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Solar power in the UK |
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| Solar systems are not new, and have been used
the world over for decades. With over 40,000 systems installed
in the UK solar power is becoming the leading source of providing
alternative energy. Richard Hanson-Graville, Technical director
at DPS Heatbanks, a leading manufacturer and supplier of water
heating systems, examines the varying types of solar systems
that are available.
We receive sufficient sunshine to provide up to 70% of our
annual domestic hot water needs from a well designed solar
installation and such a system will also reduce harmful emissions
by up to 500 kg per square metre of panel per year. Solar
hot water and heating systems are becoming cheaper and more
efficient, helped by government initiatives such as the reduction
in VAT to 5% on both parts and labour for domestic installations
that can demonstrate an improvement to energy efficiency.
Now is the time for UK plumbers to get to grips with solar
technology, and start presenting it as a serious option to
customers. |
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There are four main parts to a typical pumped
solar system: solar panels, a water store, connecting pipework,
and controls, typically comprising of a pump and a controller.
The panels fitted to the roof collect heat from the sun, using
it to heat water. This water is then circulated to the store,
which can be either a vented or unvented cylinder, or a thermal
store, typically fitted with a dedicated coil to transfer heat
into the store. The controller monitors the temperatures in
the panel and in the store, via dedicated temperature sensors,
and will activate the solar pump to circulate heat from the
panel to the store when available (typically when the panel
is around 5°C hotter than the store). The solar system can
be thought of as a separate primary circuit, installed as either
a sealed system (using an expansion vessel, filling loop and
pressure relief valve, or as a vented system connected to a
feed and expansion tank (higher than the panels). All pipework
should be well insulated. |
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| Solar panels
The size of panels required for a domestic property is anywhere
between 2m2 and 7m2, with panels fitted to the roof either
using simple support brackets, or by recessing into the roof.
The water in the solar system must usually be dosed with anti-freeze
to prevent frost damage to the panels, however certain systems
can overcome this. |
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Flat plate collectors
Flat plate collectors are made of a metal collector plate,
usually copper, coated in special materials and fitted into
an insulated box. Tubes running along the collector carry
water, and heat is conducted from the surface of the collector
into the water. The range of flat plate collectors is quite
large offering very good value for money, with panels available
from AES (as fitted at our own premises), Filsol, Solar Sense,
Powertech, Viessmann, and others. Flat plate collectors can
also be 'home-made' with a number of workshops around the
country set up to provide training. |
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The solartwin panel system
The Solartwin panel system is retro-fitted to an existing
vented cylinder. The need for a controller is removed by the
use of a second small photovoltaic panel to power a solar
pump, which pumps cold water from the cold feed connection
to a cylinder, returning it hot to the top of the cylinder
via the hot water draw off. The panels can resist freezing
as the collector tubes within the panel are made of rubber
and will not burst, however protection may be needed against
lime scale build up within the solar system. |
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| Evacuated tube collectors |
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| Evacuated Tube Collectors are constructed of glass
solar collector tubes, containing a second inner tube with a
selective coating. The space between the outer tube and inner
tube is evacuated and maintained at a high vacuum eliminating
all heat loss by conduction and convection. Solar radiation
passes through the highly transparent outer glass tube and reaches
the solar selective coating on the outside of the inner tube.
The solar selective coating absorbs the solar radiation and
converts it to thermal energy. |
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The first type of evacuated collectors are Direct
Flow Evacuated Tube Collectors which heat up water flowing through
the tubes. Panels worth noting are the Riomay NEG Suntube (20
year guarantee), the Solamax from Thermomax, the Viessmann Vitosol
200, and the Seido 3 Combi-Collector from PowerTech which has
a built in 40 litre mains water store that can be used to provide
a pre-heated water supply to a standard unvented cylinder or
thermal store (also avoiding the need for a pump, controller,
or anti-freeze). |
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The second type are Evacuated Heat Pipe Collectors
and differ in that they consist of a heat pipe inside a vacuum-sealed
tube. Each tube contains a sealed copper pipe (heat pipe) that
is attached to a black copper fin absorber plate. As the sun
shines on the black surface of the fin, alcohol within the heat
tube is heated and hot vapour rises to the top of the pipe.
Water, or glycol, flows through a manifold picking up the heat,
while the alcohol condenses and flows back down into the tube.
Both the Thermomax Memotron tubes and the Viessmann's Vitosol
300 tubes work on this principle, and both have the added advantage
of built in overheat protection - when a programmed temperature
has been achieved, a 'memory metal' spring expands and pushes
a plug against the neck of the heat pipe blocking the return
of the condensed fluid and stopping heat transfer. |
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| Photo Voltaic Panels (PVs) convert sunlight into
electricity. Although not as efficient as panels used to heat
water, they have a huge variety of applications from the large-scale
production of electricity in sunny countries to recharging the
batteries in watches and calculators, however the only time
they apply to plumbing is when used as an electricity source
for pumps or controls. |
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| The solar store |
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| The choice of store to use in conjunction with
panels is dependent on the type and area of panels used. As
the heat input from the panels cannot usually be limited, the
store needs to be large enough to absorb all the heat delivered
by the panels. Protection against overheating may be needed,
however correctly matching the panel size to the size of the
store should overcome this (the larger the store the less chance
of overheat). A general rule for sizing stores is 60 litres
per square metre of panel, however this will depend upon the
panel efficiencies. |
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The most common type of store is a twin coil cylinder,
with one coil at the bottom of the store dedicated to solar,
and the second connected to a standard boiler system to provide
heat when the solar input is not enough. Alternatively, two
stores can be used, one heated by the solar, and one by the
boiler. Solar versions of nearly all types of cylinder are available,
and many panels can be supplied with their own dedicated solar
store. |
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Drain-back systems Drain-back
systems overcome problems associated with freezing or overheat
by allowing the water to drain out of the panels back into
a thermal store during freezing or overheat. During normal
operation, the solar pump forces water up to the panels. On
some systems, the pump must be on to keep water in the panels,
whereas some may make use a control valve to allow water to
drain. Thermostats on the panel or pipework can be used to
sense freezing or overheat conditions respectively. With drain-back
systems, special care must be taken to avoid air locks within
the solar pipework. |
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| Plate heat exchangers |
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| Plate heat exchangers can be used to heat a hot
water store that is not fitted with a dedicated solar coil,
and as such can be used to convert cylinders to use solar. Ideally
the store should have two spare bosses for connection to the
heat exchanger. The exchanger transfers heat from pumped panel
water to the store water, also pumped from the store. Heat exchangers
are very efficient at transferring heat and are particularly
suited to district or commercial applications. |
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| Controls The choice of which
solar controller to use will depend upon the type of system.
A Basic system can make use of straightforward controllers,
which are no more difficult to install than a central heating
timeclock. Many include a digital read-out of water temperatures
in the panels and store, and one should rarely need to use
the more advanced controllers available. The controller will
typically need wiring to two sensors (one on the solar panel,
and one on the solar store or adjoining pipework), to the
solar pump, and to the mains electrical supply. Pre-assembled
controls packs are also available, such as Solar Sense's Consol
unit, as well as fully pre-assembled units such as our own
DPS Cel-F Solar Heat Bank. |
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| Summary: The choice of which
system to use comes down to a balance of panel efficiency,
size, cost, the guarantees on the panels, the appearance of
the panels, and simplicity of installation. One should obtain
quotes and performance data for a variety of panel types,
as well as looking at the cost of associated controls, before
making a final decision. Although the range of systems available
may seem a bit daunting at first, do not worry. They are all
pretty similar in general operation, and should prove as easy
as installing a typical boiler system. |
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