Environmental Engineering Reference
In-Depth Information
Figure 4.58
Efficiency over a Range of Relative Photovoltaic
Generator Powers
It considers the typical part-load operation for irradiation regimes in central
Europe.
The ideal energy yield
E
ideal
of a photovoltaic system with photovoltaic
area
A
PV
, module efficiency
η
PV
and solar irradiation
H
Solar
is given by:
(4.121)
Assuming a 10 m
2
inclined area with an annual solar irradiation of 1100
kWh/m
2
, modules with an efficiency of 10 per cent could generate 1100 kWh
per year in the ideal case. In reality, the energy yield of the photovoltaic system
is much lower. The average real efficiency of solar modules is lower than the
nominal efficiency due to soiling, shading and elevated operating temperatures.
Inverter losses reduce the yield further. The so-called
performance ratio PR
describes the ratio of the real and ideal energy:
(4.122)
Good systems have PRs of more than 0.75 = 75 per cent. This value can be
used for designing a new system if no shading losses are to be considered. Very
good systems can even reach PRs that are higher than 0.8. Problematic systems
can have PRs below 0.6. Inverter failures, MPP tracking problems, module
failure or solar generator shading are the main reasons for low PR values.
Figure 4.59 shows the connection of solar modules to a central inverter. A
number of modules are connected in series to form a string until the desired
voltage is reached. Several identical strings can be connected in parallel to
increase the power of the generator. Blocking diodes that were often used in
older systems can be omitted because their protection capability is low and
they cause permanent forward losses.
