DESCRIPTION

Photovoltaic systems convert energy from the sun into electricity through
semi conductor cells. Systems consist of semi-conductor cells connected
together and mounted into modules. Modules are connected to an
inverter to turn their direct current (DC) output into alternating current
(AC) electricity for use in buildings. Photovoltaics supply electricity to the
building they are attached to or to any other load connected to the
electricity grid. Excess electricity can be sold to the National Grid when
the generated power exceeds the local need. PV systems require only
daylight, not sunlight to generate electricity (although more electricity is
produced with more sunlight), so energy can still be produced in overcast
or cloudy conditions. Photovoltaics are generally blue/grey in colour and
can be used successfully in all parts of the UK.

WHERE CAN PHOTOVOLTAICS BE USED?

Photovoltaic panels come in modular panels which can be fitted to the
top of roofs (looking similar to a roof light) and in slates or shingles
which are an integral part of the roof covering (looking similar to normal
roof tiles). Photovoltaic cells can be incorporated into glass for atria walls
and roofs or used as cladding or rain screen on a building wall - this is
particularly suitable for prestige offices. They can also be attached to
individual items such as street lights, parking meters, motorway noise
barriers or the sides of bridges.

Photovoltaic systems can be disc reet through being designed as an
integral part of the roof. An ‘invisible’ design using slates or shingles as
opposed to an architectural statement is likely to be preferable if in a
sensitive area.

Ideally, photovoltaics should face between south-east and south-west, at
an elevation of about 30-40°. However, in the UK even flat roofs receive
90% of the energy of an optimum system. They are particularly suited to
buildings that use electricity during the day - offices, retail and schools.

COST RANGES

There is rarely a standard photovoltaic project so it is hard for manufacturers /installers to quote average prices.
Ball park figures for fully installed costs are:

• £5,000-£8,000/kWp for a 1kWp (the peak output of the panel) roof
mounted system (the Energy Saving Trust show the average installed
cost to be £6,000)

• £10,000 -£15,000/kWp for façade or atrium systems.

There are economies of scale to be had, both in size of system and
number of systems bought.

For example, one supplier suggests a 10%
reduction for a 5kWp system and a 20% reduction for a 50kWp system
(from the 1kWp system price). Discounts will need to be negotiated on an individual basis with suppliers for each project.
Grants are available.

OUTPUT RANGES

The size of a photovoltaic installation is expressed by its kilowatt peak
(kWp) potential, which is an indication of how much electricity the
system could generate at peak or optimum conditions.
As a rough rule of thumb the Department of Trade and Industry estimates
that a typical 1kWp system in the UK could be expected to produce
between 700-750 kWh/yr of electricity (new data is pending), although
some technologies will generate considerably more than that. A high
performance system in London might be expected to produce a maximum
of 850kWh/yr.
The performance depends more on location, orientation and whole
system design than it does on cell type.
Photovoltaic cells come in a number of types with varying operating
efficiencies and therefore different areas of panel are required to produce
the same output:

 A gas heated 2 bed/4 person flat (built to 2002 Building Regulations)
uses approximately 1,500kWh/year in electricity (for lights and
appliances), so a typical 1kWp system would provide approximately 45%
of the dwelling’s electricity needs.

A gas heated 4 bed/7 person 3 storey terraced house (built to 2002
Building Regulations) uses approximately 2,500kWh/yr (for lights and
appliances), so a typical 1kWp system would provide approximately 28%
of the dwelling’s electricity needs.

From the DTI domestic field trial performance analysis of 113 domestic
systems, the average contribution of PV system to the electrical load
has been 43%. The contribution clearly depends on the size of the
system installed but is also a function of the available sunlight
throughout the year.

TECHNICAL FEASIBILITY ISSUES OR CONSTRAINTS

• Photovoltiacs should ideally face between south-east and south-west,
at an elevation of about 30-40° although arrays pointing east or west
are acceptable (albeit at lower performance) especially for roofs
inclined at a low angle to the horizontal.

• Arrays should NOT be horizontal as the rain will be unable to wash
them clean.

• Systems should be in locations that will be unshaded at all times of
day if possible. Gable roofs, chimneys, cables, TV aerials, trees and
other buildings in the vicinity should be identified as potentially
shading the modules, particularly in the early morning or early
afternoon. The performance of a whole panel will be affected even if
only part of it is shaded.

• Photovoltaics need to be ventilated (behind the modules) so that they
don’t heat up - their efficiency decreases as their temperature rises.
Suitable ventilation is easier to ensure for bolt-on systems. Rear
ventilation is less important for some thin film modules which can be
mounted directly onto the roof cover.

• If retrofitting systems to existing buildings, the system must be
carefully positioned on the roof to take account of the loading
capacity of the roof.

• The potential for vandalism should be assessed if the system can be
seen from the ground or if it is accessible due to raised pavements or
other buildings. Sometimes it is necessary to cover panels with heavy
duty perspex to protect them from flying objects.

• Care must be taken if the systems are to be fitted to social housing
properties (or other properties) with pre-payment meters as some
meters do not allow the export of electricity and can be damaged by
attempted export.

• If the area is known for a bird population (e.g. seagulls) they may
need to be discouraged from perching near the systems to prevent the
need for regular cleaning.

• If a photovoltaic system cannot be installed at time of build, the
electrical systems of the building (this is particularly relevant for
dwellings) should be prepared to facilitate later retrofitting. This would
include an extra 3 ways in the fuse board and an extra fuse spur
extending into the roof space. The design of a new building should always,
where practicable, be suitable to support solar technologies at a later
date.

Grid connection

The Local Distribution Network Operator (DNO) is an entity licensed to
distribute electricity through cables and has a duty to provide connections
to premises. For outline details on how to connect a photovoltaic system into the gridconsult OFGEM guidance ‘How to get connected to electricity supply’

www.ofgem.gov.uk/temp/ofgem/cache/cmsattach/5329_howtogetconn
ectedtoelectricitysupply.pdf

For more detail, consult ‘A Technical Guide to the Connection of
Generation to the Distribution Network’ www.energynetworks.org/
pdfs/FES_00318_v040211.pdf published in February 2004, funded by
the DTI New and Renewable Energy Programme.

For small systems, the DNO must only be told about the installation at
the time of commissioning but for larger systems or a number of small
systems, discussion should start as soon as possible with the DNO to
ensure the project goes to time. It is likely that DNO personnel would
need to be present at the time of commissioning for larger projects.

MAINTENANCE REQUIREMENTS

There should be very little maintenance required as the technology has
no moving parts. If there is a large bird population in the area, they
should be discouraged from perching on or near the PV cells as quantities
of bird excrement on the panels will affect their performance. Bird
excrement is unlikely to be washed away by rain.

The output of the panel should be monitored so that if the output is
much lower than expected, the panels and set up can be inspected and if
necessary cleaned.

ADDITIONAL BENEFITS OF TECHNOLOGY

The two main benefits of incorporating photovoltaics into a development
are the production of free electricity and the consequent saving of
carbon emissions. As a technology it also has a number of other benefits:

• Straightforward to install as modular and light

• Technically reliable - they are generally guaranteed to last between
20-25 years but are expected to last longer

• Avoidance of climate change levy for non-domestic buildings

• Architectural integration - photovoltaics can be added almost invisibly
to buildings, can be used as a design element or can lead the
architectural concept of a building

• Marketing impact - a clear statement about renewable energy

• Excess electricity can be sold to the grid. The price obtained will need
to be negotiated with an energy supplier

• Money can be saved where the PV panels displace other
construction materials such as roof tiles, or prestige cladding
materials such as marble

• Renewable Obligation Certificates, or Renewable Energy Certificates of
Origin can be obtained depending on the amount of electricity
generated by the panels - these can be then be traded.

PLANNING AND LEGISLATIVE ISSUES

See solar water heating for details regarding planning permission, as it is the same for photovoltaic systems. As with solar water heating more discreet designs using slates or shingles as opposed to an architectural statement are likely to be preferable in sensitive areas.

Grid connection

There are no licensing requirements relating to photovoltaic systems,
however the consent of the local Distribution Network Operator (DNO) is
required to connect the system to the grid.

Single installations of small photovoltaic systems (up to 5kW) will come
under the scope of Engineering Recommendation G83/1 -
‘Recommendations for the connection of small-scale embedded
generators (up to 16a per phase) in parallel with public low-voltage
distribution networks’.

Larger systems (above 5kW) and multiple small systems a re required to
meet Engineering Recommendation G59 - ‘Recommendations for the
connection of embedded generating plant to the regional companies’
distribution systems’.

LINKS TO SOURCES OF MORE DETAILED TECHNICAL INFORMATION

• Companion Guide to PPS 22: The technical annexes. Available at
www.odpm.gov.uk/planning.
• The British Photovoltaic Association, National Energy Centre, Davy
Avenue, Knowlhill, Milton Keynes, MK5 8NG, T: 01908 442291,
F: 0870 0529193, E: enquiries@pv-uk.org.uk, W: www.pv-uk.org.uk.
• Photovoltaics in Buildings: A Design Guide Report No ETSU
S/P2/00282/REP. March 1999. Available from DTI publications
orderline 0870 150 2500, www.dti.gov.uk/energy/renewables/
publications/pdfs/pvguide.pdf.
• Photovoltaics in Buildings: Testing, Commissioning and Monitoring
Guide Report No ETSU S/P2/00290/REP, available from DTI as above.

FREQUENTLY ASKED QUESTIONS

Will a PV roof affect my National House-Building Council (NHBC) warranty?
If a photovoltaic system is being installed by the developer as part of a
new build project, then the roof will be covered by the warranty as
normal (although the panels themselves will not be covered). If PV panels
are being retrofitted to an existing roof covered by an NHBC warranty,
then if there is any damage caused as a result of the panel installation
(for example, water ingress) then the resulting damage will not be
covered by the NHBC warranty.

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