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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.[1][2]
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.[3] In large-scale installations, mirrors may concentrate sunlight into a smaller collector.[original research?]
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.[4] Barbados, Austria, Cyprus, Israel and Greece are the leading countries by capacity per person.[4]
History
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Records of solar collectors in the United States date to before 1900,[5] 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 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.
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.[6]
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.[7] 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).[8] 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),[9] estimated to save the country 2 million barrels (320,000 m3) of oil a year.[10][11]
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.[12]
Asia
After 1960, systems were marketed in Japan.[5]
Australia has a variety of national and state and regulations for solar thermal starting with MRET in 1997.[13][14][15]
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.[16]
Design requirements
 | This section needs additional citations for verification. (January 2016) |
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.
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.
Structure and working
Simple designs include a simple glass-topped insulated box with a flat solar absorber made of dark-colored sheet metal, attached to copper heat exchanger pipes, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In industrial cases a parabolic mirror can concentrate sunlight on the tube. Heat is stored in a hot water storage tank. The volume of this tank needs to be larger with solar heating systems to compensate for bad weather[clarification needed] and because the optimum final temperature for the solar collector[clarification needed] is lower than a typical immersion or combustion heater. The heat transfer fluid (HTF) for the absorber may be water, but more commonly (at least in active systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor delivers heat to the tank through a heat exchanger (commonly a coil of copper heat exchanger tubing within the tank). Copper is an important component in solar thermal heating and cooling systems because of its high heat conductivity, atmospheric and water corrosion resistance, sealing and joining by soldering and mechanical strength. Copper is used both in receivers and primary circuits (pipes and heat exchangers for water tanks).[17]
The 'drain-back' is another lower-maintenance concept.[18] No anti-freeze is required; instead, all the piping is sloped to cause water to drain back to the tank. The tank is not pressurized and operates at atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes empty before freezing can occur.
Residential solar thermal installations fall into two groups: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting, ensuring that hot water is always available. The combination of solar water heating and back-up heat from a wood stove chimney[19] can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.
When a solar water heating and hot-water central heating system are used together, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will replace the lower heating element and the upper element will remain to provide for supplemental heat. However, the primary need for central heating is at night and in winter when solar gain is lower. Therefore, solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched. In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy. When combined with storage, large scale solar heating can provide 50-97% of annual heat consumption for district heating.[20][21]
Heat transfer
Direct
Direct or open loop systems circulate potable water through the collectors. They are relatively cheap. Drawbacks include:
- They offer little or no overheat protection unless they have a heat export pump.
- They offer little or no freeze protection, unless the collectors are freeze-tolerant.
- Collectors accumulate scale in hard water areas, unless an ion-exchange softener is used.
The advent of freeze-tolerant designs expanded the market for SWH to colder climates. In freezing conditions, earlier models were damaged when the water turned to ice, rupturing one or more components.
Indirect
Indirect or closed loop systems use a heat exchanger to transfer heat from the "heat-transfer fluid" (HTF) fluid to the potable water. The most common HTF is an antifreeze/water mix that typically uses non-toxic propylene glycol. After heating in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable water. Indirect systems offer freeze protection and typically overheat protection.
Propulsion
Passive
Passive systems rely on heat-driven convection or heat pipes to circulate the working fluid. Passive systems cost less and require low or no maintenance, but are less efficient. Overheating and freezing are major concerns.
Active
Active systems use one or more pumps to circulate water and/or heating fluid. This permits a much wider range of system configurations.
Pumped systems are more expensive to purchase and to operate. However, they operate at higher efficiency and can be more easily controlled.
Active systems have controllers with features such as interaction with a backup electric or gas-driven water heater, calculation and logging of the energy saved, safety functions, remote access and informative displays.
Passive direct systems
An integrated collector storage (ICS or batch heater) system uses a tank that acts as both storage and collector. Batch heaters are thin rectilinear tanks with a glass side facing the Sun at noon. They are simple and less costly than plate and tube collectors, but they may require bracing if installed on a roof (to support 400–700 lb (180–320 kg) lbs of water), suffer from significant heat loss at night since the side facing the sun is largely uninsulated and are only suitable in moderate climates.
A convection heat storage unit (CHS) system is similar to an ICS system, except the storage tank and collector are physically separated and transfer between the two is driven by convection. CHS systems typically use standard flat-plate type or evacuated tube collectors. The storage tank must be located above the collectors for convection to work properly. The main benefit of CHS systems over ICS systems is that heat loss is largely avoided since the storage tank can be fully insulated. Since the panels are located below the storage tank, heat loss does not cause convection, as the cold water stays at the lowest part of the system.
Active indirect systems
Pressurized antifreeze systems use a mix of antifreeze (almost always low-toxic propylene glycol) and water mix for HTF in order to prevent freeze damage.
Though effective at preventing freeze damage, antifreeze systems have drawbacks:
- If the HTF gets too hot the glycol degrades into acid and then provides no freeze protection and begins to dissolve the solar loop's components.
- Systems without drainback tanks must circulate the HTF – regardless of the temperature of the storage tank – to prevent the HTF from degrading. Excessive temperatures in the tank cause increased scale and sediment build-up, possible severe burns if a tempering valve is not installed, and if used for storage, possible thermostat failure.
- The glycol/water HTF must be replaced every 3–8 years, depending on the temperatures it has experienced.
- Some jurisdictions require more-expensive, double-walled heat exchangers even though propylene glycol is low-toxic.
- Even though the HTF contains glycol to prevent freezing, it circulates hot water from the storage tank into the collectors at low temperatures (e.g. below 40 °F (4 °C)), causing substantial heat loss.
A drainback system is an active indirect system where the HTF (usually pure water) circulates through the collector, driven by a pump. The collector piping is not pressurized and includes an open drainback reservoir that is contained in conditioned or semi-conditioned space. The HTF remains in the drainback reservoir unless the pump is operating and returns there (emptying the collector) when the pump is switched off. The collector system, including piping, must drain via gravity into the drainback tank. Drainback systems are not subject to freezing or overheating. The pump operates only when appropriate for heat collection, but not to protect the HTF, increasing efficiency and reducing pumping costs.[22]
Do-it-yourself (DIY)
Plans for solar water heating systems are available on the Internet.[23] DIY SWH systems are usually cheaper than commercial ones, and they are used both in the developed and developing world.[24] [25]
