Page 106 - Advances in Renewable Energies and Power Technologies

2. Intermittent and Stochastic Renewable Energy Production 79

After a paragraph on the accuracy evaluation of the models, the various available
forecasting methodologies for solar irradiation will be presented according to the
forecast horizon. Finally, new perspectives will be reported in a last paragraph.

2. INTERMITTENT AND STOCHASTIC RENEWABLE ENERGY
PRODUCTION IN AN ELECTRICAL GRID
It is important to explain why the introduction of intermittent and stochastic electri-
cal power into an electrical network induces some difficulties for the energy man-
ager, to better understand the need to have efficient forecasting tools to estimate
the wind and PV powers at various temporal horizons.
Before getting into details, it is worth reminding ourselves some principles on the
electrical network management, especially on the challenge to reach at every
moment a precise balance between production and consumption.

2.1 THE PRODUCTION/CONSUMPTION BALANCE: A DIFFICULT TASK
EVEN WITH CONVENTIONAL ENERGY PRODUCTION MEANS
The produced electricity must be immediately used whenever it is sent to the elec-
trical network. The energy manager must have all the means necessary to produce
electrical energy and must especially ensure a precise balance between electricity
generation and demand. In a power system, this power balance must be maintained
at any moment, i.e., the electricity demand must be balanced by the electricity gen-
eration at all times. The electrical network must be working at a stabilized frequency
(50 Hz, sometimes 60 Hz); thus, the production systems must adapt their production
to the power consumption either in the electrical stations or in the dispatching center
at all time.
In normal mode, there is a situation of balance: the production is always equal to
the consumption and the electricity frequency slightly fluctuates around the refer-
ence frequency as seen in Fig. 3.1.
In case decrease of the electrical production happens suddenly because of the
loss of a means of production (or if a shadow of a cloud moves across a PV plant
or the wind speed drops off in front of the wind turbine), this balance is broken
and the frequency falls below the reference frequency. In these conditions, a rapid
increase of the produced electrical power by a connected production means is
required (taking into account the ramp rate e rate of power rise) or the start-up of
a new production means should be added (according to the start-up time). But the
power ramp rate of an energy plant and its starting time are not instantaneous as
seen in Table 3.2 [5]. As the run-up time is long, the activation of a new production
system should be anticipated; only a hydraulic plant starts rapidly, followed by a
light-fuel turbine; internal combustion engines (fuel or gas) need about 45 min
before starting to produce electricity (see Table 3.2).