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Hot water is required for many purposes and the sun can be used effectively, efficiently and economically to provide this heat. The warming effect of solar radiation is obvious and it is well known that a container of cold water, left exposed to the sun will be raised in temperature. Solar water heating systems are designed to make convenient use of this phenomenon.
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== Introduction ==
Solar water heaters generally employ a solar collector and a storage tank. The solar water heating collector is by far the most widespread solar energy conversion device and there are several millions in use around the world. There are many simple designs of collectors and water heating systems. Construction and manufacture is easily achievable in most developing countries.
Hot water is required for many purposes and the sun can be used effectively, efficiently and
economically to provide this heat. The warming effect of solar radiation is obvious and it is well
known that a container of cold water, left exposed to the sun will be raised in temperature. Solar
water heating systems are designed to make convenient use of this phenomenon.


Solar water heaters generally employ a solar collector and a storage tank. The solar water
'''Benefits of Solar Water Heaters'''
heating collector is by far the most widespread solar energy conversion device and there are
 
several millions in use around the world. There are many simple designs of collectors and water
Solar Waster Heater can provide multiple benefits, including:
heating systems. Construction and manufacture is easily achievable in most developing
 
countries.
* Low maintenance costs.
* High durability.
* Hot water all throughout the year.
* Reduction in the utility bills as it can provide a third of your hot water needs.
* Compatibility with the existing heating, hot water boiler, heat pumps and biomass.
* Reduction of carbon footprint by saving between 230 kg and 510 kg of CO2 per year.<ref name="Greenmatch"><https://www.greenmatch.co.uk/solar-energy/solar-water-heating></ref>


== Energy for heating water ==
== Energy for heating water ==
The energy required to raise the temperature of a substance is a physical property known as the
 
specific heat of the particular substance. The specific heat of water is 4.2J/g/ oC, i.e. 4.2 joules
The energy required to raise the temperature of a substance is a physical property known as the specific heat of the particular substance. The specific heat of water is 4.2J/g/ oC, i.e. 4.2 joules of energy are required to raise the temperature of one gram of water by one degree centigrade. Using larger and more familiar units:
of energy are required to raise the temperature of one gram of water by one degree centigrade.
Using larger and more familiar units:


Energy required (kJ) = 4.2 x volume (litres) x temperature rise (oC)
Energy required (kJ) = 4.2 x volume (litres) x temperature rise (oC)


Thus, in order to consider energy sources for water heating, the parameters that must be known
Thus, in order to consider energy sources for water heating, the parameters that must be known are the volume of water required in a given time period (hour, day), the temperature of the 'cold' water, and the required delivery temperature. Hot water may be used for a variety of purposes, but as an example, domestic use is considerable. Here usage varies widely; in industrialised countries an average of around 50 litres per person per day is normal, in developing countries the more wealthy inhabitants may use this amount or more, while the [[poor]] may not use hot water at all.
are the volume of water required in a given time period (hour, day), the temperature of the 'cold'
water, and the required delivery temperature. Hot water may be used for a variety of purposes,
but as an example, domestic use is considerable. Here usage varies widely; in industrialised
countries an average of around 50 litres per person per day is normal, in developing countries
the more wealthy inhabitants may use this amount or more, while the [[poor]] may not use hot
water at all.


== Example ==
== Example ==
It is more convenient for the examples that follow to measure energy in kWh. To convert
megajoules (MJ) into kWh divide by 3.6 (1kWh = 3.6MJ) thus 16.8MJ = 4.7kWh.


This is the amount of energy that must be put into the water. With many water-heating systems
It is more convenient for the examples that follow to measure energy in kWh. To convert megajoules (MJ) into kWh divide by 3.6 (1kWh = 3.6MJ) thus 16.8MJ = 4.7kWh.
not all the energy used goes into heating the water, i.e. there are heat losses - the process is not
100% efficient. Some examples are given below.


Electric resistance heating is almost 100% efficient. Hence, to heat the water in the above
This is the amount of energy that must be put into the water. With many water-heating systems not all the energy used goes into heating the water, i.e. there are heat losses - the process is not 100% efficient. Some examples are given below.
example - 100 litres through a temperature rise of 40 oC - would require 4.7kWh of electricity, or
 
for example a 1kW rated electric immersion heater running for nearly five hours.
Electric resistance heating is almost 100% efficient. Hence, to heat the water in the above example - 100 litres through a temperature rise of 40 oC - would require 4.7kWh of electricity, or for example a 1kW rated electric immersion heater running for nearly five hours.


Although electricity is efficient at heating water it is expensive and not available everywhere.
Although electricity is efficient at heating water it is expensive and not available everywhere.


Water is usually heated by burning fuel. An oil (kerosene) or gas fuelled water heater has an
Water is usually heated by burning fuel. An oil (kerosene) or gas fuelled water heater has an efficiency of around 50%, while heating water on an open fire has an efficiency of about 10%. In this latter case to heat the 200 litres of water through 40 oC would require fire wood with a calorific value of nearly This is equivalent to about 10kg of low (15%) moisture wood 100kWh.
efficiency of around 50%, while heating water on an open fire has an efficiency of about 10%. In
this latter case to heat the 200 litres of water through 40 oC would require fire wood with a
calorific value of nearly This is equivalent to about 10kg of low (15%) moisture wood 100kWh.


By comparison, a simple solar water heater might have an efficiency of around 30%. On a very
By comparison, a simple solar water heater might have an efficiency of around 30%. On a very sunny day the solar energy received might be 6kWh/m2. Thus, to heat 100 litres of water through 40 oC would require a solar collector with an area of 4.7/(6 x 0.3) = 2.6m2.
sunny day the solar energy received might be 6kWh/m2. Thus, to heat 100 litres of water
through 40 oC would require a solar collector with an area of 4.7/(6 x 0.3) = 2.6m2.


In general:
In general:
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Collector area required = (Energy demand) (Solar energy per m2) x (collector efficiency)
Collector area required = (Energy demand) (Solar energy per m2) x (collector efficiency)


The above example is intended only to give a rough indication. The energy available from the
The above example is intended only to give a rough indication. The energy available from the sun and the performance characteristics of solar collectors vary in a complex way and generalisations should be used with caution.
sun and the performance characteristics of solar collectors vary in a complex way and
generalisations should be used with caution.


== The availability of solar energy ==
== The availability of solar energy ==
The power density of solar energy reaches a maximum of about 1000W/m2 at sea level. This is
made up of two components, the radiation in the direct beam from the sun, and diffuse radiation
from the sky (radiation that has been scattered by the atmosphere). On a clear day diffuse
energy may amount to 15-20% of the global irradiance whereas on a cloudy day it will be 100%.


Global irradiance varies throughout the course of the day because the path length of the solar
The power density of solar energy reaches a maximum of about 1000W/m2 at sea level. This is made up of two components, the radiation in the direct beam from the sun, and diffuse radiation from the sky (radiation that has been scattered by the atmosphere). On a clear day diffuse energy may amount to 15-20% of the global irradiance whereas on a cloudy day it will be 100%.
radiation through the atmosphere changes. For the same reason, there are variations with
season and latitude. The total solar energy received in a day (known as the insolation or solar
irradiation) can vary from 0.5kWh m2/ in the UK winter to 5kWh/ m2 in the UK summer and can
be as high as 7kWh/m2 in desert regions of the world. Many tropical regions do not have large
seasonal variations and receive an average 6kWh/ m2/day throughout the year.


This variability is an important aspect of solar energy because it influences system design and
Global irradiance varies throughout the course of the day because the path length of the solar radiation through the atmosphere changes. For the same reason, there are variations with season and latitude. The total solar energy received in a day (known as the insolation or solar irradiation) can vary from 0.5kWh m2/ in the UK winter to 5kWh/ m2 in the UK summer and can be as high as 7kWh/m2 in desert regions of the world. Many tropical regions do not have large seasonal variations and receive an average 6kWh/ m2/day throughout the year.
solar energy economics. The size of the solar collector required for a particular application is
 
dependent on the location under consideration.
This variability is an important aspect of solar energy because it influences system design and solar energy economics. The size of the solar collector required for a particular application is dependent on the location under consideration.


== The technology ==
== The technology ==
When radiant energy strikes the surface of an object, a proportion (depending upon the angle of
incidence and the nature of the surface) is reflected, part is absorbed and part may be
transmitted through the object. With a few important exceptions, such as photovoltaic cells, the
energy of the absorbed radiation is degraded rapidly to heat.


The balance between the absorbed input energy and the heat loss to the environment
When radiant energy strikes the surface of an object, a proportion (depending upon the angle of incidence and the nature of the surface) is reflected, part is absorbed and part may be transmitted through the object. With a few important exceptions, such as photovoltaic cells, the energy of the absorbed radiation is degraded rapidly to heat.
determines the temperature attained. The heat loss increases with the temperature and limits
the ultimate temperature attained by a collector system. It also reduces the proportion of useful
heat extractable from the system. Maximum temperatures and maximum useful power outputs
are therefore obtained when a highly absorbent, well-insulated body is exposed to a high
intensity of solar radiation. A wide range of systems, designed to meet a variety of needs and
situations, have been developed and many are available commercially.


The best known solar heating device is the flat-plate collector, which is widely used for water
The balance between the absorbed input energy and the heat loss to the environment determines the temperature attained. The heat loss increases with the temperature and limits the ultimate temperature attained by a collector system. It also reduces the proportion of useful heat extractable from the system. Maximum temperatures and maximum useful power outputs are therefore obtained when a highly absorbent, well-insulated body is exposed to a high intensity of solar radiation. A wide range of systems, designed to meet a variety of needs and situations, have been developed and many are available commercially.
heating in many parts of the world. The flat-plate collector absorbs as much as possible of the
incident solar energy that falls upon it. Since the collector is normally fixed in position, the plate
is close to perpendicular to the beam of sunlight (and therefore maximum absorption) for only
part of the time, and the level of energy received therefore varies more strongly with time and
season than does the actual intensity of the solar radiation. Because of the large areas over
which heat can be lost, the retention of heat and hence the collection efficiency, falls off rapidly
with increase in collection temperature. Since domestic water is normally needed at only about
50oC this is not normally a problem.


A simple flat-plate collector is shown in Figure 1.<br>
The best known solar heating device is the flat-plate collector, which is widely used for water heating in many parts of the world. The flat-plate collector absorbs as much as possible of the incident solar energy that falls upon it. Since the collector is normally fixed in position, the plate is close to perpendicular to the beam of sunlight (and therefore maximum absorption) for only part of the time, and the level of energy received therefore varies more strongly with time and season than does the actual intensity of the solar radiation. Because of the large areas over which heat can be lost, the retention of heat and hence the collection efficiency, falls off rapidly with increase in collection temperature. Since domestic water is normally needed at only about 50oC this is not normally a problem.
This consists of:<br>
• an absorber that is painted black and from
which heat is removed by a heat transfer
fluid<br>
• a cover which is transparent to solar
radiation<br>
• insulation at the back and sides of the
absorber<br>
• a casing to protect the absorber and its
insulation.<br>
[[Image:flat_plate_collector.jpeg|thumb|500px|right|Figure 1: A simple flat plate collector]]


The absorber may be made from one of a wide
A simple flat-plate collector is shown in Figure 1.<br>This consists of:<br>• an absorber that is painted black and from which heat is removed by a heat transfer fluid<br>• a cover which is transparent to solar radiation<br>• insulation at the back and sides of the absorber<br>• a casing to protect the absorber and its insulation.<br>
range of materials, including copper, stainless
steel, galvanised steel, aluminium and plastics.
When choosing an absorber material, it is
important to ensure that it is compatible, from the
point of view of corrosion, with the other
components in the system and with the heat
transfer fluid used. The absorber must also be
able to withstand the highest temperature that it
might reach on a sunny day when no fluid is flowing in the collector (known as the stagnation
temperature).


The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate, or
[[File:flat plate collector.jpeg|thumb|500px|right|Figure 1: A simple flat plate collector]]
may form an integral part of the absorber.


Experience has shown that simple mechanical clamping of tubes to an absorber plate is likely to
The absorber may be made from one of a wide range of materials, including copper, stainless steel, galvanised steel, aluminium and plastics. When choosing an absorber material, it is important to ensure that it is compatible, from the point of view of corrosion, with the other components in the system and with the heat transfer fluid used. The absorber must also be able to withstand the highest temperature that it might reach on a sunny day when no fluid is flowing in the collector (known as the stagnation temperature).
result in an absorber with a poor efficiency. A good thermal bond, such as a braze, weld or high
temperature solder is required for tube and plate designs, in order to ensure good heat transfer
from the absorbing surface into the fluid.


Matt black paints are commonly used for absorber surfaces because they are relatively cheap,
The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate, or may form an integral part of the absorber.
simple to apply and may be easily repaired. Paints, however, have the disadvantage that they
are usually strong emitters of thermal radiation (infrared), and at high temperature this results in
significant heat losses from the front of the collector.


Heat losses from the collector can be substantially reduced by the use of absorber coatings
Experience has shown that simple mechanical clamping of tubes to an absorber plate is likely to result in an absorber with a poor efficiency. A good thermal bond, such as a braze, weld or high temperature solder is required for tube and plate designs, in order to ensure good heat transfer from the absorbing surface into the fluid.
known as 'selective surfaces'. These surfaces may be applied by electroplating or by dipping a
metal absorber in appropriate chemicals to produce a thin semi-conducting film over the surface.
The thin film will be transparent to solar radiation but at the same time appear opaque to
thermal radiation. However, these surfaces cannot be produced or applied easily.


Flat-plate collectors usually have a transparent cover made of glass or plastic. The cover is
Matt black paints are commonly used for absorber surfaces because they are relatively cheap, simple to apply and may be easily repaired. Paints, however, have the disadvantage that they are usually strong emitters of thermal radiation (infrared), and at high temperature this results in significant heat losses from the front of the collector.
required to reduce heat losses from the front of the collector and to protect the absorber and the
insulation from the weather. Most covers behave like a greenhouse. They permit solar radiation
to pass into the collector, but they absorb the thermal radiation emitted by the hot absorber.


A solar water heating system consists essentially of a flat-plate collector and a water storage
Heat losses from the collector can be substantially reduced by the use of absorber coatings known as 'selective surfaces'. These surfaces may be applied by electroplating or by dipping a metal absorber in appropriate chemicals to produce a thin semi-conducting film over the surface. The thin film will be transparent to solar radiation but at the same time appear opaque to thermal radiation. However, these surfaces cannot be produced or applied easily.
tank. The simplest arrangement is illustrated in Figure 2. In this, the tank is placed at a higher
level than the collector so that the heated water will run from the collector to the tank and induce
natural circulation by convection (thermosyphon). These systems can be very reliable provided
the pipe-work diameter is adequate and there are not sharp bends. This is the most common
design of solar water heaters used in developing countries.


At night it is possible for the collector to lose heat
Flat-plate collectors usually have a transparent cover made of glass or plastic. The cover is required to reduce heat losses from the front of the collector and to protect the absorber and the insulation from the weather. Most covers behave like a greenhouse. They permit solar radiation to pass into the collector, but they absorb the thermal radiation emitted by the hot absorber.
by radiation and the circulation will be in the
opposite direction, so the water will cool. This
can be overcome by use of a suitable non-return
valve. However, there is a danger with solar
collectors when used under clear night conditions
(e.g. in arid and semi arid regions) that they can
actually freeze even when the ambient
temperature is above freezing point. In such
conditions it may be necessary to have a primary
circuit through the collector filled with antifreeze
and a separate indirect hot water cylinder where
the water from the collector passes through a
copper coil to heat the main water supply. This
problem will only apply in certain desert regions
in the cold season or at high altitudes in the
tropics and sub-tropics.
[[Image:solar_water_system.jpeg|thumb|500px|right|Figure 2: solar water heating system]]
It is also possible to pump the water between the
collector and tank. This allows the two
components to be more widely separated and the tank does not have to be at a level higher than
the collector, however, these systems are much more complex and electricity is required to
power the circulating pump.


== Costs ==
A solar water heating system consists essentially of a flat-plate collector and a water storage tank. The simplest arrangement is illustrated in Figure 2. In this, the tank is placed at a higher level than the collector so that the heated water will run from the collector to the tank and induce natural circulation by convection (thermosyphon). These systems can be very reliable provided the pipe-work diameter is adequate and there are not sharp bends. This is the most common design of solar water heaters used in developing countries.
Flat-plate solar collectors typically cost £150 per square metre and a professionally installed
 
system costs around £2,000 or more. In the UK, the economics are marginal because savings
At night it is possible for the collector to lose heat by radiation and the circulation will be in the opposite direction, so the water will cool. This can be overcome by use of a suitable non-return valve. However, there is a danger with solar collectors when used under clear night conditions (e.g. in arid and semi arid regions) that they can actually freeze even when the ambient temperature is above freezing point. In such conditions it may be necessary to have a primary circuit through the collector filled with antifreeze and a separate indirect hot water cylinder where the water from the collector passes through a copper coil to heat the main water supply. This problem will only apply in certain desert regions in the cold season or at high altitudes in the tropics and sub-tropics.
on fuel will be approximately £100 a year. In sunnier locations, however, payback periods of just
 
a few years are possible. The use of solar water heating displaces the burning of other fuels and
[[File:solar water system.jpeg|thumb|500px|right|Figure 2: solar water heating system]]
hence is beneficial to the environment.


== Further information ==
It is also possible to pump the water between the collector and tank. This allows the two components to be more widely separated and the tank does not have to be at a level higher than the collector, however, these systems are much more complex and electricity is required to power the circulating pump.
The following publications provide more detailed, easy to understand, information on solar water
heating.<br>
• Solar Water Heating: A D-I-Y Guide CAT Publications - 1999<br>
• Heating Water by the Sun UK-ISES 1981<br>
• Solar Domestic Hot Water by Plante R H Wiley and Sons - 1983<br>
• Practical Solar Heating McCarthy K/Ford B - - Prism Press - 1978<br>


James and James produce quite a few [[renewable energy]] publications including; The World
== Costs ==
Directory of Renewable Energy, Renewable Energy World, and Solar Energy Houses.


Their contact details are:
Flat-plate solar collectors typically cost £150 per square metre and a professionally installed system costs around £2,000 or more. In the UK, the economics are marginal because savings on fuel will be approximately £100 a year. In sunnier locations, however, payback periods of just a few years are possible. The use of solar water heating displaces the burning of other fuels and hence is beneficial to the environment.


James & James (Science Publishers) Ltd. <br>
== Further information ==
35 - 37 William Road <br>
London <br>
NW1 3ER<br>
United Kingdom<br>
Tel: +44 (0)20 7387 8558<br>
Fax: +44 (0)20 387 8998<br>
E-mail: orders@jxj.com<br>
Website: http://www.jxj.com


Information can also be obtained from the following organisations:
The following publications provide more detailed, easy to understand, information on solar water heating.


The Solar Energy Society<br>
* Solar Water Heating: A D-I-Y Guide CAT Publications - 1999
School of Engineering, Oxford Brooks University<br>
* Heating Water by the Sun UK-ISES 1981
Gipsy Lane Campus, Headington, Oxford<br>
* Solar Domestic Hot Water by Plante R H Wiley and Sons - 1983
OX3 0BP<br>
* Practical Solar Heating McCarthy K/Ford B - Prism Press - 1978
United Kingdom<br>
Tel: +44 (0)1865 484367<br>
Fax: +44 (0)1865 484263<br>
E-mail: uk-ises@brookes.ac.uk<br>
Website: http://www.brookes.ac.uk/uk-ises


The Solar Energy Society is a non-profit organisation. It is a forum for all those interested in the
James and James produce quite a few [[renewable energy]] publications including; The World Directory of Renewable Energy, Renewable Energy World, and Solar Energy Houses.
advancement of the utilisation of the sun's energy. Members of the Society are drawn from
industry, government, academic institutions, architectural and engineering practices, as well as
the general public: academic qualifications are not a pre-requisite for membership.


NEF Renewables,<br>
Their contact details are:
The National Energy Foundation,<br>
Davy Avenue, Knowlhill, Milton Keynes<br>
MK5 8NG<br>
United Kingdom<br>
Tel: +44 (0)1908 665555<br>
Fax: +44 (0)1908 665577<br>
Email: http://www.greenenergy.org.uk/


The Solar Trade Association Ltd*<br>
* James & James (Science Publishers) Ltd. - 35 - 37 William Road - London - NW1 3ER - United Kingdom - Tel: +44 (0)20 7387 8558 - Fax: +44 (0)20 387 8998 - E-mail: orders@jxj.com - Website: http://jxj.com/
Pengillan<br>
Lerryn<br>
Lostwithiel<br>
Cornwall<br>
PL22 0QE<br>
United Kingdom<br>
Tel: +44 1208 873518<br>
Fax: +44 1208 873518<br>
Website: http://www.greenenergy.org.uk/sta/


Trade association for the UK. Manufactures and installers of solar equipment.<br>
Information can also be obtained from the following organizations:
The Information Officer *<br>
Solar Energy Unit<br>
University College of Wales, Cardiff<br>
Newport Road<br>
Cardiff<br>
CF2 1TA<br>
United Kingdom<br>
Can supply a list of solar collector manufacturers.* <br>


* The Solar Energy Society - School of Engineering, Oxford Brooks University - Gipsy Lane Campus, Headington, Oxford - OX3 0BP - United Kingdom - Tel: +44 (0)1865 484367 - Fax: +44 (0)1865 484263 - E-mail: uk-ises@brookes.ac.uk - Website: http://www.brookes.ac.uk/uk-ises - The Solar Energy Society is a non-profit organisation. It is a forum for all those interested in the advancement of the utilisation of the sun's energy. Members of the Society are drawn from industry, government, academic institutions, architectural and engineering practices, as well as the general public: academic qualifications are not a pre-requisite for membership.
* NEF Renewables, - The National Energy Foundation, - Davy Avenue, Knowlhill, Milton Keynes - MK5 8NG - United Kingdom - Tel: +44 (0)1908 665555 - Fax: +44 (0)1908 665577 - Email: http://www.greenenergy.org.uk/
* The Solar Trade Association Ltd* - Pengillan - Lerryn - Lostwithiel - Cornwall - PL22 0QE - United Kingdom - Tel: +44 1208 873518 - Fax: +44 1208 873518 - Website: http://www.greenenergy.org.uk/sta/ - Trade association for the UK. Manufactures and installers of solar equipment.
* The Information Officer * - Solar Energy Unit - University College of Wales, Cardiff - Newport Road - Cardiff - CF2 1TA - United Kingdom - Can supply a list of solar collector manufacturers.* -


{{attrib PATB}}
{{Page data
{{Includes content from}}
| keywords = solar, water, energy, water heating
| ported-from = https://practicalaction.org
| sdg = SDG06 Clean water and sanitation, SDG07 Affordable and clean energy
}}


[[Category:Solar]]
[[Category:Water]]
[[Category:Energy]]
[[Category:Energy]]
[[Category:Water heating]]
[[Category:Water heating]]

Latest revision as of 14:45, 23 October 2023

This page or section should probably be merged with Solar hot water.See the talk page here or there for more.Read more...

Hot water is required for many purposes and the sun can be used effectively, efficiently and economically to provide this heat. The warming effect of solar radiation is obvious and it is well known that a container of cold water, left exposed to the sun will be raised in temperature. Solar water heating systems are designed to make convenient use of this phenomenon.

Solar water heaters generally employ a solar collector and a storage tank. The solar water heating collector is by far the most widespread solar energy conversion device and there are several millions in use around the world. There are many simple designs of collectors and water heating systems. Construction and manufacture is easily achievable in most developing countries.

Benefits of Solar Water Heaters

Solar Waster Heater can provide multiple benefits, including:

  • Low maintenance costs.
  • High durability.
  • Hot water all throughout the year.
  • Reduction in the utility bills as it can provide a third of your hot water needs.
  • Compatibility with the existing heating, hot water boiler, heat pumps and biomass.
  • Reduction of carbon footprint by saving between 230 kg and 510 kg of CO2 per year.[1]

Energy for heating water[edit | edit source]

The energy required to raise the temperature of a substance is a physical property known as the specific heat of the particular substance. The specific heat of water is 4.2J/g/ oC, i.e. 4.2 joules of energy are required to raise the temperature of one gram of water by one degree centigrade. Using larger and more familiar units:

Energy required (kJ) = 4.2 x volume (litres) x temperature rise (oC)

Thus, in order to consider energy sources for water heating, the parameters that must be known are the volume of water required in a given time period (hour, day), the temperature of the 'cold' water, and the required delivery temperature. Hot water may be used for a variety of purposes, but as an example, domestic use is considerable. Here usage varies widely; in industrialised countries an average of around 50 litres per person per day is normal, in developing countries the more wealthy inhabitants may use this amount or more, while the poor may not use hot water at all.

Example[edit | edit source]

It is more convenient for the examples that follow to measure energy in kWh. To convert megajoules (MJ) into kWh divide by 3.6 (1kWh = 3.6MJ) thus 16.8MJ = 4.7kWh.

This is the amount of energy that must be put into the water. With many water-heating systems not all the energy used goes into heating the water, i.e. there are heat losses - the process is not 100% efficient. Some examples are given below.

Electric resistance heating is almost 100% efficient. Hence, to heat the water in the above example - 100 litres through a temperature rise of 40 oC - would require 4.7kWh of electricity, or for example a 1kW rated electric immersion heater running for nearly five hours.

Although electricity is efficient at heating water it is expensive and not available everywhere.

Water is usually heated by burning fuel. An oil (kerosene) or gas fuelled water heater has an efficiency of around 50%, while heating water on an open fire has an efficiency of about 10%. In this latter case to heat the 200 litres of water through 40 oC would require fire wood with a calorific value of nearly This is equivalent to about 10kg of low (15%) moisture wood 100kWh.

By comparison, a simple solar water heater might have an efficiency of around 30%. On a very sunny day the solar energy received might be 6kWh/m2. Thus, to heat 100 litres of water through 40 oC would require a solar collector with an area of 4.7/(6 x 0.3) = 2.6m2.

In general:

Collector area required = (Energy demand) (Solar energy per m2) x (collector efficiency)

The above example is intended only to give a rough indication. The energy available from the sun and the performance characteristics of solar collectors vary in a complex way and generalisations should be used with caution.

The availability of solar energy[edit | edit source]

The power density of solar energy reaches a maximum of about 1000W/m2 at sea level. This is made up of two components, the radiation in the direct beam from the sun, and diffuse radiation from the sky (radiation that has been scattered by the atmosphere). On a clear day diffuse energy may amount to 15-20% of the global irradiance whereas on a cloudy day it will be 100%.

Global irradiance varies throughout the course of the day because the path length of the solar radiation through the atmosphere changes. For the same reason, there are variations with season and latitude. The total solar energy received in a day (known as the insolation or solar irradiation) can vary from 0.5kWh m2/ in the UK winter to 5kWh/ m2 in the UK summer and can be as high as 7kWh/m2 in desert regions of the world. Many tropical regions do not have large seasonal variations and receive an average 6kWh/ m2/day throughout the year.

This variability is an important aspect of solar energy because it influences system design and solar energy economics. The size of the solar collector required for a particular application is dependent on the location under consideration.

The technology[edit | edit source]

When radiant energy strikes the surface of an object, a proportion (depending upon the angle of incidence and the nature of the surface) is reflected, part is absorbed and part may be transmitted through the object. With a few important exceptions, such as photovoltaic cells, the energy of the absorbed radiation is degraded rapidly to heat.

The balance between the absorbed input energy and the heat loss to the environment determines the temperature attained. The heat loss increases with the temperature and limits the ultimate temperature attained by a collector system. It also reduces the proportion of useful heat extractable from the system. Maximum temperatures and maximum useful power outputs are therefore obtained when a highly absorbent, well-insulated body is exposed to a high intensity of solar radiation. A wide range of systems, designed to meet a variety of needs and situations, have been developed and many are available commercially.

The best known solar heating device is the flat-plate collector, which is widely used for water heating in many parts of the world. The flat-plate collector absorbs as much as possible of the incident solar energy that falls upon it. Since the collector is normally fixed in position, the plate is close to perpendicular to the beam of sunlight (and therefore maximum absorption) for only part of the time, and the level of energy received therefore varies more strongly with time and season than does the actual intensity of the solar radiation. Because of the large areas over which heat can be lost, the retention of heat and hence the collection efficiency, falls off rapidly with increase in collection temperature. Since domestic water is normally needed at only about 50oC this is not normally a problem.

A simple flat-plate collector is shown in Figure 1.
This consists of:
• an absorber that is painted black and from which heat is removed by a heat transfer fluid
• a cover which is transparent to solar radiation
• insulation at the back and sides of the absorber
• a casing to protect the absorber and its insulation.

Figure 1: A simple flat plate collector

The absorber may be made from one of a wide range of materials, including copper, stainless steel, galvanised steel, aluminium and plastics. When choosing an absorber material, it is important to ensure that it is compatible, from the point of view of corrosion, with the other components in the system and with the heat transfer fluid used. The absorber must also be able to withstand the highest temperature that it might reach on a sunny day when no fluid is flowing in the collector (known as the stagnation temperature).

The fluid passageways of the absorber may consist of tubes bonded to an absorbing plate, or may form an integral part of the absorber.

Experience has shown that simple mechanical clamping of tubes to an absorber plate is likely to result in an absorber with a poor efficiency. A good thermal bond, such as a braze, weld or high temperature solder is required for tube and plate designs, in order to ensure good heat transfer from the absorbing surface into the fluid.

Matt black paints are commonly used for absorber surfaces because they are relatively cheap, simple to apply and may be easily repaired. Paints, however, have the disadvantage that they are usually strong emitters of thermal radiation (infrared), and at high temperature this results in significant heat losses from the front of the collector.

Heat losses from the collector can be substantially reduced by the use of absorber coatings known as 'selective surfaces'. These surfaces may be applied by electroplating or by dipping a metal absorber in appropriate chemicals to produce a thin semi-conducting film over the surface. The thin film will be transparent to solar radiation but at the same time appear opaque to thermal radiation. However, these surfaces cannot be produced or applied easily.

Flat-plate collectors usually have a transparent cover made of glass or plastic. The cover is required to reduce heat losses from the front of the collector and to protect the absorber and the insulation from the weather. Most covers behave like a greenhouse. They permit solar radiation to pass into the collector, but they absorb the thermal radiation emitted by the hot absorber.

A solar water heating system consists essentially of a flat-plate collector and a water storage tank. The simplest arrangement is illustrated in Figure 2. In this, the tank is placed at a higher level than the collector so that the heated water will run from the collector to the tank and induce natural circulation by convection (thermosyphon). These systems can be very reliable provided the pipe-work diameter is adequate and there are not sharp bends. This is the most common design of solar water heaters used in developing countries.

At night it is possible for the collector to lose heat by radiation and the circulation will be in the opposite direction, so the water will cool. This can be overcome by use of a suitable non-return valve. However, there is a danger with solar collectors when used under clear night conditions (e.g. in arid and semi arid regions) that they can actually freeze even when the ambient temperature is above freezing point. In such conditions it may be necessary to have a primary circuit through the collector filled with antifreeze and a separate indirect hot water cylinder where the water from the collector passes through a copper coil to heat the main water supply. This problem will only apply in certain desert regions in the cold season or at high altitudes in the tropics and sub-tropics.

Figure 2: solar water heating system

It is also possible to pump the water between the collector and tank. This allows the two components to be more widely separated and the tank does not have to be at a level higher than the collector, however, these systems are much more complex and electricity is required to power the circulating pump.

Costs[edit | edit source]

Flat-plate solar collectors typically cost £150 per square metre and a professionally installed system costs around £2,000 or more. In the UK, the economics are marginal because savings on fuel will be approximately £100 a year. In sunnier locations, however, payback periods of just a few years are possible. The use of solar water heating displaces the burning of other fuels and hence is beneficial to the environment.

Further information[edit | edit source]

The following publications provide more detailed, easy to understand, information on solar water heating.

  • Solar Water Heating: A D-I-Y Guide CAT Publications - 1999
  • Heating Water by the Sun UK-ISES 1981
  • Solar Domestic Hot Water by Plante R H Wiley and Sons - 1983
  • Practical Solar Heating McCarthy K/Ford B - Prism Press - 1978

James and James produce quite a few renewable energy publications including; The World Directory of Renewable Energy, Renewable Energy World, and Solar Energy Houses.

Their contact details are:

  • James & James (Science Publishers) Ltd. - 35 - 37 William Road - London - NW1 3ER - United Kingdom - Tel: +44 (0)20 7387 8558 - Fax: +44 (0)20 387 8998 - E-mail: orders@jxj.com - Website: http://jxj.com/

Information can also be obtained from the following organizations:

  • The Solar Energy Society - School of Engineering, Oxford Brooks University - Gipsy Lane Campus, Headington, Oxford - OX3 0BP - United Kingdom - Tel: +44 (0)1865 484367 - Fax: +44 (0)1865 484263 - E-mail: uk-ises@brookes.ac.uk - Website: http://www.brookes.ac.uk/uk-ises - The Solar Energy Society is a non-profit organisation. It is a forum for all those interested in the advancement of the utilisation of the sun's energy. Members of the Society are drawn from industry, government, academic institutions, architectural and engineering practices, as well as the general public: academic qualifications are not a pre-requisite for membership.
  • NEF Renewables, - The National Energy Foundation, - Davy Avenue, Knowlhill, Milton Keynes - MK5 8NG - United Kingdom - Tel: +44 (0)1908 665555 - Fax: +44 (0)1908 665577 - Email: http://www.greenenergy.org.uk/
  • The Solar Trade Association Ltd* - Pengillan - Lerryn - Lostwithiel - Cornwall - PL22 0QE - United Kingdom - Tel: +44 1208 873518 - Fax: +44 1208 873518 - Website: http://www.greenenergy.org.uk/sta/ - Trade association for the UK. Manufactures and installers of solar equipment.
  • The Information Officer * - Solar Energy Unit - University College of Wales, Cardiff - Newport Road - Cardiff - CF2 1TA - United Kingdom - Can supply a list of solar collector manufacturers.* -
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Keywords solar, water, energy, water heating
SDG SDG06 Clean water and sanitation, SDG07 Affordable and clean energy
Authors Steven Medina, Nd
License CC-BY-SA-3.0
Ported from https://practicalaction.org (original)
Language English (en)
Related 0 subpages, 4 pages link here
Impact 1,491 page views
Created February 26, 2009 by Steven Medina
Modified October 23, 2023 by Maintenance script
Cite as Steven Medina, Nd (2009–2023). "Solar Water Heating". Appropedia. Retrieved April 24, 2024.
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