Heat is the easy part
Turning sunlight into electricity took decades of physics and materials work. Turning it into heat takes a dark surface and some water. Anyone who has felt a hose left in the sun has met the principle. Because of this, solar water heating has a much longer and simpler history than the solar cell, and today it remains one of the most widely used ways of collecting energy from the sun.
Early experiments
The Swiss scientist Horace-Bénédict de Saussure is often credited with building a “hot box” in the 1760s, a wooden box with glass covers that trapped heat from the sun and reached temperatures high enough to cook food. It was not a water heater, but it demonstrated the greenhouse effect on which later collectors depend: glass lets sunlight in and slows the escape of heat.
In the 1800s, French inventor Augustin Mouchot built solar-driven devices using curved reflectors to boil water and run small steam engines, and showed them at exhibitions. They worked as demonstrations but were not an economic rival to coal, which was cheap at the time. Coal also fed a growing network of electric lighting and gas supply, while sunlight needed large collecting surfaces.
The 1890s: tanks on roofs
The first widely cited commercial solar water heater is the Climax, patented by Clarence Kemp of Baltimore in 1891. It was essentially a set of iron tanks painted dark and set in a glass-covered box, so the water warmed in place. Households could use the warmed water directly. These are known as batch heaters or integral collector storage systems. They had a weakness: since the water tank was exposed on a roof, it cooled quickly after sunset, and the heat was often lost overnight.
Separating collector and tank
In 1909, William Bailey of California introduced a design that separated the collector from a storage tank. Hot water from the collector rose by natural circulation into an insulated tank placed above it, and cooler water returned to the bottom of the collector. This is called thermosiphon, since the movement needs no pump. Because the hot water was kept in an insulated tank, it stayed hot through the night. His business, sometimes known by the name “Day and Night,” sold many units in California, according to the usual accounts.
Solar water heaters became popular in parts of California in the early twentieth century and then in Florida in the 1920s and 1930s, until cheap natural gas and electricity pushed them out of favor in many areas. This pattern, in which interest rose when fuel was expensive and fell when it was cheap, repeated for decades.
Spread around the world
The oil price rises of the 1970s led to renewed interest in several countries. Israel is the example most often cited, where solar water heaters became very common on rooftops and a regulation passed around 1980 required them on most new residential buildings. Other places with strong sun, such as Cyprus and Greece, also adopted them widely.
In more recent decades, China has become the largest user by far, with many households using vacuum-tube designs. Exact figures change from year to year, so it is safest to say that a large share of the world’s installed solar water heating is in China.
How modern systems work
Modern systems fall into a few families.
- Flat-plate collectors use a dark metal absorber under glass, in an insulated frame, with pipes carrying water or another fluid.
- Evacuated tube collectors place absorbers inside glass tubes from which air has been removed. The vacuum greatly reduces heat loss, which is useful in cold or cloudy climates.
- Passive systems such as thermosiphon units use natural circulation, with a tank above the collector.
- Active systems use pumps and controls, and are common where the tank sits inside the building.
In colder regions, an antifreeze fluid is often used in a closed loop with a heat exchanger, so that pipes do not freeze. Most systems include a backup, such as an electric element or gas boiler, for cloudy days.
How it compares with solar electricity
Solar water heating converts a larger share of incoming sunlight into usable energy than a photovoltaic panel does, because heat is easier to collect. It does only one job, though. It cannot power appliances or feed the grid. When the price of photovoltaic panels fell, some households started to heat water with electricity from panels instead, much as electric cooking and other electric appliances had earlier moved work away from fuels, a choice that depends on local prices, roof space and rules.
The solar water heater is a reminder that renewable energy is more than electricity. Some of its oldest successes have come from collecting heat in a way that early inventors could have understood, and in many countries it has cut the amount of fuel needed for heating water, though savings depend on climate and household use.