Introduction to Solar Photovoltaics (PV)
Solar cells are the small dark segments of silicon that you can see behind the glass of a solar module. The cells are the building blocks for photovoltaic (PV) solar panels. Solar cells use light energy (photons) from the sun to generate electricity through the photovoltaic effect. The majority of cells are wafer-based crystalline silicon cells or thin-film cells based on cadmium telluride or silicon.
Busbars are a thin strip of copper or aluminium placed between cells. It separates solar cells and conducts the direct current the cells collect from solar photons to the junction box mounted in the rear of the panel.
Solar modules (or sometimes referred to as panels) are what you see on the roof or mounted in frames on the ground. They encapsulate the cells, busbars and diodes (if used) between a layer of glass and insulating backing material. The module generates electrical energy when exposed to sunlight. Most solar modules are rigid, but semi-flexible ones are available, based on thin-film cells. Bypass diodes may be incorporated or used externally, in case of partial module shading, to maximize the output of module sections still illuminated.
Strings or stringing is the used to describe how array modules are electrically connected together. Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired current capability.
Solar collector is more commonly used in relation to solar thermal technologies which use the suns thermal energy to produce hot water, but do not generate electricity. These can either be used in new-build or retro-fit situations and comprise of a collector which is typically mounted on the roof and is connected via a pump to the hot water tank. Indirect solar thermal systems require a twin-coil tank, direct systems use do not need a separate coil.
Retrofit: This is generally where the PV modules or solar thermal collectors are mounted above the tiles or slates on pitched roofs or mounted on frames where the roof is flat.
Ground Mounted: Ground mounted systems tend to be much larger and are commonly used for systems of MW scale. All that is needed in this instance is a relatively flat, unshaded area of ground and permission from the local planning authority.
Building integrated: Building Integrated Photovoltaics (BIPV) can be used as part of the fabric of the building. This can be in the form of in-roof (instead of the roof fabric of the building), façades or glass substitution. These solutions are popular with architects, with facades, atriums and canopies also offering the added benefit of providing shade. Canopies are increasing be being used in canopies.
Which is the better solar PV cell technology?
The cell technology refers to the materials and techniques used to construct the solar cells. There are many varieties of cell technology, each have their own advantages and disadvantages.
In the past, silicon based technologies (monocrystalline and polycrystalline) have been able to achieve higher efficiencies, ranging between 13% – 20%. Statistics show that monocrystalline modules perform better in direct south-facing applications whilst polycrystalline technologies perform equally to monocrystalline systems and even occasionally better in more easterly and westerly applications. Monocrystalline are thought to perform better in clear sky conditions whereas polycrystalline modules perform well in more diffuse light. Although monocrystalline modules have been used for decades and are proven to be reliable long term, polycrystalline is now cheaper option due to lower manufacturing costs.
Thin film technology efficiencies range between 4% – 14%. Thin film technologies are the more efficient choice when used in higher temperatures and perform well in diffuse light. Technologies producing only 4% efficiency will often be found on flexible substrates, for example the thin photovoltaic film used on products such as backpacks and roll mats. Thin film technologies are often frameless, which removes the efficiency offset from the high embodied energy of the aluminium frames used for some other systems.
Hybrid technologies are also available, combining multiple solar cell technologies to improve the performance of a single solar module through a range of sunlight conditions. For example, combining monocrystalline and thin film technologies extends the cells performance in a wider range of the UV spectrum.
Continual international investment in R&D to solar cell technologies is encouraging new players in the market. Pervoskite solar cells are progressing in leaps and bounds, with reported efficiencies of 3.8% in 2009 to nearly 18% achieved in 2014. Although it is still has a way to go before becoming commercially available it is a good example of new solar technologies on the horizon.
Is solar energy viable in the UK?
There are now 100,000's of solar systems installed in the UK which is testament to the fact that this is now a well-established technology. A typical domestic 4kW PV system, installed on a south facing roof, with a slope of 35°, could be expected to generate around 3,600 – 4000 kWh per year. In comparison, the average UK household uses approximately 3300kWh per year, so more than enough to power a typical home. As there is more light in the summer generation will be higher, so excess electricity can be exported back to the grid and for which you will be paid. With energy storage solutions becoming available it will be possible for homes and businesses to store and consume more of their electricity.
Solar thermal uses the sun’s rays to create usable heat; there is plenty of sun in the UK to make it effective. Heat cannot be exported to the grid so needs to be accompanied by an efficient storage system in the form of a hot water tank.
How long will my system last?
Solar module manufacturers usually offer warranties of at least 20 years on their products. Indeed there are modules still working that have been installed for considerably longer than this.
Will my PV system continue to work during a power cut?
In the UK, all grid connected solar inverters must meet G83 or G59 regulations which means they automatically shut down during a grid power cut and automatically start-up again when the grid comes back online. Therefore all grid connected solar pv systems must be disabled when the grid goes down, thus protecting engineers working on the grid to rectify the fault.
How much maintenance do solar power systems require?
A well designed and installed system needs little if any maintenance. If the system is located near trees or a heavily industrialised energy then occasion cleaning the glass surface of the modules maybe necessary to maintain output. The inverter has a shorter life than the modules and may need replacing every 10-15 years.
Large Scale Solar Systems
Do I need planning permission for a commercial PV system?
The installation of solar panels on non-domestic buildings and land may be 'permitted development' with no need to apply to the Local Planning Authority for planning permission. There are, however, important limits and conditions, detailed on the following pages, which must be met to benefit from these permitted development rights. Non domestic buildings and land for the purposes of these permitted development rights includes businesses and community buildings. Permitted development rights are also available for domestic properties. For more information please visit the Planning Portal.
Is there planning guidance for ground mounted systems?
Yes, the BRE National Solar Centre has published ‘Planning Guidance for the Development of Large Scale Ground Mounted Solar PV systems’ which provides best practice planning guidance in respect of how large ground mounted arrays are developed, setting out planning considerations and requirements.
Can I keep livestock in the same field as a pv system?
Yes, it is possible to graze small livestock such as sheep on land where a solar pv system is installed. There are benefits from doing this such as keeping the weeds down, and the solar arrays providing shade for the animals.
The BRE National Solar Centre has published two documents regarding the symbiotic relationship between plants, animals and large scale solar PV systems. The BRE NSC’s ‘Biodiversity Guidance for Solar Developments’ helps various stakeholders better understand how to support biodiversity on solar farms. A further document, ‘Agricultural Good Practice Guidance for Solar Farms’, details the principles of good practice for the management of small livestock in solar farms.
Do I need to contact someone about connecting to the grid?
This is something your installer will do for you.
Building Integrated PV
How do I integrate PV into my building?
There are many different ways to integrate solar PV into your building for example; roof tiles, shutters, anti-glare measures, balconies, brise soleil and car ports. The easiest way to incorporate BIPV is into newly built structures but it is possible to retrofit them to existing buildings too.
Any MCS Certified installer is certified to install BIPV. As with any contractor we would recommend that you request references or view a previously installed system.
Is planning permission necessary for BIPV systems?
The installation of solar panels and equipment on residential buildings and land may be 'permitted development' with no need to apply to the Local Planning Authority for planning permission. There are, however, important limits and conditions, detailed on the following pages, which must be met to benefit from these permitted development rights. It is likely to require planning permission to install If the building is listed or within a designated area. Building integrated solar power systems are more likely to get planning permission in these areas but always seek guidance from your local authority before going commencing an installation.
Does my roof need to be south facing?
An unobstructed system facing due south inclined to around 35o is likely to yield the high output. However, pitched roofs facing east and/or west can also be an effective arrangement for a solar power system. On average an east or west facing system can typically achieve up to two thirds the energy yield of a south facing system (assuming a typical roof slope). Systems spilt between east and west-facing roofs produce an evenly distributed energy profile because they capture morning, midday and evening sunshine rather than generating a large peak of electricity at midday. If only flat roof spaces are available solar technologies can be mounted on frames or even installed flat onto the roof.
Do I need planning permission?
The installation of solar panels and equipment on residential buildings and land may be 'permitted development' with no need to apply to the Local Planning Authority for planning permission. There are, however, important limits and conditions which must be met to benefit from these permitted development rights. For more information please visit the Planning Portal. You should discuss with the Local Planning Authority for your area whether all of the limits and conditions will be met.
What is the Microgeneration Certification Scheme (MCS)?
Microgeneration Certification Scheme (MCS) is a nationally recognised quality assurance scheme, supported by the Department of Energy and Climate Change. MCS certifies microgeneration technologies used to produce electricity and heat from renewable sources. MCS is also an eligibility requirement for the Government's financial incentives, which include the Feed-in Tariff and the Renewable Heat Incentive. It is an industry-led and nationally recognised quality assurance scheme, supported by the Department of Energy and Climate Change (DECC). MCS itself is a BS EN ISO/IEC 17065:2012 Scheme and was launched in 2008.
MCS certifies microgeneration products used to produce electricity and heat from renewable sources. MCS also certifies installation companies to ensure the microgeneration products have been installed and commissioned to the highest standard for the consumer. The certification is based on a set of installer standards and product scheme requirements which are available in the MCS Standards section of their website.
How much does a system cost?
Since Solar pv costs have more than halved over the past 5 years and it is now possible to get a 4kW system installed for less than £6,000.
Feed-in-Tariff (FiT) scheme exists for PV and the Renewable Heat Incentive (RHI) for solar thermal. These are the main methods of receiving income from most solar installations. The latest FiT rates can be seen here Ofgem Tariff Tables and the RHI rates can be found on the Government’s RHI Webpage.
Is an Energy Performance Certificate (EPC) needed to claim the FiT?
Yes, to qualify for the FiT at the standard rate for solar power your property must have an EPC rating of D or above. If your property is not rated as D or above, you can improve your EPC rating by installing energy efficiency measures, or you can opt to receive the FiT at the lower rate for the lifetime of the tariff.
What is a ‘rent a roof’ arrangement?
Some companies offer to install a solar pv system which they continue to own, but claim the FiT payments for the electricity generated and also the value of the export to the grid. These arrangements are usually termed ‘rent a roof’. The property owner benefits by receiving free electricity when the system is generating. Sometimes an additional rental payment is also offered depending on the size of the system. It is advisable to check to see whether these schemes are right for you as their maybe alternatives that offer a better return.
How can I tell if my system is working?
You should regularly check your wireless monitoring facility, if installed, to ensure generation is in line with expectations and there are no warning messages. If there is no wireless monitoring facility than a regular check of the generation meter would be worthwhile. You can contact your solar installer for more details about the monitoring options available on your system and how to access them and read them correctly.
How do I make a complaint against a solar company?
The Renewable Energy Consumer Code was set up by the Renewable Energy Association. Its aim is to guarantee a high quality experience for consumers wishing to buy or lease small-scale energy generation systems for their homes. The Renewable Energy Consumer Code logo is a sign that the company has agreed to abide by the high standards set out in our Consumer Code. All MCS accredited installers must be a member of a recognised consumer code such as RECC.
AC (Alternating Current) – an electrical current that regularly changes the direction in which it moves. Mains electricity in the UK is supplied at about 230 volts.
Big Six Energy Suppliers – Britain’s largest gas and electricity suppliers to over 50 million homes and businesses in Britain, with over 90% share of domestic customers. They consist of EDF Energy, nPower, British Gas, Scottish Power, SSE and E.ON UK.
CSR (Corporate Social Responsibility) – a company’s sense of responsibility towards the community and environment (both environmental and social) in which it operates.
DC (Direct Current) – an electrical current which flows in only one direction. DC is produced by batteries, generators and fuel cells.
DNO (Distribution Network Operator) – companies licensed to distribute electricity in Great Britain by the Office of Gas and Electricity Markets.
Energy Performance Contract – a partnership between the customer and business to improve the energy efficiency of your buildings and facilities.
HH (Half Hourly) – meters which record total consumption every half-hour and pass the information to the energy supplier.
IPMVP (International Performance Measurement & Verification Protocol) – framework which defines standard terms and suggests best practise for quantifying the results of energy efficiency investments.
KVA (Kilovoltampere) – an electrical unit, equal to 1000 volt-amperes.
kWh (Kilowatt Hour) – the electricity consumed or generated every hour. A kilowatt comprises of 1000 watts.
kWp (Kilowatt Peak) – the great amount of possible electricity generation from a system.
kWth (Kilowatt Thermal) – kWth is the unit of heat supply capacity measuring the potential output from a heating plant.
M&V (Measurement & Verification) – term given to the process for quantifying savings delivered by an Energy Conservation Measure (ECM), as well as the sub-sector of the energy industry involved with this practice.
MWh (Megawatt Hour) – is a unit of energy equal to 1,000 watt-hours.
MPAN (Meter Point Administration Number) – unique reference number for your electricity supply.
MPRN (Meter Point Reference Number) – unique reference number for your gas supply.
Non-Big Six – alternatives to the Big Six energy suppliers.
Single phase – refers to a two wire Alternating Current (AC) power circuit. Typically there is one live wire and one neutral wire.
Standing charge – fixed costs associated with providing your energy supply such as meter reading, maintenance and the cost of keeping you connected to the network.
Three phase – electric power systems have at least three conductors carrying alternating current voltages. It is much more efficient for high-capacity installations, but costly to install.
CIBSE (Chartered Institution of Building Services Engineers) – professional body that supports the science, art and practice of building services engineering. CIBSE have a dedicated energy centre and provide benchmarking for energy efficiency projects.
DECC (Department of Energy and Climate Change) – responsible for ensuring the UK has secure, affordable energy and for the UK’s role in domestic and international efforts to avoid dangerous climate change.
National Grid – high-voltage electric power transmission network connecting power stations and major substations and ensuring that electricity generated anywhere in England, Scotland and Wales.
Ofgem (Office of Gas and Electricity Markets) – protects the interests of gas and electricity consumers in Great Britain by promoting competition and regulating monopoly network companies.
Capacity Market – aims to ensure there is enough power generation in the system to meet peak demand.
CfDs (Contracts for Difference) – comprised of a ‘Reference Price’ and a ‘Strike Price’. The reference price is the price the Government believes the generator receives for their electricity sales in the market. The strike price is the level of support the Government determines is acceptable to a generator to encourage investment in generation.
EDR (Electricity Demand Reduction) – a government incentive offering an initial £20m of funding for energy efficiency projects.
FITS (Feed in Tariff Scheme) – a government programme designed to promote the uptake of a range of small-scale renewable and low-carbon electricity generation technologies.
RHI (Renewable Heat Incentive) – pays participants who generate and use renewable energy to heat their buildings.
ROCs (Renewable Obligation Certificates) – certificates used to trade renewable electricity under the Renewables Obligation.
UK Solar PV Strategy – sets out government ambition to see solar rolled out more widely and its potential to support tens of thousands of jobs.
ROO-FIT Pending Data: Applications (these figures are extracted from Ofgem’s Renewables and CHP Register and include all applications currently pending accreditation. The covering note states that the data have not been quality assured and probably contain inaccuracies).
DECC's explanatory note on Degression and Preliminary Accreditation Processes (August 2012)
- September 2013 Energy Trends article: New Solar Photovoltaics deployment Table
- September 2014 Energy Trends article: Analysis of Feed-in Tariff generation data
- December 2014 Energy Trends article: Energy usage in households with Solar PV installations