Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are more specific examples.

In many climates, a solar heating 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.

In the southern regions of Africa like Zimbabwe, solar water heaters have been gaining popularity, thanks to the Austrian and other EU-funded projects that are promoting more environmentally friendly water heating solutions.

Residential solar thermal installations can be subdivided into two kinds of systems: compact and 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 such as 50 °C. Hence, hot water is always available. The combination of solar hot water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates without the supplemental heat requirement of a solar hot water system being met with fossil fuels or electricity.

Among pumped options, there is an important distinction to be made regarding the sustainability of the design of the system. This relates to what source of energy powers the pump and its controls. The type of pumped solar thermal systems which use mains electricity to pump the fluid through the panels are called low carbon solar because the pumping negates the carbon savings of the solar by about 20%, according to data in a report called "Side by side testing of eight solar water heatings" by DTI UK. However, zero-carbon pumped solar thermal systems use solar electricity which is generated onsite using photovoltaics to pump the fluid and to operate its control electronics. This represents a zero operational carbon footprint and is becoming an important design goal for innovative solar thermal systems.

History

Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he started NerYah Company, Israel's first commercial manufacturer of solar water heaters.^[4][5] Due to the abundance of sunlight in Israel, solar water heaters were used by some 20% of the population by 1967. Following the energy crisis in the 1970s, the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result, Israel is now the world leader in the use of solar energy per capita (3% of the primary national energy consumption).^[6]

During this time, there was some resurgence of interest in solar heating in North America. Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar hot water heating has become the norm in countries with an abundance of solar radiation, like Cyprus, Israel and Greece, as well as in Japan and Austria, where there is less.

Solar hot water systems have become popular in China, where basic models start at around 1,500 yuan (US$190), much cheaper than in Western countries (around 80% cheaper for a given size of collector). It is said that at least 30 million Chinese households now have one, and that the popularity is due to the efficient evacuated tubes which allow the heaters to function even under gray skies and at temperatures well below freezing.

In 2005, Spain became the first country in the world to require the installation of photovoltaic electricity generation in new buildings, and the second (after Israel) to require the installation of solar hot water systems.

Usage

Hot water heated by the sun can be used to:

• Heat water (e.g. for sanitary purposes such as showering, washing, ...)
• Generate electricity

Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology for these places. The global solar thermal market is dominated by China, Europe, Japan and India.

The typical 50 gallon electric water heater uses 11.1 barrels of oil a year, which translates into the same amount oil used by a typical 4 door sedan driven by the average consumer.

Electric utility companies often provide electricity by burning and releasing energy from fuels such as oil, coal and nuclear energy. An electrical home hot water heater sits on an electrical grid and may be driving the use of unclean fuels on the other end of the grid.

Israel

Solar water heater on a rooftop in Jerusalem

Israel's use of solar water heaters is estimated to save the country two million barrels of oil a year, and the country has the highest per capita use in the world.In the 1950s there was a fuel shortage in the new Israeli state, and the government forbid heating water between 10 p.m. and 6 p.m. As the situation worsened, engineer Levi Yissar proposed that instead of building more electrical generators, homes should switch to solar water heaters. He built a prototype in his home, and in 1953 he started NerYah Company, Israel's first commercial manufacturer of solar water heaters.By 1967 around one in twenty households heated their water with the sun and 50,000 solar heaters had been sold. However, cheap oil from Iran and from oil fields captured in the Six Day War made Israeli electricity cheaper and the demand for solar heaters dropped. With the 1970s oil crisis, Harry Zvi Tabor, the father of Israel's solar industry, developed the prototype solar water heater that is now used in over 90% of Israeli homes.

In 1980, the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result, Israel is now the world leader in the use of solar energy per capita (3% of the primary national energy consumption).

As of the early 1990s, all new residential buildings were required by the government to install solar water-heating systems, and Israel's National Infrastructure Ministry estimates solar panels for water-heating already satisfy 4% of the country's total energy demand.Israel and Cyprus are the per capita leaders in the use of solar hot water systems with over 90% of homes using them.

Technique

A thermodynamic water heater installed on a house in Belgium

In order to heat water using solar energy, a collector is fastened to the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid is either pumped (active system) or driven by natural convection (passive system) through it.

The collector could be made of a simple glass topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In some cases, before the solar energy is absorbed, a parabolic mirror is used to concentrate sunlight on the tube. Some systems are capable of converting light to heat and therefore are not as reliant on outside temperature.

A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for an entire family.

Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.

The working fluid for the absorber may be the hot water from the tank, but more commonly (at least in pumped systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger (commonly a coil of copper tubing within the tank). Another lower-maintenance concept is the 'drain-back': 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 is open to atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes empty by the time when freezing could occur.

When a solar hot water and hot-water central heating system are used in conjunction, 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 be lower in the tank than the hotter one. However, the main need for central heating is at night when there is no sunlight 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.

The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump.

Economics, energy and system costs

A laundromat in California with panels on the roof providing hot washing water.

In sunny, warm locations, where freeze protection is not necessary, a batch type solar hot water heater can be extremely cost effective. In higher latitudes, there are often additional design requirements for cold weather, which add to system complexity. This has the effect of increasing the initial cost (but not the life-cycle cost) of a solar hot water system, to a level much higher than a comparable hot water heater of the conventional type. When calculating the total cost to own and operate, a proper analysis will consider that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters (and also in conjunction with an existing hot water heater). In addition, federal and local incentives can be significant.

As an example, a 56 ft.² solar water heater can cost US $7,500, but that initial cost is reduced to just $3,300 in the US State of Oregon due to federal and state incentives. The system will save approximately US $230 per year, with a payback of 14 years. Lower payback periods are possible based on maximizing sun exposure. As energy prices rise, payback periods decrease. In cooler locations, solar heating used to be less efficient. Usable amounts of domestic hot water were only available in the summer months, on cloudless days, between April and October. During the winter and on cloudy days, the output was poor. Independent surveys have shown that modern systems do not suffer these limitations. There are cases of households in cool climates getting all of their domestic hot water year round from solar alone. Systems have been show to efficiently work as far north as Whitehorse, Yukon (latitude of 60 B 43' N ).

The installation costs in the UK used to be prohibitive, on average about £9,000. This is reduced in more recent years to £3,000, with payback period reduced, with the rise in the gas price, to 12 years . As energy prices rise, payback periods shorten accordingly.

According to ANRE (a Flemish energy agency, subsidised by the Flemish or Belgian government, a complete, commercial (active) solar hot water system composed of a solar collector (3-4 m²; this is large enough for 4 people), pipes and tank (again large enough for 4 people) costs around 4000 euro. The installation by a recognised worker costs another 800 euro. Electrabel's home magazine Eandismagazine stated in 2008 that a complete system (including 4m² of solar collectors and a supply barrel of 200-240 liters) to cost 4500 euro. The system would then pay back itself in 11 years , when the returns are weighed off against a regular electric boiler. Calculation was as follows: a saving of 1875 kWh (which is 50% of the energy requirements in domestic hot water production) x 0.10 euro/kWh = 187, 5 euros. This multiplied by 11.6 years made 2175 euros (or the cost of the system with deducted regional tax benefits).

In Australia, the cost for an average solar hot water system fully installed is between $1,800 and $2,800. This is after tax rebates (there is a federal rebate, some state rebates and Renewable Energy Certificates). According to the Department of Environment and Water Resources, the yearly electricity savings are between $300 and $700. This brings the payback period to under 2 years in the best case and under 10 years in the worst case. Easy Being Green has a program available where consumers can acquire a system for free (with government rebates) excluding the cost of installation.

A monobloc solar heater in Cirque de Mafate, La Réunion

Types

Solar hot water systems can be classified in different ways:

• The type of collector used (see below)
• The location of the collector - roof mount, ground mount, wall mount
• The location of the storage tank in relation to the collector
• The requirement for a pump - active vs. passive
• The method of heat transfer - open-loop or closed-loop (via heat exchanger)
• Photovoltaic thermal hybrid solar collectors can be designed to produce both hot water and electricity.

Compact systems (passive systems)

A compact system also known as a monobloc has a tank for the heated water and a solar collector mounted on the same chasis. Typically these systems will function by natural convection (thermosiphon) or heatpipes to transfer the heat energy from the collector to the tank.

A integrated collector storage system

A special type of compact system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than intense plate and tube collectors but only suitable in moderate climates with good sunshine. A step up from the ICS is the Convection Heat Storage Unit. These are plate type intense collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from heater to tank. Neither pumps or electricity are used. It is more efficient than an ICS as the intense collector heats a small(er) amount of water that is constantly rising to the tank. It can be used in areas with less sunshine than the ICS.

A passive open loop system

Direct ('open loop') compact systems, if made of metals are not suitable for cold climates. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreeze is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). However, direct ('open loop') systems are slightly cheaper and more efficient.

A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosiphon principle).

Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.

Pumped systems (active systems)

How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces the carbon savings of a system by 10% to 20%.

Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. Most systems in northern Europe are of this type. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.

The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.

Newer zero carbon solar water heating systems are powered by solar electric (photovoltaic or PV) pumps. These typically use a 5-20W PV panel which faces in the same direction as the main solar heating panel and a small, low power diaphragm pump or centrifugal pump to circulate the water.

The most commonly used solar collector is the insulated glazed flat panel. Less expensive panels, like polypropylene panels (for swimming pools) or higher-performing ones like evacuated tube collectors, are sometimes used.

Types of thermal collector

It has been suggested that this article or section be merged into Solar thermal collector. (Discuss)

There are three main kinds of solar thermal collectors in common use. In order of increasing cost they are: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors. The efficiency of the system is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heating energy that can be usefully obtained from insulation). Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or as the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass e.g. flat collectors or providing a vacuum seal e.g. evacuated tube collector. The choice of collector is determined by the heating requirements and environmental conditions in which it is employed.

Formed plastic collector

Formed plastic collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries, heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year-round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.

Flat plate collector

A flat plate collector

A flat plate collector consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover.

Fluid is circulated, using either mains or solar electricity, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank, sometimes directly or otherwise to a heat exchanger or to some other device for using the heated fluid. Some fabricants have a completely flooded absorber consisting of 2 sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.

As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one using heat exchangers. By dispensing with a heat exchanger in these flat plate panel, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be somewhat more efficient, particularly at low light levels.

As with evacuated tubes, most flat plate collectors have a life expectancy of over 25 years.

Evacuated tube collector

Evacuated (or vacuum) tubes panel.

Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). In some cases, the tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. Some systems use foam insulation in the manifold. Soda-lime glass is used in the higher quality evacuated tubes manufacture.

Lower quality evacuated tube systems use the glass coated absorber. Due to the extreme temperature difference of the glass under stagnation temperatures, the glass sometimes shatters. The glass is a lower quality boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube. Moisture entering the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water.

Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square metre than flat panels. However, they are more expensive and fragile than flat panels. Evacuated heat tubes perform better than flat plate collectors in cold climates because they only rely on the light they receive and not the outside temperature. The high stagnation temperatures can cause antifreeze to break down, so careful consideration must be used if selecting this type of system in temperate climates.Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter.

For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. The absorber area only occupied about 50% of the collector panel on early designs, however this has changed as the technology has advanced to maximize the absorption area. In extremely hot climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays of 20 to 30 or more, the efficient but costly evacuated tube collectors have net benefit in winter and also give real advantage in the summer months. They are well suited to extremely cold ambient temperatures and work well in situations of consistently low-light. They are also used in industrial applications, where high water temperatures or steam need to be generated. Properly designed evacuated tubes have a life expectancy of over 25 years which greatly adds to their value.

Solar thermal cooling

Solar thermal cooling can be achieved via absorption refrigeration cycles, desiccant cycles and solar-mechanical processes.

The absorption cycle solar cooling system works like a refrigerator in that it uses hot water to compress a gas that, once expanded, will absorb energy, which cools the air. The main problem currently is that the absorber machine works with liquid at 90 °C, a fairly high temperature to be reached with pumped solar panels with no auxiliary power supply.

The same pumped solar thermal installation can be used for producing hot water for the whole year. It can also be used for cooling in the summer and partially heating the building in winter.

DIY systems

With an ever-rising do-it-yourself-community and their increasing environmental awareness, people have begun building their own (small-scale) solar hot water systems from scratch or buying easy to install kits. Plans for solar hot water systems are available on the Internet. and people have set about building them for their own domestic requirements. DIY solar hot water systems are usually much cheaper than commercial ones, and installation costs can sometimes be avoided as well. The DIY-solar hot water systems are being used both in the developed world, as in the developing world, to generate hot water. Rather than build systems from scratch, many DIY solar enthusiasts are buying simple off-the-shelf solar DIY kits. In particular the new freeze tolerant, zero-carbon PV pumped systems, are becoming common in parts of Europe, since their simplicity enables them to be plumbed in quickly and safely without the need of a mains electrician.