Solar Water Heating

Solar water heating
From Wikipedia, the free encyclopedia
Solar water heating (SWH) is the conversion of sunlight into
renewable energy for water heating using a solar thermal collector.
Solar water heating systems comprise various technologies that are
used worldwide increasingly.
In a "closecoupled" SWH system the storage tank is horizontally
mounted immediately above the solar collectors on the roof. No
pumping is required as the hot water naturally rises into the tank
through thermosiphon flow. In a "pumpcirculated" system the
storage tank is ground or floormounted and is below the level of
the collectors; a circulating pump moves water or heat transfer fluid Roofmounted closecoupled
between the tank and the collectors. thermosiphon solar water heater.
SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes
may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is
used to heat the water.
Contents
1 Overview
2 History
2.1 Mediterranean
2.2 AsiaPacific
3 System design requirements
3.1 Freeze protection
3.2 Overheat protection
4 Types of solar water heating systems
4.1 Direct and indirect systems
4.2 Passive and active systems
4.3 Passive direct systems
4.4 Active indirect systems: drainback and antifreeze
4.5 Powering a heatpump hot water heater via Solar PV panels
4.6 A rough comparison of solar hot water systems
5 Collectors used in modern domestic SWH systems
6 Heating of swimming pools
7 Economics, energy, environment, and system costs
7.1 Energy production
7.2 System cost
7.3 Operational carbon/energy footprint and life cycle assessment
7.3.1 Terminology
7.3.2 Carbon/energy footprint
7.3.3 Life cycle carbon/energy assessment
8 Doityourself (DIY) systems
9 System specification and installation
10 Standards
10 Standards
10.1 Europe
10.2 United States
10.3 Australia
11 APPENDIX 1. Worldwide use
11.1 Top countries worldwide
11.2 Solar heating in European Union + Switzerland
12 See also
13 References
14 External links
Overview
Water heated by the sun is used in various ways. While perhaps best known in a residential setting to
provide domestic hot water, solar hot water also has industrial applications, e.g. to generate electricity.[1]
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.
In order to heat water using solar energy, a collector, often fastened
to a roof or a wall facing the sun, heats a working fluid that is either
pumped (active system) or driven by natural convection (passive
system) through it.[2] The collector could be made of a simple glass
topped insulated box with a flat solar absorber made of sheet metal,
attached to copper heat exchanger pipes and darkcolored, or a set of
metal tubes surrounded by an evacuated (near vacuum) glass
A solar water heater installed on a cylinder. In industrial cases a parabolic mirror can concentrate
house in Belgium 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 antifreeze and a corrosion inhibitor which delivers heat to
the tank through a heat exchanger (commonly a coil of copper heat exchanger tubing within the tank).
Copper is an important component in solar thermal heating and cooling systems because of its high heat
conductivity, resistance to atmospheric and water corrosion, sealing and joining by soldering, and
mechanical strength. Copper is used both in receivers and primary circuits (pipes and heat exchangers for
water tanks).[3]
Another lowermaintenance concept is the 'drainback': no antifreeze 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) which 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 backup heat from a wood stove chimney to heat water[4]
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 hotwater 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 How a Solar Hot Water system works
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 nonelectric 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,[5] comprising a blackpainted 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 flatplate 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
especially after the 1973 oil crisis.
See Appendix 1 at the bottom of this article for a number of
countryspecific statistics on the "Use of solar water heating An advertisement for a Solar Water
worldwide". Wikipedia also has countryspecific articles about solar Heater dating to 1902
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.
Mediterranean
Israel, Cyprus and Greece are the per capita leaders in the use of solar water heating systems with over
30%–40% of homes using them.[6]
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 pm and 6 am. Levi
Yissar built the first prototype Israeli solar water heater and in 1953 he launched the NerYah Company,
Israel's first commercial manufacturer of solar water heating.[7] 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).[8] 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),[9] estimated to save the country 2 million barrels (320,000 m3) of oil
a year, the highest per capita use of solar energy in the world.[10]
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.[11]
AsiaPacific
The world saw a rapid growth of the use of solar warm water after
1960, with systems being marketed in Japan and Australia.[5]
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,[12] Japan, and Australia.
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) Passive (thermosiphon) solar water
which prescribed that the system must be operational in cities like heaters on a rooftop in Jerusalem
Bogotá where there are more than 200 days overcast. The ultimate
designs were so successful that Las Gaviotas offered a 25year
warranty on any of its installations in 1984. Over 40,000 were
installed and still function a quarter of a century later.
Australia has a variety of incentives (national and state) and
regulations (state) for solar thermal introduced starting with MRET
in 1997.[13][14][15]
Solar water heating systems have become popular in China, where
basic models start at around 1,500 yuan (US$235), 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
New solar hot water installations
at temperatures well below freezing.[16] during 2007, worldwide.
System design requirements
The type, complexity, and size of a solar water heating system is mostly determined by:
Changes in ambient temperature and solar radiation between summer and winter.
The changes in ambient temperature during the daynight cycle.
The possibility of the potable water or collector fluid overheating.
The possibility of the potable water or collector fluid freezing.
The minimum requirements of the system are typically determined by the amount or temperature of hot
water required during winter, when a system's output and incoming water temperature are typically at their
lowest. The maximum output of the system is determined by the need to prevent the water in the system
from becoming too hot.
Freeze protection
Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid.
Drainback systems drain the transfer fluid from the system when the pump stops. Many indirect systems
use antifreeze (e.g. Propylene glycol) in the heat transfer fluid.
In some direct systems, the collectors can be manually drained when freezing is expected. This approach is
common in climates where freezing temperatures do not occur often, but is somewhat unreliable since the
operator can forget to drain the system. Other direct systems use freezetolerant collectors made with
flexible polymers such as silicone rubber.
A third type of freeze protection is freezetolerance, where low pressure polymer water channels made of
silicone rubber simply expands on freezing. One such collector now has European Solar Keymark
accreditation, following extra durability testing.
Overheat protection
When no hot water has been used for a day or two, the fluid in the collectors and storage can reach very
high temperatures in all systems except for those of the drainback variety. When the storage tank in a
drainback system reaches its desired temperature, the pumps are shut off, putting an end to the heating
process and thus preventing the storage tank from overheating.
One method of providing over heat protection is to dump the heat into a hot tub.
Some active systems deliberately cool the water in the storage tank by circulating hot water through the
collector at times when there is little sunlight or at night, causing increased heat loss. This is most effective
in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors,
due to their superior insulation. No matter the collector type, however, they may still overheat. High
pressured sealed solar thermal systems versions ultimately rely on the operation of temperature and
pressure relief valves. Low pressure, open vented ones have simpler, more reliable safety controls, typically
an open vent.
Types of solar water heating systems
Direct and indirect systems
Direct or open loop systems circulate potable water through the collectors. They are relatively cheap but
can have the following drawbacks:
They offer little or no overheat protection unless they have a heat export pump.
They offer little or no freeze protection, unless the collectors are freezetolerant.
Collectors accumulate scale in hard water areas, unless an ionexchange softener is used.
Until the advent of freezetolerant solar collectors, they were not considered suitable for cold climates
since, in the event of the collector being damaged by a freeze, pressurized water lines will force water to
gush from the
freezedamaged
collector until the
problem is noticed
and rectified.
Indirect or closed
loop systems use a
heat exchanger that
separates the
potable water from
the fluid, known as
the "heattransfer
fluid" (HTF), that
circulates through Direct systems: (A) Passive CHS system with tank above collector. (B) Active system with
the collector. The pump and controller driven by a photovoltaic panel
two most common
HTFs are water and an antifreeze/water mix that typically uses nontoxic propylene glycol. After being
heated in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable
water. Though slightly more expensive, indirect systems offer freeze protection and typically offer overheat
protection as well.
Passive and active systems
Passive systems rely on heatdriven convection or heat pipes to circulate water or heating fluid in the
system. Passive solar water heating systems cost less and have extremely low or no maintenance, but the
efficiency of a passive system is significantly lower than that of an active system. Overheating and freezing
are major concerns.
Active systems use one or more pumps to circulate water and/or heating fluid in the system.
Though slightly more expensive, active systems offer several advantages:
The storage tank can be situated lower than the collectors, allowing increased freedom in system
design and allowing preexisting storage tanks to be used.
The storage tank can be hidden from view.
The storage tank can be placed in conditioned or semiconditioned space, reducing heat loss.
Drainback tanks can be used.
Superior efficiency.
Increased control over the system.
Modern active solar water systems have electronic controllers that offer a wide range of functionality, such
as the modification of settings that control the system, interaction with a backup electric or gasdriven water
heater, calculation and logging of the energy saved by a SWH system, safety functions, remote access, and
informative displays, such as temperature readings.
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
active system, 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 approaches 3–5 °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. (In direct systems this "on differential" can be reduced to around 4 °C
because there is no heat exchanger impediment.)
Some active SWH systems use energy obtained by a small photovoltaic (PV) panel to power one or more
variablespeed DC pump(s). To ensure proper performance and longevity of the pump(s), the DCpump and
PV panel must be suitably matched. Some PV pumped solar thermal systems are of the antifreeze variety
and some use freezetolerant solar collectors. The solar collectors will almost always be hot when the
pump(s) are operating (i.e., when the sun is bright), and some do not use solar controllers. Sometimes,
however, a differential controller (that can also be powered by the DC output of a PV panel) is used to
prevent the operation of the pumps when there is sunlight to power the pump but the collectors are still
cooler than the water in storage. One advantage of a PVdriven system is that solar hot water can still be
collected during a power outage if the sun is shining. Another advantage is that the operational carbon
clawback of using mains pumped solar thermal (which typically negates up to 23% of its carbon savings) is
completely avoided.
An active solar water heating system can be equipped with a bubble
pump (also known as geyser pump) instead of an electric pump. A
bubble pump circulates the heat transfer fluid (HTF) between
collector and storage tank using solar power, 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 at low temperature
as it is heated by the sun. The steam bubbles form a geyser pump,
causing an upward flow. The system is designed such that the
bubbles are separated from the hot fluid and condensed at the
The bubble separator of a bubble
pump system
highest point in the circuit, after which the fluid flows downward
toward the heat exchanger caused by the difference in fluid
levels.[17][18][19] The HTF typically arrives at the heat exchanger at
70 °C and returns to the circulating pump at 50 °C. In frostprone climates the HTF is water with propylene
glycol antifreeze 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, which depends on the efficiency of the heat
exchanger, the temperature of the water being heated, and the total solar energy available.
Passive direct systems
An integrated collector storage (ICS or Batch Heater) system uses a tank that acts as both storage and
solar collector. Batch heaters are basically thin rectilinear tanks with a glass side facing the position of the
sun at noon. They are simple and less costly than plate and tube collectors, but they sometimes require extra
bracing if installed on a roof (since they are heavy when filled with water [400–700 lbs],) suffer from
significant heat loss at night since the side facing the sun is largely uninsulated, and are only suitable in
moderate climates.
A convection heat storage unit (CHS) system is similar to an ICS system, except the storage tank and
collector are physically separated and transfer
between the two is driven by convection. CHS
systems typically use standard flatplate type or
evacuated tube collectors, and the storage tank must
be located above the collectors for convection to work
properly. The main benefit of a CHS systems over an
ICS system is that heat loss is largely avoided since
(1) the storage tank can be better insulated, and (2)
since the panels are located below the storage tank,
heat loss in the panels will not cause convection, as
the cold water will prefer to stay at the lowest part of
the system.
Active indirect systems: drainback and
antifreeze
Pressurized antifreeze or pressurized glycol systems
An integrated collector storage (ICS) system
use a mix of antifreeze (almost always nontoxic
propylene glycol) and water mix for HTF in order to
prevent freeze damage.
Though effective at preventing freeze damage, antifreeze systems have many drawbacks:
If the HTF gets too hot (for example, when the homeowner is on vacation,) the glycol degrades into
acid. After degradation, the glycol not only fails to provide freeze protection, but also begins to eat
away at the solar loop's components: the collectors, the pipes, the pump, etc. Due to the acid and
excessive heat, the longevity of parts within the solar loop is greatly reduced.
Most do not feature drainback tanks, so the system must circulate the HTF – regardless of the
temperature of the storage tank – in order to prevent the HTF from degrading. Excessive
temperatures in the tank cause increased scale and sediment buildup, possible severe burns if a
tempering valve is not installed, and, if a water heater is being used for storage, possible failure of the
water heater's thermostat.
The glycol/water HTF must be replaced every 3–8 years, depending on the temperatures it has
experienced.
Some jurisdictions require doublewalled heat exchangers even though propylene glycol is nontoxic.
Even though the HTF contains glycol to prevent freezing, it will still circulate hot water from the
storage tank into the collectors at low temperatures (e.g. below 40 °F (4 °C)), causing substantial heat
loss.
A drainback system is an indirect active system where the HTF (almost always pure water) circulates
through the collector, being driven by a pump. The collector piping is not pressurized and includes an open
drainback reservoir that is contained in conditioned or semiconditioned space. If the pump is switched off,
the HTF drains into the drainback reservoir and none remains in the collector. Since the system relies upon
being able to drain properly, all piping above the drainback tank, including the collectors, must slope
downward in the direction of the drainback tank. Installed properly, the collector cannot be damaged by
freezing or overheating.[20] Drainback systems require no maintenance other than the replacement of failed
system components.
Powering a heatpump hot water heater via Solar PV panels
With the drastic drop in the prices of Photovoltaics circa 2010 it became increasingly popular in residential
settings with low hotwater demands to consider heating water via an electric heat pump hot water heater
powered by a solar PV array. This has the following advantages: 1) simpler/cheaper installation and
maintenance, 2) excess energy collected can be used for household electricity use or put back into the grid,
and 3) the heatpump dehumidifies the living space. See for example: Getting into Hot Water — Part 1
Marc Rosenbaum (http://www.greenbuildingadvisor.com/blogs/dept/guestblogs/gettinghotwaterpart1)
A rough comparison of solar hot water systems
Comparison of SWH systems[21]
ICS Active Active Bubble
Characteristic Thermosiphon Drainback
(Batch) direct indirect Pump
Low profileunobtrusive
Lightweight collector
Survives freezing weather
Low maintenance
Simple: no ancillary control
Retrofit potential to existing
store
Space saving: no extra
storage tank
Collectors used in modern domestic SWH systems
Solar thermal collectors capture and retain heat from the sun and use it to heat a liquid.[22] 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.[23]
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, and radiation all occur more
rapidly over large thermal gradients.[23] We may term this the 'deltat 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.[5] However, this setup would be
inefficient due to an oversight of the equilibrium effect, above: as soon as heating of the tank and water
begins, the heat gained starts to be lost back into the environment, and this continues until the water in the
tank reaches the ambient temperature. The challenge is therefore to limit the heat loss from the tank, thus
delaying the time when thermal equilibrium is regained.
ICS or batch collectors reduce heat loss by placing the water tank in a thermally insulated box.[1][24] This
is achieved by encasing the water tank in a glasstopped box that allows heat from the sun to reach the
water tank.[25] However, the other walls of the box are thermally insulated, reducing convection as well as
radiation to the environment.[26] 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.
Standard ICS collectors have a characteristic that strongly limits the efficiency of the collector: a small
surfacetovolume ratio.[27] 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 surfacetovolume ratio and most modern collectors attempt to increase
this ratio for efficient warming of the water in the tank. There are many variations on this basic design, with
some ICS collectors comprising several smaller water containers and even including evacuated glass tube
technology, a type of ICS system known as an Evacuated Tube Batch (ETB) collector.[1]
Flat plate collectors are an extension of the basic idea to place a
collector in an 'oven'like box with glass in the direction of the
Sun.[1] Most flat plate collectors have two horizontal pipes at the top
and bottom, called headers, and many smaller vertical pipes
connecting them, called risers. The risers are welded (or similarly
connected) to thin absorber fins. Heattransfer fluid (water or
water/antifreeze mix) is pumped from the hot water storage tank
Flatplate solar thermal collector,
(direct system) or heat exchanger (indirect system) into the
viewed from rooflevel
collectors' bottom header, and it travels up the risers, collecting heat
from the absorber fins, and then exits the collector out of the top
header. Serpentine flat plate collectors differ slightly from this "harp" design, and instead use a single pipe
that travels up and down the collector. However, since they cannot be properly drained of water, serpentine
flat plate collectors cannot be used in drainback systems.
The type of glass used in flat plate collectors is almost always lowiron, tempered glass. Being tempered,
the glass can withstand significant hail without breaking, which is one of the reasons that flatplate
collectors are considered the most durable collector type.
Unglazed or formed collectors are similar to flatplate collectors, except they are not thermally insulated
nor physically protected by a glass panel. Consequently these types of collectors are much less efficient for
domestic water heating. For pool heating applications, however, the water being heated is often colder than
the ambient roof temperature, at which point the lack of thermal insulation allows additional heat to be
drawn from the surrounding environment.[28]
Evacuated tube collectors (ETC) are a way in which heat loss to the environment,[1] 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.[29] 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 ETC is therefore surrounded by two concentric tubes of 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.[30] Life of the vacuum varies from collector to collector,
anywhere from 5 years to 15 years.
Flat plate collectors are generally more efficient than ETC in full sunshine conditions. However, the energy
output of flat plate collectors is reduced slightly more than evacuated tube collectors in cloudy or extremely
cold conditions.[1] Most ETCs are made out of annealed glass, which is susceptible to hail, breaking in
roughly golf ball sized hail. ETCs made from "coke glass," which has a green tint, are stronger and less
likely to lose their vacuum, but efficiency is slightly reduced due to reduced transparency.
Heating of swimming pools
Both pool covering systems floating atop the water and separate solar thermal collectors may be used for
pool heating.
Pool covering systems, whether solid sheets or floating disks, act as insulation and reduce heat loss. Much
of a pool's heat loss occurs through evaporation, and using a cover provides a barrier against evaporation.
Using a pool cover will supplement the solar thermal collectors discussed below. See Swimming Pool
Covers for a detailed discussion.
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.[31] Once the pool
water has reached the required temperature, a diverter valve is used to return pool water directly to the pool
without heating.[32] 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 groundmounted 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 ruleofthumb for the required panel area needed is 50% of the
pool's surface area.[32] 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.[28] 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
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/m2 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/m2 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 A laundromat in California with
overcast. Useful calculators for estimating insolation at a site can be panels on the roof providing hot
found with the Joint Research Laboratory of the European washing water.
Commission[33] and the American National Renewable Energy
Laboratory.[34][35]
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 °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.[36]
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
Direct Indirect Indirect Direct
Configuration Thermosiphon
active active active 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 5.3 3.9 3.3 4.8 4.0
(temperate)
– e.g. Zurich, Switzerland
– Insolation 6.5 kW.h/m2/day
(tropical) 11.2 8.8 7.1 9.9 8.4
– e.g. Phoenix, USA
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 °C. In
the tropical scenario the equivalent heating would be by some 33 °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[37] 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.[26] 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
lifecycle cost) of a solar water heating system, to a level much higher than a comparable water heater of
the conventional type. The biggest single consideration is therefore the large initial financial outlay of solar
water heating systems.[38] Offsetting this expense can take several years[39] and the payback period is
longer in temperate environments where the insolation is less intense.[40] 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 water
heaters (also in conjunction with an existing water heater). At higher latitudes, solar heaters may be less
effective due to lower solar energy, possibly requiring larger and/or dualheating systems.[40] In addition,
government 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
Price of electricity use for mains pumping (if this is used)
Price of water heating fuel (e.g. gas or electricity) saved per kW.h
Amount of water heating fuel used per month by a household
Upfront state or government subsidy for installation of a solar water heater
Recurrent or annual tax rebates or subsidy for operating renewable energy
Annual maintenance cost of SWH system (e.g. antifreeze or pump replacements)
Savings in annual maintenance 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 200 kW.h per month (about 6.57 kW.h/day) due to SWH. Unfortunately payback times
can vary greatly due to regional sun, extra cost due to frost protection needs of collectors, household hot
water use etc. so more information may be needed to get accurate estimates for individual households and
regions. For instance in central and southern Florida the payback period could easily be 7 years or less
rather than the 12.6 years indicated on the chart for the US.[41]
Costs and payback periods for residential SWH systems with savings of 200 kW.h/month (using
2010 data)
System Effective Electricity Electricity Payback
Country Currency Subsidy(%)
cost cost cost/kW.h savings/month period(y)
Brazil BRL 2500[42] 0 2500 0.25 50 4.2
South
Africa
ZAR 14000 15[43] 11900 0.9 180 5.5
Australia AUD 5000[44] 40[45] 3000 0.18[46] 36 6.9

Belgium EUR 4000[47] 50[48] 2000 0.1[49] 20 8.3
United
States
USD 5000[50] 30[51] 3500 0.1158[52] 23.16 12.6
United
Kingdom
GBP 4800[53] 0 4800 0.11[54] 22 18.2
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.[40] 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,[39] thus extending the payback period of such a
system.
Solar leasing is now available in Spain for solar water heating systems from Pretasol[55] 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.[45]
Operational carbon/energy footprint and life cycle assessment
Terminology
Operational energy footprint (OEF) is also called energy parasitics ratio (EPR) or coefficient of
performance (CoP).
Operational carbon footprint (OCF) is also called carbon clawback ratio (CCR).
Life cycle assessment is usually referred to as LCA.
Carbon/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 mains electricity to pump the
fluid through the panels are called 'low carbon solar'. In most systems the pumping cancels the energy
savings by about 8% and the carbon savings of the solar by about 20%.[56] However, some new low power
pumps will start operation with 1W and use a maximum of 20W.[57][58] 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 12hour sunny day, the potentially negative effect of such a pump can be reduced to about 3% of
the total power produced.
The carbon footprint of such household systems varies substantially, depending on whether electricity or
other fuels such as natural gas are being displaced by the use of solar. Except where a high proportion of
electricity is already generated by nonfossil fuel means, natural gas, a common water heating fuel, in many
countries, has typically only about 40% of the carbon intensity of mains electricity per unit of energy
delivered. Therefore the 3% or 8% energy clawback in a gas home referred to above could therefore be
considered 8% to 20% carbon clawback, a very low figure compared to technologies such as heat pumps.
However, PVpowered active solar thermal systems typically use a 5–30 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 reduces the operational carbon and energy footprint: a growing design goal for
solar thermal systems.
Work is also taking place in a number of parts of the world on developing alternative nonelectrical
pumping systems. These are generally based on thermal expansion and phase changes of liquids and gases,
a variety of which are under development.
Life cycle carbon/energy assessment
Now looking at a wider picture than just the operational environmental impacts, recognised standards can
be used to deliver robust and quantitative life cycle assessment (LCA). LCA takes into account the total
environmental 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.[39]
In terms of energy, some 60% of the materials of a SWH system goes into the tank, with some 30% towards
the collector[59] (thermosiphon flat plate in this case) (Tsiligiridis et al.). In Italy,[60] 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 energyrelated 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 according to this southern European study.
Moving further north into colder, less sunny climates, the energy payback time of a solar water heating
system in a UK climate is reported as only 2 years.[61] This figure was derived from the studied solar water
heating system being: direct, retrofitted to an existing water store, PV pumped, freeze tolerant and of 2.8
sqm aperture. For comparison, a solar electric (PV) installation took around 5 years to reach energy
payback, according to the same comparative study.
In terms of CO2 emissions, a large degree of the emissionssaving 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 Ecoindicator 99 points system as a yardstick (i.e. the yearly environmental load of an average
European inhabitant) in Greece,[59] a purely gasdriven 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[60] 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 emissionscostly activity when the heat transfer fluid (glycolbased) 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,[39] 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.
Analysing their lower impact retrofit freezetolerant solar water heating system, Allen et al. (qv) report a
production CO2 impact of 337 kg, which is around half the environmental impact reported in the Ardente et
al. (qv) study.
Where information based on established standards are available, the environmental transparency afforded
by life cycle analysis allows consumers (of all products) to make increasingly wellinformed product
selection decisions. As for identifying sectors where this information is likely to appear first, environmental
technology suppliers in the microgeneration and renewable energy technology arena are increasingly being
pressed by consumers to report typical CoP and LCA figures for their products.
In summary, the energy and emissions cost of a SWH system forms a small part of the life cycle cost and
can be recovered fairly rapidly during use of the equipment. Their environmental impacts can be reduced
further by sustainable materials sourcing, using nonmains circulation, by reusing existing hot water stores
and, in cold climates, by eliminating antifreeze replacement visits.
Doityourself (DIY) systems
People have begun building their own (smallscale) solar water heating systems from scratch or buying kits.
Plans for solar water heating systems are available on the Internet.[62] and people have set about building
them for their own domestic requirements. DIY SWH systems are usually cheaper than commercial ones,
and they are used both in the developed and developing world.[63]
System specification and installation
Except in rare instances it will be insufficient to install a SWH system with no electrical or gas or
other fuel backup. Many SWH systems have a backup electric heating element in the integrated tank,
the operation of which may be necessary on cloudy days to ensure a reliable supply of hot water.
The temperature stability of a system is dependent on the ratio of the volume of warm water used per
day as a fraction of the size of the water reservoir/tank that stores the hot water. If a large proportion
of hot water in the reservoir is used each day, a large fraction of the water in the reservoir needs to be
heated. This brings about significant fluctuations in water temperature every day, with possible risks
of overheating or underheating, depending on the design of the system. Since the amount of heating
that needs to take place every day is proportional to hot water usage and not to the size of the
reservoir, it is desirable to have a fairly large reservoir (i.e. equal to or greater than daily usage,)
which will help prevent fluctuations in water temperature.
If ample storage is preexisting or can otherwise be reasonably acquired, a large SWH system is more
efficient economically than a small system.[59] This is because the price of a system is not linearly
proportional to the size of the collector array, so the price per square meter of collector is cheaper in a
larger system. If this is the case, it pays to use a system that covers nearly all of the domestic hot
water needs, and not only a small fraction of the needs. This facilitates more rapid cost recovery.
Not all installations require new replacement solar hot water stores. Existing stores may be large
enough and in suitable condition. Direct systems can be retrofitted to existing stores while indirect
systems can be also sometimes be retrofitted using internal and external heat exchangers.
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.
The most efficient PV pumps are designed to start very slowly in very low light levels, so if
connected uncontrolled, they may cause a small amount of unwanted circulation early in the morning
– for example when there is enough light to drive the pump but while the collector is still cold. To
eliminate the risk of hot water in the storage tank from being cooled that way this is very important.
solar controller may be required.
The modularity of an evacuated tube collector array 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.
Particularly in locations further towards the poles than 45 degrees from the equator, roof mounted sun
facing collectors tend to outperform wall mounted collectors in terms of total energy output.
However, it is total useful energy output which usually matters most to consumers. So arrays of
sunny wall mounted steep collectors can sometimes produce more useful energy because there can be
a small increase in winter gain at the expense of a large unused summer surplus.
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.
United States
OG300: OG300 Certification of Solar Water Heating Systems.[64]
Australia
Renewable Energy (Electricity) Act 2000
Renewable Energy (Electricity) (Largescale Generation Shortfall Charge) Act 2000
Renewable Energy (Electricity) (Smallscale Technology Shortfall Charge) Act 2010
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 relevant participants of the Largescale Renewable Energy Target and Smallscale Renewable Energy
Scheme must comply with the above Acts.[65]
APPENDIX 1. Worldwide use
Top countries worldwide
Solar hot water system installed on
low cost housing in the Kouga Local
Municipality, South Africa
Top countries using solar thermal power, worldwide: GWth[11][66][67][68][69][70][71]
# Country 2005 2006 2007 2008 2009 2010 2011 2012 2013
1 China 55.5 67.9 84.0 105.0 101.5 117.6
– EU 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
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
5 Australia 1.2 1.3 1.2 1.3 5.0 5.8
6 Brazil 1.6 2.2 2.5 2.4 3.7 4.3
7 Japan 5.0 4.7 4.9 4.1 4.3 4.0
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
World (GWth) 88 105 126 149 172 196
Solar heating in European Union + Switzerland
Solar thermal heating in European Union (MWth)[72][73][74]
# Country 2008 2009 2010[68] 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 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 (MWth) 19,08 21,60 23.49 25.55 29.66 31.39
* = estimation, F = France as a whole
See also
Australia: Solar hot water in Australia
Solar thermal collector
Wikimedia Commons has
Solar air heating media related to Solar water
Solar air conditioning heating.
Concentrating solar power
Passive solar
Renewable heat
Solar combisystem
Solar energy
Solar thermal energy
Renewable energy commercialization
Sustainable design
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External links
Parts of a solar heating system (http://www.nrel.gov/docs/fy04osti/34279.pdf)
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