how does pv work
‘Photovoltaic’ is a marriage of two words: ‘photo’, from Greek roots, meaning light, and ‘voltaic’, from ‘volt’, which is the unit used to measure electric potential at a given point.
Photovoltaic systems use cells to convert solar radiation into electricity. The cell consists of one or two layers of a semi-conducting material. When light shines on the cell it creates an electric field across the layers, causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity is.
The most common semi conductor material used in photovoltaic cells is silicon, an element most commonly found in sand. There is no limitation to its availability as a raw material; silicon is the second most abundant material in the earth’s mass.
A photovoltaic system therefore does not need bright sunlight in order to operate. It can also generate electricity on cloudy days.
PV Technologies: Cells and Modules
PV cells are generally made either from crystalline silicon, sliced from ingots or castings, from grown ribbons or thin film, deposited in thin layers on a low-cost backing.
The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. A typical commercial solar cell has an efficiency of 15% about one-sixth of the sunlight striking the cell generates electricity. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the PV industry.
Crystalline silicon technology:
Crystalline silicon cells are made from thin slices cut from a single crystal of silicon (monocrystalline) or from a block of silicon crystals (polycrystalline), their efficiency ranges between 11% and 19%. This is the most common technology representing about 90% of the market today.
Three main types of crystalline cells can be distinguished:
- Monocrystalline (Mono c-Si)
- Polycrystalline (or Multicrystalline) (multi c-Si)
- Ribbon sheets (ribbon-sheet c-Si)
Thin Film technology:
Thin film modules are constructed by depositing extremely thin layers of photosensitive materials onto a low-cost backing such as glass, stainless steel or plastic.
Thin Film manufacturing processes result in lower production costs compared to the more material-intensive crystalline technology, a price advantage which is counterbalanced by lower efficiency rates (from 4% to 11%). However, this is an average value and all Thin Film technologies do not have the same efficiency.
Four types of thin film modules (depending on the active material used) are commercially available at the moment:
- Amorphous silicon (a-Si)
- Cadmium telluride (CdTe)
- Copper Indium/gallium Diselenide/disulphide (CIS, CIGS)
- Multi junction cells (a-Si/m-Si)
Other cell types:
There are several other types of photovoltaic technologies developed today starting to be commercialised or still at the research level, the main ones are:
- Concentrated photovoltaic:
Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. Efficiencies are in the range of 20 to 30%.
- Flexible cells:
Based on a similar production process to thin film cells, when the active material is deposited in a thin plastic, the cell can be flexible. This opens the range of applications, especially for Building integration (roofs-tiles) and end-consumer applications.
The Photovoltaic technology can be used in several types of applications:
Grid-connected domestic systems
This is the most popular type of solar PV system for homes and businesses in developed areas. Connection to the local electricity network allows any excess power produced to feed the electricity grid and to sell it to the utility. Electricity is then imported from the network when there is no sun. An inverter is used to convert the direct current power produced by the system to alternative power for running normal electrical equipments.
Grid-Connected power plants
These systems, also grid-connected, produce a large quantity of photovoltaic electricity in a single point. The size of these plants range from several hundred kilowatts to several megawatts. Some of these applications are located on large industrial buildings such as airport terminals or railways stations. This type of large application makes use of already available space and compensates a part of the electricity produced by these energy-intensive consumers.
Off-grid systems for rural electrification
Where no mains electricity is available, the system is connected to a battery via a charge controller. An inverter can be used to provide AC power, enabling the use of normal electrical appliances. Typical off-grid applications are used to bring access to electricity to remote areas (mountain huts, developing countries). Rural electrification means either small solar home system covering basic electricity needs in a single household, or larger solar mini-grids, which provide enough power for several homes. More information is available on CCRES.
Off-grid industrial applications
Uses for solar electricity for remote applications are very frequent in the telecommunications field, especially to link remote rural areas to the rest of the country. Repeater stations for mobile telephones powered by PV or hybrid systems also have a large potential. Other applications include traffic signals, marine navigation aids, security phones, remote lighting, highway signs and waste water treatment plants. These applications are cost competitive today as they enable to bring power in areas far away from electric mains, avoiding the high cost of installing cabled networks.
Photovoltaic cells are used in many daily electrical appliances, including watches, calculators, toys, battery chargers, professional sun roofs for automobiles. Other applications include power for services such as water sprinklers, road signs, lighting and phone boxes.
The most important feature of solar PV systems is that there are no emissions of carbon dioxide - the main gas responsible for global climate change - during their operation. Although indirect emissions of CO2 occur at other stages of the lifecycle, these are significantly lower than the avoided emissions. PV does not involve any other polluting emissions or the type of environmental safety concerns associated with conventional generation technologies. There is no pollution in the form of exhaust fumes or noise.
Decommissioning a system is unproblematic. Although there are no CO2 emissions during operation, a small amount does result from the production stage. PV only emits 21,65 grams CO2/kWh, however, depending on the PV technology. The average emissions for thermal power in Europe, on the other hand, are 900g CO2/kWh. By substituting PV for thermal power, a saving of 835879 g/kWh is achieved.
The benefit to be obtained from carbon dioxide reductions in a country's energy mix is dependent on which other generation method, or energy use, solar power is replacing. Where off-grid systems replace diesel generators, they will achieve CO2 savings of about 1 kg per kilowatt-hour. Due to their tremendous inefficiency, the replacement of a kerosene lamp will lead to even larger savings, of up to 350 kg per year from a single 40 Wp module, equal to 25kg CO2/kWh. For consumer applications and remote industrial markets, on the other hand, it is very difficult to identify exact CO2 savings per kilowatt-hour.
Recycling of PV modules is possible and raw materials can be reused. As a result, the energy input associated with PV will be further reduced.
If governments adopt a wider use of PV in their national energy generation, solar power can therefore make a substantial contribution towards international commitments to reduce emissions of greenhouse gases and their contribution to climate change.
By 2030, according to the EPIA-Greenpeace Solar Generation Advanced Scenario, solar PV would have reduced annual global CO2 emissions by just over 1,6 billion tonnes. This reduction is equivalent to the output from 450 coal-fired power plants (average size 750 MW).
Cumulative CO2 savings from solar electricity generation between 2005 and 2030 will have reached a level of 9 billion tonnes.
Carbon dioxide is responsible for more than 50% of the man-made greenhouse effect, making it the most important contributor to climate change. It is produced mainly by the burning of fossil fuels. Natural gas is the most environmentally sound of the fossil fuels, because it produces roughly half as much carbon dioxide as coal, and less of other polluting gases. Nuclear power produces very little CO2, but has other major safety, security, proliferation and pollution problems associated with its operation and waste products.
A popular belief still persists that PV systems cannot "pay back" their energy investment within the expected lifetime of a solar generator - about 25 years. This is because the energy expended, especially during the production of solar cells, is seen to outweigh the energy eventually generated.
Data from recent studies shows, however, that present-day systems already have an energy payback time (EPBT) - the time taken for power generation to compensate for the energy used in production - of 1 to 3.5 years, well below their expected lifetime. With increased cell efficiency and a decrease in cell thickness, as well as optimized production procedures, it is anticipated that the EPBT for grid-connected PV will decrease further.
The figure hereafter shows energy payback times for different solar cell technologies (thin film, ribbon, multicrystalline and monocrystalline) at different locations (southern and northern Europe). The energy input into a PV system is made up of a number of elements, including the frame, module assembly, cell production, ingot and wafer production and the silicon feedstock. The energy payback time for thin film systems is already less than a year in southern Europe. PV systems with monocrystalline modules in northern Europe, on the other hand, will pay back their input energy within 3.5 years.
Figure - Energy payback times for range of PV systems (rooftop system, irrad. 1700 resp. 1000 kWh/m2/year)
10 good reasons to switch to solar photovoltaic electricity
Photovoltaic is emerging as a major power source due to its numerous environmental and economic benefits and proven reliability:
The fuel is free.
The sun is the only resource needed to power solar panels. And the sun will keep shining until the world's end. Also, most photovoltaic cells are made from silicon, and silicon is an abundant and non-toxic element (the second most abundant material in the earth's mass).
It produces no noise, harmful emissions or polluting gases.
The burning of natural resources for energy can create smoke, cause acid rain, pollute water and pollute the air. Carbon dioxide or CO2, a leading greenhouse gas, is also produced. Solar power uses only the power of the sun as its fuel. It creates no harmful by-product and contributes actively to reduce the global warming.
From : Externe project, 2003; Kim and Dale, 2005; Fthenakis and Kim, 2006; Fthenakis and Kim, 2007; Fthenakis and Alsema, 2006.
PV systems are very safe and highly reliable.
The estimated lifetime of a PV module is 30 years. Furthermore, the modules' performance is very high providing over 80% of the initial power after 25 years which makes photovoltaics a very reliable technology in the long term. In addition, very high quality standards are set at a European level which guarantees that consumers buy reliable products.
The energy pay-back time of a module is constantly decreasing.
This means that the time required for a PV module to produce as much energy as it needs to be manufactured is very short, it varies between 1,5 years to 3 years. This is between 6 to 18 times more energy than the energy needed to be manufactured (depending on the technology, the type of system and the location).
PV Modules can be recycled and therefore the materials used in the production process (silicon, glass, aluminium, etc.) can be reused.
Recycling is not only beneficial for the environment but also for helping to reduce the energy needed to produce those materials and therefore the cost of fabrication. More information is available on the following website: CCRES.
It requires low maintenance.
Solar modules are almost maintenance-free and offer an easy installation.
It brings electricity to remote rural areas.
Solar systems give an added value to rural areas (especially in developing countries where electricity is not available). House lighting, hospital refrigeration systems and water pumping are some of the many applications for off-grid systems. Telecommunication systems in remote areas are also well-known users of PV systems.
It can be aesthetically integrated in buildings (BIPV).
Systems can cover roofs and façades contributing to reduce the energy buildings consume. They don't produce noise and can be integrated in very aesthetic ways. .European building legislations have been and are being reviewed to make renewable energies as a required energy source in public and residential buildings. This fact is accelerating the development of ecobuildings and positive energy buildings (E+ Buildings) which opens up many opportunities for a better integration of PV systems in the built environment. More information is available on CCRES.
It creates thousands of jobs.
The PV sector, with an average annual growth of 40% during the past years is increasingly contributing to the creation of thousands of jobs in Europe and worldwide.
It contributes to improving the security of Europe's energy supply.
In order to cover 100% of the electricity demand in Europe, only the 0.7% of the total land of Europe would be needed to be covered by PV modules. Therefore Photovoltaics can play an important role in improving the security of Europe's energy supply.
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Croatian Center of Renewable Energy Sources (CCRES)