[covered porches] while in summer the path of the sun is right over our heads and above the roof, so we have shade.”
Archaeological excavations between 500 BCE through 200 BCE confirm that both rural and urban builders in the Hellenic world conformed to Socrates’ advice.
Yang-sui were used in China to concentrate the sun’s rays to start fire.
2. In the last decade, troves of solar concentrators called yang-suis (solar ignitors) by the Chinese, dating to about 1000 BCE, have been unearthed by archaeologists in China. One museum made a mold of a yang-sui to conform to its concave interior and cast a copy in bronze, the metal used in the original. After polishing its curved surface, a museum employee, acting on a hunch, focused sunlight onto small pieces of wood, just as the ancients had done thousands of years before, and lit a flame. (See related photos: “Spanish Solar Energy.”)
Confucius had written that it was the eldest son’s duty to use the yang-sui to light the evening cooking fire. Every morning, according to the sage, tradition required him to attach a yang-sui to his belt when he dressed for the day.
The yang-sui, remarked the discovering archaeologist, “should be considered as one of the great inventions of ancient Chinese history.”
A solar hot box.
3. The French mathematician Joseph Fourier in the early nineteen hundreds modeled global warming with the solar hot box built by Horace de Saussure almost a century earlier. The solar hot box consisted of a well-insulated box with the top covered by layers of glass. When exposed to the sun, Saussure obtained temperatures inside the box exceeding 240 degrees Fahrenheit (116 degrees Celsius).
As Fourier observed, if the atmosphere allowed sunlight to pass through but trapped the consequent solar heat, as did Saussure’s device, “the lower levels of the atmosphere would acquire elevated temperatures.” The analogy of the hot box and global warming rings true today as well. The solar hot box also became the prototype for all relatively low-temperature solar heating devices such as hot air and water collectors.
Augustin Mouchot’s Largest Sun Machine.
Workers inspect the first commercial solar power plant
4. The first solar steam engine was built and tested by Augustine Mouchot, a French engineer, in 1866. He focused a parabolic mirror onto a one-inch tube in which the water was turned into steam. He went on to use concentrators to produce ice and electricity.
Mouchot’s work ignited a number of inventors to develop solar motors over the latter part of the nineteenth century. One of those pioneers was John Ericsson, who had helped save the Union during the Civil War by designing the first iron-clad battleship. Ericsson devoted the last twenty years of his life to solar engineering, believing that because “a great portion of our planet enjoys perpetual sunshine, the field therefore awaiting the application of the solar engine is almost beyond computation, while the source of its power is boundless.”
By 1914, the first commercial solar plant producing steam went up in Egypt and compared well in cost and production with its coal-fired competition. The developer could now announce, “Sun power is now a fact and no longer [just] a beautiful possibility.” But the great dream disintegrated with outbreak of World War I and the consequent discoveries of cheap oil in the formerly fuel-short but sunny parts of the world.
Advertisement for the Climax Solar Water Heater
5. The first commercial solar water heater consisted of several 25-gallon cylindrical water tanks painted black inside a hot box, which inventor Clarence Kemp began marketing in 1891 as the Climax Solar Water Heater. They became most popular in the area in and around Los Angeles, California, at the turn of the nineteenth century. Improvements made in 1911, including pipes exposed to the sun that heated the water and a remote insulated storage tank to which the solar-heated water entered, enabled people to enjoy sun-heated water day and night.
The new water heaters spread throughout California. But skepticism sometimes remained so the company challenged doubters to hold their hand in water coming out of the solar heater. One guy did, recalled a rancher, “and the poor guy got an awful roasting. He almost lost his skin.”
Although the California market for solar water heaters plummeted due to the discovery of cheap natural gas in the 1920s, they migrated to other parts of the world. China, for example has more than 60 million installed.
Selenium bars, which when light shone on them produced electricity.
6. The discovery of the direct conversion of sunlight into electricity began with the realization in 1873 that selenium loses its electrical resistance when exposed to sunlight. The insight led to a flurry of interest in the material throughout Europe.
In one 1876 experiment, William Grylls Adams and Richard E. Day lit a candle inches away from bars of selenium. The needle on their measuring device immediately jumped. Screening the candle had the opposite effect. They therefore concluded, “It was clear that a current could be started in the selenium by the action of the light alone.”
Eight years later, Charles Fritts moved the technology forward by putting up the first photovoltaic array on a New York rooftop. Fritts showed that, due to its modular nature, a solar system can be placed on site where the electricity is needed and tailored to the exact needs of the consumer. Or as John Ericsson articulated, “One precious virtue of this new energy source is that it can produce energy without occupying useful spaces.”
An apartment complex with solar cells mounted on every rooftop.
7. Albert Einstein did not receive his Nobel Prize for relativity. He won it for discovering a special aspect of light: that not only does light travel as a wave but also as a particle (a photon), whose power depends on each wave length. Together with the new science of the day called quantum mechanics, Einstein’s light particle discovery explains how a solar cell works. Advances in materials research and science led to the discovery of the first practical solar cell in 1953. (See related story: “Japan Solar Energy Soars, But Grid Needs to Catch Up.”)
The selenium solar cells of the late nineteenth century could only convert .4 percent of the incoming sunlight into electricity, hardly enough to power anything electric. But in the 1950s, the start of the semiconductor revolution spawned the first solar cells capable of directly converting enough sunlight to generate useful amounts of power. Invented along with the silicon transistor at Bell Laboratories, the silicon solar cell had an efficiency 15 times greater than its selenium predecessor.
The first practical use of solar cells occurred five years later, to power satellites. Solar-powered satellites not only serve the military to guide missiles and provide reconnaissance, but they have also revolutionized trade and commerce, speeding up ATM and credit card transactions, allowing for instantaneous television reporting from the field to the station, providing GPS to everyone, and a thousand other applications. The success of photovoltaics in space is now being replicated on the ground with millions of rooftops being covered with solar panels.
John Perlin is an analyst in the physics department of the University of California, Santa Barbara. He oversees solar installations at the university, and he writes, speaks, and lectures about solar energy. His new book is Let It Shine: The 6000-Year Story of Solar Energy.