Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 286

formance. However, it must also be observed that efficiencies are considerably
enhanced starting from low radiation with increasing irradiance due to the open-
circuit voltage that increases logarithmically with irradiance. Fig. 6.31 also clearly
illustrates this fact within the section of low radiation sums.
According to Fig. 6.31 for the multi-crystalline cells the arising solar radiation
onto the module surface generates an increase of the direct current (DC). Yet,
doubled irradiance does not exactly double the direct current amount due to the
discussed cell temperature rise. The conditions are similar for the mono-
crystalline cells also illustrated in Fig. 6.31; however, in the latter case the area-
specific power generation is higher due to the higher cell efficiencies.
Fig. 6.31 reveals that for a given radiation the respective area-specific alternat-
ing current (AC) power generation is slightly lower when compared to the corre-
sponding direct current (DC) generation. This is due to the losses within the in-
verter.
6.3 Economic and environmental analysis
Within the following sections the specific power production costs and selected
environmental burdens are explained and discussed for state-of-the-art grid con-
nected photovoltaic generators.
Yet, the economic and environmental assessment of stand-alone photovoltaic
systems is much more difficult and largely depends on the respective site condi-
tions. For instance, the economic and environmental parameters of hybrid systems
are largely influenced by the distribution of the generated power among the indi-
vidual power generating units. Furthermore, there is no fixed easily determinable
comparison facilitating the economic and environmental comparison. Whereas for
grid-connected systems, the parameters mentioned above are generally compared
to power plant alternatives, the assessment of stand-alone systems is much more
difficult. For instance, light generated by photovoltaic solar home systems is often
used in replacement of candles, kerosene lamps and lead batteries. Additionally,
stand-alone systems are generally only applied in regions where they have an
economic edge over grid extension. The economic assessment quickly reveals that
not so much the power production costs but rather the power distribution cost
account for the major share of the consumer end price. For this reason, such sys-
tems will not be discussed in more detail throughout the following sections.
6.3.1 Economic analysis
Currently grid-connected photovoltaic power generation is mainly performed by
means of roof-mounted systems as well with an increasing importance by means
of so called photovoltaic power plants. Hence, a typical photovoltaic system of an
electric nominal capacity of 3 kW, installed on a slanting roof, will be analysed.