Solar Thermal John Perlin*, the author of From Space to Earth - The Story of Solar Electricity, and co-author [with Ken Butti] A Golden Thread - 2500 years of Solar Architecture and Technology, provides here a short summary of the evolution of solar thermal - Solar Thermal refers to the direct conversion of sunlight to heat.

SOLAR HOT WATER HEATING

A cross-section of a hot box. Eighteenth- and nineteenth-century scientists used the hot box to test how much sunheat glass-covered enclosures could trap.

Horace de Saussure, a noted Swiss naturalist, observed in the 1760s, "It is a known fact, and a fact that has probably been known for a long time, that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass. To determine the effectiveness of trapping heat with glass covers, de Saussure built a rectangular box out of half-inch pine, insulated the inside, and had the top covered with glass, and had two smaller boxes placed inside. When exposed to the sun, the bottom box heated to 228 degrees F (109 degrees C) or 16 degrees F (9 degrees C) above the boiling point of water. de Saussure was unsure of how the sun heated the glass boxes. Today we can better explain what happened. Sunshine penetrated the glass covers. The black inner lining absorbed the sunlight and converted it into heat. Though clear glass allows the rays of the sun to easily enter through it, it prevents heat from doing the same. As the glass trapped the solar heat in the box, it heated up. Its inventor realized that someday the hot box might have important practical applications, as "it is quite small, inexpensive and easy to make." Indeed, the hot box has become the prototype for the solar collectors that have provided sun-heated water to millions since 1892.

The first solar water heaters were bare metal tanks painted black containing water and tilted to face the sun.

In the nineteenth century, no easy way existed to heat water. People generally used a cook stove for this purpose. Wood had to be chopped or heavy hods of coal lifted, then the fuel had to be kindled and the fire periodically stocked. In cities, the wealthier heated their water with gas manufactured from coal. Still, the fuel didn't burn clean and the heater had to be lit each time someone wanted to heat water. If someone forgot to extinguish the flame, the tank would blow up. To add to the problem of heating water, in many areas, wood or coal or coal-gas cost a lot and many times could not be easily obtained. To circumvent these problems, many handy farmers or prospectors or other outdoors men devised a much safer, easier, and cheaper way to heat water - placing into the sun a metal water tank painted black to absorb as much solar energy as possible. These were the first solar water heaters on record. The downside was that even on clear, hot days it usually took from morning to early afternoon for the water to get hot. And as soon as the sun went down, the tanks rapidly lost their heat because they had no protection from the night air.

1891 - World's First Solar Solution

Advertisement for the Climax Solar-Water Heater, the world's first commercial solar water heater, patented in 1891.

The shortcomings of the bare tank solar water heaters came to the attention of Clarence Kemp, who sold, in Baltimore, Maryland, the latest home heating equipment. In 1891, Kemp patented a way to combine the old practice of exposing metal tanks to the sun with the scientific principle of the hot box, thereby increasing the tanks' capability to collect and retain solar heat. He called his new solar water heater the Climax - the world's first commercial solar water heater. Kemp originally marketed his invention to eastern gentlemen whose wives had gone off with their maids to summer at some resort, leaving their husbands to fend for themselves. The solar water heater, Kemp advertised, would simplify housekeeping duties for this class of men already burdened by their wives and domestic staffs absence and unaccustomed to such work as lighting the gas furnace or stove to heat water.

1896 - Sunshine States Use Advantage
 

The home of Walter van Rossem, overlooking the Pasadena Rose Bowl. In 1896, the van Rossem home had a Climax Solar Water Heater placed on the roof.

In California and other such temperate states, having greater amounts of sunshine throughout the year and higher fuel costs (than in places like Maryland) made it essential for residents to take their solar assets seriously and not waste them. The Climax sold well in such areas. Sixteen hundred had gone up in homes throughout Southern California by 1900, including the one installed for Walter van Rossem's mother in Pasadena where three years earlier a third of the households in this California city heated their water with the sun.

Early 1900s - New Invention Revolutionizes Business

This Pomona Valley, California family had switched in 1911 from the Climax tanks in a glass-covered box system to a solar panel for their hot water needs.

Cutaway drawing shows how the Pomona Valley installation worked. The sun-heated water flowing through pipes attached to metal backing inside a glass-covered box. The heated water, lighter than the incoming cold water, naturally and immediately rose through the pipes to an insulated storage tank where the sun-heated water was kept warm for use both day and night. Notice, too, the connection of the furnace to the storage tank, guaranteeing hot water even after several rainy days.

From the turn of the century to 1911, more than a dozen inventors filed patents that improved upon the Climax. But none changed the fact that the heating unit and the storage unit were one and the same and both laid exposed to the weather and the cold night air. Hence, water heated by the sun the night before never stayed hot enough to do the wash the next morning or to heat the bath. In 1909, William J. Bailey patented a solar water heater that revolutionized the business. He separated the solar water heater into two parts: a heating element exposed to the sun and an insulated storage unit tucked away in the house so families could have sun heated water day and night and early the next morning. The heating element consisted of pipes attached to a black-painted metal sheet placed in a glass-covered box. Because the water to be heated passed through narrow pipes rather than sat in a large tank, Bailey reduced the volume of water exposed to the sun at any single moment and therefore, the water heated up faster. Providing hotter water for longer periods put Bailey's solar hot water heater, called the Day and Night, at a great advantage over the competition. Soon the Climax went out of business. From 1909, when Bailey started up his business, through 1918, his company had sold more than 4,000 Day and Night Solar Hot Water Heaters.

1920s to 1940s - Nation's Use of Solar in Flux

Workman installing solar water heater on the roof of the laundry room in a Florida subdivision going up in the 1930s. Like most housing in Florida, every house in this tract used solar energy to heat its water.

Because people had to rely on expensive imported coal or wood for fuel, many found solar a cheaper alternative. The huge discoveries of natural gas in the Los Angeles basin during the 1920s and 1930s killed the local solar water heater industry. Rather than lose money from the energy changes in the Southland, Bailey took the innovations he had made in solar and applied them to develop the thermostatically-controlled gas water. His Day and Night Gas Water Heater made him his second fortune. He also sold the patent rights of the Day and Night Solar Water Heater to a Florida firm. A building boom in Florida during the 1920s had tripled, but just as in California before the great oil strikes, people had to pay a pretty penny to heat water. The high cost of energy combined with the tropical climate and the great growth in housing stock created a big business for those selling solar water heaters. By 1941, more than half the population heated its water with the sun! Declining electric rates after World War II, in tandem with an aggressive campaign by Florida Power and Light to increase electrical consumption by offering electric water heaters at bargain prices, brought Florida's once flourishing solar water heater industry to a screeching halt.

1960s and 70s - Japanese Embrace the Sun

Unlike America during the post World War II years, the Japanese lacked cheap and abundant energy to supply hot water on demand. Rice farmers in particular yearned for a hot bath after working long hours in the hot humid patties. But to heat water, they had to burn rice straw, which they could have otherwise used to feed their cattle or fertilize the earth. So when a Japanese company began marketing a simple solar water heater consisting of a basin with its top covered by glass, more than 100,000 were in use by the 1960s. People living in the towns and cities bought either a plastic solar water heater that resembled an inflated air mattress with a clear plastic canopy or a more expensive but longer lasting model that resembled the old Climax Solar Water Heaters - cylindrically shaped metal water tanks placed in a glass-covered box. Close to 4,000,000 of these solar water heaters sat on roof tops by 1969.

The advent of huge oil tankers in the 1960s allowed the Japanese access to new oil fields in the Middle East, supplying them with cheap, abundant fuel. As had happened in California and Florida, the solar water heater industry collapsed. But not for long. The Oil Embargo of 1973 and the subsequent dramatic increase in the price of petroleum revived the local solar water heater industry. Annual sales of greater than 100,000 units continued to hold steady from 1973 until the second oil shock of 1979. Sales then jumped to almost half a million that year and leaped to nearly a million the following year. By this time, the Japanese favored solar water heaters closely resembling the type introduced to California in 1909 by William J. Bailey with the heating and storage units separated. As the price of oil began to stabilize in 1985 and then drop sharply in subsequent years, so did the sales of solar water heaters; still the Japanese purchase around 250,000 each year. Today, more than 10,000,000 Japanese households heat their water with the sun.

1970s - Australia Hops Aboard

Solahart, the leading Australian manufacturer of solar water heaters, chose in the 1970s an integral collector-tank configuration for easy installation on pitched roofs commonly found in Australia. The new design also saved money by eliminating extensive piping and the need for a heavy storage tank in the attic.

From the 1950s to the early 1970s, a few thousand Australians relied on the sun to heat their water. The numbers grew phenomenally as a consequence of two huge spikes in oil prices in 1973 and 1979. Interestingly, purchasing of solar water heaters during these heady years varied from state to state. While 40 to 50% of those living in Australia's Northern Territory heated their water with the sun, the percentage dropped to around 15% in Western Australia and sunk to below 5% in the more populated eastern states. The sharp difference had more to do with the cost of electricity than the amount of sun available. People in the Northern Territory and Western Australia bought electricity generated by imported and increasingly costly petroleum while those in the eastern states of New South Wales, Queensland and Victoria had their electricity produced by locally mined and very cheap coal. In the late 1980s, the Australian solar water heater market began to stagnate. Pipelines bringing newly discovered natural gas to previously fuel-short regions such as the Northern Territory and Western Australia has braked any growth in these once fertile markets for solar water heaters. Exports now account for more than 50% of the sales made by Solahart, Australia's leading manufacturer of solar water heaters.

Israel Heats Up

Levi Yissar, who brought solar water heating to Israel, stands next to his prototype. It closely resembled the type introduced in California in the first decade of the twentieth century with heating and storage separated. The headline in the this 1953 issue of Israel's principle newspaper, Maariv, reads, "Heating Water by the Sun Begins."

Unlike the United States and much of Europe, Israel, like Japan, found itself without sufficient fuel supplies in the early 1950s. The power situation became so bleak in the early days of the Jewish State that the government had to forbid heating water between 10 p.m. and 6 p.m. Despite mandatory domestic rationing of electricity, power shortages worsened, causing pumping stations to fail and threatening factory closures. A special committee impaneled by the government could only suggest the purchase of more centralized generators to overcome the problem. This conclusion raised the ire of Israeli engineer, Levi Yissar, who complained, "How about an already existing energy source which our country has plenty of - the sun. Surely we need to change from electrical energy to solar energy, at least to heat our water." Yissar put his money where his mouth was, becoming Israel's first manufacturer of solar water heaters. By 1967 about one in twenty households heated their water with the sun. But cheap oil coming from Iran in the late 1960s as well from oil fields captured during the Six Day War drastically reduced the price of electricity and the number of people purchasing solar water heaters.

Israeli success in the Yom Kippur War brought on the infamous oil boycott of 1973. The Israelis responded by mass purchasing of solar water heaters. By 1983, 60% of the population heated their water with the sun. When the price of oil dropped in the mid 1980s, the Israeli government did not want people backsliding in their energy habits as has happened in the rest of the world. It therefore required its inhabitants to heat their water with the sun. Today, more than 90% of Israeli households own solar water heaters.

Pool Owners Get in the Swim

Two swimmers enjoy pool water heated by the Climax Solar Water Heater.

Solar swimming pool water heaters rank as the most successful yet least heralded commercial solar application. The use of solar energy for pool heating and the equipment and need of pool owners make a perfect match. The storage unit for the solar heated water already exists - the swimming pool. The pump needed to push water through the solar collector also must be bought irrespective of the technology used to heat the water. The pool owner merely has to purchase the solar collectors. Since those using the pool only want the temperature of the pool to reach no greater than 80 or so degrees F (27 degrees C), the solar collector does not require a costly glass cover or expensive metal sheeting and piping. In fact, in the 1970s, American Freeman Ford developed low-cost plastic to act as the solar collector. Exposed to the sun, water would pass through narrow ducts in the plastic and heat up sufficiently to warm the pool. Of course, the outdoor swimming season harmoniously coincides with the maximum output of the solar collectors. Even with other forms of energy selling very cheaply, the pool owner buying a solar unit starts to save money very quickly. In the United States alone, solar swimming pool heaters have produced the energy output equivalent to running ten nuclear power plants.

*John Perlin lives in Santa Barbara, California and can be contacted at Perlin@rahus.org