What are Photovoltaics (PV)?
Photovoltaic means electricity from light. The photovoltaic (PV) process converts free solar energy - the most abundant energy source on the planet - into direct current electricity. After conversion into higher voltage alternating current, photovoltaic systems can help to power ordinary electrical equipment, such as household appliances or computers, or they can export surplus electricity to the national grid. PV systems are also valuable in remote locations which are not connected to the mains grid; in this case electricity is often stored in battery banks. Indeed the first commercial applications of PV for powering buildings were over 40 years ago on lighthouses in Japan.
A PV cell consists of two or more thin layers of semi-conducting material, most commonly silicon. When the silicon is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC). The electrical output from a single cell is small, so multiple cells are connected together to form a "string", which produces a direct current. In many roof-integrated applications, strings are then encapsulated (usually behind glass) to form a module (sometimes referred to as a "panel"). The PV module is the principal building block of a PV system and any number of modules can be connected together to give the desired electrical output.
PV equipment has no moving parts and as a result requires minimal maintenance. It generates electricity without producing emissions of greenhouse or any other gases, and its operation is silent, although if - as in most cases - the direct current produced by a PV panel needs converting to AC for connection to the UK grid, it needs to be passed through an inverter, which may produce a small mains "hum".
There are five commercially available types of solar cell:
The EurActive Roofer project focuses on commercial applications on roofs and so does not consider other types of cells that are mainly used in space or are still in development. There is a page on this micro-site that gives more information about commonly found types of PV cell.
- Thick Film Silicon
- Thin Film Silicon
- Other Thin Films (CIGS and CdTe)
Is PV a new technology?
The photovoltaic effect was discovered by the French physicist Alexandre-Edmond Becquerel as long ago as 1839. Initially seen as something of a curiosity, it was realised in the 1960s that PV could be used to power satellites, and was initially commercialised for this purpose. It was soon being used as well for some off-grid applications, such as lighthouses, where regular access to refuel diesel generators was difficult and expensive. PV first became available as a consumer product on solar powered calculators, where the low demand for power enabled them to be cost-effective at a relatively early stage of development.
Other than for research purposes, building integrated and roof-mounted photovoltaics only started to be seen in the early 1990s, as costs fell with advances in clean manufacturing (driven in part by the growing use of silicon semiconductors in the computer industry). Widespread support for PV on buildings commenced in the late 1990s, with Government led initiatives such as the "1,000 solar roofs" programme in Germany running from 1990-95, which was later increased 100-fold to become the "100,000 solar roofs" scheme starting in January 1999. In contrast, the UK has been slow to pick up the challenge of mass installations, although Leicester has begun a 1,000 solar roof campaign.
This church roof in Freiburg shows that not all early German installations were aesthetically pleasing.
These older projects do give assurance that photovoltaics are a reliable and long-lived technology. Most manufacturers will guarantee panels for at least 25 years, although they should remain productive for over 40; there may be a need to replace or upgrade the associated inverters midway through these periods. Panels need no regular maintenance; occasional cleaning may be necessary, depending on local environmental conditions, but in many places rain is sufficient to keep panels in good shape. However because of their long life, there are some doubts expressed about whether panels encapsulated into double glazed units will last as long, given the typical life of standalone double glazing - but this may not be an issue as PV will only usually be incorporated into the highest quality double glazed units.
PV is a straightforward technology, and in selecting a roof for PV cells, there are just three basic requirements:
It is possible to place PV panels in other directions, and some early installations had them facing at angles between East and West and still produced a reasonable energy yield. However for optimal output, a direction as close to South as possible should be selected. On flat roofs, it is possible to mount PV on frames that can be angled correctly. If the PV is to be mounted on a vertical façade the orientation should preferably be between South East and South West. North facing orientations should be avoided.
- The roof should ideally face between Southeast and Southwest
- The roof should not be subject to overshading (eg. from trees or other buildings) for most of the day
- The inverter (changing the photovoltaic direct current into 230V AC) should be placed close to the PV panels to minimise transmission losses
A tilted array will receive more light than a vertical array, but any angle between vertical and 15° off horizontal can be used. A minimum tilt of 15° off horizontal is recommended to allow the rain to wash dust off the array. The optimal tilt angle is 30° - 40° for a south facing array in the UK, but a typical UK house roof titled at around 32° will be almost ideal. Shallower tilt angles are better for east or west facing arrays as the sun is lower in the sky as it moves away from due South. (Peak output is obtained when the plane of the array is perpendicular to the sun. Although the peak production would be obtained in Britain with an array angled at 52° at midday, to maximise output during daylight hours the tilt should be lower to average out the changing sun angle.)
Shadows cast by tall trees and neighbouring buildings must also be considered. Even minor shading can result in significant loss of energy, as if only one cell is blocked from a string, the output from the entire string falls sharply. For this reason it is best to lay modules so that any encroaching shadows affect an entire string at a time, rather than to lay them at right angles when the shadows might affect the end cell of several strings.
As noted above, most systems convert direct current produced by the PV cells into 230V AC before use. As losses are much grater with low voltage DC, the inverter should be located as close as reasonably possible to the panels - the loft space in a home is ideal.
How much Energy will a PV array generate?
PV panels are rated in terms of kWp - that is maximum power output expressed as kilowatts (peak). However most of the time they will produce rather less than peak output (and obviously at night produce no energy at all). A rule of thumb in the UK is that the average output over a year will be around 750kWh per kilowatt-peak of panel installed.
The chart above shows output from a 6.47kWp South-facing array in Milton Keynes over the life of the EurActive Roofer project. Over the final two years between July 2006 and July 2008, a total of 10,202kWh was generated, equivalent to an output of 788kWh per kWp per annum. PV output is not affected as much as might be expected by the solar insolation - that is, by the amount of solar energy falling on each square metre of the earth's surface. The map below shows how insolation does vary quite significantly across the UK, but in reality actual PV output will depend more on local features such as the angle and tilt of the array. PV output is also slightly better away from the West of the country, where higher levels of cloud cover reduce the light wavelengths on which PV works more than total solar insolation. (Source: PVGIS © European Communities, 2001-2008)
How large should a PV array be?
The area required for mounting a PV array depends on the output power desired and the type of module used. For non-domestic applications, it may be determined by the base load (lights, PCs, staff room fridge, etc.) on a summer afternoon, unless it is intended to generate a surplus that can be exported to the grid as part of a policy of moving towards energy neutrality (where exported electricity on sunny days is balanced by imported electricity in winter and on dull days). Typically, an area of around 8 m2 will be required to mount an array with a rated power output of 1kWp if monocrystalline modules are used (the most efficient type). If polycrystalline modules are used an area of around 10m2 will be required for a 1kWp system and if amorphous modules are used an area of up to 20m2 will be required. These areas can be scaled up or down depending on the output power desired. Most domestic systems installed in the UK have a rated power output of 1 - 3 kWp, although smaller or larger systems can be installed depending on budget. PV is not a cheap technology, so users are not advised to over-specify the quantity.
The panel shown here is rated at around 325Wp, and its size can be seen by the installer holding it.
Under English planning laws, a General Permitted Development Order (GPDO) issued in April 2008 grants rights to carry out certain limited forms of development on the home, without the need to apply for planning permission. The scope incudes Solar PV and solar thermal (roof mounted) which is permitted unless
PV roofs are also eligible for limited grant support under the UK Government's Low Carbon Buildings Programme. Some electricity companies offer favourable rates for photovoltaic electricity exported by their customers back to the grid; others however will only pay a minimal amount.
- the panels when installed protrude more then 200mm, or
- they would be placed on the principal elevation facing onto or visible from the highway in buildings in Conservation Areas and World Heritage Sites.
The information given is for guidance only.