Solar water heating

Solar water heating (SWH) is the conversion of sunlight into heat for water heating using a solar thermal collector . A variety of configurations are available in different climates and latitudes. SWHs are widely used for residential and some industrial applications. [1]

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. [2] In large-scale facilities, mirrors may concentrate sunlight onto a smaller collector.

The global solar thermal market is dominated by China, Europe, Japan, and India , while Israel was one of the first countries to mandate installation of SWH in 1980, leading to a flourishing industry. [3]


Frank Shuman ‘s sunengine on the March 1916 cover of Hugo Gernsback’ s The Electrical Experimenter

Records of solar collectors in the US date to before 1900, [4] 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 power station in Maadi, Egypt , using parabolic troughs to power a 60-70 horsepower engine that pumped 6,000 gallons 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 .

See Appendix 1 for country-specific statistics on the “Use of Solar Water Heating Worldwide”. Solar power is in use in Australia , Canada , China , Germany , India , Israel , Japan , Portugal , Romania , Spain , the United Kingdom and the United States .


Israel, Cyprus and Greece are the per capita leaders in the use of solar water heating systems Supporting 30% -40% of homes. [5]

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. [6] Solar water heaters were used by 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980, the United States required the installation of solar water heaters in all new homes. [7] As a result, Israel became the world leader in the use of solar energy per capitawith 85% of households using solar thermal systems, [8] estimated to save the country 2 million barrels (320,000 m 3 ) of oil a year. [9]

In 2005, Spain became the world’s first country to introduce 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. [10]


After 1960 systems were marketed in Japan. [4]

MRET in 1997. [11] [12] [13]

Solar water heating systems are popular in China, where 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 efficient and effective. [14]

Latin America

Colombia developed a local solar water heating industry thanks to the designs of Las Gaviotas , directed by Paolo Lugari. Driven by a desire to reduce costs in social housing, the team studied the best systems from Israel and made adaptations to meet the specifications set by Banco Central Hipotecario (BCH) which required the system to operate in cities such as Bogotá that are overcast for more than 200 days annually. The ultimate designs were so successful that the Gaviotas offered a 25-year warranty on its facilities in 1984. Over 40,000 were installed and still function a quarter of a century later.

Design requirements

The type, complexity and size of a solar water heating system is determined by:

  • Changes in ambient temperature and solar radiation between summer and winter
  • Changes in ambient temperature during the day-night cycle
  • Possibility of drinking water or collector fluid overheating or freezing

The minimum requirements of the system are generally determined by the amount of the temperature when the temperature is high, and the temperature of the product is usually lower than that. The maximum output of the system is determined by the need to prevent 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(eg, 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 many cases where it is relatively unreliable since it is unreliable.

A third type of freeze protection is freeze-tolerance, where low pressure polymer water channels 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 collector at the time of the day, losing heat. This is most effective in direct or thermal storage. 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 open wind.


Sample designs include a simple glass-topped insulated box with a flat solar absorber made of sheet metal, attached to copper heat exchanger pipes and dark-colored, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In industrial cases a mirror parabolic 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 final optimum temperature for the solar collector clarification needed ]is a typical immersion immersion or combustion heater. The heat transfer fluid (HTF) for the absorption May be water, drank 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 has 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 in both primary and secondary circuits (pipes and heat exchangers for water tanks). [15]

Another lower-maintenance concept is the ‘drain-back’. No anti-freeze is required; instead, all the piping is sloped to water to drain back to the tank. The tank is not pressurized and operates at atmospheric pressure. As soon as the pump shuts off, for 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. The invention relates to an application of the invention to the application of the invention to the application of the principle of the application of the present invention to the application of the present invention (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 [16] can be used in a hot water system 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 solar heating is at night when solar gain is lower. Therefore, solar water heating is better than the best of the world. 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 thermalsystems. 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 . [17] [18]

Heat transfer


Direct systems: (A) Passive CHS system with tank above collector. (B) Active system with pump and controller driven by a photovoltaic panel.

Direct or open loop circulate drinking 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 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 have been damaged when the water is turned over.


Indirect or closed loop systems use a heat exchanger to transfer heat from the heat-transfer fluid (HTF) fluid to the drinking 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 exchanger exchanger, where its heat is transferred to the drinking water. Indirect systems offer freeze protection and typically overheat protection.



Passive systems rely on heat-driven convection or heat pipes to circulate the working fluid. Passive systems cost less and require less, but are less efficient. Overheating and freezing are major concerns.


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 operate. However, they can be more easily controlled.

Active systems with remote-controlled 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 both collector and storage. Batch heaters are thin rectilinear tanks with a glass facing the sun at noon . They are more likely to be installed on a roof (to support 400-700 lb (180-320 kg) lbs of water) The sun is largely uninjured and only suitable for moderate climates.

convection heat storage unit (CHS) system is similar to an ICS system, except the storage tank and collector are separated by convection. CHS systems typically use standard flat-plate gold type 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, the heat sink does not cause convection, as the cold water stays at the lowest part of the system.

Active indirect systems

Pressurized antifreeze systems use a mixture of antifreeze (almost always non-toxic propylene glycol) and water mix for HTF in order to prevent freeze damage.

Effective anti-freeze damage, antifreeze systems have drawbacks:

  • If the HTF gets too hot the glycol degrades into acid and then provides no protection and opens up 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 non-toxic.
  • Even though the HTF contains glycol to prevent freezing, it circulates at low temperatures (eg below 40 ° F (4 ° C)), causing substantial heat loss.

drainback system is an indirect active system where the HTF (usually pure water) circulates through the collector, driven by a pump. The collector piping is not included in the scope of the invention. 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. [19]

Do-it-yourself (DIY)

Plans for solar water heating systems are available on the Internet. [20] DIY SWH systems are usually cheaper than commercial ones, and they are used both in the developed and developing world. [21]


Characteristic ICS (Batch) thermosiphon Active direct Active indirect drainback Bubble Pump
Low profile-unobtrusive
Lightweight collector
Survives freezing weather
Low maintenance
Simple: no ancillary control
Retrofit potential to existing store
Space saving: no extra storage tank
Comparison of SWH systems. Source: Solar Water Heating ‘ [22]



Main article: Solar thermal collector

Solar thermal collectors capture and retain heat from the sun and use it to heat a liquid. [23] Two important physical principles govern the technology of solar thermal collectors:

  • Any hot object Ultimately returns to thermal equilibrium with ict environment, due to heat loss from conduction , convection and radiation. [24] Efficiency (the proportion of heat energy retained for a predefined time period) is directly related to heat loss from the collector surface. Convection and radiation are the most important sources of heat loss. Thermal insulation is used to slow heat loss from a hot object. This follows the second law of thermodynamics (the ‘equilibrium effect’).
  • Heat is lost more quickly if the temperature difference between a hot object and its environment is larger. Heat loss is predominantly governed by the thermal gradient between the collector surface and the ambient temperatures. Conduction, convection and radiation all over Occur more Rapidly wide thermal gradients [24] (the delta- t effect).

Flat plate

Flat plate collectors are an extension of the idea to place a collector in an oven-like box with glass directly facing the Sun. [1]Most flat pipe collectors have two horizontal pipes, called headers, and many smaller vertical pipes connecting them, called risers. The risers are welded (or similarly connected) to thin absorbing purposes. Heat-transfer fluid (water or water / antifreeze mix) is pumped from the hot water storage or tank heat exchanger into the collectors’ bottom header, and it travels up the riser, collecting heat from the absorbing purposes, And Then exits the collector out of the top header. Serpentine flat flat collectors differ from this “harp” design, and instead use a single pipe that travels up and down the collector. However, since they can not be properly drained of water, flat serpentine collectors can not be used in drainback systems.

The type of glass used in flat collectors is always low-iron, tempered glass . Such glass can withstand significant hail without breaking, which is one of the reasons that flat collectors are considered the most durable collector type.

Unglazed or collectors are similar to flat-collectors, except they are not thermally insulated by a glass panel. Consequently, these types of collectors are much less efficient. For pool heating applications, the water to be heated is Often colder than the ambient temperature roof, at the point the qui Lack of thermal insulation Allows additional heat to be drawn from the surrounding environment. [25]

Evacuated tube

Evacuated tube collectors (ETC) are a way to reduce the heat loss, [1] inherent in flat plates. Since heat loss due to convection can not cross a vacuum, it forms an efficient insulation mechanism to keep heat inside the collector pipes. [26] Since two flat glass sheets are in the vacuum range, the vacuum is created between two concentric tubes. Typically, the water piping in an ETC is therefore Surrounded by two concentric tubes of glass separated by a vacuum That allowed on heat from the sun (to heat the pipe) That objective limits heat loss. The inner tube is coated with a thermal absorb. [27] Vacuum life varies from collector to collector, from 5 years to 15 years.

Flat plate collectors are more efficient than ETC in full sunshine conditions. However, the energy output of flat plate collectors is slightly more than ETCs in cloudy or extremely cold conditions. [1] Most ETCs are made out of annealed glass, qui est susceptible to hail , failing Given Roughly golf ball -sized particles. ETCs made from “Coke glass,” which: has a green held, are stronger and less Likely hjälper Their vacuum, goal efficiency is due to Slightly Reduced Reduced transparency. ETCs can gather energy from the sun all day long at low angles due to their tubular shape. [28]


PV pump

One way to power an active system is via a photovoltaic (PV) panel . To ensure proper pump performance and longevity, the (DC) pump and PV panel must be suitably matched. Although a PV-powered pump does not operate at night, the controller must ensure that it does not work.

PV pumps offer the following advantages:

  • Simpler / cheaper installation and maintenance
  • Excess PV output can be used for household electricity use or put back into the grid.
  • Can dehumidify living space. [29]
  • Can operate during a power outage.
  • Avoids the carbon consumption from using grid-powered pumps.

Bubble pump

A bubble pump (also known as a geyser pump) is suitable for flat panel and vacuum tube systems. In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil at low temperature and the sun heats it. The steam bubbles form a geyser, causing an upward flow. The flow of the fluid in the flow of the fluid and the condensate at the highest point in the circuit, after which the fluid flows downward towards the heat exchanger caused by the difference in fluid levels. [30] [31] [32] The HTF typically arrived at the heat exchanger at 70 ° C and returned to the circulating pump at 50 ° C. Pumping typically starts at about 50 ° C and reaches the sun until equilibrium is reached.


differential controller senses temperature differences between water and the solar collector and the water in the storage tank near the heat exchanger. The controller starts the pump when the water in the collector is about 8-10 ° C and the temperature difference reaches 3-5 ° C. This ensures that you save water when you’re in the water. Excessive cycling on and off. (In direct systems the pump can be triggered with a difference around 4 ° C because they have no heat exchanger.)


The simplest collector is a water-filled metal tank in a sunny place. The sun heats the tank. This was how the first systems worked. [4] This setup would be inefficient due to the equilibrium effect, and it would have a positive impact on the environment. The challenge is to limit the heat loss.

  • The storage tank can be located lower than the collectors, allowed increased freedom in system design and allowed pre-existing storage tanks to be used.
  • The storage tank can be hidden from view.
  • The storage tank can be placed in a conditioned or semi-conditioned space, reducing heat loss.
  • Drainback tanks can be used.

Insulated tank

ICS or batch collectors reduce heat loss by thermally insulating the tank. [1] [33] This is achieved by the tank in a glass-topped box that allows heat from the sun to reach the water tank. [34] The other walls of the box are thermally insulated, reducing convection and radiation. [35] The box can also have a reflective surface on the inside. This one looks at the tank back to the tank. In a simple way one could consider an ICS solar water heater as a water tank that has been enclosed in a type of ‘oven’ that retains heat from the sun as well as the heat of the water in the tank. Using a box does not eliminate the heat from the tank to the environment, but it greatly reduces this loss.

Standard ICS collectors have a feature that strongly limits the efficiency of the collector: a small surface-to-volume ratio. [36] Since the amount of heat can be absorbed by the sun, it can not be . These collectors have an inherently small surface-to-volume ratio. Collectors attempt to increase this ratio for efficient warming of the water. Variations on this basic design include collectors that combine smaller water containers and evacuated glass tube technology, a type of ICS system known as an Evacuated Tube Batch (ETB) collector. [1]


Evacuated tube

ETSCs can be more useful than other solar collectors during winter season. ETCs can be used for heating and cooling purposes in industries such as pharmaceuticals and pharmaceuticals, paper, leather and textile and also for residential houses, hospitals nursing home, hotels swimming pool etc.

An ETC can operate at a range of temperatures from medium to high for solar hot water, swimming pool, air conditioning and solar cooker.

Higher temperature (up to 200 ° C (392 ° F)).

Swimming pools

Floating pool covering systems STCs are used for pool heating.

Pool covering systems, whether solid sheets or floating disks, Much heat loss occurs through evaporation, and using a cover slows evaporation.

STCs for nonpotable pool water are often made of plastic. Pool water is mildly corrosive due to chlorine. Water is circulated through the panels using the existing pool or supplemental pump. In mild environments, unglazed plastic collectors are more efficient as a direct system. In cold or windy environments evacuated tubes or flat plates in an indirect configuration are used in conjunction with a heat exchanger. This reduces corrosion. A fairly simple differential temperature controller is used to direct the water to the panels or heat exchanger or by turning a valve or operating the pump. Once the pool has reached the temperature, it has to be separated from the pool without heating. [37]Many systems are configured as draining systems where the water drains into the pool when the water pump is switched off.

The collector panels are usually mounted on a roof, or ground-mounted on a tilted rack. Due to the low temperature difference between the air and the water, the panels are often formed collectors or unglazed flat plate collectors. A simple rule-of-thumb is required for 50% of the pool’s surface area. [37] This is for areas where pools are used in the summer season only. Adding solar collectors to a conventional outdoor pool, in a cold climate, can easily extend the pool. [25] An active solar energy system analysis program can be used to optimize the solar heating system before it is built.

Energy production

A laundromat in California .

The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place ( insolation ). In the tropics insolation can be relatively high, eg 7 kWh / m2 per day, versus eg, 3.2 kWh / m2 per day in temperate areas. Even at the same latitude average insolation can vary the weather and the amount of overcast. Calculators are available for estimating insolation at a site. [38] [39] [40]

Below is a table that gives a rough indication of the specifications and energy that could be expected from a solar water heating system involving some 2 m 2 of absorbing area of ​​the collector, demonstrating two evacuated tube and three flat plate solar water heating systems. Certification information or figures calculated from those data. The bottom two rows for energy production (kWh / day) for a tropical and a temperate scenario. These temperatures are for heating water at 50 ° C above ambient temperature.

With most solar water heating systems, the energy output scales linearly with the collector surface area. [41]

Daily energy production (kW th .h) of five solar thermal systems. The evac tube systems 20 tubes
Technology Flat plate Flat plate Flat plate ETC ETC
Configuration Direct active thermosiphon Indirect active Indirect active Direct active
Overall size (m 2 ) 2.49 1.98 1.87 2.85 2.97
Absorber size (m 2 ) 2.21 1.98 1.72 2.85 2.96
Maximum efficiency 0.68 0.74 0.61 0.57 0.46
Energy production (kWh / day): 
– Insolation 3.2 kWh / m 2 / day ( temperate )
– eg Zurich, Switzerland
5.3 3.9 3.3 4.8 4.0
– Insolation 6.5 kWh / m 2 / day (tropical)
– eg Phoenix, USA
11.2 8.8 7.1 9.9 8.4

The figures are fairly similar between the above collectors, yielding some 4 kWh / day in a temperate climate and some 8 kWh / day in a tropical climate when using a collector with a 2 m 2 absorb. In the temperate scenario this is enough to heat 200 liters of water by some 17 ° C. In the tropical scenario the equivalent heating would be 33 ° C. Many thermosiphon systems have comparable energy output to equivalent active systems. The efficiency of evacuated tube collectors is somewhat lower than that of flat collectors because of the absorbers are made of the tubes and the tubes having space between them, resulting in a larger percentage of inactive overall collector area. Some methods of comparison [42]calculate the efficiency of evacuated tube collectors based on the table of contents. Efficiency is reduced at higher temperatures.


In sunny, warm rentals, where freeze protection is not necessary, an ICS (batch type) solar water heater can be cost effective. [35] In higher latitudes, design requirements for cold weather add to system complexity and cost. This Increases initial costs, but not life-cycle costs. The biggest single consideration is therefore the large initial financial outlay of solar water heating systems. [43] Offsetting this expense can take years. [44] The payback period is longer in temperate environments. [45] Since solar energy is free, operating costs are small. At higher latitudes, solar heaters may be more effective than lower insolation, possibly larger and / or dual-heating systems. [45] In some countries government incentives can be significant.

Cost factors (positive and negative) include:

  • Price of solar water heater (more complex systems are more expensive)
  • Efficiency
  • Installation cost
  • Electricity used for pumping
  • Price of water heating fuel (eg gas or electricity) saved per kWh
  • Amount of water heating fuel used
  • Initial and / or recurring government subsidy
  • Maintenance cost (eg antifreeze or pump replacements)
  • Savings in maintenance of water heating system (electric / gas / oil)

Payback times can vary greatly from regional sun, extra cost due to frost protection needs of collectors, household hot water use etc. For instance in central and southern Florida, the period was 12 years or more indicated for the US [46]

Costs and payback periods for residential SWH systems with savings of 200 kWh / month (using 2010 data)
Country Currency System cost Subsidy (%) Effective cost Electricity cost / kWh Electricity savings / month Payback period (y)
 brazil BRL 2500 [47] 0 2500 0.25 50 4.2
 South Africa ZAR 14000 15 [48] 11900 0.9 180 5.5
 australia AUD 5000 [49] 40 [50] 3000 0.18 [51] 36 6.9
 belgium EUR 4000 [52] 50 [53] 2000 0.1 [54] 20 8.3
 United States USD 5000 [55] 30 [56] 3500 0.1158 [57] 23.16 12.6
 United Kingdom GBP 4800 [58] 0 4800 0.11 [59] 22 18.2

The payback period is shorter given greater insolation. However, even in temperate areas, solar water heating is cost effective. The payback period for photovoltaic systems has historically been much longer. [45] Costs and payback period are shorter if not complementary / backup system is required. [44] thus extending the payback period of such a system.


Australia operates a system of Renewable Energy Credits, based on national renewable energy targets. [50]

The Toronto Solar Neighborhoods Initiative offers subsidies for the purchase of solar water heating units. [60]

Energy footprint and life cycle assessment

Energy footprint

The source of electricity in an active SWH system determines the extent to which a system contributes to atmospheric carbon during operation. Active solar thermal systems that use electricity to pump the fluid through the panels are called ‘low carbon solar’. In most systems, the savings of 20%. [61] However, low power pumps operate with 1-20W. [62] [63] Assuming a solar collector panel delivering 4kWh / day and a pump running intermittently from the hands of electricity for a total of 6 hours during a 12-hour sunny day, the potential negative effect of such a pump can be reduced to about 3% of the heat produced.

However, PV-powered active solar thermal systems typically use a 5-30 W PV panel and a small, low power diaphragm pump or centrifugal pump to circulate the water. This reduces the operational carbon and energy footprint.

Alternative non-electrical pumping systems may employ thermal expansion and phase changes of liquids and gases.

Life Cycle Energy Assessment

Recognized standards can be used to deliver robust and quantitative life cycle assessments (LCA). LCA considers the financial and environmental costs of acquisition of raw materials, manufacturing, transportation, using, servicing and disposal of equipment. Elements include:

  • Financial costs and gains
  • Energy consumption
  • CO 2 and other emissions

In terms of energy consumption, some 60% goes into the tank, with 30% towards the collector [64] (thermosiphon flat plate in this case). In Italy, [65] some 11 giga-joules of electricity are used in the production of 35% towards the collector. The main energy-related impact is emissions. The energy used in manufacturing is recovered within the first 2-3 years of use (in southern Europe).

By contrast the energy payback time in the UK is reported to only 2 years. This figure is for a direct system, a retrofitted to an existing water store, a PV pumped, a freeze tolerant and 2.8 sqm aperture. For comparison, a PV installation took around 5 years to reach energy payback, according to the same comparative study. [66]

In terms of CO 2 emissions, a large fraction of the emissions is dependent on the degree to which it is used to supplement the sun. Using the Eco-indicator 99 points system a yardstick (ie the annual environmental load of an average European inhabitant) in Greece, [64] has a purely gas-driven system that has fewer emissions than a solar system. This calculation assumes that the solar system produces a household.

A test system in Italy produced about 700 kg of CO 2 , considering all the components of manufacture, use and disposal. Maintenance was identified as an emissions-costly activity when the heat transfer fluid (glycol-based) was replaced. However, the emissions have been recovered within two years of the equipment. [65]

In Australia, life cycle emissions were also recovered. The tested SWH system had about 20% of the impact of an electrical water heater and half that of a water heater. [44]

Analyze their lower impact retrofit freezing-tolerant solar water heating system, Allen et al. (qv) reported a production CO 2 impact of 337 kg, which is around the environmental impact reported in the Ardente et al. (qv) study.

System specification and installation

  • Most SWH installations require backup heating.
  • The amount of hot water consumed each day. In a solar-only system, a significant fraction of the water in the reservoir. The larger the reservoir the smaller the daily temperature variation.
  • SWH systems offer large scale savings in collector and tank costs. [64] Thus the most economically efficient scale meets 100% of the heating needs of the application.
  • Direct systems (and some indirect systems using heat exchangers) can be retrofitted to existing stores.
  • Equipment components must be insulated to achieve full system benefits. The installation of efficient insulation significantly reduces heat loss.
  • The most efficient PV pumps start slowly in low light levels, so they may cause a small amount of unwanted circulation while the collector is cold. The controller must get stored hot water from this cooling effect.
  • Evacuated tube collector arrays can be adjusted by removing / adding tubes or their heat pipes, allowing customization during / after installation.
  • Above 45 degrees latitude, roof mounted sun-facing collectors tend to outproduce wall-mounted collectors. However, arrays of wall-mounted energy can be used in the past.



  • EN 806: Specifications for installations inside buildings. General.
  • EN 1717: Protection against pollution of drinking water in water installations and general requirements of devices to prevent pollution by backflow.
  • EN 60335: Specification for safety of household and similar electrical appliances. (2-21)
  • UNE 94002: 2005 Thermal solar systems for domestic hot water production. Calculation method for heat demand.

United States

  • OG-300: OG-300 Certification of Solar Water Heating Systems. [67]


  • CAN / CSA-F378 Series 11 (Solar Collectors)
  • CAN / CSA-F379 Series 09 (Domestic Solar Hot Water Systems)
  • SRCC Standard 600 (Minimum Standard for Solar Thermal Concentrating Collectors)


  • Renewable Energy (Electricity) Act 2000
  • Renewable Energy (Electricity) Act 2000 Large-scale Generation Shortfall Charge
  • Renewable Energy (Electricity) Act 2010 Small-scale Technology Shortfall Charge
  • Renewable Energy (Electricity) Regulations 2001
  • Renewable Energy (Electricity) Regulations 2001 – STC Calculation Methodology for Solar Water Heaters and Air Source Heat Pump Water Heaters
  • Renewable Energy (Electricity) Amendment (Transitional Provision) Regulations 2010
  • Renewable Energy (Electricity) Amendment (Transitional Provisions) Regulations 2009

All participants in the Large-Scale Renewable Energy Target and Small-Scale Renewable Energy Scheme must comply with the above Acts. [68]

Worldwide use

Solar hot water system installed on Kouga Local Municipality , South Africa
Top countries using solar thermal power, worldwide (GW th ) [10] [69] [70] [71] [72] [73] [74]
# Country 2005 2006 2007 2008 2009 2010 2011 2012 2013
1  china 55.5 67.9 84.0 105.0 101.5 117.6 262.3 [75]
 US 11.2 13.5 15.5 20.0 22.8 23.5 25.6 29.7 31.4
2  United States 1.6 1.8 1.7 2.0 14.4 15.3 16.8 [75]
3  germany 7.8 8.9 9.8 10.5 11.4 12.1
4  turkey 5.7 6.6 7.1 7.5 8.4 9.3 11.0 [75]
5  australia 1.2 1.3 1.2 1.3 5.0 5.8 5.8 [75]
6  brazil 1.6 2.2 2.5 2.4 3.7 4.3 6.7 [75]
7  Japan 5.0 4.7 4.9 4.1 4.3 4.0 3.2 [75]
8  austria 2.5 3.0 3.2 2.8 3.4 3.5
9  greece 2.7 2.9 2.9 2.9 2.9 2.9
10  Israel 3.3 3.8 3.5 2.6 2.8 2.9 2.9 [75]
World (GW th ) 88 105 126 149 172 196


Solar thermal heating in European Union (MW th ) [76] [77] [78]
# Country 2008 2009 2010 [71] 2011 2012 2013
1  germany 7,766 9.036 9,831 10,496 11.416 12.055
2  austria 2,268 3,031 3,227 2,792 3,448 3,538
3  greece 2,708 2,853 2,855 2,861 2,885 2,915
4  italy 1,124 1,410 1,753 2,152 2,380 2,590
5  spain 988 1,306 1,543 1,659 2,075 2,238
6  la France 1,137 1,287 1,470 1,277 1,691 1,802
7  poland 254 357 459 637 848 1,040
8  Portugal 223 395 526 547 677 717
9  Czech Republic 116 148 216 265 625 681
10 switzerland 416 538 627
11  Netherlands 254 285 313 332 605 616
12  denmark 293 339 379 409 499 550
13  cyprus 485 490 491 499 486 476
14  UK 270 333 374 460 455 475
15  belgium 188 204 230 226 334 374
16  sweden 202 217 227 236 337 342
17  Ireland 52 85 106 111 177 196
18  slovenia 96 111 116 123 142 148
19  hungary 18 59 105 120 125 137
20  Slovakia 67 73 84 100 108 113
21  Romania * 66 80 73 74 93 110
22  Bulgaria * 22 56 74 81 58 59
23  Malta * 25 29 32 36 34 35
24  Finland * 18 20 23 23 30 33
25  Luxembourg * 16 19 22 25 23 27
26  Estonia * 1 1 1 3 10 12
27  Latvia * 1 1 1 3 10 12
28  Lithuania * 1 2 2 3 6 8
Total EU27 + Sw (GW th ) 19,08 21,60 23.49 25.55 29.66 31.39
* = estimate, F = France as a whole

See also

  • Solar thermal collector
  • Solar air heating
  • Solar air conditioning
  • Concentrating solar power
  • Passive solar
  • Renewable heat
  • Solar combisystem
  • Solar energy
  • Solar thermal energy
  • Renewable energy commercialization
  • Sustainable design


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