supplement power from other sources and pave the way for more widespread use. Photovoltaic panels are now common sights on residential rooftops and community installations. In 2016, more than 14,000 megawatts of capacity were installed within the US photovoltaic sector, doubling the capacity installed in 2015, according to the Solar Energy Industries Association, and those numbers only continued to rise. By 2024, the solar capacity in the US was an impressive 139 gigawatts, with over 4.7 million photovoltaic systems installed in the United States—enough solar to power over 22 million homes.

A more visionary application for photovoltaics is a satellite solar power station (SSPS). Such a satellite would be a gigantic platform in geosynchronous orbit, giving large arrays of solar cells twenty-four-hour-a-day exposure to solar energy undiluted by passage through the atmosphere. The power would be beamed back to receiving sites on Earth as microwaves and there reconverted to electricity. However, such concepts pose severe hazards, such as the loss of proper alignment of the satellite power transmitter.

Photolytic Technology

The most challenging of all solar energy technologies may ultimately be the most rewarding. Research has shown that a small percentage of photons from the sun are energetic enough to split water molecules, from which three atoms—two hydrogen and one oxygen—are liberated. If this process becomes feasible on a large scale, oxygen and hydrogen could be readily stored and either burned as fuels or recombined as water in a reaction that releases electricity. Again, the space age has shown the way through development of fuel cells, which combine hydrogen and oxygen with almost 100 percent efficiency, yielding significant amounts of fresh water and electricity in the process. If photolysis, as it is called, could draw upon the abundant salt water of the oceans to produce hydrogen, oxygen, and electricity—with fresh water as an added benefit—humanity’s quest for acceptable energy sources would move beyond “renewable” to involve environmentally enhancing power.

Competitiveness with Other Energy Sources

The energy crisis precipitated by the Arab oil embargo of 1973 forced industrialized nations to make major adjustments in energy use and policy. Through the Energy Research and Development Administration (ERDA) and the Solar Energy Research Institute (SERI), both created in 1974, the United States began a program of government-sponsored research designed to help meet an announced goal of 10 quads of energy from solar power by the year 2000—enough to replace about 5 million barrels of oil daily. The oil embargo, however, also stimulated a sharp, but unfortunately temporary, reduction in the demand for energy. After years of steady 6 percent annual increases, new energy construction almost came to a standstill. On top of this, oil prices fell sharply again in the 1980s, and environmental constraints on the use of fossil fuels were eased. The net effect was a loss of urgency in the quest for solar power, and federal assistance for research and application was cut to a small fraction of previous levels. Meanwhile, serious problems developed in the fledgling solar power industry, as complaints about inflated pricing and exaggerated savings undermined public confidence and legislative support. Many of the young solar energy ventures that were launched in the 1980s failed.

At the turn of the twenty-first century, solar energy had yet to make a widespread impact on energy consumption for several reasons, including the high initial costs of solar heating systems, the tendency of building contractors to be conservative, and the fact that fossil fuels remained relatively inexpensive during the 1990s. As fossil fuels became depleted and their prices continued to rise in the twenty-first century, however, solar power became an increasingly viable energy resource.

With the passage in the United States of the Solar Investment Tax Credit (ITC) in 2006 and its multiple extensions over the subsequent decades, including in 2022, the solar energy industry grew by more than 200 percent since its implementation. The industry showed an average annual growth rate of 68 percent during the decade between 2006 and 2016. During that time, the number of solar photovoltaic installations increased as the cost of solar energy dropped considerably, falling 70 percent between 2010 and 2016. The industry became competitive in 2016 and surpassed all other sources of new electricity, with 39 percent of all new electric generating capacity. According to the Solar Energy Industries Association, by 2023, 55 percent of all new electric capacity, the largest in history, came from solar energy, and it held more than 6 percent of the country's total energy generation. This marked the first time that a majority of newly added electric capacity to the grid was provided by a renewable energy resource. Furthermore, the solar industry in the US employed nearly 280,000 individuals in an industry valued at approximately $60.1 billion in 2023.

Principal Terms

barrel: a measure of energy consumption, 1 barrel is equal to the energy in an average barrel (42 gallons) of crude oil; about 5.8 million Btu

Btu (British thermal unit): the amount of heat necessary to raise the temperature of 1 pound of water 1 degree Fahrenheit; equivalent to about 0.25 calorie

insolation: the radiation from the sun received by a surface; generally expressed in terms of power per unit area, such as watts per square meter

kilojoule: a unit of electrical energy equivalent to the work done to raise a current of electricity flowing at 1,000 amperes for 1 second (1,000 coulombs) by 1 volt; equivalent to 4.184 calories; approximately the energy needed to raise 100 kilograms one meter

kilowatt: 1,000 watts, or about 1.34 horsepower

megawatt: 1 million watts, or about 1,340 horsepower

photolytics: the technology that makes use of sunlight’s ability to alter chemical compounds in ways that can produce energy, fuels, or both

photovoltaics: the technology employed to convert radiant solar energy directly into an electric current, using devices called solar cells

quad: 1 quadrillion Btu; equivalent to 8 billion barrels of gasoline

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