Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 283

Decentralised systems Quasi centralised systems Centralised systems

~

DC

AC

~

DC

AC

~

DC

AC

~ ~

Public power grid

Fig. 6.28 Concepts for grid-connected photovoltaic plants (see also /6-10/)

− Centralised systems with several 100 kW or few MW (Fig. 6.28) are typically
mounted on the ground or on very large roofs such as those of fairs. Solar
modules may either be stationary mounted or tracked to the current solar alti-
tude by single-axis or two-axis tracking systems. The energy generated by
photovoltaics is fed into the low or medium voltage grid by means of one or
several inverters and a transformer. Photovoltaic plants of this type show cur-
rently electric capacities between some 100 kW and up to 5 MW. However,
even higher capacities are achievable from a technical point of view without
any problems.

6.2.5 Energy conversion chain, losses, and characteristic power curve

Energy conversion chain. Grid-connected photovoltaic power generation is
aimed at the supply of grid-compatible alternating current (AC), which is provi-
ded from radiated solar energy at several energy conversion levels illustrated in
Fig. 6.29.
As shown in the diagram, solar radiation energy (i.e. diffuse and direct radia-
tion), and thus the photons energy content, is first converted into potential energy
of the electrodes of the semi-conductor material; they are now able to move freely
within the crystal lattice. If recombination does not immediately take place, and
thus no energy in the form of heat is released into the crystal lattice, photovoltaic
cells supply this energy as direct current (DC). This direct current is then trans-
formed within grid-connected photovoltaic generators into alternating current
(AC) by means of a post-connected inverter customising to properties of the solar
energy to the relevant specifications to allow the feeding of the energy into the
power grid. Whereas small plants directly feed into the low-voltage grid, larger
systems feed into the medium voltage grid.