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.
As of 2017, global solar hot water (SHW) thermal capacity is 472 GW and 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.
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, using parabolic troughs to power a engine that pumped 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.
Solar power is in use in Australia, Canada, China, Germany, India, Israel, Japan, Portugal, Romania, Spain, the United Kingdom and the United States.
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 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.