Photovoltaics
In just one hour the sun delivers more energy to Earth than the world uses in an entire year. This solar energy can be utilised in many ways, through the use of photovoltaics (PV), for example. German PV technology is in use on every continent on the globe, supplying electricity to grid-connected and off-grid systems.
- Diagram of a solar cell:
1.) negative electrode
2.) positive electrode
3.) n-silicon
4.) p-silicon
5.) barrier layer
Technologies and applications
Photovoltaic cells enable sunlight to be converted directly into electrical energy. Positive and negative charges are separated by radiation energy in the solar cell and collected for use at the two poles of the cell, the same way as in a battery. A specific number of solar cells (e.g. 48) are combined and connected to form one solar module. Thus, the solar module is the heart of a PV system, and the cell is the heart of the solar module.
Over 90 % of the solar cells in use around the world consist of crystalline silicon, which has proven itself over decades of use. In future, thin-film cells will also play a more important role thanks to cheaper production materials, more homogenous surfaces and other operative features when compared to crystalline solar cells. The fall in the price of solar silicon in 2009 has significantly reduced the differential between crystalline and thin-film modules. Nevertheless, thin-film technology promises great future potential for extending the range of possible applications for photovoltaics. This is also true for technologies that are still being researched and tested, such as organic photovoltaics (OPV), which copy photosynthesis processes observed in nature.
Thin-film cells still operate at a lower efficiency factor however, so they require a larger installation area to achieve the same output capacity as standard modules. When selecting photovoltaic modules it is therefore important to consider not only basic module costs (price per kilowatt) but also the system costs (“production costs”) per kilowatt-hour produced. Locations exposed to high levels of direct solar radiation make investments in this technology more profitable. Grid-connected solar power systems are currently experiencing the strongest growth worldwide. These systems use inverters to convert solar power into a grid-compatible alternating current and feed it into the public power grid.
Grid-connected photovoltaic systems are available in a wide range of power classes, ranging from small systems on apartment buildings with an output of 1 kWp (kilowatt peak) and a solar module surface area of approximately 10 square metres, up to large, free-standing systems with outputs of up to 100 MWp (megawatt peak). Small systems with typical outputs of 3 – 4 kWp can be integrated easily into existing buildings. Medium-size systems with outputs ranging from approx. 30 to 100 kWp are often mounted on factory and office buildings, farm buildings, schools, town halls and other public buildings. There are even a few industrial rooftop systems in the megawatt range. However, large systems with outputs from 1 to 60 MW are usually constructed as free-standing systems in open areas.
Photovoltaic systems also make it possible to generate and use electrical energy independently of existing power grids, dispensing with the high cost of constructing power grids over long distances. Solar systems with integrated battery systems are also suitable as back-up systems for regions with unreliable power supplies. The simplest method of off-grid application is to use the direct current generated by the solar energy to operate electrical equipment locally. Photovoltaics can, however, also be used to create off-grid ‘island’ systems. Such ‘mini-grids’ can supply electricity to facilities ranging in size from individual buildings up to several small towns. In order to feed the supply into mini-grids, an inverter has to first convert the electricity into alternating current. To ensure that electricity is available when required, even during periods of insufficient solar radiation, it is advisable to integrate a storage module (e.g. battery) into the mini-grid during construction.
A long-term, convenient and cost-effective version of off-grid electricity supply using such island systems could be created by combining photovoltaic systems with wind farms and hydropower plants and/or electricity generators powered by diesel and bio fuels (hybrid systems). PV off-grid systems may make it possible to save on the fuel (e.g. diesel) used to generate electricity in generators in rural areas, fuel that often has to be transported over great distances.
