Technical aspects

home >  faqs > Last updated: 6 February 2002

What is a photovoltaic power system?
The photovoltaic (PV) process converts sunlight - the most abundant renewable energy source on the planet - directly into electricity. The equipment required for this process has no moving parts and as a result requires minimal maintenance. In addition, the electricity is generated with no emissions and no noise. The basic power-generating element is a photovoltaic module. Modules are made out of photovoltaic cells. A photovoltaic cell consists of semiconducting material, most commonly silicon. When the cell is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (d.c.). The electrical output from a single cell is small (around 0,6 V d.c.), therefore multiple cells are connected together to provide a more useful output. Photovoltaic cells connected in this way are encapsulated (usually behind glass) to form a weatherproof photovoltaic module.
Multiple photovoltaic modules can likewise be connected together (then called a photovoltaic array) in order to provide sufficient power for common electrical uses.
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What is a grid-connected photovoltaic power system, and what are its main features?
A grid-connected photovoltaic power system consists mainly of the photovoltaic (PV) modules that convert sunlight into direct current electricity and an inverter that converts the direct current to alternating current to make the electricity compatible with the local electricity grid. Various mounting systems can be used to support the photovoltaic modules on the roof, on the ground or on a building façade. Increasingly architects and builders are integrating the PV system into the building design. Most grid-connected photovoltaic power systems do not require storage batteries.
About ten square metres of photovoltaic modules would rate a power of 1,2 kW and, depending on the mountings used, weighs about 150 kg. Silicon, glass and metal would typically be the main construction materials involved. Over a period of time such a photovoltaic array would produce roughly one third to a half of the energy requirements of a typical household in northern Europe. Photovoltaic power systems can be large or small to suit different applications (for example a house or commercial/industrial building), and can also be sized according to the proportion of electricity that is to be provided by the system.
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Could one power a house and become independent from the electricity utility grid?
Installing a photovoltaic array on top of one's roof or in place of a roof is possible and there are around 70 000 houses in OECD countries equipped that way. Nevertheless one still needs grid-connection to power the house when the weather is bad. So one should not disconnect from the electricity grid. Over the whole year a photovoltaic power system might produce the amount of electricity consumed (excluding electric heating). In the case of net metering the electricity bill would be zero and one may earn money if the electricity buy-back rate proposed by the utility company is attractive.
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Can one heat a house with a photovoltaic power system?
Theoretically yes since photovoltaic power systems produce electric current. But it is wiser and cheaper to install a solar thermal hot water system.
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How much will I need to power a home?
A typical domestic photovoltaic power system of 1,5 kW (12 m2) would produce around a third (northern Europe) to a half of the annual demand of an average urban family household (taking the average demand to be around 10 kWh per day). However, calculating the system size depends on many factors, for example whether the system is grid-connected, energy demand of the household, etc.
These factors will all be taken into account by a system installer when designing a photovoltaic power system.
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Is the technology reliable and how long does it last?
Although photovoltaics is a relatively young technology in terms of market application, the experience accumulated confirms photovoltaic energy as a proven technology able to generate electricity for 25 years or more without intervention. This, and extensive testing, allows the module manufacturers to offer extensive guarantees of performance. Photovoltaic modules are tested to national and International Standards such as those of Technical Committee 82 (TC82) of the International Electrotechnical Commission (IEC, http://www.iec.ch).
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What factors should one consider before installing a photovoltaic power system?
The decision to install a photovoltaic power system should take into account the following:
The photovoltaic modules (whether mounted on the roof or elsewhere) should have access to considerable direct sunlight all year. This means mainly unimpeded exposure to a semi-circle of most of the sky when looking south (northern hemisphere) or north (southern hemisphere), with a low likelihood of significant trees or buildings close by in this direction in the future.
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How can one connect a photovoltaic power system to the grid?
Connecting a photovoltaic power system to the electricity network will require permission from the Network Operator. The operators have different policies when it comes to connecting photovoltaic systems to their networks, and also different rates will be paid for exported electricity. The system installer will make the necessary arrangements for grid-connection, in many cases.
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What must one do to operate the photovoltaic array effectively?
Once installed, the photovoltaic array (module assembly) requires very little attention from the householder or building owner. The photovoltaic modules contain no moving parts and cost nothing to run, using no fuel or consumables. The photovoltaic array operates silently and safely, produces no wastes and requires only minimal maintenance (such as cleaning dust from the module surface in dry dusty places).
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Will a grid-connected photovoltaic power system still work when the grid is out of order?
No, in case of a lack of power coming from the grid, the inverter of the photovoltaic power system will shut off to protect people repairing the grid (an inverter is an electronic device converting PV module d.c. current into a.c. current compatible with the grid operator requirements). But it is possible to have a system with a self-regulating inverter and a storage battery back up to operate independently from the grid. These photovoltaic power systems are normally more expensive than simple grid-connected systems.
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Will photovoltaic modules produce electricity even when there is no sun (cloudy sky or fog)?
Yes, photovoltaic modules work also under these conditions but will produce much less electricity than with direct sun light.
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Do photovoltaic modules need to be cleaned and how often?
Usually no. Rain does the job. In dry areas and low latitudes sweeping dust might be necessary. The method will depend upon situation and application.
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To install a photovoltaic power system, who should one contact?
System installers usually carry out installations of domestic photovoltaic power systems. Contact Professional associations.
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What is the land area covered by a centralized photovoltaic power station?
Pilot photovoltaic power plants such as Erga's plant in southern Italy occupy 1,2 ha (12 000 m²) per megawatt of power installed. A project within the IEA  PVPS cooperative programme is carefully studying all the implications (economical, technical, sociological, financial, etc.) of equipping desert areas with large photovoltaic power plants.
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How much energy does a photovoltaic power system produce per year?
Whether a centralized photovoltaic power plant system or a grid-connected distributed "photovoltaic roof", the electricity production will depend upon the geographic situation. In Italy, the Erga 3,3 MW demonstration photovoltaic pilot centralized power system produces annually 1 300 kWh of electricity per kilowatt of installed power. With photovoltaic systems using concentrated light (such as the pilot plant installed in the Canary Islands) the annual energy production reaches 1 400 kWh per kilowatt of power installed. A practical limit is 1 600 kWh per kilowatt of power installed per year. The power ratio is 75% to 80%.
In Japan a photovoltaic power system of 1 kW will produce in average 1 200 kWh of electricity per year. In Germany, 600 kWh to 1 000 kWh of electricity per year is produced per kilowatt installed.
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The photovoltaic modules never pay back the amount of energy they consume in the process of their construction, do they?
False. Studies show that photovoltaic modules made out of crystalline silicon pay back their energy content in 3 to 5 years, while new type of modules made out of thin films pay back in 2 to 3 years. Technological progresses will considerably shorten this energy payback time. Most of the companies guarantee their photovoltaic modules for 25 years.
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What is the lifetime of photovoltaic modules?
There is no consumption of matter in the process of converting solar light into electricity through the semiconductors the photovoltaic cells and the photovoltaic modules are made of. So the lifetime is theoretically infinite. The only limitation is the way the photovoltaic modules (weather proof encapsulated photovoltaic cells) are able to withstand the aggressive environment they are put in. Some manufacturers guarantee their modules up to 25 years. There are many photovoltaic modules installed by the beginning of the photovoltaic terrestrial era in 1970s and still working well. Humid and salty climates are very restricting (water and electricity do not like each other) nevertheless photovoltaic modules resist well. International Standards like IEC 61215 and IEC 61646 test the resistance of photovoltaic modules under harsh conditions. When buying a photovoltaic module it is recommended to refer to International Standards of the International Electrotechnical Commission (http://www.iec.ch).
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Does photovoltaic technology need bright sunshine to work properly?
The electrical output of a photovoltaic module is dependent upon the intensity of the light to which it is exposed. So photovoltaic modules will tend to generate more electricity on bright days than when skies are overcast. However, photovoltaics do not need to be in direct sunlight to work, so even on overcast days a photovoltaic module will be generating some electricity.
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Are photovoltaic power systems suitable for use in northern countries?
Photovoltaic power systems have been used in Canada, Germany and Finland over the last 20 years or more for many applications, particularly in remote areas where grid-connection is impractical, such as weather monitoring stations, marine navigation aids, week-end cottages, etc.
Over the last few years photovoltaic technology has also started to be introduced into urban areas, incorporated into the roofs and façades of homes, offices and factories.
A modest sized domestic grid-connected photovoltaic power system will provide a substantial portion of a household's electricity needs for over 6 months of the year and installations on commercial buildings are particularly suitable, contributing towards the daytime demands of an office.
Over 400 MW of grid-connected photovoltaic power systems are already installed in OECD countries (2000).
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What applications are there for photovoltaic power systems?
Photovoltaic technology has many applications, both for off-grid systems in remote areas and grid-connected in the case of integration onto buildings situated in the urban environment. Photovoltaic power systems have been used for many years in applications such as powering remote dwellings, telecommunication repeater stations, water pumping stations, monitoring stations, to name just a few examples.
Photovoltaic technology is also widely used in the developing world. The technology is particularly suited where electricity grids are unreliable or non-existent, with remote locations often making photovoltaic power supply the most economic option. In addition, many developing countries have a high level of solar irradiation year round.
In more recent years, photovoltaics has become more widely used in urban areas, where it can be integrated into new buildings or mounted onto existing buildings. This is a rapidly growing application. Photovoltaic technology is ideally suited to the urban environment, providing pollution and noise free electricity without using extra space.
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How long will photovoltaic power systems last?
The average lifetime of a photovoltaic module can be in excess of 25 years, crystalline silicon modules in particular have a very long life span. In addition, they require very little maintenance. Other system elements will have a varied lifespan; for example storage batteries in stand-alone systems can last between 2 years and 12 years depending on type, price and maintenance schemes. Lead-acid accumulators are mostly used.
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What is the difference in energy coming from the sun, between places near the equator and further north or south?
In the Sahara a typical annual value is 2 500 kWh/m2 and in for example Sweden a typical annual value is around 1 000 kWh/m2. So, even quite far from the equator photovoltaic modules can generate electricity.
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What is the theoretical maximal efficiency for photovoltaic cells?
The practical limit of the conversion efficiency for photovoltaic cells is around 30% (single junction: at laboratory level in 2000 the record efficiency is 25%). This is due to material properties of the photovoltaic cell, which limit the absorption of the sunlight. The energy from the sun (photons) has a spectral distribution and only a limited range of this distribution can be absorbed effectively (around 400 nm - 800 nm, visible light). Some of this absorbed energy will also be lost as heat. Under concentrated light conversion efficiency more than 40% is achievable with multijunction cells.
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What are peak power and peak-watt (Wp)?
Peak power and peak-watt (Wp) are the terms used by photovoltaic professionals. These terms refer to measurement of the power output of a cell or a module under standard test conditions (STC): incident irradiance G0= 1 000 W×m-2 (bright sunshine) with the reference solar spectral irradiance air mass 1,5 and a cell or module temperature of 25°C. The power of an installed photovoltaic power system refers to STC. The SI unit is watt (W) but professionals might use the word peak watt (Wp).
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