Solar energy

• 1. Introduction

  ›

  Solar energy is the radiant light and heat from the Sun, and the Earth catches an astonishing amount of it. Every moment, 174 petawatts of solar radiation strike the upper atmosphere. Roughly 30 percent bounces back into space. The remaining 122 petawatts soaks into clouds, oceans and land. In 2002, that one-year haul exceeded everything humanity used in twelve months, captured in a single hour. By 2019 the math had shifted only slightly, to one hour and twenty-five minutes.

  The question is not whether the energy exists. It is how to catch it. Mirrors that boiled water beside the Nile. Black bottles that purify drinking water in the tropics. Walls in French orchards built to trap warmth during the Little Ice Age. A single-seat plane that flew around the world without a drop of fuel. This documentary follows the many shapes that one resource takes, from the steam engines of Egyptian cotton fields to molten salt glowing at 566 degrees Celsius in the Chilean desert.

• 2. Steam Beside The Nile

  ›

  In 1878, at the Universal Exposition in Paris, Augustin Mouchot demonstrated a solar steam engine. He could not push the work further because coal was cheap and other factors stood in the way. The early promise of sun power kept colliding with the economics of fossil fuel.

  Frank Shuman, a US inventor and engineer, took up the problem in 1897. He built a small demonstration engine that reflected solar energy onto square boxes filled with ether. Ether boils at a lower temperature than water, and black pipes inside the boxes drove a steam engine. In 1908 he formed the Sun Power Company to build something larger.

  Shuman worked with his technical advisor A.S.E. Ackermann and the British physicist Sir Charles Vernon Boys. Together they used mirrors to focus sunlight on collector boxes, raising the heat enough that water could replace ether. By 1912 Shuman had patented the entire solar engine system. Between 1912 and 1913 he built the world's first solar thermal power station in Maadi, Egypt. Its parabolic troughs powered a 45-52 kW engine that pumped more than 22,000 litres of water per minute from the Nile to neighboring cotton fields.

  World War I and the discovery of cheap oil in the 1930s smothered the momentum. Yet Shuman believed in what he had built. In 1916 he was quoted saying that after the world's stores of oil and coal are exhausted, the human race can receive unlimited power from the rays of the Sun. His basic design waited until the 1970s to find its second life.

• 3. Heat Without A Flame

  ›

  Solar hot water systems heat water using nothing but sunlight, and in the right latitudes they do much of a household's work. Between 40 degrees north and 40 degrees south, solar heating can supply 60 to 70 percent of domestic hot water, reaching temperatures up to 60 degrees Celsius. The most common collectors are evacuated tubes at 44 percent of the market and glazed flat plates at 34 percent, with unglazed plastic collectors at 21 percent used mainly for swimming pools.

  China leads the world in this technology by a wide margin. As of 2015, global installed capacity of solar hot water sat near 436 thermal gigawatts, and China held 309 of them, a 71 percent share. Israel and Cyprus lead per capita, with more than 90 percent of homes using solar hot water. In the United States, Canada and Australia, heating swimming pools dominates, with 18 thermal gigawatts installed as of 2005.

  The oldest trick of all needs no machinery: thermal mass. Stone, cement and water absorb the Sun's heat during the day and release it at night. In arid and warm temperate regions this kept buildings cool. In cold areas the same materials hold warmth. A solar chimney pushes the idea further, using a warmed vertical shaft to pull air through a building by updraft.

  Nature offers its own controls. Deciduous trees planted on a building's Equator-facing side shade it in summer, while bare winter limbs let light through. Because leafless trees still block one-third to one-half of incident radiation, the gain is a balance. In climates with heavy heating loads, planters keep these trees to the east and west to preserve winter sun.

• 4. Cooking And Clean Water

  ›

  The simplest solar cooker is the box cooker, first built by Horace de Saussure in 1767. It is an insulated container with a transparent lid, and it works even under partly overcast skies. Panel cookers add a reflective surface to direct light onto the container. Reflector cookers use dishes, troughs or Fresnel mirrors to concentrate light, reaching 315 degrees Celsius and above, though they need direct sun and must be turned to track it.

  Concentrating geometries also deliver industrial process heat. The Solar Total Energy Project in Shenandoah, Georgia, was the first commercial system, where 114 parabolic dishes met half the process heating, air conditioning and electrical needs of a clothing factory. This cogeneration plant produced 400 kW of electricity, 401 kW of steam and 468 kW of chilled water, with a one-hour peak load thermal store. Elsewhere, perforated sun-facing walls called unglazed transpired collectors preheat ventilation air, raising it by up to 22 degrees Celsius and paying for themselves in three to twelve years. By 2003 more than 80 such systems covered 35,000 square metres worldwide, including an 860-square-metre wall in Costa Rica for drying coffee beans and a 1,300-square-metre one in Coimbatore, India, for drying marigolds.

  Water is the other great use. Solar distillation makes saline or brackish water drinkable, a method first recorded by 16th-century Arab alchemists. The first large plant rose in 1872 in the Chilean mining town of Las Salinas, with a collection area of 4,700 square metres, producing up to 22,700 litres a day for 40 years. A simpler approach, solar water disinfection, exposes water-filled PET bottles to sunlight for six hours to two days. Recommended by the World Health Organization, it serves over two million people in developing countries for their daily drinking water.

• 5. Salt That Holds The Sun

  ›

  Molten salt solves the oldest complaint about solar power: it stops at sundown. By storing the heat collected by a solar tower or trough, a concentrated solar plant can make electricity at night or in bad weather. The idea was proven in the Solar Two project, which ran from 1995 to 1999 and reached an annual storage efficiency near 99 percent.

  The most common mixture blends sodium nitrate, potassium nitrate and calcium nitrate. It is non-flammable, non-toxic, and already familiar to the chemical and metals industries as a heat-transport fluid. The salt melts at 131 degrees Celsius. It waits in an insulated cold tank at 288 degrees Celsius, then gets pumped through a solar collector where focused light heats it to 566 degrees Celsius before it moves to a hot tank insulated well enough to hold the heat for up to a week.

  When power is needed, the hot salt feeds a conventional steam generator, driving a turbine just as coal, oil or nuclear plants do. A 100-megawatt turbine would need a tank about 9.1 metres tall and 24 metres across to run for four hours. Several parabolic trough plants in Spain use the method, and the Solana Generating Station in the United States carries six hours of storage.

  Chile pushed the concept furthest. The Cerro Dominador plant pairs a 110 MW solar-thermal tower with molten salts, transferring heat through an exchanger to make superheated steam and drive a turbine on the Rankine cycle. Its storage lets it generate electricity for up to 17.5 hours without direct sunlight, and the project secured sales of up to 950 gigawatt-hours per year. Further north in the Antofagasta region, the planned María Elena plant is a 400 MW thermo-solar complex built on the same technology.

• 6. Sun In Motion

  ›

  Building a solar-powered car has been an engineering goal since the 1980s. The World Solar Challenge runs teams from universities and companies 3,021 km across central Australia, from Darwin to Adelaide. When it was founded in 1987, the winner averaged 67 km/h. By 2007 the winning average had climbed to 90.87 km/h. The North American Solar Challenge and the planned South African Solar Challenge reflect the same international pursuit.

  Water came next. The first practical solar boat was built in England in 1975, and by 1995 passenger boats carrying PV panels began to appear. In 1996 Kenichi Horie made the first solar-powered crossing of the Pacific Ocean. The Sun21 catamaran made the first solar-powered Atlantic crossing in the winter of 2006-2007.

  The sky proved the hardest frontier and the richest. The unmanned AstroFlight Sunrise made the first solar flight in 1974. On the 29th of April 1979, the Solar Riser carried a person in the first fully controlled solar-powered flight, reaching 40 feet. The Gossamer Penguin flew on photovoltaics alone in 1980, and the Solar Challenger crossed the English Channel in July 1981. In 1990 Eric Scott Raymond flew from California to North Carolina in 21 hops. Unmanned craft then set the records, with the Helios reaching 29,524 metres in 2001 and the Zephyr, built by BAE Systems, flying 54 hours in 2007. From March 2015 to July 2016, the single-seat Solar Impulse circled the globe, able to take off under its own power and stay aloft for several days.

• 7. Cities, Crops And Fuels

  ›

  Greek and Chinese builders first oriented their buildings toward the south for light and warmth, an early form of solar architecture. Passive solar design rests on orientation to the Sun, compact proportions, selective shading and thermal mass, and Socrates' Megaron House is a classic example. Modern designers tie lighting, heating and ventilation together with computer modeling.

  Urban heat islands run hotter than their surroundings because asphalt and concrete absorb solar energy, holding more heat and reflecting less. Painting buildings and roads white and planting trees counters the effect. A hypothetical cool communities program in Los Angeles projected a roughly 3 degree Celsius drop at a cost of one billion US dollars, with estimated annual benefits of 530 million US dollars from lower air conditioning and healthcare savings.

  Farmers have long bent sunlight to their needs. During the Little Ice Age, French and English farmers built fruit walls that acted as thermal masses and sped ripening. In 1699 Nicolas Fatio de Duillier even proposed a tracking mechanism to pivot a wall toward the Sun. Greenhouses date back to Roman times, when primitive versions grew cucumbers year-round for the emperor Tiberius, and the first modern greenhouses rose in 16th-century Europe to hold exotic plants brought from distant explorations.

  Solar energy can also become fuel. Solar chemical processes drive reactions that store the Sun's energy in transportable form. Hydrogen has been a major research target since the 1970s, whether by electrolysis or by thermochemical routes that split water at 2,300 to 2,600 degrees Celsius. The Solzinc process under development at the Weizmann Institute of Science uses a 1 MW solar furnace to break down zinc oxide above 1,200 degrees Celsius, yielding pure zinc that can later react with water to release hydrogen.

• 8. From Scarcity To Cheapest Power

  ›

  Solar technologies of the 1860s were born from a fear that coal would run out. As coal and petroleum grew cheaper and more available, that early development stalled in the early 20th century. Commercial solar water heaters appeared in the United States in the 1890s and spread until the 1920s before cheaper fuels displaced them.

  The 1973 oil embargo and the 1979 energy crisis forced governments to rethink energy policy and revived interest in the Sun. Incentive programs followed, including the Federal Photovoltaic Utilization Program in the United States and the Sunshine Program in Japan. Research centers were founded, among them SERI in the United States, now NREL, NEDO in Japan, and the Fraunhofer Institute for Solar Energy Systems ISE in Germany. Interest in solar water heating dipped in the 1980s as petroleum prices fell, then grew steadily through the 1990s, averaging 20 percent annual growth since 1999.

  The economics finally turned decisively. In 2021 Lazard estimated the levelized cost of new unsubsidized utility-scale solar electricity at less than 37 dollars per megawatt-hour, with existing coal-fired power costing more. New solar also beat new gas-fired power, though not generally existing gas.

  The scale of what waits is hard to grasp. In 2021 the Carbon Tracker Initiative estimated that powering all human energy from solar alone would need 450,000 square kilometres, roughly the area of Sweden, Morocco or California, and just 0.3 percent of Earth's land. A 2011 report from the International Energy Agency found that solar could supply a third of the world's energy by 2060 if politicians commit to limiting climate change. As one assessment put it, the strength of solar is the incredible variety and flexibility of applications, from small scale to big scale.