Solar water heating (SWH) is heating water by sunlight, using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications. A Sun-facing collector heats a working fluid that passes into a storage system for later use. SWH are active (pumped) and passive (convection-driven). They use water only, or both water and a working fluid. They are heated directly or via light-concentrating mirrors. They operate independently or as hybrids with electric or gas heaters. In large-scale installations, mirrors may concentrate sunlight into a smaller collector. At the end of 2023, global solar hot water thermal capacity was 560 GWth, a 3% increase from 2022. The market is dominated by China, the United States and Turkey. Barbados, Austria, Cyprus, Israel and Greece are the leading countries by capacity per person. There were 122 million solar hot water systems in operation at the end of 2022.
History
Records of solar collectors in the United States date to before 1900, involving a black-painted tank mounted on a roof. In 1896 Clarence Kemp of Baltimore enclosed a tank in a wooden box, thus creating the first 'batch water heater' as they are known today. Frank Shuman built the world's first solar thermal power station in Maadi, Egypt between 1912 and 1913, using parabolic troughs to power a 45 to 52 kilowatts (60 to 70 horsepower) engine that pumped 23,000 litres (6,000 US gal) of water per minute from the Nile River to adjacent cotton fields. Flat-plate collectors for solar water heating were used in Florida and Southern California in the 1920s. Interest grew in North America after 1960, but especially after the 1973 oil crisis.
Mediterranean
Israel, Cyprus and Greece are the per capita leaders in the use of solar water heating systems supporting 30%–40% of homes. Flat plate solar systems were perfected and used on a large scale in Israel. In the 1950s a fuel shortage led the government to forbid heating water between 10 pm and 6 am. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he launched the NerYah Company, Israel's first commercial manufacturer of solar water heating. Solar water heaters were used by 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 Israel required the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result, Israel became the world leader in the use of solar energy per capita with 85% of households using solar thermal systems (3% of the primary national energy consumption), estimated to save the country 2 million barrels (320,000 m3) of oil a year. In 2005, Spain became the world's first country to require the installation of photovoltaic electricity generation in new buildings, and the second (after Israel) to require the installation of solar water heating systems, in 2006.
Asia
After 1960, systems were marketed in Japan. Australia has a variety of national and state and regulations for solar thermal starting with MRET in 1997. Solar water heating systems are popular in China, where basic models start at around 1,500 yuan (US$235), around 80% less than in Western countries for a given collector size. At least 30 million Chinese households have one. The popularity is due to efficient evacuated tubes that allow the heaters to function even under gray skies and at temperatures well below freezing.
Design requirements
The type, complexity and size of a solar water heating system is mostly determined by:
Changes in ambient temperature and solar radiation between summer and winter Changes in ambient temperature during the day-night cycle Possibility of the potable water or collector fluid overheating or freezing The minimum requirements of the system are typically determined by the amount or temperature of hot water required during winter, when a system's output and incoming water temperature are typically at their lowest. The maximum output of the system is determined by the need to prevent the water in the system from becoming too hot.
Freeze protection Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Drainback systems drain the transfer fluid from the system when the pump stops. Many indirect systems use antifreeze (e.g., propylene glycol) in the heat transfer fluid. In some direct systems, collectors can be manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often but can be less reliable than an automatic system as it relies on an operator. The third type of freeze protection is freeze-tolerance, where low-pressure water pipes made of silicone rubber simply expand on freezing. One such collector now has European Solar Keymark accreditation. Notably, while the need for freeze protection has traditionally been a complicating factor, systems using evacuated tube collectors may be used in climates with moderate frosts without any need for drainback or antifreeze. This is because an evacuated tube system only has water in the thermally insulated header, not in the evacuated tubes themselves. Additional protection is also provided by the controller: if header temperature drops below a set value (typically 2 to 4'C) the circulating pump is automatically turned on for a short period to pass warmer water to the header.
Overheat protection When no hot water has been used for a day or two, the fluid in the collectors and storage can reach high temperatures in all non-"drainback" systems. When the storage tank in a "drainback" system reaches its desired temperature, the pumps stop, ending the heating process and thus preventing the storage tank from overheating. Some active systems deliberately cool the water in the storage tank by circulating hot water through the collector at times when there is little sunlight or at night, losing heat. This is most effective in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors, due to their superior insulation. Any collector type may still overheat. High pressure, sealed solar thermal systems ultimately rely on the operation of temperature and pressure relief valves. Low pressure, open vented heaters have simpler, more reliable safety controls, typically an open vent.
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