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To
download this factsheet on PV in pdf
format please click here
Introduction
to Solar Photovoltaics
The
sun’s energy is the most abundant form
of energy
on the planet, and has the potential to
provide for
all our energy needs many times over. As a
source of
energy it is free and widely available.
Solar Photovoltaic
(Solar PV) panels convert the sun’s
energy
directly to electrical energy. On houses
the panels
are usually erected on a south facing roof
where they
receive the maximum amount of sunlight,
and the
electricity produced can then be used to
power
appliances, charge batteries, or be fed
back into
the grid. Although direct sunlight is not
essential, electrical
output increases with greater light
intensity.
FIGURE 1:
ROOF MOUNTED SOLAR INSTALLATION
1.
The technology
Solar
PV panels consist of a series of solar
cells, made
of silicon, each one capable of producing
around half a
volt of electricity. Individual PV cells
will be connected together to form a
module or panel, and then individual
panels are then
linked together to meet a particular need.
When a
photon (a unit of light energy) hits a PV
cell, this increases
the energy level of the electrons in the
silicon, which
creates a flow of electrons – i.e.
electricity.
Solar cells come in three main forms –
monocrystalline,
polycrystalline, or thin film – each
working at
different levels of efficiency and
suitable for
different types of installations.
FIGURE
2: LARGE-SCALE PV INSTALLATION (SOURCE
– SOLAR CENTURY)
Solar panels produce
direct current (DC) electricity. The
electricity we use for most applications
is at 240 volt
alternating current (AC), so the
electricity produced
from panels will need to be converted.
Solar panels
will only produce electricity during
sunlight
hours, so a back up system of power may
also be
needed. If
sites are not grid connected, the
electricity will be used
to charge a bank of batteries connected to
an inverter.
This converts the DC into 240 volt AC,
providing
usable current for most applications.
If a site is grid
connected the panels can be connected
to a two-way meter, which measures the
current
imported into the building from the grid,
and the
Solar PV current exported out into the
grid. This allows
the householder to sell the electricity to
the grid
when the panel is producing more energy
than needed
and to import energy from the grid when
the panel
is not producing enough electricity (i.e.
at night).
Therefore there will be a constant supply
of electricity,
and the solar electricity produced will
not be
wasted.
2.
Area of panels
With
solar PV panels being low power, quite a
large area
of panels will be needed in order to
provide a reasonable
amount of usable power. A large unshaded
south facing
roof is ideal. But flat roofs can also be
used if they are not in the shade. Panels
are then
mounted on frames at the correct angle.
6m2
of high
efficiency panels will produce around 1kW
at peak output. A typical system for a
house will cover
between 10-15m2
and be
rated at around 1.5-2kWp, providing for
around a third of a household’s electricity
needs (as long as the house is not heated
by
electricity). The size of a system will
depend upon the
area of roof available for panels, so that
systems providing
for much larger buildings than purely
domestic
dwellings can be created.
FIGURE
3: DOMESTIC SOLAR PV (SOURCE - SOLAR
CENTURY)
3.
Cost
Solar
PV panels are relatively expensive for the
amount of
power they produce, and so cost will be a
major
consideration in these installations. A
domestic system
producing 1.5-2kWp will cost from around
£6000.
Payback periods will be quite long. Yet as
these systems
increase in popularity the price will
fall, and
therefore the economics will get better
and better.
There is also significant government
funding for
solar PV systems of all sizes, which will
reduce the
financial burden considerably.
FIGURE
4: INTEGRATED SOLAR TECHNOLOGY
Modern
solar panels can also play a functional role in
buildings as a roofing
or cladding material – solar cladding
panels, solar roofing tiles, solar roofing panels
and solar glass are all products that can be
integrated into a
building. This will not only produce electricity,
but also save on the cost of conventional roofing
and cladding materials. This means that the
economics of solar
systems integrated into new buildings
or refits can work out favourably, even if integrated
solar components can be more expensive than
conventional sit-on-top solar panels.
To download this factsheet on
solar thermal in pdf format please click here
Introduction
to Solar Thermal Energy
The
sun’s energy is free and abundant, and can be used to provide green
and renewable hot water for a variety of domestic and commercial
situations. For Solar Thermal Water Heating systems solar collector
panels are installed on roofs facing between South West and South East
(the more south facing the better the performance). These collectors
absorb the sun’s energy and change it into heat, which is then used
to heat water. There are two main types of collector plates, and
several different ways these can be used to provide hot water.
Ensuring you have the right type of plate and set-up is crucial if
your solar hot water system is to be as economically and
environmentally efficient as possible.
1.
Types of Solar Collector Plates
a)
Flat Plate Collectors
The
most common panels for solar water heating are
flat-plate collectors. These consist of a thin
metal box with insulated sides and back, a glass
or plastic cover (the glazing), and a dark
coloured absorber plate. The glazing allows most
of the solar energy into the box whilst preventing
much of the heat gained escaping. The insulation
on the sides and back minimizes further heat loss
to the surroundings. The absorber plate is in the
box painted with a selective dark coloured
coating, designed to maximize the amount of solar
energy absorbed as heat. Running through the
absorber plate are many fine tubes (usually made
of copper), through which water is pumped. As the
water travels through these tubes it ‘absorbs’
the heat – this heated water is then gathered in
a larger collector pipe through which it can be
transported into the household’s hot water
system. A viable domestic flat plate collector
will need to be around 3-4 m2.
Figure
1
:
Installation Of Evacuated Tubes (source – solar flair)
b)
Evacuated Tube Collectors
These
are a more modern and more efficient design of
solar energy collector that can heat water to much
higher temperatures and require less area, yet
they are also correspondingly more expensive.
Instead of an absorber plate, water is
pumped though absorber tubes (metal tubes with a
selective solar radiation absorbing coating),
gaining heat before going into the collector pipe.
Each absorber tube is housed inside a glass tube,
from which the air has been evacuated, forming a
vacuum. The glass tube allows solar radiation
through to the absorber tube where it can be
turned into heat. The vacuum eliminates convective
and conductive heat loss - virtually all heat
absorbed is transferred to the water. The circular
shape of each evacuated tube means that solar
radiation is perpendicular to the collector for a
much greater time than with
flat plate collectors, further improving
efficiency, meaning that an evacuated tube
collector need be only around 2m2 to be
effective.
2.
System Requirements
There
are several necessary features that each system
must have. Systems must have freeze protection -
if the water in a system freezes it can damage the
solar collector, so this needs to be guarded
against.
Systems
using evacuated tube collectors should also have
boil protection to prevent water becoming too hot,
as they can heat the water to much higher
temperatures.
Any
system will also require an electric pump to push
the water around the system. This pump can be run
by electricity provided by a solar PV panel
incorporated in the roof installation – this
saves on mains electricity, reduces CO2 emissions
further, and in some systems helps to regulate the
temperature of the water produced. The low power
produced by these panels’ means a slow pump
speed, which makes it a more suitable solution for
flat-plate installations that require a slower
water speed. Evacuated tube systems require more
energy to pump the water around, and so generally
will need a pump run off mains power. If solar PV
is not provided, or is not suitable, then the pump
will have to be run off the mains – this will
incur slight running costs and offset the
environmental benefits (reducing CO2
abatement by as much as 20 percent in some
evacuated tube systems).
3.
System set-up
Each
solar hot water system will require a solar collector, plumbing to
join it to the existing system, a pump to circulate water around the
system (either mains electricity or solar PV), and either modification
to your existing hot water storage cylinder or a new cylinder.
Each
system will be tailored to the individual building, the existing hot
water system and hot water needs, but can be put into one of two
generalized categories, ‘open’ or ‘closed’. Again depending on
the situation (space, cost, needs) systems can either be single or
dual cylinder – the solar water can be stored in a large cylinder
along with conventionally heated water, or it can be stored in its own
separate solar hot water cylinder.
An
open system heats the water that will actually be used as the
household’s domestic hot water – this means that the hot water
travelling through the solar collector is the water that will come out
of the taps. Such systems are especially suitable if you already have
a low-pressure vented hot water system – usually identifiable by a
cold storage tank in the attic.
A
closed system will heat water that is passed though a heat exchanger
in a hot water storage cylinder, thus heating the water coming out of
your taps indirectly. Such systems can be used if your hot water is
kept at mains or high pressure, usually identifiable if your system
provides pressurized hot water for things like showers without the
need for separate pumps.
4.
Costs
A
small-scale solar hot water heater installation will cost from £2000-£5000,
with payback times varying from as little as 5 years, to over 50
years. Accurate payback figures are dependent upon a wide range of
variables and are very different for each system installed.
To
ensure that a system is as economically beneficial as possible it is
important that the system’s size is calculated in relation to
current hot water needs. A good solar water heater installation should
have a running capacity producing around 50-70% of non-space heating
hot water needs – the bigger the running capacity the greater the
savings but there will be a limit to how much energy a system can
provide.
To
minimize payback period and maximize savings you need to aim for the
biggest possible running capacity for the minimal possible investment
in solar collectors – installers should be able to work out the
optimal
size needed in respect to your household hot water needs and usage.
For these systems anything
too small is unlikely to provide significant benefit, anything too
large and it is unlikely that the greater investment will be justified
by any additional energy savings. Any quote should provide a detailed
breakdown of the specification and cost of the proposed system, and be
able to explain how they have calculated the size of the system to be
appropriate for specific hot water needs.
For
each situation there are many factors influencing which system will be
best to install – considering these factors will help you choose the
best system for your needs. The cheapest short-term option may not be
the best or cheapest in the longer term.
- Type
and efficiency of solar collector – flat-plate collectors may be
of lower efficiency and so require a larger area, but can still
work out cheaper.
- New
and additional hot water cylinders – will a system need new hot
water cylinders for twin tank systems, or a new bigger cylinder
for a single tank system, and will this be included in the price?
Is the hot water system being changed or upgraded anyway?
- Size
of cylinder and its performance – the larger the cylinder
generally the greater the amount of solar energy that can be
stored. They will cost more though, and require more space.
- Installation
costs – installing the system requires roof working, plumbing
and often electrical skills, and the more of such work required
the more expensive the installation. Simple but less efficient
systems are generally cheaper to install, more complex and
efficient systems cost more to install but can produce more heat.
Self-installing a system will reduce costs dramatically, but
requires the relevant practical skills.
- Planning
permission – most panels are governed by the same guidelines as
roof light windows, generally not requiring permission. It is
worth checking with your council, especially if you live in a
listed building or conservation area.
- Hard
water – many systems will not function if the water is too hard,
so check with the installer and your water company.
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