What is a photovoltaic power system?
A photovoltaic (PV) power system converts the sun’s energy into electricity. The main component of a PV system is a photovoltaic module which consists of a number of photovoltaic cells. Photovoltaic cells are made of a semi-conducting material, most commonly silicon. When the cell is exposed to light the photons generate electrical charges inside the material. The electrical output from a single cell is small (around 0,6 V d.c.), therefore multiple cells are connected together within a PV module. The PV module is the electricity producing unit designed to withstand adverse environmental conditions (heat cold, humidity, etc.). The module is the commercial product (one does not buy or install a PV cell). Multiple PV modules are normally connected together to form a photovoltaic (PV) array with a greater power output. Direct current (d.c.) produced by the photovoltaic modules can be transformed into alternating current (a.c.) through a specifically designed inverter. Photovoltaic modules and inverter and all the ancillary control and management equipment are making a photovoltaic power system. PV systems vary in size to suit different applications (for example houses, commercial and industrial buildings or centralized production).

What applications are there for photovoltaic power systems?
Photovoltaic technology has many applications, both for off-grid systems in remote areas and grid-connected systems 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.
Photovoltaic technology is also widely used in the developing world. The technology is particularly suited where electricity grids are unreliable or non-existent. In remote locations photovoltaic power is often the most economic energy supply option. In addition, many developing countries have a high level of solar irradiation year round.
In more recent years, photovoltaics have 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 any additional space.

Do photovoltaic modules need bright sunshine to work properly? Will they produce electricity even when it is cloudy?
The electrical output of a photovoltaic module is dependent upon the intensity of the light to which it is exposed. Hence 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.

What must one do to operate the photovoltaic array effectively? Do PV modules need cleaning? Is any maintenance required?
Once installed, the photovoltaic modules (or the photovoltaic array) require 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. The modules are often self cleaning with rain however in dry areas it may be necessary to clean dust from the module surface.

Where can a PV array be installed?
A photovoltaic (PV) array (modules interconnected) can be installed on the roof, on the ground or as a building façade. Increasingly architects are integrating the PV system into the building itself; this is known as Building Integrated Photovoltaics (BIPV). For existing buildings it is more common for PV arrays to be fixed to the roof surface. Both flat and pitched roofs are suitable for PV systems; the type of mounting system will depend on the roof type and slope.

What factors should one consider before installing a photovoltaic array?
For a PV system to operate most effectively the PV modules should have an 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 shading this area in the future. Another important factor to consider is if the roof is structurally adequate to support the modules and mounting system.

How much PV will I need to power a home?
The system size required to power a home is dependant on many factors, especially the geographical location of the home, the energy demand of the household and any shading of the PV array. These factors will all be taken into account by a system installer when designing a photovoltaic power system. Typically, residential PV systems in most countries are found in the PV power range of 1 kW to 5 kW.

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/array that convert daylight into direct current electricity and a specifically designed inverter that converts the direct current to alternating current electricity which is compatible with the electricity grid. The system is grid-connected therefore storage batteries are not normally required. Excess electricity generated by the PV system can be exported to the grid, or if the electricity produced by the PV system does not meet the household electricity demand electricity can be imported from the grid.

How can one connect a photovoltaic power system to the electricity grid?
Connecting a photovoltaic power system to the electricity grid will require permission from the local electricity utility. The utilities have different policies when it comes to connecting photovoltaic systems to their electricity grids, and also different rates will be paid for exported electricity. In many cases the system installer will make the necessary arrangements for grid-connection.

Will a grid-connected photovoltaic power system still work when the electricity grid is out of order?
No; in case of a lack of power coming from the electricity grid, the inverter of the photovoltaic power system will shut off the PV system to protect people repairing the grid. It is possible to have a system with a self-regulating inverter and a storage battery back-up to operate independently from the electricity grid. However these photovoltaic power systems are normally more expensive than simple grid-connected systems.

Could one power a house and become independent from the electricity utility grid?
Over a year a PV system may produce sufficient energy to match a household’s annual electricity demand, if the PV system is sized appropriately. However the PV system only produces electricity during daylight and will not generate electricity overnight. Also, in many countries, there may be minimal production during the winter months when daylight is limited.

In remote areas it is possible to have an off-grid PV system which uses storage batteries to store energy when the electricity produced by the PV system exceeds electricity demand. This stored energy can then be used when the PV system is producing insufficient energy to meet demand.

What is the lifetime of photovoltaic modules?
The lifetime of a photovoltaic module can exceed 25 years - crystalline silicon modules in particular have a very long life span. There are many photovoltaic modules installed at the beginning of the 1970s that are still working well. The only limitation is the way the photovoltaic modules are able to withstand the aggressive environment they are installed in. Humid and salty climates are challenging nevertheless photovoltaic modules can still perform well.

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 arrangements.

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 with minimal 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).

Are photovoltaic power systems suitable for use in high latitudes?
Photovoltaic power systems have been used in high latitudes 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 and weekend cottages. Photovoltaic technology has also started to be introduced into high latitude 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 energy demands of an office.

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 solar energy value is 2 500 kWh/m² and in Sweden, for example, a typical annual value is around 1 000 kWh/m². Photovoltaic modules can still generate electricity in locations far from the equator.

What is the efficiency of photovoltaic cells and modules?
The practical limit of the conversion efficiency (proportion of sunlight energy that the cell converts to electrical energy) for photovoltaic cells is around 30 % (40 % is achieved under concentrated sunlight). 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.

Crystalline silicon PV modules have commercial efficiencies between 13 % and 18 %. Thin film technology modules are constructed by depositing extremely thin layers of photovoltaic semi-conductor materials onto a backing material such as glass, stainless steel or plastic. Module conversion efficiencies reported for thin film PV are currently ranging from 5 % to 10 %.

Further research and development is being carried out to improve the efficiency of all the basic types of cells with laboratory efficiencies for single crystal cells over 25 %, and for thin film technologies over 19 % being achieved.

What are the terms used to describe power from a photovoltaic system?
The basic electrical power of PV modules is given in watts (W). This represents the rated power of a PV device under standard test conditions (STC) of 1 000 W/m² irradiance, 25°C cell junction temperature and solar reference spectrum AM 1,5.

What is the land area covered by a centralized photovoltaic power station?
Photovoltaic power plants with wafer based crystalline silicon technology occupy about 1,2 ha (12 000 m²) per MW (one million watts) of power installed.

How much energy does a photovoltaic power system produce per year?
The electricity production depends upon the geographic situation and the system size. For example, in Sweden a photovoltaic power system with a rated power of 1 kW can produce about 600 kWh to 800 kWh of electrical energy per year. The corresponding figures for Austria are 850 kWh to 950 kWh; for Japan, 1 000 kWh to 1 100 kWh; for Italy, 1 000 kWh to 1 500 kWh; and for Australia, 1 000 kWh to 2 000 kWh per year.

 

Economic aspects

How much does it cost to install a photovoltaic power system?
On average, installed system prices for the lowest price off-grid applications are double those for the lowest price grid-connected applications. This is because off-grid systems require storage batteries and associated equipment.

In 2007 the lowest system prices in the off-grid sector, irrespective of the type of application, typically ranged from about 10 USD/W to 15 USD/W. The average price of grid-connected systems was about 7 USD/W. The trend in PV prices over the last decade can be found in the National Survey Reports and the Trends in Photovoltaic Applications reports on this website.

Why use photovoltaic power systems when they are currently expensive to buy?
The demand for grid-connected PV systems comes from electricity customers, electricity utilities and also indirectly from policy makers and governments – each having their own set of preferences when it comes to the benefits provided by PV that are in addition to the supply of electricity.

For residential customers the most important values are environmental aspects, image and prestige, reliability of PV systems, system modularity, independence with regard to their power supply and the corresponding price benefits.

Commercial customers are interested in not only making money with PV (if local promotion policies allow it) but also additional values such as prestige, image or supply security.

Architects identify photovoltaics mainly as a building element with multifunctional characteristics such as shading, roofing or material saving. The innovative design features, for example colour, shape or transparency, or the thermal performance characteristics of PV such as heating, ventilation or insulation, may also inspire this group to make use of PV systems. Another important value is the prestige associated with a PV system.

Some characteristics of photovoltaics could become more important for electricity utilities in the future, for example PV as an opportunity for new markets and business, or to improve the image and prestige of the utility. An increasing number of utilities are offering green power products as a distinguishing element in liberalized and competitive markets. The potential for PV to contribute to peak electricity demand reduction is also an important value from the electricity utility point of view.

Whereas the benefits of PV outlined above are in fact a part of the sum of societal benefits, policy makers and governments mainly focus on the issue of avoiding fossil fuels in order to contribute to supply security and to secure the environmental benefits, in addition to the promotion of industry development and creation of new job opportunities.

What is a feed-in tariff?
The term feed-in tariff (FiT) simply refers to a market support measure in which an explicit monetary reward is provided for producing PV electricity; paid (usually by the electricity utility) at a rate per kWh somewhat higher than the retail electricity rates being paid by the customer – which is why the measure is often more correctly termed an enhanced FiT. In principle the measure encourages efficient production of PV electricity with the output from the PV system being monitored and recorded, and is described as the performance-based market support measure. Some form of FiT is now being applied, or considered, in countries belonging to the IEA PVPS programme.

Is the price of electricity produced by a photovoltaic power system competitive with conventional sources, and what does grid-parity mean?
At present photovoltaic electricity is not competitive with conventional sources of energy in urban environments. Nevertheless, in places where the electricity grid is not available, stand-alone (off-grid) photovoltaic power systems may provide a more cost-effective energy service than other sources of energy.

Grid-parity - the point in time when the cost of electricity from PV systems matches the price paid by consumers for retail electricity – is anticipated to occur within five to ten years in a number of countries. This is because the price of PV electricity should fall as manufacturing costs decrease, while the retail electricity price from conventional electricity suppliers should rise in response to a number of factors – incorporation of a carbon price in electricity prices, investments in ageing electricity infrastructure, further commercialization of the electricity industry and so on.

 

Environmental and social aspects

Are photovoltaic cells completely environmental friendly?
During their electricity generation lifetime (more than 25 years) photovoltaic modules do not have any impact on the environment. During production and especially material purification there are some negative impacts on the environment. For silicon photovoltaic cells the silicon has to be very pure and this purification process is energy demanding; however developments in the manufacturing process have reduced this energy demand.

The majority of photovoltaic cells used are made out of silicon, which is non toxic. Photovoltaic modules additionally incorporate glass, plastic and in some cases an aluminium frame. Some photovoltaic cells not made out of crystalline silicon. Thin film materials commercially used are amorphous silicon (a-Si), cadmium telluride (CdTe), and copper-indium-gallium-diselenide (CIGS). Cadmium in particular needs to be handled with care.

Are developing countries able to make use of photovoltaic technology?
Not only are developing countries able to make use of the technology, in many cases it will increasingly play a vitally important role. Bringing basic electricity services to rural areas, pumping drinking water and powering telecommunication repeater stations are already popular applications.

PV offers the ability, sometimes uniquely, to provide electricity to populations remote from electricity grids and also to enhance the quality of existing electricity supplies. Although not directly mentioned in the UN Millennium Development Goals access to electricity is a recognized prerequisite for meeting these goals.

What do countries do to promote the use of photovoltaic electricity?
Local, national and international policies, as well as availability of suitable standards and codes and the perception of the general public, electricity utilities and others, all govern the rate of deployment of PV systems worldwide. The ways that different countries promote PV are described in the National Survey Reports found on this website, and are summarized in the report Trends in Photovoltaic Applications, also found on this website.