Solar Photovoltaics (PV)
This online Renewable Technology Briefing will help to give you an understanding of the technology and application of grid connected solar photovoltaics (PV) to provide electricity for domestic and small commercial buildings.
This briefing will give the key outline and link you directly to fuller explanations on the internet (these are not official B&ES endorsed links), and downloadable documents (in case you want to know more about any aspect). Additionally there will be references to key books and pamphlets that can give you better understanding of the subject – many freely available.
Solar cells, or photovoltaic (PV) cells, convert sunlight directly into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage) - the PV effect that was practically applied just 60 years ago.
The rigid cells (traditionally made of silicon) are about 150mm square and produce a small amount of electricity (at about 0·5 volts) so, to generate a higher voltage, a number of cells (typically 36 or 72) are connected in series to form a PV panel. The panels are then connected together to create a photovoltaic array (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.
A fuller explanation of the different types of cells can be seen here and a 2 minute video of how a PV cell captures the sun's energy here
PV panels are quite different to solar thermal panels that capture the sun's heat to produce hot water although some panels are now manufactured that combine both a PV array and a solar thermal collector as described in this article.
The estimated lifetime of a PV module is 30 years and the performance would be expected to remain over 80% of the initial power after 25 years.
The carbon footprint of manufacturing PV has decreased by approximately 50% in the last 10 years due to performance improvements, raw material savings and manufacturing process improvements.
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 will reduce as they increase in temperature. 1 kWp of well sited PV 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. The chart shows average proportions of the available annual solar radiation in the UK. To obtain a more precise estimation of the potential generated energy for a particular site (anywhere in Europe) look to this online calculator. 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).
The are three principle types of PV cells. 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). They are typically dark coloured with a 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 and glass panels.
The crystalline PVs currently account for over 90% of installed systems.
The cost of the materials is generally highest for the more efficient types of cell although the integration of thin-film technologies into building elements adds to their cost.
PVs can power systems that are totally disconnected from the grid (particularly in rural locations) however the surge in interest in the 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 as well as providing the normal grid supply to the premises when there is insufficient PV generation.
There is relatively little mechanical work associated with the installation of PV panels - their is controlled work that requires properly trained electricians. There is a mature market for the supply of flexible mounting systems. The installation of roof mounted PV panels would normally require a properly trained 'roofing' installer as well as electricians.
PV modules generate electricity whenever they are exposed to daylight and individual modules cannot be switched off so, unlike most other electrical installations, installing a PV system involves working on a live system.
The main types of installation are ...
- roof or wall mounted of framed PVs - the PVs are mounted in framing (that should protrude less than 200mm to satisfy normal planning requirements)
- roof or wall integrated PVs - where the PV 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 the standard tiles/slates
- surface mounted (thin film PV) - the PV is attached to a building component - anything from a glass panel to a flat roof can be covered with semi-flexible thin-film PV.
- 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.
These are illustrated and discussed on the NEF site. Typical installations of PV modules will weigh 10–13 kg/m².
More recent developments have led to a variety of forms of integrated solar collector, including;
- solar shingles (mounted flat on boarded roofs) or slates (mounted on battens) that can replace standard roof components
- solar glass laminates (where the PV takes the form of semi-transparent glazing)
These can offer opportunties to include PV in a project where previously aesthetic considerations would have prevented their use.
In a grid connected system, DC power from the PV modules is fed into an inverter for conversion to 240 V AC power for connection to local electricity network through the consumer unit. The inverter (or 'power control unit') will preferably 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 PV 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. The DTI document contains detailed connection information.
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 PV 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) - normally systems designed for grid systems are not usable directly with battery storage.
PV 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 PVs.
A recent report from the International Energy Agency (IEA) indicated that the life expectancy of the equipment associated with a PV installation was ....
- PV panels: 30 years for mature module technologies
- Inverters: 15 years for small plants (residential PV)
- Structure: 30 years for roof-top and façades, and between 30- to 60-years for ground mount installations on metal supports
- Cabling: 30 years
The 2006 UK Photovoltaic Domestic Field Trial showed that for fairly small system sizes of around 1.6kWp, a significant fraction of the building demand can be met by the PV system with the majority of systems providing between 20 and 80% of the building annual load, with an average of 51%.
In the UK fixing solar panels to a single dwelling house that is not listed building or in a conservation area is considered to be ‘permitted development’ under planning law with no need to apply for planning permission - see the Planning Portal for more detailed guidance. PV installations are notifiable for building regulations purposes and the local building control should be informed.
The District Network Operator (DNO) must be consulted about the 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 - details that guide the design and installation of PV installations. This is essential if government funding is being sought for a project of less than 50kW but does, in any case, provide a useful source of information for larger installations.
PV 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.
FITs comprise 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). Currently (in 2012) in the absence of smart meters to measure this amount of energy being fed back into the grid it is assumed that this will be 50% of the energy produced.
If PVs are installed and the property owner/operator receives FITs, and subsequently the property is passed to another owner, the FITs will remain with the installation and accrue to the new owner.
If a business has PVs installed and is already involved in the Carbon Reduction Commitment (CRC) energy efficiency scheme the output from PV systems (that are registered for FITs) will not be counted as 'zero emission' for CRC purposes but must be accounted for at the grid average. To use PVs to gain CRC credit, FITs can not be claimed
Great resources that will provide more detail on the background and application of PVs
References to buy or borrow
CIBSE TM25 Understanding Building Photovoltaics provides a comprehensive reference in the design and aplication 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
Web Sites and freely downloadable resources
National Energy Foundation have an excellent set of pages covering many aspects of PV and its installation
Energy Saving Trust has a page with up to date guidance and links to appropriate legislation and government web resources as well as a downloadable, and ver readable Buyers Guide to Solar Electricity.
There is an excellent site Your Sun Your Energy produced by the European Photovoltaic Industry Association (EPIA) and the European Commission's European Photovoltaic Technology Platform. A document that provides comprehensive coverage of forecasts for future technology is the Solar photovoltaic electricity empowering the world 2011
The EC's Photovoltaic Geographical Information System (PVGIS) provides detailed estimation of the annual electricity yield from a PV installation
IEA Photovoltaic Power Systems Programme (PVPS) - this has a few relevant downloads including a detailed 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 - A great place to catch up with all the government funding and guidance
The US DoE has a good site with basic introductions to all renewable technologies including Small Scale Solar Systems
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.
Standards and Regulations
There are many British standards that affect PVs - the Microgeneration Installation Standard: MIS 3002 (available at no cost) provides the requirements for installation of solar PV to comply with the MCS scheme and provides a reference to other required standards.