Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 268

tion at the optimal operating point. For this purpose a sufficiently dimensioned
buffer capacitor is required at the inverter entry.
− Excess voltages, for instance caused by idle solar generators at low tempera-
tures and high solar radiation, but also by distant lightning strikes, must not
cause any defects.
− Inverters are generally designed for a slightly lower nominal power than that of
the photovoltaic generator (e.g. factor varying from 0.8 to 0.9). This is due the
fact that solar generators only rarely reach their nominal power because during
full solar radiation the resulting temperature increase of the modules reduces
the overall cell efficiency. Additionally, smaller inverters show a lower self-
consumption. Therefore an optimum can be found regarding the overall effi-
ciency rate. Nevertheless, if overloading of the inverter does occur, the inverter
power input must be precisely limited by displacing the operating point to-
wards the open-circuit voltage. In the ideal case, the admissible power is
adapted to the current passive cooling element temperature. Another, however
not optimal solution, is to disconnect and periodically restart the inverter in
case of over-load.
− Grid-connected inverters should be supplied with energy by the solar generator
itself, so that no energy from the grid is consumed at night-time. Furthermore,
the inverter should already start and perform reliably at very low solar radiation
levels.
− High conversion efficiency should already be achieved for small capacities
(> 90 % at 10 % nominal power). The so-called "European Efficiency" allows
for a simple comparison of different types of inverters by considering the typi-
cal distribution of energy generated by solar generators according to the typical
Central European climate. For this purpose inverter efficiency is weighted dif-
ferently for six different power levels. Medium capacity levels are taken with a
higher contribution into consideration because the efficiency curve should yield
high values within this section. European efficiencies of smaller inverters
(< 1 kW) should be above 90 % and between approximately 95 and 97 % for
larger inverters.
− Grid-connected inverters should be provided with integrated self-monitoring
systems equipped with user-friendly displays and interfaces to a communica-
tion system, if required. The latter allows for a permanent monitoring and re-
mote diagnosis, which cannot necessarily be provided by the average user.
Mounting systems. Energy yields of photovoltaic modules are proportional to
radiated solar energy. This is why the orientation of module surfaces towards the
sun is of major importance. In this respect, fixed mounting systems and one or
two axes tracking systems are distinguished. In general such tracking systems
allow increasing the electricity generation compared to installations without a
tracking device. For sun-concentrator photovoltaic systems such tracking systems
are indispensable because these installations allow only the use of direct radiation.