Solar cells, or photovoltaic (PV) cells, convert sunlight directly into electricity. Photovoltaics gets its name from the process of converting light (photons) to electricity (voltage). Photovoltaic panels are quite different to solar thermal panels that capture the sun's heat to produce hot water, although some panels now combine both a PV array and a solar thermal collectors.
Rigid photovoltaic cells (traditionally made of silicon) are generally approximately 150mm square and produce a small amount of electricity (at about 0·5 volts). This means that in order to generate a higher voltage, a large number of cells, typically 36 or 72, are connected in series to form a photovoltaic panel. The panels are then connected together to create a photovoltaic array. Generally an array will consist of around 10 panels for a domestic application.
Thin film solar cells use layers of semiconductor materials a few microns thick. Being semi-flexibile they can be used as part of building elements such as roof tiles and glazing systems. New technologies are producing solar cells applied by printing press technologies using solar dyes, and integrated with conductive plastics.
The estimated lifetime of a photovoltaic module is 30 years and performance would be expected to remain at over 80% of the initial power after 25 years.
The carbon footprint of manufacturing photovoltaic has decreased by approximately 50% in the last 10 years due to performance improvements, raw material savings and manufacturing process improvements.
 Transforming sunlight to electricity
Solar panels are rated in terms of peak power (kWp). This is the potential power output in bright sunlight (1000W/m²) and an air temperature of 25 ºC (the output of panels reduces as they increase in temperature). 1 kWp of well-sited photovoltaic array in the UK should be able to produce 700-800 kWh of electricity per year.
The amount of incident solar radiation will depend on the latitude of the site, the direction that the panels face and the panels tilt angle. An online calculator is available for obtaining estimates of the potential generated energy for sites in Europe (ref EU Joint Research Centre). Even on cloudy days the resulting diffuse light will provide useful electricity, however the performance will be reduced if the site is regularly shaded (for example by adjacent buildings or vegetation).
 Photovoltaic cells
There are three principal types of photovoltaic cell. Their individual percentage efficiencies indicate how much of the incoming solar radiation will convert into electricity leaving the cell (there will be further losses in the control systems and cabling):
- Monocrystalline: Typical efficiency of 15% (about 100kWh per m² per year in the UK). These are typically dark coloured with close lines of thin conductors.
- Polycrystalline: Typical efficiency of at least 13% (around 100kWh per m² per year in the UK). These are likely to have a truly crystalline appearance.
- Thin film: Typical efficiency of 7% (about 60kWh per m² per year in the UK). These may be integrated onto brise-soleil, roof tiles or glass panels.
Crystalline photovoltaics currently account for over 90% of installed systems.
 Using the energy from photovoltaics
Photovoltaics can power systems that are totally disconnected from the grid (particularly in rural locations) however the surge in interest in UK applications is for grid-connected systems that are eligible for government funding. This allows any excess power produced to be sold to the electrical supply company, and provides a normal grid supply to the premises when there is insufficient photovoltaic generation.
 Photovoltaic installation
There is relatively little mechanical work associated with the installation of photovoltaic panels, and there is a mature market for the supply of flexible mounting systems. However, there is controlled work that requires properly trained electricians, and installation of roof-mounted panels would normally require a trained 'roofing' installer. Photovoltaic modules generate electricity whenever they are exposed to daylight and individual modules cannot be switched off so, unlike most other electrical installations, installing a photovoltaic system involves working on a live system.
Typical installations of photovoltaic modules will weigh 10–13 kg/m².The main types of installation are:
- Roof or wall mounting of framed photovoltaics. The photovoltaics are mounted in framing that should generally protrude less than 200mm to satisfy normal planning requirements.
- Roof or wall-integrated photovoltaics, where the panel is a weathered section of the surface.
- Roof slates and tiles. These will be more expensive as individual components but may reduce expenditure by displacing standard tiles/slates.
- Surface-mounted (thin-film) photovoltaics. Semi-flexible thin-film photovoltaics are attached to a building component such as a glass panel or flat roof.
- Framed installations (freestanding or attached to building). Plastic or metal frames can sit on roofs or on the ground to provide optimum tilt and orientation.
More recent developments have led to a variety of forms of integrated solar collector, including:
- Solar shingles mounted flat on boarded roofs.
- Solar slates mounted on battens that can replace standard roof components.
- Solar glass laminates, where the photovoltaics take the form of semi-transparent glazing.
These can offer opportunities to include photovoltaics in a project where, previously, aesthetic considerations would have prevented their use.
In a grid connected system, DC power from the photovoltaic modules is fed into an inverter for conversion to 240 V AC power for connection to the local electricity network through the consumer unit. Ideally, the inverter (or 'power control unit') will be sited close to the panels to reduce DC transmission losses (it needs ventilation and may hum). In larger applications several inverters may be used to provide improved resilience against failure. The inverter unit will normally control the connection of the photovoltaic system to the grid (as well as to the building loads). If there is a mains power outage the inverters automatically switch off to protect any engineers working on the power lines.
In off-grid systems the DC power is fed into a charge controller before being supplied to a storage medium, such as lead acid batteries. A grid-connected solar photovoltaic system requires no batteries. Specialised solutions may be used to provide hybrid systems allowing grid-based systems to work safely off-grid (in combination with batteries), but normally, systems designed for the grid are not usable directly with battery storage.
Photovoltaic systems do not generally require special precautions for lightning protection although taking precautions against lightning and excess voltage may be advisable to protect the investment in the photovoltaics.
- Photovoltaic panels: 30 years for mature module technologies.
- Inverters: 15 years for small plants (residential photovoltaics).
- Structures: 30 years for roof-tops and façades, and between 30 to 60 years for ground mounted installations on metal supports.
- Cabling: 30 years.
The BRE's UK Photovoltaic Domestic Field Trial in 2006 showed that for fairly small system sizes of around 1.6kWp, a significant fraction of the building demand can be met by the photovoltaic system with the majority of systems providing between 20 and 80% of the building annual load, with an average of 51%.
 Planning and Regulation
In the UK, fixing solar panels to a single dwelling house that is not a listed building and is not in a conservation area is considered to be a 'permitted development' and so there is no need to apply for planning permission. Photovoltaic installations are however notifiable for building regulations purposes and the local building control authority should be informed.
The District Network Operator (DNO) must be consulted about connection to the local grid (and an agreement put in place) although normally systems of up to 16 amps per phase can be installed without asking permission of the DNO. Extensive guidance on the electrical requirements is given in the DTI publication: Photovoltaics in buildings: guide to the installation of PV systems.
The Microgeneration Certification Scheme (MCS) has provided Microgeneration Installation Standard: MIS 3002 that guides the design and installation of photovoltaics. This is essential if government funding is being sought for a project of less than 50kW and provides a useful source of information for larger installations.
 Government Funding
Photovoltaic installations are recognised as a small-scale (less than 5MW) 'renewable energy' technology by the UK Government. As such they attract Feed in Tariff payments (FITs) for installations up to 5MW for a duration of 25 years. Microgeneration systems (less than 50kW) must be installed under the auspices of the MCS to attract funding.
Feed in Tariffs consist of 2 elements:
- Generation tariff. A payment for each unit (kWh) of electricity generated.
- Export tariff. Electricity that is not used on site can be exported back to the grid and a payment is paid as an additional payment (on top of the generation tariff).
If photovoltaics are installed and the property owner/operator receives Feed in Tariffs, and subsequently the property is passed to another owner, the Feed in Tariffs will remain with the installation and accrue to the new owner.
If a business has photovoltaics installed and is already involved in the Carbon Reduction Commitment (CRC) Energy Efficiency Scheme the output from photovoltaic systems (that are registered for Feed in Tariffs) will not be counted as 'zero emission' for CRC purposes but must be accounted for at the grid average. To use photovoltaics to gain CRC credit, Feed in Tariffs cannot be claimed. Detailed guidance on the Feed in Tariff scheme for businesses is available in some clearly written fact sheets on the Carbon Trust website.
This article has been reproduced here with the kind permission of the Building & Engineering Services Association. The original article, along with other resources, can be seen on their website: www.b-es.org
 Related articles on Designing Buildings Wiki
- BRE National Solar Centre.
- BRE photovoltaic certification scheme.
- Brise soleil.
- Carbon Reduction Commitment.
- Code of practice for grid connected solar photovoltaic systems.
- Feed in tariff.
- Fuel cell.
- Future of electricity in domestic buildings.
- Geothermal energy.
- Green Deal.
- Large scale solar thermal energy.
- PV inverter.
- Solar thermal systems.
- Tau - the solar powered island.
- Zero carbon homes.
- Zero carbon non-domestic buildings.
 External references
- BRE National Solar Centre.
- CIBSE TM25 Understanding Building Photovoltaics provides a comprehensive reference in the design and application of PV.
- BRE's 8 page Information Paper 8/11 Photovoltaic Systems on Dwellings - Key Factors For Successful Installations, provides one of the most concise guides for PV application in the UK.
- BSRIA BG 1/2008 - Renewable Technologies has a short section on PVs
- National Energy Foundation have an excellent set of pages covering many aspects of PVs.
- Energy Saving Trust has a page with up to date guidance and links to appropriate legislation and government web resources as well as a Buyers Guide to Solar Electricity.
- Your Sun Your Energy produced by the European Photovoltaic Industry Association (EPIA) and the European Commission's European Photovoltaic Technology Platform.
- EPIA: Solar photovoltaic electricity empowering the world 2011.
- The EC's Photovoltaic Geographical Information System (PVGIS) provides detailed estimations of the annual electricity yield from a PV installation.
- IEA Photovoltaic Power Systems Programme (PVPS) has relevant downloads including: Review of the life cycle assessment of PV (Report IEA-PVPS T12-03:2011 November 2011).
- Department of Energy & Climate Change - DECC Solar PV Resource.
- The US DoE Small Scale Solar Systems.
- The Carbon Trust Solar PV.
- The DTI Guide to the installation of PV systems provides extensive electrical installation details.
- Solar Trade Association. A well-established trade organisation with some resources.
- British Photovoltaic Association. A relatively new manufacturer and installers based organisation that provides a growing resource of materials.
- The IET's Code of Practice for Grid Connected Solar Photovoltaic Systems
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