Solar water heating

Solar collectors (water heating panels) for heating a swimming pool in the Netherlands

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Solar water heating or solar hot water is water heated by the use of solar energy. Solar heating systems are generally composed of solar thermal collectors, a water storage tank or another point of usage, interconnecting pipes and a fluid system to move the heat from the collector to the tank. This thermodynamic approach is distinct from semiconductor photovoltaic (PV) cells that generate electricity from light; solar water heating deals with the direct heating of liquids by the sun where no electricity is directly generated.

A solar water heating system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The water can be heated 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 common examples of solar water heating.

A solar water heating system can form part of a solar thermal cooling system, promoting efficient temperature control of buildings or parts thereof. During cool conditions, the same system can provide hot water. Solar heating of buildings in temperate climates has a season-problem: In winter, when most heating is needed, least is available from the sun. This can often be solved by storing solar heat in the ground or in groundwater (Seasonal thermal store).

Overview

Hot water heated by the sun is used in many ways. While perhaps best known in a residential setting to provide hot domestic water, solar hot water also has industrial applications, e.g. to 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.

A solar hot water heater installed on a house in Belgium

In order to heat water using solar energy, a collector, often fastened to a roof or a wall facing the sun, heats working fluid that 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 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 in order to allow for bad weather, and because the optimum final temperature for the solar collector is lower than a typical immersion or combustion heater. The heat transfer fluid (HTF) for the absorber may be the hot water from the tank, but more commonly (at least in active 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 are empty before freezing could 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 such as 55°C. Hence, hot water is always available. The combination of solar 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 water heating system being met with fossil fuels or electricity.

When a solar water heating 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 replace the lower heating element and the upper element will remain in place to provide for any heating that solar cannot provide. 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.

History

There are records of solar collectors in the United States dating back to before 1900, comprising a black-painted tank mounted on a roof. In 1896 Clarence Kemp of Baltimore, USA enclosed a tank in a wooden box, thus creating the first 'batch water heater' as they are known today. Although flat-plate collectors for solar water heating were used in Florida and Southern California in the 1920s there was a surge of interest in solar heating in North America after 1960, but specially after the 1973 oil crisis.

Work in Israel

Solar water heater on a rooftop in Jerusalem

Flat plate solar systems were perfected and used on a very large scale in Israel. In the 1950s there was a fuel shortage in the new Israeli state, and the government forbade heating water between 10 p.m. and 6 a.m.. 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. Despite the abundance of sunlight in Israel, solar water heaters were used by only 20% of the population by 1967. Following the energy crisis in the 1970s, 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 with 85% of the households today using solar thermal systems (3% of the primary national energy consumption), estimated to save the country two million barrels of oil a year, the highest per capita use of solar energy in the world..

Other countries

New solar hot water installations during 2007, worldwide.

The world saw a rapid growth of the use of solar warm water after 1960, with systems being marketed also in Japan and Australia Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar water heating has become the norm in countries with an abundance of solar radiation, like the Mediterranean, and Japan and Austria, where there 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 of Gaviotas studied the best systems from Israel, and made adaptations as to meet the specifications set by the Banco Central Hipotecario (BCH) which prescribed that the system must be operational in cities like Bogotá where there are more than 200 days overcast. The ultimate designs were so successful that Las Gaviotas offered in 1984 a 25 year warranty on any of its installations. Over 40,000 were installed, and still function a quarter of a century later.

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 water heating systems in 2006.

Australia has a variety of incentives (national and state) and regulations (state) for solar thermal introduced starting with MRET in 1997 .

Solar water heating 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 . Israel and Cyprus are the per capita leaders in the use of solar water heating systems with over 30%-40% of homes using them.

See Appendix 1 at the bottom of this article for a number of country-specific statistics on the "Use of solar water heating worldwide". Wikipedia also has country-specific articles about solar energy use (thermal as well as photovoltaic) in Australia, Canada, China, Germany, India, Israel, Japan, Portugal, Romania, Spain, the United Kingdom and the United States.

Types of Solar Water Heating (SWH) systems

A monobloc (thermosiphon) solar heater in Cirque de Mafate, La Réunion

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

• The changes in ambient temperature during the day-night cycle.
• Changes in ambient temperature and solar radiation between summer and winter.
• The temperature of the water required from the system.

The minimum efficiency of the system is determined by the amount or temperature of hot water required during winter (when the largest amount of hot water is often required). The maximum efficiency of the system is determined by the need to prevent the water in the system from becoming too hot (to boil, in an extreme case). There are two main categories of solar water heating systems. Passive systems rely on convection or heat pipes to circulate water or heating fluid in the system, while active systems use a pump. In addition, there are a number of other system characteristics that distinguish different designs:

• 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 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.

Passive systems

An integrated collector storage (ICS) system

A special type of passive 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 plate and tube collectors but are only suitable in moderate climates with good sunshine.

A step up from the ICS is the Convection Heat Storage unit (CHS or thermosiphon). These are often plate type or evacuated tube collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from collector to tank. Neither pumps nor electricity are used to enforce circulation. It is more efficient than an ICS as the collector heats a small(er) amount of water that constantly rises back to the tank. It can be used in areas with less sunshine than the ICS. An CHS also known as a compact system or monobloc has a tank for the heated water and a solar collector mounted on the same chassis. Typically these systems will function by natural convection or heat pipes to transfer the heat energy from the collector to the tank.

File:DirectSolarSystems.jpg

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

Direct ('open loop') passive systems use water from the main household water supply to circulate between the collector and the storage tank. When the water in the collector becomes warm, convection causes it to rise and flow towards the water storage tank. They are often not suitable for cold climates since, at night, the water in the collector can freeze and damage the panels.

Indirect ('closed loop') passive systems use a non-toxic antifreeze heat transfer fluid (HTF) in the collector. When this fluid is heated, convection causes it to flow to the tank where a passive heat exchanger transfers the heat of the HCF to the water in the tank.

The attraction of passive solar water heating systems lies in their simplicity. There are no mechanical or electrical parts that can break or that require regular supervision or maintenance. Consequently the maintenance of a passive system is simple and cheap. The efficiency of a passive system is often somewhat lower than that of an active system and overheating is largely avoided by the inherent design of a passive system.

Active systems

File:IndirectSystemSchematics2.jpg

Indirect active systems: (C) Indirect system with heat exchanger in tank; (D) Drainback system with drainback reservoir. In these schematics the controller and pump are driven by mains electricity

Active solar hot water systems employ a pump to circulate water or HTF between the collector and the storage tank. Like their passive counterparts, active solar water heating systems come as two types: direct active systems pump water directly to the collector and back to the storage tank, while indirect active systems pump transfer fluid (HTF), the heat of which is transferred to the water in the storage tank. Because the pump should only operate when the fluid in the collector is hotter than the water in the storage tank, a controller is required to turn the pump on and off. The use of an electronically controlled pump has several advantages:

• The storage tank can be situated lower than the collectors. In passive systems the storage tank must be located above the collector so that the thermosiphon effect can transport water or HCF from collector to tank. The use of a pump allows the storage tank to be located lower than the collector since the circulation of water or HCF is enforced by the pump. A pumped system allows the storage tank to be located out of sight.

• Because of the fact that active systems allow freedom in the location of the storage tank, the tank can be located where heat loss from the tank is reduced, e.g. inside the roof of a house. This increases the efficiency of the solar water heating system.

• New active solar water heating systems can make use of an existing warm water storage tanks ("geysers"), thus avoiding duplication of equipment.

• Reducing the risk of overheating. If no water from the solar hot water system is used (e.g. when water users are away), the water in the storage tank is likely to overheat. Several pump controllers avoid overheating by activating the pump at night. This pumps hot water or HTF from the storage tank through the collector (that is cold at night), thus cooling the water in the storage tank.

• Reducing the risk of freezing. For direct active systems in cold weather, the pump controller can pump hot water from the water storage tank through the collector in order to prevent the water in the collector from freezing, thus avoiding damage to the system

Active systems can tolerate higher water temperatures than would be the case in an equivalent passive system. Consequently active systems are often more efficient than passive systems but are more complex, more expensive, more difficult to install and rely on electricity to run the pump and controller.

Active systems with intelligent controllers

Modern active solar water systems have electronic controllers that permit a wide range of functionality such as full programmability; interaction with a backup electric or gas-driven water heater; measurement of the energy produced; sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display of error messages or alarms; remote display panels; and remote or local datalogging.

File:SolarController2.gif

A typical programmable differential controller

The most popular pump controller is a differential controller that senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. In a typical configuration, the controller turns the pump on when the water in the collector is about 8-10°C warmer than the water in the tank and it turns the pump off when the temperature difference approached 0 °C. This ensures the water always gains heat from the collector when the pump operates and prevents the pump from cycling on and off too often.

Although the pumps of most active systems are driven by mains electricity, many active solar systems obtain energy to power the pump by a photovoltaic (PV) panel. The PV panel converts sunlight into electricity, which in turn drives the direct current (DC) pump. In this way, water flows through the collector only when the sun is shining. The DC-pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes. Several DC-pumps are intelligent and employ maximum power point (MPP) tracking to optimise pump rate, for instance during periods of small amounts of electricity from the PV panel during cloudy weather. The controller is sometimes used to prevent the pump from running when there is sunlight to power the pump but the collector is still cooler than the water in storage. The main advantage of a PV-driven pump is that hot water is always available during a power outage. The pump is operated by the sun and is completely independent from mains electricity. Some differential controllers use power from the PV panel when sunlight is available, but use mains electricity when light is not available.

The low /variable flow from PV powered pumps for domestic hot water only (no heating) is best matched with a temperature maximising solar absorber of the serpentine type. This in conjunction with a stratified hot water tank design maximises a small quantity of hot water that reduces the need for the standby heating system to operate. This stategy has been found to maximise efficiency.

Active systems with drainback

A drain-back system is an indirect active system where heat transfer fluid circulates through the collector, being driven by a pump. However the collector piping is not pressurised and includes an open drainback reservoir. If the pump is switched off, all the heat transfer fluid drains into the drainback reservoir and none remains in the collector. Consequently the collector cannot be damaged by freezing or overheating. This makes this type of system well-suited to colder climates.

Active systems with a bubble pump

The bubble separator of a bubble-pump system

An active solar water heating system can be equipped with a bubble pump instead of an electric pump. A bubble pump circulates the heat transfer fluid (HTF) between collector and storage tank using solar power and without any external energy source and is suitable for flat panel as well as vacuum tube systems. In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil as it is heated. The steam bubbles lead to a reduced density, causing an upward flow. The system is designed such that the bubbles are separated from the hot fluid and condensed at the highest point in the circuit, after which the fluid flows downward towards the heat exchanger. The HTF typically arrives at the heat exchanger at 70 °C and returns to the circulating pump at 50 °C. In frost prone climates the HTF is water with propylene glycol anti-freeze added, usually in the ratio of 60 to 40. Pumping typically starts at about 50°C and increases as the sun rises until equilibrium is reached depending on the efficiency of the heat exchanger, the temperature of the water being heated and the strength of the sun.

Freeze protection

Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Some systems drain the transfer fluid from the system when the pump stops. In indirect systems (where the transfer fluid is separated from the heated water), this is called drainback and in direct systems (where the heated water is used as the transfer fluid) it is called draindown. Many indirect systems use anti-freeze (e.g. glycol) in the heat transfer fluid. This approach is simpler and more reliable than drainback and is common in climates where freezing temperatures occur often.

In both direct and indirect systems, automatic recirculation may be used for freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.

In some direct systems, the collectors are manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often.

Overheat protection

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. Almost all systems have pressure relief valves through which excessive water pressure or steam can be vented. Active systems often cool the water in the storage tank by circulating hot water through the collector at night (when solar energy does not heat the collector).

A rough comparison of solar hot water systems

Comparison of SWH systems
Characteristic	ICS (Batch)	Thermosyphon	Active direct	Active indirect	Drainback
Low profile-unobtrusive			X	X	X
Lightweight			X	X	X
Freeze tolerant				X	X
Low maintenance	X	X	X
Simple: no ancillary control	X	X
Space saving	X	X

Collectors used in modern domestic solar water heating systems

Main article: Solar thermal collector

Solar thermal collectors capture and retain heat from the sun and transfer this heat to a liquid. Two important physical principles govern the technology of solar thermal collectors:

• Any hot object ultimately returns to thermal equilibrium with its environment, due to heat loss from the hot object. The processes that result in this heat loss are conduction, convection and radiation. The efficiency of a solar thermal collector is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heat energy that can be retained for a predefined period of time). Within the context of a solar collector, convection and radiation are the most important sources of heat loss. Thermal insulation is used to slow down heat loss from a hot object to its environment. This is actually a direct manifestation of the Second law of thermodynamics but we may term this the 'equilibrium effect'.
• Heat is lost more rapidly if the temperature difference between a hot object and its environment is larger. Heat loss is predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Conduction, convection as well as radiation occur more rapidly over large thermal gradients. We may term this the 'delta-t effect'.

The most simple approach to solar heating of water is to simply mount a metal tank filled with water in a sunny place. The heat from the sun would then heat the metal tank and the water inside. Indeed, this was how the very first SWH systems worked more than a century ago. However, this setup would be inefficient due to an oversight of the equilibrium effect, above: once when the tank and water has started to be heated, the heat gained would be lost back into the environment, ultimately until the water in the tank would assume the ambient temperature. The challenge is therefore to limit the heat loss from the tank, thus delaying the time until thermal equilibrium is reached.

ICS or batch collectors overcome the above problem by putting the water tank in a box that limits the loss of heat from the tank back into the environment. This is achieved by encasing the water tank in a glass-topped box that allows heat from the sun to reach the water tank. However, the other walls of the box are thermally insulated, reducing convection as well as radiation to the environment. In addition, the box can also have a reflective surface on the inside. This reflects heat lost from the tank back towards 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 heat of the water in the tank. Using a box does not eliminate heat loss from the tank to the environment, but it largely reduces this loss. There are many variations on this basic design, with some ICS collectors comprising several smaller water containers and even including evacuated glass tube technology. This because ICS collectors have a characteristic that strongly limits the efficiency of the collector: a small surface-to-volume ratio. Since the amount of heat that a tank can absorb from the sun is largely dependent on the surface of the tank directly exposed to the sun, it follows that a small surface would limit the degree to which the water can be heated by the sun. Cylindrical objects such as the tank in an ICS collector inherently have a small surface-to-volume ratio and most modern collectors attempt to increase this ratio for efficient warming of the water in the tank.

File:Collectors flatplate evactube.jpg

Flat plate and evacuated tube collectors side-by-side.

Flat plate collectors is an extension of the basic idea to place a collector in an 'oven'-like box. Here, a pipe is connected to the water tank and the water is circulated through this pipe and back into the tank. The water tank is now outside the collector that only contains the pipes. Since the surface-to-volume ratio increases sharply as the diameter of a pipe decreases, most flat-plate collectors have pipes less than 1 cm in diameter. The efficiency of the heating process is therefore sharply increased. The design of a flat-plate collector therefore typically takes the shape of a flat box with a robust glass top oriented towards the sun, enclosing a network of piping. In many flat-plate collectors the metal surface of the pipe is increased with flat metal flanges or even a large, flat metal plate to which the pipes are connected. Since the water in a flat-plate collector usually reaches temperatures much higher than that of an ICS, the problem of radiation of heat back to the environment is very important, even though a box-like 'oven' is used. This is because the 'delta-t effect' is becoming important. Formed collectors are a degenerate modification of a flat-plate collector in that the piping of the collector is not enclosed in a box-like 'oven'. Consequently these types of collectors are much less efficient for domestic water heating. However, since water colder than the ambient temperature is heated, these collectors are efficient for that specific purpose.

Evacuated tube collectors is a way in which heat loss to the environment, inherent in flat plates, has been reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes. Since two flat sheets of glass are normally not strong enough to withstand a vacuum, the vacuum is rather created between two concentric tubes. Typically, the water piping in an evacuated tube collector is therefore surrounded by two concentric tubes of borosilicate glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent.

Flat plate collectors are generally more efficient than evacuated tube collectors in full sunshine conditions. However, the energy output of flat plate collectors drop off rapidly in cloudy or cool conditions compared to the output of evacuated tube collectors that decrease less rapidly. In-depth discussion of different solar collector types and their respective applications and performance, also those used in industrial applications, can be found in the Wikipedia article on Solar thermal collectors.

Heating of swimming pools

Solar thermal collectors for nonpotable pool water use are often made of plastic. Pool water, mildly corrosive due to chlorine, is circulated through the panels using the existing pool filter 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 do not have pool water pumped through them, they are used in conjunction with a heat exchanger that transfers the heat to pool water. This causes less corrosion. A fairly simple differential temperature controller is used to direct the water to the panels or heat exchanger either by turning a valve or operating the pump.. Once the pool water has reached the required temperature, a diverter valve is used to return pool water directly to the pool without heating. Many systems are configured as drainback systems where the water drains into the pool when the water pump is switched off.

The collector panels are usually mounted on a nearby 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 for the required panel area needed is 50% of the pool's surface area. This is for areas where pools are used in the summer season only, not year 'round. Adding solar collectors to a conventional outdoor pool, in a cold climate, can typically extend the pool's comfortable usage by some months or more if an insulating pool cover is also used. An active solar energy system analysis program may be used to optimize the solar pool heating system before it is built.

Economics, energy, environment, and system costs

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

Energy production

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 (the insolation). In tropical places the insolation can be relatively high, e.g. 7 kW.h per day, whereas the insolation can be much lower in temperate areas where the days are shorter in winter, e.g. 3.2 kW.h per day. Even at the same latitude the average insolation can vary a great deal from location to location due to differences in local weather patterns and the amount of overcast. Useful calculators for estimating insolation at a site can be found with the Joint Research Laboratory of the European Commission and the American National Renewable Energy Laboratory.

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 m² of absorber area of the collector, demonstrating two evacuated tube and three flat plate solar water heating systems. Certification information or figures calculated from those data are used. The bottom two rows give estimates for daily energy production (kW.h/day) for a tropical and a temperate scenario. These estimates are for heating water to 50 degrees C above ambient temperature.

With most solar water heating systems, the energy output scales linearly with the surface area of the absorbers. Therefore, when comparing figures, take into account the absorber area of the collector because collectors with less absorber area yield less heat, even within the 2 m² range. Specifications for many complete solar water heating systems and separate solar collectors can be found at Internet site of the SRCC.

Daily energy production (kWth.h) of five solar thermal systems. The evac tube systems used below both have 20 tubes
Technology	Flat plate	Flat plate	Flat plate	Evac tube	Evac tube
Configuration	Direct active	Thermosiphon	Indirect active	Indirect active	Direct active
Overall size (m2)	2.49	1.98	1.87	2.85	2.97
Absorber size (m2)	2.21	1.98	1.72	2.85	2.96
Maximum efficiency	0.68	0.74	0.61	0.57	0.46
Energy production (kW.h/day): - Insolation 3.2 kW.h/m2/day (temperate) - e.g. Zurich, Switzerland	5.3	3.9	3.3	4.8	4.0
- Insolation 6.5 kW.h/m2/day (tropical) - e.g. Phoenix, USA	11.2	8.8	7.1	9.9	8.4

The figures are fairly similar between the above collectors, yielding some 4 kW.h/day in a temperate climate and some 8 kW.h/day in a more tropical climate when using a collector with an absorber area of about 2m² in size. In the temperate scenario this is sufficient to heat 200 litres of water by some 17 degrees C. In the tropical scenario the equivalent heating would be by some 33 degrees C. Many thermosiphon systems are quite efficient and have comparable energy output to equivalent active systems. The efficiency of evacuated tube collectors is somewhat lower than for flat plate collectors because the absorbers are narrower than the tubes and the tubes have space between them, resulting in a significantly larger percentage of inactive overall collector area. Some methods of comparison calculate the efficiency of evacuated tube collectors based on the actual absorber area and not on the 'roof area' of the system as has been done in the above table. The efficiency of the collectors becomes lower if one demands water with a very high temperature.

System cost

In sunny, warm locations, where freeze protection is not necessary, an ICS (batch type) solar 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 water heating system, to a level much higher than a comparable hot water heater of the conventional type. The biggest single consideration is therefore the large initial financial outlay of solar water heating systems. Offsetting this expense can take several years and the payback period is longer in temperate environments where the insolation is less intense. 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 (also in conjunction with an existing hot water heater). At higher latitudes, solar heaters may be less effective due to lower solar energy, possibly requiring larger and/or dual-heating systems. In addition, federal and local incentives can be significant.

The calculation of long term cost and payback period for a household SWH system depends on a number of factors. Some of these are:

• Price of purchasing solar water heater (more complex systems are more expensive)
• Efficiency of SWH system purchased
• Installation cost
• State or government subsidy for installation of a solar water heater
• Price of electricity per kW.h
• Number of kW.h of electricity used per month by a household
• Annual tax rebates or subsidy for using renewable energy
• Annual maintenance cost of SWH system
• Savings in annual maintenenance of conventional (electric/gas/oil) water heating system

The following table gives some idea of the cost and payback period to recover the costs. It does not take into account annual maintenance costs, annual tax rebates and installation costs. However the table does give an indication of the total cost and the order of magnitude of the payback period. The table assumes an energy savings of 140 kW.h per month (about 4.6 kW.h/day) due to SWH.

Costs and payback periods assuming a household electricity savings of 140 kW.h/month due to SWH (using 2010 data)
Country	Currency	System cost	Subsidy(%)	Effective cost	Electricity cost/kW.h	Electricity savings/month	Payback period(y)
Template:Country data Australia	$Aus	5000	40	3000	0.18	25	9.9
Template:Country data Belgium	Euro	4000	50	2000	0.1	14	11.9
Template:Country data Brazil	Real	2500	0	2500	0.25	35	6.0
Template:RSA	ZA Rand	14000	15	11900	0.9	126	7.9
United Kingdom	UK Pound	4000	10	3600	0.11	15.4	19.4
United States	US$	5000	30	3500	0.10	14	20.8

Two points are clear from the above table. Firstly, the payback period is shorter in countries with a large amount of insolation and even in parts of the same country with more insolation. This is evident from the payback period less than 10 years in most southern hemisphere countries, listed above. This is partly because of good sunshine, allowing users in those countries to need smaller systems than in temperate areas. Secondly, even in the northern hemisphere countries where payback periods are often longer than 10 years, solar water heating is financially extremely efficient. This is partly because the SWH technology is efficient in capturing irradiation. The payback period for photovoltaic systems is much longer. In many cases the payback period for a SWH system is shortened if it supplies all or nearly all of the warm water requirements used by a household. Many SWH systems supply only a fraction of warm water needs and are augmented by gas or electric heating on a daily basis, thus extending the payback period of such a system.

Solar leasing is now available in Spain for solar water heating systems from Pretasol with a typical system costing around 59 euros and rising to 99 euros per month for a system that would provide sufficient hot water for a typical family home of six persons. The payback period would be five years.

Australia has instituted a system of Renewable Energy Credits, based on national renewable energy targets. This expands an older system based only on rebates.

Operational Carbon Footprint and Life Cycle Assessment

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 mains electricity to pump the fluid through the panels are called 'low carbon solar'. In older systems the pumping cancels the carbon savings of the solar by about 20%. However, modern pumps use around 20 W. Assuming a solar collector panel delivering 4 kW.h/day and a pump running intermittently from mains electricity for a total of 6 hours during a 12-hour sunny day, the negative effect of the pump is about 3% of the total power produced. The carbon footprint of all modern household systems could therefore be considered as very low. However, zero-carbon active solar thermal systems typically use a 10-30 W 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. This represents a zero operational carbon footprint and is becoming an important design goal for innovative solar thermal systems.

A more robust and quantitative apparoach is a life cycle assessment (LCA) that takes into account the total cost of acquisition of raw materials, manufacturing, transport, using, servicing and disposing of the equipment. There are several aspects to such an assessment, including:

• The financial costs and gains incurred during the life of the equipment.
• The energy used during each of the above stages.
• The CO₂ emissions due to each of the above stages.

Each of these aspects may present different trends with respect to a specific SWH device.

Financial assessment. The table in the previous section as well as several other studies suggest that the cost of production is gained during the first 5–12 years of use of the equipment, depending on the insolation, with cost efficiency increasing as the insolation does.

In terms of energy, some 60% of the materials of a SWH system goes into the tank, with some 30% towards the collector (thermosiphon flat plate in this case) (Tsiligiridis et al.). In Italy, some 11 GJ of electricity are used in producing the equipment, with about 35% of the energy going towards the manufacturing the tank, with another 35% towards the collector and the main energy-related impact being emissions. The energy used in manufacturing is recovered within the first two to three years of use of the SWH system through heat captured by the equipment.

In terms of CO₂ emissions, a large degree of the emissions-saving traits of a SWH system is dependent on the degree to which water heating by gas or electricity is used to supplement solar heating of water. Using the Eco-indicator 99 points system as a yardstick (i.e. the yearly environmental load of an average European inhabitant) in Greece, a purely gas-driven system may be cheaper in terms of emissions than a solar system. This calculation assumes that the solar system produces about half of the hot water requirements of a household. The production of a test SWH system in Italy produced about 700 kg of CO₂, with all the components of manufacture, use and disposal contributing small parts towards this. Maintenance was identified as an emissions-costly activity when the heat transfer fluid (Glycol-based) was periodically replaced. However, the emissions cost was recovered within about two years of use of the equipment through the emissions saved by solar water heating. In Australia, the life cycle emissions of a SWH system are also recovered fairly rapidly, where a SWH system has about 20% of the impact of an electrical water heater and half of the emissions impact of a gas water heater.

In summary, the energy and emissions cost of a SWH system forms a relatively small part of the life cycle cost and are recovered fairly rapidly during use of the equipment.

DIY systems

With an ever-rising do-it-yourself-community and their increasing environmental awareness, people have begun building their own (small-scale) solar water heating systems from scratch or buying easy to install kits. Plans for solar water heating systems are available on the Internet. and people have set about building them for their own domestic requirements. DIY solar water heating systems are usually much cheaper than commercial ones, and installation costs can sometimes be avoided as well. The DIY solar water heating 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 active 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, although they may lack sufficient control in periods of low solar irradiation.

Considerations during the installation of a solar water heating (SWH) system

• Except in rare instances it will be inefficient to install a SWH system with no electrical or gas or other fuel backup. Many SWH systems (e.g. thermosiphon systems) have an integrated electrical heater in the integrated tank. Conversely, many active solar systems incorporate a conventional "geyser". But even in a tropical environment there are rainy and cloudy days when the insolation is low and the temperature of the water in the tank increases very little on account of solar heating. Electrical or other backup ensures a reliable supply of hot water.
• The installation of a SWH system needs to be complemented with efficient insulation of all the water pipes connecting the collector and the water storage tank, as well as the storage tank (or "geyser") and the most important warm water outlets. The installation of efficient lagging significantly reduces the heat loss from the hot water system. The installation of lagging on at least two meters of pipe on the cold water inlet of the storage tank reduces heat loss, as does the installation of a "geyser blanket" around the storage tank (if inside a roof). In cold climates the installation of lagging and insulation is often performed even in the absence of a SWH system.
• If a photovoltaic (PV) panel is used to drive the pump in an active system, the installation of a controller is crucial. This prevents the pump from switching on early in the morning when there is light to drive the pump but while the collector is still cold. This causes the hot water in the storage tank to be cooled. Some modern pumps can operate even in fairly low light levels, causing unwanted circulation through the collector.
• Usually a large SWH system is more efficient economically than a small system . This is because the price of a system is not linearly proportional to the size of the collector, so a square meter of collector is cheaper in a larger system. If this is the case, it pays to use a system that covers all or nearly all of the domestic hot water needs, and not only a small fraction of the needs. This facilitates more rapid cost recovery.
• Due to the modularity of an evacuated tube collector panel, this technology allows the adjustment of the collector size by removing some tubes or their heat pipes. Budgeting for a larger than required array of tubes therefore allows for the customisation of collector size to the needs of a particular application, especially in warmer climates.

Standards

Europe

• EN 806: Specifications for installations inside buildings conveying water for human consumption. General.
• EN 1717: Protection against pollution of potable water in water installations and general requerements 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.

APPENDIX 1. Use of solar water heating worldwide

Top countries worldwide

Top countries using solar thermal power, worldwide: GWth
#	Country	2005	2006	2007	2008
1	Template:CHN	55.5	67.9	84.0	100.0
2	Template:EU	11.2	13.5	15.5	17.4
3	Template:TUR	5.7	6.6	7.1	7.8
4	Template:JPN	5.0	4.7	4.9	5.0
5	Template:ISR	3.3	3.8	3.5	3.55
6	Template:Country data Brazil	1.6	2.2	2.5	2.8
7	United States	1.6	1.8	1.7	1.8
8	Template:Country data Australia	1.2	1.3	1.2	1.3
9	Template:IND	1.1	1.2	1.5	1.7
10	Template:MEX	?	?	?	0.7
	World (GWth)	88	105	126	145

Solar heating in European Union + CH

Solar thermal heating in European Union + CH (MWth)
		New installations			All installations
#	Country	2006	2007	2008	Total 2008
1	Template:DEU	1,050	665	1,470	7,766
2	Template:GRE	168	198	209	2,708
3	Template:Country data Austria	205	197	243	2,268
4	Template:ITA	130	172	295	1,124
5	Template:FRA	154	179	272	1,137
6	Template:ESP	123	183	304	988
7	Template:CYP	42	46	48	485
8	Template:SUI	36	46	60	416
9	Template:DEN	18	16	23	293
10	Template:NED	10	14	18	254
11	United Kingdom	38	38	57	270
12	Template:SWE	20	18	19	202
13	Template:POR	14	18	60	223
14	Template:POL	29	47	91	254
15	Template:Country data Belgium	25	30	64	188
16	Template:CZE	15	18	25	116
17	Template:SLO	5	8	11	96
18	Template:SVK	6	6	9	67
19	Template:ROM	0	0	6	66
20	Template:IRL	4	11	31	52
21	Template:MLT	3	4	4	25
22	Template:BUL	2	2	3	22
23	Template:FIN	2	3	3	18
24	Template:LUX	2	2	3	16
25	Template:HUN	1	6	8	18
26	Template:LAT	1	1	1	5
27	Template:LTU	0	0	1	3
28	Template:EST	0	0	0	1
EU27 + CH (MWth)		2,100	1,920	3,330	19,083
2004-2006

See also

• Solar thermal collector
• Solar air heating
• Solar air conditioning
• Concentrating solar power
• Passive solar
• Seasonal thermal store
• Solar combisystem
• Solar energy
• Solar pool panels
• Solar thermal energy
• Sustainable design

External links

• Using Excel to predict solar water heating, and heating of other objects
• Parts of a solar heating system
• Solar water heating at the Open Directory Project
• Online Simulation of Solar Thermal Collector Power Output

Template:Solar energy

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