Solar thermal water heating
An enormous market in China, ambitious targets in the EU … but what
are the basics of solar water heating? Volker Quaschning describes the
principles and technology for using solar energy to heat water, and looks
at applications for domestic hot water and space heating.
The history of solar thermal applications goes back a long way, dating back at least
as far as Archimedes’ use of a concave mirror to heat water in 214 BC. As a term,
‘solar thermal’ encompasses all thermal uses of solar energy, and represents a number of
different technology options. This article deals with non-concentrating solar collector
systems used for domestic water heating (concentrating collector systems were
covered in REW Nov–Dec 2003).
At the heart of a solar thermal system is the solar collector. It absorbs solar radiation,
converts it into heat, and transfers useful heat to the solar system. There are a number
of different design concepts for collectors: besides simple absorbers used for swimming
pool heating, more sophisticated systems have also been developed for higher
temperatures, such as integral storage collector systems, flat-plate collectors,
evacuated flat-plate collectors and evacuated-tube collectors. Although
commercial integral storage collectors do exist, no significant numbers have been sold,
and so these are not described in detail here.
The majority of solar collectors that are sold in many countries are of the flat-plate variety.
The main components of these are a transparent front cover, collector housing and
an absorber. The absorber, inside the flatplate collector housing, converts sunlight to
heat and transfers it to water in the absorber tubes. As the collector can reach stagnation
temperatures up to 200°C (i.e. when no water flows through), all the materials used must be
able to resist such heat. Therefore, the absorber is usually made of metal materials
such as copper, steel or aluminium. The collector housing can be made of
plastic, metal or wood, and the glass front cover must be sealed so that heat does not
escape, and dirt, insects or humidity do not get into the collector itself. Many collectors
also have controlled ventilation, so as to avoid condensation inside the glass front
cover. The collector housing is highly insulated at the back and sides, keeping heat
losses low. However, there are still some collector heat losses, mainly due to the
temperature difference between the absorber and ambient air, and these are subdivided
into convection and radiation losses. The former are caused by air movements, while
the latter are caused by exchange of heat by radiation between the absorber and the
A sheet of glass covers the collector as it faces the sun, and this helps to prevent most
of the convection losses. Furthermore, it reduces heat radiation from the absorber into
the environment in a similar way as a greenhouse does. However, the glass also
reflects a small part of the sunlight, which does not then reach the absorber at all.
Figure 1 shows the processes occurring at a flat-plate collector.
FIGURE 1. Processes at a flat-plate collector
Black materials absorb sunlight very well, and heat up as a result. Since metallic
materials do not naturally have a black surface, they need to be coated for selective
absorption. Black, temperature-resistant lacquer can serve this purpose, but there are
much better materials for absorber coating. If a black surface heats up, it emits part of the
heat energy again as heat radiation, as can be shown with electrical hotplates: when the
hotplate is on, heat radiation can be felt on the skin without touching the hotplate itself.
A black lacquered absorber shows the same effect, transferring only part of the absorbed
heat to the water that flows through the absorber tubes, while radiating some heat
back into the environment.
So-called selective coatings absorb the sunlight almost as well as black lacquered
surfaces, and re-emit a much smaller amount of heat radiation. While the coating processes
needed for these materials are more complicated than those for lacquering, this is
compensated for by much higher efficiencies. As a result, many absorbers today have
selective coatings, with materials used including black chrome, black nickel or TiNOX.
Convection heat loss due to air movements inside the collector can be significantly
reduced by maintaining a vacuum between the front cover and the absorber of a flatplate
collector. As the ambient air pressure would then force the front cover against the
absorber, small supports must be used between the back of the collector and the
cover, to keep the cover itself in shape. It is difficult to maintain the vacuum over a
long period of time, since ambient air will always find a way between the glass and the
housing to get into the collector, and an evacuated flat-plate collector therefore has to
be evacuated again from time to time. These disadvantages can be avoided
with evacuated-tube collectors. The high (almost complete) vacuum inside the
closed glass tube of the evacuated-tube collector is more stable over a long period of
time than in an evacuated flat-plate collector. Due to their shape, glass tubes can better
resist the ambient air pressure, and therefore no supports are needed between the back
and front sides.
An evacuated-tube collector comprises a closed glass tube, inside which is a metal
absorber sheet with a heat pipe in the middle, containing a temperature-sensitive
medium such as methanol. The sun heats up and vaporizes this heat pipe fluid, and the
vapour then rises to the condenser and heat exchanger at the end of the pipe. There, the
vapour condenses, and transfers heat to the heat carrier of the solar cycle, water with
antifreeze agent. The condensed fluid flows back to the bottom of the heat pipe where
the sun begins heating it up again. To work properly, the pipes must have a minimum
angle of inclination, in order for the vapour to rise and the fluid to flow back. A cross-section
of the evacuated-tube collector and the principle of its operation is shown in Figure 2.
With some evacuated-tube collectors, the heat pipe passes through the end of the
glass tube, so that the heat transfer medium of the solar cycle can flow directly through it.
A heat exchanger is not needed with this type of collector, and the collector does not
have to be mounted at the minimum angle of inclination.
FIGURE 2. Principle of an evacuated tube collector with heat pipe; view from top
A significantly higher energy gain can be obtained with evacuated tube collectors,
especially during the cooler months of the year. Thus, a solar system using evacuatedtube
collectors requires a smaller collector area than one using standard flat-plate
On the other hand, the specific collector price for evacuated-tube collectors is higher than that
for flat-plate systems. A further consideration is that tube collectors cannot be directly
integrated into a roof, so they must always be installed on top of it, reducing their
PHOTO. Connections of evacuated tubes to the solar cycle
In order to compare collectors, test institutions usually estimate efficiency curves
based on measurements of collector performance. These curves are given for
different irradiances E and a variety of temperature differences between collector TC
and ambient air TA. The commonly used empirical equation for the collector efficiency
The three parameters eta0, a1 and a2 are
estimated by collector test measurements; eta0
is also referred to as optical efficiency. Figure 3 shows typical collector efficiencies
for a flat-plate collector. The thermal losses increase as temperature difference between
collector and ambient air rises. At low solar irradiances, the efficiency decreases at a
faster rate; for instance, at a solar irradiance of only 200 W/m², the output of Figure 3's
sample collector becomes zero even at a lower temperature difference (about 40°C).
FIGURE 3. Collector efficiencies at different irradiances and temperature differences
Water Heating Methods
For storing water overnight or on cloudy days, a storage tank is needed. A very simple
way of doing this, making use of gravity is shown in Figure 4 - the thermosyphon
system. The principle of the thermosyphon system is that cold water has a higher
specific density than warm water, and so being heavier will sink down. Therefore, the
collector is always mounted below the water storage tank, so that cold water from the
tank reaches the collector via a descending water pipe. If the collector heats up the
water, the water rises again and reaches the tank through an ascending water pipe at the
upper end of the collector. The cycle of tank–water pipe–collector ensures the water
is heated up until it achieves an equilibrium temperature. The consumer can then make
use of the hot water from the top of the tank, with any water used being replaced by cold
water at the bottom. The collector then heats up the cold water again. Due to higher
temperature differences at higher solar irradiances, warm water rises faster than it
does at lower irradiances. Therefore, the circulation of water adapts itself almost
perfectly to the level of solar irradiance. A thermosyphon system’s storage tank
must be positioned well above the collector, otherwise the cycle can run backwards
during the night and all the water will cool down. Furthermore, the cycle does not work
properly at very small height differences. In regions with high solar irradiation and flatroof
architecture, storage tanks are usually installed on the roof.
Thermosyphon systems operate very economically as domestic water heating
systems, and the principle is simple, needing neither a pump nor a control. However,
thermosyphon systems are usually not suitable for large systems, that is, those with
more than 10 m² of collector surface. Furthermore, it is difficult to place the tank
above the collector in buildings with sloping roofs, and single-circuit thermosyphon
systems are only suitable for frost-free regions.
FIGURE 4. A thermosyphon system
In contrast to thermosyphon systems, an electrical pump can be used to move water
through the solar cycle of a system by forced circulation. Collector and storage tank can
then be installed independently, and no height difference between tank and collector
is necessary. Figure 5 shows a system using forced circulation with a conventional boiler
for back-up heating.
Two temperature sensors monitor the temperatures in the solar collector and the
storage tank. If the collector temperature is above the tank temperature by a certain
amount, the control starts the pump, which moves the heat transfer fluid in the solar
cycle; ‘switch-on’ temperature differences are normally between 5°C and 10°C. If the
temperature difference decreases below a second threshold, the control switches off the
In regions where there is a danger of frost, a double-circuit system is usually used.
Drinking water is kept inside the storage tank, while the water in the solar cycle is
mixed with an antifreeze agent. A heat exchanger transfers the heat of the solar
cycle to the storage tank, and keeps the drinking water separate from the antifreeze
Forced-circulation systems can be used for room heating as well as domestic
water heating. In this case, collectors and storage tanks must be much larger than with
simple domestic water heating systems, where a collector surface of about 4 m² is
sufficient for most households. Larger systems have also been realized successfully
with two or more storage tanks.
FIGURE 5. A double-cycle system with forced circulation with a conventional boiler for back-up heating
Solar District Heating
If an entire housing estate should be fitted with solar systems, one solution is a solar
district heating system (see Figure 6). The collectors are either distributed on the
houses, or replaced by a large, central solar collector. The collectors then heat up a big
central storage tank, from which much of the heat is distributed back to the houses. The
surface-to-volume ratio of a central storage tank is much better than that for distributed
storage systems, so the storage losses are much lower, and even permit seasonal heat
storage. Solar district heating is also an option if room heating is to be covered by
solar energy. There are higher piping losses with a central tank, but some solar district
heating demonstration systems have already been successfully tested.
FIGURE 6. A solar district heating system
Solar Collector Markets
China is by far the world’s largest manufacturer and user of solar water heating.
By the end of 2002, the total installed area of solar domestic hot water systems
there was about 40 million m²; annual production and sales volume reached about
8 million m² in 2002. There are now more than 1000 manufacturers producing and
selling solar thermal systems, and a total turnover of more than €1 billion has been
achieved. Evacuated-tube collectors dominate the Chinese domestic and export
Elsewhere, about 1 million m² of collector surface was installed in the US by 2001;
almost all of this was made up of unglazed absorbers used for swimming pool heating.
In Europe, there was about 1.1 million m² of collector area installed by 2002, and the flatplate
collector dominates installations. About half of these installations have been realized
in Germany. The solar collector market mainly depends on political conditions in a particular
country, and in Germany, for instance, the collector market dropped by 40% in 2002
due to uncertain political conditions, though it recovered again in 2003. However, the EU
has very ambitious targets for collector installations, with a target of 100 million m²
by 2010. Solar thermal systems will then play an important role in the struggle against