Introduction, Current Technology, What are Photovoltaics, How do Photovoltaics generate, Power, PV materials, How much electricity will be produced, PV systems on housing, Economics, Social Implications, Environmental Aspects, Discussions, Conclusions
Photovoltaic technology can make use of the sun’s energy in many different applications. PV technology has its origins in space technology, designed to power space satellites; the International Space Station has the largest solar power system ever taken into space; it measures 73 metres tip-to-tip and provides enough electricity for 15 homes. There have been advancements made from initial semiconductor research in the 1950s and same technology has found uses in other places , using semi-conducting materials to convert sunlight to electricity.
PV cells are also used in smaller, every day, devices such as solar calculators, watches, torches but are also finding more uses in other objects such as parking meters, telephone boxes, street lighting, as ways of generating their own electricity and reducing demands from the grid.
The ability to generate electricity from sunlight is a relatively new and exciting technology that offers many new opportunities in generating ‘green’ electricity. This technology is called solar photovoltaics or more simply, PV. Also referred to as solar electric, PV offers the ability to generate electricity in a clean, quiet and renewable way. It makes use if the abundant energy from the sun, to generate electricity without the production of harmful carbon dioxide (CO2) emissions, one of the main gases affecting climate change.
There are many possible applications for PV technology but first it is important to understand how the process works.
The photovoltaic process converts sunlight – the most abundant energy source on the planet, directly into electricity. The sun emits photons (light), which generate electricity when they strike a photovoltaic cell. So in the same way a photovoltaic cell, made from a semi-conducting material, is a device that converts light into electricity.
Solar cells are made of silicon, a special type of melted sand, consisting of two or more thin layers of semi conducting material, usually silicon. The layers are given opposite charges – one positive, one negative. When sunlight strikes the solar cell, electrons are knocked loose and move toward the treated front surface of the solar cell. This creates an electron imbalance between the front and back of the cell and causes electricity to flow – the greater the intensity of light, the greater the flow of electricity.
For more information on how PV’s work see the Solar Century Website http://www.solarcentury.co.uk/content.jsp?sectno=0&subno=6
As discussed silicon is the most common material used for PV applications, partly due to the large surplus from the electronics industry. This silicon can vary in states and quality some of which are shown below
Mono-crystalline - PV cells that are made from pure mono-crystalline silicon with almost no defects or impurities
Efficiency approx. 15%
Polycrystalline PV – Is produced using numerous grades of mono-crystalline silicon. This is less expensive to manufacturing due to simpler processes involved in production compared with mono-crystalline.
Efficiency approx. 12%
Amorphous Silicon composed of silicon atoms in a thin layer rather than a crystal structure. Absorbs light more effectively than crystalline so cells can be thinner. Thin film technology can be used in rigid, flexible, curved and foldaway modules. The have a lower cost than crystalline cells but have a lower efficiency
Thin-film PV – Is the most efficient material in poor light conditions, whilst also being an extremely sturdy, vandal-proof PV.
Efficiency approx. 6%
Thick-film PV – Again an efficient cell in poor light conditions with very low embodied energy, it is most environmentally-friendly form of PV.
Pictures sourced from www.solarcentury.co.uk
The electrical output of solar cell is establishes at Standard Test Conditions (STC), where the irradiance is 1000W/m2 and at a cell temperature of 25°C. With the cell operating at these conditions the open circuit voltage (Voc), short circuit current (Isc), voltage at maximum power point (Vmax) and current at maximum power point (Imax) are recorded. From this the maximum power delivered by the cell can be calculated as well as the fill factor (FF) – the cell conversion efficiency.
FF = Vmax x Imax
Voc x Isc
Pmax = Voc x Isc x FF
Cell operating temperatures have an effect on cell conversion efficiencies – an inverse linear relationship. As cell temperature increases the cell efficiency decreases. The hotter the temperature of the cell, the lower the cell efficiency
For more information on how PV’s work see the Acre Website
There are some main components that are key to any PV system; assuming the PV array is mounted and the necessary cables and switches are in place. This equipment is often referred to as balance of system equipment required to complete the system. This includes some of the equipment explained below, and wires, conduit, a grounding circuit, fuses, safety disconnects, outlets, metal structures for supporting the module and any additional parts that are part of the PV system.
Charge Controller regulates the flow of electricity from the PV modules to the battery and the load. This helps keep the battery fully charged without overcharging it.
Batteries are sometimes necessary to store electricity for use during times when it is needed. Or to help meet loads when PV modules are not generating sufficient power to meet load requirements
An Inverter is needed to convert direct current (DC) into alternating current (AC). The power produced tend to be at a low voltage (12V) and household application require 240V
A Meter is required to ensure that the system owner is credited for any excess power that is generated by the PV system and fed into the mains grid.
Pictures sourced from www.solarcentury.co.uk
The effectiveness and efficiency of the system can depend on a series of variables, some of which are listed below:
Location – the amount of sunlight available will affect the amount of energy produced.
Orientation – the orientation of a building and positioning of solar arrays are vital factors to maximise energy production. In Scotland, and the northern hemisphere, a south facing façade will collect most light throughout the year and so produce the most energy.
Tilt Angle – inclining or tilting solar panels towards the sun can increase the levels of light falling on the surface and therefore cause an increase in electricity output. The appropriate angle of inclination will depend upon the latitude of the proposed site – solar modules located on the equator will produce the most energy when they are laid horizontally.
Optimum tilt angles will vary from city to city around the world
Glasgow - 36°, Athens - 30°, Singapore - 0°, Miami - 25°, Nairobi - 5°
In order to best understand how PV systems can work it is intended to illustrate some the applications that they are used for:
At present many people turn only to PV technology as a last resort, usually in remote locations that are difficult to access or where the cost of connection to the grid is expensive. In these locations stand-alone systems are already proving economically viable. Such systems are independent of the grid and any excess energy produced would usually be stored in a battery.
As part of a refurbishment to a shepherd's house on Mindrum Farm, Northumberland, Solar Century designed and installed a stand- alone PV/ diesel hybrid power system. The system was able to provide secure power for the house’s domestic loads whilst also being able to cope with occasional surges from agricultural activities. Before refurbishment the house had no electricity and it would have cost £35,000 to connect it to the grid. By comparison the total cost for the PV / diesel system was £27,500.
Grid connection – The awareness of PV technology is rising and there is an increasing number of systems beginning to appear. As concern rises for the need of ‘clean’ electricity and more individuals and businesses keen to implement such systems, a larger number are being connected to local electricity networks, this is known as embedded generation. In such a system the grid acts as the ‘storage’ facility. During the day electricity generated can either be used immediately (which is normal for PV applications on offices or commercial buildings) or it can be sold to an electricity supply company (more typical of domestic systems where the occupier may be out during the day). In the evening when the solar system is not able to generate the electricity required power can be bought back from the network.
In the UK there is currently no obligation for utility suppliers to purchase electricity generated by PV applications. Customers that have installed PV systems with the capacity to export electricity to the grid usually have two meters installed. One meter recording how much electricity they buy and another to record what they sell. While customers pay in the region of 7-8p/kWh for electricity, they may only be paid around 2-3p/kWh for electricity exported to the grid. In this way there is little incentive for individuals to generate electricity from PV systems in Scotland.
One method used in other countries, to encourage people to install PV systems, is Net Metering. A single meter is installed, running normally when electricity is being drawn from the grid, and it effectively runs in reverse when power is exported to the mains supply. In this way the owner is paid the same price for electricity exported as they pay the electricity supply company for the mains electricity.
Building integrated PV - Use of photovoltaics on buildings is also growing substantially, with increasing examples in operation in the UK. PV systems can be incorporated into buildings in many ways, with south facing rooftops or facades an ideal site for mounting modules. Also they can be incorporated into the building fabric, roof tiles, cladding materials, canopies and skylights.
Doxford International Business Park, Sunderland is the first speculatively constructed office building to incorporate building-integrated photovoltaics. It main façade has a cell area of 532m2, the 73kW (peak) PV facade at the Doxford Solar Office is the largest in Europe to date and will provide an estimated 55,100kWh of electricity a year.
The building was designed in accordance with passive solar principles to minimise its energy requirement. The low-energy measures include generous ceiling heights, which encourage day lighting and cross- ventilation, a well-insulated and well-sealed building envelope to minimize winter heat losses, and the provision of effective and responsive environmental controls.
“Britain is losing out to countries that have created a large home market, by introducing market stimulation measures and low manufacture costs.”
BRITISH PV ASSOCIATION
PV technology has a long way to go before establishing itself competitively with conventional electricity and other Renewables. Photovoltaic technology costs typically range from 60-70p/kWh and is viewed by the government as a long term project with anticipated price by 2020 of 10–16 p/kWh based on the current learning rate and market growth rate, with the possibility of becoming cost competitive with retail electricity in the UK around 2025 - current costs for onshore wind in good sites are in the region of from 2.5–3.0 p/kWh and around 8p/kWh for energy crop.
The British Photovoltaic Association aims to increase the market penetration of PV to 15% by 2010, by installing a 300MWp capacity within the UK, how much of this – if any would be planned for Scotland is unknown.
However, if such a venture were to prove successful there would be potential for
· Potential turnover of £1.2 billion
· Employment increase in the PV sector to around 19,000 by British companies
See http://www.pv-uk.org.uk/ for more information
A typical domestic system is around 1.5-2kWp, comprising of 20-30 modules depending on the technology used and the orientation with respect to the sun. Such a system would typically occupy between 12-20m2 of surface area, depending on the efficiency of the array – amorphous silicon would require nearly 50m2. The costs of a 2kWp system can range from £8,000 - £15,000. At such prices it can be assumed that PV is not going to play an significant part in the near future, but as interest and development improves this could change. Each system will produce between 750kWh and 1500kWh of electricity per year, depending on the orientation.
If we consider the use of integrated PV within buildings, prices compare unfavourably with £50-100/m2 for domestic roofing, with £150-800/m2 for curtain walling. Installed costs of integrated PV are in the range of £500-1000/m2 this is comparable with high quality facades used on prestige buildings where marble and other polished stones can cost £1000/m2. In this way PV panels on commercial buildings, are already cost-effective when compared with prestige cladding materials.
The client has to be convinced that not only is he paying for a high quality finish but is also getting some added value with electricity being generated in conjunction with the impressive new façade. It is unlikely that photovoltaics will benefit from the Renewables Obligation in the period to 2010 unless costs fall more rapidly than is currently anticipated but this is not to say it they could not play an important future role in reducing atmospheric pollution.
“The total number of people employed in the PV sector in the UK is less than 400. Jan 1999 “ BRITISH PV ASSOCIATION
Countries, such as Germany, the Netherlands, Japan and the USA have created programmes in order to establish a market for PV technologies and offset the high costs, which has led to employment opportunities in the PV field, as well as associated professions.
The lack of market demand in the UK is one of the major factors preventing the growth of PV, and subsequent employment, in the UK on a larger scale. There is a need for more government investment and new policy, alongside commitment from industry to provide a driving force for a future domestic market, which would lead to, increased employment opportunities.
The British Photovoltaic Association predict if they are successful in increasing market by installing systems totalling 300MWp, there would be an employment increase in the PV sector to around 19,000 by British companies, how many of these would be in Scotland is unknown.
During operation, PV and solar thermal technologies produce no air pollution, little or no noise, and require no transportable fuels.
One environmental worry with solar technologies is the lead-acid batteries that are used with some systems for storage purposes. A second environmental concern with solar technologies is the difficulty or recycling heavy metals such as cadmium, which are used in PV cells. Just as there is a large worry about the large amount of discarded personal computers that may pile up and leach cadmium, mercury, and lead into the environment, there is a worry that the cadmium used in discarded PV panels may also be an environmental threat. The use of cadmium sulphide in the production of PV panels is on the rise (replacing the more expensive silicon) this is an issue that should be considered. The environmental impact of silicon PV cells in manufacture is insignificant, however some fossil fuels are used.
PV technologies have several characteristics in their favour:
• No CO2 emissions in use – 1kWp of solar cells displaces 1000 kg of CO2.
• No moving parts – requiring minimal maintenance and predicted life spans of 15-20 years, comparable with that of fossil fuel plants.
• Operation is virtually silent – unless very large transformers are required.
• Generation is at point of use, thus avoiding transmission and distribution costs.
• Cost effective in remote locations where grid connection is to expensive.
Since the environmental impact of solar technologies is relatively small, it is perhaps more beneficial to take a look at the enormous amount of pollution that is prevented due to the use of solar technologies. There is a significant benefit of solar power is the impact on air quality and other aspects of the environment.
The amount of emissions that can be prevented through the use of a small PV system is surprising. A one-kilowatt solar electric system generates enough energy to save one tonne of CO2 emissions every year in the UK when compared to coal power station emissions.
Emissions are avoided annually by using a one-kilowatt solar electric system.
2.27 kg of Nitrogen Oxides (NOx)
6.35 kg of Sulphur Dioxide (SO2)
1000 kg of Carbon Dioxide (CO2)
Many countries have tried to advance PV technology through their own initiatives. Countries, including Germany, Japan, the Netherlands, Norway and the USA have initiated programmes of government investment, in collaboration with industry, which have lead to thousands of solar electric homes being built around the world.
For example; Germany’s “1,000 roofs” programme launched in 1990 has resulted in more than 3,000 houses being fitted with solar electric panels; So much so that the Government went onto introduce a 10,000 roofs programme and between 1990-1996, they quadrupled there installed PV capacity to 23 MWp. By 1997 a further 10MWp was installed creating a positive environment for PV. Germany has established themselves as world leaders through such measures and extensive R&D and technological developments.
Scotland, and the UK, do not currently have a suitably large home market to display its expertise in PV technology or the financial incentives to develop this market further.
The UK 100 PV Roofs Domestic Field Trial - In February 1999, the DTI announced it would provide funding for a field trial of 100 domestic PV installations. Such initiatives will help raise awareness of PV technologies, but much greater effort is needed for systems to be mass produced which will lead to significant savings. More widely available subsidies are needed for this technology to become market competitive, and this will include a favourable buy out rate for the electricity generated. Funding is on the increase for PV technologies and as awareness increases more people may become interested and willing to invest in the technology
When Scotland, and the UK, is compared with Germany the gulf is clear to see. German Government have provided subsides of around 70% to help ease the capital costs of solar electric and the utilities are legally obliged to pay a fairer price – around 90% of the price they charge for conventional electricity.
Photovoltaic technology itself is still expensive due to lack of demand and also due to lack of information available. Despite this obvious price disadvantage other obstacles have to be overcome for PV technologies:
· Conversion efficiencies are currently low only 5-18% but may improve with further research and development.
· Domestic PV systems do not necessarily deliver power when it is needed. Electricity is generated during the day when occupants are out and not during the evening when families are at home.
· Storage costs are high but can be avoided by connection to the mains whereby surplus can be exported to the grid – but the number of new connections the grid could cope with is not clear – assuming it is of a high enough quality.
· PV electricity is DC, not AC and as such an inverter is required for conversion adding further costs to the system.
· If PV cells get hot electricity generation falls significantly. Integrated PV arrays need ventilation to cool then at the back, typically natural draught.
· Grid Connection currently has associated problems and permission is required from the local distribution network operator – the quality of electricity has to be considered when exporting to the grid
For more information regarding Grid Connection
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The technology is reliable and relatively simple to install and easy to maintain. Considering the expertise that exists in the UK it is strange that PV does not play a greater part in our lives. One of the main reasons is that electric power generated from PV is too expensive to compete in Scotland and the UK due to the low prices of fossil-fuel, nuclear and even wind power.
With this absence of any significant government support for PV in Scotland, PV is struggling to compete economically with traditional methods of generation and other renewables until further research and development drives costs down.
The recently published Energy Review; Feb 2002: by the Performance and Innovation Unit suggests that any long-term framework for British energy “needs to consider the role of renewable energy, including solar water heating, especially in electrically heated buildings”. It states that “building integrated photovoltaics (BIPV) have the most potential”. It has been highlighted that these are currently expensive, but are already economic as an alternative to expensive cladding. Analysis indicates that BIPV will approach cost-effectiveness sometime after 2020.
It has been highlighted that there is a vast resource available and PV technology is one of the most feasible renewable energy’s for electricity generation within the urban environment. Successful deployment of PV cells on building facades or roofs will greatly reduce the need for additional land for electricity generation from new generation stations.
The Government is committed to expanding its supporting programme for renewables including research, development, demonstration and dissemination. The main current hurdle preventing large-scale manufacture in the UK is the current market or lack of it. The understanding and potential of photovoltaics is improving, but further Research and Development is required to capture cost-reductions. It is important that strong partnerships are established between industry and government.
Increasing environmental concerns and the need to achieve emission reduction targets should help the technology to become further established as a marketable and economically viable product. The British Government signed up to Agenda 21, a global environment and development action plan, at the Rio Earth Summit in 1992. One of the key areas addressed by Agenda 21 was the issue of sustainable development; installing PV systems could be one way to encourage such progress. PV produced clean electricity can displace power generated from fossil fuels; it is this benefit that could lead to its future success.
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