Watt Peak
Watt peak ( English peak "peak") is an area in photovoltaic common but non-standard term for the electric power (unit: watts ) of solar cells . Multiples such as kilowatt peak , megawatt peak and gigawatt peak are also common . W p , kW p , MW p or GW p are used as abbreviations . The unit is used to compare different solar cells and solar modules under standardized test conditions.
definition
The watt peak is the electrical power output by solar modules under standard test conditions (STC) with the following parameters:
- Cell temperature = 25 ° C
- Irradiance = 1000 W / m²
- Sunlight spectrum according to AM = 1.5
The International Bureau of Weights and Measures, which administers the Si units , advises against using the designation and symbol to describe physical quantities.
use
Formulations such as “The photovoltaic system has an output of 10 kW p ” or “This is a 1.2 MWp open-space solar system” are colloquial. Formally correct, it should read “The photovoltaic system has a nominal output according to standard test conditions of 10 kW” or “This is a 1.2 MW open-space solar installation (nominal output according to standard test conditions)”.
The statement " An area of about 6 to 10 m² is required per kW p " means that an area of about 6 to 10 m² is required for a desired system output of 1 kW under standard test conditions.
Accordingly, the notation “ P nom = 1 kW” for photovoltaic systems is to be preferred to the notation “ P = 1 kW p ”, as adding additions to unit symbols does not conform to the standard.
Practical relevance in Germany
The irradiance of 1000 W / m² is a transient value under real conditions . The clearer the air, the closer you go towards the equator and the higher you are above sea level, the more often it is reached. It also depends on how close the sun is to the highest point. In Germany, it is usually only reached at noon on a cloudy day.
Frequency distributions of the irradiance in Germany measured every minute also show values above this. These can also reach up to 1500 W / m² through reflection and scattering. Due to the short availability and the fact that inverters are usually designed for an irradiance of 1000 W / m² and below ( economic maximum ), they are rarely used. The maximum of the irradiance at the edge of the earth's atmosphere corresponds to the solar constant E 0 and is 1367 W / m².
In normal operation, solar modules or solar cells with an irradiation of 1 kW / m² usually have a significantly higher operating temperature than the 25 ° C provided in the test and therefore up to 20% lower efficiency and a correspondingly lower power output. Due to the usually rigid alignment of a stationary photovoltaic system , the cells are seldom aligned exactly perpendicular to the incident light, which means that the irradiance has to be multiplied by the sine of the angle of incidence - which only has the value 1.0 at 90 ° - and is therefore generally reduced.
For example, the maximum sun height in summer in Munich is 65.3 °, in Flensburg 58.7 °; the sine for this results in a reduction to 90.85 or 85.45% by the factor. At the winter solstice, the performance in the example cities is even limited to 31.73 and 20.62% of the theoretical maximum.
The specification in watt peak is used to compare solar modules with the same area from different production in terms of their efficiency and the dimensioning of the different components of a solar system. It cannot be used as the sole information to characterize a photovoltaic system, as parameters such as the type of construction (open space, roof, tracked) and the installation site are essential for the energy yield and the economic efficiency of the system. H. the latitude and the associated mean irradiance or the climatic conditions prevailing at the installation site, such as temperature, are not taken into account.
In summary, it applies to an actually implemented photovoltaic system that the specification of the nominal power in watt peak corresponds neither to a maximum power nor to a continuous power. Since the radiation conditions are often worse and the modules are usually significantly warmer than under standard test conditions, the peak performance is only achieved sporadically in practice and is even seldom exceeded.
Individual evidence
- ↑ photovoltaikbuero.de: The confusion about the watt peak August 14, 2009.
- ↑ Physikalisch-Technische Bundesanstalt: PTB-Mitteilungen 117 (2007), issue 2, p. 22, chapter 5.3.2. "The International System of Units" ( hs-heilbronn.de ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , PDF, 1.4 MB).
- ↑ Mike Zehner, Mario Moll, Stefan Thaler, Oliver Wardrobe, Matthias Hartmann, Bernhard Mayer, Thomas Betts, Ralph Gottschalg, Klaus Behrens, Wolfgang Riecke, Werner Knaupp, Bodo Giesler, Gerd Becker, Oliver Mayer: Quantification of excessive irradiation in high-resolution DWD Data sets for various locations in Germany . In: 27th PV Symposium, Banz Monastery (Bad Staffelstein) . March 1, 2012 ( docplayer.org ).
- ↑ What does kilowatt peak (kWp) actually mean? ( Memento from October 28, 2012 in the Internet Archive ) SMA Solar Technology AG
- ↑ Volker Quaschning : Renewable energies and climate protection. 3rd edition, Munich 2013, p. 127.
