18 Glossary
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18.7
Global/total radiation
Denotes the irradiation of the sun towards a horizontal surface on the earth. It is composed of the direct
irradiation and the reflected irradiation (e.g.: by snowfields).
18.8
Short-circuit current of solar cells
Current that circulates, if the positive and the negative pole of the PV generator is connected, that means when
there is no resistance or load between the two poles. It is not dangerous for the generator, but a disconnection
might cause a light arc which is dangerous for health and environment!
18.9
Mismatching
The connection of weak and better modules in one string, whereby the weaker module in a row affects the
current of the string and in that way impairs the total power.
18.10
MPP
Denotes the point of maximum power in the I-V-curve. A solar cell has a point of maximum power in its I-V-
characteristic depending on the current irradiation, temperature and light spectrum. The product of usable
voltage and corresponding current of a solar cell does not always have the same value.
18.11
Tracking
The module-surfaces of 'trackers' follow the state of the sun during the course of the day. The tracking can be
carried out by one or two axis, whereby the production of two-axle tracking is more effective. Compared to a
permanent south orientated plant the annual yield increases with up to approximately 30%. Tower mounted,
tracked and elevated solar plants allows the choice of an optimum position. That way, irrespective of structural
circumstances, a panning range of 180° free from shadowing is made possible.
18.12
Peak Power
To guarantee the comparability of power output of PV-modules it was generally accepted that the nominal
power of a module is measured at a cell temperature of 25°C, an irradiation of 1000W/m² and a spectrum of
light according to AM=1,5 and named “Peak Power” (some producers also characterise it as “nominal value”).
The boundary conditions are called Standard Test Conditions (STC).
Unfortunately the natural appearance of the STC is rare, that is why the relevant measurements so far were
only realised in laboratories, where the conditions had to be produced under high effort.
With the new methods of the Peak Power measuring devices and curve tracers series PVPM, it is now possible
to realise these measurements under current ambient conditions - the measuring device automatically converts
the results to STC. Thus after just one I-V-curve measurement the Peak Power P
pk
, the internal series
resistance R
s
and the parallel resistance R
p
are available as a result. This result is, in comparison with the
specified value, an indication for different errors in the PV-Generator, that is how they simplify quality control
and error detection.
18.13
Performance Ratio
In the photovoltaic the “Performance Ratio” means the ratio of available profit and specified value of a plant. The
Performance Ratio is the quotient of the yield of alternating current and nominal yield of generator DC. It
indicates what proportion of the current produced by the generator is in real terms available. PV-plants of high
capacity reach a Performance Ratio of more than 70%. The Performance Ratio is often denominated as quality
factor (Q). Solar modules on the basis of crystalline cells reach a quality factor of 0,85 to 0,95, network coupled
plants average out at 70% to 75%.
18.14
Phox-sensor
The principle of the Phox (
PHO
tovoltaic Lu
X
) -sensor is that the current from the photovoltaic cell, which is
almost linear to the irradiation of the cell, is conducted through a so called shunt, a particular precision resistor.
The voltage which drops at the resistor is linear depending from the irradiation and can easily be measured with
Operation manual for PVPM devices
PV-Engineering GmbH
